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

LiGdF4 and LiTbF4 single crystals are grown by the Bridgman–Stockbarger method from the nonstoichiometric melt comprising 32 mol% RF3 (R = Gd, Tb) and 68 mol% LiF, respectively. Incongruent melting of the single crystals LiGdF4 and LiTbF4 is confirmed by differential scanning calorimetry. For the first time, the crystal structures of LiGdF4 and LiTbF4 are determined by Rietveld method. Also, rare‐earth lithium fluoride syntheses from lithium nitrate melts at 400 °С are performed for LiTbF4, LiGdF4, LiYF4, and LiDyF4 compounds. For terbium, the product is a mixture of LiTbF4 and TbF3; and for gadolinium, GdF3 is thus produced. With reference to the related literature, it is concluded that LiGdF4 and LiTbF4 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 LiGdF4 and LiTbF4, however, does not prevent the use of their single crystals as photonic materials. It is well known that crystals can be stable and unstable at the same time. The most striking example is diamond, which, despite the thermodynamic instability under standard conditions, is successfully used both in engineering and as a gemstone. LiGdF4 and LiTbF4 also belong to this class of crystalline materials. LiGdF4 is a thermodynamically stable phase in the range from 750 to 425 °С.