RAMAN-SPECTRA OF PHASE-A AT VARIOUS PRESSURES AND TEMPERATURES

Variations of the Raman spectra of phase A (Mg7Si2O14H6) were investig ated up to about 400 kbar at room temperature and in the range 80-853 K at atmospheric pressure. Phase A appears to undergo a reversible pha se transition around 180 kbar at room temperature, and it becomes amor phous above 813 K at atmospheric pressure. The Raman frequencies of th e two strong OH bands of phase A decrease linearly with increasing pre ssure, but increase non-linearly with increasing temperature. The freq uencies of the other Raman bands of phase A increase non-linearly with increasing pressure but decrease both linearly and non-linearly with increasing temperature within the experimental uncertainties and the r ange investigated. The trends of the pressure and temperature effects on the Raman frequencies of phase A parallel those observed in the hyd rous beta-phase, but nonlinear behaviour was not observed in the latte r. (C) 1997 Elsevier Science Ltd. All rights reserved.