Morphotropic phase boundary (MPB) compositions separating rhombohedral and tetragonal phases in the (1-x-y)Pb(Mg 1/3 Ta 2/3)O 3-yPbZrO 3-xPbTiO 3 (PMT-PZ-PT100x) ternary solid solution system were characterized using X-ray diffraction and dielectric, piezoelectric properties. This work focused on compositions with a PZ content fixed at y=0.2, with an MPB composition found to be located at x=0.4. Piezoelectric coefficients and dielectric permittivity were found to be on the order of d 33=580 pC/N and 4100, respectively. Acceptor modification using manganese was found to induce a "hardening" effect in 0.4PMT-0.2PZ-0.4PT, with decreased piezoelectric coefficients d 33 and dielectric loss and increased mechanical quality factor Q. Piezoelectric coefficients d 33, Q values, and dielectric loss were found to be 500 pC/N, 2000, and 0.4%, respectively, for 0.4PMT-0.2PZ-0.4PT with MnO 2 dopant levels around 0.5 wt%. The figure of merit (product of Q and d 33) was found to be on the order of 1 �� 10 6, significantly higher when compared with other hard piezoelectric PZT materials. Specifically, the PMT-PZ-PT materials may be attractive candidates for high-power ultrasonic applications, particularly fine-scale components that require relating high permittivities. �� 2008 The American Ceramic Society.