Low pressure and temperature dependent phase transitions of a potassium exchanged zeolite A

Abstract

Phase transitions of an A-type zeolite with the chemical composition K 10.6Na 0.7[(SiO 2) 12(AlO 2) 11.9]-23.3 H 2O were investigated at atmospheric pressure and during evacuation (10 −6 ⩽ p ⩽ 1 bar) and at temperatures from 20 to 650 K using X-ray diffraction. At room temperature and down to a pressure of 1 mbar the structure is cubic ( a = 24.5 A ̊ ) with the space group Fm 3c . From 1 to 0.1 mbar a new cubic phase having the primitive space group Pm 3n appears without a change of the lattice constant. At lower pressures the space group is again Fm 3c . Dehydration by heating in vacuum exhibits a rhombohedral phase with space group R 3m . All transitions show hysteresis.