The phase tra?r; tion in single crystals of CsCl mixed with CsBr, RbCl, and KC1 was investigated by means of ionic conductivity measurements and differential thermal analysis. Adding RbCl or KC1 to CsCl reduces both the transition temperature, Ttr, and the enthalpy oftransformation, AHt,. In the system CsC1-CsBr Tt, shifts to higher values relative to that of pure CsCl, whereas AH,, decreases considerably. A thermodynamical consideration of the phase transition temperatures in the CsC1-CsBr system shows that pure CsBr should exhibit a transition from the B2 to the B1 structure at 880 "C with an enthalpy variation of (300 + 100) cal/mol. From the DTA experiments one should expect an enthalpy of (100 & 50) cal/mol for the transformation of pure CsBr.Die Strukturumwandlung in CsC1-Einkristallen, gemischt mit CsBr, RbCl und KCl, wurde mittels Ionenleitfiihigkeitsmessungen und differentieller thermischer Analyse untersucht. Der Zusatz von RbCl oder KCl zu CsCl erniedrigt die Umwandlungstemperatur Tt,, sowie die Umwandlungsenthalpie AHt,; bei den CsC1-CsBr-Mischkristallen dagegen nimmt T,, mit steigender CsBr-Konzentration zu, wiihrend AHt, abnimmt. Eine thermodynamische Betrachtung der Umwandlungstemperaturen des CsC1-CsBr-Systems zeigt, daB in reinem CsBr bei 880 "C ein Ubergang von der B2-in die B1-Struktur stattfinden sollte, mit einer Umwandlungsenthalpie von (300 f 100) cal/mol. Aus den DTA-Messungen wiirde man auf eine Enthalpie (100 f 50) cal/mol fur reines CsBr schlielen.

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Experimental results are given on the kinetics of the B2⇆B1 phase transition in single crystals of pure CsCl. Conductivity and dilatometric measurements have been carried out employing uniform heating and cooling rates q varying from 0.07 to 3.8 deg/min. The kinetics can be described satisfactorily using the Johnson‐Mehl and Avrami formalism for nucleation and growth processes, y = 1 — exp (−kn tn), where y is the fraction transformed, t the time, n is the time exponent, and k the rate constant of the reaction. In the B2 → B1 transformation, occurring at 476°C, two regions have been observed. In the first region (0 < y < 0.98) n = 2.5 and in the second region (y > 0.98) n = 1 was found. The rate constant strongly depends upon the heating rate. Combining the results from the conductivity experiments and from dilatometry it can be concluded that the growing particles have an ellipsoidal shape in the first stage. The starting temperature of the B1 → B2 transition is strongly influenced by the cooling rate: 470 °C for q = −0.1 deg/min and 450 °C for q = −2.4 deg/min. The transformation process showed cooperative phenomena. Within a few seconds about 40% of the material are transformed. Repeated passes through the transformation region revealed that the number of nuclei present before the beginning of the transition was strongly enhanced by the thermal treatment. The overall kinetic behaviour remained unchanged.