  The structural phase diagram of the Pb-free ferroelectric
(Na1/2Bi1/2)1-xBaxTiO3 (NBT-BT), x<0.1, has been explored by Raman spectroscopy
at temperatures from 10 to 470 K. The data provide clear evidence for a
proposed temperature-independent morphotropic phase boundary at x \approx
0.055. However, there is no evidence for a structural phase transition across T
\approx 370 K for x > 0.055, where bulk-property anomalies appear to signal a
transition to a nonpolar or antiferroelectric phase. The results identify that
the phase above 370 K shows short-range ionic displacements that are identical
to those in the long-range-ordered phase below 370 K. These conclusions provide
a natural interpretation of the weak piezoelectric response in this system and
have important implications for the search for Pb-free piezoelectrics.
