Thermal Stability of LiRF₄ (R = Gd, Tb) Compounds

Abstract LiGdF 4 and LiTbF 4 single crystals are grown by the Bridgman–Stockbarger method from the nonstoichiometric melt comprising 32 mol% RF 3 (R = Gd, Tb) and 68 mol% LiF, respectively. Incongruent melting of the single crystals LiGdF 4 and LiTbF 4 is confirmed by differential scanning calorimetry. For the first time, the crystal structures of LiGdF 4 and LiTbF 4 are determined by Rietveld method. Also, rare‐earth lithium fluoride syntheses from lithium nitrate melts at 400 °С are performed for LiTbF 4 , LiGdF 4 , LiYF 4 , and LiDyF 4 compounds. For terbium, the product is a mixture of LiTbF 4 and TbF 3 ; and for gadolinium, GdF 3 is thus produced. With reference to the related literature, it is concluded that LiGdF 4 and LiTbF 4 are the thermodynamically stable phases in the ranges from 750 ± 5 to 425 ± 15 °С, and from 782 ± 5 to 400 ± 15 °С, respectively. The metastability of LiGdF 4 and LiTbF 4 , however, does not prevent the use of their single crystals as photonic materials.