Infrared study of α-SiO2 single crystal under high pressure

We have measured high pressure alpha-quartz reflectance spectra in the mid infrared. We used single crystals, taking full profit of polarization. The quality of the spectra allows fitting the reflectance spectra. We have characterized the pressure evolution of E and A(2) modes with increased precision, even in the spectral regions where they overlap. In addition, we have determined the TO-LO splitting of each mode. Some of the A(2) modes show dramatic pressure variations of the LO-TO splitting, which cannot be explained only by changes in length and ionicity of individual bonds, requiring a new mechanism. We suggest that rotation of the SiO4 tetrahedra plays a fundamental role. We have also determined the evolution of the electronic dielectric constant under high pressure. We find that its pressure increment is mainly a volume effect, although the small increase in birefringence points to secondary changes associated to the electronic resonances. (C) 2016 AIP Publishing LLC.