Dimorphism of RF3 (R = La–Nd) crystals based on the data of X-ray diffraction studies

Abstract

The ratio of two forms of tysonite in nominally pure single crystals of RF₃ (R = La–Nd) obtained from melts under identical conditions (the as-grown state) is studied for the first time by X-ray diffraction. Crystals of RF₃ with R = La–Nd belong to the β-LaF₃ structural type (space group \(P\bar 3c1\), Z = 6) and form twins. Samples 0.2–0.4 in diameter contain inclusions of the high-temperature a form (space group P6₃/mmc, Z = 2). It is shown that twinning and dimorphism of the RF₃ crystals (R = La–Nd) have a common structural basis.