Potassium Lanthanum Nitrate System: Phase Equilibria and Thermal and Structural Properties

Abstract

The thermal decompositions of K2La(NO3)5· 2H2O and K3La2(NO3)9have been studied under vacuum and under nitrogen by means of temperature-dependent X-ray diffraction and conventional thermal analysis. It is shown that the complexity of the decomposition processes arises from the formation of liquid phases. Interpretation of the complete decomposition scheme demanded the concurrent determination, by differential scanning calorimetry, of the phase relationships in the two binary systems LaONO3–KNO3and K3La2(NO3)9–KNO3. In addition, the crystal structure of K3La2(NO3)9has been refined from single-crystal diffraction data (cubic symmetry,a=13.6607(5) Å, space groupP4132). In the diagram LaONO3–KNO3, the definite compound LaONO3· 13 KNO3has been identified. It undergoes a phase transformation at 310°C (ΔtrH0=9700 J mol−1). The X-ray diffraction patterns of the two varieties have been indexed with orthorhombic symmetry, and, from cell dimension analogies, a crystal structure derived from the structure of LaONO3, with species like “KNO3” located between layers (LaO)n+n, is suggested. A polymorphic variety of LaONO3has been observed above 167°C. In the phase diagram K3La2(NO3)9–KNO3a eutectic has been found at 0.906 and 250°C. The decomposition of K2La(NO3)5· 2H2O takes place with the successive occurrence of the mixture of phases [K3La2(NO3)9, KNO3], the eutectic K3La2(NO3)9· 9.62KNO3, the phase LaONO3· 13KNO3and, surprisingly, the formation of cubic La2O3, which in turn transforms into the hexagonal variety at higher temperature. The decomposition of K3La2(NO3)9proceeds through the successive formation of LaONO3· 13KNO3and cubic La2O3. From a Rietveld refinement, it has been shown that the crystal structure of cubic La2O3[a=11.414(3) Å at 540°C] is isostructural with the cubic rate earth sesquioxides.