Theoretical Investigation of Ferroelastic Phase Transition of Pure and Mixed Alkali Cyanide Systems by the Elastic Dipole Model. I. The Cubic‐Orthorhombic Phase Transition of Pure Potassium Cyanide

Abstract

The kinetics of elastic ordering of KCR controlled by a thermally activated process is studied by considering the interaction between elastic CN-dipoles. The lattice constants obtained and reorientational activation energy of dipoles in the elastically ordered lattice agree quantitatively with experiments. The calculation indicates that the orientation ordering of elastic dipoles is induced by the strain field due to ordering alignment of dipoles. The elastic interaction between close neighbors of dipoles has little influence. The first-order character of the phase transition is obtained, and it is found that an initial degree of order depending on the temperature is necessary to start the ordering process. It is concluded that this corresponds to the nucleation process of the first-order phase transition.La cinetique de rearrangement 6lastique de KCN controlbe par un processus de thermoactivation est Btudike en considerant l'interaction entre les dipdles blastiques CN-. Les constantes du reseau obtenues et 1'6nergie d'activation de la reorientation des dipBles dans le reseau 6lastique ordonne sont en accord quantitatif avec les experiences. Le calcul indique que la reorientation des dipdles hlastiques est induite par le champ principal du aux dip8les realignes. L'interaction Blastique entre les proches voisins des dip6les a peu d'influence. Le caractere de premier ordre de la transition de phase a At6 obtenu et nous trouvons qu'un degre initial d'ordre dependant de la temperature est necessaire pour demarrer le processus de rearrangement. Nous concluons que cela correspond a un processus de nucleation de la transition de phase du premier ordre. l ) Nanjing, People's Republic of China. ?) Salt Lake City, UT 84112, USA.

Evolution of molecular ordering and phase transitions in C60/C70solid solutions

“…In ferroelastic ordering of CN − molecules in KCN, orientational ordering is induced by the strain field due to ordering alignment of elastic dipoles, with little influence coming from elastic interaction between close neighbours. If the later interaction becomes more important than the former, then the resulting ordering turns out to be antiferroelastic like in RbCN [31]. We make the assumption that in C 70 ordering is also induced by the ordering strain field as in KCN.…”

“…We follow closely the procedures and notation given in Refs. [31,32] except that here we consider the case of dipole alignment in the 〈111〉 direction of the original cubic cell. In the rhombohedral elastically ordered phase (R3m), C 70 molecules aligned in 〈111〉 direction can lie in four different orientations q = 1 − 4.…”

“…The ordering stress and strain fields when the crystal is in an elastically ordered state described by a certain w q distribution are [31]: …”

“…An equation for the ordering kinetics can be established by considering that the dipoles change from orientation 1 to any other orientation q through a thermally activated process, therefore [31 …”

“…The potential energy U of dipoles in the ordering stress field is calculated as described in Ref. [31] and yields for each of the four q = 1 − 4 orientations along 〈111〉 that:…”