Synthesis and pressure-induced reversible phase transition of a crystalline solid europium germanate NaEuGeO ₄

The crystalline solid europium germanate NaEuGeO ₄ 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 NaEuGeO ₄ as well as changes of the luminescent properties of Eu ³⁺ 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 NaEuGeO ₄ (NaEuGeO ₄-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 NaEuGeO ₄ transformed to an unidentified phase NaEuGeO ₄-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.