ISSN 1087-6596, Glass Physics and Chemistry, 2016, Vol. 42, No. 4, pp. 337–348. © Pleiades Publishing, Ltd., 2016. Original Russian Text © R.S. Bubnova, A.P. Shablinskii, S.N. Volkov, S.K. Filatov, M.G. Krzhizhanovskaya, V.L. Ugolkov, 2016, published in Fizika i Khimiya Stekla.
Crystal Structures and Thermal Expansion of Sr1 – xBaxBi2B2O7 Solid Solutions R. S. Bubnovaa, b, A. P. Shablinskiia, b, S. N. Volkova, S. K. Filatovb, M. G. Krzhizhanovskayab, and V. L. Ugolkova aGrebenshchikov Institute of Silicate Chemistry, Russian Academy of Sciences, nab. Makarova 2, St. Petersburg, 199034 Russia bSt.
Petersburg State University, Universtitetskaya nab. 7/9, St. Petersburg, 199034 Russia e-mail:
[email protected] Received February 4, 2016
Abstract—A series of Sr1 – xBaxBi2B2O7 solid solutions (x = 0–1) have been synthesized by crystallization from glass ceramics. The crystal structures of solid solutions at x = 0.65 and x = 1 were solved by direct methods of X-ray diffraction from single crystal data and refined in a reduced cell (aBa = aSr/√3) relative to SrBi2B2O7 in the same space group P63. Using the Rietveld method, the phase transition range at x = 0.65– 0.70 with a reduction of the unit cell has been revealed. As was shown by method of high-temperature powder X-ray diffraction, the solid solutions demonstrated an anisotropic thermal expansion (TE) upon heating in air. Keywords: bismuth borates, crystal structure, thermal expansion, immiscibility range DOI: 10.1134/S1087659616040040
INTRODUCTION Borates of the system BaO–Bi2O3–B2O3 are generally well-known because of their nonlinear optical (NLO) properties: β-BaB2O4 [1–3], α-BiB3O6 [4, 5], and BaBiBO4 [6]. Phase equilibria in the systems MO–Bi2O3–B2O3, where M = Ca, Sr, and Ba, were investigated for the first time in [7–12]. According to the Inorganic Crystal Structure Database (ICSD) [13], in these systems one can also identify MBi2B2O7 (M = Ca, Sr) [14], BaBi2B4O10 [15], SrBi2B4O10 [16], Ba2Bi3B25O44 [17], Ba3BiB9O18 [18], Ba3Bi2(BO3)4 [19] etc. Noncentrosymmetric borates MBi2B2O7 (M = Ca, Sr) are of special interest, since CaBi2B2O7 generates a signal of the second harmonic [14]. CaBi2B2O7 and SrBi2B2O7 have similar crystal structures despite crystallizing in different orthorhombic: CaBi2B2O7 in orthorhombic (space group Pna2), and SrBi2B2O7 in hexagonal (space group P63). Both structures consist of isolated boron–oxygen triangular radicals BO3 linked through MO6 polyhedra. The borate BaBi2B2O7 was mentioned in [20], but incorrectly characterized there by X-ray methods. Glass ceramic samples of the same stoichiometry were obtained and studied using the methods of transmission electron microscopy and powder X-ray diffraction [21]. The present work describes the synthesis and presents the results of crystal structures resolution and studies of thermal expansion (TE) of representatives of
a new series of solid solutions Sr1 – xBaxBi2B2O7. Structural transformations and order–disorder processes were investigated based on the data of single crystal X-ray diffraction analysis and powder high-temperature X-ray diffraction. EXPERIMENTAL Synthesis. Solid solutions Sr1 – xBaxBi2B2O7 (x = 0.00, 0.25, 0.50, 0.65, 0.75, 0.85, 1.00) were obtained by crystallization from the melt. The initial substances (Bi2O3, BaCO3, SrCO3 (all of the special purity grade) and H3BO3 (chemically pure grade)) were compacted into pellets and annealed at 600°C for 2 h, after which the pellets were placed into a Pt crucible, molten at 900–950°C and held for 30 min. The melt was poured on a steel plate at room temperature. According to the electron microscopy study, the samples comprised glass ceramics that had been earlier studied in [21]. Thereafter, the melt was crystallized at 600°C for 1–30 h. Thermal analysis was performed using a NETZSCH STA 429 device in a platinum crucible in air. The sample weight was 20 mg and the heating rate was 20°C/min in the temperature range 20–1000°C. The samples were studied in the mode of heating with subsequent cooling. According to the thermogravimetry (TG) data, no changes occurred in the samples weights. Determination of crystal structures. Single crystal X-ray diffraction analysis data. Single crystals are taken from polycrystalline aggregates of compositions with
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Table 1. Crystallographic parameters of solid Sr1 – xBaxBi2B2O7 solutions Parameter а, Å с, Å V, Å3 Space group μ, mm–1 Z Dmeasured, g/cm3 Crystal size, mm3 θ-range, deg Reflections number Nonequivalent reflections Nonequivalent reflections (Fo) ≥ 4σF
x = 0 [14]
x = 0.5
x = 0.7
x=1
9.1404(4) 13.0808(6) 946.44(7) P63
9.1937(14) 13.299(3) 973.5(3) P63
5.3246(3) 13.4013(8) 329.04(1) P63
5.3378(8) 13.583(2) 335.15(9) P63
63.39
61.78
6 6.796
2 6.140
2 6.825
0.03 × 0.04 × 0.05
0.02 × 0.05 × 0.07
0.04 × 0.04 × 0.03
2.98–31.50 6961 3155 694
4.67–36.42 4934 1009 463
3–24.93 1872 398 261
6 6.73
691
Rint
0.061
0.031
R1
0.140
0.061
0.040
0.033
wR2
0.051
0.041
wR2 (|Fo ≥ 4σF )
0.040
0.037
1.23
1.48
R1 (|Fo | ≥ 4σF )
0.057
S
R1 = Σ||Fo| – |Fc||/Σ|Fo|; wR2 = {Σ[w( Fo2 – Fc2 )2]/Σ[w( Fo2 )2]}1/2; w = 1/[σ2(|Fo|) + (0.0001Fo)2]; S = {Σ[w( Fo2 – Fc2 )]/(n – p)}1/2, where n is the number of reflections and p is the number of refined parameters.
x = 0.5, 0.7, and 1.0 obtained through crystallization from glass at 600°C/10 h and 600°C/0.5 h were used in the X-ray diffraction analysis.
atom coordinates and equivalent atomic displacement parameters are shown in Table 2, while the bond lengths are shown in Table 3.
The X-ray diffraction analysis (Table 1) was carried out using a Bruker Kappa APEX DUO diffractometer (CCD detector, step ω = 0.5) with monochromatized MoKα-radiation. The unit cell parameters were refined using the least-square method. Corrections taking into account the Lorentz factor, polarization, absorption, and background radiation were introduced into the intensities. Superstructural reflections were revealed on diffraction patterns (reciprocal space cross sections) of the solid solutions. For compositions with x > 0.65, the superstructural reflections become very weak (just slightly exceeding the background level) and diffuse (Fig. 1), as a result of which the refinement of crystal structures at x = 0.70 and x = 1 was performed on a reduced cell aBa = aSr/√3 (see Figs. 2, 3). For the single crystal at х = 0.5, only the cell parameters were determined because of the poor quality of the crystal. However, as was shown by the analysis of cross sections of the reciprocal space, it should have been induced in the superstructural cell relative to х = 1. The structures at x = 0.70 and x = 1 were resolved by direct methods and refined in the SHELXL-97 [22] and Jana 2006 [23] programs. The
Determination of the crystal structures by the Rietveld method. The crystal structures of the solid solutions (x = 0.65, 0.70, and 1) were studied by the Rietveld method in air in the range 2θ = 5–100°. Measurement was carried out using Rigaku Ultima IV (CuKα1 + 2radiation, 40 kV, 35 mA, Bragg–Brentano geometry, DTEX/ULTRA detector) and STOE STADI P (CuKα1-radiation, 40 kV, 30 mA, transmission geometry, position-sensitive detector) diffractometers. The crystal structures of two compositions (x = 0.7 and x = 1.0) were corroborated by the Rietveld method in the Topas 4.2 program, which allowed demonstrating the coincidence of the powder data for the whole sample with single-crystal data (Fig. 2). Some samples contain an insignificant quantity of the admixture of Bi4B2O9. As was found as a result of refining the crystal structures by the Rietveld method, peaks with the indexes h ± 2k ≠ 3n disappeared at x = 0.70. The calculated data (Table 4) corroborate the correctness of the structural model. Thus, according to the Rietveld method data, it became possible to single out the range of phase transition x = 0.65–0.70 with a reduction of the unit cell: solid solutions at x = 0.7–1.0 crys-
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(a)
x = 1.0
100
110
x = 0.5 000 100 010 110 200
020
010
000
339
1
120
2 1
b 100
(b)
020
000 010 120
aH
a
bH
Fig. 1. Diffraction patterns of the samples at x = 0.5 and 1: real pattern (a), scheme (b). (1) Reflections (aBa = aSr/√3), (2) superstructural reflections (aSr).
tallize in the reduced unit cell, whereas solid solutions at x = 0.65 crystallize in the superstructural unit cell (Fig. 2). High-temperature X-ray diffraction. Samples of the series Sr1 – xBaxBi2B2O7 (x = 0.00, 0.25, 0.50, 0.65, 0.70, 1.00) were studied using the method of the hightemperature X-ray diffraction. Measurements were carried out using a Rigaku Ultima IV diffractometer (CuKα1 + 2-radiation, 40 kV, 40 mA, reflection geometry, DTEX/ULTRA detector, temperature increment 25°C, average heating rate 40°C/h). Samples were prepared by precipitation from a heptane suspension on a Pt plate. The unit cell parameters were refined using the least-square method at different temperatures. The parameters’ temperature dependences were described by quadratic polynomials. The unit cell parameters, the experimental data processing, and the calculation of the TE coefficients were performed within the Theta–To–Tensor program [24]. RESULTS AND DISCUSSION Solid solution structures. The structural motifs of extreme members of the Sr1 – xBaxBi2B2O7 series are similar, but their unit cell sizes are different (Table 1, Figs. 2, 3). The structures of Sr1 – xBaxBi2B2O7 are composed of isolated BO3 triangles with 3 cation positions (M1, M2, and M3) between them (Fig. 4). In the structure of SrBi2B2O7, all cation positions are of the general type (Wyckoff positions 6c). The Bi3+ and Sr atoms are ordered on the positions: Bi occupies the GLASS PHYSICS AND CHEMISTRY
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two largest positions (M1 and M2), while Sr occupies the smallest position (M3) [14]. In the structure of BaBi2B2O7, the Bi3+ and Ba atoms are distributed over the positions almost statistically—the occupancy in each position is about 2/3 Bi3+ and 1/3 Ba2+. All the cation positions are split (Table 2). Upon substitution of Ba into Sr, the Sr atoms occupy the smallest position (M3) in the solid solutions. Splitting of positions. The Ba atom in the M(1)O10 polyhedron (Fig. 4b) is located on a third-order axis, whereas the Bi atom is located in the common position. Due to splitting, one can single out coordination polyhedra independently for Ba and Bi: in the case of Ba, it is an isometric ten-vertex polyhedron with bond lengths 2.52–2.98 Å, while for Bi it is an asymmetric seven-vertex one with bond lengths 2.18–2.73 Å; the extra bonds length is 3.26 Å (Fig. 4). The M(2)O10 position (Fig. 4b) is also located on the third-order axis: Ba forms an isometric ten-vertex polyhedron with bond lengths 2.55–2.94 Å, whereas Bi is coordinated sharply asymmetrically with the formation of the BiO3 polyhedron with bond lengths 2.11–2.45 Å. From the difference in bond lengths, one can identify a seven-vertex coordination with bond lengths 2.11–3.03 Å; the next three bond lengths are 3.24–3.51 Å. Ba in the M3 position (Fig. 4b) has a coordination of the trigonal prism (2.49–2.57 Å) with three additional long bonds (3.08 Å). This position is occupied by the Sr atoms at x = 0.65; it is coordinated by six oxy2016
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BaBi2B2O7, 100%
10 000
5000
0
Sr0.70Ba0.30Bi2B2O7, 94.45% Bi4B2O9 5.55%
80 000
40 000
0
80 000
Sr0.65Ba0.35Bi2B2O7, 95.93% Bi4B2O9 4.07%
** 29
32
40 000
**
0
10
20
30 2θ, CuKα
40
50
Fig. 2. Experinetal X-ray images juxtaposed with calculated ones. Peaks forbidden in the reduced cell are marked.
gen atoms with bond lengths 2.44–2.55 Å; the next three bond lengths are equal to 3.07 Å. Bi is coordinated in an umbrella-like five-vertex structure with
bond lengths 2.38–2.64 Å; the next four bond lengths are in the range 2.82–3.33 Å (Fig. 4). The polyhedra distortion caused by the presence of a lone electron
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b a
αb αa
αb αa
Fig. 3. Transition into the reduced cell (SrBi2B2O7 → BaBi2B2O7). The crystal structure is juxtaposed with the cross section of the TE tensor.
Table 2. Atom coordinates, atomic shifts parameters (Å2), and occupancy of positions—site occupancy factors (SOFs) in Sr0.30Ba0.70Bi2B2O7 and BaBi2B2O7 Atom
Position
x/a
y/b
z/c
SOF
U iso/eq
Sr0.3Ba0.70Bi2B2O7 Ba1 Bi1 Ba2 Bi2 Sr3 Bi3 O1 O2 O3 B1 B2
2b 6c 2b 2b 2b 6c 6c 6c 2b 2a 2a
1/3 0.372(2) 2/3 2/3 2/3 0.622(1) 0.151(2) 0.300(2) 2/3 0 0
2/3 0.7318(6) 1/3 1/3 1/3 0.240(1) 0.854(2) 0.148(2) 1/3 0 0
0.809(1) 0.8428(4) 0.681(1) 0.6523(5) 0.000a 0.000a 0.630(1) 0.858(1) 0.499(2) 0.633(1) 0.871(2)
0.283(11) 0.239(4) 0.371(11) 0.629(11) 0.345(16) 0.218(5) 1 1 1 1 1
0.0162(14) 0.0164(11) 0.0316(16) 0.0504(11) 0.033(4) 0.0360(14) 0.037(4) 0.056(5) 0.079(4) 0.015(3) 0.039(5)
0.8115(9) 0.841233 0.6797(10) 0.6544(3) 0.000a 0.9929(6) 0.6325(12) 0.8588(15) 0.494(4) 0.626(2) 0.847(4)
0.377(14) 0.208(5) 0.291(11) 0.709(11) 0.333(18) 0.222(6) 1 1 1 1 1
0.032(3) 0.0110(14) 0.025(4) 0.059(2) 0.052(4) 0.0351(15) 0.033(4) 0.053(5) 0.088(6) 0.008(5) 0.057(13)
BaBi2B2O7 Ba1 Bi1 Ba2 Bi2 Ba3 Bi3 O1 O2 O3 B1 B2
2b 6c 2b 2b 2b 6c 6c 6c 2b 2a 2a
1/3 0.3703(12) 2/3 2/3 2/3 0.6188(10) 0.140(3) 0.296(5) 2/3 0 0
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Table 3. Some bond lengths and balance of valence (BV, v.u.) for Sr0.30Ba0.70Bi2B2O7 and BaBi2B2O7 Bond Length (Å) Bond Length (Å) BV Bond Length (Å) Bond Length (Å) Sr0.30Ba0.70Bi2B2O7
BV
BaBi2B2O7 BO3
B1–O1 × 3 BVS B2–O2 × 3 BVS Bi1–O3 Bi1–O2 Bi1–O2 Bi1–O2 Bi1–O2 Bi1–O2 Bi1–O2 Bi1–O1 Bi1–O1 Bi1–O1 〈Bi1–O〉7 Distortion BVS
Bi2–O3 Bi2–O1 × 3 Bi2–O1 × 3 Bi2–O2 × 3 〈Bi2–O〉7 Distortion BVS
Bi3–O1 Bi3–O1 Bi3–O2 Bi3–O2 Bi3–O3 Bi3–O1 Bi3–O2 Bi3–O3 Bi3–O3 〈Bi3–O〉7 Distortion BVS
1.37(1)
1.00 3.00 0.97 2.88
B1–O1 × 3 BVS B2–O2 × 3 BVS M1O10
1.36(2)
Bi1–O3 Bi1–O2 Bi1–O2 Bi1–O2 Bi1–O2 Bi1–O2 Bi1–O2 Bi1–O1 Bi1–O1 Bi1–O1 〈Bi1–O〉7
2.11(5) 2.37(2) 2.45(3) 2.65(4) 2.72(4) 2.96(3) 3.03(2) 3.24(2) 3.25(2) 3.51(2) 2.61 0.26
Ba1–O3 Ba1–O2 × 3 Ba1–O2 × 3 Ba1–O1 × 3
2.48(5) 2.71(3) 2.79(3) 2.98(2)
〈Ba1–O〉10
2.79 0.16
2.44(3) Bi2–O3 2.741(13) Bi2–O1 × 3 2.766(14) Bi2–O1 × 3 2.910(16) Bi2–O2 × 3 〈Bi2–O〉7 2.77 0.08 2.34 Distortion 2.64 BVS 2.67* M3O9
2.18(5) 2.66(2) 2.73(2) 3.26(2) 2.62 0.13
Ba2–O3 Ba2–O1 × 3 Ba2–O1 × 3 Ba2–O2 × 3 〈Ba2–O〉10
2.38(3) 2.39(2) 2.44(2) 2.46(2) 2.643(4) 2.82(2) 2.88(2) 3.321(6) 3.325(4) 2.58 0.18
Ba3–O1 × 3 2.49(2) Ba3–O2 × 3 2.57(2) Ba3–O2 × 3 3.083(1)
1.395(10)
2.12(3) 2.424(11) 2.435(15) 2.632(19) 2.716(19) 2.945(11) 2.946(14) 3.240(14) 3.265(14) 3.481(13) 2.60 0.24
Ba1–O3 Ba1–O2 × 3 Ba1–O2 × 3 Ba1–O1 × 3
2.55(3) 2.733(17) 2.758(17) 2.939(15)
〈Ba1–O〉10
2.78 0.09
2.05(3) 2.671(8) 2.696(7) 3.233(15) 2.59 0.16
Ba2–O3 Ba2–O1 × 3 Ba2–O1 × 3 Ba2–O2 × 3 〈Ba2–O〉10
2.316(13) 2.332(14) 2.434(16) 2.442(14) 2.646(4) 2.755(13) 2.849(13) 3.301(4) 3.317(6) 2.56 0.18
Sr3–O1 × 3 Sr3–O2 × 3 Sr3–O2 × 3
2.439(12) 2.547(13) 3.0742(1)
〈Sr3–O〉9
2.69 0.26
2.32 Distortion 2.54 BVS 2.67* M2O10
Bi3–O2 Bi3–O2 Bi3–O1 Bi3–O1 Bi3–O3 Bi3–O2 Bi3–O1 Bi3–O3 Bi3–O3 〈Bi3–O〉7 2.34 Distortion 2.44 BVS 2.67*
1.03 3.09 0.98 2.94
1.38(2)
〈Ba3–O〉9
2.52(5) 2.72(2) 2.79(2) 2.98(2) 2.80 0.11
2.71 0.22
2.37 2.61 2.67*
2.05 2.27 2.67*
2.40 2.64 2.67*
*Expected BVS for the position.
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Table 4. Parameters of solid solutions according to the Rietveld method data BaBi2B2O7
Composition
Sr0.3Ba0.7Bi2B2O7
Syngony
Sr0.35Ba0.65Bi2B2O7
SrBi2B2O7*
hexagonal
Space group
P63
P63
P63
P63
2 5.3435 13.5851 355.92 0.065
2 5.3209 13.4045 328.67 0.058
6 9.2053 13.3727 981.35 0.069
6 9.1404(4) 13.0808(6) 946.44(6) 0.115
Rp
0.043
0.041
0.049
–
Rexp
0.024
0.023
0.022
0.057
RB
0.010
0.040
0.042
0.063
Sample purity
100%
95.93%
94.45%
–
Z a, Å c, Å V, Å3 RwP
*Parameters are taken from [14].
pair by Bi3+ was estimated in accordance with the Baur formula [25] (Table 3). Additional oxygen. The understanding of the versatility of splitting the positions is facilitated by the presence of an additional oxygen atom (O3) in the structure, which is not a part of the boron–oxygen triangles. All three splitting types are present in the oxocentered OM5 five-vertex polyhedron (a rather rare oxo-centered polyhedron) formed around the O3
atom (Fig. 5a). In each of these three cases, trivalent bismuth atoms appear to be closer to the central oxygen atoms than the divalent barium atoms. The latter can be considered natural from the point of view of the ratio between the average bond lengths ((Bi3+—O) = 2.290 Å < (Ba2+—O) = 2.738 Å) [26, p. 778] and in terms of the positions of the electrostatic interactions, since the Bi3+ cation is the most electropositive among the atoms surrounding oxygen in the O3 position. Interestingly, both extreme members of the series have
(a) B(1)O3
(b) O2
O2 O2
Bi1
B(2)O3 Ba2 O1
M1 M2
Bi2
αc M3
M2
Ba1
O3
O1
O3
M1
αa
Ba3 O3
Bi
Bi3
c a
Ba M3
b
O2
Fig. 4. Crystal structure of BaBi2B2O7. Juxtaposition of the crystal structure with the TE tensor (a) and positons splitting: M1, M2, and M3 (b). GLASS PHYSICS AND CHEMISTRY
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(a) Ba2
(b)
Bi2 2.18
Ba3
Sr
Ba3 2.6 4
Bi3
Sr 3.65
Bi3
O3
2.89
O3 2.15
Sr
2.11
Ba3
2.61
2.64
Bi3
2.19
Bi2
Bi1 Bi1 Ba1 Fig. 5. Oxo-centered polyhedra in the structures of BaBi2B2O7 (a) and SrBi2B2O7 (b).
an additional oxygen atom in similar coordination (Fig. 5). The splitting of the positions appears as a result of the necessity to narrow the limits of the isomorphic miscibility of the Bi3+ (contains a lone stereo-active electron pair) and Ba2+ (spherical ion of the inert gas type) atoms occurring upon cooling that are different in size and electron structure. In the same system of positions under high-temperature synthesis conditions, in the course of a temperature decrease, Bi and Ba must be separated or become ordered on different Table 5. Melting and crystallization points of Sr1 – xBaxBi2B2O7 from the DSC data at heating and cooling Heating, °C
Cooling, °C
Tmelt
Tcryst
SrBi2B2O7
776
777
Sr0.75Ba0.25Bi2B2O7
755
745
Sr0.5Ba0.5Bi2B2O7
740
699
Sr0.35Ba0.65Bi2B2O7
675
620
Sr0.25Ba0.75Bi2B2O7
687
637
Sr0.15Ba0.85Bi2B2O7
648
563
BaBi2B2O7
630
*
Composition
*Not found.
positions. This can be achieved through splitting their common position into bismuth and barium subpositions. In the structure from x = 0, the Sr atoms completely occupy the M3 position and are coordinated by oxygen on the vertices of the trigonal prism. The transition from the six-vertex polyhedron to the nine-vertex one takes place with the substitution of Sr by Ba. Phase transition in Sr1 – xBaxBi2B2O7 solid solutions. Thermal analysis results. The following thermal effects were revealed on the DSC curves: glass transition temperature, crystallization from glass, and melting upon heating or crystallization upon cooling. The temperatures of the commencement of the effects upon heating or cooling determined from the changes in the first derivative are shown in Table 5. Based on the obtained data, one can conclude that the melting point temperature decreases along with the increase of the barium fraction in the composition of solid solutions. As seen from Table 5, the temperature of the commencement of the melting effect upon heating virtually coincides with the temperature of the start of crystallization upon cooling for х = 0 and 0.25 (SrBi2B2O7 and Sr0.25Ba0.75Bi2B2O7), whereas the difference increases by more than 100°C along with the barium content increase. The difference must have been caused by the fact that compounds in the SrO–B2O3–Bi2O3 system manifest a less expressed tendency to glass formation than those in the BaO–B2O3–Bi2O3 system, which is in agreement with [10, 12]. Powder X-ray diffraction study at room temperature. The solid solutions were studied using the method of powder X-ray diffraction. The parameter and volume of the unit cell (Fig. 6) increase on the substitution of
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a/Å 9.3 9.2 9.1 c/Å 13.6
13.3
13.0 V/Å3 1000 970 940
0
0.2
0.4
0.6 x, Ba
0.8
1.0
Fig. 6. Dependence of parameters and volume of the unit cell of solid solutions on the chemical composition.
Sr by Ba. For the parameters, one observes an insignificant nonlinearity around x = 0.65. The dependence of unit cell parameters on the chemical composition was approximated by a linear function: ax = (9.128 + 0.122x) Å, cx = (13.053 + 0.521x) Å, Vx = (941.4 + 65.0x) Å3.
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The origin of the nonlinearity of the changes in the parameters becomes clear after analyzing the superstructural reflections in the single crystal data: their intensities sharply decrease to the background level in the range x = 0.7–1.0 (see 2.3.1.). As a result, a reduced cell with the parameters aBa = aSr/√3 is formed. The reduced cell is corroborated by the Rietveld method. Thermal expansion. The solid solutions Sr1 – xBaxBi2B2O7 manifest a sharp anisotropic TE: the с parameter increases more intensively than a (Fig. 7). All the dependences of the lattice parameters on temperature are approximated by second-order polynomials (Table 6). According to the high-temperature crystal chemistry of borates, the TE of borates with triangular radicals is determined by the predominant orientation of these triangles [27]: TE is minimal in the plane of the triangles (ab) and maximal along the normal to it. The coefficients of the TE of solid solutions are shown in Table 7. The comparison of the parameters temperature dependences and the unit cell volume for a series of solid solutions Sr1 – xBaxBi2B2O7 (Fig. 7) demonstrates that the а parameters change identically for all the compositions, except the solid solution with х = 0.65, and its changes are sharper than for the other compositions. It is assumed that in this sample at temperatures close to room temperature two solid solutions exist and cannot be discerned: the Ba-depleted phase is predominant, because the а parameter is close to it at х = 0.5; along with the temperature increase, the а parameter increases sharper than others, and its value approaches that for the composition at х = 0.7, which indicates the homogenization of solid solutions with x = 0.65. Phase transition and order–disorder processes in solid Sr1 – xBaxBi2B2O7 solutions. A specific feature in the behavior of the solid solutions under study was identified. Upon substitution of Sr by Ba around x =
Table 6. Coefficient of the equation a0 + a1T + a2T2 of the temperature dependence of the lattice parameters for solid Sr1 – xBaxBi2B2O7 solutions a(T) xBa a0
a1 ×
10–3
c(T) a2 ×
10–6
a0
a1 ×
10–3
V(T) a2 ×
10–6
a0
a1 ×
10–3
a2 ×
10–6
Range, °C
0
9.125
0.033
0.031
13.052
0.290
0.090
941.34
27.5
1.4
25-750
0.25
9.165
0.021
0.051
13.174
0.296
0.100
958.15
25.8
1.9
25-750
0.50
9.195
0.028
0.041
13.310
0.288
0.126
974.66
26.9
1.9
25-700
0.65
9.198
0.042
0.037
13.389
0.255
0.135
981.01
27.4
1.8
25-650
0.70
9.211
0.028
0.048
13.400
0.282
0.145
984.68
26.6
2.2
25-650
1
9.247
0.050
0.021
13.582
0.260
0.162
1005.8
30.1
1.7
25-600
a0 is the parameter value at 0°С, T is temperature (0°C). GLASS PHYSICS AND CHEMISTRY
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Table 7. Coefficient of TE α (10–6 °C–1) for solid Sr1 – xBaxBi2B2O7 solutions at different temperatures αa
xBa
αc
αV
25°С 0 0.25 0.5 0.65 0.7 1
3.8 2.6 3.3 3.9 3.1 5.6
0 0.25 0.5 0.65 0.7 1
5.8 5.9 6 7.3 6.5 6.9
0 0.25 0.5 0.65 0.7 1
7.9 9.3 8.6 10.6 9.6 8.3
23 23 22 18 21 20
30 28 29 26 27 31
325°С 27 27 27 26 28 27
38 39 40 40 40 41
625°С 30 32 33 33 34 34
46 50 50 54 53 50
a, Å
0.65–0.70, the superstructural reflections disappear, so that the a parameter of the unit cell reduces. Some nonlinearity is observed in the dependences of the lattice parameters on temperature and chemical composition and on the DSC curves (melting effect). Here, an assumption on the existence of a narrow immiscibility range was made. The existence of such a range could result from pure structural reasons. Indeed, in the crystal structure of SrBi2B2O7, Sr and Ba atoms are ordered on three positions (M1, M2, and M3), whereas they are almost completely disordered in the BaBi2B2O7n structure. Refinement of the crystal structures of the series’ intermediate members promoted an understanding of the way the transition from an ordered to a disordered structure takes place at the structural level. In the solid solution at x = 0.70, the smallest position (M3) contains only Sr atoms, while the same position contains Ba and Bi atoms in the BaBi2B2O7 structure. Here, the Ba and Bi atoms are statistically distributed over three positions at the ratio corresponding to the compound chemical composition (1 : 2). The 1/3 strontium occupancy of the smallest position and the formation of the immiscibility range at the same Sr quantity is not accidental. On the increase of the content of Sr contained in this position as the smallest one, the structural transition induced by the disruption of the statistical distribution of the ions over the positions takes place (Fig. 8).
V, Å3
c, Å x=1
9.28
13.8 1020
0.7 0.65
9.24
13.6 1000
0.5
9.20
0.25
9.16
x=0
9.12
0
400
800
13.4
980
13.2
13.0
960
0
400 T, °C
800
940
0
400
800
Fig. 7. Dependence of parameters and volume of the unit cell on temperature for solid Sr1 – xBaxBi2B2O7 solutions. GLASS PHYSICS AND CHEMISTRY
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CRYSTAL STRUCTURES AND THERMAL EXPANSION
M1
M2
347
M3
Sr SrBi2B2O7 Ba
Bi
x = 0.65
BaBi2B2O7 Fig. 8. Cation distribution over M1, M2, and M3 positions in Sr1 – xBaxBi2B2O7 from the data of X-ray diffraction analysis.
CONCLUSIONS
ACKNOWLEDGMENTS
The solid Sr1 – xBaxBi2B2O7 solutions were obtained through crystallization from glass ceramics. The crystal structures (x = 0.7, 1.0) have been solved by the direct methods and refined in the unit cell reduced relative to SrBi2B2O7. The cation positions are split into Bi-, Sr-, and Ba-subpositions. The additional oxygen in the crystal structure of this oxoborate forms a new type of the oxo-centered polyhedron (OBi5). Upon the splitting of the positions, the following regularity is observed: Bi3+ in each positon is located closer to the additional oxygen atom than Ba2+. It is natural, since Bi3+ and O2– form a strong electrostatic interaction in the structure, and the Bi3+–O2– bond length is shorter than that of Ba2+–O2–. A special phase transition range between x = 0.65 and x = 0.70, within which the unit cell’s parameters are reduced, was revealed in the series of solid solutions.
The work was financially supported by the Russian Foundation for Basic Research (project no. 14-0332076-mol). The authors used the equipment of the Center of X-ray Diffraction Studies at St. Petersburg State University.
The phase transition proceeds through the presumed range of immiscibility of the solid solutions caused by the restructuring of the crystal structure. The structure of SrBi2B2O7 contains three independent positions for cations, two of which are occupied by the Bi atoms and one by the Sr atoms. Upon substitution of 2/3 Sr by Ba, one observes the redistribution of the cations over the positions, resulting in the unit cell’s reduction, which is closely correlated to the transition range x = 0.65–0.70. Here, Sr always occupies the smallest positon (M3) in the structure. GLASS PHYSICS AND CHEMISTRY
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REFERENCES 1. Fröhlich, R., Crystal structure of the low-temperature form of BaB2O4, Z. Kristallogr., 1984, vol. 168, pp. 109– 112. 2. Xue, D. and Zhang, S., Structure and non-linear optical properties of beta-barium borate, Acta Crystallogr., Sect. B: Struct. Sci., 1998, vol. 54, pp. 652–656. 3. Fedorov, P.P., Kokh, A.E., and Kononova, N.G., Barium borate β-BaB2O4 as a material for nonlinear optics, Russ. Chem. Rev., 2002, vol. 71, no. 8, pp. 6511–671. 4. Fröhlich, R., Bohatý, L., and Liebertz, J., Die kristallstruktur von wismutborat, BiB3O6, Acta Crystallogr., Sect. C: Cryst. Struct. Commun., 1984, vol. 40, pp. 343– 344. 5. Hellwig, H., Liebertz, J., and Bohaty, L., Linear optical properties of the monoclinic bismuth borate BiB3O6, J. Appl. Phys., 2000, vol. 88, no. 1, pp. 240– 244. 6. Barbier, J., Penin, N., Denoyer, A., and Cranswick, L., BaBibO4, a novel non-centrosymmetric borate oxide, Solid State Sci., 2005, vol. 7, no. 9, pp. 1055–1061. 7. Egorysheva, A.V. and Kargin, Yu.F., Phase equilibria in the Bi2O3–BaB2O4–B2O3 system in the subsolidus region, Russ. J. Inorg. Chem., 2006, vol. 51, no. 7, pp. 1106–1110. 2016
348
BUBNOVA et al.
8. Egorysheva, A.V., Volodin, V.D., and Skorikov, V.M., Calcium bismuth borates in the CaO–Bi2O3–B2O3 system, Inorg. Mater., 2008, vol. 44, no. 1, pp. 70–75. 9. Egorysheva, A.V., Volodin, V.D., Skorikov, V.M., Yurkov, G.Yu., and Sorokin, N.I., Synthesis of nanocomposites based on MO–Bi2O3–B2O3 (M = Ca, Sr, Ba) glasses, Inorg. Mater., 2010, vol. 46, no. 4, pp. 434– 438. 10. Egorysheva, A.V., Skorikov, V.M., Volodin, V.D., Myslitskii, O.E., and Kargin, Yu.F., Phase equilibria in the BaO–Bi2O3–B2O3 system, Russ. J. Inorg. Chem., 2006, vol. 51, no. 12, pp. 1956–1960. 11. Egorysheva, A.V., Volodin, V.D., and Milenov, T., et al., Glass formation in the CaO–Bi2O3–B2O3and SrO–Bi2O3–B2O3 systems, Russ. J. Inorg. Chem., 2010, vol. 55, no. 11, pp. 1810–1817. 12. Egorysheva, A.V., Volodin, V.D., and Skorikov, V.M., Subsolidus phase equilibria in the SrO–Bi2O3–B2O3 system, Russ. J. Inorg. Chem., 2009, vol. 54, no. 11, pp. 1811–1815. 13. Inorganic Crystal Structure Database (ICSD). National Institute of Standards and Technology. Gaithersburg, 2014, version II. 14. Barbier, J. and Cranswick, L.M.D., The non-centrosymmetric borate oxides, MBi2B2O7 (M = Ca, Sr), J. Solid State Chem., 2006, vol. 179, pp. 3958–3964. 15. Bubnova, R.S., Krivovichev, S.V., Filatov, S.K., Egorysheva, A.V., and Kargin, Y.F., Preparation, crystal structure and thermal expansion of a new bismuth barium borate, BaBi2B4O10, J. Solid State Chem., 2007, vol. 180, no. 2, pp. 596–603. 16. Krzhizhanovskaya, M.G., Bubnova, R.S., Egorysheva, A.V., Kozin, M.S., Volodin, V.D., and Filatov, S.K., Synthesis, crystal structure and thermal behavior of a novel oxoborate SrBi2B4O10, J. Solid State Chem., 2009, vol. 182, pp. 1260–1264. 17. Krivovichev, S.V., Bubnova, R.S., Volkov, S.N., Krzhizhanovskaya, M.G., Egorysheva, A.V., and Filatov, S.K., Preparation, crystal structure and thermal expansion of a novel layered borate, Ba2Bi3B25O44, J. Solid State Chem., 2012, vol. 196, pp. 11–16.
18. Cai, G.M., Li, M., Liu, J., Jin, S.F., Wang, W.Y., Zheng, F., and Chen, X.L., Crystal structure and Eu3+/Tb3+ doped luminescent properties of a new borate Ba3BiB9O18, Mater. Res. Bull., 2009, vol. 44, pp. 2211–2216. 19. Volkov, S.N., Bubnova, R.S., Filatov, S.K., and Krivovichev, S.V., Synthesis, crystal structure and thermal expansion of a novel borate, Ba3Bi2(BO3)4, Z. Kristallogr.—Cryst. Mater., 2013, vol. 228, pp. 436–443. 20. Hovhannisyan, M., Hovhannisyan, R., Alexanyan, H., and Knyazyan, N., Synthesis of new stoichiometric barium bismuth borate BaBi2B2O7, BaBi10B6O25, BaBi8B2O16, Acta Crystallogr., Sect. A: Found. Crystallogr., 2010, vol. 66, pp. s161–s162. 21. Shablinskii, A.P., Drozdova, I.A., Volkov, C.N., Krzhizhanovskaya, M.G., and Bubnova, R.S., Preparation and study of glass ceramics in the system of Sr1–xBaxBi2B2O7, Fiz. Khim. Stekla, 2012, vol. 38, no. 6, pp. 886–889. 22. Sheldrick, G.M., SHELXL-97, Program for the Refinement of Crystal Structures, Univ. Göttingen, 1997. 23. Petricek, V., Dusek, M., and Palatinus, L., JANA2006. The Crystallographic Computing System, Prague: Inst. Phys., Acad. Sci. Czech Republic, 2006. 24. Bubnova, R.S., Firsova, B.A., and Filatov, S.K., Software for determining the thermal expansion tensor and the graphic representation of its characteristic surface (Theta to Tensor-TTT), Glass Phys. Chem., 2013, vol. 39, no. 3, pp. 347–351. 25. Baur, W.H., The geometry of polyhedral distortions. Predictive relationships for the phosphate group, Acta Crystallogr., Sect. B: Struct. Crystallogr. Cryst. Chem., 1974, vol. 30, pp. 1195–1215. 26. International Tables for Crystallography (2006), vol. C, ch. 9.4, p. 778; ch. 9.8, p. 907. 27. Bubnova, R.S. and Filatov, S.K., High-temperature borate crystal chemistry, Z. Kristallogr.—Cryst. Mater., 2013, pp. 395–428.
GLASS PHYSICS AND CHEMISTRY
Translated by D. Marinin
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