NaEu₃(GeO₄)₂(OH)₂: A high-pressure-stable photoluminescent lanthanide germanate

The europium germanate NaEu₃(GeO₄) ₂(OH)₂ was prepared under mild hydrothermal conditions. In situ synchrotron X-ray diffraction analysis and photoluminescence spectroscopic measurement in a diamond anvil cell (DAC) under high pressure were performed to study its structural stability as well as the luminescent properties of the Eu³⁺ ions. The synchrotron X-ray diffraction analysis revealed that the structure of NaEu₃(GeO₄)₂(OH)₂ was stable under pressure that ranged from 1 atm to 20 GPa with only a little distortion, and the unit-cell compression occurred more readily along the a axis than along the c axis. When the pressure was reduced to ambient pressure, the distortion and the unit cell recovered to the original state. Photoluminescence spectroscopic studies under high pressure showed that the redshift of the ⁵D₀ → ⁷F_J (J = 0, 1, 2) transitions and the pressure imposed on the sample had a good linear relationship. Also, the intensity of the red emission lines was independent of the pressure in the range from 9 to about 20 GPa.