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, ThH 2 , 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 ThH 2 up to 200 GPa, including an experimentally observed I4/mmm phase and two newly predicted monoclinic phases (C2/m and C/2c phase). ThH 2 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 P6 3 / m (<105 GPa), C2/c (105-270 GPa), Cmca-12 (270-385 GPa), and Cmca (385-490 GPa), followed by I4 1 /amd. When ZP effects are included, C2/c becomes stable below 240 GPa and Cmca-12 above 240 GPa. However, whether ZP effects completely change the phase diagram in binary metal hydrides has not yet been determined. For SnH 4 , 24 a heavy metal hydride, inclusion of ZP effects does not change the phase transition sequence but extends the stability field of the Ama2 phase to be 96-180 GPa, compared with 108-158 GPa without ZP effects. In a light metal hydride, BeH 2 , 8 inclusion of ZP effects in the phase diagram slightly shifts the trans- formation pressures into the phase III and IV, whereas the phase transition sequence remains unaltered. Thorium hydride has potential use for advanced nuclear fuels and has been widely investigated experimentally and theoret- ically. 30-38 Unfortunately, the scarcity of knowledge on thorium hydride under high pressures hinders its practical applications. This is partially due to the fact that high pressure is not easily accessible and controllable in diamond anvil cell. Development of reliable theoretical methods simulating physical and chemical properties of thorium hydride would significantly help, especially in the field of the nuclear materials, for which adequate experimental data are missing. In this work, we conducted a thorough theoretical investigation on the high- pressure behavior of ThH 2 . The structural properties of ThH 2 under high pressure were explored using global structural searching scheme in combination of ab initio calculations. 2. COMPUTATIONAL METHODS The ground state of a material under pressure usually corresponds to the global minimum of the Gibbs free energy surface, and finding this state is essentially a minimization problem and may be solved by searching for structures with Received: April 2, 2015 Revised: May 21, 2015 Published: May 29, 2015 Article pubs.acs.org/JPCC © 2015 American Chemical Society 13465 DOI: 10.1021/acs.jpcc.5b03195 J. Phys. Chem. C 2015, 119, 13465-13471