Pressure-driven structural instability in TbNbO4: Experimental and theoretical study

In this research article, we report the pressure dependent structural and vibrational study on fergusonite type terbium orthoniobate (TbNbO4). The compound under compression was examined using synchrotron-based powder x-ray diffraction and Raman spectroscopic technique. Both the experimental methods indicate presence of pressure-induced phase transition near 19 GPa. Raman measurements indicate the emergence of a second-high pressure phase around 30 GPa. The pressure evolution of the lattice parameters shows anisotropic compression in the ambient phase with the strongest compressibility along the b-axis. Density functional theory based first principles calculations at different pressures predict a sequence of transformations from monoclinic (Space group: I2/a) to monoclinic (Space group: P2/c) at approximately 22 GPa and then to a tetragonal (Space group: P4/nbm) phase near 26 GPa. The possible mechanism resulting from anisotropic compression leading to linear arrangement of both the cations in these phase transitions is described in this manuscript. The bulk modulus of the low-pressure phase was determined from both experimental and theoretical data utilizing Birch-Murnaghan equation of state. The pressure dependence of Raman-active modes and their corresponding Gruneisen parameters are also discussed, providing insight into the vibrational response of TbNbO4 to compression.