Understanding the Path to Stability: Structural Phase Transition and Bonding Behavior in Yb:Lu₂O₃ Nanocrystals under Compression

The incorporation of small amounts of lanthanide cationic ions into rare earth sesquioxides can advantageously induce significant improvements in both their properties and crystal morphology under compression. In this study, ytterbium (Yb)-doped Lu2O3 nanocrystalline samples with a cubic structure were synthesized via the water/ethanol solvent coprecipitation. We systematically investigated the phase transition mechanism, compression properties, and equation of state of the synthesized samples under compression using in situ high-pressure synchrotron X-ray diffraction experiments combined with first-principles calculations. The isostructural phase transition observed in cubic (C-type) Yb:Lu2O3 nanocrystals is highlighted herein. The phase sequence of low-pressure C-type (LPC) Yb:Lu2O3 under compression is LPC -> high-pressure C-type (HPC) -> monoclinic (B-type), with corresponding transition onset pressures of 9.7 and 13.8 GPa, respectively. The results show that no additional high-pressure crystalline phases occur up to a pressure of 51 GPa. The isothermal zero-pressure bulk modulus of LPC, HPC, and B-type Yb:Lu2O3 was determined by fitting the pressure-volume (P-V) data to the Birch-Murnaghan equation of state. The results show that Yb doping increases the incompressibility and structural stability of Lu2O3. Furthermore, the physical mechanisms driving the two structural phase transitions observed in C-type Yb:Lu2O3 under compression are comprehensively elucidated. The significant deviatoric strains generated during compression play an active role in the isostructural phase transition from LPC to HPC. During the transition from HPC to B-type, the elastic strain energy has a dominant influence on the coherent crystalline surfaces, leading to the formation of reconstructed phases. The enthalpy changes of the two-phase transitions were determined using the onset transition pressure and corresponding volume change in conjunction with the fundamental thermodynamic equations. Moreover, the pressure dependence of the bonding behavior was investigated. The transition from HPC to B-type Yb:Lu2O3 was accompanied by substantial changes in multiple Lu-O and Yb-O polyhedral features. Particularly, an increase in Yb-O coordination was observed during this transition. This study provides valuable insights into the pressure-driven phase transition mechanisms of nanocrystal materials.