PHASE-TRANSITION IN SOLID ACETONITRILE

The structure of the low temperature phase of acetonitrile was determined from neutron powder diffraction data. The lattice parameters were measured as a function of temperature and the structure of the high temperature phase, as previously determined, was confirmed. At 12 K, the structure is orthorhombic, Cmc2(1), a = 5.9895(3) angstrom, b = 5.2079(2) angstrom, c = 7.7317(4) angstrom. A transformation to a monoclinic phase, P2(1)/c, a 4.1182(7) angstrom, b = 8.2865(10) angstrom, c = 7.9946(11) angstrom, beta = 100.4(6)-degrees, takes places between 220 and 229 K. Equilibrium structures for the two phases were calculated using simple model potentials and the calculated structures are very similar to those observed. The nature of the phase transition was investigated by means of these model potentials.