Hydrothermal synthesis and pressure-induced reversible phase transition of holmium germanate NaHoGeO4

Abstract

NaHoGeO₄ crystals were successfully synthesized under hydrothermal conditions. The crystals had petal-like morphologies with dimensions of approximately 300 μm. The crystals displayed excellent luminescent properties, appearing yellow under natural light and pale red under UV light irradiation. Compared with solid-state synthesis method, these experiments require a shorter time and can be operated at a lower temperature. To assess the structural integrity of NaHoGeO₄ and its luminescent characteristics under varying pressure conditions, a diamond anvil cell (DAC) was utilized to perform high-pressure Raman and photoluminescence (PL) spectroscopic studies. The results of high-pressure PL spectroscopy demonstrated that NaHoGeO₄ probably underwent a structural transformation into a new phase within 5.7–9.0 GPa. High-pressure Raman analysis indicated that notable alterations occurred throughout the assessed pressure spectrum, indicating that a phase transition took place. When the pressure was increased to approximately 15.0 GPa, the high-pressure phase of NaHoGeO₄ (NaHoGeO₄-HP) remained stable. Once the pressure was reduced, the spectra reverted to their initial forms, suggesting that the phase transition could be reversed. This study provides a new approach for the study of the luminescent properties of rare-earth germanate compounds, with significant potential for practical applications.