Electronic interactions across vacancies in halide perovskites
Keywords: Chemistry.

This doctoral research made use of high-pressure techniques and compositional substitution to rationally modify the structures of a subset of halide perovskites, called halide double perovskites. These structural modifications impact the light absorption and electrical conductivity properties of halide double perovskites, enabling the observation of structure--property relationships across a series of structural modifications. Of particular interest was the use of high-pressure lattice compression to strengthen the degree of orbital interactions across crystallographic vacancies. This research interest led to the study of the high-pressure properties of Cs2AgITlIIIBr6, Cs2SnIVBr6, Cs2PdIVBr6, Cs2TeIVI6, Cs2SnIVI6, and Cs4SbIIISbVCl12. In several cases, compression of halide perovskites with vacancies results in greater than 100,000,000-fold increases in electrical conductivity as well as large decreases in the onset energy of light absorption. In Cs2AgTlBr6, the high-pressure variations in conductivity are understood as tuning of the ionization energy of the bromine vacancy defect, while the changes in bandgap are rationalized with relative strengthening and weakening of orbital interactions that give rise to the valence band maximum and conduction band minimum. In Cs2SnBr6, Cs2PdBr6, Cs2TeI6, and Cs2SnI6, alternating B-site vacancies are preferentially compressed at high-pressures such that the evolution of conductivity and bandgap can largely be related to across-vacancy orbital interactions of [SnBr6]2--, [PdBr6]2--, [TeI6]2--, and [SnI6]2--, respectively. Furthermore, Cs4SbIIISbVCl12 exhibits intervalence charge transfer that corresponds to electron transfer from [SbIIICl6]3-- octahedra to [SbVCl6]-- octahedra across a B-site vacancy. The thermal and optical energy barrier to intervalence charge transfer in Cs4SbIIISbVCl12 decreases rapidly at high pressures, eventually leading to an absorption onset below 0.08 eV, electrical conductivities above 1 S·cm--1, and activation energies below 30 meV. We tentatively assign the electronic structure above 33 GPa as Cs4SbIVSbIVCl12, corresponding to non-distinct Sb sites and non-metallic conductivity arising from rapid electron exchange facilitated by compression. Lastly, the electronic dimensionality of Cs2SnI6 was probed by manipulation of the distances between [SnI6]2-- octahedra. Pressures up to 8 GPa decrease the across-vacancy I--I distances by ca. 14% while maintaining the cubic structure. Alternatively, large substitution of the Cs A-site with increasingly bulk organic cations induces separation of the [SnI6]2-- octahedra. Analysis of the bandgaps and absorption onsets at high-pressure and across the series of A2SnI6 demonstrates that across-vacancy interactions in Cs2SnI6 are responsible for reducing the bandgap from ca. 1.5 eV to ca. 1.3 eV. At high-pressures, these across-vacancy interactions are considerably strengthened, reducing the bandgap to ca. 0.8 eV. Taken together, this work on halide perovskites with vacancies demonstrates that orbital interactions across vacancies are active at ambient pressure and are readily strengthened at high pressure, giving rise to emergent electrical properties