The temperature behavior of various relaxor-PT piezoelectric single crystals was investigated. Owing to a strongly curved morphotropic phase boundary, the usage temperature of these perovskite single crystals is limited by TR-T--the rhombohedral to tetragonal phase transformation temperature--which occurs a significantly lower temperatures than the Curie temperature Tc. Attempts to modify the temperature usage range of Pb(Zn1/3Nb2/3)O3--PbTiO3 (PZNT) and Pb(Mg1/3Nb2/3)O3-PbTiO3 (PMNT) rhombohedral crystals (Tx to approximately 150-170 degrees C, TR-T to approximately 60-120 degrees C) using minor dopant modifications were limited, with little success. Of significant potential are crystals near the mor photropic phase boundary in the Pb(Yb1/2Nb1/2)O3-PbTiO3 (PYNT) system, with a Tc > 330 degrees C, even thoug TR-T was found to be only half that value at approximately 160 degrees C. Single crystals in the novel BiScO3-PbTiO3 system offer significantly higher Tcs > 400 degrees C, while exhibiting electromechanical coupling coefficients k33 > 90% being nearly constant till the TR-T temperature around 350 degrees C, which greatly increases the temperature range for transducer applications.