Polarizability of Si-O Bonds in Olivine as a Function of Temperature: a Raman Study

Olivine undergoes no phase transition between 300 K and 850 K. Hence, Raman band intensities and band positions should vary monotonously upon heating. We find that, upon heating to 670 K, Raman bands in the 700-1000 cm-1 range decrease monotonously in intensity and positions. The effect is reversible. Above 670 K, however, the Raman bands increase again in intensity. Upon cooling from 850 K to 300 K there is at first a hysteresis and then full recovery. A prior study of the dielectric polarization of olivine at the limit of 0 Hz indicates a similar 2-step T-dependent behavior, which correlates well with the 2-step break-up of peroxy defects in the olivine structure. The break-up of the peroxy bonds leads to an extended delocalization of the wavefunction associated with the O- state. Although the concentration of peroxy defects is probably only in the 100-500 ppm range, their effect is felt across the entire olivine structure. As the wavefunction associated with the hole state spreads out, all oxygens loose some of their electron density and their polarizability increases. This effect can explain the anomalous T-dependent behavior of the Raman bands.