Polymorphic Phase Transition in a Potassium Chloride Crystal

Abstract

The phase transition data a t 298 "K of potassium chloride crystals are analysed on the basis of two alternative models, (a) the Tosi-Fumi prescription (TFP) in which overlap parameters are taken to be different in the two phases and (b) the three-body potential model (TBPM) used by Sarkar and Sengupta [l]. The effect of finite temperature and that of zero point energy is included using the Einstein approximation. It is found that, the transition data are well reproduced in both the models with T F P giving slightly better values. The temperature coefficient of transition pressure comes out with the right sign but has too high a magnitude in both the models. I n elastic data, however, T F P gives poor agreement while for TBPM the agreement is much better.Die Phasenubergangswerie bei 298 OK von Kaliumchloridkristallen werden auf der Grundlage zweier Alternativmodelle analysiert : a) der Tosi-Fumi-Vorschrift (TFP), bei der die Uberlappungsparameter in den beiden Phasen als verschieden angesehen werden, und b) dem Drei-Korper-Potential-Model1 (TBPM) von Sarkar und Sengupta [1]. Der EinfluB der endlichen Temperatur und der Nullpunktsenergie wird mit der Einstein-Naherung berucksichtigt. Es wird gefunden, da13 die Ubergangswerte in beiden Modellen gut wiedergegeben werden, wobei T F P leicht bessere Werte liefert. Der Temperaturkoeffizient des Ubergangsdruckes ergibt sich mit dem richtigen Vorzeichen, jedoch mit zu hohen Absolutwerten in beiden Modellen. Bei den elastischen Werten liefert jedoch T F P schlechtere ubereinstimmung, wahrend die Ubereinstimmung fur TBPM vie1 be,:. nser ist.

On the Polymorphic Phase Transition in Alkali Halides

“…The results are shown in Table 3. The comparison shows in fact that the eonclusions arrived a t in [14] regarding our model as compared to that of Tosi and E'unii (TFP) are valid quite generally. The equation of state and the phase transition data are slightly better represented by TFP.…”

“…We have calculated the required pressures for the desired compressions following the method described in detail in [14]. The transition pressures along with the temperature coefficients of transition pressure have also been calculated.…”

“…I n an earlier paper [14] we demonstrated how the use of a suitable three-body interaction could give a fairly good pressure-volume relationship and the phase transition pressure of potassium chloride. The present 1) Calcutta 12, India.…”

“…One way out, of this difficulty was suggested by Tosi and Fumi [13] who used different overlap potential parameters for the two phases. As in [14] we shall designate this procedure briefly as…”