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A wet chemical methodology has been developed to synthesize the undoped and rare‐earth‐doped A2SnCl6 samples (A = K, Rb, and Cs) in cubic symmetry. The double perovskite (K2PtCl6) structure of these samples was confirmed from their PXRD, SAED, FTIR, and Raman spectra. The crystallites showed octahedral morphology in the FESEM and HRTEM images. The optical band gap of undoped A2SnCl6 increased with an increase in the size of the alkali metal ion. Eu3+ and Tb3+ (5 mol‐%) doped A2SnCl6 showed intense red‐orange and green emission when excited at 380 nm. The various radiative properties of the emission bands were analyzed using the Judd‐Ofelt theory. Yb3+/Er3+ co‐doped A2SnCl6 showed multicolor upconversion under 980 nm excitation, and the emission region varied with the alkali metal. These samples show bright emission with optimum CCT and CIE coordinates, indicating that the rare‐earth‐doped A2SnCl6 samples are suitable in applications involving Near‐UV and Near‐IR excitations.

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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+.

Heavily doped nanocrystals of host KLaF 4 with rare earth (RE 3+ = Er 3+ , Tm 3+ , and Yb 3+ ) ions prepared by a simple one-step template-free wet-chemical route have been reported. Prepared KLaF 4 nanocrystals reveal phase-pure cubic structures (lattice constant a = 5.931Å) with space group Fm 3 m . Precisely defined molar ratios of heavily dopant RE 3+ ions allow us to achieve wide color upconversion (UC) emission tunability (blue, green to yellow–orange–red) and white light, without any morphology and structure changes. The enhanced red emission by a factor of ∼120 has been achieved in 20% Yb 3+ and 5% Tm 3+ ions in KLaF 4 :1% Er 3+ nanocrystals, which is due to an efficient sensitizer–acceptor (Yb 3+ to Er 3+ and Tm 3+ ions) energy transfer and interexchange energy process between acceptors. For the first time, the key role of sensitizer (Yb 3+ ) for UC emission energy transfer to Er 3+ and/or Tm 3+ is experimentally demonstrated. The evidence of upconversion photoluminescence excitation spectra reveals a broad safe biological excitation window (690–1040 nm), which can be well demonstrated by low-cost NIR diode lasers/LEDs. The applicability of these cubic nanophosphors is demonstrated as light-emitting polymer composite coatings and blocks for LEDs and solar cell panels. These well-dispersed UC nanocrystals can also be found to have greater use in bioimaging and spectral studies.