Structure, phase transitions, and thermal expansion of ethane C2H6

X-Ray investigations of polycrystalline ethane in the temperature interval 6-90 K have been performed. A previous suggestion that the structure of the orientationally ordered low-temperature phase III is monoclinic is confirmed, and the presence of an orthorhombic II and cubic I BCC phases near the melting temperature is also confirmed. It is shown that the intermediate phase II possesses a unit cell with the parameters a=4.289 A, b=5.660 A, and c=5.865 A. The volume jump Delta V at the III-II phase transition is found to be 0.21 cm(3)/mole (0.5%). It is suggested on the basis of the lack of reproducibility in the heating and cooling regimes that the phase II is metastable. It is determined that the volume change at the monoclinic-BCC phase transition in the interval 89.5-90 K reaches Delta V=3.05 cm(3)/mole or 7.1%. The temperature dependences of the parameters and the volume of the low-temperature monoclinic phase III are studied for the first time. The linear and cubic thermal expansion coefficients are determined. It is found that the linear thermal expansion is anisotropic in the monoclinicity plane ac, increasing substantially as the phase transition temperature is approached. The specific heat of ethane at constant volume C-V and the Gruneisen constant are calculated and the difference C-P-C-V is determined. It is shown that at temperatures above 50 K C-P-C-V increases substantially as a result of an intensification of the rotational motion of the molecules.