Breathing-mode Distortions Research Articles

Sr-Doping-Modulated Metal-Insulator Transition in NdNiO3 Epitaxial Films

Abstract Perovskite-structured nickelates, ReNiO3 (Re = rare earth), have long garnered significant research interest due to their sharp and highly tunable metal-insulator transitions (MITs). Doping the parent compound ReNiO3 with alkaline earth metal can substantially suppress this MIT. Recently, intriguing superconductivity has been discovered in doped infinite-layer nickelates (ReNiO2), while the mechanism behind A-site doping-suppressed MIT in the parent compound ReNiO3 remains unclear. To address this problem, we grew a series of Nd1−x Sr x NiO3 (NSNO, x = 0–0.2) thin films and conducted systematic electrical transport measurements. Our resistivity and Hall measurements suggest that Sr-induced excessive holes are not the primary reason for MIT suppression. Instead, first-principles calculations indicate that Sr cations, with larger ionic radius, suppress breathing mode distortions and promote charge transfer between oxygen and Ni cations. This process weakens Ni–O bond disproportionation and Ni2+/Ni4+ charge disproportionation. Such significant modulations in lattice and electronic structures convert the ground state from a charge-disproportionated antiferromagnetic insulator to a paramagnetic metal, thereby suppressing the MIT. This scenario is further supported by the weakened MIT observed in the tensile-strained NSNO/SrTiO3(001) films. Our work reveals the A-side doping-modulated electrical transport of perovskite nickelate films, providing deeper insights into novel electric phases in these strongly correlated nickelate systems.

Onsite and intersite electronic correlations in the Hubbard model for halide perovskites

Halide perovskites (HPs) are widely viewed as promising photovoltaic and light-emitting materials for their suitable band gaps in the visible spectrum. Density functional theory (DFT) calculations employing (semi)local exchange-correlation functionals usually underestimate the band gaps for these systems. Accurate descriptions of the electronic structures of HPs often demand higher-order levels of theory such as the Heyd-Scuseria-Ernzerhof (HSE) hybrid density functional and GW approximations that are much more computationally expensive than standard DFT. Here, we investigate three representative types of HPs, ABX3 halide perovskites, vacancy-ordered double perovskites (VODPs), and bond disproportionated halide perovskites (BDHPs), using DFT+U+V with onsite U and intersite V Hubbard parameters computed self-consistently without a priori assumption. The inclusion of Hubbard corrections improves the band gap prediction accuracy for all three types of HPs to a similar level of advanced methods. Moreover, the self-consistent Hubbard U is a meaningful indicator of the true local charge state of multivalence metal atoms in HPs. The inclusion of the intersite Hubbard V is crucial to properly capture the hybridization between valence electrons on neighboring atoms in BDHPs that have breathing-mode distortions of halide octahedra. In particular, the simultaneous convergence of both Hubbard parameters and crystal geometry enables a band gap prediction accuracy superior to HSE for BDHPs but at a fraction of the cost. Our work highlights the importance of using self-consistent Huabbard parameters when dealing with HPs that often possess intricate competitions between onsite localization and intersite hybridization.

Influence of the triangular Mn-O breathing mode on magnetic ordering in multiferroic hexagonal manganites

We use a combination of symmetry analysis, phenomenological modeling, and first-principles density functional theory to explore the interplay between the magnetic ground state and the detailed atomic structure in the hexagonal rare-earth manganites. We find that the magnetic ordering is sensitive to a breathing mode distortion of the Mn and O ions in the $ab$ plane, which is described by the ${\mathrm{K}}_{1}$ mode of the high-symmetry structure. Our density functional calculations of the magnetic interactions indicate that this mode particularly affects the single-ion anisotropy and the interplanar symmetric exchanges. By extracting the parameters of a magnetic model Hamiltonian from our first-principles results, we develop a phase diagram to describe the magnetic structure as a function of the anisotropy and exchange interactions. This in turn allows us to explain the dependence of the magnetic ground state on the identity of the rare-earth ion and on the ${\mathrm{K}}_{1}$ mode.

Fermi surface nesting and the Lindhard response function in the kagome superconductor CsV3Sb5

The recently discovered kagome net compounds AV3Sb5 (A = K, Rb, and Cs) become superconducting on cooling, in addition to displaying interesting topological features in the electronic structure. They also exhibit charge density wave ordering, which manifests as a breathing-mode distortion in the kagome layers. It has been suggested that such ordering derives from nesting between saddle points on the Fermi surface. In aid of the evolving understanding of this intriguing materials class, we present calculations of Fermi surface nesting and Lindhard susceptibility of CsV3Sb5. The breathing mode distortions appear to not display a simple link with Fermi surface nesting (FSN) and do not display the signatures of a Peierls-like transition. The FSN is agnostic to changes along kz and is only mildly impacted by small shifts of the Fermi level. The results suggest that FSN is largely independent of specific features in the saddle point.

Metal-insulator transition and local-moment collapse in negative charge transfer CaFeO3 under pressure

We compute the electronic structure, spin and charge state of Fe ions, and the structural phase stability of paramagnetic ${\mathrm{CaFeO}}_{3}$ under pressure using a fully self-consistent in charge density $\mathrm{DFT}+$ dynamical mean-field theory method. We show that at ambient pressure ${\mathrm{CaFeO}}_{3}$ is a negative charge transfer insulator characterized by strong localization of the Fe $3d$ electrons. It crystallizes in the monoclinic $P{2}_{1}/n$ crystal structure with a cooperative breathing mode distortion of the lattice. While the Fe $3d$ Wannier occupations and local moments are consistent with robust charge disproportionation of Fe ions in the insulating $P{2}_{1}/n$ phase, the physical charge density difference around the structurally distinct Fe $A$ and Fe $B$ ions with the ``contracted'' and ``expanded'' oxygen octahedra, respectively, is rather weak, of $\ensuremath{\sim}0.04$. This implies the importance of the Fe $3d$ and O $2p$ negative charge transfer and supports the formation of a bond-disproportionated state characterized by the Fe $A 3{d}^{5\ensuremath{-}\ensuremath{\delta}}{\underline{L}}^{2\ensuremath{-}\ensuremath{\delta}}$ and Fe $B 3{d}^{5}$ valence configurations with $\ensuremath{\delta}\ensuremath{\ll}1$, in agreement with strong hybridization between the Fe $3d$ and O $2p$ states. This complex interplay between electronic correlations, strong covalency, and lattice effects, resulting in bond disproportionation, is in many ways reminiscent of the behavior of rare-earth nickelates, $R{\mathrm{NiO}}_{3}$ $(R=\text{rare earth})$. Upon compression, ${\mathrm{CaFeO}}_{3}$ undergoes the metal-to-insulator phase transition (MIT) which is accompanied by a structural transformation into the orthorhombic $Pbnm$ phase. The phase transition is accompanied by suppression of the cooperative breathing mode distortion of the lattice and, hence, results in the melting of bond disproportionation of the Fe ions. Our analysis suggests that the MIT transition is associated with orbital-dependent delocalization of the Fe $3d$ electrons and leads to a remarkable collapse of the local magnetic moments. Our results imply the crucial importance of the interplay of electronic correlations and structural effects to explain the properties of ${\mathrm{CaFeO}}_{3}$.

Interplay between breathing-mode distortions and magnetic order in rare-earth nickelates from ab initio magnetic models

We use density-functional theory calculations to explore the magnetic properties of perovskite rare-earth nickelates, $\mathcal{R}$NiO$_3$, by constructing microscopic magnetic models containing all relevant exchange interactions via Wannierization and Green's function techniques. These models elucidate the mechanism behind the formation of antiferromagnetic order with the experimentally observed propagation vector, and explain the reason previous DFT plus Hubbard $U$ calculations favored ferromagnetic order. We perform calculations of magnetic moments and exchange-coupling parameters for different amplitudes of the $R_1^+$ breathing mode distortion, which results in expanded and compressed NiO$_6$ octahedra. We find that the magnetic moment vanishes for the "short bond" nickels, i.e., the ones in the compressed octahedra. The inclusion of spin-orbit coupling demonstrates that the magnetic anisotropy is very small, while the magnetic moment of the short bond nickel atoms tend to zero even for the noncollinear case. Our results provide a clear picture of the trends of the magnetic order across the nickelate series and give insights into the coupling between magnetic order and structural distortions.

Electronic and optical properties of Y-doped $$\hbox {BaBiO}_{3}$$

We investigate the nature of bond disproportionation in $$\hbox {BaBi}_{0.9}\hbox {Y}_{0.1}\hbox {O}_{3}$$ (BYBO) and its manifestation in the valence band and the optical absorption spectra. Our room-temperature (RT) structural results show that both the parent and the doped (BYBO) compounds stabilise in monoclinic structure but with a change in space group from I2/m to P21/n on Y doping. The Raman scattering studies suggest decrement in the breathing mode distortion of $$\hbox {BiO}_{6}$$ octahedra. The valence band spectra show an increase in the gap and close to the Fermi edge, we observe significant modifications in the fine structures on Y doping. These were studied using band structure calculations under TB-mBJ. The results show that the electronic states of the Y ions play significant role in driving the electronic structure. We also observe the importance of O 2p holes and also transfer of electrons from O 2p to Bi1 6s, Bi2 6s, and Y states to bring about bond disproportionation in the doped compound. The intensity variations and the features in the optical absorption spectra were understood based on the E vs k point calculations performed in the irreducible part of the Brillouin zone. The behaviour of the gap on doping is also in line with the calculations and optical absorption studies. The behaviour of the optical absorption spectra suggests possible use in solar cell and optical sensor applications. A composite of the doped compound with $$\hbox {BaTiO}_{3}$$ may find applications in the field of high-energy density capacitor. We believe that our results will be helpful in understanding the role of the electronic states of the dopants in superconductivity, especially in comparison with the monoclinic phase exhibited in the superconducting phase of $$\hbox {BaPb}_{0.75}\hbox {Bi}_{0.25}\hbox {O}_{3}$$ .

Effect of electron-lattice coupling on charge and magnetic order in rare-earth nickelates

We investigate the impact of electron-lattice coupling on the stability of various magnetic orders in rare-earth nickelates. We use the Hartree-Fock approximation, at zero temperature, to study an effective, two-band model with correlations characterized by a Hubbard $U$ and a Hund's $J$. This is coupled to breathing-mode distortions of the octahedral oxygen cages, described semi-classically, with a Holstein term. We analyze the effect of the various parameters on the resulting phase diagram, in particular on the charge disproportionation and on the magnetic order. We confirm that the coupling to the lattice cooperates with Hund's coupling and thus encourages charge disproportionation. We also find that it favors the fully disproportionated, 4-site periodic magnetic order of type $\Uparrow 0 \Downarrow 0$. Other convergent magnetic phases, such as the collinear $\uparrow\uparrow\downarrow\downarrow$ and non-collinear $\uparrow\rightarrow\downarrow\leftarrow$ states, do not couple to the lattice because of their lack of charge disproportionation. Novel phases, e.g. with charge disproportionation but no magnetic order, are also found to be stabilized in specific conditions.

Strain control of electronic phase in rare-earth nickelates

We use density functional plus $U$ methods to study the effects of a tensile or compressive substrate strain on the charge-ordered insulating phase of ${\mathrm{LuNiO}}_{3}$. The numerical results are analyzed in terms of a Landau energy function, with octahedral rotational distortions of the perovskite structure included as a perturbation. Less than $4%$ tensile or compressive strain leads to a first-order transition from an insulating structure with large-amplitude breathing-mode distortions of the ${\mathrm{NiO}}_{6}$ octahedra to a metallic state in which breathing-mode distortions are absent but Jahn-Teller distortions in which two Ni-O bonds become long and the other four become short are present. Compressive strain produces uniform Jahn-Teller order with the long axis aligned perpendicular to the substrate plane, while tensile strain produces a staggered Jahn-Teller order in which the long bond lies in the plane and alternates between two nearly orthogonal in-plane directions, forming a checkerboard pattern. In the absence of the breathing-mode distortions and octahedral rotations, the tensile-strain-induced transition to the staggered Jahn-Teller state would be of second order.

Crystal and magnetic structure of single-crystal La1-x Sr x MnO 3 (x ≈ 1/8)

A neutron powder diffraction (NPD) study on the crystal and magnetic structure of a crushed La1-xSrxMnO3 (x ≈ 1/8) single crystal has been performed. The sample belongs to orthorhombic (Pnma, O) above the Jahn-Teller (JT) transition temperature (TJT) and monoclinic (P121/c1, M′) in the JT regime. We have also refined the NPD data below the charge/orbital ordering (CO/OO) temperature (TCO/OO) with a monoclinic (P121/c1, M′′) model because the experimental resolution was insufficient to clearly identify a triclinic $(P\bar {1})$ structure. The refined lattice parameters show an obvious breathing-mode distortion between TCO/OO and TJT, accompanied by a large deviation of the monoclinic angle β from 90°, signifying a very strong cooperative JT distortion. A ferromagnetic (FM) moment of 3.43(5)μ B/Mn besides an A-type antiferromagnetic (A-AFM) moment of 0.54(2) μ B/Mn is directed mainly along the b axis in P121/c1 symmetry at 5 K. With increasing temperature, the A-AFM domains transform into FM ones above ~100 K and the FM spin orientation turns from the b to the c axis in crystallographic b-c plane below Tc = 187(1) K. The magnetization measurements show typical anomalies around TCO/OO and TJT. The measured saturation moment of 3.9(1)μ B/Mn at 70 kOe and 5 K is well consistent with the sum 3.97(5)μ B/Mn of the refined FM and A-AFM moments at 5 K, implying the A-AFM spins are aligned in field direction at 70 kOe. The applied magnetic field can affect the paramagnetic insulating (PMI) state in the range of magnetic polarons. Based on the size of JT distortion and the bond-valence sums (BVS’s), the CO/OO phenomenon is being discussed.

Evolution of the spin state of a 3d ion in a pyramidal complex

The evolution of the spin state of a metallic 3d ion in a pyramidal MeO5 environment is studied. Ions of metals with 3d6 (Fe2+,Co3+) and 3d4 configurations (Mn3+,Cr2+) are studied. The change in the spin state of the metal ion from S=0 to S=1 and S=2 with a variation of the effective charge Zeff of the metal ion and the displacements of the oxygen ions is investigated. The stability of the spin state as a function of the magnitude and symmetry of the displacements of the ligands is checked. Breathing-mode distortions, displacements of the 3d ion along the Z axis, Jahn-Teller distortions of the basal plane of the pyramid, and fluting distortions of the plane of the pyramid are studied. It is shown that different types of distortions produce different behaviors of the terms corresponding to a change in the spin states of all three types. It is found that the spin state of the coordination complex MeO5 with the 3d4 configuration of the metal ion is most sensitive to breathing-mode distortions and Jahn-Teller type distortions stabilize the state with spin S=1 for the 3d6 configuration.

Manganites at quarter filling: Role of Jahn-Teller interactions

We have analyzed different correlation functions in a realistic spin-orbital model for half-doped manganites. Using a finite-temperature diagonalization technique the CE phase was found in the charge-ordered phase in the case of small antiferromagnetic interactions between ${t}_{2g}$ electrons. It is shown that the ingredient responsible for stabilization of the CE-type spin and orbital-ordered state is the cooperative Jahn-Teller (JT) interaction between next-nearest ${\mathrm{Mn}}^{+3}$ neighbors mediated by the breathing mode distortion of ${\mathrm{Mn}}^{+4}$ octahedra and displacements of ${\mathrm{Mn}}^{+4}$ ions. The topological phase factor in the Mn--Mn hopping leading to gap formation in one-dimensional models for the CE phase as well as the nearest neighbor JT coupling are not able to produce the zigzag chains typical for the CE phase in our model.

EXAFS observation of two distinct Bi–O distances below Tc for a Ba 0.6K 0.4BiO 3 single crystal

In order to find two distinct Bi–O distances as evidence for the breathing mode distortion due to an electron–phonon interaction below T c, extended X-ray absorption fine structure of Bi L 3-edge is measured from 14 to 300 K for a high quality single-crystal Ba 0.6K 0.4BiO 3. The two distinct Bi–O bond lengths are obtained from data analyzed by a two-shell fit, and the Bi–O distances are similar to that in BaBiO 3, independent of temperature. They come from a remnant phase of BaBiO 3. Bi–O peaks coming from the superconducting phase are not analyzed because the Bi–O peaks overlap. Further, the Debye–Waller factor (σ 2) for Bi–O bonds does not fit Einstein model with increasing temperature.

Structural and transport properties of the BaBi 1−xIn xO 3 system

The structures of In-doped perovskites BaBi 1−xIn xO 3, 0 ≤ x ≤ 0.5, have been investigated by powder X-ray and neutron diffraction. Two phase transitions were observed. As the In concentration is increased, the parent monoclinic BaBiO 3 first loses the breathing-mode distortions (x ≈ 0.1) and then changes the direction of octahedral tilts (x ≈ 0.4), resulting in the orthorhombic ( Ibmm) and tetragonal ( I4/ mcm) symmetry, respectively, with random arrangement of Bi(III) and Bi(V) ions. The transport measurements show an increase of conductivity with increasing indium concentration, but the system remains semiconducting.

The Breathing-Mode Distortion around Transition-Metal Defects in Semiconductors

The breathing-type distortion around transition-metal ions incorporated into semiconductors is determined in the approximation where the ligands are treated as point charges. It is shown how the crystal-field parameter can be found along with the value of the breathing-mode distortion energy from zero-phonon line measurements. The change of the transition-metal–ligand distance with respect to the equilibrium distance of the perfect crystal is also calculated.

Lattice effects and magnetic order in the canted ferromagnetic insulator La0.875>Sr0.125MnO 3+ delta.

The structural, magnetic, and transport behavior of orthorhombic, insulating La{sub 0.875}Sr{sub 0.125}MnO{sub 3+{delta}} were studied as a function of temperature from 20 to 300 K. Upon cooling from room temperature, we observe the rapid development of a large breathing-mode distortion coincident with large positive and negative thermal expansions for the {ital c} and {ital b} axes, respectively. The onset of canted ferromagnetic ordering at 220 K is correlated with the relaxation of this distortion, which reaches values similar to those at room temperature when the magnetic ordering saturates. We observe no first-order-type discontinuity in the volume at the ferromagnetic transition. These results demonstrate a strong coupling between magnetic ordering and crystal structure even though there is no metal-insulator transition associated with the magnetic transition. {copyright} {ital 1996 The American Physical Society.}

Cooperative ordering of holes and spins in La2NiO4.125.

${\mathrm{La}}_{2}$${\mathrm{NiO}}_{4.125}$ exhibits a cooperative ordering of dopant-induced holes and Ni spins below a transition temperature of 110 K. There is also an ordering of interstitial oxygens that occurs near room temperature. We present a comprehensive analysis of experimentally observed superlattice intensities in terms of structural models, justifying previous identifications. The model for the interstitial order involves a 3a\ifmmode\times\else\texttimes\fi{}5b\ifmmode\times\else\texttimes\fi{}5c unit cell with an ideal interstitial density of \ensuremath{\delta}=2/15 per formula unit. In the spin and charge ordered state, the magnetic moments are sinusoidally modulated within the ${\mathrm{NiO}}_{2}$ planes. The moments point transverse to the modulation direction, with nearest neighbors antiparallel, and have a maximum amplitude that is g80% of that observed in undoped ${\mathrm{La}}_{2}$${\mathrm{NiO}}_{4}$. A corresponding modulation of atomic positions within ${\mathrm{NiO}}_{2}$ planes involves breathing-mode distortions consistent with a modulation of the charge density. The results are compared with theoretical models and with experimental observations on related systems.

First-principles study of the atomic structure and local vibrational modes of the DX center in GaAs under pressure.

We investigate the structural and dynamical properties of the DX center in Si-doped GaAs under pressure through self-consistent ab initio pseudopotential calculations. The negatively charged broken-bond configuration for the DX center is found to be the ground state against the shallow-donor level at pressures above 24 kbar, in good agreement with measured values. This configuration is also found to be more stable than a tetrahedrally symmetric breathing-mode distortion of the donor neighboring atoms, which induces a localized level. The calculated local vibrational frequencies for both the shallow donors and deep DX centers are generally underestimated compared with experimental results, while their pressure dependences are in good agreement with experiments. However, the pressure coefficient of the donor vibrating mode in the breathing-mode distortion is found to be about twice as large as measured values.

Structural phase diagram of the Ba1-xKxBiO3 system.

The structures of five phases in the ${\mathrm{Ba}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{K}}_{\mathit{x}}$${\mathrm{BiO}}_{3}$ system for 0\ensuremath{\le}x\ensuremath{\le}0.5 and temperatures below 473 K have been determined by neutron powder diffraction. Bulk superconductivity occurs only in a cubic perovskite phase which exists for x\ensuremath{\ge}0.37 (at 10 K). At room temperature, as the potassium concentration is decreased, the cubic structure distorts first by ${\mathrm{BiO}}_{6}$ octahedral tilting and then by symmetric oxygen breathing-mode distortions. Semiconducting behavior for the monoclinic phase at 0\ensuremath{\le}x\ensuremath{\le}0.1 can readily be explained on the basis of a commensurate charge-density wave. The semiconducting orthorhombic phase, which extends from the monoclinic phase to the cubic superconducting phase, contains only octahedral tilting distortions that would not be expected to destroy metallic behavior. Thus, this commensurate structure provides no explanation for the metal-insulator transition. An incommensurate modulation extending across the semiconducting region of the phase diagram has been observed by electron diffraction, but is not observed by neutron diffraction. It is not clear whether this incommensurate modulation is responsible for the nonmetallic behavior.

Structural phase diagram of the Ba 1−x K x BiO 3 system

Abstract We have determined the structural phase diagram of the Ba1−xKxBiO3 system for 0≤x≤0.5 and temperatures from 10K to above 400K. At room temperature the undoped compound, BaBiO3, is monoclinic. With increasing x, the monoclinic phase successively transforms to orthorhombic, and then to cubic phases. As the symmetry increases, first the breathing-mode distortion and then the rotation of BiO6 octahedra are eliminated. The superconducting composition, x≥0.37, is cubic at all temperatures.