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.

629 phys. stat. sol. (b) 159, 629 (1990) The elastic dipole model and results presented in two previous papers are applied to study the phase transitions of mixed and diluted alkali cyanide systems. The effect of impurities in the crystal on the phase transition temperature is analyzed at first. It is supposed that part of the CN-ions may be preferentially aligned in some directions and can not rotate freely due to the elastic interaction between CN-ions and impurities. When the concentration of CNions, taking part in the ordering process, is less than that corresponding to the suddenly changed amount of order parameter at the phase transition point, the maximum ordering energy in the lattice will not be large enough to permit the ordering process to take place, and the crystal will remain in a disordered state at all temperatures. With this model the experimental results obtained in these systems can be explained reasonably well. The effects of hydrostatic pressure on the phase transition temperature are attempted to be analysed too.Le modele du dipBle tlastique et les resultats prtsentes dans les deux precedents articles sont utilists dans l'etude de la transition de phase des cyanures d'alcalins mixtes et dilues. L'effet des impuretes du cristal sur la temperature de la transition de phase est analyse en premier. I1 est suppose qu'une part des ions CN-peuvent dtre alignes preferentielement dans certaines directions et ne peuvent pas tournes librement a cause de I'interaction 6lastique entre les ions CN-et les impuretes. Quand la concentration en ions CN-, prenant part au processus de rearrangement, est plus faible que celle correspondant au soudain changement du parametre d'ordre a la transition de phase, l'energie maximum de rearrangement dans le reseau ne sera pas assez importante pour permettre au processus de rearrangement de prendre place, et le cristal restera dans un &tat de desordre a haute temperature. Avec ce modele les resultats experimentaux obtenus pour ce systeme s'expliquent plut8t bien. Nous tentons d'analyser les effets de la pression hydrostatique sur la temperature de la transition de phase. ') Nanjing, People's Republic of China. ' ) 07738 Mexico D.F., Mexico. 3, Salt Lake City, UT 84112, USA. 4, Parts I and I1 see phys. stat. sol. (b) 154, 167 and (b) 154, 181 (1989). 41 physicd (b) 159/2 41'

Using a simple microscopic model, the kinetic equations of the orientational ordering phase transition are established and the general characters of the kinetic behavior of the first-order and the second-order phase transitions are obtained. The results about the equilibrium state obtained as a special case agree completely with those obtained from the phenomenological theories of the first-order and the second-order phase transitions. The different behavior of these two kinds of phase transitions is explained by a unified model. Meanwhile, the relation between them is given and the possibility of a system transiting from one to another caused by external influence is discussed. Lastly, it is shown that in such a non-equilibrium process, a "potential function" corresponding to the thermodynamic force can be established which is equivalent to the free energy function in the thermodynamics in the equilibrium state and can be used to give the direction and speed of the phase transition in the whole non-equilibrium process too.Avec un simple modele microscopique, on etablis les equations dynamiques de phase-transition en orientation ordre et obtient caracteres geniraux de comportement dynamique du premier ordre et deuxitme ordre phase-transition. Le resultat sur l'etat-equilibre obtenu de lui comme un cas special s'accorde completement avec lequel obtenu par theories phenomenologiques pour phase-transition de l'ordre premier et l'ordre deuxitme. Le comportement different de ces deux genre de phase-transitions est explique avec un modele unifie. En m&me temps, la relation entre eux est exprimbe, et la possibilite d'un systeme transitant de I'un a I'autre cause par l'influence exterieure est discutee. En dernier, il est montre que dans un tel processus non-tquilibre, une ))fonction potentiellea correspondant a la force thermodynarnique peut Btre etablie laquelle est equivalent a la fonction-energy-libre en thermodynamique dans l'btat equilibre mais peut Btre utiliste pour donner la direction et la vitesse de phase-transition dans processus non-kquilibre entier.

The series expension of elastic Green's function of anisotropic cubic crystal is calculated and the expansion coefficients are given under the second order approximation. Applying the results to elastic dipole model, one obtains the expressions of elastic displacement field due to a symmetrical center and the interaction between two symmetrical centers. For strongly anisotropic cubic crystals such as K and Cu, it is surprising that the numerical results of the displacement field of the symmetrical center and the interaction between them are basically the same as those obtained by using lattice statics, which is based on the discrete native of the lattice, although the convergence is not very satisfactory. This seems to indicate that our analytical expression of the elastic Green's function leads to a simple and easy method, which can be used generally to describe some mechanical behaviour of cubic crystals correctly.

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