BaTiF₆:Mn⁴⁺ Red Phosphor: Synthesis of Single Crystals at Room Temperature and the High Hydrolysis-Resistant Property

Due to the low solubility of BaF₂, the BaTiF₆:Mn⁴⁺ phosphor for whitelight-emitting diodes application has been generally synthesized by the hydrothermal route, during which process the valence of the manganese dopant is difficult to be controlled as tetravalent. In this paper, a new synthesis method that proceeded at room temperature was reported. This method uses BaTiOF₄ as the precursor and allows for the control of the phase transformation rate from BaTiOF₄ to BaTiF₆ in the K₂MnF₆/HF acid solution. Benefitting from that, we successfully prepared red-emitting BaTiF₆:Mn⁴⁺ elongated crystals with a single-crystal nature up to a record-breaking length of 200-300 μm. The effects of the crystallinity of the BaTiOF₄ precursor on its phase transformation rate into BaTiF₆ and on the optimal Mn⁴⁺ doping concentration were studied. The BaTiF₆:Mn⁴⁺ single-crystal phosphor exhibits relatively excellent hydrolysis-resistant behavior after being immersed in water for 3 h, at which condition the commercial K₂SiF₆:Mn⁴⁺ has become brown. This study may inspire the room-temperature preparation of other hydrolysis-resistant alkali earth fluorotitanate or fluorosilicate phosphors with stable tetravalent manganese doping.