Garg A B, Errandonea D, Rodríguez-Hernández P, Muñoz A
High Pressure and Synchrotron Radiation Physics Division, Bhabha Atomic Research Centre, Mumbai 400085, India.
Homi Bhabha National Institute, Anushaktinagar, Mumbai 400094, India.
J Phys Condens Matter. 2021 Apr 26;33(19). doi: 10.1088/1361-648X/abe478.
In this paper we perform a high-pressure (HP) study of fergusonite-type HoNbO. Powder x-ray diffraction experiments anddensity-functional theory (DFT) simulations provide evidence of a phase transition at 18.9(1.1) GPa from the monoclinic fergusonite-type structure (space group I2/a) to another monoclinic polymorph described by space group P2/c. The phase transition is reversible and the HP structural behavior is different than the one previously observed in related niobates. The HP phase remains stable up to 29 GPa. The observed transition involves a change in the Nb coordination number from 4 to 6, and it is driven by mechanical instabilities. We have determined the pressure dependence of unit-cell parameters of both phases and calculated their room-temperature equation of state. For the fergusonite-phase we have also obtained the isothermal compressibility tensor. In addition to the HP studies, we report ambient-pressure Raman and infrared (IR) spectroscopy measurements. We have been able to identify all the active modes of fergusonite-type HoNbO, which have been assigned based upon DFT calculations. These simulations also provide the elastic constants of the different structures and the pressure dependence of the Raman and IR modes of the two phases of HoNbO. According tocalculations, the reported phase transition is related to a mechanical instability and a phonon softening.
在本文中，我们对弗格森矿型HoNbO₄进行了高压（HP）研究。粉末X射线衍射实验和密度泛函理论（DFT）模拟提供了证据，表明在18.9(1.1) GPa时发生了从单斜弗格森矿型结构（空间群I2/a）到由空间群P2/c描述的另一种单斜多晶型的相变。该相变是可逆的，且高压下的结构行为与之前在相关铌酸盐中观察到的不同。高压相在高达29 GPa时仍保持稳定。观察到的相变涉及铌的配位数从4变为6，并且是由机械不稳定性驱动的。我们确定了两个相的晶胞参数对压力的依赖性，并计算了它们的室温状态方程。对于弗格森矿相，我们还获得了等温压缩率张量。除了高压研究外，我们还报告了常压拉曼光谱和红外（IR）光谱测量结果。我们已经能够识别弗格森矿型HoNbO₄的所有活性模式，这些模式是根据DFT计算确定的。这些模拟还提供了不同结构的弹性常数以及HoNbO₄两相的拉曼和红外模式对压力的依赖性。根据计算，所报道的相变与机械不稳定性和声子软化有关。
