The aim of the proposed experiment is to determine the temperature dependence of the coercive stress, i.e. the critical stress required to induce ferroelastic domain switching, in rhombohedral PZT (Lead Zirconate Titanate) ceramics. This characteristic property of ferroelectric ceramics is particularly relevant to the use of such materials in piezoelectric transducers, where a static compressive stress is often applied in order to avoid failure in service by fracture or long-term fatigue. Neutron diffraction patterns will be measured on ENGIN-X under various combinations of applied compressive stress and temperature. The change in coercive stress as a function of temperature will be correlated with the change in the rhombohedral angle alpha, also determined from the diffraction patterns, and compared with a potential model for the domain switching mechanism based on twinning deformation.