Octahedral Rotation Preferences in Perovskite Iodides and Bromides

Phase transitions in <i>ABX</i>₃ perovskites are often accompanied by rigid rotations of the corner-connected <i>BX</i>₆ octahedral network. Although the mechanisms for the preferred rotation patterns of perovskite oxides are fairly well recognized, the same cannot be said of halide variants (i.e., <i>X</i> = Cl, Br, or I), several of which undergo an unusual displacive transition to a tetragonal phase exhibiting in-phase rotations about one axis (<i>a</i>⁰<i>a</i>⁰<i>c</i>⁺ in Glazer notation). To discern the chemical factors stabilizing this unique phase, we investigated a series of 12 perovskite bromides and iodides using density functional theory calculations and compared them with similar oxides. We find that in-phase tilting provides a better arrangement of the larger bromide and iodide anions, which minimizes the electrostatic interactions, improves the bond valence of the <i>A</i>-site cations, and enhances the covalency between the <i>A</i>-site metal and Br^– or I^– ions. The opposite effect is present in the oxides, with out-of-phase tilting maximizing these factors

Atomic Scale Design of Polar Perovskite Oxides without Second-Order Jahn–Teller Ions

Demands for low-power and high-efficiency electronic devices have spurred an increased interest in new ferroelectric oxides, which display spontaneous electric polarizations. There are only a few mechanisms, however, capable of producing ordered dipoles in solid-state materials. Using first-principles density functional calculations, we extend the current repertoire and identify the required rotational patterns conducive to “geometric” ferroelectricity in (A,A′)­B₂O₆ perovskite oxides with A cation order along [001]-, [111]-, and [110]-directions. For the polar oxides, we show that electric polarizations arise through a geometric, “rotation-induced” mechanism and are greater than those induced by spin-driven mechanisms. We also discuss the energetics of each ordered arrangement and explain how competing centrosymmetric phases can lead to potential complications in thin-film growth of these materials. Finally, we generalize these results to a simple set of structural chemistry guidelines, which may be used to design other artificial oxides without inversion symmetry

Lithium Niobate-Type Oxides as Visible Light Photovoltaic Materials

Lithium Niobate-Type Oxides as Visible Light Photovoltaic Material

Ferroelectric Oxides with Strong Visible-Light Absorption from Charge Ordering

The applications of transition metal oxides as photovoltaic and photocatalytic materials are mainly impeded by their poor visible light absorption, low photogenerated carrier mobility, and low valence band position, which originate from the generally large band gap (≥3 eV), narrow transition metal <i>d</i> states, and deep oxygen 2<i>p</i> states. Here, we conceive a design strategy to realize small band gap polar oxides with high carrier mobilities by combining small radii <i>A</i> cations with Bi³⁺/Bi⁵⁺ charge disproportion. We show that these cation sizes and chemical features shift the valence band edge to higher energies and therefore reduce the band gap, promoting the formation of highly dispersive Bi 6<i>s</i> states near the Fermi level as a byproduct. By means of advanced many-electron-based first-principles calculations, we predict a new family of thermodynamically stable/metastable polar oxides <i>A</i>BiO₃ (<i>A</i> = Ca, Cd, Zn, and Mg), which adopt the Ni₃TeO₆-type (space group <i>R</i>3) structure and exhibit optical band gaps of ∼2.0 eV, as promising single phase photovoltaic and photocatalytic materials operating in the visible light spectrum

Structural Diversity from Anion Order in Heteroanionic Materials

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

Crystal-Chemistry Guidelines for Noncentrosymmetric A₂BO₄ Ruddlesden−Popper Oxides

Noncentrosymmetric (NCS) phases are seldom seen in layered A₂BO₄ Ruddlesden–Popper (214 RP) oxides. In this work, we uncover the underlying crystallographic symmetry restrictions that enforce the spatial parity operation of inversion and then subsequently show how to lift them to achieve NCS structures. Simple octahedral distortions alone, while impacting the electronic and magnetic properties, are insufficient. We show using group theory that the condensation of <i>two</i> distortion modes, which describe suitable symmetry unique octahedral distortions or a combination of a single octahedral distortion with a “compositional” A or B cation ordering mode, is able to transform the centrosymmetric aristotype into a NCS structure. With these symmetry guidelines, we formulate a data-driven model founded on Bayesian inference that allows us to rationally search for combinations of A- and B-site elements satisfying the inversion symmetry lifting criterion. We describe the general methodology and apply it to 214 iridates with A²⁺ cations, identifying RP-structured Ca₂IrO₄ as a potential NCS oxide, which we evaluate with density functional theory. We find a strong energetic competition between two closely related polar and nonpolar low-energy crystal structures in Ca₂IrO₄ and suggest pathways to stabilize the NCS structure

Role of Acentric Displacements on the Crystal Structure and Second-Harmonic Generating Properties of RbPbCO₃F and CsPbCO₃F

Two lead fluorocarbonates, RbPbCO₃F and CsPbCO₃F, were synthesized and characterized. The materials were synthesized through solvothermal and conventional solid-state techniques. RbPbCO₃F and CsPbCO₃F were structurally characterized by single-crystal X-ray diffraction and exhibit three-dimensional (3D) crystal structures consisting of corner-shared PbO₆F₂ polyhedra. For RbPbCO₃F, infrared and ultraviolet–visible spectroscopy and thermogravimetric and differential thermal analysis measurements were performed. RbPbCO₃F is a new noncentrosymmetric material and crystallizes in the <i>achiral</i> and <i>nonpolar</i> space group <i>P</i>6̅<i>m</i>2 (crystal class 6̅<i>m</i>2). Powder second-harmonic generation (SHG) measurements on RbPbCO₃F and CsPbCO₃F using 1064 nm radiation revealed an SHG efficiency of approximately 250 and 300 × α-SiO₂, respectively. Charge constants <i>d</i>₃₃ of approximately 72 and 94 pm/V were obtained for RbPbCO₃F and CsPbCO₃F, respectively, through converse piezoelectric measurements. Electronic structure calculations indicate that the nonlinear optical response originates from the distorted PbO₆F₂ polyhedra, because of the even–odd parity mixing of the O 2<i>p</i> states with the nearly spherically symmetric 6<i>s</i> electrons of Pb²⁺. The degree of inversion symmetry breaking is quantified using a mode-polarization vector analysis and is correlated with cation size mismatch, from which it is possible to deduce the acentric properties of 3D alkali-metal fluorocarbonates

Interplay of Cation Ordering and Ferroelectricity in Perovskite Tin Iodides: Designing a Polar Halide Perovskite for Photovoltaic Applications

Owing to its ideal semiconducting band gap and good carrier-transport properties, the fully inorganic perovskite CsSnI₃ has been proposed as a visible-light absorber for photovoltaic (PV) applications. However, compared to the organic–inorganic lead halide perovskite CH₃NH₃PbI₃, CsSnI₃ solar cells display very low energy conversion efficiency. In this work, we propose a potential route to improve the PV properties of CsSnI₃. Using first-principles calculations, we examine the crystal structures and electronic properties of CsSnI₃, including its structural polymorphs. Next, we purposefully order Cs and Rb cations on the A site to create the double perovskite (CsRb)­Sn₂I₆. We find that a stable ferroelectric polarization arises from the nontrivial coupling between polar displacements and octahedral rotations of the SnI₆ network. These ferroelectric double perovskites are predicted to have energy band gaps and carrier effective masses similar to those of CsSnI₃. More importantly, unlike nonpolar CsSnI₃, the electric polarization present in ferroelectric (CsRb)­Sn₂I₆ can effectively separate the photoexcited carriers, leading to novel ferroelectric PV materials with potentially enhanced energy conversion efficiency

Microscopic Origins of Optical Second Harmonic Generation in Noncentrosymmetric–Nonpolar Materials

We use a symmetry-based structural analysis combined with an electronic descriptor for bond covalency to explain the origin of the second-order nonlinear optical response (second harmonic generation, SHG) in noncentrosymmetric nonpolar ATeMoO₆ compounds (where A = Mg, Zn, or Cd). We show that the SHG response has a complex dependence on the asymmetric geometry of the AO₆ and AO₄ functional units and the orbital character at the valence band edge, which we are able to distinguish using an A–O bond covalency descriptor. The degree of covalency between the divalent A-site cation and the oxygen ligands dominates over the geometric contributions to the SHG arising from the acentric polyhedra, and this can be understood from considerations of the local static charge density distribution. The use of a local dipole model for the polyhedral moieties (AO₄/AO₆, MoO₄, and TeO₄) can account for a nonzero SHG response, even though the materials exhibit nonpolar structures; however, it is insufficient to explain the change in the magnitude of the SHG response upon A-cation substitution. The atomic scale and electronic structure understanding of the macroscopic SHG behavior is then used to identify hypothetical HgTeMoO₆ as a candidate telluromolybdate with an enhanced nonlinear optical response

RbMgCO₃F: A New Beryllium-Free Deep-Ultraviolet Nonlinear Optical Material

A new deep-ultraviolet nonlinear optical material, RbMgCO₃F, has been synthesized and characterized. The achiral nonpolar acentric material is second harmonic generation (SHG) active at both 1064 and 532 nm, with efficiencies of 160 × α-SiO₂ and 0.6 × β-BaB₂O₄, respectively, and exhibits a short UV cutoff, below 190 nm. RbMgCO₃F possesses a three-dimensional structure of corner-shared Mg­(CO₃)₂F₂ polyhedra. Unlike other acentric carbonate fluorides, in this example, the inclusion of Mg²⁺ creates pentagonal channels where the Rb⁺ resides. Our electronic structure calculations reveal that the denticity of the carbonate linkage, monodentate or bidendate, to the divalent cation is a useful parameter for tuning the transparency window and achieving the sizable SHG response