Microscopic origin of pressure-induced isosymmetric transitions in fluoromanganate cryolites

Using first-principles density functional theory calculations, we investigate the hydrostatic pressure-induced reorientation of the Mn--F Jahn-Teller bond axis in the fluoride cryolite Na$_3$MnF$_6$. We find a first-order isosymmetric transition occurs between crystallographically equivalent monoclinic structures at approximately 2.15 GPa, consistent with earlier experimental studies. Analogous calculations for isostructural $3d^0$ Na$_3$ScF$_6$ show no evidence of a transition up to 6.82 GPa. Mode crystallography analyses of the pressure-dependent structures in the vicinity of the transition reveals a clear evolution of the Jahn-Teller bond distortions in cooperation with an asymmetrical stretching of the equatorial fluorine atoms in the MnF$_6$ octahedral units. We identify a change in orbital occupancy of the $e_g$ manifold in the $3d^4$ Jahn-Teller active Mn(III) to be responsible for the transition, which stabilizes one monoclinic $P2_1/n$ variant over the other.Comment: 10 pages, 9 figure

Nonlinear Phononic Control and Emergent Magnetism in Mott Insulating Titanates

Optical control of structure-driven magnetic order offers a platform for magneto-optical terahertz devices. We control the magnetic phases of $d^1$ Mott insulating titanates using nonlinear phononics to transiently perturb the atomic structure based on density functional theory (DFT) simulations and solutions to a lattice Hamiltonian including nonlinear multi-mode interactions. We show that magnetism is tuned by indirect excitation of a Raman-active phonon mode, which affects the amplitude of the TiO$_6$ octahedral rotations that couple to static Ti--O Jahn-Teller distortions, through infrared-active phonon modes of LaTiO$_3$ and YTiO$_3$. The mode excitation reduces the rotational angle, driving a magnetic phase transition from ferromagnetic (FM) to $A$-type antiferromagnetic (AFM), and finally a $G$-type AFM state. This novel $A$-AFM state arises from a change in the exchange interactions and is absent in the bulk equilibrium phase diagram, but it emerges as a dynamically accessible optically induced state under multi-mode excitations. Our work shows nonlinear phononic coupling is able to stabilize phases inaccessible to static chemical pressure or epitaxial strain.Comment: 6 pages, 4 figure

Inducing Spontaneous Electric Polarizations in Double Perovskite Iodide Superlattices for New Ferroelectric Photovoltaic Materials

In this work, we use density functional theory calculations to demonstrate how spontaneous electric polarizations can be induced \textit{via} a hybrid improper ferroelectric mechanism in iodide perovskites, a family well-known to display solar-optimal band gaps, to create new materials for photoferroic applications. We first assemble three chemically distinct ($A$$A^{\prime}$)($B$$B^{\prime}$)I$_6$ double perovskites using centrosymmetric $AB$I$_3$ perovskite iodides (where $A$ = Cs, Rb, K and $B$ = Sn, Ge) as building units. In each superlattice, we investigate the effects of three types of $A$- and $B$-site cation ordering schemes and three different $B$I$_6$ octahedral rotation patterns. Out of these 27 combinations, we find that 15 produce polar space groups and display spontaneous electric polarizations ranging from 0.26 to 23.33 $\mu$C/cm$^2$. Furthermore, we find that a layered $A$-site/rock salt $B$-site ordering, in the presence of an $a^0a^0c^+$ rotation pattern, produces a chiral "vortex-like" $A$-site displacement pattern. We then investigate the effect of epitaxial strain on one of these systems, (CsRb)(SnGe)I$_6$, in layered and rock salt ordered configurations. In both phases, we find strong competition between the cation ordering schemes as well as an enhancement of the spontaneous polarization magnitude under tensile strain. Finally, using advanced functionals, we demonstrate that these compounds display low band gaps ranging from 0.2 to 1.3 eV. These results demonstrate that cation ordering and epitaxial strain are powerful ways to induce and control new functionalities in technologically-useful families of materials

Crystal structure stability and electronic properties of layered nickelate La4_4Ni3_3O10_{10}

We investigate the crystal structure and the electronic properties of the trilayer nickelate La$_4$Ni$_3$O$_{10}$ by means of quantum mechanical calculations in the framework of the density functional theory. We find that, at low temperature, La$_4$Ni$_3$O$_{10}$ undergoes a hitherto unreported structural phase transition and transforms to a new monoclinic $P2_1/a$ phase. This phase exhibits electronic properties in agreement with recent angle-resolved photoemission spectroscopy data reported in H.\ Li \emph{et al}., Nat.\ Commun.\ \textbf{8}, 704 (2017) and should be considered in models focused on explaining the observed $\sim$140\,K metal-to-metal phase transition

Octahedral Engineering of Orbital Polarizations in Charge Transfer Oxides

Negative charge transfer $AB$O$_3$ oxides may undergo electronic metal--insulator transitions (MIT) concomitant with a dilation and contraction of nearly rigid octahedra. On both sides of the MIT are in-phase or out-of-phase (or both) rotations of adjacent octahedra that buckle the $B$--O--$B$ bond angle away from 180$^\circ$. Using density functional theory with the PBEsol$+U$ approach, we describe a novel octahedral engineering avenue to control the $B$ 3d and O $2p$ orbital polarization through enhancement of the $B$O$_6$ rotation "sense" rather than solely through conventional changes to the $B$--O bond lengths, \emph{i.e.} crystal field distortions. Using CaFeO$_3$ as a prototypical material, we show the flavor of the octahedral rotation pattern when combined with strain--rotation coupling and thin film engineering strategies offers a promising avenue to fine tune orbital polarizations near electronic phase boundaries.Comment: 6 pages, 5 figure

Designing a robustly metallic noncenstrosymmetric ruthenate oxide with large thermopower anisotropy

The existence of approximately 30 noncentrosymmetric metals (NCSM) suggests a contraindication between crystal structures without inversion symmetry and metallic behavior. Those containing oxygen are especially scarce. Here we propose and demonstrate a design framework to remedy this property disparity and accelerate NCSM-oxide discovery: The primary ingredient relies on the removal of inversion symmetry through displacements of atoms whose electronic degrees of freedom are decoupled from the states at the Fermi level. Density functional theory calculations validate this crystal--chemistry strategy, and we predict a new polar ruthenate exhibiting robust metallicity. We demonstrate that the electronic structure is unaffected by the inclusion of spin-orbit interactions (SOI), and that cation ordered SrCaRu$_2$O$_6$ exhibits a large thermopower anisotropy ($\left|\Delta\mathcal{S}_\perp\right|\sim6.3 \mu \textrm{V K}^{-1}$ at 300 K) derived from its polar structure. Our findings provide chemical and structural selection guidelines to aid in the search of new NCS metals with enhanced thermopower anisotropy.Comment: Revised manuscript close to published version. Supplemental information available upon reques

Interplay of octahedral rotations and breathing distortions in charge ordering perovskite oxides

We investigate the structure--property relationships in $AB$O$_3$ perovskites exhibiting octahedral rotations and cooperative octahedral breathing distortions (CBD) using group theoretical methods. Rotations of octahedra are ubiquitous in the perovskite family, while the appearance of breathing distortions -- oxygen displacement patterns that lead to approximately uniform dilation and contraction of the $B$O$_6$ octahedra -- are rarer in compositions with a single, chemically unique $B$-site. The presence of a CBD relies on electronic instabilities of the $B$-site cations, either orbital degeneracies or valence-state fluctuations, and often appear concomitant with charge order metal--insulator transitions or $B$-site cation ordering. We enumerate the structural variants obtained from rotational and breathing lattice modes and formulate a general Landau functional describing their interaction. We use this information and combine it with statistical correlation techniques to evaluate the role of atomic scale distortions on the critical temperatures in representative charge ordering nickelate and bismuthate perovskites. Our results provide new microscopic insights into the underlying structure--property interactions across electronic and magnetic phase boundaries, suggesting plausible routes to tailor the behavior of functional oxides by design.Comment: 14 pages, 8 figure

Electron-lattice instabilities suppress cuprate-like electronic structures in SrFeO3_3/SrTiO3_3 superlattices

Using {\it ab initio} density functional theory we explore the behavior of thin layers of metallic $d^4$ SrFeO$_3$ confined between the $d^0$ dielectric SrTiO$_3$ in a superlattice geometry. We find the presence of insulating SrTiO$_3$ spacer layers strongly affects the electronic properties of SrFeO$_3$: For single SrFeO$_3$ layers constrained to their bulk cubic structure, the Fermi surface is two-dimensional, nested and resembles the hole-doped superconducting cuprates. A Jahn-Teller instability couples to an octahedral tilt mode, however, to remove this degeneracy resulting in insulating superlattices.Comment: 4 pages, 4 figure

Structure and properties of functional oxide thin films: Insights from electronic-structure calculations

The confluence of state-of-the-art electronic-structure computations and modern synthetic materials growth techniques is proving indispensable in the search for and discovery of new functionalities in oxide thin films and heterostructures. Here, we review the recent contributions of electronic-structure calculations to predicting, understanding, and discovering new materials physics in thin-film perovskite oxides. We show that such calculations can accurately predict both structure and properties in advance of film synthesis, thereby guiding the search for materials combinations with specific targeted functionalities. In addition, because they can isolate and decouple the effects of various parameters which unavoidably occur simultaneously in an experiment -- such as epitaxial strain, interfacial chemistry and defect profiles -- they are able to provide new fundamental knowledge about the underlying physics. We conclude by outlining the limitations of current computational techniques, as well as some important open questions that we hope will motivate further methodological developments in the field

Assessing exchange-correlation functional performance in the chalcogenide lacunar spinels GaM4_4Q8_8 (M = Mo, V, Nb, Ta; Q = S, Se)

We perform systematic density functional theory (DFT) calculations to assess the performance of various exchange-correlation potentials $V_{xc}$ in describing the chalcogenide GaM$_4$Q$_8$ lacunar spinels (M=Mo, V, Nb, Ta; Q=S, Se). We examine the dependency of crystal structure (in cubic and rhombohedral symmetries), electronic structure, magnetism, optical conductivity, and lattice dynamics in lacunar spinels at four different levels of $V_{xc}$: the local density approximation (LDA), generalized gradient approximation (GGA), meta-GGA, and hybrid with fractional Fock exchange. We find that LDA significantly underperforms GGA and higher level functionals in predicting lattice constants. LDA also fails to properly describe the magnetism in the family, and for some compositions it does not find the ground state distorted crystal structure. The Perdew-Burke-Ernzerhof (PBE) and PBE revised for solids (PBEsol) GGA functionals perform reasonably well in predicting lattice constants as well as the electronic structures. We find that the GGA with an on-site Coulomb interaction (GGA$+U$) is unnecessary to produce a semiconducting state in the distorted polar $R3m$ phase. Plus Hubbard $U$ values ranging from 1$\sim$2 eV, however, improve the quantitative performance of the GGA functionals. The meta-GGA functional SCAN predicts reasonable lattice constants and electronic structures; it exhibits behavior similar to the GGA$+U$ functionals for small $U$ values. The hybrid functional HSE06 is accurate in predicting the lattice constants, but leads to a band gap of $\approx$1 eV in the rhombohedral phase, which is higher than the experimental estimation of 0.2 eV. Our findings suggest that accurate qualitative and quantitative simulations of the lacunar spinel family with DFT requires careful attention to the nuances of the exchange-correlation functional and considered spin structures.Comment: 16 Pages, 13 Figures, 5 Table