Geometric Analysis and Formability of the Cubic A₂BX₆ Vacancy-Ordered Double Perovskite Structure

Abstract

A geometric analysis of the cubic A2BX6 structure commonly formed by metal halides is presented. Using the 'hard sphere' approximation, where the ions are represented by spheres of a fixed radius, we derive four limiting models that each constrain the distances between constituent ions in different ways. We compare the lattice parameters predicted by these four models with experimental data from the Inorganic Crystal Structure Database (ICSD). For the fluorides, the maintenance of the AX bond length at the sum of the A and X radii gives the best approximation of the lattice parameter, leading to structures with widely separated BX6 octahedra. For the heavier halides, a balance between forming an A site cavity of the correct size, and maintaining suitable anion-anion distances determines the lattice parameter. It is found that in many A2BX6 compounds of heavier halides, the neighbouring octahedra show very significant anion-anion overlap. We use these models to predict a compound with A site rattling and use DFT to confirm this prediction. Lastly, we use the geometric models to derive formability criteria for vacancy ordered double perovskites.

Degradation mechanism of hybrid tin-based perovskite solar cells and the critical role of tin (IV) iodide

“…To date, reports describing the degradation mechanism of Sn perovskites remain limited, in contrast with Pb-based analogues 47 – 53 . For example, some studies have shown that ASnX 3 (A = Cs + , MA + or FA + ; X = I - , Br - ) can decompose in air to form A 2 SnX 6 (vacancy-ordered double perovskite 54 ) and SnO 2 31 , 55 – 57 , while others revealed that FASnI 3 and (PEA) 2 SnI 4 (where PEA refers to phenylethylammonium) degrade in the presence of oxygen to SnI 4 , SnO 2 and FAI/PEAI 58 , 59 . Whilst these findings provide plausible air-mediated decomposition routes of Sn-based perovskites, a more detailed understanding of the degradation mechanism as well as knowledge of inconspicuous reaction pathways is required.…”

Assessing the environmental effects related to quantum dot structure, function, synthesis and exposure

“…Perovskite QDs may be organic-inorganic hybrids or fully inorganic; and are represented by the chemical formula ABX 3 (e.g., CsPbI 3 or CsPbBr 3 ) 50,51 in which A is an inorganic monovalent cation (such as caesium, Cs + ) or an organic cation (such as methylammonium, CH 3 NH 3 + , or formamidinium, (NH 2 ) 2 CH + ), 52 B is an inorganic-metal cation such as lead (Pb) or tin (Sn); and X is an anion, such as oxygen, halogens, or alkali metals. 49,[53][54][55][56] 1.2.2 Structure. Structurally, metallic QDs occur in three basic forms: the core, core/shell or alloyed types (Fig.…”

Optoelectronic and transport properties of Rb/ Cs 2 TeI 6 defective perovskites for green energy applications

“…16 Theo. 17 F I G U R E 1 Volume optimization plot for Rb/Cs 2 TeI 6 n(λ) in wavelength range of 250-750 nm as visible range is 400-700 nm. Figure 3A elaborates the dispersion behavior of Rb/Cs 2 TeI 6 in term of Ɛ 1 (λ).…”

“…Brik and Kityk 16 calculated lattice constant for Cs 2 TeI 6 from their developed analytical technique. Rahim et al 17 also calculated the lattice constant for Cs 2 TeI 6 at analytical level. Up till now, the optical and thermoelectric properties neither reported experimentally nor theoretically while electronic properties were not studied theoretically.…”