Thermal Properties of Metals with Low-Temperature Structural Instabilities.

To examine the low temperature properties of these materials, a calorimeter was designed and constructed capable of performing both accurate absolute heat capacity measurements and high resolution relative heat capacity measurements on samples as small as a few milligrams over the temperature range 4 K to 300 K. Two high transition temperature superconductors of the A15 structure were studied, niobium-tin and niobium-germanium. The lattice transformation at 41 K in niobium-tin was examined with high resolution calorimetry to determine its relationship to the 17.8 K superconducting transition. The superconducting transition was found to correspond to the low temperature tetragonal phase of niobium-tin, with no unusual effects due to sample size. Developments in chemical-vapor-deposition techniques made it possible to measure the heat capacity of thin films of niobium-germanium.