Thermal Expansion and Isothermal Compressibility of Solid Nitrogen

The coefficients of linear thermal expansion and of isothermal compressibility have been measured for solid nitrogen in the temperature intervals from 4.2 degrees to 40 degrees K and 8 degrees to 40 degrees K, respectively. Length changes of the sample were detected with a resolution of 10(-9) cm using a dilatometer of the three-terminal capacitor variety contained inside a pressure vessel. Apparatus and experimental technique are described in detail. The thermal expansion and compressibility of alpha-N-2 increase rapidly with increasing temperature above 20 degrees K, and in the former case the length increase is exponential in 1/T above 24 degrees K. A reproducible value for the volume discontinuity at the alpha-beta crystallographic transformation at 35.6 degrees K was obtained only if many hours were taken to effect the transformation. The data are discussed in relation to recent experimental and theoretical information obtained from studies of Raman and infrared spectra, nuclear quadrupole resonance, and specific heat. The anomalous thermal expansion is attributed to excited librational and translational states of large amplitude and appreciable anharmonic content. The adiabatic compressibility calculated from the experimental data agrees well with published values from sound velocity experiments. The Gruneisen parameter of alpha-N-2 is found to be strongly temperature dependent.