Neutron diffraction study of the α- to β-phase transition in BaD2 under high pressure

Abstract

We report high-pressure neutron diffraction, and Raman spectroscopy data from the -(Pnma) to -(P6 3 /mmc) phase transition in BaD 2 . The transition was observed at 2.53(5) GPa on pressure increase, and 1.65(5) GPa on pressure decrease, with a relatively narrow 0.20(5) GPa region of phase coexistence. We report the isothermal equations of state for the -phase at 300 K and the -phase at both 300 K and 480 K, and have calculated the entropy change through the transition. The Raman data show that a low energy Ba-vibration is seen to soften with applied pressure; DFT calculations suggest that this is predominantly due to an instability in the Ba atoms prior to the first-order transition. The shift in deuterium positions in the high-pressure phase suggest that the Wyckoff 4f-model (where one of the deuterium atoms is split between two sites) is a better fit at lower pressures, but that the model tends towards the Wyckoff 2d-model (where D1 is localised on a single site) with increased pressure. These results are used to discuss implications on the reported increases in ionic-conductivity over the transition.

Geometric Frustration and Concerted Migration in the Superionic Conductor Barium Hydride

“…Note that the 4f split site has approximately half the ADP as the 2d. Verbraeken et al and Ridley et al both found that the D1 site showed a relatively large ADP in the HT phase. , Due to the covariance of U , occupancy, and coordinates, it is possible to find different ADPs. Specifically, Verbraeken et al.…”

“…24 In the high pressure Ni 2 In form, the cations have been suggested to occupy the special 2c, 2a, and 2d sites by analogy with BaF 2 . 21 Previous studies at high temperature 1 or high pressure 24 on BaD 2 have shown that the D1 atom can be placed on either the 2d or 4f split site. 1,24 We also refined the NPD data using both the 2d and 4f models and found that both returned satisfactory fits.…”

“…21 Previous studies at high temperature 1 or high pressure 24 on BaD 2 have shown that the D1 atom can be placed on either the 2d or 4f split site. 1,24 We also refined the NPD data using both the 2d and 4f models and found that both returned satisfactory fits. We note here that full refinement of the occupancies, atomic displacement parameters (ADPs, U), and coordinates created unstable fits due to the strong covariances between the three parameters (Figure S2 shows the resulting occupancies from these refinements).…”

“…A similar transition from Cotunnite to Ni 2 In structure type is also observed in ambient temperature, high pressure studies of both BaH 2 and BaF2, 21 and indeed this pressure driven transition is also observed for the important hcp anion analogues Na 2 S 22 and Li 2 S. 23 Such transitions to higher symmetry phases under pressure have been previously reported and linked to differing compressibilities within the sublattices. 24 In the high pressure Ni 2 In form, the cations have been suggested to occupy the special 2c, 2a, and 2d sites by analogy with BaF 2 . 21 Previous studies at high temperature 1 or high pressure 24 on BaD 2 have shown that the D1 atom can be placed on either the 2d or 4f split site.…”

Temperature Dependent Local Atomic Structure and Vibrational Dynamics of Barium Hydride and Calcium Hydride

“…This is similar to the evolution of the atomic positions under high pressure, where the largest changes observed corresponded to the D(2) sites followed by the Ba/ Ca sites. 27,28 The high compressibility of the a-axis was attributed to the movement of the D(2) sites, while Raman spectroscopy showed that there was mode softening that was a result of instability in the Ba sites. Similar to the high pressure studies, these high temperature results show that modifications to the D(2) sites, and to a lesser degree the Ba sites, are main contributing factors that lead to the phase transition in BaD 2 .…”