Thermal-expansion studies on the low-T phase diagram of α-(BEDT-TTF)₂KHg(SCN)₄

High-resolution thermal-expansion measurements on high-quality single crystals of the title compound reveal thermodynamic evidence for a second-order phase transition at T_A = 7.9K. According to the Ehrenfest relation the negative signs of the discontinuities in the coefficient of thermal expansion for directions parallel and perpendicular to the conducting planes correspond to negative uniaxial-pressure dependences of T_A with a predominant effect along the intralayer direction. At a temperature slightly above T_A, i.e. around 12K, measurements perpendicular to the planes reveal indications for a second anomaly which is field-independent for B ≤ 9.5T but shows a peculiar dependence on the thermal history.