We characterized the temperature dependent ( ~ 25–200°C) electromechanical properties and crystal structure of Pb(In _1/2 Nb _1/2 )O ₃ -Pb(Mg _1/3 Nb _2/3 )O ₃ -PbTiO ₃ single crystals using in situ electrical measurement and neutron diffraction techniques. The results show that the poled crystal experiences an addition phase transition around 120°C whereas such a transition is absent in the unpoled crystal. It is also found that the polar order persists above the maximum dielectric permittivity temperature at which the crystal shows a well-defined antiferroelectric behavior. The changes in the electrical properties and underlying crystal structure are discussed in the paper.