Relaxor-PT based ferroelectric single crystals Pb(Zn₁/₃)Nb(₂/₃)O₃-PbTiO₃ (PZNT) and Pb(Mg₁/₃)Nb(₂/₃)O₃-PbTiO₃ (PMNT) offer high performance with ultra-high electromechanical coupling factors k₃₃ > 0.9 and piezoelectric coefficients d₃₃s > 1500 pC/N. However, the usage temperature range of these perovskite single crystals is limited by T(RT)-the rhombohedral to tetragonal phase transition temperature, which occurs at significantly lower temperatures than the Curie temperature T(C), a consequence of curved morphotropic phase boundaries (MPBs). Furthermore, these <001>-oriented crystals exhibit low mechanical quality Q and coercive fields, restricting their usage in high-power applications. In this survey, recent developments on binary and ternary perovskite relaxor-PT crystal systems are reviewed with respect to their temperature usage range. General trends of dielectric and piezoelectric properties of relaxor-PT crystal systems are discussed in relation to their respective T(C)/T(RT). In addition, two approaches have been implemented to improve mechanical Q, including acceptor dopants, analogous to hard polycrystalline ceramics, and anisotropic domain engineering, enabling low-loss crystals with high coupling for high-power applications.