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A phase-diagram for the Na0.5K0.5NbO3-LiTaO3 solid solution series (NKN-LT) is presented for compositions ≤10 mol% LT, based on the combined results of temperature-variable X-ray powder diffraction and dielectric measurements. In addition to the reported orthorhombic and tetragonal polymorphs of NKN-LT, a monoclinic phase is revealed. Changes to electrical properties as a function of LT substitution are correlated to phase content. Increasing the LT content from 5 to 7 mol% LT led to improved temperature stability of piezoelectric properties because of the avoidance of the monoclinic-tetragonal polymorphic phase transition during thermal cycling (at >;25°C). For 7 mol% LT samples: d33 = 200 pC/N; Tc = 440°C; εr = 550 and tan δ = 0.02 (at 20°C). Modification of this composition by solid solution with BiScO3 led to a decrease in d33 values. Transmission electron microscopy of a sample of 0.95[0.93 NKN-0.07LT]-0.05BiScO3 indicated a core-shell grain structure which led to temperature-stable dielectric properties.