Promethium trifluoride: polymorphism and the structure of modifications on the model crystal 61(Ce0.5Gd0.5)F3. Part I. Structural and chemical modelling of “pseudo 61PmF3” in a short quasi-system “from CeF3 to GdF3”

Abstract

PmF₃ is a member of the family of 17 fluorides of rare earth elements (REE – R = Sc, Y, La and 14 lanthanides with Ln = Ce–Lu). The Pm element is “synthetic”. PmF₃ is practically unavailable for research. In the classification of 14 Ln (1976) PmF₃ is placed in the structural subgroup (SS) A (LaF₃–PmF₃), since polymorphic transformation (PT) could not be detected in it. The chemistry of 120 RF₃–R′F₃ systems is the basis of fluoride photonics, in which Ln³⁺ lanthanide activator ions are the leaders. Analysis of multiple LnF₃–Ln′F₃ systems requires their chemical classification (ChCl) (2019). Its first level consists of LnF₃ components. The place of PmF₃ in the SS of LnF₃, depending on the presence (absence) of PT, radically changes the ChCl. For the first time, indirect data pro et contra PT in PmF₃ were collected, obtained over almost fifty years of research. All of them, except for an indefinite theoretical one, are for the “hidden” PT in PmF₃ and its belonging to SS B. The method of structural and chemical modeling was used, in which a radioactively stable two-component composition ⁶¹(Ce_0.5Gd_0.5)F₃ is synthesized in a quasi-system (QS) “from CeF₃ to GdF₃”. The model crystal has the structural properties of “pseudo ⁶¹PmF₃”. By in situ X-ray diffraction (XRD) at +20–188 °C, a dimorphism of “pseudo ⁶¹PmF₃” was revealed and its belonging to SS B LnF₃ (Ln = Pm–Gd) was established. The lattice parameters of the “pseudo ⁶¹PmF₃” modification of type β-YF₃ was determined for the first time. The PT β ↔ t of the ⁶¹(Ce_0.5Gd_0.5)F₃ crystal occurs at −148 °C with ΔT_trans = 0. A jump in the formula volume (ΔV_form = 5.1%) of the giant negative thermal expansion effect of type II (NTE-II) at PT β- → t-⁶¹(Ce_0.5Gd_0.5)F₃ (compression at heating) was calculated.