Jahn-Teller phase transitions in TbcGd1−cVO4

The temperatures at which the cooperative Jahn-Teller phase transitions occur in Tb_cGd_1?cVO₄ have been determined from Raman and optical birefringence measurements. Over a narrow range of c there are two transition temperatures; between these temperatures the crystal is distorted, but a higher and lower temperatures the crystal has the same undistorted structure. This unusual behaviour was predicted earlier, and is accurately described by molecular field theory.