  In piezoelectric ceramics the changes in the phase stabilities versus stress
and temperature in the vicinity of the phase boundary play a central role. The
present study was dedicated to the classical piezoelectric,
lead-zirconate-titanate (PZT) ceramic with composition
Pb(Zr$_{0.54}$Ti$_{0.46}$)O$_3$ at the Zr-rich side of the morphotropic phase
boundary at which both intrinsic and extrinsic contributions to
piezoelectricity are significant. The pressure-induced changes in this
two-phase (rhombohedral $R3c$+monoclinic $Cm$ at room temperature and
$R3c+P4mm$ above 1 GPa pressures) system were studied by high-pressure neutron
powder diffraction technique. The experiments show that applying pressure
favors the $R3c$ phase, whereas the $Cm$ phase transforms continuously to the
$P4mm$, which is favored at elevated temperatures due to the competing entropy
term. The $Cm\rightarrow R3c$ phase transformation is discontinuous. The
transformation contributes to the extrinsic piezoelectricity. An important
contribution to the intrinsic piezoelectricity was revealed: a large
displacement of the $B$ cations (Zr and Ti) with respect to the oxygen anions
is induced by pressure. Above 600 K a phase transition to a cubic phase took
place. Balance between the competing terms dictates the curvature of the phase
boundary. After high-pressure experiments the amount of rhombohedral phase was
larger than initially, suggesting that on the Zr-rich side of the phase
boundary the monoclinic phase is metastable.
