Experimental and theoretical study of “random walk” disorder in mercury(II) amidobromide

Abstract

Mercury(II) amidobromide can exist in two forms. The stable form at room temperature has an ordered, orthorhombic lattice, whereas in the unstable cubic form the mercury atoms are believed to be disordered with respect to their positions in a way equivalent to a type of random walk disorder in the crystal. The heat capacities of the two forms have been measured from ∼ 10 to ∼ 300 K, giving values of the calorimetric entropy at 25°C. The difference between the actual entropy of the two forms at 25°C has been found from the relation: ΔG=ΔH–TΔS, where ΔH has been determined from heat of solution measurements and ΔG from the e.m.f. of a cell. The value of 131.01 J K^–1 mol^–1 for the calorimetric entropy of cubic mercury amidobromide at 25°C agrees, within the limits of experimental uncertainty, with that of 130.17 J K^–1 mol^–1 obtained from the study of the equilibrium between the two forms, which suggests that the cubic form is ordered at the absolute zero.

A theoretical study of the random walk disorder believed to exist in cubic mercury amidobromide at room temperature has been made by means of a simple Monte Carlo method, using lattices with sides of lengths from 3 up to 13 units. These calculations show that if the random walk disorder of cubic mercury amidobromide were retained at the absolute zero this salt might be expected to have a residual entropy of about 6.7 J K^–1 mol^–1.