Mn-doped 0.15Pb(In 1/2 Nb 1/2 )O 3 –0.55Pb(Mg 1/3 Nb 2/3 )O 3 –0.30PbTiO 3 single crystals were grown by modified Bridgman technique. High dielectric peak temperature of 161°C, low dielectric loss of 0.03% and enhanced pyroelectric coefficient of 10.1×10 −4 C/m 2 K are observed. Effects of oxygen vacancies related to annealing atmosphere have been investigated to explain enhanced dielectric and pyroelectric properties. Detectivity figure of merit of 34.1×10 −5 Pa −1/2 is achieved for oxygen-annealed crystals, which is the highest value among reported PIMNT crystals. Two different linear relationships between the coercive field and frequency are observed to obey Merz’s law, which reveals the domain and polarization switching mechanism.