Two-Component Rare-Earth Fluoride Materials with Negative Thermal Expansion Based on a Phase Transition-Type Mechanism in 50 R F3- R'F3 (R = La-Lu) Systems

Abstract

The formation of materials with negative thermal expansion (NTE) based on a phase transition-type mechanism (NTE-II) in 50 T-x (temperature-composition) RF₃-R'F₃ (R = La-Lu) systems out of 105 possible is predicted. The components of these systems are "mother" RF₃ compounds (R = Pm, Sm, Eu, and Gd) with polymorphic transformations (PolTrs), which occur during heating between the main structural types of RF₃: β-(β-YF₃) → t-(mineral tysonite LaF₃). The PolTr is characterized by a density anomaly: the formula volume (V_form) of the low-temperature modification (V_β-) is higher than that of the high-temperature modification (V_t-) by a giant value (up to 4.7%). In RF₃-R'F₃ systems, isomorphic substitutions chemically modify RF₃ by forming R_1-xR'_xF₃solid solutions (ss) based on both modifications. A two-phase composite (β-ss + t-ss) is a two-component NTE-II material with adjustable parameters. The prospects of using the material are estimated using the parameter of the average volume change (ΔV/V_av). The V_av at a fixed gross composition of a system is determined by the β-ss and t-ss decay (synthesis) curves and the temperature T. The regulation of ΔV/V_av is achieved by changing T within a "window ΔT". The available ΔT values are determined using phase diagrams. A chemical classification (ChCl) translates the search for NTE-II materials from 15 RF₃ into an array of 105 RF₃-R'F₃ systems. Phase diagrams are divided into 10 types of systems (TypeSs), in four of which NTE-II materials are formed. The tables of the systems that comprise these TypeSs are presented. The position of T_trans of the PolTr on the T scale for a short quasi-system (QS) "from PmF₃ to TbF₃" determines the interval of the ΔT_trans offset achievable in the RF₃-R'F₃ systems: from -148 to 1186 ± 10 °C. NTE-II fluoride materials exceed known NTE-II materials by almost three times in this parameter. Equilibrium in RF₃-R'F₃ systems is established quickly. The number of qualitatively different two-component fluoride materials with the giant NTE-II can be increased by more than ten times compared to RF₃ with NTE-II.

Keywords: negative thermal expansion; phase diagrams; polymorphic transformation; rare-earth trifluorides.