Abstract We report high-pressure neutron diffraction, and Raman spectroscopy data from the α- (Pnma) to β- (P63/mmc) phase transition in BaD2. 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 α-phase at 300 K and the β-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 D positions in the high-pressure phase suggest that the Wyckoff 4f-model (where one of the D 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.

中文翻译：

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BaD2 高压下α-β-相变的中子衍射研究

摘要 我们报告了 BaD2 中从 α- (Pnma) 到 β- (P63/mmc) 相变的高压中子衍射和拉曼光谱数据。在压力增加时在 2.53(5) GPa 处观察到转变，在压力减小时观察到 1.65(5) GPa，具有相对较窄的 0.20(5) GPa 相共存区域。我们报告了 300 K 下 α 相和 300 K 和 480 K 下 β 相的等温状态方程，并计算了跃迁过程中的熵变。拉曼数据表明，低能 Ba 振动会随着施加的压力而软化；DFT 计算表明，这主要是由于一阶跃迁之前 Ba 原子的不稳定性。高压相中 D 位置的变化表明 Wyckoff 4f 模型（其中一个 D 原子在两个位置之间分裂）在较低压力下更适合，但该模型倾向于 Wyckoff 2d 模型（其中 D1 位于单个位置）压力增加。这些结果用于讨论对所报告的过渡期间离子电导率增加的影响。