Structural Diversity from Anion Order in Heteroanionic Materials

Abstract

Heteroanionic materials leverage the advantages offered by two different anions coordinating the same or different cations to realize unanticipated or enhanced electronic, optical, and magnetic responses. Beyond chemical variations offered by the anions, the ability to control the anion order present within a single transition metal polyhedron via anion-sublattice engineering offers a potentially transformative strategy in tuning material properties. The set of design rules for realizing and controlling anion order, however, are incomplete, which is due in part to the limited anion-ordered diversity in known structures. This aspect makes formulating such principles from experiment alone challenging. Here, we demonstrate how computational methods at multiple levels of theory are effective at investigating the anion site order dependencies in heteroanionic materials, HAMs, and enable the construction of crystal-chemistry principles. Our approach relies on a database of anion ordered structure variants in which we manipulate the lattice degrees of freedom through the incorporation of structural distortions. Structure–property relationships and anion-order descriptors are data mined from group theoretical techniques and density functional theory calculations. Using our combined computational scheme, we uncover a hybrid improper mechanism to stabilize polar phases, propose the chemical link between local and long ranger anion order, and detail the sequence of order–disorder/displacive transitions observed experimentally in the oxyfluoride Na3MoO3F3. Our method is scalable and transferable to many heteroanionic chemistries and crystal families, facilitating the construction of heteroanionic materials design principles.

Symmetry and the Role of the Anion Sublattice in Aurivillius Oxyfluoride Bi2TiO4F2

“…Nevertheless, by using penalties for both cation and anion sites, most structures refined to give Ti–O bonds shorter than Ti–F bonds. This is consistent with structures reported for NdNiO 2 F, 75 α- A 3 MoO 3 F 3 ( A = Rb and K), 1 , 4 and other oxyfluorides. 19 For both Γ 3 – and M 3 – apical ordering, the GII results in Table 2 and Figure 7 suggest that, in general, the fac structures are less strained than the equivalent mer structures, which is again consistent with the importance of pπ dπ bonding between oxygen and d 0 cations, favoring cis and fac isomers with shorter Ti–O bonds.…”

Engineering of Ferroic Orders in Thin Films by Anionic Substitution

“…Although the cis ‐type coordination maximizes the overlap between the nitrogen 2p and transition metal d orbitals and it is energetically preferred over the trans ‐type configuration, [ 19–21 ] most bulk oxynitrides do not show long‐range anion order. The coexistence of both cis‐ and trans ‐type structures provides a new dimensionality that can be used to engineer multiferroism.…”

Predicting stable lithium iron oxysulphides for battery cathodes

“…In the meantime, anionic substitution, or mixed anion materials have been largely unexplored until recently 15 . With different sizes, valence charge, electronegativity and polarizability of the anions, hetero-anionic materials are known to form layered structures due to preferential cation-anion bonding 16,17 .…”

“…Computational materials discovery, and in particular, high-throughput screening studies, have previously been applied to many fields including battery cathodes 27,28 . Those studies, however, rely heavily on existing experimental data, as hypothetical materials are constructed from simple substitutions of the known materials 17,29,30 . For lithium iron oxysulphides, which remain largely unexplored so far, the available experimental data are scarce.…”