Improved Phase Stability and Enhanced Luminescence of Calcite Phase LuBO₃:Ce³⁺ through Ga³⁺ Incorporation

The application of LuBO₃:Ce³⁺ (LBO:Ce) crystal as an excellent scintillation material has been limited due to its poor phase stability at high temperature or high pressure, so improving the phase stability is essential for promoting its development. Ga stabilized LuBO₃:Ce³⁺ (LGBO:Ce) is synthesized by solid-state reaction at 1200 °C. Powder X-ray diffraction patterns and Raman spectra at ambient pressure show that all the samples are pure calcite phase. In situ high-pressure synchrotron radiation XRD patterns illustrate that calcite phase LGBO:Ce exhibits more excellent phase stability than that of LBO:Ce under high pressure due to the superior compressibility of the [GaO₆] octahedral unit. The optical band gap of LGBO decreases from 5.58 to 4.64 eV after introducing 10% Ga, which leads to the decreased nonradiative transition and about double luminescence intensity as expected. More interestingly, the charge transition from O^2- to Ce⁴⁺ is observed at about 290 nm in the absorption spectra. The X-ray photoelectron spectroscopy spectra indicate the ratio of Ce⁴⁺/Ce³⁺ increases with increasing concentration of Ga³⁺, which can be attributed to the variation of energy separation between the 4f ground state of Ce³⁺ and the Fermi energy level position. In contrast to the enhancement of PL intensity, the integrated X-ray excited luminescence intensity decreases after Ga³⁺ incorporation attributing to the result of both decreased effective atomic number and ionization energy between 5d₁ level and conduction band. The thermal luminescence spectra show that after the incorporation of Ga³⁺ the oxygen vacancy and intrinsic defects in LBO remain unchanged but that the concentration of oxygen vacancy significantly reduces. The mechanism of Ga³⁺ incorporation on phase stability and luminescence properties of LBO:Ce has been proposed and discussed systematically.