Unique properties of ZnTiF6·6H2O:Mn4+ red-emitting hexahydrate phosphor

The structural and luminescence properties of ZnTiF6·6H2O:Mn4+ red-emitting hexahydrate phosphor were investigated using X-ray diffraction (XRD) measurement, photoluminescence (PL) analysis, PL excitation spectroscopy, electron paramagnetic resonance, and Raman scattering spectroscopy. The ZnTiF6·6H2O:Mn4+ phosphor exhibited remarkable change in the XRD traces and PL spectra after thermal annealing at temperatures (Ta) below or above ∼150°C. After heating below ∼150°C, the hexahydrate phosphor showed structural phase transition, probably from the room-ambient stable trigonal symmetry (R3¯) to the low-temperature stable monoclinic symmetry (P21/c). Surprisingly, similar XRD and PL data were obtained when the phosphor was vacuum-evacuated in a cryostat. This is also a novel finding of the present work and is supported by the Raman scattering data. The partially dehydrated state returned to the rehydrated state by putting the sample in room ambient. Annealing above a Ta of ∼200°C changed the hexahydrate phosphor to the dehydrated one (ZnTiF6:Mn4+) with the trigonal R3¯m symmetry and gave clearly different PL spectra from its hexahydrate counterpart. Photo-induced degradation in the PL efficiency of ZnTiF6·6H2O:Mn4+ was also observed by exposing the samples to visible-near-UV light and could be explained by the photooxidation and/or disproportionation reactions of the Mn ions.