Probing structural and optical modulations in metal-ion co-doped gadolinium vanadate: a combined spectroscopic and diffraction study

Upconversion (UC) luminescence enhancement in trivalent lanthanide-doped materials – particularly the ytterbium (Yb$^{3+}$)/erbium (Yb$^{3+}$) ion pair – remains challenging due to complex, interdependent mechanisms involving structural modifications and defect formation. Here, we present a systematic investigation of UC enhancement in relation to defect formation in GdVO$_4$:Yb$^{3+}$/Er$^{3+}$ microcrystals through strategic co-doping with optically inactive ions (Sc$^{3+}$, Cd$^{2+}$, Zn$^{2+}$). We employ a novel multi-technique approach combining positron annihilation lifetime spectroscopy (PALS) – to quantify vacancy-type defects in UC materials – with X-ray diffraction and photoluminescence quantum yield (PLQY) measurements to establish direct structure–defect–property relationships. Our results reveal that dopant valence and ionic radius dictate distinct defect formation patterns: isovalent Sc$^{3+}$ substitution primarily induces lattice contraction with minimal point defect generation, while aliovalent Cd$^{2+}$ and Zn$^{2+}$ doping creates extensive Gd vacancy networks through charge compensation mechanisms. Critically, we demonstrate that defect cluster size and spatial distribution, rather than total defect concentration, govern UC efficiency. ... mehrZn$^{3+}$ co-doping achieves a remarkable PLQY enhancement through progressive suppression of large vacancy clusters at grain boundaries, coupled with favorable redistribution of oxygen vacancies within crystallite interiors. In contrast, Cd$^{2+}$ doping, despite similar charge compensation requirements, produces extended defect clusters that act as efficient non-radiative quenching centers, limiting PLQY improvement. These findings establish defect engineering as a powerful strategy for UC optimization and provide a quantitative framework for rational design of high-efficiency lanthanide-based photonic materials.