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arXiv 2203.12261cond-mat.mtrl-sci
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高压下BaO的晶格不稳定性、非谐性和Raman光谱:第一性原理研究

Lattice instability, anharmonicity and Raman spectra of BaO under high pressure: A first principles study

K. Lavanya, N. Yedukondalu, S. C. Rakesh Roshan, Shweta D. Dabhi, Suresh Sripada, M. Sainath, Lars Ehm, John B. Parise

摘要:

碱土金属氧化物,特别是MgO和CaO在地球下地幔中占主导地位,因此,探索这类化合物的高压行为具有重要的地球物理研究意义。在所有这些化合物中,BaO在0- 1.5Mbar压力范围内表现出丰富的多晶型。静态焓计算表明,在5.1,19.5,120 GPa压力下,BaO分别经历了NaCl型(B1)$\rightarrow$ NiAs型(B8)$\rightarrow$畸变CsCl型(d-B2)$\rightarrow$ CsCl型(B2)的压致结构相变。发现B1 $\rightarrow$ B8 $\&$ B8 $\rightarrow$ d-B2跃迁本质上是一级跃迁,而d-B2 $\rightarrow$ B2是二级或弱一级相变。有趣的是,d-B2相在宽的压力范围内显示出稳定性,$\sim$ 19.5-113 GPa。力学和动力学的稳定性的环境和高压相证明通过计算的弹性常数和声子色散曲线,分别。在高压下,B8,d-B2和B2相分别观察到明显的声子软化和沿M方向的软声子模。压力依赖的拉曼光谱表明,在压力下的相变从d-B2拉曼非活性相。总之,本研究提供了一个全面的了解背后的压力引起的结构相变的BaO的机制。

英文摘要:

Alkaline-earth metal oxides, in particular MgO and CaO dominate Earths lower mantle, therefore, exploring high pressure behavior of this class of compounds is of significant geophysical research interest. Among all these compounds, BaO exhibits rich polymorphism in the pressure range of 0-1.5 Mbar. Static enthalpy calculations revealed that BaO undergoes a pressure induced structural phase transition from NaCl-type (B1) $\rightarrow$ NiAs-type (B8) $\rightarrow$ distorted CsCl-type (d-B2) $\rightarrow$ CsCl-type (B2) at 5.1, 19.5, 120 GPa respectively. B1 $\rightarrow$ B8 $\&$ B8 $\rightarrow$ d-B2 transitions are found to be first order in nature whereas d-B2$\rightarrow$ B2 is a second order or weak first order phase transition. Interestingly, d-B2 phase shows stability over a wide pressure range, $\sim$19.5-113 GPa. Mechanical and dynamical stabilities of ambient and high pressure phases are demonstrated through computed elastic constants and phonon dispersion curves, respectively. Under high pressure, significant phonon softening and soft phonon mode along M-direction are observed for B8, d-B2 and B2 phases, respectively. Pressure dependent Raman spectra suggest a phase transition from d-B2 to Raman inactive phase under pressure. Overall, the present study provides a comprehensive understanding of underlying mechanisms behind pressure-induced structural phase transitions in BaO.

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