TECHNOLOGY FOCUS - INORGANIC CHEMISTRY - Preferred Components: The nanophosphor: comprises a core which comprises nanoparticles comprising a second compound that is sodium yttrium gadolinium fluoride compound of formula (NaY1-w-z-xyGdwLz) and a shell which comprises the first compound and covers the core, where the second compound is fluoride-based one which is co-doped with ytterbium 3+ and erbium3+; and is 1-50 nm in size. The nanoparticle has hexagonal structure. The first compound has crystalline structure. The core is 1-30 nm in size.x = 0.1-0.9;y = greater than 0 to less than or equal to 0.1;x+y = greater than 0.1 to less than or equal to 1;w, z = 0-1;x+y+w+z = greater than 0.1 to less than or equal to 1; andL = lanthanide elements.The gadolinium precursor is gadolinium acetate, gadolinium chloride and/or gadolinium chloride hexahydrate. The cerium precursor is cerium acetate, cerium chloride and/or cerium chloride heptahydrate. The terbium precursor is terbium acetate, terbium acetate tetra hydrate, terbium chloride and/or terbium chloride hexahydrate. The first solution further comprises a lanthanide precursor consisting yttrium, lanthanum, praseodymium, neodymium, promethium, samarium, europium, dysprosium, holmium, erbium, thulium, ytterbium and/or lutetium. Preferred Method: The heat treatment in step (d) is performed at 200-370 degrees C. The nanoparticle is prepared by (e) preparing a fifth solution comprising yttrium precursors, ytterbium precursors, erbium precursors, oleic acid and 1-octadecene, (f) preparing a sixth solution comprising lanthanide complex compounds by heating the fifth solution (g) preparing a second reaction solution by mixing the sixth solution and a seventh mixed solution comprising sodium precursors and fluorine precursors and (h) forming nanoparticles comprising the second compound by heat treatment of the second reaction solution. EXAMPLE - 0.2 mmol Yttrium chloride hexahydrate, 0.6 mmol gadolinium hexahydrate, 0.18 mmol ytterbium chloride hexahydrate and 0.02 mmol erbium chloride hexahydrate were mixed with 6 ml solvent, oleic acid, and 15 ml 1-octadecene to obtain mixed solution. The mixed solution was heated to 150 degrees C. Lanthanide compounds were dissolved in solvent to form a transparent solution to form a solution including lanthanide complex compound. Mixed solution containing 2.5 mmol sodium hydroxide, 4 mmol ammonium fluoride and methanol was added to the solution including lanthanide complex compound and stirred to obtain reactive solution. Heat treatment was performed at 300 degrees C for 90 minutes to give sodium yttrium gadolinium fluoride compound containing (beta-NaY0.2Gd0.5F4:Yb3+0.18, Er3+0.02) nanoparticles. 0.8 mmol Gadolinium chloride hexahydrate, 0.15 mmol cerium chloride heptahydrate, 0.05 mmol terbium chloride hexahydrate and 3.1 mmol sodium oleate were added to water, ethanol and hexane and mixed to obtain first solution. Heat treating was performed at 60 degrees C for 4 hours to obtain second solution including oleate complex compounds including gadolinium oleate. The second solution was added to third solution containing oleic acid and 1-octadecene and adding the nanoparticles to solution containing gadolinium and stirring to obtain forth solution. 10 ml Methanol solution including 2.5 mmol sodium hydroxide and 4 mmol ammonium fluoride was added and stirred to obtain first reactive solution. After removing methanol from first reactive solution, heat treatment was performed to obtain nanophosphor having core/shell structure.