Phase transitions in [Mg(H2O)6](BF4)2 studied by differential scanning calorimetry§

Abstract

Three solid phases of [Mg(H2O)6](BF4)2 have been detected by differential scanning calorimetry (DSC). The phase transition temperatures: = 259.0 ± 1.5 K and = 143.3 ± 2.0 K on heating, and at: = 255.3 ± 0.3 K and = 140.6 ± 0.2 K on cooling, were determined as the mean values from results obtained for two samples independently synthetized in different ways. The presence of ca 3.7 K and 2.7 K wide hysteresis of the phase transition temperatures: T C1 and T C2, respectively, suggests that both detected phase transitions are of the first order type. Mean values of entropy changes at the phase transitions: ΔS 1 = 17.9 ± 1.3 kJ mol−1K−1 and ΔS 2 = 8.3 ± 0.2 kJ mol−1 K−1 indicate that both phase transitions are of order--disorder type. At ca T m = 370.2 ±3.1 K [Mg(H2O)6](BF4)2 partially melts (dissolves in water). Temperature of the sample crystallization depends on the scanning rate of the cooling. §This article is dedicated to Professor J. A. Janik on the occasion of his 80th birthday.