Polymorphous nature of cubic halide perovskites

Abstract

It has been long known that numerous halide and oxide perovskites can have non-ideal octahedra, showing tilting, rotation, and metal atom displacements. It has also been known that compounds that have at low temperatures a single structural motif ("monomorphous structures") could become disordered at higher temperatures, resulting in non-ideal octahedra as an entropy effect. What is shown here is that in many cubic halide perovskites and some oxides compounds a distribution of different low-symmetry octahedra ("polymorphous networks") emerge already from the minimization of the systems internal energy, i.e., they represent the intrinsic, preferred low temperature pattern of chemical bonding. Thermal disorder effects build up at elevated temperatures on top of such low temperature polymorphous networks. Compared with the monomorphous counterparts, the polymorphous networks have lower predicted total energies (enhanced stability), larger band gaps and dielectric constants now dominated by the ionic part, and agrees much more closely with the observed pair distribution functions. The nominal cubic perovskites (Pm-3m) structure deduced from X-Ray diffraction is actually a macroscopically averaged, high symmetry configuration, which should not be used to model electronic properties, given that the latter reflect a low symmetry local configuration.

The crystal structure and temperature dependence of the elpasolite Tl2LiYCl6

“…It is worth considering this work in the context of recent evidence on the polymorphous nature of some cubic perovskites (Zhao et al, 2020). Simply put, there is growing evidence that small unit cells (such as those presented in this work) do not precisely capture the atom positions.…”

Group Theory Analysis to Study Phase Transitions of Quasi-2D Sr3Hf2O7

“…Moreover, the Hf site may be studied using the Hf PAC probe. In addition, and as recently mentioned by Zao et al [ 56 ], a local description of oxides is fundamental in the understanding of their properties, since polymorphs should be present in a given sample. In this sense, hyperfine quantities are especially well suited to probe very local environments.…”

Bulk and Nanocrystalline Cesium Lead-Halide Perovskites as Seen by Halide Magnetic Resonance

“…In the following, we establish the halide magnetic resonance, both NMR and NQR, as versatile tools for studying CsPbX 3 in bulk microcrystalline forms ( Figure 2 b) 41 , 42 and for ligand-capped colloidal NCs ( Figure 2 b–d). 43 Such a comparative study allowed us to probe the effects of structural dynamics 44 , 45 and disorder, 46 as well as the surface defects and ligands. 47 , 48 The effects of structure dynamics on NMR and NQR spectra at various temperatures are herein corroborated via AIMD simulations.…”

“…Twinned orthorhombic subdomains were proposed based on X-ray scattering analysis. 71 An alternative description could invoke the polymorphous nature of perovskite materials 46 and, as already discussed above for CsPbCl 3 NCs, the radial gradient of octahedral tilt angles. The DFT-based structural optimization of a ∼4.6 nm CsPbBr 3 NC, identical to that performed for the CsPbCl 3 NC above, confirms that the structural disorder of the NC surface also occurs to a similar extent in CsPbBr 3 NCs (see Figure S10 ).…”