THE PROPERTIES OF SALT-FILLED SODALITES .1. THERMAL-DECOMPOSITION REACTIONS OF HYDROXOBORATE SODALITE

Differential scanning calorimetric investigations of tetrahydroxoborate sodalite Na8[AlSiO4]6[B(OH)4]2 and low temperature polarization microscopic studies indicate structural phase transitions of sluggish character of different samples, covering the temperature range 270-300 K. Sodalite samples show cubic symmetry (ao = 9.010(1) Å at 310 K) above 300 K but superstructure characteristics on single-crystal precession and Guinier powder diffraction photographs below 270 K. This pattern could be indexed to orthorhombic symmetry (a0 = 25.510(1) Å, b0 = 12.750(1) Å, c0 = 9.020(1) Å at T ≤ 270 K). Variations of the transition characteristics in the DSC curves of the species from the high temperature run (773 K) and those synthesized at 573 K are discussed. Thermoanalysis (thermogravimetry, differential thermogravimetry and differential thermoanalysis), coupled with mass spectrometry, of the sodalite samples shows a significant expansion of the framework at elevated temperatures (573-825 K), when a total of four molecules of H2O are released in a two-step decomposition reaction. The decomposition behaviour indicates the existence of a hydrogen bonding system between framework and non-framework oxygens and is discussed in terms of the destruction of these bonds during heating. © 1990.