Raman spectroscopy and theoretical modeling of BeO at high pressure

Abstract

The pressure dependence of the vibrational properties of BeO has been studied by Raman spectroscopy with a diamond-anvil, high-pressure cell. Five fundamental bands in the Raman spectrum were measured at room temperature for both a single crystal compressed with an argon medium to 41.5 GPa and a polycrystalline sample with no medium to 55 GPa. The frequencies of four modes [$A_1$(TO), $A_1$(LO), $E_1$(TO), and $E_2$] have a strong, positive pressure dependence, whereas that of the lowest-frequency mode ($E_2$ symmetry at 338 ${\mathrm{cm}}^{\mathrm{-}1}$) is weakly positive. No pressure-induced phase transitions were observed in BeO to the highest pressure of the experiments. These results are compared with parameter-free calculations using the potential-induced breathing (PIB) model. A transition from the wurtzite structure (B4) to the rocksalt structure (B1) is predicted at 40 GPa and is consistent with the experimental result since a pressure greater than the equilibrium transition pressure would be required to nucleate the B1 phase. The Raman frequencies and mode Grüneisen parameters calculated with PIB lattice dynamics agree closely with the experimental measurements.