Structural and vibrational properties of GdTaO₄ under compression: An insight from experiment and first principles simulations

Polycrystalline GdTaO4, synthesized by solid state reaction route at 1300°C, adopts an M′ fergusonite crystal structure (space group P2/ c) with GdO8 and TaO6 as constituent units. The compression behavior of the compound has been investigated in a diamond anvil cell by powder x-ray diffraction and Raman spectroscopic techniques. Both the techniques indicate pressure driven first order isosymmetric phase transition in the compound around 19 GPa. X-ray diffraction data show nearly 6% volume discontinuity at the phase transition and a change in oxygen coordination around the Ta atom from six in the ambient phase to eight in a high pressure phase. Experimental data collected in the process of decompression confirm the reversible nature of phase transition. Bulk modulus obtained by fitting the pressure–volume data to the 3rd-order Birch–Murnaghan equation of state shows a higher value of bulk modulus for the high pressure phase compared to the low pressure phase, which is consistent with increased density due to volume collapse at the phase transition. The pressure dependence of unit cell parameters and Raman active modes along with Grüneisen parameters are also reported. Density functional theory based first principles simulations performed on compound corroborate the experimental findings. In low pressure phase, the simulated volumes of the constituent polyhedra under pressure indicate that the major contribution in the bulk modulus comes from lower valence rare earth polyhedra; however, for a high pressure phase, both the polyhedra units (GdO8 and TaO8) have almost similar contribution to the bulk modulus of the compound.