Anomalous thermal expansion in PMN–PSN solid solutions

The temperature evolution of the volumetric coefficient of thermal expansion (CTE) in single crystals $(1-x)$PbMg$_{1/3}$Nb$_{2/3}$O$_3$+$(x)$PbSc$_{1/2}$Nb$_{1/2}$O$_3$ (PMN–PSN), at $x$ = 0.35 (PMN–PSN35), $x$ = 0.8 (PMN–PSN80) and pure PSN has been studied. In PSN and PMN–PSN80, an abrupt transition from a smooth decrease in CTE with reaching zero in the cubic phase typical for relaxors to a negative CTE in the rhombohedral phase was observed. Negative CTE exists in the temperature range of about 40 K. In the PMN–PSN35 crystal CTE smoothly passes from positive to negative values. At $T\approx$ 250 K a phase transition (PT) to a low-symmetry (presumably monoclinic) phase was found. Below this temperature the CTE smoothly returns to positive values.