Lattice dynamics across the ferroelastic phase transition in Ba₂ZnTeO₆: a Raman and first-principles study

Structural phase transitions drive several unconventional phenomena including some illustrious ferroic attributes which are relevant for technological advancements. On this note, we have investigated the ferroelastic structural transition of perovskite-type trigonal Ba₂ZnTeO₆ across T_c ∼ 150 K. With the help of Raman spectroscopy and density-functional theory (DFT)-based calculations, we report new intriguing observations associated with the phase transition in Ba₂ZnTeO₆. We observed the presence of a central peak (quasi-elastic Rayleigh profile), huge softening in the soft mode, hysteretic phonon behavior, and signatures of coexistent phases. The existence of a central peak in Ba₂ZnTeO₆ is manifested by a sharp rise in the intensity of the Rayleigh profile concomitant with the huge damping (or softening) of the soft mode (at ∼31 cm^-1) near T_c, shedding light on the lattice dynamics during the phase transition. This is further corroborated by our phonon calculations that show that the soft mode (E_g) in the high-symmetry structure involving TeO₆ octahedral rotation (with Ba and Zn translation) condenses into A_g and B_g modes in the C2/m low-symmetry phase. While most of the phonon bands split below T_c confirming the phase transition, we observed thermal hysteretic behavior of phonon modes, which signifies the first-order nature of the transition and the presence of coexisting phases as corroborated by our temperature-dependent X-ray diffraction and specific heat measurements.