197 research outputs found
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 NaMnF. 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 NaScF 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
octahedral units. We identify a change in orbital occupancy of the
manifold in the Jahn-Teller active Mn(III) to be responsible for the
transition, which stabilizes one monoclinic 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 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 octahedral rotations that
couple to static Ti--O Jahn-Teller distortions, through infrared-active phonon
modes of LaTiO and YTiO. The mode excitation reduces the rotational
angle, driving a magnetic phase transition from ferromagnetic (FM) to -type
antiferromagnetic (AFM), and finally a -type AFM state. This novel -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
()()I double perovskites using
centrosymmetric I perovskite iodides (where = Cs, Rb, K and =
Sn, Ge) as building units. In each superlattice, we investigate the effects of
three types of - and -site cation ordering schemes and three different
I 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 C/cm. Furthermore, we find
that a layered -site/rock salt -site ordering, in the presence of an
rotation pattern, produces a chiral "vortex-like" -site
displacement pattern. We then investigate the effect of epitaxial strain on one
of these systems, (CsRb)(SnGe)I, 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
Octahedral Engineering of Orbital Polarizations in Charge Transfer Oxides
Negative charge transfer O 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
--O-- bond angle away from 180. Using density functional theory
with the PBEsol approach, we describe a novel octahedral engineering avenue
to control the 3d and O orbital polarization through enhancement of
the O rotation "sense" rather than solely through conventional changes
to the --O bond lengths, \emph{i.e.} crystal field distortions. Using
CaFeO 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
Crystal structure stability and electronic properties of layered nickelate LaNiO
We investigate the crystal structure and the electronic properties of the
trilayer nickelate LaNiO by means of quantum mechanical
calculations in the framework of the density functional theory. We find that,
at low temperature, LaNiO undergoes a hitherto unreported
structural phase transition and transforms to a new monoclinic 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 140\,K metal-to-metal phase
transition
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 SrCaRuO exhibits a large
thermopower anisotropy ( 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 O 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 O octahedra -- are rarer in compositions
with a single, chemically unique -site. The presence of a CBD relies on
electronic instabilities of the -site cations, either orbital degeneracies
or valence-state fluctuations, and often appear concomitant with charge order
metal--insulator transitions or -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 SrFeO/SrTiO superlattices
Using {\it ab initio} density functional theory we explore the behavior of
thin layers of metallic SrFeO confined between the dielectric
SrTiO in a superlattice geometry. We find the presence of insulating
SrTiO spacer layers strongly affects the electronic properties of
SrFeO: For single SrFeO 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 GaMQ (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 in
describing the chalcogenide GaMQ 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 : 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) is unnecessary to produce a
semiconducting state in the distorted polar phase. Plus Hubbard
values ranging from 12 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 functionals for small values. The hybrid functional HSE06 is
accurate in predicting the lattice constants, but leads to a band gap of
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
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