Polymorphic transformations of NaYGeO4 olivine under high pressure and high temperature.

Two new high-pressure modifications of sodium yttrium germanate NaYGeO 4 are described. A reversible pressure-induced phase transition of olivine-type NaYGeO 4 to new phases has been studied in the pressure range of 4–10 GPa at temperatures from 400 to 1000°C. It was shown that NaYGeO 4 olivine is stable at 400–600°C up to 10 GPa. A high-pressure polymorph of NaYGeO 4 with NaNdSiO 4 crystal structure is formed at 800°C starting from 6 GPa. Along with this polymorph, another new high-pressure modification of NaYGeO 4 with NaSmSiO 4 crystal structure was found in small amount. In addition, a yet undescribed polymorph of Y 2 Ge 2 O 7 with pyrochlore-type structure was isolated at 8 GPa and 800°C. The crystal structures of the new phases were determined by X-ray powder diffraction. The results of DFT calculations confirmed the experimental crystallography and gave a preliminary insight into the relative thermodynamic stability and electronic properties for all three new compounds. [ABSTRACT FROM AUTHOR]