Tricolor Ho3+ Photoluminescence Enhancement from Site Symmetry Breakdown in Pyrochlore Ho2Sn2O7 after Pressure Treatment

The photoluminescence (PL) characterization spectrum has been widely used to study the electronic energy levels. ${\mathrm{Ho}}^{3+}$ 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 ${\mathrm{Ho}}_{2}{\mathrm{Sn}}_{2}{\mathrm{O}}_{7}$ through pressure treatment at room temperature, which makes a non-PL material to a strong multiband PL material with ${\mathrm{Ho}}^{3+}$ at the regular lattice site with 18.2% concentration. Under a high pressure compression-decompression treatment up to 78.0 GPa, the ${\mathrm{Ho}}_{2}{\mathrm{Sn}}_{2}{\mathrm{O}}_{7}$ undergoes pyrochlore ($Fd\text{ }3m$), to cotunnite ($Pnma$), then amorphous phase transition with different ${\mathrm{Ho}}^{3+}$ 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 ${\mathrm{Ho}}^{3+}$ $4f$ 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 (${C}_{1}$) site, in which two distinct ${\mathrm{Ho}}^{3+}$ emission centers ($S$ center and $L$ center) are present.