Chemical and physical pressure meet: deciphering the polymorphism and morphology of α- and δ-KY3F10 induced by Eu3+ doping

Abstract

An atomic-level understanding of the local effects on the structure and electronic properties provoked by chemical and/or physical pressure is essential; however, their intricate relationship is poorly understood, which poses a challenge for the design of new inorganic materials with tailored properties. In this work, we report the synthesis and comprehensive characterization of pure α- and δ-KY₃F₁₀ polymorphs with varying Eu³⁺-doping levels (10–40%) revealing how the chemical (substitution of Y³⁺ by Eu³⁺)–physical pressure effects can be separated to provide fundamental insight into the stability, electronic properties, and morphology of both polymorphs. Our results consist of XRD, ICP-MS, FT-IR, and HRSEM measurements in combination with DFT calculations. Experimental and theoretical findings disclose a coupling mechanism in α- and δ-KY₃F₁₀ polymorphs, despite their otherwise near-identity, in which the negative pressure effect of the δ-KY₃F₁₀ polymorph is accompanied by subtle structural distortions and changes in the electronic configuration associated with a shift in the local coordination of Eu³⁺ at the [EuF₈] cluster from C_2v to C_4v symmetry. We assess the local atomic arrangements and stability of the (100), (110) and (111) surfaces of both polymorphs. The morphologies observed in the HRSEM images and the expected pathways and corresponding barrier heights connecting them are reproduced with remarkable accuracy. This work provides mechanistic insights into the transition between α- and δ-KY₃F₁₀ polymorphs at low concentrations of Eu³⁺-doping and offers a theoretical basis to disentangle the chemical and physical pressure, providing a novel perspective for the rational design of high-performance KY₃F₁₀-based structures.