Orthorhombic Unit Cell Research Articles

The new Ca(Fe,Al)2O4 phase with calcium ferrite-type structure, a likely carrier of Al in the transition zone and lower mantle

Synchrotron-based high-pressure single-crystal X-ray diffraction experiments were conducted on a new CaFe1.2Al0.8O4 phase at ambient temperature and up to a maximum pressure of 61 GPa. Crystals of CaFe1.2Al0.8O4 were synthesized at 24 GPa and 1500 °C. This phase has orthorhombic unit cell parameters (a = 8.9785(1) Å, b = 2.9158(1) Å, c = 10.4253(6) Å, V = 286(3) Å3, Z = 8), crystallizes with a space group Pnma, and exhibits K0,T of 195(2) GPa. At pressures ∼50 GPa, unit cell volume of CaFe1.2Al0.8O4 drops by about 7% over a small pressure interval. By combining results from synchrotron-based high-pressure single-crystal X-ray diffraction and electronic structure calculations, it is shown that these changes in the compression behaviour are associated with changes of the electronic state of iron in the octahedral site. Thus, CaFe1.2Al0.8O4 is stable under the conditions of the transition zone and lower mantle and, therefore, may be considered as a likely Al carrier in the Earth's deep geospheres.

Correlation of Structural, Microstructural, Dielectric, and Impedance Properties in Nd Doping on Bi4Ti2FeO12 Multiferroic Aurivillius Ceramic

Bi4−x Nd x FeTi2O12 (BNFTO, x = 0, 0.5, 1.0, and 1.5) Aurivillius three‐layered ceramics are prepared via a solid‐state reaction method. The crystal structure of the compounds reveals that the compounds exhibit a major orthorhombic structure with the B2cb space group. Slight distortions in the orthorhombic unit cell and an increase in cell volume with the doping of Nd are observed. Highly dense randomly oriented plate‐like nonuniform grains are observed in the micrographs of the ceramics. A decrease in energy bandgap values is noticed in Nd‐doped ceramics from 2.88 to 2.18 eV. An increase in polarization values is observed with Nd doping due to the improved grain boundaries. The appearance of a weak ferromagnetic nature at low temperature followed by a decline in magnetic interaction at room temperature (RT) is observed for all compounds. The frequency and temperature variations of the dielectric constant are investigated. The frequency and temperature variations of the impedance measurements indicate that compounds exhibit non‐Debye‐type electrical relaxations and a negative temperature coefficient of resistance. Furthermore, single‐peak relaxations are predominant for all measured samples, mainly the contribution of intragranular grain regions. This makes Nd‐dopant BNFTO Aurivillius ceramics have a great research value for RT multiferroic applications.

Feiite: Synthesis, stability, and implications for its formation conditions in nature

Abstract Feiite (Fe3TiO5) is a high-pressure Fe-Ti oxide mineral recently discovered in martian meteorite Shergotty. Feiite is isostructural with Fe4O5, a high-pressure iron oxide stable at pressures >10 GPa. The stability of feiite has yet to be studied, as it has not previously been synthesized in the laboratory. To determine the minimum pressure at which feiite can be synthesized, we have conducted multi-anvil experiments at 1200 °C and at pressures ranging from 7 to 12 GPa. Major element compositions and XRD patterns indicate that we successfully synthesized feiite with an orthorhombic unit cell (Cmcm structure) in experiments conducted at pressures 8 GPa or greater. Relative to A2B2O5 phases with similar structure, feiite can be synthesized at lower pressures. The coexistence of feiite and liuite (FeTiO3-perovskite) in Shergotty indicates that the upper pressure limit of feiite stability is above 15 GPa. To investigate the effect of oxygen fugacity on the composition and stability of feiite, we conducted an additional series of experiments at 1200 °C and 10 GPa pressure in which we varied the Fe3+/Fetotal ratio of the experimental starting materials. In doing so, we identified a minimum Fe3+ content necessary to stabilize the feiite structure (Fe3+/Fetotal = 0.26 at 10 GPa and 1200 °C). The importance of Fe3+ for feiite stability suggests this phase would not form in lunar or HED meteorites, where iron-titanium oxides contain little to no ferric iron. Though our experimental results can only place a lower limit on the shock pressures experienced in Shergotty, the determined pressure stability indicates feiite could also be present in diamond-bearing terrestrial rocks sourced from the upper mantle or transition zone. Additionally, the presence of feiite would be an indicator of source Fe3+/Fetotal.

Orthorhombic C32: A topological semimetal with nodal ring

We identify by ab initio calculations a new topological semimetal with a nodal ring in an sp2-sp3 hybridized carbon network, with a 32-atom base-centered orthorhombic unit cell in Cccm (D2h20) symmetry, termed as bco-C32 carbon. The dynamical stability of bco-C32 carbon has been confirmed with phonon mode calculations and the thermal stability has been confirmed by molecular dynamics simulations. Electronic band structures calculations show that bco-C32 carbon is a topological nodal line semimetal with a closed nodal ring centering the high-symmetric Γ point inside the first Brillouin zone, and the drumhead-like topological non-trivial surface states can be seen inside or outside the projected nodal ring in the (001) surface depending on the different surface terminations. Moreover, the calculated optical properties show that bco-C32 carbon is a dark black carbon phase like as graphite. Our findings have identified a new sp2-sp3 hybridized carbon allotrope and provide insights for its physical properties.

Structural, thermal and superconducting properties of Ag2O-doped YBa2Cu3O7-x composite materials

A composite material with superconducting properties, consisting of YBa2Cu3O7-x doped with Ag2O, was obtained after 233 h of sintering at 960 °C and followed by a special cooling program in oxygen atmosphere. Preferential orientation of YBa2Cu3O7-x (YBCO-123) crystallites was evidenced through X-ray diffraction (XRD) analysis and scanning electron microscopy (SEM). Small increase of c parameter for orthorhombic YBCO-123 crystalline unit cell together with a slight decrease of oxygen content (7-x) versus Ag content, were encountered. The dimensions of YBCO-123 crystallites are dependent on Ag content. Thermogravimetric analysis (TGA) during heating in flowing air atmosphere from room temperature (RT) to 1100 °C, showed that total mass loss is depending on Ag content. The cooling (from 1100 °C to RT, in air atmosphere) endows optimal oxygen accumulation only for 0 and 5 wt% Ag, while final masses for 10 and 20 wt% of Ag content, are more than 10% less. Differential scanning calorimetry (DSC) showed peritectic melting points for YBa2Cu3O7-x dependent on Ag percentage. DSC graphics also highlighted the endothermic effects: silver melting; melting and decomposition of Ag2O (in 225.81–345.12 °C temperature range) only for 10 and 20 wt%, but completely missing in the case of 5 wt% Ag content. We concluded that some of Cu sites from YBCO-123 superconductor were substituted by Ag ions. Quasi de Almeida-Thouless lines in the range of magnetic field amplitudes from 0.4 to 800 A/m, in the temperature range from 77 K to RT, were also investigated. Two cases are found concerning intergranular critical currents (Jci): depinning mechanism available for 5 wt% Ag content; the decoupling mechanism for 10, and 20 wt% Ag content.

Giant Thermoelectric Power Factor Anisotropy in PtSb1.4Sn0.6.

We report on the giant anisotropy found in the thermoelectric power factor (S2σ) of marcasite structure-type PtSb1.4Sn0.6 single crystal. PtSb1.4Sn0.6, synthesized using an ambient pressure flux growth method upon mixing Sb and Sn on the same atomic site, is a new phase different from both PtSb2 and PtSn2, which crystallize in the cubic Pa3̅ pyrite and Fm3̅m fluorite unit cell symmetry, respectively. The large difference in S2σ for heat flow applied along different principal directions of the orthorhombic unit cell stems mostly from anisotropic Seebeck coefficients.

Charge conduction mechanism and non-debye type relaxation in LaCrO3 perovskite orthochromite

The orthochromite LaCrO3 (LCO) was synthesized via the sol-gel auto-combustion route and sintered at 900 °C. The Rietveld refined XRD profile confirmed the single-phase perovskite structure of the compound crystallized in the orthorhombic unit cell with Pbnm space group symmetry. The electrical studies were conducted by complex impedance spectroscopy in 40-5 × 106 Hz and 133–363 K experimental ranges of frequency and temperature, respectively. The complex impedance data were modeled by (RGQG)(RGBQGB) equivalent circuit that revealed the contributions of grains and grain boundaries to the electrical transport mechanism of the material. The AC conductivity analysis proposed that the electrical conduction in the LCO perovskite was governed through Mott's variable range hopping (VRH) mechanism. The imaginary modulus spectra were fitted by the Kohlrausch–Williams–Watts (KWW) relaxation function that resolved the presence of two thermally activated relaxation phenomena. The dielectric permittivity followed the non-Debye relaxation model which characterized the nature of dielectric relaxation in the LCO chromite.

Thermo analytical characterization of tantalum oxide in the process for the development of tantalum nitride photoelectrodes

Thermogravimetric analysis and differential scanning calorimetry techniques were used to estimate the kinetic parameters of the oxidation of a tantalum metallic foil in the Ta3N5 photoelectrodes preparation process. Model-free methods were used to determine the pre-exponential and activation energy of the Arrhenius equation related to the tantalum oxidation. The obtained kinetic model considers the existence of two independent oxidation events: [A→FnB] and [C→AnD→AnE].X-ray powder diffraction analysis was used to identify the crystalline phases present in each sample. The sample oxidized at 5 °C·min−1 (500 °C), contained two tantalum oxide polymorphs, Ta4O (25 ± 5) % and Ta2O (25 ± 2) %, and metallic tantalum showing a cubic structure. All the remaining samples presented a monophasic Ta2O5 with orthorhombic unit cells which showed a similar cell volume, but crystallite size increased with the temperature.The heating rate used in the oxidation of the Ta foils was verified to influence the photoelectrochemical performance of the prepared Ta3N5 photoelectrodes. Samples prepared with increasing heating rates evidenced a shift in onset potential towards higher potentials, and a more pronounced dark current. For the considered nitridation conditions of 8 h at 850 °C, the highest photocurrent performance was obtained with a heating rate of 5 °C·min−1 to 700 °C.

The superconducting properties of YBa2Cu3Oy ceramics fabricated using ultrahigh dilution technology

YBa2Cu3Oy (Y-123) is one of the most intensively studied high-temperature superconductors (HTS) and has great potential for applications in electronics. Its superconducting properties significantly depend on the oxygen index (y), the peculiarities of the atomic crystal and microgranular structure, the type and concentration of contaminating atoms. Therefore, of particular interest are the techniques of HTS synthesis that significantly affect the superconducting properties, as well as modifications of these techniques. The aim of this study was to investigate the effect of ultrahigh dilution (UHD) technology on the Y-123 HTS properties.We applied UHD technology to treat Y2O3·4BaO2·6CuO composition (before and/or after calcination) and used solid-phase reaction to obtain batches of ceramic samples of the Y-123 phase. Main characteristics of the synthesized HTS specimens (the orthorhombic unit cell parameters, temperature dependence of electrical resistance, Meissner effect, and critical current density) were studied.X-ray phase analysis demonstrated that the samples were practically single-phase and consisted of the Y-123 phase. Depending on the treatment method, the modifying effect varied. Positive and most significant results were demonstrated for specimens obtained from precursor powder mixture treated with UHD. For some of such specimens an increase in the temperature of the beginning of the transition to the superconducting state by 3.8 K, the Meissner effect by 9%, and the density of the intergranular critical current by 35% were observed. We conclude that UHD technology can be used to improve properties of HTS. However, methodology for implementation of UHD technology should depend on which specific superconductor property needs to be changed.

Crystal Growth from Anhydrous HF Solutions of M2+ (M = Ca, Sr, Ba) and [AuF6]-, Not Only Simple M(AuF6)2 Salts.

Crystal growth from anhydrous HF solutions of M2+ (M = Ca, Sr, Ba) and [AuF6]− (molar ratio 1:2) gave [Ca(HF)2](AuF6)2, [Sr(HF)](AuF6)2, and Ba[Ba(HF)]6(AuF6)14. [Ca(HF)2](AuF6)2 exhibits a layered structure in which [Ca(HF)2]2+ cations are connected by AuF6 units, while the crystal structure of Ba[Ba(HF)]6(AuF6)14 exhibits a complex three-dimensional (3-D) network consisting of Ba2+ and [Ba(HF)2]2+ cations bridged by AuF6 groups. These results indicate that the previously reported M(AuF6)2 (M = Ca, Sr, Ba) compounds, prepared in the anhydrous HF, do not in fact correspond to this chemical formula. When the initial M2+/[AuF6]− ratio was 1:1, single crystals of [M(HF)](H3F4)(AuF6) were grown for M = Sr. The crystal structure consists of a 3-D framework formed by [Sr(HF)]2+ cations associated with [AuF6]− and [H3F4]− anions. The latter exhibits a Z-shaped conformation, which has not been observed before. Single crystals of M(BF4)(AuF6) (M = Sr, Ba) were grown when a small amount of BF3 was present during crystallization. Sr(BF4)(AuF6) crystallizes in two modifications. A high-temperature α-phase (293 K) crystallized in an orthorhombic unit cell, and a low-temperature β-phase (150 K) crystallized in a monoclinic unit cell. For Ba(BF4)(AuF6), only an orthorhombic modification was observed in the range 80–230 K. An attempt to grow crystals of Ca(BF4)(AuF6) failed. Instead, crystals of [Ca(HF)](BF4)2 were grown and the crystal structure was determined. During prolonged crystallization of [AuF]6– salts, moisture can penetrate through the walls of the crystallization vessel. This can lead to partial reduction of Au(V) to A(III) and the formation of [AuF4]− byproducts, as shown by the single-crystal growth of [Ba(HF)]4(AuF4)(AuF6)7. Its crystal structure consists of [Ba(HF)]2+ cations connected by AuF6 octahedra and square-planar AuF4 units. The crystal structure of the minor product [O2]2[Sr(HF)]5[AuF6]12·HF was also determined.

Crystallinity of Bitumen via WAXD and DSC and Its Effect on the Surface Microstructure

It is well documented that most bituminous binders contain crystallisable material. This crystallisable fraction, often referred to as paraffinic or natural wax, is associated with the bitumen’s origin and has an influence on its rheological performance. In the literature, Differential Scanning Calorimetry (DSC) is the primary technique used to determine the melting and crystallisation behaviour of this waxy fraction. However, in bitumen, thermal transitions in DSC are typically very broad and can be combined with recrystallisation effects upon reheating. This work explores the potential crystallisation and melting process of a waxy and a wax-free bitumen via three different approaches: DSC, Wide-Angle X-ray Diffraction (WAXD) and Confocal Laser Scanning Microscopy (CLSM). The findings reveal that the DSC transitions of the waxy bitumen are in good agreement with the corresponding occurrence of WAXD signals and to some extent with the formation and disappearance of the surface microstructures which were followed at two cooling and heating rates. WAXD results additionally demonstrate that the crystalline material in bitumen is organised in an orthorhombic unit cell, typical for straight chain aliphatic structures. On the other hand, DSC and WAXD support the lack of crystallinity for the wax-free bitumen which could explain its featureless CLSM surface. Overall, the originality of this work resides in the disclosure of connections between crystallographic properties, thermal transitions and the surface micromorphology of bitumen.

Fe[4-(3-Phenylpropyl)Pyridine]2[Fe(CN)5NO]: A 2D Coordination Polymer with Thermally-Induced Spin Transition and Nature of Its Asymmetric Hysteresis Loop

The titled material crystallizes with an orthorhombic unit cell, in the P21212 space group (Nr. 18). Its crystal structure was solved and refined from powder XRD patterns. The solid framework is formed by stacked undulated sheets of inorganic nature, Fe[Fe(CN)5NO], separated by bimolecular organic pillars, [4-(3-Phenylpropyl)pyridine]2, which remain coordinated to the axial coordination sites for the iron atom. The molecules forming these pillars remain coupled through C–H⋯π and dispersive interactions between neighboring molecules. When this solid is cooled and then warmed, a reversible spin transition, from high to low spin (HS → LS), and vice versa, is observed. This transition occurs in the temperature interval of 135–165 K, with a hysteresis between them of about 30 K. That hysteresis loop appears with a pronounced asymmetry when the slopes for the HS → LS and LS → HS transitions are compared. This effect is discussed in terms of the related structural changes in the solid structure during the spin transitions. The transition was also monitored from IR and Raman spectra recorded at 80 and 300 K. Relevant information on the electronic structure for both, the LS and HS phases of this material, was derived from the corresponding XPS spectra recorded at 114 and 270 K. This contribution emphasizes the role of the nitrosyl group (NO) as an electron buffer for tuning the bonding properties of the inorganic layer at the CN 5σ orbital to make possible the observed thermally-induced SCO behavior. The thermally-induced spin transition in this solid shows an asymmetric hysteresis loop, which was ascribed to the nature of the pillar molecule.

Oxide Ion Conductivity, Proton Conductivity, and Phase Transitions in Perovskite-Derived Ba3-x Sr x YGa2O7.5 0 ≤ x ≤ 3 Materials.

We report the synthesis, structural characterization, and oxide ion and proton conductivities of the perovskite-related Ba3–xSrxYGa2O7.5 family. Single-phase samples are prepared for 0 ≤ x ≤ 3 and show a complex structural evolution from P2/c to C2 space groups with an increase in x. For 1.0 ≲ x ≲ 2.4, average structures determined by X-ray and neutron powder diffraction show metrically orthorhombic unit cells, but HAADF-STEM imaging reveals this is caused by microstructural effects due to intergrowths of the Ba- and Sr-rich structure types. Variable-temperature powder diffraction studies suggest that 0 ≲ x ≲ 2.4 compositions undergo a phase transition upon being heated to space group Cmcm that involves disordering of the oxygen substructure. Thermal expansion coefficients are reported for the series. Complex impedance studies show that the Ba-rich samples are mixed proton and oxide ion conductors under moist atmospheres but are predominantly oxide ion conductors at high temperatures or under dry atmospheres. Sr-rich samples show significantly less water uptake and appear to be predominantly oxide ion conductors under the conditions studied.

Microwave-Assisted Solution Synthesis of Metastable Intergrowth of AgInS2 Polymorphs.

The intergrowth of stable and metastable AgInS2 polymorphs was synthesized using a microwave-assisted synthesis. The samples were synthesized in water and in a deep eutectic solvent (DES) consisting of choline chloride and thiourea. An increase in the metal precursor concentration improved the crystallinity of the synthesized samples and affected the particle size. AgInS2 cannot be synthesized from crystalline binary Ag2S or In2S3 via this route. The solution synthesis reported here results in the intergrowth of the thermodynamically stable polymorph (space group I2d, chalcopyrite structure) and the high-temperature polymorph (space group Pna21, wurtzite-like structure) that is metastable at room temperature. A scanning transmission microscopy (STEM) study revealed the intergrowth of tetragonal and orthorhombic polymorphs in a single particle and unambiguously established that the long-thought hexagonal wurtzite polymorph has pseudo-hexagonal symmetry and is best described with the orthorhombic unit cell. The solution-synthesized AgInS2 polymorphs intergrowth has slightly lower bandgap values in the range of 1.73 eV–1.91 eV compared to the previously reported values for tetragonal I2d (1.86 eV) and orthorhombic Pna21 (1.98 eV) polymorphs.

Crystal and molecular structure of V-amylose complexed with butan-1-ol

Chain-folded lamellar V-amylose single crystals were prepared by adding butan-1-ol to hot dilute aqueous solutions of native amylose. The base-plane electron and X-ray diffraction patterns recorded from hydrated specimens agreed with a P212121 space group and an orthorhombic unit cell. A molecular model resulting from an exhaustive search of helix position and restrained structure refinement against diffraction data was proposed. The unit cell contains 4 antiparallel left-handed 6-fold amylose single helices with 1 butan-1-ol molecule located inside each of the helical cavities. In addition, 4 butan-1-ol and 20 water molecules are distributed in the 4 interstitial spaces. The amylose helices form rows along the b-axis that are alternatively shifted by ±b/8 with respect to one another, which validates the geometrical model proposed by Helbert and Chanzy in 1994.

Structural domains in antiferromagnetic LaFeO3 thin films

Abstract We have grown epitaxial LaFeO3 thin films on different oxide substrates by molecular beam epitaxy and investigated them using transmission electron microscopy. The films reveal structural domains that strongly depend on the substrate material, substrate orientation, and the film thickness. On (001) perovskite substrates of SrTiO3 and LaAlO3, the long c-axis of the orthorhombic unit cell was found always to lie in-plane parallel to one of the <100> axes of the perovskite forming 90° rotational twins with the rotation axis parallel to the [110] axis of LaFeO3 and to the substrate normal. In contrast, MgO and MgAl2O4 substrates lead to the in-plane and the out-of-plane orientation of the c-axis and give rise to smaller domains. The results are discussed with respect to the correlation between the crystallographic and the antiferromagnetic structure of LaFeO3.

The Structural and Dielectric Properties of Bi3−xNdxTi1.5W0.5O9 (x = 0.25, 0.5, 0.75, 1.0)

A new series of layered perovskite-like oxides Bi3−xNdxTi1.5W0.5O9 (x = 0.25, 0.5, 0.75, 1.0) was synthesized by the method of high-temperature solid-state reaction, in which partial substitution of bismuth (Bi) atoms in the dodecahedra of the perovskite layer (A-positions) by Nd atoms takes place. X-ray structural studies have shown that all compounds are single-phase and have the structure of Aurivillius phases (APs), with close parameters of orthorhombic unit cells corresponding to space group A21am. The dependences of the relative permittivity ε/ε0 and the tangent of loss tgσ at different frequencies on temperature were measured. The piezoelectric constant d33 was measured for Bi3−xNdxTi1.5W0.5O9 (x = 0.25, 0.5, 0.75) compounds of the synthesized series.

Novel Fluoridoaluminates from Ammonothermal Synthesis: Two Modifications of K2AlF5 and the Elpasolite Rb2KAlF6

Two new modifications of the pentafluoridoaluminate K2AlF5 were obtained from ammonothermal synthesis at 753 K, 224 MPa and 773 K, 220 MPa, respectively. Both crystallize in the orthorhombic space group type Pbcn, with close metric relations and feature kinked chains of cis-vertex-connected AlF6 octahedra resulting in the Niggli formula ∞1{[AlF2/2eF4/1t]2−}. The differences lie in the number of octahedra necessary for repetition within the chains, which for K2AlF5-2 is realized after four and for K2AlF5-3 after eight octahedra. As a result, the orthorhombic unit cell for K2AlF5-3 is doubled in chain prolongation direction [001] as compared to K2AlF5-2 (1971.18(4) pm versus 988.45(3) pm, respectively), while the unit cell parameters within the other two directions are virtually identical. Moreover, the new elpasolite Rb2KAlF6 is reported, crystallizing in the cubic space group Fm3¯m with a = 868.9(1) pm and obtained under ammonothermal conditions at 723 K and 152 MPa.

Molybdenum Oxide Coatings Deposited on Plasma Nitrided Surfaces

Molybdenum trioxide (MoO3) thin films are generally deposit on metallic surfaces to increase their tribological performance. In this manuscript, MoOx coatings were for the first time deposited on plasma nitrided surfaces. Vickers microhardness tests, X-ray diffractometry, and micro abrasive wear tests characterized the samples. It was investigated the effect of temperature in hardness and tribological properties of austenitic stainless steel surfaces. When the temperature of plasma nitriding increases, the coating thickness and the nitrogen diffusion on the treated surface also increases, favoring the hardness of the coatings. After MoOx deposition, the predominant MoO3 phase with monoclinic and orthorhombic unit cells was observed. MoOx coatings after plasma nitriding demonstrated the highest wear resistance.

Crystal structure, microstructure, piezoelectric and dielectric properties of high-temperature piezoceramics Bi-=SUB=-3-x-=/SUB=-Nd-=SUB=-x-=/SUB=-Ti-=SUB=-1.5-=/SUB=-W-=SUB=-0.5-=/SUB=-O-=SUB=-9-=/SUB=- (x=0, 0.1, 0.2)

A new series of perovskite-like oxides Bi3-xNdxTi1.5W0.5O9 (x=0, 0.1, 0.2) has been synthesized by the method of high-temperature solid-state reaction. The X-ray diffraction study showed that the compounds are single-phase and have the structure of the family of Aurivillius phases with parameters close to the orthorhombic unit cell corresponding to the space group A21am. The relative permittivity ε/ε0 and loss angle tangent tgsigma are measured as functions of temperature at different frequencies. The piezoelectric modulus d33 was measured for the synthesized compounds. The microstructure of Bi3-xNdxTi1.5W0.5O9 (x=0, 0.1, 0.2) was obtained. The study of the microstructure shows that the crystallites have a shape characteristic of Aurivillius phases. Keywords: Aurivillius phases, Bi2.8Nd0.2Ti1.5W0.5O9, activation energy Ea, Curie temperature TC.