Pressure enhances Tricolor Ho3+ Pyrochlore photoluminescence A tricolor PL achieved in pyrochlore FIGURE 1 HoSnO through pressure treatment, which2 2 7 makes a non-PL material to a strong PL material. The HoSnO undergoes Fd-3m,2 2 7 to Pnma, then amorphous phase transition with different Ho coordination and site3+ symmetries. The PL emerged once phase transition took place with the breakdown of site symmetry and enhanced hybridization of Ho 4f and 5d orbitals. Upon3+ decompression, an amorphous state with EXPERIMENTAL RESULTS a partial retaining of the defected Pnma, The pristine HoSnO is a non-PL material with cubic symmetry2 2 7 accompanied with a large enhancement at low pressure (LP). Upon compression above 32 GPa, the local Ho site symmetry is broken from D to C seen in Fig.2 which3+ of red-dominant tricolor PL in the low- leads to a tri-color PL emergence. Upon decompression, the HP3d s symmetry (C) site, in which two distinct1 phase becomes unstable and partial turns to amorphous state, Ho emission centers (S center and L center)3+ which reduces the Ho site symmetry further to C with partial13+ are present. retaining of HP phase. Surprisingly the recovered HoSnO2 2 7 mixture largely enhances the tri-color PL, especially the red-PL becomes a dominant component through the dual-site emission Lanthanide compounds Ln2BOare excellent refractories while2 7 centers (S-center inherited from HP phase and L-center from some Ln-containing ones exhibit interesting PL behavior which ion pair cross-relaxation in the amorphous phase). can act as a laser. Ho is one of the commonly used Ln doping3+ ions to provide the PL, but the concentration is limited to 1% FIGURE 2 (atom). We explored the PL evolution of concentrated materials EuSnOand LaSnO under pressure, and found that change2 2 227 7 of the local Euor Sn3+ 2+site symmetry can largely explain the suppress and absence of PL. However, few of these materials meet the criteria that the quenched materials can bring in the high pressure (HP) treatment history information. Inspired by this, the non-PL HoSnO is selected and subjected to HP2 2 7 treatment to alter the local site symmetry. Surprisingly, Ho3+ PL is successfully generated by the pressure-induced phase transition in Fig.1, and upon decompression, the minority red emission is largely enhanced (fourfold) along with the other two bands due to the Ho site symmetry breakdown and ion pair3+ cross-relaxation. 70