Hybrid Organic–Inorganic Halide Derivatives of the 2H Hexagonal Perovskite Structure

Four quaternary hybrid halide perovskites have been synthesized
in hydrohalic acid solutions under hydrothermal conditions. The structures
of (CH3NH3)2AgRhX6 and
(CH3NH3)2NaRhX6 (X = Cl–, Br–) consist of infinite one-dimensional
chains of face-sharing metal halide octahedra. The structure is closely
related to the 2H hexagonal perovskite structure, but the space group
symmetry is lowered from hexagonal P63/mmc to trigonal P3m1 by site ordering of the Rh3+ and Ag+/Na+ cations. All compositions demonstrate broad-spectrum
visible light absorption with optical transitions arising from rhodium
d-to-d transitions and halide-to-rhodium charge transfer transitions.
The bromides show a 0.2 eV red-shift in the optical transitions compared
to the analogous chlorides. Crystal field splitting energies were
found to be 2.6 and 2.4 eV for the chloride and bromide compositions,
respectively. Band structure calculations for all compositions give
rather flat valence and conduction bands, suggesting a zero-dimensional
electronic structure. The valence bands are made up of crystal orbitals
that are almost exclusively Rh 4d–Cl 3p (Br 4p) π* in
character, while the conduction bands have Rh 4d–Cl 3p (Br
4p) σ* character.

四种四元杂化卤化物钙钛矿（perovskite）已在氢卤酸溶液（hydrohalic acid solutions）中，于水热条件（hydrothermal conditions）下成功合成。(CH3NH3)2AgRhX6与(CH3NH3)2NaRhX6（X = Cl–, Br–）的晶体结构由无限延伸的共面金属卤化物八面体一维链构成。该结构与2H六方钙钛矿结构密切相关，但因Rh3+与Ag+/Na+阳离子发生位点有序化，其空间群对称性（space group symmetry）从六方晶系的P63/mmc降至三方晶系的P3m1。所有组分均表现出宽光谱可见光吸收特性，其光学跃迁源于铑的d-d电子跃迁以及卤化物到铑的电荷转移跃迁。相较于对应氯化物，溴化物的光学跃迁存在0.2 eV的红移。氯化物与溴化物组分的晶体场分裂能分别为2.6 eV与2.4 eV。针对所有组分开展的能带结构计算（band structure calculations）结果显示，其价带（valence band）与导带（conduction band）均呈现显著平坦的特征，表明该类材料的电子结构属于零维类型。价带主要由几乎完全具有Rh 4d–Cl 3p（Br 4p）反键π*轨道特性的晶体轨道构成，而导带则以Rh 4d–Cl 3p（Br 4p）反键σ*轨道特性的晶体轨道为主要组成部分。