RESUMEN
Composite crystals SrxLi2+xAl2-xO4:Eu2+ were synthesized and their structures were determined using single-crystal X-ray diffraction. The commensurate structure with a modulation wavevector q = 5c*/6 was analyzed in a conventional manner in 3D space, while a structure model in (3+1)-dimensional superspace was used for the other two crystals with modulation wavevectors slightly differing from 5c*/6. The superstructure of the commensurate phase was described using the space group P4/n and a common superspace group I4/m(00γ)00 was used for the (3+1)D structures of all three crystals. The whole structure of each crystal consists of two substructures. Basis vectors a and b are common, but c is different for the two substructures. The first substructure is a host framework constructed by (Li/Al)O4 tetrahedra sharing edges. A linear connection of cavities is seen to be channel-like, in which Sr ions locate as guest cations forming the second substructure. The crystal of q = 5c*/6 contains five Sr ions per six cavities in a channel. Sr ions are distributed at seven sites, some of which are partially occupied. Statistical disorder of local structure models for the location of Sr ions in the channel was assumed to explain the results. Such a partially disordered character was also seen in the incommensurate phases and properly embodied by a (3+1)D model containing an atomic domain of the Sr ion with occupational modulation. Plots of the occupation factor, interatomic distances and the bond valence sum at each metal site as functions of t (= x4 - q·r) are roughly identical in the three crystals, which are considered as members of the same series of composite crystals.
RESUMEN
In the field of photonics, alkali copper(I) halides attract considerable attention as lead-free emitters. The intrinsic quantum confinement effects originating from low-dimensional electronic structure lead to high photoluminescence quantum yields (PLQYs). Among them, Cs3 Cu2 I5 is the most promising candidate, satisfying both high PLQY and air stability. In this study, a strategy to explore a new material meeting these requirements through the use of the mixed-anions of I- and Cl- is proposed. The expectation is maintained that the large difference in ionic radii between them likely results in the formation of a novel compound. Consequently, Cs5 Cu3 Cl6 I2 with a 1D zigzag chain structure is discovered. This material exhibits blue emission (≈462 nm) with a near-unity quantum yield of 95%. An electronic structure calculation reveals that the localized nature of the valence band maximum is crucial in obtaining efficient self-trapped exciton emission. Moreover, the iodine-bridged 1D connectivity significantly enhances the chemical stability of Cs5 Cu3 Cl6 I2 , compared with the pure chloride phase. The present findings provide a new perspective for developing air-stable alkali copper(I) halides with highly efficient luminescence.