Two-Component Rare-Earth Fluoride Materials with Negative Thermal Expansion Based on a Phase Transition-Type Mechanism in 50 RF(3)-R’F(3) (R = La-Lu) Systems

The formation of materials with negative thermal expansion (NTE) based on a phase transition-type mechanism (NTE-II) in 50 T–x (temperature–composition) RF(3)-R’F(3) (R = La-Lu) systems out of 105 possible is predicted. The components of these systems are “mother” RF(3) compounds (R = Pm, Sm, Eu, and Gd) with polymorphic transformations (PolTrs), which occur during heating between the main structural types of RF(3): β-(β-YF(3)) → t-(mineral tysonite LaF(3)). 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 RF(3)-R’F(3) systems, isomorphic substitutions chemically modify RF(3) by forming R(1−x)R’(x)F(3) solid 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 RF(3) into an array of 105 RF(3)-R’F(3) 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 PmF(3) to TbF(3)” determines the interval of the ΔT(trans) offset achievable in the RF(3)-R’F(3) 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 RF(3)-R’F(3) 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 RF(3) with NTE-II.