Vacancy-ordered double perovskites of the general formula A2BX6 are a family of perovskite derivatives composed of a face-centered lattice of nearly isolated [BX6] units with A-site cations occupying the cuboctahedral voids Despite the presence of isolated octahedral units, the close-packed iodide lattice provides significant electronic dispersion, such that Cs2SnI6 has recently been explored for applications in photovoltaic devices To elucidate the structure–property relationships of these materials, we have synthesized solid-solution Cs2Sn1–xTexI6 However, even though tellurium substitution increases electronic dispersion via closer I–I contact distances, the substitution experimentally yields insulating behavior from a significant decrease in carrier concentration and mobility Density functional calculations of native defects in Cs2SnI6 reveal that iodine vacancies exhibit a low enthalpy of formation, and that the defect energy level is a shallow donor to the conduction band rendering the material 

http://pubs.acs.org/doi/pdf/10.1021/jacs.6b03207

Defect Tolerance to Intolerance in the Vacancy-Ordered Double Perovskite Semiconductors Cs2SnI6 and Cs2TeI6

Lead halide perovskites have emerged as promising semiconducting materials for different applications owing to their superior optoelectronic properties. Although the community holds different views toward the toxic lead in these high-performance perovskites, it is certainly preferred to replace lead with nontoxic, or at least less-toxic, elements while maintaining the superior properties. Here, the design rules for lead-free perovskite materials with structural dimensions from 3D to 0D are presented. Recent progress in lead-free halide perovskites is reviewed, and the relationships between the structures and fundamental properties are summarized, including optical, electric, and magnetic-related properties. 3D perovskites, especially A2 B+ B3+ X6 -type double perovskites, demonstrate very promising optoelectronic prospects, while low-dimensional perovskites show rich structural diversity, resulting in abundant properties for optical, electric, magnetic, and multifunctional applications. Furthermore, based on these structure-property relationships, strategies for multifunctional perovskite design are proposed. The challenges and future directions of lead-free perovskite applications are also highlighted, with emphasis on materials development and device fabrication. The research on lead-free halide perovskites at Linkoping University has benefited from inspirational discussions with Prof. Olle Inganas.

Structural and Functional Diversity in Lead-Free Halide Perovskite Materials

The application of pressure on halide perovskite materials provides key insights into their structural properties and the collective motions of their basic structural units. Here, we use synchrotron X-ray diffraction and Raman spectroscopy measurements combined with density functional theory calculations to perform a comprehensive study on the structural and vibrational properties of the so-called defect halide perovskites Cs2SnX6 (X = Cl, Br, I) under high hydrostatic pressures up to 20 GPa. We find that, while Cs2SnCl6 and Cs2SnBr6 retain a face-centered cubic (FCC) structure for all studied pressures, Cs2SnI6 undergoes successive phase transformations initially to a more disordered structure and secondly to a low-symmetry monoclinic I2/m phase. The first transition is only evidenced by certain features emerging in the Raman spectra at ∼3.3 GPa, whereas the latter happens in a pressure window of around 8–10 GPa and involves tilting and elongation of SnI6 octahedra and hysteretic behavior with pressure r...

Wulf Brendel

Papers

TG and DTA investigations on hexaiodometallates