Aliovalent-substitution defect chemistry, crystalline

Solid State Ionics 278 (2015) 157–165

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Aliovalent-substitution defect chemistry, crystalline-glassy phase separation and ionic conductivity in fresnoite Ba2TiSi2O8-based materials Quanchao Wang a, Shuaibo Liu a, Xiaoming Wang b, Hui Fu b, Jungu Xu a, Fengqi Lu a, Emmanuel Véron c,e, Mathieu Allix c,e, Florence Porcher d, Xiaojun Kuang a,⁎ a Guangxi Ministry-Province Jointly-Constructed Cultivation Base for State Key Laboratory of Processing for non-Ferrous Metal and Featured Materials, Guangxi Universities Key Laboratory of Non-ferrous Metal Oxide Electronic Functional Materials and Devices, College of Materials Science and Engineering, Guilin University of Technology, Guilin 541004, PR China b Beijing National Laboratory for Molecular Sciences, The State Key Laboratory of Rare Earth Materials Chemistry and Applications, College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, PR China c UPR3079 CEMHTI, 1D Avenue de la Recherche Scientifique, 45071 Orléans Cedex 2, France d CEA/Saclay, Laboratoire Léon Brillouin, F-91191 Gif Sur Yvette, France e Université d'Orléans, Faculté des Sciences, Avenue du Parc Floral, 45067 Orléans Cedex 2, France

a r t i c l e

i n f o

Article history: Received 27 March 2015 Received in revised form 21 May 2015 Accepted 3 June 2015 Available online xxxx Keywords: Ionic conductivity Glass 29 Si NMR Powder diffraction Atomistic simulation Defect chemistry

a b s t r a c t Ba2TiSi2O8 fresnoite, containing a layered mixed TiO5 and SiO4 polyhedral network with pentagonal tunnels similar to melilite, displayed extremely limited solubility for both K+ and La3+ in Ba2+ sites. Atomistic static lattice simulations revealed high energetic costs for K+ and La3+ substitutions for Ba2+ in Ba2TiSi2O8. These results emphasize the rigidity of the mixed layered polyhedral network of Ba2TiSi2O8 fresnoite, which is not suitable for stabilizing the oxygen vacancies and interstitials. In contrast with the La-substituted compositions forming crystalline mixtures, the Ba2 − xKxTiSi2O8 − 0.5x compositions showed phase separation into crystalline Ba2TiSi2O8 and amorphous K2TiSi2O7. However, the addition of potassium enhanced the ionic conductivity of Ba2 − xKxTiSi2O8 − 0.5x compositions, which mainly arises from the potassium conduction in the glass component K2TiSi2O7 as the oxide ion conduction was found to be limited. Measurements on the pure K2TiSi2O7 glass entirely reproduced the electrical and crystallization behaviors observed in Ba2 − xKxTiSi2O8 − 0.5x composites, confirming experimentally the responsibility of the glassy material for the enhanced potassium ionic conductivity in Ba2 − xKxTiSi2O8 − 0.5x composites. This study contributes to the further understanding of oxide ionic conductivity of SrSiO3-based materials, which is currently under debate. © 2015 Elsevier B.V. All rights reserved.

1. Introduction Given the increasing concern over the energy and ecological issues, there is significant interest in developing clean energy sources, for which solid oxide fuel cell (SOFC) is a major candidate technology because of its high efficiency and fuel flexibility [1]. Lowering the operating temperature of SOFCs requires electrolytes with high oxide ion conductivity (exceeding 10−2 S/cm) at low temperature range (500– 700 °C) [1–3]. Oxide mobility in solids is associated with defects, either oxide anions vacancies or interstitial oxide anions [3,4]. Oxide vacancies are the charge carriers in most cases, for example, the leading electrolytes including yttrium-stabilized zirconium and Gd-doped ceria [5] fluorites and Sr, and Mg-doped lanthanum gallate perovskites [6,7]. The mobile oxygen interstitials are relatively rare and often observed in tetrahedral prototypes including apatite [8], scheelite [9], β-SnWO4 [10] and melilite [4,11], where the tetrahedral cations have variable coordination geometry and are readily bonding with the extra oxygen to ⁎ Corresponding author. E-mail address: [email protected] (X. Kuang).

http://dx.doi.org/10.1016/j.ssi.2015.06.002 0167-2738/© 2015 Elsevier B.V. All rights reserved.

stabilize the interstitials. Tetrahedral units with non-bridging terminal oxygens are key components for transporting oxygen interstitials given their deformation and rotation flexibility [4,11]. Creation of oxygen vacancies in tetrahedral prototypes has been also proved to be practical for developing oxide ion conductors. Kendrick et al. [12] have reported that in the La1 − xBa1 + xGaO4 − 0.5x system that contains isolated tetrahedra units, oxygen vacancies in the GaO4 tetrahedra introduced via Ba2+-doping on the La site can be stabilized via sharing corners with the neighboring GaO4 tetrahedra, forming Ga2O7 tetrahedral dimer units to maintain the four-coordinate geometry of Ga. The vacancies migrate via a cooperative mechanism involving the continuous breaking and re-forming of Ga2O7 units assisted by tetrahedral deformation and rotation [12]. Recently, Goodenough et al. have also reported new oxygen vacancy-conducting Na/K-doped SrMO3 (M = Si, Ge) containing isolated corner-shared three-fold cyclical tetrahedral trimers M3O9 [13,14]. However a debate arose on questioning the oxide ion conductivity of the SrSiO3-based materials from a direct oxide-ion diffusivity measurement on the Sr0.8K0.2Si0.5Ge0.5O2.9 composition [15]. The results showed very low levels of oxide ion conductivity, which is unable to correlate with the high overall conductivity observed

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on these materials, so that other charge species for the total conductivity have to be considered [15]. The low oxide ion conductivity in these materials was correlated with the inhomogeneous chemical composition nature shown on Sr0.8 K0.2Si0.5Ge0.5O2.9, which revealed the presence of amorphous potassium silicate component and questioned the presence of potassium inside the crystalline SrSi0.5Ge0.5O3 phase to produce the oxygen vacancies. Very few amount of sodium in the crystalline SrSiO3 phase and existence of amorphous materials were also found in the Nadoped SrSiO3 [16–18]. More recently, a density functional theory (DFT) calculation has showed that high energetic costs associated with the oxygen vacancy formation in the SrSiO3-based materials do not favor sodium and potassium substitution of the strontium sites [18]. In short words, the role of the glass phase has been inferred to enhance the conductivity in the SrSiO3-based materials but with Na+/K+ as major diffusion charge carriers [17]. The layered tetrahedral network of the melilite has also demonstrated capability to accommodate mobile oxygen interstitials as in La1 + xSr1 − xGa3O7 + 0.5x compositions [4]. The oxygen excess, introduced by La3+ substitution for Sr2+, enters into pentagonal tunnels formed in the (3,4)-linked tetrahedral Ga3O7 layers at tetrahedral Ga3 + cationic level, and is incorporated into the environment of one of the five tetrahedra defining the pentagonal tunnel. More recently the Sr2MgSi2O7 melilite [19] was reported to show the ability to create mobile oxygen vacancies upon the acceptor-doping of Na+ in Sr2+ sites, in contrast with oxygen deficient La1 − xSr1+ xGa3O7 − 0.5x melilite showing insulating behavior [20]. Ba2TiSi2O8 (BTS) adopts noncentrosymmetric (P4bm) tetragonal fresnoite structure and displays good ferroelectric, piezoelectric and pyroelectric properties [21,22]. The Fresnoite structure consists of twodimensional mixed tetrahedral and square pyramidal network [23,24] alternating with Ba2+ cationic layers along the c-axis (Fig. 1), which has common structural features with the melilite structure e.g. the (3,4)-linkage of polyhedra with pentagonal rings to accommodate the large Ba2 + cations, and the corner-sharing tetrahedral dimers [4]. In order to gain oxide ion conductivity in the layered fresnoite Ba2TiSi2O8 by creating the oxygen vacancies or interstitials through the acceptor/ donor-doping, we investigated the phase formation and conductivities of potassium and lanthanium-substituted Ba2TiSi2O8 compositions Ba2 − xKxTiSi2O8 − 0.5x and Ba2 − xLaxTiSi2O8 + 0.5x in this study. We found that Ba2TiSi2O8 has limited solubility for both potassium and lanthanium but the addition of potassium enhanced the ionic conductivity of Ba2 − xKxTiSi2O8 − 0.5x compositions even with limited potassium solubility in Ba2TiSi2O8, similar to the SrMO3-based materials [15–18,25]. Ba2 − xKxTiSi2O8 − 0.5x compositions showed phase separation into Ba-rich crystalline and K-rich amorphous phases. We experimentally proved for the first time that the ionic conductivity enhancement originates from the formation of glassy phase in the potassium-substituted Ba2TiSi2O8 composites. Complementary characterizations using 29Si NMR, neutron diffraction and variable temperature X-ray diffraction techniques as well as the theoretically atomistic static lattice simulations for the defect formation were carried out on Ba2 − xKxTiSi2O8 − 0.5x for probing the defect chemistry in the crystalline phase and thermal behavior of the glassy phase. 2. Experimental section 2.1. Synthesis Ba2 − xKxTiSi2O8 − 0.5x (x = 0.0–1.6) and Ba2 − xLaxTiSi2O8 + 0.5x (x = 0–1) samples were synthesized by solid state reaction route using BaCO3 (99.9%), K2CO3 (99.9%), La2O3 (99.99%), TiO2 (99.9%) and SiO2 (99.9%) as starting materials. These materials were weighed according to the correct stoichiometries and mixed thoroughly in ethanol in agate mortar. For the potassium-substituted compositions, the samples were calcined at 1000 °C (x = 0–0.3), 900 °C (x = 0.4–0.7), or 800 °C (x = 0.8–1.6) for 16 h with heating and cooling rates of 5 °C/

Fig. 1. Structure of the Ba2TiSi2O8 fresnoite: projections along c-axis (a) and b-axis (b). TiO5 square pyramids and Si2O7 tetrahedral dimers are drawn in yellow and cyan, respectively. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

min. After calcination, Ba2 − xKxTiSi2O8 − 0.5x powders were ground, pressed into pellets and fired at 1300 °C (x = 0), 1250 °C (x = 0.1– 0.3), 1100 °C (x = 0.4–0.7), 990 °C (x = 0.8–1.1), 960 °C (x = 1.2– 1.6) for 16 h to obtain the final materials. The pellet samples were prepared for compositions up to x = 1.4 in Ba2 − xKxTiSi2O8 − 0.5x. No pellets could be obtained for higher potassium contents as the samples started to melt at 960 °C and reducing the firing temperature resulted in mixed crystalline phases. For neutron powder diffraction, about 12 g of Ba0.4K1.6TiSi2O7.2 were fired at 960 °C for 16 h with heating and cooling rates of 5 °C/min. The K2TiSi2O7 glass was prepared according to the following procedure: the mixed stoichiometric raw materials K 2 CO 3 , TiO 2 and SiO 2 were pre-heated at 500 °C for 2 h then fired at 1200 °C for 20 min to get the melt, which was subsequently quenched in air to room temperature, leading to colorless and transparent glass. For the lanthanium-substituted compositions, the samples were calcined at 1100 °C (x = 0.1–0.4), and 1000 °C (x = 0.6–1) for 8 h with heating and cooling rates of 5 °C/min and were subsequently ground, pressed into pellets. The La-substituted pellets were then fired at 1320 °C for 16 h (x = 0.1), 1250 °C for 16 h and 1300 °C for 16 h (x = 0.2, 0.3, 0.4), 1150 °C for 16 h and 1200 °C for 16 h (x = 0.6, 0.8, 1) to ensure reaching phase equilibrium. 2.2. Characterization The phase formation of both powder and pellet samples were examined by X-ray diffraction (XRD) performed on a Panaytical X'pert Pro diffractometer with CuKα radiation. The variable temperature Xray diffraction (VTXRD) data over the 5–80° 2θ range were collected


3. Atomistic static lattice simulation The atomistic static lattice simulations on defect formation in Ba2TiSi2O8 were performed using the General Utility Lattice Program (GULP) [27,28] based on the interatomic potential approach [29]. The Buckingham potential function [30] was used to model interaction between ions with the shell model [31] to describe the electronic polarizability for the structure modeling of Ba2TiSi2O8. At the first stage, the potential parameters were taken from the previous studies and it was found that none of the parameter data sets provided the ideal reproduction of the cell and crystal structure of Ba2TiSi2O8. Then, fitting procedures were attempted to obtain good potential parameters not only to reproduce the crystal structure but also to obtain reasonable physical properties (e.g. static dielectric constant and elastic constants). The structural parameters of Ba2TiSi2O8 obtained in this study from the Rietveld refinement of both NPD and XRD data were used for comparison with the calculated ones. The best potential parameters obtained from the relaxed fitting procedures are listed in Table 1, among which the interatomic Si–O and O–O potential parameters were modified and the interatomic potential parameters for Ba–O [32] and Ti–O [33] are fixed as those from the previous studies. The extrinsic and intrinsic

defect formation energies were then calculated based on appropriate combinations of vacancy, interstitial and dopant energy terms.

4. Results and discussion 4.1. Phase formation XRD measurements detected single crystalline fresnoite phase in Ba2 − xKxTiSi2O8 − 0.5x (x = 0–1.6) samples (Fig. 2). However, the cell parameters of the Ba2 − xKxTiSi2O8 − 0.5x samples do not show strong dependency on the composition (Fig. S1), which is not consistent with the substitution of K+ cation (radius is 1.51 Å in 8-fold coordination [34]) for the smaller Ba2+ cation (radius is 1.42 Å in the same coordination). The average bulk compositions of the Ba2 − xKxTiSi2O8 − 0.5x pellets (Table S2 in supporting information) from the SEM-EDS elementary analyses on more than ten different regions are close to the nominal compositions, confirming the presence of potassium in the samples and suggesting that the volatilization of potassium is not significant during the preparation. However, the SEM-EDS elementary analyses showed compositional inhomogeneousness for the Ba2 − xKxTiSi2O8 − 0.5x pellets. Elemental mapping analysis of the x = 1 composition (Fig. 3) clearly confirmed the inhomogeneous composition distribution in the sample. Careful examination of the background profiles in the XRD data revealed the existence of an amorphous phase in the highly potassium-substituted compositions. The TEM-EDS elementary analysis from the individual grains revealed very little potassium presence in the fresnoite phases and confirmed the existence of large amount of an amorphous material containing potassium, titanium and silicon elements. Thermogravimetric analysis (TGA) measurements were performed on the highly potassium-substituted composition Ba0.4K1.6TiSi2O7.2. The asmade Ba0.4K1.6TiSi2O7.2 lost ~ 0.53% mass upon heating to 700 °C, and only ~ 0.6% mass loss was seen on the Ba0.4K1.6TiSi2O7.2 sample after being exposed in air for 24 h (Fig. S2 ), showing non-hygroscopy nature for the potassium-substituted compositions. For the Ba2 − xLaxTiSi2O8 + 0.5x La-substituted compositions, both XRD and SEM-EDS elementary analysis results revealed that it is also difficult to substitute La3 + for Ba2 + to create oxygen interstitials in Ba2TiSi2O8. XRD data shows that compositions with x ≥ 0.3 contained secondary phases (La2Si2O7 and La2Ti2SiO9) (Fig. S3) and the refined cell parameters (Fig. S1) of Ba2 − xLaxTiSi2O8 + 0.5x give similar compositional dependency with that for the Ba2 − xKxTiSi2O8 − 0.5x compositions. The EDS elementary analysis was carried out on the x = 0.2 pellet composition where only very limited amount of impurity was detected from XRD data. The elemental mapping (Fig. S4) showed inhomogenous cationic distribution and the compositions

x = 1.6 x = 1.4 x = 0.8

Intensity(a.u.)

from room temperature to 850 °C with a temperature increment of 50 °C and with a collection time of 5 min at each temperature. Constant-wavelength (λ = 1.2255 Å) neutron powder diffraction (NPD) data were collected on the 3T2 diffractometer (Laboratoire Léon Brillouin, France). The bulk compositions and individual grain compositions were analyzed by energy-dispersive X-ray spectroscopy (EDS), that was performed on a Hitachi S4800 field-emission scanning electron microscope (SEM) and a JEOL-2100F field-emission transmission electron microscope (TEM) with an accelerating voltage of 200 kV, respectively. Rietveld analysis was performed using the Topas-Academic software [26]. To determine the amorphous contents in the composite samples, the materials were mixed with known mass percentages (10%, 20%, 30% and 40%) of a crystalline corundum phase Al2O3 as internal standards [16]. Alternating-current impedance spectroscopy measurements in air as well as O2 and N2 flows were carried out using a Solartron 1260A impedance/gain-phase analyzer over a frequency range of 10−1–107 Hz. The pellets of Ba2 − xKxTiSi2O8 − 0.5x were all coated with gold paste on each face, and fired at 500 °C for 30 min to burn out the organic components in the paste to form gold electrodes. The K2TiSi2O7 glass sample of was cut and polished into colorless and transparent cuboids with the sizes of ~3.8 mm × 3.5 mm × 1.7 mm for the impedance measurements. The impedance data were analyzed by Zview software. Solid state NMR experiments were carried out using a Bruker AVANCE III 400 MHz WB spectrometer, giving a frequency of 79.51 MHz for 29Si. Single pulse magic angle spinning (MAS) NMR experiments were carried out at a MAS of 10.0 kHz using a 4 mm o.d. zirconia rotor. The 29Si π/2 pulse was 6.0 μs, and the relaxation delay was 300 s. Thermogravimetric analysis (TGA) was performed using a TA Instruments TGA Q500. The temperature was ramped to 700 °C at a heating rate of 20 °C/min with a nitrogen flow during the experiments.

x = 0.5 x = 0.3 x=0

Table 1 Buckingham potential and shell model parameters employed in this study. Interaction

A (eV)

ρ (Å)

C (eV Å6)

Ba2+–O2− [32] Ti4+–O2− [33] Si4+–O2− O2−–O2− K+–O2− [47] La3+–O2− [48]

2096.8 2088.107 1162.4592 145,016.5 1000.3 4579.23

0.3522 0.2888 0.270843 0.228728 0.36198 0.30437

8.0 0.0 0.0 477.84679 10.569 0.0

Y (e) 1.848 2.89 −2.86902

159

PDF #72-0375 k (eV Å−2) 29.1 253.6 74.92

20

25

30 2Theta(degree)

35

40

Fig. 2. XRD patterns of Ba2 − xKxTiSi2O8 − 0.5x samples. The data are compared to the powder pattern of Ba2TiSi2O8 (PDF #72-0375) from the database of International Center for Diffraction Data (ICDD).

160


Fig. 3. SEM-EDS Elemental mapping over an area of 120 × 88 μm2, showing inhomogeneous elemental distribution in the BaKTiSi2O7.5 composition.

obtained from the Ba-rich regions showed limited presence of La in this sample. 4.2. Structural analysis NPD data was collected on the highly potassium-substituted composition Ba0.4K1.6TiSi2O7.2, in which the background shape (Fig. 4) clearly

Intensity (a.u.)

a

showed the presence of a large amount of amorphous phase in the sample. Combined Rietveld refinement of the crystalline fresnoite phase in the sample on both XRD and NPD data was carried out. The original structural model of fresonite (Fig. 1, space group P4bm) contains one Ba site (4c), one Ti site (2a), one Si site (4c) and four distinctly crystallographic sites O1 (2b, bridging site in the Si2O7 dimer), O2 (4c, terminal site in the Si2O7 dimer), O3 (8d, bridging site between Si2O7 and TiO5) and O4 (2a, terminal site in TiO5) [24]. The Rietveld refinement confirmed that the potassium content in the crystalline phase is as low as ~1.0(7) % and the oxygen sites are almost fully occupied (Table 2), which is consistent with the TEM-EDS elementary analysis results. Fig. 4 shows the Rietveld refinement plots of XRD and NPD data of Ba0.4K1.6TiSi2O7.2. The resulting bond lengths and angles of the crystalline fresnoite phase in the Ba0.4K1.6TiSi2O7.2 sample are listed in Table S2.

4.3. Amorphous content in Ba2 − xKxTiSi2O8 − 0.5x samples

10

20

30

40

50

60

70

80

2Theta (degree)

Intensity (a.u.)

b

As showed by the EDS elemental mapping results (Fig. 3), the BaKTiSi2O7.5 sample contains significant amorphous phase. In order to quantitatively describe the content of the amorphous phase in the Ba2 − xKxTiSi 2O8 − 0.5x composites, Rietveld analysis of XRD data of the samples mixed with known amounts of crystalline corundum phase (Al2O3) were performed. Such quantitative analysis of the amorphous content has been successfully applied on the sodium-doped SrSiO3 compositions [16]. Given that no K presence was found in the crystalline BTS phase, the phase separation according to the reaction Ba2− xKxTiSi2O8− 0.5x → (2 − x)/2 Ba2TiSi2O8 + (x/2) K2TiSi2O7 is expected, i.e. the amorphous material is K2TiSi2O7 in the Ba2− xKxTiSi2O8− 0.5x composites. Therefore the theoretical amorphous content may be calculated as a function of the substitution level (x), as

Table 2 Final refined structural parameters for the crystalline Ba2TiSi2O8 phase* in the Ba0.4K1.6TiSi2O7.2 composition from a combined NPD and XRD refinement.

10

20

30

40 50 60 2Theta (degree)

70

80

Fig. 4. Rietveld refinement plots of the XRD (a) and NPD data (b) collected on the Ba0.4K1.6TiSi2O7.2 composition. Reliability factors are: Rwp ~1.3%, Rp ~ 1.0%, RB ~ 0.3% for NPD data, and Rwp ~ 3.6%, Rp ~ 2.6%, RB ~ 1.3% for XRD data. Experimental and calculated data are given as crosses and the line, respectively. The difference is given as the curve underneath. Ticks indicate the positions of Bragg reflections.

Atom

Site

x

y

z

Occupancy

Biso(Å2)

Ba/K Ti Si O1 O2 O3 O4

4c 2a 4c 2b 4c 8d 2a

0.32717(9) 0 0.1267(3) 0 0.1254(5) 0.2917(5) 0

0.82717(9) 0 0.6267(3) 1/2 0.6254(5) 0.5780(6) 0

0 0.540(1) 0.514(2) 0.627(2) 0.217(1) 0.6446(9) 0.208(2)

0.989(7)/0.011(7) 1 1 0.99(1) 0.99(1) 0.99(1) 0.99(1)

1.18(1) 0.83(8) 1.19(8) 1.1(1) 0.87(9) 1.03(5) 1.0(1)

*a = b = 8.52881(7) Å, c = 5.21488(6) Å, V = 379.334(9) Å3, Z = 2, space group: P4bm; the occupancies were refined subject to the constraint of charge neutrality.


4.4.

29

Si NMR data of Ba2 − xKxTiSi2O8 − 0.5x materials

-82.08

x=1.3 x=1 x=0.5

29

Si MAS NMR was used to examine if the connectivity of SiO4 in Ba2 − xKxTiSi2O8 − 0.5x compositions is maintained upon the potassium-addition, where Qn is used to describe the bridging oxygen atoms of SiO4 tetrahedra. In the fresnoite structure, each Si from Si2O7 dimers has three bridging oxygen atoms: one oxygen atom in the Si–O–Si bond and two oxygen atoms in the Si–O–Ti bonds where Ti is a 5-coordinated center (Fig. 1). Fig. 6 shows solid state 29 Si NMR data recorded on Ba2 − xK x TiSi 2O 8 − 0.5x (x = 0, 0.5, 1, 1.3, 1.6) samples. All the compositions displayed a common sharp peak at − 82.08 ppm chemical shift position, which is ascribed unambigously to the tetrahedral Q3-linked Si centers in Si2O7 dimers. A previous theoretical study has predicated that the Ti for Si substitution in zeolites would cause a shift of the 29Si NMR signal of neighboring Si nuclei to lower fields (~ 1 ppm per second neighboring Ti) [35]. The chemical shift position for this Q3 -linked Si signal in Ba 2TiSi2 O8 is much lower than the normal chemical shift range (from − 88 to − 98 ppm) of the Q3-linked Si in the silicates [36,37], meaning that the Ti–O–Si bond in Ba2 TiSi 2O 8 provides much less 29 Si shielding than the Si–O–Si bond. The position and width of this Q 3-linked Si signal did not change with the potassium content in Ba2 − xKxTiSi2O8 − 0.5x, suggesting the formation of pure Ba2TiSi2O8 phase, independently on the potassium addition and consistently with no potassium substitution in Ba2TiSi2O8 as revealed by the diffraction and EDS elementary analysis experiments above. In addition to the sharp Q3-linked peak located at − 82.08 ppm, an extra broad peak centered at ~ 89 ppm was observed in Ksubstituted Ba2TiSi2O8 compositions. This broad feature of the 29Si NMR signal is a typical behavior of Si centers in amorphous phases [16,17], consistent with the presence of potassium-rich glass materials in Ba2 − xKxTiSi2O8 − 0.5x. The intensity of the broad peak at ~ 89 ppm increased with the potassium content, in good agreement with the increased amorphous contents with potassium amount in Ba2 − xKxTiSi2O8 − 0.5x (Fig. 5). The Si environment of the ~89 ppm 29Si signal from the amorphous phase is difficult to assign unambiguously without the proper references but could be due to Q2 or Q3-linked Si

-89

x=1.6

Intensity ( a.u. )

represented by the dash line in Fig. 5. Measurements performed on the x = 1.6 sample mixed with four different contents of Al2O3 gave an amorphous content of 79.1(1) wt.%, slightly higher than the calculated value (70 wt.%). Fig. 5 shows the contents of the amorphous materials from the Rietveld analysis in the x = 0.5, 1.0, 1.3, 1.6 samples, which are close to calculated percentages, confirming the hypothesis that all the potassium is present in the amorphous phase.

161

x=0

-60

-80

-100

Fig. 6. 29Si MAS NMR spectra of Ba2 − xKxTiSi2O8 − 0.5x.

centers in the glass according to the combined 29Si and 17O NMR results of amorphous (TiO2)x(SiO2)1 − x sol–gel materials [38]. 4.5. VTXRD data of Ba2 − xKxTiSi2O8 − 0.5x VTXRD experiments were performed on the x = 0.5 and 1 compositions of Ba 2 − x Kx TiSi 2O 8 − 0.5x from room temperature up to 850 °C. The VTXRD data (Fig. 7 for x = 1 and Fig. S5 for x = 0.5) showed the appearance of extra reflections from a K 2TiSi3 O9 -like phase upon heating up to 800 °C. There was no apparent change on the cell parameters for the Ba 2 TiSi2 O 8 phase before and after the VTXRD experiments. Therefore the appearance of extra reflection is ascribed to the crystallization of the potassium-rich amorphous materials instead of a Ba2 − xKxTiSi2O8 − 0.5x phase decomposition. 4.6. Ionic conductivity of Ba2 − xKxTiSi2O8 − 0.5x AC impedance spectra were measured on Ba2 − xKxTiSi2O8 − 0.5x pellets while heating up to 700 °C, below the crystallization temperature of the glassy phase in the composites. The complex impedance data of the Ba2TiSi2O8 parent material comprises one large and one small semicircular arcs (Fig. 8a) above 500 °C, corresponding to the bulk and grain boundary responses, respectively. The large semicircular arc can be modeled with parallel resistor (R) and capacitor (C) elements. The intercept of the large semicircular arc at low frequency was extracted as the bulk resistivity Rb. The capacitance estimated from the

100

: K2TiSi3O9

80

crystallization of glass

Intensity (a.u.)

Amorphous content(wt%)

-120

Chemical shift (ppm)

60 40

o

25 C - after VT o

850 C o

800 C o

750 C o

25 C - before VT

20

PDF #72-0375

0 0.0

0.4

0.8 1.2 x in Ba2-xKxTiSi2O8-0.5x

1.6

2.0

Fig. 5. Amorphous mass content in Ba2− xKxTiSi2O8− 0.5x (x = 0.5, 1, 1.3, 1.6). The dashed line represents the calculated contents (wt.%) if all the potassium was present in the amorphous phase.

15

20

25

30

35

40

2Theta (degree) Fig. 7. VTXRD Patterns of the BaKTiSi2O7.5 sample on heating. The ∇ symbols mark the reflections from K2TiSi3O9 phase. The data are compared to the powder pattern of Ba2TiSi2O8 (PDF #72-0375) from ICDD database.

162


equation ωRC = 1 (ω = 2πfmax, where fmax is the frequency at the maximum imaginary impedance Z″max for the semicircular arc) is 2.29 pF/ cm, consistent with the bulk response [39]. Below 500 °C, only the semicircular arc from the bulk response was observed on the Ba2TiSi2O8 pellet. For the potassium-containing compositions, the electrode responses were clearly seen in addition to the bulk and grain boundary responses (Fig. 8b and c). The x = 1.4 sample displayed significant Warburg-type

a)

b)

c)

electrode response (Fig. 8c), showing large capacitance within 10−7– 10−5 F/cm from 0.01 Hz to 103 Hz, suggesting ionic conduction in the material. Fig. 9a shows the Arrhenius plots of the total conductivity of the Ba2 − xKxTiSi2O8 − 0.5x materials on heating, displaying enhanced ionic conductivity with the potassium content in Ba2 − xKxTiSi2O8 − 0.5x. The typical compositional dependency of the total conductivity at 600 °C is shown in Fig. 9b: when small amount of potassium is present in the material (x = 0.1), the conductivity displayed a jump by more than 2 orders of magnitude. Beyond this point, the conductivity was enhanced by the potassium incorporation at a slower rate. The Ba0.6K1.4TiSi2O7.3 composition reached the highest conductivity (σb ~ 6 × 10−3 S/cm at 600 °C) among the measured Ba2 − xKxTiSi2O8 − 0.5x compositions. The activation energies in the low temperature region below 500 °C are essentially identical (0.83–0.87 eV) for all the compositions. For x ≥ 0.8 compositions, the Arrhenius plot of the conductivity starts to show curvature above 500 °C, leading to higher activation energies (~1.2 eV) in the high temperature region compared with those in low temperature region. The conductivity measured on cooling from 700 °C to RT is essentially identical to the one observed on heating. However, when the temperature increased above 800 °C, where the crystallization of the glass phase occurred, the conductivity on cooling became lower than that on heating (Fig. S6 shows the data for the Ba1.3K0.7TiSi2O7.65 composition). This result reveals that the crystallization of the glassy component is detrimental to the ionic conductivity. The electrode response observed in Ba2 − xKxTiSi2O8 − 0.5x samples did not show apparent collapsing at high temperature, which is not the typical impedance behavior for an oxide ion conductor [4,40]. For a good oxide ion conductor, at high temperature and in air or O2 atmosphere, the electrode response arc could collapse down to semicircular arc [4,40]; while the electrode polarization resistance in N2 with low parital oxygen pressure (pO2) would be enhanced significantly compared with that in O2 because of the reduced kinetic for the oxygen ion diffusion and charge transfer reaction of O2 + 2e ↔ O2− along the sample-electrode interface [39] in the low-pO2 environment; therefore the shape of the electrode response arc would become much steeper even revert back to a Warburg-type spike when the atmosphere was changed from O2 to low-pO2 N2 [41–43]. Such behavior of electrode response with the partial oxygen pressure is commonly observed in the oxide ion conductors and is used to diagnose the oxide ion conduction [40–42]. In order to examine whether the ionic conduction in Ba2 − xKxTiSi2O8 − 0.5x arises from the oxide anion, impedance measurements of Ba0.8K1.2TiSi2O7.4 under O2 and N2 flows were performed and the electrode responses under the O2 and N2 flows were compared. The measurements showed that the electrode response arc slightly changed with the atmosphere change from O2 to N2 with the total resistivity unmodified (Fig. S7). This suggests that the oxide ionic conduction is limited in the Ba2 − xKxTiSi2O8 − 0.5x composites. Therefore the ionic conduction observed in the Ba2 − xKxTiSi2O8 − 0.5x composites can be ascribed to the potassium conduction, which should be associated with the amorphous material since the crystalline phase contains little potassium. 4.7. K2TiSi2O7 glass

Fig. 8. Complex impedance plots for the Ba2 − xKxTiSi2O8 − 0.5x: x = 0 (a), x = 0.5 (b), x = 1.4 (c). The numbers label the selected frequency marked by filled circles. Rb and Rgb denote bulk and grain boundary resistivities, respectively. The low-frequency tails in (b) and (c) are ascribed to sample-electrode interface responses from the ionic conduction.

The Rietveld refinements of NPD and XRD data together with the TEM and solid 29Si NMR data revealed that there is very little potassium presence in the crystalline Ba2TiSi2O8 phase and large amounts of amorphous material are present in the Ba2 − xKxTiSi2O8 − 0.5x composites. The amorphous contents from the quantitative Rietveld analysis of XRD data are consistent with the K2TiSi2O7 composition for the glass component. Given that the Ba2TiSi2O8 crystalline phase is an insulator, the enhanced ionic conductivity in the potassium-substituted compositions Ba2 − xKxTiSi2O8 − 0.5x may be ascribed to the presence of the K2TiSi2O7 glass in the composite materials. Therefore the K2TiSi2O7 glass was synthesized and its characterizations were carried out to reproduce the electrical and crystallization behavior observed in the


Ba2 − xKxTiSi2O8 − 0.5x composites. SEM-EDS elemental analysis of the K2TiSi2O7 glass gave a bulk composition of K2Ti1.12(2)Si2.32(5), close to the nominal composition. The elemental mapping analysis showed homogeneous elemental distribution for the K2TiSi2O7 glass (Fig. S8). Impedance measurements indicated that the K2TiSi2O7 glass also displayed ionic conduction behavior and its conductivity (Fig. 10) is slightly higher than that observed on the Ba0.4K1.6TiSi2O7.2 composite containing 79 wt.% of amorphous material, which is reasonable as the glass component is diluted in the composite. Similar to the K-rich Ba2 − xKxTiSi2O8 − 0.5x compositions, the K2TiSi2O7 glass displayed curvature on the Arrhenius conductivity plots (Fig. 10) around 500 °C, leading to higher activation energy above 500 °C. The K2TiSi2O7 glass impedance measurements under O2 and N2 flows also indicated limited oxide ion conduction (Fig. S9). The VTXRD data of the K2TiSi2O7 glass showed the occurrence of crystallization ≥ 800 °C, leading to the K2TiSi3O9 phase (Figs. S10–11). The K2TiSi2O7 glass was annealed at 850 °C for 6 h and the Rietveld analysis was performed on the XRD data of the post-annealing product, which confirmed the existence of the K2TiSi3O9 phase (Fig. S11). The impedance measurement of the K2TiSi2O7 glass on a heating and cooling cycle up to 700 °C gave essentially the same conductivities on heating and cooling (Fig. 10). On the contrary, the measurement on a heating and cooling cycle up to 850 °C showed degraded conductivity on cooling (Fig. S12), similar to the Ba2 − xKxTiSi2O8 − 0.5x composites, which may be also ascribed to the crystallization of the glassy phase at temperature ≥ 800 °C during the measurement. In short, the measurement on the pure K2TiSi2O7 glass showed entire reproduction of the electrical and crystallization behaviors of the Ba2 − xKxTiSi2O8 − 0.5x composites, thus proving

o

T ( C)

a -2

800

600

400

200

10

x=0 x = 0.1 x = 0.2 x = 0.4 x = 0.8 x = 1.4

1.19(4)

-3

10

1.17(3)

-4

-1

σt (Scm )

10

-5

10

-6

10

-7

10

0.84(3) 0.84(3)

-8

10

-9

0.84(1) 0.87(1)

0.86(3)

0.83(3)

10

-10

10

1.0

b

1.2

1.4

1.6 1.8 -1 1000/T (K )

2.0

2.2

2.4

163

experimentally that the K2TiSi2O7 glassy component is responsible for the enhanced ionic conductivity in Ba2 − xKxTiSi2O8 − 0.5x composites. 4.8. Energetics of defect formation in Ba2TiSi2O8 The atomistic static lattice simulations based on the interatomic potential methods are a powerful computational tool on probing the defect chemistry, which have been successfully applied in many oxide ion conductors including the perovskite [29], apatite [8], melilite-type [4] materials. In this study, the atomistic static lattice simulations were performed for the first time on the fresnoite structure for addressing the energetic costs of defect formation associated with oxide anions. The potential parameters listed in Table 1 reproduced well the crystal structure of Ba2TiSi2O8 (Table 3) and gave reasonable static dielectric constants, elastic constants and thermal expansion coefficients compared to the experimental values (Table S3) [44]. For example, the lattice parameters were reproduced to within 0.01 Å and the bond lengths within 0.07 Å for the most of bonds except for one relatively large difference ~ 0.13 Å for one Si–O (Table 3). The solution energy of the following process for the acceptor-doping of potassium in Ba2TiSi2O8 was derived according to the defect reaction equation × •• K2O + 2Ba× Ba + OO → 2KBa′ + VO + 2BaO. The lowest solution energy (~7.3 eV) for the potassium substitution for Ba is associated with the vacancy formation on the terminal O4 site in TiO5 pyramid, comparable to those from the DFT calculation on the Na/K-doped SrSiO3 compositions [18]. For the oxygen interstitial defect formation via the La3+ substitution for Ba2 +, the lowest energy was found to be related with center of the void in the pentagonal tunnels between two Ba atoms, similar to the melilite case [4,11,45]. The calculated solution energy for the donor-doping of lanthanium in Ba2TiSi2O8 is ~6.0 eV. The magnitudes on these solution energies suggest that the occurrence of K+ and La3+ doping in Ba2TiSi2O8 is unlikely to occur for creating respectively the oxygen vacancies and interstitials, which is consistent with experimental results. The calculation showed that the oxygen Frenkel defects displayed the lowest formation energy ~ 4.6 eV among these extrinsic and intrinsic defects (in Table 4). However, such energy still represents a high barrier, meaning that the oxygen Frenkel defects could be minor defects in the materials even when the defect reaction occurred. Apart from the oxygen Frenkel defects, the other intrinsic Frenkel and Schottky defects in Ba2TiSi2O8 have higher formation energies than those for the extrinsic defects of oxygen vacancies and interstitials from the K+/La3+ doping (Table 4). The unsuccessfull potassium substitution and the phase separation in Ba2 − xKxTiSi2O8 − 0.5x composites appear similar to the Na/Kdoped SrSiO3 case showing isolated Si3O9 tetrahedral rings [15–18].

o

T ( C) -2

10

-1

800

600

200

10

Ba0.6K1.4TiSi2O7.3

-3

10

-2

10

-4

10

K2TiSi2O7 heating K2TiSi2O7 cooling

-3

10 σ t (Scm )

-1

σt (Scm )

400

-1

-5

10

-6

10

-4

10

-5

10

-6

-7

10

-8

10

10

-7

10

0.0

0.4

0.8

1.2

1.6

x Fig. 9. (a) Arrhenius plots of the total conductivities of Ba2 − xKxTiSi2O8 − 0.5x. The activation energies (eV) are given for each composition. (b) Compositional (x) dependency of the total conductivity at 600 °C. For the Ba2TiSi2O8 parent composition, the bulk conductivity data are given in both plots.

-8

10

1.0

1.2

1.4 1.6 1.8 -1 1000/T (K )

2.0

2.2

Fig. 10. Arrhenius plots of the total conductivities on heating and cooling of the K2TiSi2O7 glass in comparison with Ba0.6K1.4TiSi2O7.3 composite.

164


Table 3 Calculated and experimental structural parameters for Ba2TiSi2O8. Parameters (Å)

Calculated

Experimental

Δ(Calc. − Exp.)

a c Ba–O1 Ba–O2 (×2) Ba–O2 Ba–O3(×2) Ba–O3(×2) Ba–O4 (×2) Mean Ba–O Ti–O3 (×4) Ti–O4 Mean Ti–O Si–O1 Si–O1 Si–O3 (×2) Mean Si–O

8.5205 5.2096 2.787 2.760 2.645 3.074 2.838 3.319 2.941 1.930 1.684 1.881 1.763 1.592 1.657 1.667

8.5288 5.2149 2.851 2.816 2.685 3.007 2.834 3.337 2.952 1.973 1.733 1.853 1.638 1.544 1.619 1.605

−0.0083 −0.0053 −0.064 −0.056 −0.040 0.067 0.004 −0.018 −0.011 −0.043 −0.049 0.028 0.125 0.048 0.038 −0.062

The SrSiO3 structure does not accommodate oxygen vacancies presumably because the large separation (the inter-ring Si–Si distance ≥ 4.69 Å) and rigidity of the silica-rings in the structure do not permit the opening or distortion of the rings for corner-sharing while maintaining the 4coordinate geometry for Si [46]. In the tetrahedral prototype, the terminal oxygen site is more energy-favorable to accommodate the vacancy than the bridging oxygen site if the distortion of the tetrahedron was allowed to share corner with the neighboring tetrahedron [12,13]. In fresnoite, all the terminal oxygen atoms in the 2D polyhedral network are placed at same level and the bridging oxygen atoms form an individual layer along the c-axis (Fig. 1), in contrast with the melilite structure. Removing the terminal oxygen (O2) in the Si2O7 dimer Ba2TiSi2O8 would be difficult as the large inter-layer Si–Si separation (the shortest interlayer Si–Si distance is ~5.21 Å, equal to the c parameter) would not allow sharing the terminal oxygen of SiO4 in the neighboring layer without distorting the polyhedral layer network significantly. The impossibility to dope Ba2TiSi2O8 with potassium implies high rigidity of the mixed corner-shared TiO5 and SiO4 polyhedral framework, which does not allow stabilization of oxygen vacancies not only at the level of the terminal oxygen sites (O2) of the Si2O7 dimer but also at the terminal oxygen sites (O4) in the TiO5 square pyramid. The high energy barrier for the solubility of La3+ in Ba2TiSi2O8 emphasizes the rigidity of the mixed layered polyhedral network in fresnoite again, being not suitable for stabilizing the oxygen interstitials as well in the pentagonal tunnels in a way that works in the gallate melilite materials [4,11]. 5. Conclusion The Ba2TiSi2O8 fresnoite displayed extremely limited solubility for both K+ and La3+ in Ba2+ sites. In contrast with the crystalline-mixture nature in the La-substituted compositions, the Ba2 − xKxTiSi2O8 − 0.5x compositions showed phase separation into crystalline Ba2TiSi2O8 and amorphous K2TiSi2O7. The addition of potassium enhanced the ionic conductivity of Ba2 − xKxTiSi2O8 − 0.5x, which contains limited oxide ion conduction contribution so that the ionic conductivity Table 4 Formation energies of extrinsic and intrinsic defects in Ba2TiSi2O8. Defect type

Equation

Acceptor-doping Donor-doping O Frenkel

× •• K2O + 2Ba× Ba + OO → 2KBa′ + VO + 2BaO • ″ La2O3 + 2Ba× Ba → 2LaBa + Oi + 2BaO

Ba Frenkel Si Frenkel Ti Frenkel Full Schottky

″ •• Ba× Ba → VBa + Bai '''' + Si•i • • • Si× Si → VSi '''' + Ti•i • • • Ti× Ti → VTi × × × .. '''' 8O× O + TiTi + 2BaBa + 2SiSi →8VO + 2VSi + V'Ti' ' ' + 2V'Ba' + Ba2TiSi2O8

″ •• O O →VO þ Oi

Energy (eV) 7.31 6.14 4.62 11.0 28.2 27.0 59.8

presumably originates from the potassium condution in the glass component. Crystallization of the glass component reduced the ionic conductivity. Measurements on the pure K 2 TiSi 2 O 7 glass reproduced the electrical and crystallization behaviors observed in Ba 2 − x K x TiSi 2 O 8 − 0.5x composites, fully confirming the role of the glass material in Ba2 − xKxTiSi2O8 − 0.5x composites on being responsible for the ionic conductivity. The atomistic static lattice simulations revealed high energetic costs for the K+ and La3+ substitutions for Ba2+ in Ba2TiSi2O8, supporting the limited solubility of K+ and La3+ in Ba2TiSi2O8, and emphasizing the rigidity of the mixed layered polyhedral network in fresnoite, being not suitable for stabilizing the oxygen vacancies and interstitials. This study with combination of the complimentary characterizations and defect modeling together with the experimental measurement on the glass confirms the role of glassy phase on the enhanced ionic conductivity in the so-called acceptorsubstituted silicate composites, which contribute to the understanding of the SrSiO3-based conductors currently on the debating focus of whether they are good oxide ion conductors as well as more materials based on the tetrahedral prototypes. Acknowledgments This work is funded by the National Natural Science Foundation of China (No. 21101174), Natural Sceince Foundation of Guangxi (2014GXNSFGA118004), the Research Project (No. 213030A) of Chinese Ministry of Education, Program for New Century Excellent Talents in University (No. NCET-13-0752), and Guangxi Ministry-Province Jointly-Constructed Cultivation Base for State Key Laboratory of Processing for non-Ferrous Metal and Featured Materials (No. 13AA-8). The authors acknowledge the support of the Laboratoire Léon Brillouin at CEA/ Saclay (beamline 3T2) for the neutron beamtime. X. K. also thanks Guilin University of Technology and Chinese Ministry of Education Scientific Research Foundation for Returned Scholars for the Start-up Funds. Prof. M. Saiful Islam (University of Bath, UK) and Prof. Xing Tang (Guilin University of Techonology) are acknowledged for the valuable discussion on the atomistic simulation results. Appendix A. Supplementary data Cell parameters, (variable temperature) XRD data, SEM-EDS elementary analysis results, conductivities and complex impedance plots under O2 and N2 flows of Ba2 − xKxTiSi2O8 − 0.5x and Ba2 − xLaxTiSi2O8+ 0.5x compositions as well as the K2TiSi2O7 glass; TGA data of the Ba 0.4K 1.6 TiSi2 O7.2 composition; bond lengths and angles of the fresnoite phase in the Ba0.4 K 1.6 TiSi2 O 7.2 composite from the combined NPD and XRD refinement; Rietveld refinement plot of the XRD data of the K 2 TiSi2 O 7 glass post annealing at 850 °C for 6 h; the calculated and experimental physical properties of Ba2TiSi 2O8. Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.ssi.2015.06.002. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14]

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