Local Order and Non Equilibrium Behaviour in Glassy Crystal Cyanoadamantane

During isothermal annealing in the glassy crystal phase. cyanoadamantane relaxes towards a metastable state. The microstructure of the latter is described. I t is di f ferent from t h a t o f the stable phase at the same temperature. The kinetics of ordering which are coupled to a volume contraction are analyzed in terms of molecular relaxation. The "glassy crystals" / 1 / which are obtained by quenching the orientationally disordered phase of some molecular crystals [plastic crystals1 are very attract ive models of the glassy state as their thermal behaviour reveals the typical features of a classical glass transition whereas the frozen disorder is mainly orientational. Such a behaviour has recently been suspected in cyanoadamantane [CN-ADMI which deserves special attention on account of i ts simple structure /2/. When CN-ADM i s r a p i d l y cooled from the room temperature f-c-c phase [I] to a low enough temperature. the f i rst order transition to the stable antiferro ordered monoclinic phase [ I l l is avoided [Tc' 283 K at thermal equilibriuml. While reheating. a heat capacity jump which could indicate a glassy transition is observed at about Tg = 170 K. The macroscopic crystal symmetry as wel l as the t ime averaged structure remain unchanged : the dipolar axis can randomly take six orientations closely located along the directions. But NMR and dielectric experiments /3/ suggest a freezing of this tumbling while the fast uniaxial reorientation is almost unchanged at Tg. Besides. low temperature experiments have revealed characteristic anomalies of amorphous solids [ two level systems1 /4/. We have recently pointed out that annealings of plastic single crystals quenched down in the 150-170 K range. show slow growing of a local order /5.6/. I t is characterized by X-ray superstructure spots at the X-boundary points of the Bril louin zone [Fig.l.b.l. We give here some elements of a molecular analysis of this local order [§ 11 and non equilibrium behaviour 15 21. The X-ray measurements were performed on an automatic diffractometer in experimental conditions which have been described previously /6/. A, I