Zeyu Deng1 Fengxia Wei1 2 Federico Brivio1 Yue Wu1 Shijing Sun1 Paul Bristowe1 Anthony Cheetham1

1, University of Cambridge, Cambridge, , United Kingdom
2, Institute of Materials Research and Engineering, Singapore, , Singapore

Hybrid halide perovskites AMIIX3 (A = amine or alkali metal cation; MII = divalent cation; X = Cl, Br and I) have emerged as potentially useful light-absorbing materials for solar cell applications.1,2 However, stability issues and the toxicity of Pb have led to a search for Pb-free alternatives. One of the ways of achieving this is to form a halide double perovskite A2MIMIIIX6 in which MI and MIII are monovalent and trivalent cations. Compared with single perovskites, double perovskties have a broader chemical diversity because both MI and MIII sites can be modified. Recent efforts have focused on Bi and In based double perovskites: Cs2AgBiBr6, Cs2AgBiCl6, Cs2AgInBr6, (CH3NH3)2KBiCl6, (CH3NH3)2TlBiBr6, (CH3NH3)2AgBiBr6 and (CH3NH3)2AgSbI6.3–9 In this paper we show that it is also possible to incorporate rare earth elements (Y and Gd) into the perovskite framework, forming (CH3NH3)2KYCl6 and (CH3NH3)2KGdCl6.10 Both perovskites possess large direct band gaps and exhibit a rhombohedral to face centered cubic phase transition at high temperature. Our work expands the scope of hybrid perovskites to rare-earth containing materials, enabling the possibility of future applications in solid-state lighting and magnetism.
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