  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.
