Thermochemistry of hydrated rare earth orthophosphates

Abstract

The hydrated rare earth orthophosphates LnPO4 ·xH2O (Ln=La-Dy) contain zeolitic water in the structural channels, which is released reversibly up to 300°. The thermal stabilities of the hydrates depend upon the nature of the Ln atom. The dehydration temperature decreases with decreasing ionic radiusr of Ln3+, according to the general equationθ=(r−a)/b (whereθ is the DSC and/or DTG dehydration peak temperature, anda andb are empirical constants depending on the experimental conditions). The contracting-area rate equation was valid for linearization of the isothermalα vs. time plots in the rangeα ∈ 〈0; 0.65〉. Kinetic values ofE, A, ΔH + and ΔS+ were calculated and the kinetic stabilities of the hydrates are discussed.