Introduction
Classification of Perovskites MaterialsPerovskites are classes of compounds whose crystal structure is similar to that of CaTiO 3 rock minerals discovered in 1839. [1] Based on empirical formula of the mother mineral, perovskites can be defined as compounds whose empirical formula is ABX 3 ; where A is an inorganic/organic cation (e.g., Ca 2þ , CsMn 2þ , Sn 2þ , Sr 2þ , Pb 2þ , Bi 2þ , Cu 2þ , Ge 2þ , Cd 2þ ) whose size is less than that of A and X can be oxide (O 2À ), halide (Cl À , I À , and Br À ), nitride (N 3À ), sulphide (S 2À ), hydride (H À ), selenide (Se 2À ), or teluride (Te 2À ) anions. [2][3][4] By permutating between ions at A and B sites, perovskites can be classified as allorganic, all-inorganic, or organic-inorganic materials depending on choice of cations in the ABX 3 crystal structure, as displayed in Figure 1. The least popular of the three categories is all-organic perovskites, where organic cations are placed at A and B sites. [5] A. Stoddart recently reported allorganic perovskite for ferroelectric applications by placing divalent organic cation (e.g., N-methyl-N 0 -diazabicyclo[2.2.2] octonium) at A site, ammonium ion (NH 4 þ ) at B site and halides at X-site. [6] All-organic perovskite can also be classified as metalfree perovskites since there are no metallic ions in their crystal structure. All-inorganic perovskites are formed by placing metallic cations at A and B sites. Examples include BaTiO 3 , [7] CsPbI 3 , [8] LaYS 3 , [9] and LaWN 3 . [10] By replacing X with borohydride (BH 4 À ) anionic radical, Schouwink et al. simulated group of stable AB(BH 4 ) 3 perovskites for ferroelectric applications. [11] Examples of allinorganic perovskites of this form include KCa(BH 4 ) 3 , CsCa(BH 4 ) 3 , and RbCa(BH 4 ) 3 . [11,12] Lastly, we have organicinorganic or hybrid perovskites where organic cations occupy A-site and inorganic cations occupy B site or vice versa. Examples of perovskites in this category include CH 3 NH 3 PbX 3 , (CH 3 NH 3 ) 3 Bi 2 X 9 , CH(NH 2 ) 2 PbX 3 where X-sites are mostly occupied by halides. [2,13,14] Perovskites can also be classified based on the type of anions at X-site. Thus, we can have oxide-, nitride-, hydride-, sulfide-, telluride-, selenide-, or halidebased perovskites depending on type of anions at X-site. In that sense, BaTiO 3 is an oxide-based, LaYS 3 is a sulfide-based, LaWN 3 is a nitride-based, KCa(BH 4 ) 3 and KMgH 3 are hydride-based while CH 3 NH 3 PbX 3 is an organic-inorganic metal halide perovskites, also sometimes called hybrid halide or organometallic halide perovskites. In some cases, more than one type of halides occupies X-site to form mixed halide perovskites, e.g., ABI x Cl 3Àx , ABI x Br 3Àx , or ABBr x Cl 3Àx , where A and B are the predefined cations. In other situations, oxide-halide, halide-nitride, halidesulfide, or other similar combinations occupy X-sites to form double anions perovskites. [15] SrFeO 3À0.05À0.10 F 0.05 and SrFe 0.9 Ti 0.1 O 3À0.05À0.10 F 0.05 are examples of double anions perovskites synthesized u...