Mix and Match: Organic and Inorganic Ions in the Perovskite Lattice

Abstract

X-site anions are possible, with only two general requirements that must be fulfilled: i) charge neutrality in the ABX 3 unit (typically, X is a divalent or monovalent anion, hence A and B cations with oxidation states between +1 and +5 can be combined) and ii) the ionic radii of the chosen composition must allow the above coordination, or different phases become more stable. Both of these simple design rules have a certain flexibility.The electronic argument i) can be stretched by allowing the combination of multiple ABX 3 units to form an overall charge neutral unit. For example, this is the case when considering A 2 BB′X 6 or AA′B 2 X 6 systems. Such double perovskite systems are known with B and B′ (A and A′) being the same elemental species (disproportion in oxidation state, e.g., Cs 2 Au + Au 3+ I 6 ) [20] or different ones (e.g., SrBaTi 2 O 6 [21] and SrPbTi 2 O 6 [22] for A-site disproportion or K 2 NbTaO 6 [23] and Bi 2 FeCrO 6 [24] for the B-site). Of course, this concept can also be used to combine three ABX 3 units (e.g., (CaTiO 3 ) n (SrTiO 3 ) l (BaTiO 3 ) m [25-27]) and much more complex stoichiometries with mixed ratios (e.g., (Pb 2 InNbO 6 ) 3/4 (Ba 2 InBiO 6 ) 1/4 [28] or [CaMn 3 3+ ][Mn 3 3+ Mn 4+ ] O 12 ] [29] ). Furthermore, although the bonding character in oxide perovskites is often mainly ionic, some compositions have an increased level of covalency (e.g., halides, sulfides, Bi-and Pbbased oxide perovskites). Thus, the electronic shell configuration and other effects like the steric demands of a metal lonepair will influence the structure and stability of ABX 3 perovskite compounds.In addition, the geometric argument ii) allows for some flexibility. For a perfectly cubic perovskite, the (effective) ionic radii r must fulfill the relation of Goldschmidt's tolerance factor 2( ) 1 A X B X t r r r r = + + = . [30,31] However, perovskites can exist also with t being not ideal, usually in a range of about 0.75 < t < 1.05. [8] Whenever t ≠ 1, the crystal distorts from an ideal cubic lattice, usually in form of off-center displacements, octahedral rotations, or a combination of both. The classification of Glazer [32,33] can be used to describe such distortions from a perfectly cubic perovskite. [34] It is further possible that ABX 3 compounds are stable whose t does not fall into the interval that is required for the perovskite crystal structure. In these cases that are no longer stabilized by deformations and tilting, one observes other phases where the connectivity of BX 6Materials science evolves to a state where the composition and structure of a crystal can be controlled almost at will. Given that a composition meets basic requirements of stoichiometry, steric demands, and charge neutrality, researchers are now able to investigate a wide range of compounds theoretically and, under various experimental conditions, select the constituting fragments of a crystal. One intriguing playground for such materials design is the perovskite structure. While a game of mixing and matching ions has been pla...

Polymorphism in metal halide perovskites

“…15 Estimated effective ionic radii generally fall in the range of 2.03 to 2.38 Å for MA+ and 2.24 to 2.53 Å for FA+. 15,[49][50][51] We note there are some discrepancies in the literature on phase transition temperatures and space groups observed for specific compositions. In 1987, for example, Poglitsch and Weber reported the space groups of the low temperature phases of MAPbCl 3 , MAPbBr 3 and MAPbI 3 to be P222 1 , Pna2 1 , and Pna2 1 , respectively.…”

In Situ Spectroelectrochemical Investigation of Perovskite Quantum Dots for Tracking Their Transformation

“…Ever since Mitzi et al described perovskites and their unique properties for optoelectronic applications in the 1990s, hybrid organic-inorganic perovskites have attracted significant attention owing to their versatile and outstanding photophysical properties (Mitzi et al, 1995;Kagan et al, 1999;Mitzi et al, 1999). Various ABX 3 -type hybrid perovskites (A MA + , FA + , and Cs + , where MA + CH 3 NH 3 + and FA + CH(NH 2 ) 2+ , B Pb 2+ and Sn 2+ , and X Cl − , Br − , and I − ) have been studied for their intrinsic structural (Gebhardt and Rappe, 2019) and photophysical properties (Chen et al, 2018). They have been found to exhibit a large grain layering up to the micron level (Chen et al, 2015b), fast charge transport (Senanayak et al, 2017), long diffusion lengths in the range of several micrometers (Dong et al, 2015), absorption cross-sections larger than 10 5 cm -1 (Wang et al, 2019), and low exciton binding energies (16-32 meV), which are comparable to the thermal energy (k B T) at room temperature and thus can facilitate free carrier generation (Saba et al, 2014).…”

Cs3Cu4In2Cl13 Nanocrystals: A Perovskite-Related Structure with Inorganic Clusters at A Sites

“…), 10 with rare examples of divalent organic cations, such as piperazine-1,4-diium (C4H12N4 2+ ) and a few others. [11][12][13][14] In our efforts to explore new metal halide materials, we have discovered a novel perovskite-related structure with a general formula Cs3Cu4In2Cl13, in the form of colloidal nanocrystals (NCs). The structure of these NCs was determined using three-dimensional electron diffraction (3D ED) on single NCs in combination with powder x-ray diffraction (XRD), as is discussed in detail below.…”