Promethium trifluoride: polymorphism and the structure of modifications on the model crystal ⁶¹(Ce_0.5Gd_0.5)F₃. Part II. In situ X-ray diffraction study of t- and - "pseudo⁶¹PmF₃" crystal structures near the temperature of a polymorphic transformation

Crystal structures of high-temperature t- (LaF3-type) and low-temperature & beta;- (& beta;-YF3-type) modifications of (61)((Ce0.5Gd0.5)-Ce-58-Gd-64)F-3 - "pseudo(61)PmF(3)" near the temperature of a polymorphic transformation (PolTr) (-148 & PLUSMN; 5 & DEG;C) were studied by in situ X-ray diffraction (XRD). During the t- & RARR; & beta;- "pseudo(61)PmF(3)" 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 direction and compression in the (0.8%) and (0.9%) directions. The structural changes in t- and & beta;- "pseudo(61)PmF(3)" at heating from -148 to +20 & DEG;C represent changes (decrease and increase) in cation-anion bond lengths (BL) without changing the shape of the coordination polyhedron (CP). "Pseudo(61)PmF(3)" is a material with giant negative thermal expansion at PolTr (NTE-II). The decrease in volume during the & beta;- & RARR; t- "pseudo(61)PmF(3)" PolTr (at heating) is 3.8%.