Further adventures of the perovskite family
The well known perovskite structure consists of `B-site' nodes connected by `X-site' linkers into a cubic network with an `A-site' ion sitting at each interstice [Fig. 1(a)]. It is justly celebrated among crystallographers because it combines an extremely stable topology with a rather precarious geometry (Salje, 1989). In other words, although the connectivity just described is extremely common, its most symmetrical realization, the aristotype structure, is comparatively rare. Even the mineral perovskite itself, CaTiO3, is orthorhombic under ambient conditions, only transforming to the cubic phase (via a tetragonal intermediate) above 1635 K (Yashima & Ali, 2009).
This observation is more than simply a crystallographic curiosity, since it is precisely this instability that causes these materials to respond readily to external stimuli. This in turn gives functionality including dielectric, piezoelectric and ferroelectric behaviour (Bhalla et al., 2000), and oxide, proton and electronic conduction (Kreuer, 2003; Richter et al., 2009). Entire communities of scientists have grown up around each of these properties, working on exploiting these materials' rich behaviour for functional applications (Tilley, 2016).
Group-theoretical analysis has contributed significantly to understanding the possible distortions from the aristotype symmetry (Salje, 1976; Howard & Stokes, 1998, 2005). In fact, the perovskite family was an important historical motivation for developing this sort of analysis. Perovskites were a very early example of the work of Bärnighausen on the trees that now bear his name (Fig. 2). Even the very words `aristotype' and `hettotype' were coined by Megaw (1973) to describe the perovskite structure.
Importantly, the propensity to structural distortion is genuinely a property of the perovskite structure, rather than the mineral perovskite itself. It is straightforward to replace – either outright or as a solid solution – the ion at the A, B or X sites, producing a large family of materials with varying chemistry and, therefore, different active distortion modes. Thus the structural flexibility of this family of materials is accompanied by a corresponding compositional flexibility.
In particular, none of the A, B or X ions need be a single atom. It has been well known for at least fifty years that (poly)methylammonium ions can be inserted on the A site of halide-bridged perovskites (Weber, 1978a,b). At around the same time, materials with longer, bidentate X-site ions such as formate (HCO2−) were first reported (Sletten & Jensen, 1973), although these were not initially described as perovskites. Thus, while the term `organic–inorganic' or `hybrid' perovskites originally referred specifically to materials where the A-site ion was organic (Mitzi, 1999), it was soon extended to include organic X linkers such as cyanide, formate or dicyanamide (Li et al., 2017). These two developments naturally complement one another: as the X-site linker becomes longer, there is more space for a larger A-site cation [Figs. 1(b), 1(c)]. This opens very powerful new possibilities for crystal engineering: we can change not only the size of these ions, but also their shape, chemistry (e.g. replacing a metal by an organic ion) and physics (e.g. incorporating directional hydrogen bonding as well as isotropic Coulomb and van der Waals interactions).
Hybrid perovskites take after their inorganic counterparts in each of the ways described above. They undergo complex structural distortions, in several cases having incommensurately modulated phases (Fütterer et al., 1995; Canadillas-Delgado et al., 2019). They have important applications – most spectacularly in solar cells (Kojima et al., 2009), but also as ferroelectrics (Guo et al., 2010) and multiferroics (Jain et al., 2009). And theoretical analysis has revealed the structural possibilities that arise from the new degrees of freedom afforded by polyatomic linkers (Boström et al., 2016, 2018).
Both inorganic and hybrid perovskites are flexible in yet another sense: small variations in ionic radius or stoichiometry will often produce related structures that are informally considered under the wider perovskite banner. Common examples of such structures include the `hexagonal perovskites', where BX6 octahedra, rather than sharing vertices, form face-sharing columns (Lander, 1951; You et al., 2017); and layered structures such as the Dion–Jacobson, Ruddlesden–Popper and Aurivillius phases (Schaak & Mallouk, 2002; Saparov & Mitzi, 2016). As a result, the boundaries of the perovskite family are, appropriately, themselves rather flexible – even if we have not quite yet reached the stage of `radical perovskite anarchy' (Palgrave, 2019).
The discussion above of hybrid perovskites has conspicuously neglected the case of molecular ions on the B site. It turns out that this is no exception, accommodating polyatomic ions as readily as the A or X sites (Bremner et al., 2002). This produces yet another new sub-family of metal-free perovskites, some of which have impressive ferroelectric properties comparable to their inorganic counterparts (Ye et al., 2018).
In this issue, Budzianowski et al. (2022) present a new dabco-based, metal-free perovskite that for the first time has not the ammonium ion but the hydronium ion, H3O+, on the B site. Once again, incorporating new chemistry into the perovskite structure has enabled new crystallography to emerge.
Both polymorphs of the new material are intriguing in different ways. The α polymorph, with the true perovskite topology, crystallizes in the chiral space group P3221, which, although analogous to the ammonium analogue, is perhaps unexpected given these materials' simple, highly symmetrical components. The β polymorph is a polar, face-sharing hexagonal perovskite. Its structure is in effect a commensurate modulation of a non-polar structure with a cell three times smaller (and the authors go to some length to demonstrate that this simpler model does not adequately describe their data). The structure of both polymorphs is clearly influenced by the varying degrees of disorder of the molecular components, both hydronium and dabco ions.
This work will be important to at least three frontiers in materials chemistry and physics. First, the topic of metal-free perovskites is rapidly evolving into a subfield in its own right (Cui et al., 2022). Second, substituting polyatomic for monatomic ions is now acknowledged as a way of achieving novel degrees of freedom beyond the specific perovskite family (Boström & Goodwin, 2021). Third and more generally still, the study of structural disorder in molecular materials such as this one is an important frontier in crystallography. Such disorder may be a desideratum in its own right (Das et al., 2020) or even harnessed as a crystal engineering tool to direct the formation of particular structures.
After so many years of intense research, one might think that little remains to be discovered about the perovskite structure. But, as Budzianowski, Petřiček and Katrusiak have demonstrated, with a subtle change in chemistry, it retains its power to surprise crystallographers once again.
Bärnighausen, H. (1975). Acta Cryst. A31, S3. Google Scholar
Bhalla, A. S., Guo, R. & Roy, R. (2000). Mater. Res. Innovations, 4, 3–26. Web of Science CrossRef CAS Google Scholar
Boström, H. L. B. & Goodwin, A. L. (2021). Acc. Chem. Res. 54, 1288–1297. Web of Science PubMed Google Scholar
Boström, H. L. B., Hill, J. A. & Goodwin, A. L. (2016). Phys. Chem. Chem. Phys. 18, 31881–31894. PubMed Google Scholar
Boström, H. L. B., Senn, M. S. & Goodwin, A. L. (2018). Nat. Commun. 9, 2380. PubMed Google Scholar
Bremner, C. A., Simpson, M. & Harrison, W. T. A. (2002). J. Am. Chem. Soc. 124, 10960–10961. Web of Science CSD CrossRef PubMed CAS Google Scholar
Budzianowski, A., Petřiček, V. & Katrusiak, A. (2022). IUCrJ, 9, 544–550. CrossRef IUCr Journals Google Scholar
Canadillas-Delgado, L., Mazzuca, L., Fabelo, O., Rodriguez-Velamazan, J. A. & Rodriguez-Carvajal, J. (2019). IUCrJ, 6, 105–115. Web of Science CSD CrossRef CAS PubMed IUCr Journals Google Scholar
Cui, Q., Liu, S. F. & Zhao, K. (2022). J. Phys. Chem. Lett. pp. 5168–5178. CrossRef Google Scholar
Das, S., Mondal, A. & Reddy, C. M. (2020). Chem. Soc. Rev. 49, 8878–8896. CrossRef CAS PubMed Google Scholar
Fütterer, K., Depmeier, W. & Petříček, V. (1995). Acta Cryst. B51, 768–779. CSD CrossRef Web of Science IUCr Journals Google Scholar
Goldschmidt, V. M. (1927). Geochemische Verteilungsgesetze Der Elemente, Vol. 8, Untersuchungen üBer Bau und Eigenschaften von Krystallen. Oslo: Skrifter Norske Videskaps-Akad. Google Scholar
Guo, M., Cai, H.-L. & Xiong, R.-G. (2010). Inorg. Chem. Commun. 13, 1590–1598. CrossRef CAS Google Scholar
Howard, C. J. & Stokes, H. T. (1998). Acta Cryst. B54, 782–789. Web of Science CrossRef CAS IUCr Journals Google Scholar
Howard, C. J. & Stokes, H. T. (2005). Acta Cryst. A61, 93–111. Web of Science CrossRef CAS IUCr Journals Google Scholar
Hu, K.-L., Kurmoo, M., Wang, Z. & Gao, S. (2009). Chem. Eur. J. 15, 12050–12064. CrossRef PubMed CAS Google Scholar
Jain, P., Ramachandran, V., Clark, R. J., Zhou, H. D., Toby, B. H., Dalal, N. S., Kroto, H. W. & Cheetham, A. K. (2009). J. Am. Chem. Soc. 131, 13625–13627. Web of Science CSD CrossRef PubMed CAS Google Scholar
Kojima, A., Teshima, K., Shirai, Y. & Miyasaka, T. (2009). J. Am. Chem. Soc. 131, 6050–6051. Web of Science CrossRef PubMed CAS Google Scholar
Kreuer, K. (2003). Annu. Rev. Mater. Res. 33, 333–359. CrossRef CAS Google Scholar
Lander, J. J. (1951). Acta Cryst. 4, 148–156. CrossRef ICSD CAS IUCr Journals Web of Science Google Scholar
Li, W., Wang, Z., Deschler, F., Gao, S., Friend, R. H. & Cheetham, A. K. (2017). Nat. Rev. Mater. 2, 16099. CrossRef Google Scholar
Megaw, H. D. (1973). Crystal Structures: A Working Approach. Philadelphia: W. B. Saunders. Google Scholar
Mitzi, D. B. (1999). In Progress in Inorganic Chemistry, Vol. 48, pp. 1–121. Chichester: John Wiley and Sons. Google Scholar
Palgrave, R. G. (2019). Twitter, https://t.co/inB8wGYYUS. Google Scholar
Richter, J., Holtappels, P., Graule, T., Nakamura, T. & Gauckler, L. J. (2009). Monatsh. Chem. 140, 985–999. CrossRef CAS Google Scholar
Salje, E. (1976). Acta Cryst. A32, 233–238. CrossRef IUCr Journals Google Scholar
Salje, E. (1989). Philos. Trans. R. Soc. Lond. A, 328(1599), 409–416. Google Scholar
Saparov, B. & Mitzi, D. B. (2016). Chem. Rev. 116, 4558–4596. Web of Science CrossRef CAS PubMed Google Scholar
Schaak, R. E. & Mallouk, T. E. (2002). Chem. Mater. 14, 1455–1471. Web of Science CrossRef CAS Google Scholar
Sletten, E. & Jensen, L. H. (1973). Acta Cryst. B29, 1752–1756. CSD CrossRef CAS IUCr Journals Web of Science Google Scholar
Tilley, R. J. D. (2016). Perovskites: Structure-Property Relationships. Chichester: John Wiley and Sons. Google Scholar
Weber, D. (1978a). Z. Naturforsch. B, 33, 862–865. CrossRef Google Scholar
Weber, D. (1978b). Z. Naturforsch. B, 33, 1443–1445. CrossRef Google Scholar
Yashima, M. & Ali, R. (2009). Solid State Ionics, 180, 120–126. Web of Science CrossRef ICSD CAS Google Scholar
Ye, H.-Y., Tang, Y.-Y., Li, P.-F., Liao, W.-Q., Gao, J.-X., Hua, X.-N., Cai, H., Shi, P.-P., You, Y.-M. & Xiong, R.-G. (2018). Science, 361, 151–155. CrossRef CAS PubMed Google Scholar
You, Y.-M., Liao, W.-Q., Zhao, D., Ye, H.-Y., Zhang, Y., Zhou, Q., Niu, X., Wang, J., Li, P.-F., Fu, D.-W., Wang, Z., Gao, S., Yang, K., Liu, J.-M., Li, J., Yan, Y. & Xiong, R.-G. (2017). Science, 357, 306–309. CrossRef CAS PubMed Google Scholar
Zhang, W., Cai, Y., Xiong, R.-G., Yoshikawa, H. & Awaga, K. (2010). Angew. Chem. Int. Ed. 49, 6608–6610. Web of Science CSD CrossRef CAS Google Scholar
This is an open-access article distributed under the terms of the Creative Commons Attribution (CC-BY) Licence, which permits unrestricted use, distribution, and reproduction in any medium, provided the original authors and source are cited.