Promethium trifluoride: polymorphism and the structure of modifications on the model crystal 61(Ce0.5Gd0.5)F3. Part II. In situ X-ray diffraction study of t- and β- “pseudo61PmF3” crystal structures near the temperature of a polymorphic transformation

Abstract

Crystal structures of high-temperature t- (LaF₃-type) and low-temperature β- (β-YF₃-type) modifications of ⁶¹(⁵⁸Ce_0.5⁶⁴Gd_0.5)F₃ – “pseudo⁶¹PmF₃” near the temperature of a polymorphic transformation (PolTr) (−148 ± 5 °C) were studied by in situ X-ray diffraction (XRD). During the t- → β- “pseudo⁶¹PmF₃” PolTr (at cooling) anions are ordered in the direction perpendicular to the hexagonal close packing (HCP) layers. This is accompanied by 13% stretching of the t-type crystal lattice in the <010> direction and compression in the <100> (0.8%) and <001> (0.9%) directions. The structural changes in t- and β- “pseudo⁶¹PmF₃” at heating from −148 to +20 °C represent changes (decrease and increase) in cation–anion bond lengths (BL) without changing the shape of the coordination polyhedron (CP). “Pseudo⁶¹PmF₃” is a material with giant negative thermal expansion at PolTr (NTE-II). The decrease in volume during the β- → t- “pseudo⁶¹PmF₃” PolTr (at heating) is 3.8%.