Solvothermal phase selection of Cr3+-doped (NH4)xScF(3+x) compounds (x = 0, 1, 3), structure, and NIR luminescence

5 at% Cr³⁺ doped ScF₃, NH₄ScF₄ and (NH₄)₃ScF₆ were selectively synthesized via solvothermal reaction of metal nitrate and NH₄F in methanol for 24 h, with the effects of NH₄F/M molar ratio (M=Sc_0.95Cr_0.05), methanol content and reaction temperature (up to 140 °C) being systematically investigated. The compounds were characterized in detail for crystal structure, microstructure, thermal behavior and vibrational properties, and broadband NIR luminescence (∼700–1300 nm) with an FWHM (full width at half maximum) as large as ∼156 nm was obtained from the as synthesized ScF₃:Cr³⁺ phosphor powder under 470 nm blue light excitation. The technique is advantageous over aqueous processing since it allows high yield production of the water-soluble ammonium fluoroscandates and was expected to be well applicable to analogous materials such as those of the Al, Ga and In elements.