Thermal expansion of niobium diselenide in the basal plane at low temperatures

The thermal expansion of a high-quality 2H-NbSe₂ single crystal is precisely measured in the basal plane and the temperature range 5.7–50 K. An anomaly of the thermal expansion in the basal plane is detected, and it is found to be caused by a superconducting transition. The change in thermal expansion coefficient α _ab is used to calculate the pressure derivative of the critical temperature (dT _c /dp _ab ) = (19.0 ± 2.0) × 10^?9 K/Pa, and this derivative agrees well with the reported data. An additional anomaly, which indicates a phase transition, is also detected in the pretransition range. The nature of the detected phase transition is discussed.