CHARACTERIZATION OF THE HIGH-TEMPERATURE MODIFICATIONS OF INCOMMENSURATE TRIDYMITE L3-T-O(MX-1) FROM 25-DEGREES-C TO 250-DEGREES-C

Incommensurate tridymite L3-T-o(MX-1) shows a cascade of five phase tr ansitions at 65, 110, 150, 200, and 380 degrees C upon heating. The X- ray diffraction patterns were investigated with a Buerger precession c amera revealing a sequence of four incommensurate phases in the range from room temperature to 200 degrees C. The phases formed between 65 a nd 110 degrees C and between 110 and 150 degrees C are new modificatio ns of tridymite. At 65 degrees C the monoclinic tridymite L3-T-o(MX-1) phase undergoes a first-order transformation to an orthorhombic phase . The incommensurate structural modulation of the room-temperature pha se with the wavevector q(1) = 0.663 a -0.498 c* flips to q(2) = 0.042 a -0.388 c*. Simultaneously, a commensurate modulation with tripled b lattice parameter is formed. The wavelengths of both modulations do not depend appreciably on the temperature. The incommensurate modulati on discontinuously disappears near 110 degrees C whereas the commensur ate modulation along the b axis becomes non-integral with a temperatur e-dependent wavelength varying between 115 and 100 Angstrom. At 150 de grees C the symmetry is reduced to monoclinic again with gamma = 90.4 degrees. Between 150 and 200 degrees C the monoclinic angle gradually decreases to 90 degrees and the wavelength of the modulation from abou t 90 to 65 Angstrom. At higher temperatures, the satellite reflections fade into weak streaks and the normal orthorhombic high-temperature m odification of tridymite is formed. The phase transitions are reversib le upon cooling except for the first transformation that is partly irr eversible for single crystals and reversible but incomplete for pulver ized material at room temperature.