Application of Raman in phase equilibrium studies: the structures of substitutional solid solutions of KNO3 by RbNO3

Raman spectroscopy was employed to investigate the KNO3-RbNO3 system. Raman studies indicated that Rb+ may substitute K+ up to 67 mol % in the solid s olutions of the KNO3 II structure (denoted as K1-xRbxNO3 (KII)) and up to 8 0 mol % in the solid solutions of the KNO3 III structure (denoted as K1-xRb xNO3 (KIII)). The substitutional crystals retained the transitions of I to III to II of KNO3 on cooling and the metastable property of KNO3 III at roo m temperature. It was found that rubidium nitrate had a considerable tenden cy to have the structure of potassium nitrate. This accounts for the fact t hat larger rubidium ions can replace many more smaller potassium ions in th e nitrate than vice versa. When the concentration of rubidium ions was more than 67 mol %, the substituted crystals underwent the mixed structural pha se transition of the KNO3 III structure to the RbNO3 IV structure, and K1-x RbxNO3 (KIII) seemed to consist of disordered R3m microstructure. (C) 1999 Kluwer Academic Publishers.