The results of a collaborative study of the ferroelectric-ferroelectric transition in rhombohedral lead zirconate-titanate are described. Firstly, experimental evidence for the first-order nature of the transition is given together with an explanation of this in pure single crystals. Secondly, the use of bismuth-doped ceramic material in pyro-electric detection is discussed. By applying an electric field on the cooling cycle, thermal hysteresis can be reduced to a minimum.