Polymorphous nature of cubic halide perovskites

Xin-Gang Zhao, Gustavo M. Dalpian, Zhi Wang, and Alex Zunger
Phys. Rev. B 101, 155137 – Published 27 April 2020

Abstract

Many common crystal structures can be described by a single (or very few) repeated structural motif (“monomorphous structures”) such as octahedron in cubic halide perovskites. Interestingly, recent accumulated evidence suggests that electronic structure calculations based on such macroscopically averaged monomorphous cubic (Pm3m) halide perovskites obtained from x-ray diffraction, show intriguing deviations from experiment. These include systematically too small band gaps, dielectric constants dominated by the electronic, negative mixing enthalpy of alloys, and significant deviations from the measured pair distribution function. We show here that a minimization of the systems T=0 internal energy via density functional theory reveals a distribution of different low-symmetry local motifs, including tilting, rotations, and B-atom displacements (“polymorphous networks”). This is found only if one allows for larger-than-minimal cell size that does not geometrically exclude low symmetry motifs. As the (super) cell size increases, the energy is lowered relative to the monomorphous cell, and stabilizes after ∼32 formula units (≥160 atoms) are included. Being a result of nonthermal energy minimization in the internal energy without entropy, this correlated set of displacements must represent the intrinsic geometry preferred by the underlying chemical bonding (lone pair bonding), and as such has a different origin than the normal, dynamic thermal disorder modeled by molecular dynamics. Indeed, the polymorphous network, not the monomorphous ansatz, is the kernel structure from which high temperature thermal agitation develops. The emerging physical picture is that the polymorphous network has an average structure with high symmetry, yet the local structural motifs have low symmetries. We find that, compared with monomorphous counterparts, the polymorphous networks have significantly lower predicted total energies, larger band gaps, and ionic dominated dielectric constants, and agree much more closely with the observed pair distribution functions. An analogous polymorphous situation is found in the paraelectric phase of a few cubic oxide perovskites where local polarization takes the role of local displacements in halide perovskites, and in the paramagnetic phases of a few 3d oxides where the local spin configuration takes that role.

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  • Received 17 November 2019
  • Revised 10 March 2020
  • Accepted 16 March 2020
  • Corrected 10 August 2020

DOI:https://doi.org/10.1103/PhysRevB.101.155137

©2020 American Physical Society

Physics Subject Headings (PhySH)

Condensed Matter, Materials & Applied Physics

Corrections

10 August 2020

Correction: The initial proof correction list inadvertently contained some omissions. Missing support information in the Acknowledgment section has been inserted. Minor fixes to wording throughout the text have been made.

Authors & Affiliations

Xin-Gang Zhao1, Gustavo M. Dalpian1,2, Zhi Wang1, and Alex Zunger1

  • 1Energy Institute, University of Colorado, Boulder, Colorado 80309, USA
  • 2Centro de Ciências Naturais e Humanas, Universidade Federal do ABC, 09210-580, Santo André, SP, Brazil

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Vol. 101, Iss. 15 — 15 April 2020

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