Pressure induced phase transitions in TiH2

Abstract

Recent room temperature experiments on TiH 2 [Kalita et al., J. Appl. Phys. 108, 043511 (2010)], an important compound in hydrogen storage research, revealed a cubic (fcc, Fm-3m) to tetragonal (bct, I4/mmm) phase transition at around 0.6 GPa, which was suggested to remain stable up to at least 90 GPa. However, the simulated X-ray diffraction (XRD) pattern of the I4/mmm structure cannot explain all the diffraction peaks observed at 90 GPa. In this article, we apply the recently developed particle swarm optimization algorithm for crystal structure prediction to propose that at 63 GPa TiH 2 presents a further phase transition from I4/mmm to P4/nmm, which we have also confirmed is dynamically and enthalpically stable up to 294 GPa. Moreover, the XRD patterns and calculated lattice parameters of the P4/nmm structure are in good agreement with the experimental available data. Above 294 GPa, we predict a monoclinic P2 1 /m structure to be stable. V C 2013 American Institute of Physics. [http://dx

The Search for Superconductivity in High Pressure Hydrides

“…111 CSP techniques, on the other hand, predicted the following sequence of transitions: I4/mmm → P 4/nmm → P 2 1 /m at 63 and 294 GPa, respectively. 112 The computed XRD patterns of the P 4/nmm structure were found to be in better agreement with experimental results up to 90 GPa than those of I4/mmm. Figure 4: Two structures often adopted by metal hydrides: (a) the CaF 2 (fcc) structure, and (b) the I4/mmm structure.…”

First-principles study on the structural and electronic properties of metallic HfH2 under pressure

“…The lattice parameters and atomic positions of selected structures at favored pressures are summarized in Table SI . In comparison, we observed that in both HfH 2 and TiH 2 21 , the locations of metal and hydrogen atoms are similar for I 4/ mmm , P 4 /nmm and P 2 1 /m , respectively. In all above structures, the nearest H-H distances are 2.149 Å at 1 atm, much longer than that in the pure H 2 .…”

“…Surprisingly, we found that Cmma and P 4 /nmm are energetically nearly degenerated in the range of 0–300 GPa, and enthalpy difference is less than 1 meV/f.u.. If I 4/ mmm , P 4 /nmm and P 2 1 /m phases satisfied the stability conditions of mechanics and dynamics, the phase transition sequence of HfH 2 is similar to the TiH 2 21 . However, we found that the P 4 /nmm is not stable both in dynamical and mechanical properties (see the later discussions).…”

“…Not only does pressure provide a different route to the synthesis of new compounds, but also enables many known materials to exhibit novel phenomena that cannot be found at normal conditions. Lately, theoretical study revealed that TiH 2 at high pressure had a structural transformations I 4/ mmm → P 4/ nmm → P 2 1 / m and the calculated pressure of phase transition were 63 and 294 GPa, respectively 21 . The structures and properties of ZrH 2 were also investigated in experiment under high pressure 22 .…”

CALYPSO structure prediction method

“…如, Zhao等人 [46] 通过CALYPSO方法确定了实 验长期未确定的硅和锗的高压亚稳相结构; Li等人 [47] 利用CALYPSO方法确定了前人实验合成的潜在超硬 材料硼化钨化合物的结构: I4/m-4u W 2 B(-phase), I4 1 / amd-8u WB(-WB或-WB), P6 3 /mmc-4u WB 2 (-phase) 和P6 3 /mmc-4u WB 3 (WB 4 ). Gao等人 [48] 利用CALYPSO 方法确定了重要的储氢材料TiH 2 的高压相结构.…”