Anomalous thermoelastic behaviour of orientationally disordered (NH4I)x(KI)1−x mixed crystals

We have incorporated the translational rotational (TR) coupling effects in the framework of three body force shell model (TSM) to develop an extended TSM (ETSM). This ETSM has been applied to reveal the second order elastic constants (C11, C12 and C44) in the dilute regimes 0≤x≤0.50. Our theoretical results have reproduced well the observed variations of C11 (x=0.14, 0.43) and C12 (x=0.14, 0.43) with temperature. The anomalous elastic behaviour observed in C44 below 150 K for x=0.43 has been predicted well by ETSM results in the orientationally disordered (NH4I)x(KI)1−x mixed crystals. Our results for C12 are in good agreement with the experimental data available for x=0.14 and 0.43. Besides, the third and fourth order elastic constants and the results on cohesive and thermophysical properties are also discussed.