Article pubs.acs.org/JPCC
Zero-Point Effects on Phase Transitions of Thorium Dihydride under High Pressure Chao Zhang,*,† Shu-Ping Guo,† Hong Jiang,† Guo-Hua Zhong,‡ and Yue-Hua Su† †
Department of Physics, Yantai University, Yantai 264005, China Center for Photovoltaics and Solar Energy, Shenzhen Institutes of Advanced Technology, Chinese Academy of Sciences and The Chinese University of Hong Kong, Shenzhen 518055, China
‡
S Supporting Information *
ABSTRACT: The crystalline structures of thorium dihydride, ThH2, under pressure are investigated by using an unbiased structure searching method coupled with ab initio calculations. Three low-enthalpy phases are found as the thermodynamic ground states of ThH2 up to 200 GPa, including an experimentally observed I4/mmm phase and two newly predicted monoclinic phases (C2/m and C/2c phase). ThH2 is predicted to undergo I4/mmm → C2/m → C/2c phase transitions without zero-point (ZP) effects, whereas it directly transforms from the I4/mmm phase to the C2/c phase with ZP effects. Phonon calculations show that these competitive phases are thermodynamically stable. There is a strengthening of the metallic characters of the chemical bonding with increased pressure. Our results highlight the role of ZP effects in the high-pressure behaviors of metal hydrides and provide insight into further studies of other compounds containing light elements under pressure.
1. INTRODUCTION Exploration of metal hydrides at extreme conditions is a central theme in physics, chemistry, and allied sciences. Under high pressure, most metal hydrides undergo phase transition and transform into new structures of higher densities and novel chemical bonding. Several of these new structures are metallic and even superconducting, despite the fact that some metal hydrides are insulator with large band gap at ambient pressure, such as alkali and alkaline earth hydrides,1−8 transition metal hydrides,9−17 and group 14 hydrides.18−25 Understanding the behavior of metal hydrides under high pressure is significant to applied research areas for providing guidance on designing improved hydrogen storage materials for transportation applications.26−28 Given the extremely light mass of the hydrogen atom, the zero-point (ZP) effect is adequate enough to affect relative stabilities of structures and vibrational properties of hydrogen and hydrides, especially with increased pressure that increases the vibrational energy of hydrogen atoms. The inclusion of ZP effects leads to a complete revision of solid hydrogen phase diagram.29 Without ZP effects, the most stable phases are P63/ m ( 1). Phys. Rev. Lett. 2011, 106, 237002. (6) Zhou, D.; Jin, X.; Meng, X.; Bao, G.; Ma, Y.; Liu, B.; Cui, T. Ab Initio Study Revealing a Layered Structure in Hydrogen-Rich KH6 under High Pressure. Phys. Rev. B 2012, 86, 014118. (7) Wang, H.; Tse, J. S.; Tanaka, K.; Iitaka, T.; Ma, Y. Superconductive Sodalite-Like Clathrate Calcium Hydride at High Pressures. Proc. Natl. Acad. Sci. U. S. A. 2012, 109, 6463−6466. (8) Wang, Z.; Yao, Y.; Zhu, L.; Liu, H.; Iitaka, T.; Wang, H.; Ma, Y. Metallization and Superconductivity of BeH2 under High Pressure. J. Chem. Phys. 2014, 140, 124707. (9) Kim, D. Y.; Scheicher, R. H.; Ahuja, R. Predicted HighTemperature Superconducting State in the Hydrogen-Dense Transition-Metal Hydride YH3 at 40 K and 17.7 GPa. Phys. Rev. Lett. 2009, 103, 077002. (10) Kim, D. Y.; Scheicher, R. H.; Mao, H. K.; Kang, T. W.; Ahuja, R. General Trend for Pressurized Superconducting Hydrogen-Dense Materials. Proc. Natl. Acad. Sci. U. S. A. 2010, 107, 2793−2796. (11) Kim, D. Y.; Scheicher, R. H.; Pickard, C. J.; Needs, R. J.; Ahuja, R. Predicted Formation of Superconducting Platinum-Hydride Crystals under Pressure in the Presence of Molecular Hydrogen. Phys. Rev. Lett. 2011, 107, 117002. (12) Gao, G.; Wang, H.; Zhu, L.; Ma, Y. Pressure-Induced Formation of Noble Metal Hydrides. J. Phys. Chem. C 2011, 116, 1995−2000. (13) Zhou, X.-F.; Oganov, A. R.; Dong, X.; Zhang, L.; Tian, Y.; Wang, H.-T. Superconducting High-Pressure Phase of Platinum Hydride from First Principles. Phys. Rev. B 2011, 84, 054543. (14) Zhang, C.; Chen, X. J.; Lin, H. Q. Phase Transitions and Electron−Phonon Coupling in Platinum Hydride. J. Phys.: Condens. Matter 2012, 24, 035701. (15) Gao, G.; Bergara, A.; Liu, G.; Ma, Y. Pressure Induced Phase Transitions in TiH2. J. Appl. Phys. 2013, 113, 103512. (16) Gao, G.; Hoffmann, R.; Ashcroft, N. W.; Liu, H.; Bergara, A.; Ma, Y. Theoretical Study of the Ground-State Structures and Properties of Niobium Hydrides under Pressure. Phys. Rev. B 2013, 88, 184104. (17) Ye, X.; Hoffmann, R.; Ashcroft, N. W. Theoretical Study of Phase Separation of Scandium Hydrides under High Pressure. J. Phys. Chem. C 2015, 119, 5614−5625. (18) Tse, J. S.; Yao, Y.; Tanaka, K. Novel Superconductivity in Metallic SnH4 under High Pressure. Phys. Rev. Lett. 2007, 98, 117004. (19) Chen, X. J.; Struzhkin, V. V.; Song, Y.; Goncharov, A. F.; Ahart, M.; Liu, Z. X.; Mao, H. K.; Hemley, R. J. Pressure-Induced Metallization of Silane. Proc. Natl. Acad. Sci. U. S. A. 2008, 105, 20−23.
mmm ThH2 under ambient pressure, the H atom gains approximately 0.733 electrons while the Th atom loses about 1.466 electrons, showing ionic character. This finding is in excellent agreement with the previous calculations.37 When compressed, the Th atom loses fewer electrons, 1.229 electrons at 30 GPa, and the H atom gains fewer electrons. In other words, some electrons resided at the H atom transfer back to the Th atom under the applied pressure. For the C2/c ThH2 at 80 GPa, the effective valency of the Th and H atoms can be represented as +0.952 and −0.476, respectively. This results indicates that the pressure-induced charge transfer accounts for the structural phase transition, which is consistent with the results of electronic DOS. In addition, the VB of both the Th and H atoms continually decreases with increased pressure. At 170 GPa, the calculated VB of Th and H atoms are 14.176 and 3.410 Å 3 in the C2/c phase, respectively, which are approximately 67% and 51% that in the I4/mmm phase at 30 GPa.
4. CONCLUSIONS The ground-state phases of TiH2 under pressure have been systemically explored by using an unbiased structure searching method coupled with ab initio calculations. Without considering ZP energy, the tetragonal I4/mmm phase is predicted to transform, via a monoclinic C2/m phase, to another monoclinic C2/c phase. These phase transitions successively take place at 60 and 166 GPa. When inclusion of ZP energy, both the phase transition pressure and sequence completely change. ThH2 is predicted to directly transform from the I4/mmm phase to the C2/c phase at 52 GPa. This is first observed in binary metal hydrides. The I4/mmm → C2/c phase transition is of the first order. Phonon calculations show that the competitive phases are thermodynamically stable due to the absence of any imaginary frequencies in their favored pressure ranges. The pressure-induced charge transfer drives the structural transition. Our results demonstrate the role of ZP effects in phase transition of metal hydrides under pressure and have great implications for compounds containing light elements.
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Table 2. Calculated Effective Atomic Charge and Volumes According to Bader Partitioning of ThH2 phase
Article
ASSOCIATED CONTENT
S Supporting Information *
Coordination number of H and Th and bond lengths for competitive phases. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.5b03195. 13470
DOI: 10.1021/acs.jpcc.5b03195 J. Phys. Chem. C 2015, 119, 13465−13471
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DOI: 10.1021/acs.jpcc.5b03195 J. Phys. Chem. C 2015, 119, 13465−13471
