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Structural and electronic properties of BaCrO4 at high-pressures
Solid State Communications 155 (2013) 45–48
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Structural and electronic properties of BaCrO4 at high-pressures Xiao-Lin Wei a,b,n, Li-Chun Xu b, Yuan-Ping Chen a, Li-Min Liu b a b
Laboratory for Quantum Engineering and Micro-Nano Energy Technology, Department of Physics, Xiangtan University, Xiangtan, Hunan 411105, China Laboratory for Green Energy, Beijing Computational Science Research Center, Beijing 100084, China
a r t i c l e i n f o
abstract
Article history: Received 22 September 2012 Received in revised form 22 October 2012 Accepted 28 October 2012 by E.G. Wang Available online 5 November 2012
Particle swarm optimization technique was applied to a structural search, yielding a high-pressure phase of BaCrO4, i.e. orthorhombic P212121 phase (stable 4 16 GPa). The P212121 structure consists of [CrO4] tetrahedra and ionized Ba, and can be viewed as a strong distortion of the low-pressure Pnma structure. Analysis of X-ray diffraction spectrum suggested that P212121 structure might be the experimentally observed high-pressure phase at pressures above 9 GPa, which was previously identified as monoclinic P21/m structure. Moreover, the calculated electronic structure suggests that P212121 structure is a semi-conductor with a band gap of 2.63 eV. Crown Copyright & 2012 Published by Elsevier Ltd. All rights reserved.
Keywords: C. Crystal structure and symmetry D. Phase transitions
1. Introduction Barium chromate (BaCrO4) has been extensively studied by experiments over the past decades due to its potential applications as oxidizing agent and photocatalyst [1–3]. It has also attracted great theoretical interest aimed at understanding the physics of its electronic band gaps and photocatalytic behavior [4]. Barium chromate is a naturally occurring chromate analog of barite (BaSO4) [5], which crystallizes in an orthorhombic structure (space group Pnma, no. 62) at ambient conditions. There is a substantial interest to explore the possible metastable high pressure polymorphs with novel structures and properties that may be quenched to ambient conditions for practical applications. Recently, Huang et al. [6] studied BaCrO4 to 25 GPa in a diamond-anvil cell (DAC) by x-ray diffraction and Raman spectroscopy methods. These spectra reveal that a transformation is abrupt near 9 GPa and the orthorhombic Pnma structure transforms to a novel high-pressure (HP) phase. This HP phase was suggested as a monoclinic P21/m structure, different from other high-pressure forms of ABO4-type compounds. Most recently, Santamarı´a-Pe´rez et al. [7] measured the x-ray diffraction data of BaSO4 in a DAC up to a pressure of 48 GPa, shown that barite transforms to a orthorhombic P212121 structure at pressures that range from 15 to 27 GPa, depending on the pressure media used, and further suggested that the previously observed HP phase of BaCrO4 should be crystallized in the same P212121 structure.
n Corresponding author at: Laboratory for Quantum Engineering and Micro-Nano Energy Technology, Department of Physics, Xiangtan University, Xiangtan, Hunan 411105, China. Tel./fax: þ 86 731 58292063. E-mail address: [email protected] (X.-L. Wei).
Since the x-ray diffraction patterns of the HP phase of BaCrO4 is incomplete and noisy at high pressures, rendering experimental structure solution difficult, there is a possibility that the HP phase of BaCrO4 adopts the P212121 structure instead of the P21/m structure. Therefore, further experimental or theoretical works on the crystal structure of the HP phase should be of interest. Here, using a particle swarm optimization (PSO) algorithm for crystal structure prediction with the only input information of chemical composition [8], we found that BaCrO4 adopts the orthorhombic P212121 structure at high-pressures. The well agreement between the simulated and the experimental x-ray diffraction patterns suggests that the P212121 structure is the experimentally observed HP phase. Details of the predicted structures, simulated x-ray diffraction patterns, and electronic properties in comparison with previously experimental and theoretical results will be made in the ensuing paragraphs.
2. Methods The variable-cell high-pressure structure predictions were performed in the pressure range 5–50 GPa with four formula units (f.u.) in the simulation cell via the PSO technique as implemented in Crystal structure AnaLYsis by Particle Swarm Optimization (CALYPSO) code [8–10]. This methodology has been successful in correctly predicting crystal structures for various systems including elements and compounds at high pressure [11–16]. The underlying ab initio structure relaxations were performed using Density Function Theory (DFT) within the Generalized Gradient Approximation (GGA) [17], as implemented in the VASP code [18]. The electron and core interactions were included by using the frozen-core all-electron projector augmented wave (PAW) method [19,20], with Ba:5s25p66s2,
0038-1098/$ - see front matter Crown Copyright & 2012 Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ssc.2012.10.039
46
X.-L. Wei et al. / Solid State Communications 155 (2013) 45–48
Fig. 1. (Color online) Crystal structures of (a) the ambient-pressure Pnma and (b) the high-pressure P212121 structure in polyhedral view. The tetrahedra and balls represent [CrO4] units and Ba atoms, respectively.
9.5
Table 1 Lattice parameters of Pnma and P212121 structure at 0 and 11 GPa, respectively. The atomic positions of the P212121 structure within GGA at 11 GPa.
P212121 Ba Cr O1 O2 O3 O4 a
˚ b or y (A)
˚ c or z (A)
Expt.a GGA LDA
9.106 9.218 8.974
5.539 5.642 5.392
7.335 7.435 7.192
GGA LDA 4a 4a 4a 4a 4a 4a
7.288 7.063 0.133 0.119 0.948 0.299 0.932 0.168
6.373 6.258 0.293 0.237 0.169 0.300 0.944 0.039
6.846 6.685 0.080 0.577 0.723 0.710 0.057 0.428
Ref. [6]
a
8.5
Lattice Parameters (Å)
Pnma
˚ a or x (A)
9.0
Obs. Huang et al. Cal. GGA Cal. LDA
8.0 7.5
a' c'
c
7.0 6.5
b'
6.0 5.5
b
5.0 4.5 0
2
4
6
8
10
12
14
16
Pressures (GPa) Cr:3s13d5 and O:2s22p4 treated as the valence electrons. The energy cutoff 520 eV and appropriate Monkhorst–Pack k meshes[21] were chosen to ensure that enthalpy calculations are well converged to 1 meV/atom.
3. Results In order to verify our calculations, the phase stability and structural properties of the well-studied Pnma phases (Fig. 1a) is considered in some detail. The Pnma structure can be described in terms of its cation subarray BaCr, which is of FeB-type. This structure consists of triangular prisms of Ba atoms that share faces along the b direction and corners in the other two directions, with the [CrO4] groups inserted into these metal prisms. The Pnma structure was successfully reproduced by the ab initio structure searching via CALYPSO with four f.u.’s in a simulating cell at 5 GPa. The calculated structural parameters along with the previous experimental data are summarized in Table 1. The well agreement between the theoretical and experimental data exhibits sufficient validity and reliability of our simulations. The orthorhombic P212121 structure (Z¼ 4) was derived from a simulation with four f.u.’s in the unit cell at 50 GPa by the CALYPSO methodology. As shown in Fig. 1b, the P212121 structure is basically a strong distortion of the initial Pnma structure (see Fig. 1a). As the Pnma structure transforms to the P212121 structure, the environment of the Cr atom almost does not change,
Fig. 2. (Color online) Evolution of the lattice parameters with pressures. Lattice constants of the Pnma and P212121 structure are shown by a, b, c and a0 , b0 , c0 , respectively. The experimental data from Ref. [6] are presented by the black circles.
however the tilting movement of the [CrO4] tetrahedra result in a change in the environment of the Ba atoms, and triangular prisms of the Ba atoms do not exist anymore. The evolution of the lattice parameters as a function of pressures is shown in Fig. 2, which reveals that the a axis contracts approximately 19.5%, the b axis expands approximately 22%, and the c axis remains nearly constant at the experimental transition pressure of 9 GPa. In view of that GGA often overestimate the lattice parameters, but the LDA underestimated them, the simulated lattice parameters of the Pnma structures are in well agreement with the experimental data at high-pressures. A volume collapse of approximate 5% at the experimental transition pressure of 9 GPa is an indicative of a first-order phase transition, which is in well accordance with the experimental observations. The enthalpy curves of the predicted P212121 structure relative to the ambient-pressure Pnma structure is shown in Fig. 3. The crossing between the two curves indicates a pressure-induced phase transition. Clearly, the Pnma structure transforms to the P212121 structure at approximate 16 GPa. It should be pointed out that the predicted transition pressure is a bit higher than the experimental observation of 9 GPa. However, in view of that the transition of BaCrO4 is not complete at 18 GPa in the experiment,
X.-L. Wei et al. / Solid State Communications 155 (2013) 45–48
0.3 Energy (eV/f.u.)
-48.0
-49.0 -49.5 -50.0
0.1
65
70
75
80
85
90
95
3
Volume A /f.u.
0.0
P 212121
Pnma -0.1 0
5
10
15
20
25
Pressures (GPa) Fig. 3. (Color online) Enthalpies of the P212121 structure with respect to the Pnma structure as a function of pressure. The inset illustrates the change of total energy for both phases.
*
8
Cal. P 212121 Cal. P 212121 Obs. Ref. 6 Cal. Pnma
Intensity (arb. units)
7 6 5
*
*
4 3 2
*
1 *
0 4
in the simulated x-ray diffraction patterns. In the angle range of 41–141 the strongest peak of the P212121 structures locates at 7.41, as indicated by the big black star in Fig. 4, which is in excellent agreement with the experimental observations. Experimentally [6], the x-ray features changed and a number of peaks disappeared at pressures above 18 GPa quite before the Pnma structure completely transforms to the P212121 structure. Thus an x-ray diffraction pattern of pure HP phase of BaCrO4 is yet unavailable, and we cannot identify the crystal structure of the HP phase directly by a complete comparison of the theoretical and experimental x-ray diffraction patterns. The incomplete x-ray diffraction spectrum is very likely to be the reason that the HP phase was previously indexed to the P21/m structure, and the corresponding atomic positions were not solved. In order to quantitatively describe the electronic and bonding behavior of the BaCrO4 at high-pressures, total and partial density of states (DOS) of the P212121 structure at 11 GPa in comparison with that of the Pnma structure at 0 GPa were calculated and shown in Fig. 5. The simulated band gaps of the Pnma structure is 2.76 eV, in general agreement with the experimental observation (2.64 eV), whereas the band gap of P212121 structure is predicted to be 2.63 eV, indicating that BaCrO4 is still a semiconductor at 11 GPa. A small reduction of 5% in the band gap was found with the pressure increasing from 0 to 11 GPa, which is mainly attributed to the slight broadening of the valance band centered at approximate 2 eV. As shown by the partial DOS, the Ba-6s and Cr-4s orbital are completely empty, revealing charge
*
6
8
10
12
14
DOS states/eV/BaCrO 4
H-H Pnma (eV/f.u.)
0.2
-48.5
2θ (degree)
18 12 6 0 12 8 4 0
Ba 5p
Cr
8 4 0 8 0
Total
Pnma
3d
O 2p
4
Fig. 4. (Color online) The theoretical and experimental x-ray diffraction profiles for the HP phase of BaCrO4 at 10.3 GPa. Vertical markers indicate Bragg reflections of the predicted orthorhombic P212121 structure (above) and the initial orthorhombic Pnma structure (below).
-10
-8
-6
-4
-2
0
2
4
2
4
Energy eV
DOS states/eV/BaCrO 4
the simulated result is quite acceptable. Except the Pnma and P212121 structures, several other structures (e.g. P213, Pbcn, Pna21, P4/nmb and P21/c structure) with competitive enthalpies were also predicted at high-pressures, but all of them were finally found to be energetically unstable and therefore the enthalpies of them were not shown in Fig. 3. The previously proposed P21/m structure was not found in our ab initio structural searching by using the global optimization method for crystal structure prediction. Since the atomic positions of the P21/m structure is unknown, the enthalpies of which were not calculated and compared to the P212121 structure. Thus the P21/m structure could not be theoretically excluded from the phase transition sequence of BaCrO4. However, the current result strongly hints that the P21/m structure is not an energetically favorable structure of BaCrO4 at high-pressures. The simulated x-ray diffraction patterns of the P212121 and Pnma structures in comparison with the experimental spectrum [6] at 10.3 GPa are shown in Fig. 4. The experimentally observed new peaks marked by the red stars have been successfully found
47
18 12 6 0 12 8 4 0
Ba 5p
Cr
8 4 0 8
3d
O 2p
4 0
Total
P 212121
-10
-8
-6
-4
-2
0
Energy eV Fig. 5. (Color online) Total and partial DOS of the Pnma structure at 0 GPa and P212121 structure at 11 GPa. The vertical dashed lines denote the Fermi level.
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X.-L. Wei et al. / Solid State Communications 155 (2013) 45–48
transformation from the metal atoms to O atoms. The d states of Cr and the p states of O lie in the energy range between 4.5 to 2.9 eV and 4.5 to 0 eV, respectively. Their hybridation causes bonding in the [CrO4] tetrahedra. This is very similar to what happens in typical ABO4 compounds, e.g. SrMoO4 [22], in which the partial covalent bonding is formed mainly because of the d–p hybridation.
4. Concluding remarks We predicted one high-pressure polymorphs of BaCrO4 with P212121 structure stable above 16 GPa. The well agreement between the experimental and theoretical x-ray diffraction pattern indicates that the P212121 structure may be the previously observed HP phase of BaCrO4. The total DOS of the P212121 structure reveals that BaCrO4 is a semiconductor with band gap of 2.63 eV at 11 GPa, whereas the partial DOS suggests the existence of an ionic interaction between Ba and [CrO4] unit and a partial covalent bonding between Cr and O atoms.
Acknowledgments This work was supported by the Open Fund based on innovation platform of Hunan colleges and universities (No. 12K045), and Hunan Science and Technology Bureau planned project (No. 2010SK3166).
References [1] Z. Zou, J. Ye, K. Sayama, H. Arakawa, Nature 414 (2001) 625–627. [2] R. Asahi, T. Morikawa, T. Ohwaki, K. Aoki, Y. Taga, Science 293 (2001) 269–271. [3] J. Yin, Z. Zou, J. Ye, Chem. Phys. Lett. 378 (2003) 24–28. [4] S.R. Thakarea, S.R. Patilb, M.D. Choudhari, Indian J. Chem., Sect A 49A (2009) 54–58. [5] S.-H. Yu, M. Antonietti, H. Colfen, J. Hartmann, Nano Lett. 3 (2003) 379–382. [6] T. Huang, S.R. Shieh, A. Akhmetov, X. Liu, C.-M. Lin, J.-S. Lee, Phys. Rev. B 81 (2010) 214117. [7] D. Santamarı´a-Pe´rez, L. Gracia, G. Garbarino, R.C.Jorda´n Beltra´n, O. Gomis, D. Errandonea, C. Ferrer-Roca, D. Martı´nez-Garcı´a, A. Segura, Phys. Rev. B 84 (2011) 054102. [8] Y. Wang, J. Lv, L. Zhu, Y. Ma, Phys. Rev. B 82 (2010) 094116. [9] J. Lv, Y. Wang, L. Zhu, Y. Ma, J. Chem. Phys. 137 (2012) 084104–084108. [10] Y. Wang, J. Lv, L. Zhu, Y. Ma, Comput. Phys. Commun. 183 (2012) 2063–2070. [11] P. Li, G. Gao, Y. Wang, Y. Ma, J. Phys. Chem. C 114 (2010) 21745–21749. [12] Y. Wang, H. Liu, J. Lv, L. Zhu, H. Wang, Y. Ma, Nat. Commun. 2 (2011) 563. [13] G. Gao, H. Wang, A. Bergara, Y. Li, G. Liu, Y. Ma, Phys. Rev. B 84 (2011) 064118. [14] J. Lv, Y. Wang, L. Zhu, Y. Ma, Phys. Rev. Lett. 106 (2011) 015503. [15] L. Zhu, H. Wang, Y. Wang, J. Lv, Y. Ma, Q. Cui, Y. Ma, G. Zou, Phys. Rev. Lett. 106 (2011) 145501. [16] H. Wang, J.S. Tse, K. Tanaka, T. Iitaka, Y. Ma, Proc. Natl. Acad. Sci. USA 109 (2012) 6463–6466. [17] J.P. Perdew, K. Burke, M. Ernzerhof, Phys. Rev. Lett. 77 (1996) 3865–3868. [18] G. Kresse, J. Furthmuller, Phys. Rev. B 54 (1996) 11169–11186. [19] G. Kresse, D. Joubert, Phys. Rev. B 59 (1999) 1758–1775. ¨ [20] P.E. Blochl, Phys. Rev. B 50 (1994) 17953–17979. [21] H.J. Monkhorst, J.D. Pack, Phys. Rev. B 13 (1976) 5188–5192. [22] V. R, Comput. Mater. Sci. 50 (2011) 2683–2687.
Contents lists available at SciVerse ScienceDirect
Solid State Communications journal homepage: www.elsevier.com/locate/ssc
Structural and electronic properties of BaCrO4 at high-pressures Xiao-Lin Wei a,b,n, Li-Chun Xu b, Yuan-Ping Chen a, Li-Min Liu b a b
Laboratory for Quantum Engineering and Micro-Nano Energy Technology, Department of Physics, Xiangtan University, Xiangtan, Hunan 411105, China Laboratory for Green Energy, Beijing Computational Science Research Center, Beijing 100084, China
a r t i c l e i n f o
abstract
Article history: Received 22 September 2012 Received in revised form 22 October 2012 Accepted 28 October 2012 by E.G. Wang Available online 5 November 2012
Particle swarm optimization technique was applied to a structural search, yielding a high-pressure phase of BaCrO4, i.e. orthorhombic P212121 phase (stable 4 16 GPa). The P212121 structure consists of [CrO4] tetrahedra and ionized Ba, and can be viewed as a strong distortion of the low-pressure Pnma structure. Analysis of X-ray diffraction spectrum suggested that P212121 structure might be the experimentally observed high-pressure phase at pressures above 9 GPa, which was previously identified as monoclinic P21/m structure. Moreover, the calculated electronic structure suggests that P212121 structure is a semi-conductor with a band gap of 2.63 eV. Crown Copyright & 2012 Published by Elsevier Ltd. All rights reserved.
Keywords: C. Crystal structure and symmetry D. Phase transitions
1. Introduction Barium chromate (BaCrO4) has been extensively studied by experiments over the past decades due to its potential applications as oxidizing agent and photocatalyst [1–3]. It has also attracted great theoretical interest aimed at understanding the physics of its electronic band gaps and photocatalytic behavior [4]. Barium chromate is a naturally occurring chromate analog of barite (BaSO4) [5], which crystallizes in an orthorhombic structure (space group Pnma, no. 62) at ambient conditions. There is a substantial interest to explore the possible metastable high pressure polymorphs with novel structures and properties that may be quenched to ambient conditions for practical applications. Recently, Huang et al. [6] studied BaCrO4 to 25 GPa in a diamond-anvil cell (DAC) by x-ray diffraction and Raman spectroscopy methods. These spectra reveal that a transformation is abrupt near 9 GPa and the orthorhombic Pnma structure transforms to a novel high-pressure (HP) phase. This HP phase was suggested as a monoclinic P21/m structure, different from other high-pressure forms of ABO4-type compounds. Most recently, Santamarı´a-Pe´rez et al. [7] measured the x-ray diffraction data of BaSO4 in a DAC up to a pressure of 48 GPa, shown that barite transforms to a orthorhombic P212121 structure at pressures that range from 15 to 27 GPa, depending on the pressure media used, and further suggested that the previously observed HP phase of BaCrO4 should be crystallized in the same P212121 structure.
n Corresponding author at: Laboratory for Quantum Engineering and Micro-Nano Energy Technology, Department of Physics, Xiangtan University, Xiangtan, Hunan 411105, China. Tel./fax: þ 86 731 58292063. E-mail address: [email protected] (X.-L. Wei).
Since the x-ray diffraction patterns of the HP phase of BaCrO4 is incomplete and noisy at high pressures, rendering experimental structure solution difficult, there is a possibility that the HP phase of BaCrO4 adopts the P212121 structure instead of the P21/m structure. Therefore, further experimental or theoretical works on the crystal structure of the HP phase should be of interest. Here, using a particle swarm optimization (PSO) algorithm for crystal structure prediction with the only input information of chemical composition [8], we found that BaCrO4 adopts the orthorhombic P212121 structure at high-pressures. The well agreement between the simulated and the experimental x-ray diffraction patterns suggests that the P212121 structure is the experimentally observed HP phase. Details of the predicted structures, simulated x-ray diffraction patterns, and electronic properties in comparison with previously experimental and theoretical results will be made in the ensuing paragraphs.
2. Methods The variable-cell high-pressure structure predictions were performed in the pressure range 5–50 GPa with four formula units (f.u.) in the simulation cell via the PSO technique as implemented in Crystal structure AnaLYsis by Particle Swarm Optimization (CALYPSO) code [8–10]. This methodology has been successful in correctly predicting crystal structures for various systems including elements and compounds at high pressure [11–16]. The underlying ab initio structure relaxations were performed using Density Function Theory (DFT) within the Generalized Gradient Approximation (GGA) [17], as implemented in the VASP code [18]. The electron and core interactions were included by using the frozen-core all-electron projector augmented wave (PAW) method [19,20], with Ba:5s25p66s2,
0038-1098/$ - see front matter Crown Copyright & 2012 Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ssc.2012.10.039
46
X.-L. Wei et al. / Solid State Communications 155 (2013) 45–48
Fig. 1. (Color online) Crystal structures of (a) the ambient-pressure Pnma and (b) the high-pressure P212121 structure in polyhedral view. The tetrahedra and balls represent [CrO4] units and Ba atoms, respectively.
9.5
Table 1 Lattice parameters of Pnma and P212121 structure at 0 and 11 GPa, respectively. The atomic positions of the P212121 structure within GGA at 11 GPa.
P212121 Ba Cr O1 O2 O3 O4 a
˚ b or y (A)
˚ c or z (A)
Expt.a GGA LDA
9.106 9.218 8.974
5.539 5.642 5.392
7.335 7.435 7.192
GGA LDA 4a 4a 4a 4a 4a 4a
7.288 7.063 0.133 0.119 0.948 0.299 0.932 0.168
6.373 6.258 0.293 0.237 0.169 0.300 0.944 0.039
6.846 6.685 0.080 0.577 0.723 0.710 0.057 0.428
Ref. [6]
a
8.5
Lattice Parameters (Å)
Pnma
˚ a or x (A)
9.0
Obs. Huang et al. Cal. GGA Cal. LDA
8.0 7.5
a' c'
c
7.0 6.5
b'
6.0 5.5
b
5.0 4.5 0
2
4
6
8
10
12
14
16
Pressures (GPa) Cr:3s13d5 and O:2s22p4 treated as the valence electrons. The energy cutoff 520 eV and appropriate Monkhorst–Pack k meshes[21] were chosen to ensure that enthalpy calculations are well converged to 1 meV/atom.
3. Results In order to verify our calculations, the phase stability and structural properties of the well-studied Pnma phases (Fig. 1a) is considered in some detail. The Pnma structure can be described in terms of its cation subarray BaCr, which is of FeB-type. This structure consists of triangular prisms of Ba atoms that share faces along the b direction and corners in the other two directions, with the [CrO4] groups inserted into these metal prisms. The Pnma structure was successfully reproduced by the ab initio structure searching via CALYPSO with four f.u.’s in a simulating cell at 5 GPa. The calculated structural parameters along with the previous experimental data are summarized in Table 1. The well agreement between the theoretical and experimental data exhibits sufficient validity and reliability of our simulations. The orthorhombic P212121 structure (Z¼ 4) was derived from a simulation with four f.u.’s in the unit cell at 50 GPa by the CALYPSO methodology. As shown in Fig. 1b, the P212121 structure is basically a strong distortion of the initial Pnma structure (see Fig. 1a). As the Pnma structure transforms to the P212121 structure, the environment of the Cr atom almost does not change,
Fig. 2. (Color online) Evolution of the lattice parameters with pressures. Lattice constants of the Pnma and P212121 structure are shown by a, b, c and a0 , b0 , c0 , respectively. The experimental data from Ref. [6] are presented by the black circles.
however the tilting movement of the [CrO4] tetrahedra result in a change in the environment of the Ba atoms, and triangular prisms of the Ba atoms do not exist anymore. The evolution of the lattice parameters as a function of pressures is shown in Fig. 2, which reveals that the a axis contracts approximately 19.5%, the b axis expands approximately 22%, and the c axis remains nearly constant at the experimental transition pressure of 9 GPa. In view of that GGA often overestimate the lattice parameters, but the LDA underestimated them, the simulated lattice parameters of the Pnma structures are in well agreement with the experimental data at high-pressures. A volume collapse of approximate 5% at the experimental transition pressure of 9 GPa is an indicative of a first-order phase transition, which is in well accordance with the experimental observations. The enthalpy curves of the predicted P212121 structure relative to the ambient-pressure Pnma structure is shown in Fig. 3. The crossing between the two curves indicates a pressure-induced phase transition. Clearly, the Pnma structure transforms to the P212121 structure at approximate 16 GPa. It should be pointed out that the predicted transition pressure is a bit higher than the experimental observation of 9 GPa. However, in view of that the transition of BaCrO4 is not complete at 18 GPa in the experiment,
X.-L. Wei et al. / Solid State Communications 155 (2013) 45–48
0.3 Energy (eV/f.u.)
-48.0
-49.0 -49.5 -50.0
0.1
65
70
75
80
85
90
95
3
Volume A /f.u.
0.0
P 212121
Pnma -0.1 0
5
10
15
20
25
Pressures (GPa) Fig. 3. (Color online) Enthalpies of the P212121 structure with respect to the Pnma structure as a function of pressure. The inset illustrates the change of total energy for both phases.
*
8
Cal. P 212121 Cal. P 212121 Obs. Ref. 6 Cal. Pnma
Intensity (arb. units)
7 6 5
*
*
4 3 2
*
1 *
0 4
in the simulated x-ray diffraction patterns. In the angle range of 41–141 the strongest peak of the P212121 structures locates at 7.41, as indicated by the big black star in Fig. 4, which is in excellent agreement with the experimental observations. Experimentally [6], the x-ray features changed and a number of peaks disappeared at pressures above 18 GPa quite before the Pnma structure completely transforms to the P212121 structure. Thus an x-ray diffraction pattern of pure HP phase of BaCrO4 is yet unavailable, and we cannot identify the crystal structure of the HP phase directly by a complete comparison of the theoretical and experimental x-ray diffraction patterns. The incomplete x-ray diffraction spectrum is very likely to be the reason that the HP phase was previously indexed to the P21/m structure, and the corresponding atomic positions were not solved. In order to quantitatively describe the electronic and bonding behavior of the BaCrO4 at high-pressures, total and partial density of states (DOS) of the P212121 structure at 11 GPa in comparison with that of the Pnma structure at 0 GPa were calculated and shown in Fig. 5. The simulated band gaps of the Pnma structure is 2.76 eV, in general agreement with the experimental observation (2.64 eV), whereas the band gap of P212121 structure is predicted to be 2.63 eV, indicating that BaCrO4 is still a semiconductor at 11 GPa. A small reduction of 5% in the band gap was found with the pressure increasing from 0 to 11 GPa, which is mainly attributed to the slight broadening of the valance band centered at approximate 2 eV. As shown by the partial DOS, the Ba-6s and Cr-4s orbital are completely empty, revealing charge
*
6
8
10
12
14
DOS states/eV/BaCrO 4
H-H Pnma (eV/f.u.)
0.2
-48.5
2θ (degree)
18 12 6 0 12 8 4 0
Ba 5p
Cr
8 4 0 8 0
Total
Pnma
3d
O 2p
4
Fig. 4. (Color online) The theoretical and experimental x-ray diffraction profiles for the HP phase of BaCrO4 at 10.3 GPa. Vertical markers indicate Bragg reflections of the predicted orthorhombic P212121 structure (above) and the initial orthorhombic Pnma structure (below).
-10
-8
-6
-4
-2
0
2
4
2
4
Energy eV
DOS states/eV/BaCrO 4
the simulated result is quite acceptable. Except the Pnma and P212121 structures, several other structures (e.g. P213, Pbcn, Pna21, P4/nmb and P21/c structure) with competitive enthalpies were also predicted at high-pressures, but all of them were finally found to be energetically unstable and therefore the enthalpies of them were not shown in Fig. 3. The previously proposed P21/m structure was not found in our ab initio structural searching by using the global optimization method for crystal structure prediction. Since the atomic positions of the P21/m structure is unknown, the enthalpies of which were not calculated and compared to the P212121 structure. Thus the P21/m structure could not be theoretically excluded from the phase transition sequence of BaCrO4. However, the current result strongly hints that the P21/m structure is not an energetically favorable structure of BaCrO4 at high-pressures. The simulated x-ray diffraction patterns of the P212121 and Pnma structures in comparison with the experimental spectrum [6] at 10.3 GPa are shown in Fig. 4. The experimentally observed new peaks marked by the red stars have been successfully found
47
18 12 6 0 12 8 4 0
Ba 5p
Cr
8 4 0 8
3d
O 2p
4 0
Total
P 212121
-10
-8
-6
-4
-2
0
Energy eV Fig. 5. (Color online) Total and partial DOS of the Pnma structure at 0 GPa and P212121 structure at 11 GPa. The vertical dashed lines denote the Fermi level.
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X.-L. Wei et al. / Solid State Communications 155 (2013) 45–48
transformation from the metal atoms to O atoms. The d states of Cr and the p states of O lie in the energy range between 4.5 to 2.9 eV and 4.5 to 0 eV, respectively. Their hybridation causes bonding in the [CrO4] tetrahedra. This is very similar to what happens in typical ABO4 compounds, e.g. SrMoO4 [22], in which the partial covalent bonding is formed mainly because of the d–p hybridation.
4. Concluding remarks We predicted one high-pressure polymorphs of BaCrO4 with P212121 structure stable above 16 GPa. The well agreement between the experimental and theoretical x-ray diffraction pattern indicates that the P212121 structure may be the previously observed HP phase of BaCrO4. The total DOS of the P212121 structure reveals that BaCrO4 is a semiconductor with band gap of 2.63 eV at 11 GPa, whereas the partial DOS suggests the existence of an ionic interaction between Ba and [CrO4] unit and a partial covalent bonding between Cr and O atoms.
Acknowledgments This work was supported by the Open Fund based on innovation platform of Hunan colleges and universities (No. 12K045), and Hunan Science and Technology Bureau planned project (No. 2010SK3166).
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