The periodicity of negative thermal expansion in rare earth trifluorides

Abstract

Negative thermal expansion (NTE) in trifluorides of rare earth elements (REEs – R) is considered. The electronic structure of R divides RF3 into two homological series: 1) short ScF3, YF3, LaF3 of d-elements and 2) long LaF3 and 14 fluorides of 4f-elements (lanthanides - Ln) from CeF3 to LuF3. The type of NTE is different for different series. The Periodic Table of RF3 (RF3 are in the locations of elements R in the Periodic Table of Elements) sets the periodicity of NTE types for the first time. In the short series only ScF3 (ReO3 type) has NTE-I in a wide temperature range. Eight LnF3 (out of 15 RF3) have polymorphic transition (PolTr). The only PolTr β- (β-YF3 type) → t- (LaF3 type) of three possible (β- → t-; β- →α-YF3; t- → α-YF3) gives denser high-temperature t-modification. It is evidence of a giant NTE-II. The unit cell parameters of the model crystal61(58Ce0.564Gd0.5)F3 («pseudo61PmF3») were measured in situ by X-ray diffraction to calculate Vform of PmF3. The unit cell parameters of β- ​«pseudo PmF3» were obtained. PmF3 belongs to the structural subgroup (SSG) B of LnF3. PolTrs in these four LnF3 are accompanied by a giant NTE-II up to ∼4.7 %. These LnF3 form a new group of single component materials with non-adjustable NTE-II parameters. Multicomponent materials with adjustable NTE-II parameters arise in LnF3-Ln’F3 systems.