Octahedral Network Research Articles

Ruddlesden-Popper interface in correlated manganite heterostructures induces magnetic decoupling and dead layer reduction

We report on the interface engineering in correlated manganite heterostructures by octahedral decoupling using embedded stacks of atomic layers that form the Ruddlesden-Popper structure. A room temperature magnetic decoupling was achieved through deposition of a (SrO)2–TiO2–(SrO)2 sequence of atomic layers at the interface between La0.7Sr0.3MnO3 and La0.7Sr0.3Mn0.9Ru0.1O3 films. Moreover, the narrowing of the interfacial dead layer in ultrathin La0.7Sr0.3MnO3 films was demonstrated by insertion of a single (SrO)2 rock-salt layer at the interface with the SrTiO3(100) substrate. The obtained results are discussed based on the symmetry breaking and disconnection of the MnO6 octahedra network at the interface that may lead to the improved performance of all-oxide magnetic tunnel junctions. We suggest that octahedral decoupling realized by formation of Ruddlesden-Popper interfaces is an effective structural mechanism to control functionalities of correlated perovskite heterostructures.

Phase transitions from semiconductive amorphous to conductive polycrystalline in indium silicon oxide thin films

The enhancement in electrical conductivity and optical transparency induced by a phase transition from amorphous to polycrystalline in lightly silicon-doped indium oxide (InSiO) thin films is studied. The phase transition caused by simple thermal annealing transforms the InSiO thin films from semiconductors to conductors. Silicon atoms form SiO4 tetrahedra in InSiO, which enhances the overlap of In 5s orbitals as a result of the distortion of InO6 octahedral networks. Desorption of weakly bonded oxygen releases electrons from deep subgap states and enhances the electrical conductivity and optical transparency of the films. Optical absorption and X-ray photoelectron spectroscopy measurements reveal that the phase transition causes a Fermi energy shift of ∼0.2 eV.

Enhancement of orbital ordering and spin polarization by controlling the dimensionality of the octahedra network

AbstractTwo-dimensional (2D) systems have been widely investigated with perovskite superlattices because they show excellent epitaxy. A challenging question is naturally raised: whether lower dimensionality, e.g., one dimension (1D) and zero dimension (0D), can be achieved by perovskites? In this work, we propose a way to control the dimensionality of the octahedra network in perovskite superlattices by selecting the substrate orientation and superlattice period. Taking SrTiO3/SrRuO3 as an example, we demonstrate that the 1D structure is in a 1D Ising state, which is paramagnetic, while the 0D structure is ferromagnetic insulator with fully saturated magnetic moment on the Ru sites. New phenomena in the magnetic and electronic properties are observed, including large strain response, half-metallicity, and orbital-selective quantum confinement effects.

Oxygen Tuned Local Structure and Phase-Change Performance of Germanium Telluride.

The effect of oxygen on the local structure of Ge atoms in GeTe-O materials has been investigated. Oxygen leads to a significant modification to the vibrational modes of Ge octahedra, which results from a decrease in its coordination. We find that a defective octahedral Ge network is the crucial fingerprint for rapid and reversible structural transitions in GeTe-based phase change materials. The appearance of oxide Raman modes confirms phase separation into GeO and TeO at high level O doping. Counterintuitively, despite the increase in crystallization temperature of oxygen doped GeTe-O phase change materials, when GeTe-O materials are used in electrical phase change memory cells, the electrical switching energy is lower than the pure GeTe material. This switching energy reduction is ascribed to the smaller change in volume, and therefore smaller enthalpy change, for the oxygen doped GeTe materials.

Interfacial Octahedral Rotation Mismatch Control of the Symmetry and Properties of SrRuO3.

Epitaxial strain can be used to tune the properties of complex oxides with perovskite structure. Beyond just lattice mismatch, the use of octahedral rotation mismatch at heterointerfaces could also provide an effective route to manipulate material properties. Here, we examine the evolution of the structural motif (i.e., lattice parameters, symmetry, and octahedral rotations) of SrRuO3 films grown on substrates engineered to have the same lattice parameters, but different octahedral rotations. SrRuO3 films grown on SrTiO3 (001) (no octahedral rotations) and GdScO3-buffered SrTiO3 (001) (with octahedral rotations) substrates are found to exhibit monoclinic and tetragonal symmetry, respectively. Electrical transport and magnetic measurements reveal that the tetragonal films exhibit higher resistivity, lower magnetic Curie temperatures, and more isotropic magnetism as compared to those with monoclinic structure. Synchrotron-based quantification of the octahedral rotation network reveals that the tilting pattern in both film variants is the same (albeit with slightly different magnitudes of in-plane rotation angles). The abnormal rotation pattern observed in tetragonal SrRuO3 indicates a possible decoupling between the internal octahedral rotation and lattice symmetry, which could provide new opportunities to engineer thin-film structure and properties.

Octahedral Rotation Preferences in Perovskite Iodides and Bromides.

Phase transitions in ABX3 perovskites are often accompanied by rigid rotations of the corner-connected BX6 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., X = Cl, Br, or I), several of which undergo an unusual displacive transition to a tetragonal phase exhibiting in-phase rotations about one axis (a(0)a(0)c(+) 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 A-site cations, and enhances the covalency between the A-site metal and Br(-) or I(-) ions. The opposite effect is present in the oxides, with out-of-phase tilting maximizing these factors.

A Cubic Non-Centrosymmetric Mixed-Valence Iron Borophosphate–Phosphite

A first member of a new family of metal borophosphate–phosphite Fe1.834IIFe0.166IIIB0.5[PO3(OH)]0.8(HPO3)2.033 has been successfully synthesized by using the boric acid–phosphorous acid flux. The compound crystallizes in the cubic crystal system in a non-centrosymmetric space group I4̅3d (No. 220) with unit cell parameters of a = 21.261(3) Å, and Z = 48, featuring a very condensed network of FeO6 octahedra and disordered phosphate-phosphite moieties with mixed valency of iron. The compound contains a novel fundamental building unit (FBU), a cyclic borophosphate-phosphite ring which is further connected to form a propeller like partial anionic structure. Metal polyhedra also form a propeller-like structure through edge-sharing and are further connected to the partial anionic structure to form the three-dimensional structure. Thermal analyses, infrared and Mössbauer spectroscopy, magnetic and second harmonic generating (SHG) measurements using 1064 nm radiation have been performed on this compound. SHG measurements indicate that the compound has an efficiency approximately equal to α-SiO2.

Approaching compositional limits of perovskite – type oxides and oxynitrides by synthesis of Mg0.25Ca0.65Y0.1Ti(O,N)3, Ca1–xYxZr(O,N)3 (0.1 ≤ x ≤ 0.4), and Sr1–xLaxZr(O,N)3 (0.1 ≤ x ≤ 0.4)

Partial substitution of cations and anions in perovskite-type materials is a powerful way to tune the desired properties. The systematic variation of the cations size, the partial exchange of O2− for N3− and their effect on the size of the optical band gap and the thermal stability was investigated here. The anionic substitution resulted in the formation of the orthorhombic perovskite-type oxynitrides Mg0.25Ca0.65Y0.1Ti(O,N)3, Ca1-xYxZr(O,N)3, and Sr1–xLaxZr(O,N)3. A two-step synthesis protocol was applied: i) (nano-crystalline) oxide precursors were synthesized by a Pechini method followed by ii) ammonolysis in flowing NH3 at T = 773 K (Ti) and T = 1273 K (Zr), respectively. High-temperature synthesis of such oxide precursors by solid–state reaction generally resulted in phase separation of the different A-site cations. Changes of the crystal structures were investigated by Rietveld refinements of the powder XRD data, thermal stability by DSC/TG measurements in oxygen atmosphere, oxygen and nitrogen contents by O/N analysis using hot gas extraction technique, and optical band gaps by photoluminescence spectroscopy. By moving from Mg0.25Ca0.65Y0.1Ti(O,N)3 via Ca1–xYxZr(O,N)3 to Sr1–xLaxZr(O,N)3, the degree of tilting of the octahedral network is reduced, as observed by an increase in the B–X–B angles caused by the simultaneously increasing effective ionic radius of the A-site cation(s). In general, increasing substitution levels on the A-site (Y3+ and La3+) are accompanied by an enhanced replacement of O2− by N3−. In all three systems, this anionic substitution resulted in a reduction of the optical band gap by approximately 1 eV (Ti) and up to 2.1 eV (Zr) compared to the respective oxides. For Mg0.25Ca0.65Y0.1Ti(O,N)3 an optical band gap of 2.2 eV was observed, appropriate for a solar water splitting photocatalyst. The Zr-based oxynitrides required a by a factor of 2 higher nitrogen contents to significantly reduce the optical band gap and the measured values of 2.9 eV–3.2 eV are larger compared to the Ti-based oxynitride. Bulk thermal stability was revealed up to T = 881 K. In general, the thermal stability decreased with increasing substitution levels due to an increasing deviation from the ideal anionic composition as demonstrated by O/N analysis.

Lattice dynamics and vibrational spectra of the orthorhombic, tetragonal, and cubic phases of methylammonium lead iodide

The hybrid halide perovskite CH3NH3PbI3 exhibits a complex structural behaviour, with successive transitions between orthorhombic, tetragonal and cubic polymorphs at ca. 165 K and 327 K. Herein we report first-principles lattice dynamics (phonon spectrum) for each phase of CH3NH3PbI3. The equilibrium structures compare well to solutions of temperature-dependent powder neutron diffraction. By following the normal modes we calculate infrared and Raman intensities of the vibrations, and compare them to the measurement of a single crystal where the Raman laser is controlled to avoid degradation of the sample. Despite a clear separation in energy between low frequency modes associated with the inorganic PbI3 network and high-frequency modes of the organic CH3NH3+ cation, significant coupling between them is found, which emphasises the interplay between molecular orientation and the corner-sharing octahedral networks in the structural transformations. Soft modes are found at the boundary of the Brillouin zone of the cubic phase, consistent with displacive instabilities and anharmonicity involving tilting of the PbI6 octahedra around room temperature.

Towards 3D Mapping of BO6 Octahedron Rotations at Perovskite Heterointerfaces, Unit Cell by Unit Cell.

The rich functionalities in the ABO3 perovskite oxides originate, at least in part, from the ability of the corner-connected BO6 octahedral network to host a large variety of cations through distortions and rotations. Characterizing these rotations, which have significant impact on both fundamental aspects of materials behavior and possible applications, remains a major challenge at heterointerfaces. In this work, we have developed a unique method to investigate BO6 rotation patterns in complex oxides ABO3 with unit cell resolution at heterointerfaces, where novel properties often emerge. Our method involves column shape analysis in ABF-STEM images of the ABO3 heterointerfaces taken in specific orientations. The rotating phase of BO6 octahedra can be identified for all three spatial dimensions without the need of case-by-case simulation. In several common rotation systems, quantitative measurements of all three rotation angles are now possible. Using this method, we examined interfaces between perovskites with distinct tilt systems as well as interfaces between tilted and untilted perovskites, identifying an unusual coupling behavior at the CaTiO3/LSAT interface. We believe this method will significantly improve our knowledge of complex oxide heterointerfaces.

Low temperature epitaxial growth of anatase TaON using anatase TiO2 seed layer

Anatase TaON is a promising semiconducting oxynitride with high Hall mobility. However, the high temperature (≥750 °C) required for its epitaxial growth on (LaAlO3)0.3–(SrAl0.5Ta0.5O3)0.7 by nitrogen-plasma-assisted pulsed laser deposition, examined in our previous work, disadvantages its practical application. In this study, we adopted two approaches to decrease the growth temperature: the use of a substrate with better lattice matching to anatase TaON and the introduction of an anatase TiO2 seed layer. The growth temperature was successfully reduced to 650 °C by the latter approach. This indicated that the edge-shared MX6 octahedral network of the anatase structure effectively stabilizes anatase TaON.

First-Principles Analysis of Proton Conduction Mechanism in Pyrochlore-Structured Lanthanum Zirconate

The proton conduction behaviors in pyrochlore-structured lanthanum zirconate, La2Zr2O7, have been evaluated theoretically in a first-principles manner based on the transition state theory. The three-dimensional proton-conducting network consisting of two hopping paths was found, which lies along the corner-shared ZrO6 octahedral network in the crystal lattice. The calculated potential barrier of the proton-conducting network in the undoped system (0.39 eV) is lower than the apparent activation energies of the experimentally reported conductivities in acceptor-doped systems (0.6–0.9 eV). The effects of alkaline earth metal and yttrium dopants on the proton mobility and conductivity have also been evaluated, so that the calculated association energies between protons and dopants are in the range of 0.1–0.5 eV depending on the dopant species and substitutional sites. This makes the slopes of the proton conductivity curves in the Arrhenius plots steeper, leading to higher activation energies (0.4–0.8 eV) in t...

ACICULAR ZONED TOURMALINE (MAGNESIO-FOITITE TO FOITITE) FROM A QUARTZ VEIN NEAR TISOVEC, SLOVAKIA: THE RELATIONSHIP BETWEEN CRYSTAL CHEMISTRY AND ACICULAR HABIT

Distinctly chemically zoned tourmaline was found in a quartz vein near Tisovec, Slovak Ore Mountains, Slovakia. The tourmaline forms radial aggregates of light grey to green thin prismatic to acicular crystals growing in cracks in the host rock. The root zone of the aggregates has dravitic to oxy-dravitic compositions, shifting to magnesio-foitite in the middle parts of the crystals and to foititic compositions in the outer parts of the aggregates. The optimized formula, considering the single-crystal structure refinement (SREF) and the chemistry of the magnesio-foititic zone, is ~ X (□ 0.5 Na 0.4 Ca 0.1 ) Y (Mg 1.1 Al 1.0 Fe 2+ 0.8 Fe 3+ 0.1 ) Z (Al 5.7 Fe 3+ 0.3 ) (Si 5.9 Al 0.1 O 18 ) (BO 3 ) 3 (OH) 3 [O 0.6 (OH) 0.3 F 0.1 ], with lattice parameters (SREF) a 15.929(2) A, c 7.163(1) A, and V 1574.0(7) A 3 . SREF data as well as a detailed study of bond lengths indicate a dominancy of Al 3+ at the Z site substituted by a small proportion of Fe 3+ , while Fe 2+ and Mg 2+ essentially occupy the Y site along with a significant amount of Al. The chemical composition is controlled by the substitutions FeMg –1 , Al□(Fe,Mg) –1 Na –1 , and AlO(Fe,Mg) –1 (OH) –1 . The AlO(Fe,Mg) –1 (OH) –1 substitution is dominant in the transition from oxy-dravite to magnesio-foitite, and the Al□(Fe,Mg) –1 Na –1 substitution prevails in the magnesio-foitite and foitite compositions. Both proton- and alkali-deficient substitutions produce the enrichment in Al which can be linked to the forming of the acicular habit. Aluminum strongly prefers the Z -site, whose octahedra form a network of chains parallel to c . Consequently, polymerization of the Z O 6 octahedral network will dominate growth in the a direction. Therefore, the chemical composition, mainly Al enrichment, which is common for most of the acicular to fibrous tourmalines, can be an important factor promoting preferential growth in the c direction. The rate and time for crystallization, which depend mostly on the temperature and cooling rate, are also factors which should be considered.

Structural investigation of fluorophosphate glasses by 19F, 31P MAS-NMR and IR spectroscopy

A series of glass samples of composition [(x)MnF2–(x–80)NaPO3–(20)ZnF2] where x=0, 10, 20, 30 and 40, has been prepared. IR and NMR spectroscopy have been performed, especially 19F MAS and 31P MAS NMR. On the basis of these measurements, glass structure may be described as a hybrid network of ZnF6 and MnF6 octahedra sharing corners and edges and also connected to tetrahedral fluorophosphate groups (PX)4 where X=O− or F−. Sodium cations fill vacancies of this network.

BaMn9(II)(VO4)6(OH)2: a homospin ferrimagnet with a broken spinel-lattice of B-sites.

A new vanadate of BaMn9(II)(VO4)6(OH)2 was synthesized by a conventional hydrothermal method. BaMn9(II)(VO4)6(OH)2 crystallizes in the cubic space group Pa3[combining macron], which exhibits an edge-sharing MnO6 octahedral network structure with cavities occupied by Ba(VO4)6(16-) groups. The lattice built by Mn(2+) ions shows the reverse triangular dipyramid Mn7 in the system, which can be considered as a broken spin lattice of B-sites in spinels. The title compound shows a ferrimagnetic behavior with TC = 18 K.

Orientations of low-energy domain walls in perovskites with oxygen octahedral tilts

Many applications of ferroic materials, such as data storage and spintronics, are achieved through the control and manipulation of their domain wall (DW) orientations and configurations. Here we propose a rotational compatibility condition to identify low-energy DWs in perovskites with oxygen octahedral tilt instability. It is derived from the strong DW energy anisotropy arising from the rigidity and corner-sharing feature of the octahedral network. We analyze quantitatively the DWs in $\mathrm{SrTi}{\mathrm{O}}_{3}$ and explain successfully the unusual ferroelectric DW width and energy in $\mathrm{BiFe}{\mathrm{O}}_{3}$.

MnTaO2N: Polar LiNbO3‐type Oxynitride with a Helical Spin Order

AbstractThe synthesis, structure, and magnetic properties of a polar and magnetic oxynitride MnTaO2N are reported. High‐pressure synthesis at 6 GPa and 1400 °C allows for the stabilization of a high‐density structure containing middle‐to‐late transition metals. Synchrotron X‐ray and neutron diffraction studies revealed that MnTaO2N adopts the LiNbO3‐type structure, with a random distribution of O2− and N3− anions. MnTaO2N with an “orbital‐inactive” Mn2+ ion (d5; S=5/2) exhibits a nontrivial helical spin order at 25 K with a propagation vector of [0,0,δ] (δ≈0.3), which is different from the conventional G‐type order observed in other orbital‐inactive perovskite oxides and LiNbO3‐type oxides. This result suggests the presence of strong frustration because of the heavily tilted MnO4N2 octahedral network combined with the mixed O2−/N3− species that results in a distribution of (super)‐superexchange interactions.

MnTaO2N: polar LiNbO3-type oxynitride with a helical spin order.

The synthesis, structure, and magnetic properties of a polar and magnetic oxynitride MnTaO2N are reported. High-pressure synthesis at 6 GPa and 1400 °C allows for the stabilization of a high-density structure containing middle-to-late transition metals. Synchrotron X-ray and neutron diffraction studies revealed that MnTaO2N adopts the LiNbO3-type structure, with a random distribution of O(2-) and N(3-) anions. MnTaO2N with an "orbital-inactive" Mn(2+) ion (d(5); S=5/2) exhibits a nontrivial helical spin order at 25 K with a propagation vector of [0,0,δ] (δ≈0.3), which is different from the conventional G-type order observed in other orbital-inactive perovskite oxides and LiNbO3-type oxides. This result suggests the presence of strong frustration because of the heavily tilted MnO4N2 octahedral network combined with the mixed O(2-)/N(3-) species that results in a distribution of (super)-superexchange interactions.

Room-temperature polar ferromagnet ScFeO3 transformed from a high-pressure orthorhombic perovskite phase.

Multiferroic materials have been the subject of intense study, but it remains a great challenge to synthesize those presenting both magnetic and ferroelectric polarizations at room temperature. In this work, we have successfully obtained LiNbO3-type ScFeO3, a metastable phase converted from the orthorhombic perovskite formed under 15 GPa at elevated temperatures. A combined structure analysis by synchrotron X-ray and neutron powder diffraction and high-angle annular dark-field scanning transmission electron microscopy imaging reveals that this compound adopts the polar R3c symmetry with a fully ordered arrangement of trivalent Sc and Fe ions, forming highly distorted ScO6 and FeO6 octahedra. The calculated spontaneous polarization along the hexagonal c-axis is as large as 100 μC/cm(2). The magnetic studies show that LiNbO3-type ScFeO3 is a weak ferromagnet with TN = 545 K due to a canted G-type antiferromagnetic ordering of Fe(3+) spins, representing the first example of LiNbO3-type oxides with magnetic ordering far above room temperature. A comparison of the present compound and rare-earth orthorhombic perovskites RFeO3 (R = La-Lu and Y), all of which possess the corner-shared FeO6 octahedral network, allows us to find a correlation between TN and the Fe-O-Fe bond angle, indicating that the A-site cation-size-dependent octahedral tilting dominates the magnetic transition through the Fe-O-Fe superexchange interaction. This work provides a general and versatile strategy to create materials in which ferroelectricity and ferromagnetism coexist at high temperatures.

Atomic-scale control of TiO₆ octahedra through solution chemistry towards giant dielectric response.

The structures of many important functional oxides contain networks of metal-oxygen polyhedral units i.e. MOn. The correlation between the configurations and connectivities of these MOn to properties is essentially important to be well established to conduct the design, synthesis and application of new MOn-based functional materials. In this paper, we report on an atomic-scale solution-chemistry approach that for the first time enables TiO6 octahedral network control starting from metastable brookite TiO2 through simultaneously tuning pH values and interfering ions (Fe3+, Sc3+, and Sm3+). The relationship between solution chemistry and the resultant configuration/connectivity of TiO6 octahedra in TiO2 and lepidocrocite titanate is mapped out. Apart from differing crystalline phases and morphologies, atomic-scale TiO6 octahedral control also endows numerous defect dipoles for giant dielectric responses. The structural and property evolutions are well interpreted by the associated H+/OH− species in solution and/or defect states associated with Fe3+ occupation within TiO6 octahedra. This work therefore provides fundamental new insights into controlling TiO6 octahedral arrangement essential for atomic-scale structure-property design.