Double Perovskite K3InF6 as an Upconversion Phosphor and Its Structural Transformation Through Rubidium Substitution

Luminescent properties including energy upconversion on rare‐earth doped cryolite (double perovskite) structured K3InF6 have been investigated by synthesizing samples (both pure and Eu3+, Tb3+, Er3+ doped and Yb3+/Er3+ co‐doped samples) solvothermally. Cryolite structure of K3InF6 was evident in its powder X‐ray diffraction (PXRD) pattern which could be refined successfully in Fd3 space group with a lattice constant of a = 17.718(3) Å. Three bands centred at 227, 311 and 496 cm–1 were present for K3InF6 in its Raman spectrum at room temperature confirming cryolite structure and phonon energy of it was estimated to be 367 cm–1 by Lorentz fitting procedure. Emissions in red and green regions were observed for Eu3+ and Tb3+ doped K3InF6 samples, respectively. The local site symmetry and nature of bonding in Eu3+ doped samples were analyzed by Judd–Ofelt (J–O) parameters. For Er3+ and Er3+–Yb3+ doped samples, upconversion emission with the laser of λ = 980 nm was carried out in addition to normal excitation and emission spectral measurements. Intra configurational f–f transitions of Er3+ ions were noticed both in normal and upconverted spectra. Emission in red region over the green dominated both in the normal and in upconverted emission spectra. These were reasoned to arise from cross relaxation (CR) energy transfer process between two nearby Er3+‐ions. Structural transformation of cryolite K3InF6 to elpasolite has also been examined by substituting K+ with Rb+.