Pressure- and Temperature-Dependence of Polar Nanoregions in KTN40 (KTa0.6Nb0.4O3) and KNbO3

Abstract

We investigated the pressure- and temperature-dependent phase transitions in KTN₄₀ (KTa_0.6Nb_0.4O₃) and KNbO₃, two members of the perovskite-type ferroelectric solid solution system KTN (KTa_xNb_1–xO₃), by heat capacity measurements, Raman spectroscopy, and second harmonic generation (SHG). The phase transition temperatures for the rhombohedral → orthorhombic → tetragonal → cubic sequence in KTN₄₀ were determined to be 180(2) K, 225(2) K, and 295(2) K, respectively, by heat capacity measurements. For KNbO₃, SHG measurements revealed orthorhombic → tetragonal → cubic transitions at 460(3) K and 720(3) K. In both compounds, SHG experiments indicated the presence of polar nanoregions within the cubic phase above the Curie temperatures of KTN₄₀ and KNbO₃. The dynamics and stability fields of these polar nanoregions were analyzed, resulting in Burns temperatures T_d of 510(5) K for KTN₄₀ and 1000(5) K for KNbO₃. Intermediate temperatures, T*, where polar nanoregions begin to merge into larger domains, were identified for KTN₄₀ and KNbO₃ to be 350(5) K and 775(5) K, respectively. The pressure-dependent tetragonal → cubic phase transition was observed at 1.3(1) GPa for KTN₄₀ and 15(0.5) GPa for KNbO₃ using SHG. SHG measurements further confirmed polar nanoregions occurring in the cubic phase above the Curie pressures of KTN₄₀ and KNbO₃. Burns pressures p_d were determined to be 6(1) GPa for KTN₄₀ and 24(1) GPa and intermediate pressures p* were found at 1.8(2) GPa and 17(1) GPa, respectively. The known phase boundaries in temperature- and pressure-dependent phase diagrams could be reproduced and extended and phase diagrams could be enhanced by the addition of stability fields of polar nanoregions. It could also be shown that the dynamics of the polar nanoregions depend on chemical composition, as temperature-induced polar nanoregions in KTN₄₀ appear to be smaller than in KNbO₃. In addition, this study contains thermodynamic data for KNbO₃, KTN₄₀ and KTaO₃. Based on the excess entropy for KTN₄₀, these show that the formation of solid solutions tends to be favored in the KTN system compared to segregation.