Journal Article

Two-Component Rare-Earth Fluoride Materials with Negative Thermal Expansion Based on a Phase Transition-Type Mechanism in 50 IR/IFsub.3-IR/I’Fsub.3 Systems

Sobolev, Boris P, Sulyanova, Elena A

2023

Overview

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[sub.3]-R’F[sub.3] (R = La-Lu) systems out of 105 possible is predicted. The components of these systems are “mother” RF[sub.3] compounds (R = Pm, Sm, Eu, and Gd) with polymorphic transformations (PolTrs), which occur during heating between the main structural types of RF[sub.3]: β-(β-YF[sub.3]) → t-(mineral tysonite LaF[sub.3]). The PolTr is characterized by a density anomaly: the formula volume (V[sub.form]) of the low-temperature modification (Vβ-) is higher than that of the high-temperature modification (Vt-) by a giant value (up to 4.7%). In RF[sub.3]-R’F[sub.3] systems, isomorphic substitutions chemically modify RF[sub.3] by forming R[sub.1−x]R’[sub.x]F[sub.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[sub.av]). The V[sub.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[sub.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[sub.3] into an array of 105 RF[sub.3]-R’F[sub.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[sub.trans] of the PolTr on the T scale for a short quasi-system (QS) “from PmF[sub.3] to TbF[sub.3]” determines the interval of the ΔT[sub.trans] offset achievable in the RF[sub.3]-R’F[sub.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[sub.3]-R’F[sub.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[sub.3] with NTE-II.

• Publisher: MDPI AG
• Subject: Fluorides