Synthesis and Pressure-induced Reversible Phase Transition of a Crystalline Solid Europium Germanate NaEuGeO4

The crystalline solid europium germanate NaEuGeO4 was prepared under hydrothermal conditions. The in-situ photoluminescence spectroscopic measurement and the synchrotron X-ray diffraction analysis in a diamond anvil cell （DAC） under high pressure were performed to study the pressure-induced phase transition of NaEuGeO4 as well as changes of the luminescent properties of Eu3＋ ions. Photoluminescence spectroscopic studies revealed that a phase transition occurred at a pressure range of 6.5--10 GPa and the high-pressure phase of NaEuGeO4 （NaEu- GeO4-HP） was still stable when the pressure raised up to about 20 GPa. As the pressure was released, the spectra returned to the original state, revealing that the pressure-induced phase transition is a completely reversible process. The synchrotron X-ray diffraction analysis demonstrated that the structure of NaEuGeO4 transformed to an uniden- tified phase NaEuGeO4-HP at a pressure higher than 10 GPa, and the phase transition is reversible. This result is consisted with that obtained by photoluminescence spectra analysis.