The rotational motion of the ammonium ion in NH 4CI at low temperature under the influence of its nearest neighbours has been studied using a computer simulation technique. The Lennard Jones potential is used as the representative interaction between NHt and CI-. Three values of the time increment At occurring in the algorithm equation are taken to illustrate the three-dimensional effect on the rotational dynamics of the NH: ion. In each case we notice a well defined transition gap around = 1·25 separating phases II and III which are known from the literature. The libration frequency of the ammonium ion is found to be 1"0; 170 em -1, corresponding to the transition temperature of 242 K, which is in agreement with the Raman spectra study by Couzi et af. (1973).
Computer simulation studies have been made to investigate the phase transition in ND 4CI at low temperature. Two different types of potential have been used to represent the interaction between ND t and CI-: (i) the Lennard Jones potential and (ii). the twelfth inverse power or r -12 repulsive potential. The present study reveals the occurrence of a phase transition in N-D 4CI in a well-defined region. The Lennard Jones potential appears to be more appropriate in depicting the two welldefined phases known as II and III in the literature. The energy pattern curves obtained in this study give us the nature of the rotational behaviour of ND:' before and after the transition at 249 K.The libration frequency of the ND t ion is estimated to be of the order of 130 em -1.
Aust. J. Phys., 1984,37, 667-74 The effects of pressure on the dynamical behaviour of an NH: ion near the A transition under the influence of its nearest neighbours in ammonium chloride have been studied by using the computer simulation technique. The Lennard Jones (6-12) potential is used as the representative interaction between NH: and Cl-. The present calculations reveal a decrease in entropy of the system with increasing pressure. The libration frequency of the NH: ion is estimated to be approximately 170, 182 and 210 cm -1 at pressures of 1 atm, 3 and 10 kbar respectively, in agreement with the Raman spectrum study of NH4 CI at high pressure (Ebisuzaki and Nicol 1969).
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