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On the phase transition in α-NH4HgCl3, a two-dimensional version of ammonium chloride

Authors
Journal
Solid State Communications
0038-1098
Publisher
Elsevier
Publication Date
Volume
19
Issue
9
Identifiers
DOI: 10.1016/0038-1098(76)90672-4

Abstract

Abstract Heat capacity of α-NH 4HgCl 3 crystal has been measured with an adiabatic calorimeter from 11 to 300 K. A sharply peaked anomaly due to an order-disorder change of the ammonium ions was found at 54.97 ± 0.04 K. The entropy and enthalpy changes were estimated to be ΔS = 5.2 ± 1.0JK −1 mol −1 and ΔH = 342 ± 65 J mol −1. In accordance withthe structural two-dimensionality of α-NH 4HgCl 3 crystal, Onsager's solution of the two-dimensional Ising model was used in calculation of the transition temperature. On the assumption that the octopole-octopole interaction is responsible for the ordering of the ammonium ions in the present crystal and in ammonium chloride, the calculation gives 74.44 K for the transition temperature. Several possibilities were discussed for explaining the remaining discrepancy between the observed and calculated transition temperatures.

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