Growth and Characterization of Multicomponent, Equimolar Cubic Solid-Solution Crystals in the CaF2–SrF2–BaF2–NdF3 System

Equimolar crystals of a high-entropy Ca0.25Sr0.25Ba0.25Nd0.25F2.25 (CaSrBaNdF9) fluoride solid solution were grown from a melt by the Bridgman technique, and their optical, electrical, and thermal properties were studied for the first time. This solid solution crystallizes in a fluorite-type structure (space group Fm-3m with lattice parameter a = 5.807 Å), is transparent over a wide spectral range, and has a refractive index of nD = 1.5035(5). In terms of ionic conductivity (σdc increases monotonically from 3.7 × 10−5 to 3.9 × 10−4 S/cm in the studied temperature range of 643–810 K), it significantly exceeds the parameters of binary and ternary NdF3-based single crystals, such as M1−xNdxF2+x (M = Ca, Sr, Ba; x = 0.24–0.25) and Ca0.58Sr0.21Nd0.21F2.21. The grown multicomponent material is a hard (HV~3.6 GPa) isomorphic-capacious crystalline matrix for various applications in solid-state ionics, optics and photonics, and opens up prospects for the development of new functional isotropic optical crystalline materials in quaternary CaF2–SrF2–BaF2–RF3 and higher-order complex fluoride systems nMF2–mRF3, where n + m ≥ 4, M and R are ions of alkaline earth and rare earth elements, respectively.