Ab initio calculation of structural stability, electronic and optical properties of Ag2Se S. Rameshkumar, G. Jaiganesh, V. Jayalakshmi, and B. Palanivel Citation: AIP Conference Proceedings 1665, 090024 (2015); doi: 10.1063/1.4918004 View online: http://dx.doi.org/10.1063/1.4918004 View Table of Contents: http://scitation.aip.org/content/aip/proceeding/aipcp/1665?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Ab initio calculation of thermodynamic, transport, and optical properties of CH2 plastics Phys. Plasmas 22, 053303 (2015); 10.1063/1.4919963 Microwave measurements and ab initio calculations of structural and electronic properties of N -Et-1,2-azaborine J. Chem. Phys. 131, 224312 (2009); 10.1063/1.3270157 Structural stability and characteristics of the metastable Ag–W phases studied by ab initio and molecular dynamics calculations J. Appl. Phys. 101, 063512 (2007); 10.1063/1.2711766 Structural, electronic, and optical properties of Zr O 2 from ab initio calculations J. Appl. Phys. 100, 104103 (2006); 10.1063/1.2386967 Ab initio calculations of structural and electronic properties of small silver bromide clusters J. Chem. Phys. 111, 8925 (1999); 10.1063/1.480237
Ab initio Calculation of Structural Stability, Electronic and Optical Properties of Ag2Se S. Rameshkumar1, G. Jaiganesh2, V. Jayalakshmi1,*and B. Palanivel3 1
2
Department of Physics, SRM University, Ramapuram Campus, Chennai – 600089, India Materials Science Group, Indira Gandhi Centre for Atomic Research, Kalpakkam – 603102, India 3 Department of Physics, Pondicherry Engineering College, Puducherry – 605014, India * E-mail:
[email protected]
Abstract. The structural stability, electronic and optical properties of Ag2Se compound is studied using ab initio packages. Ag2Se is found to crystallize in orthorhombic structure with two different space groups i.e. P212121 (No. 19) and P2221 (No. 17). For this compound in these two space groups, the total energy has been computed as a function of volume. Our calculated results suggest that the P212121 –phase is more stable than that of the P2221–phase. The band structure calculation show that Ag2Se is semimetallic with an overlap of about 0.014 eV in P212121–phase whereas is metallic in nature in P2221–phase. Moreover, the optical properties including the dielectric fuction, energy loss spectrum are obtained and analysed. Keywords: Total energy calculations, Intermetallic compounds, Structural stability, Electronic and Optical properties PACS: 71.15.Mb, 71.15.Nc, 71.20.Lp, 71.55.Ak
INTRODUCTION Ag2Se compound has attracted significant attention recently due to the important technological applications. Ag2Se is an superionic conductor in αand β-phases. It has high electronic and ionic mobility. It is used as an additive in highly conductive composite glasses for sensor, displays, photochargeable battaries and electrolytes [1, 2]. This compound has also the large and positive magenetoresistance value [3]. The theoretical study of it has considerable attention due to their structural stability and electronic properties [4]. The low temperature phase Ag2Se is known as the mineral naumannite [5]. The structure of low temperature phase Ag2Se has been determined as an primitive orthorhombic structure with two different space groups i.e. P212121 (No. 19) and P2221 (No. 17) [6]. The descrepency in the space group motivated us to study the structural stability of this compound. To understand the origin of different kind of preperties in this compound, knowledege of electronic structure becomes important. In this work we investigated the structural stability, electronic and optical properties of Ag2Se in these two space groups using ab-initio methods. The important issue addressed in this work is structural phase
possibility in naumannite Ag2Se, so that it is worthy to examine low temperature phase application in this compound. For the purpose the tight binding linear muffin-tin orbital (TB–LMTO) and full potential linearized augmented plane wave (FP–LAPW) method was used.
CRYSTAL STRUCTURE The crystal structure of Ag2Se with two differnet space groups is given in Figure 1. Both the unit cell contains four Ag2Se molecules. In the space group P212121 (P2221), there are two (three) types of structurallly differnet sites of Ag ions, while positions of Se ions are inequivalent one another. From the figure 1a (P2221) showed that the Se ions are directly coordinated with Ag2 and Ag3 ions when compared to
FIGURE 1. Crystal structure of Ag2Se compound in (a) P2221, (b) P212121 space group.
Solid State Physics AIP Conf. Proc. 1665, 090024-1–090024-3; doi: 10.1063/1.4918004 © 2015 AIP Publishing LLC 978-0-7354-1310-8/$30.00
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Ag1 ions. In P212121 space group, Ag1 ions are coordinated almost tetrahedrally by Se whereas Ag2 ions are coordinated square pyramidally by Se ion. Both the Ag ions have a four fold coordination by Se. It is clearly shown in Figure 1b.
METHODOLOGY Within the framework of density functional theory (DFT), the electronic structure calculations are performed using the TB-LMTO and WIEN2k ab initio packages. These methods have been well described in the literatures [7, 8]. The density of states are calculated by the tetrahedran method. For the calculation of P212121 and P2221 phase Ag2Se, 48 irreducible k-points were used in the entire Brillouin zone. E and k convergence are also checked carefully. To describe the exchange and correlation potential, we use the local density approximation (LDA) [9]. The primitive orthorhombic crystal is not very densely packed, it is necessary to include empty spheres in the TB-LMTO calculations. The potential and charge density inside each atomic sphere are assumed to have spherical symmetry. For WIEN2k calculations, the correct values of muffin tin spheres radii and plane wave cut off energy were chosen.
RESULTS AND DISCUSSION The electronic structure calculations were carried out for the Ag2Se compound in the P212121 and P2221 –phases. The total energy as a function of molecular volume is calculated for it in a manner similar to our earlier work [10] and is shown in Figure 2. From the total energy calculations, it is observed that it has energetically stable in P212121–phase when compared with P2221–phase. It is also suggest that there is no structural phase transition occurs between these two phases. The total energy difference between the two phases is -3.3 eV. The calculated equilibrium lattice constants and bulk modulus and its pressure derivative of it were estimated by fitting the total energies to the Birch-Murnaghan equation of state. The calculated ground state property values are listed in Table 1 and are compared with the available other results in the literature.
TABLE 1. Calculated Parameters: Lattice parameters (a, b, c) in Å, Bulk modulus (B0) in GPa and its pressure derivative (B΄), and Coheshive energy (Ecoh) in eV/atom. Ag2Se – P212121 Ag2Se – P2221 Present Exp6. Present Exp6. 4.30 4.33 7.19 7.05 A 6.99 7.06 8.00 7.85 B 7.67 7.76 4.44 4.33 C 93.17 78.75 B 5.24 5.27 B΄ Ecoh
3.70
-
3.43
-
In order to obtain a deeper insight into the changes in the electronic band compositions, we present the total density of states for this compound in Figure 3a and 3b. The density of states for this compound in both the phases lies mainly in three energy regions: (1) the lowest region streaming mainly from the anion s-like states, (2) the region at the bottom of the valence band originating from the anion p-like states and considerable amount of cation d-like states and the region near the Fermi level is quite complex; For P212121–phase shows semimetallic whereas in P2221–phase is metallic and (3) the energy region above EF dominates by unoccupied states. Moreover the lower most conduction bands, Ag s and Se p states, cross the Fermi level, while the top most valence band, Se p state, cross the Fermi level. Hence, it is a semimetal with an overlap of about 0.014 eV in P212121–phase. It also agrees well with the available other theoretical results [4].
(a) Ag2Se: P212121 (19)
(b)
FIGURE 2. Total energy vs. cell volume in Ag2Se
FIGURE 3. The total density of states of Ag2Se
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45
65
60 50
E//X E//Z
(a)
45
E//X E//Z
(b)
35
35
0.3
ε2
25 20
L(ω )
25
30
20
0.2
15
15
0.1
10
10 5
5
0 -5
0.0
0 0
1
2
3
4
5
6
7
8
9
10 11 12 13 14
0
1
2
3
4
5
Energy (eV)
6
7
8
9
10 11 12 13 14
-2
0
2
4
6
Energy (eV)
55
8
10
12
14
Energy (eV)
35
0.5
50
(d)
45
E//x E//z
30
E//X E//Z
(e)
40
E//X E//Z
(f) 0.4
25
35 30
0.3
ε2
20
L(ω )
20
25
ε1
E//X E//Z
(c)
0.4
30
40
ε1
0.5
40
55
15
0.2
15
10
10 5
0.1
5
0
0.0
-5
0 0
1
2
3
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7
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9
10 11 12 13 14
0
1
2
3
4
Energy (eV)
5
6
7
8
9
Energy (eV)
10 11 12 13 14
-2
0
2
4
6
8
10
12
14
Energy (eV)
FIGURE 4. The real part ε1 (ω), imaginary part ε2 (ω) and energy loss spectrum L (ω) of Ag2Se in P212121 and P2221–phases
The calculated real and imaginary part of the dielectric function ε(ω) for Ag2Se is shown in Figure 4 (a-f) in P212121 and P2221–phases for two polarization directions as a function of the photon energy. At the high frequencies, the zero crossing of ε1(ω) is located at 1.8 eV (E//X) and 2.1 eV (E//Z) for P212121–phase whereas is located at 4.25 eV (E//X) and 4.30 eV (E//Z) for P2221–phase. The imaginary part ε2(ω) of the dielectric function is shown in Figure (4b and 4e). Form the figure 4e one can observe easily that the maximum values are around 0.4 eV for (E//X) and (E//Z) in P2221–phase. However in P212121 phase, the maximum values are around 1.6 eV and 0.4 eV for (E//X) and (E//Z), respectively. The energy loss spectrum is related to the energy loss of a fast electron transverse in the material. It is usually large at the plasma energy. The energy loss spectrum for Ag2Se is shown in Figure 4c and 4f. It shows that the most prominent peak is located at 12 eV for (E//X) and 10 eV for (E//Z) polarization in both phases.
CONCLUSIONS In this study, using ab-initio packages we investigated the structural, electronic and optical properties of naumannite Ag2Se compound in two different space groups. The calculation shows that the P212121–phase is energetically more stable than the P2221–phase of Ag2Se compound. Our calculated results confirmed that the Ag2Se is semimetallic with an overlap of about 0.014 eV in P212121 whereas it is metallic nature in P2221–phase. We have also
presented the dielectric function and energy loss spectrum of this compound.
ACKNOWLEDGMENTS The authors are thankful to Prof. O. K. Anderson and Prof. O. Jepson, Max Planck Institute, Stuttgart, Germany for provides the TB-LMTO code and DSTSERB for providing the financial assistance.
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