High-Pressure Polymorphs of LaHO with Anion Coordination Reversal

Abstract

Two novel high-pressure polymorphs of lanthanum oxyhydride have been successfully predicted and stabilized under pressure. When reacted at 3 GPa, the fluorite structure of LaHO with anion-centered tetrahedral (HLa4/OLa4) geometry is transformed to the PbCl2-type structure involving coordination number increase of Hto five (HLa5 square pyramids). Upon further application of pressure to 5 GPa, LaHO changed into the anti Fe2P-type structure. Interestingly, the 5 GPa phase contains tetrahedral HLa4 and square-pyramidal OLa5 geometry, meaning coordination switching, as confirmed by ab initio calculations. The structural analysis shows that this unprecedented phenomenon is enabled by higher compressibility of hydride anion and emphasizes its potential in the search for new high-pressure forms of hydride-based materials.

Structure Model for Anion-Disordered Photochromic Gadolinium Oxyhydride Thin Films

“…In agreement with our previous studies, the diffraction patterns of the remaining films show fcc symmetry, best matched by the Fm 3̅ m pattern of GdH 2 (#00-50-1107). , Due to the limited number of reflections and their large FWHM it is not possible to exclude the small degree of tetragonal distortion which was reported for the similar anion-ordered LnOH ( ) and LnO 0.5 H 2 ( ) oxyhydrides. , …”

“…) and LnO 0.5 H 2 (c a / 2 0.999 = ) oxyhydrides. 10,32 The rocking curve FWHM is a relative measure of the degree of out-of-plane preferential orientation of thin films. According to the rocking curves shown in the insets of Figure 2, we find that the Gd metal film has a preferential out-of-plane orientation along the hcp c-axis.…”

Alkali-Rich Antiperovskite M3FCh (M = Li, Na; Ch = S, Se, Te): The Role of Anions in Phase Stability and Ionic Transport

“…For example, imaginary phonon modes for Li 3 FS disappear at 20 GPa, which is higher than the pressure of 5 GPa to stabilize the cubic Li 3 HS. These tendencies can be understood in terms of the high compressibility of H – , as revealed in LnHO (Ln = lanthanide) , and SrVO 2 H . In fact, the computed volumes of H – decrease more than those of F – and Ch 2– ions with increasing pressure (Figure S1, Tables S1 and S2).…”

“…For example, imaginary phonon modes for Li 3 FS disappear at 20 GPa, which is higher than the pressure of 5 GPa to stabilize the cubic Li 3 HS. These tendencies can be understood in terms of the high compressibility of H − , as revealed in LnHO (Ln = lanthanide) 25,26 and SrVO 2 H. 27 In fact, the computed volumes of H − decrease more than those of F − and Ch 2− ions with increasing pressure (Figure S1, Tables S1 and S2). Note that Kim et al 39 showed that M 3 FS and M 3 FSe are energetically (not dynamically) unstable based on the decomposition energies using ab initio calculations at ambient pressure.…”

“…From the structural perspective, the ideal cubic structure (Figure a) is unconventionally stable over a wide range of t ; only when t ≤ 0.85 does the orthorhombic Na 3 HS undergo octahedral distortions (Figure b). The size-flexibility of hydride ions at the B -site, as seen in some oxyhydrides, − is attributed as stabilizing the cubic phase, but at the same time the chalcogen anions at the A -site are polarizable. In addition, although the soft rotational phonon mode has been identified, the general link between the lattice dynamics and ionic transport has not been quantified.…”