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  Zhao, Y.; Chen, K.; Li, N.; Ma, S.; Wang, Y.; Kong, Q.; Baudelet, F.; Wang, X.; Yang, W.: Tricolor Ho³⁺ Photoluminescence Enhancement from Site Symmetry Breakdown in Pyrochlore Ho₂Sn₂O₇ after Pressure Treatment. Physical Review Letters 125 (2020), p. 245701/1-5

10.1103/physrevlett.125.245701

Abstract:
The photoluminescence (PL) characterization spectrum has been widely used to study the electronic energy levels. Ho3+ is one of the commonly used doping elements to provide the PL with concentration limited to 1% atomic ratio. Here, we present a tricolor PL achieved in pyrochlore Ho2Sn2O7 through pressure treatment at room temperature, which makes a non-PL material to a strong multiband PL material with Ho3+ at the regular lattice site with 18.2% concentration. Under a high pressure compression-decompression treatment up to 78.0 GPa, the Ho2Sn2O7 undergoes pyrochlore (Fd 3m), to cotunnite (Pnma), then amorphous phase transition with different Ho3+ coordinations and site symmetries. The PL emerged from 31.2 GPa when the pyrochlore to cotunnite phase transition took place with the breakdown of site symmetry and enhanced hybridization of Ho3+ 4f and 5d orbitals. Upon decompression, the materials became an amorphous state with a partial retaining of the defected cotunnite phase, accompanied with a large enhancement of red-dominant tricolor PL from the ion pair cross-relaxation effect in the low-symmetry (C1) site, in which two distinct Ho3+ emission centers (S center and L center) are present.