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Soft Modes and Local Structural Transitions in Pb-free Ba(Ti0.8Zr0.2)O3-x(Ba0.7Ca0.3)TiO3 (x = 0.5): Pressure- and Temperature-dependent Raman Studies

We report our Raman studies of a new lead-free relaxor ferroelectrics, Ba0.85Ca0.15Ti0.9Zr0.1O3 (BCTZO). The Raman modes of BCTZO are compared with those of BaTi0.8Zr0.2O3 (BTZO), and BaTiO3 (BTO). Also, they are compared with the eigenmodes of BTO calculated by using an ab-initio quantum-mechanical frozen-phonon method. The sharp mode at 321 cm-1 of BTO, reported as a coupled mode showing an interference effect, becomes progressively broader in BTZO and BCTZO. This behavior, together with a broadening of the 527-cm-1 mode, suggests that the mode-coupling is weakened in BTZO and BCTZO. The structural transitions of BCTZO were investigated as functions of pressure at pressures below 20 GPa and of temperature at temperatures below 600 K. Three characteristic pressure-induced transitions, on each at 2.5, 5.0, and 13.0 GPa, were found. The transitions are suggested by the drastic changes in phonon modes (two softening modes, one each at ~ 300 and ~ 530 cm-1) and by the transformation of the intensity profile. A temperature-induced transition was found at a Curie temperature of ~ 380 K, where the average structure changes from tetragonal to cubic. It is accompanied by a softening mode at ~ 530 cm-1. The phonon spectrum of BCTZO suggests that its local environment is close to that of BTZO. However, the characteristic pressures of BCTZO are close to those of BTO. The sequence of pressure-induced transitions in both BCTZO and BTZO illustrate rich interplay between the long-range averaged structure and the short-range local order such that four distinguishable phases are suggested: tetragonal, locally ordered but compensated cubic, disordered cubic, and ideal cubic. We found that the critical pressures are plausibly related to the average crystal lattice.