Chemical and physical pressure meet: deciphering the polymorphism and morphology of α-and δ-KY 3 F 10 induced by Eu 3+ doping

Abstract

An atomic-level understanding of the local effects on the structure and electronic properties provoked by chemical and/or physical pressure are 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-KY 3 F 10 polymorphs with varying Eu 3+doping levels (10-40%) revealing how the chemical (substitution of Y 3+ by Eu 3+ )-physical pressure effects can be separated to provide fundamental insight into their 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-KY 3 F 10 polymorphs, despite their otherwise near-identity, in which the negative pressure effect of the δ-KY 3 F 10 polymorph is accompanied by subtle structural distortion and changes in the electronic configuration associated with the local coordination of Eu 3+ at the [EuF 8 ] cluster, from C 2v to C 4v symmetry. We assess the local atomic arrangements and stability of (100), ( 110) and ( 111) surfaces of both polymorphs, the morphologies of HRSEM images, the expected pathways and corresponding barrier heights connecting them are reproduced with remarkable accuracy. This work delivers mechanistic insights into the transition between α- and δ-KY 3 F 10 polymorphs at low concentration of Eu 3+doping and offers a theoretical basis to disentangle the chemical and physical pressure, providing a novel perspective for the rational design of high-performance KY 3 F 10 -based structures.