Orthorhombic Imma Research Articles

Comprehensive Investigation of Ti-Doped Sr3CoSb2O9 Triple Perovskite: Structural, Morphological, Optical, and Dielectric Characteristics

We report a comprehensive investigation of the doped Ti4+ and the pristine Sr3CoSb2O9 triple perovskite material. By conducting a solid-state reaction in the air at room pressure, a stabilized single orthorhombic Immm (SG 71) phase is obtained. For series samples Sr3CoSb2−xTixO9 (x = 0.0, 0.1, 0.3, 0.5), using Rietveld refinements of powdered x-ray patterns, we reach to the conclusion that the Ti4+ doping at B-site has considerable impact on its structural parameters. The microstructure of the series samples was studied using FESEM and associated with EDS mapping for compositional analysis. The optical characteristics studied via optical absorbance measurements indicate that with an increasing doping concentration of Ti4+, the optical bandgap increases for 0.1 sample and then decreases up to 1.50 eV for 0.5 sample. As a result, they can be considered potential candidates for technological applications like photocatalytic systems and solar cells due to their suitable bandgap values and optical absorption-related applications. The FTIR analysis aimed to examine the bond forms and vibrational modes of the synthesized samples. Moreover, the space charge polarization mechanism explains a detailed analysis of temperature-dependent dielectric properties in the 1 KHz-2 MHz frequency range. The dielectric loss behavior is described by the thermally activated relaxation process tailored by the Arrhenius law and characterized by doping Ti4+ ions into the material’s structure. The existence of Co in Co2+ and Co3+ oxidation states, while Sb3+ state is stabilized with doping confirmed by X-ray photoelectron spectroscopy research. The current study thereby advances our knowledge of how these materials’ structural, morphological, optical, and dielectric characteristics are related.

Design, synthesis and Characterization of a novel antiferroelectric solid solution (1-x)PbHfO3-xBiAlO3

Antiferroelectric (AFE) materials are potentially useful for energy storage applications. Lead hafnate (PbHfO3) is one of the prototypical AFE materials. However, its critical field (Ecr) which is needed to induce the AFE to ferroelectric phase transition is close to the dielectric breakdown strength. To reduce Ecr, we prepare the solid solutions of (1-x)PbHfO3–xBiAlO3 (x = 0.00–0.04) via solid state synthesis. The crystal structures, dielectric behaviour, ferroelectric properties, and energy storage properties are investigated. A temperature-composition phase diagram is established. At room temperature, the crystal symmetry of all compositions is orthorhombic with the Pbam space group. Upon heating, the transition to the AFE orthorhombic Imma phase is observed at the temperature TC1, which slightly decreases with increasing x, followed by the transition to the cubic phase at the temperature TC2, which does not depend on x. Distinct dielectric anomalies are observed at TC1 and TC2. It is found that BiAlO3 substitution reduces Ecr and the polarization–electric field relations display characteristic double hysteresis loops. For x = 0.04, at a comparatively small applied field of 130 kV/cm, the values of recoverable energy density (Wrec) and efficiency of 0.24 J/cm3 and 84 %, respectively, are obtained. At 190 °C, a Wrec = 0.75 J/cm3 and a very high efficiency of 92 % are obtained at the field of 50 kV/cm. Thus, the prepared material can potentially be used in high-temperature pulsed power capacitors for energy storage applications within the temperature range from room temperature up to 190 °C.

Exploration of novel structures and related physical properties of Fe2Pt ordered alloys

The Fe–Pt phase diagram suggests the possible existence of ordered Fe2Pt structures, however, the ordered structures of Fe2Pt and related physical properties are unclear until now. In this work, four new low-energy ferromagnetic Fe2Pt ordered structures are predicted based on the particle swarm optimization algorithm. Their structural stability is verified by the formation enthalpy, mechanical stability and lattice dynamic stability. The order of the thermodynamic stability is Cmcm > Immm > I4/mmm > P3¯m1. All four structures are dynamically (except tetragonal I4/mmm structure) and mechanically stable. The fundamental physical properties including magnetic, elastic, phonon and thermal properties are investigated in detail. Their total magnetic moments, as well as the bulk modulus are almost the same. The shear modulus and Young's modulus of Cmcm structure are close to the P3¯m1, and larger than that of other two structures. The order of the elastic anisotropy is Immm > P3¯m1 > I4/mmm > Cmcm. The orthorhombic Immm structure can be regarded as a distorted tetragonal I4/mmm structure, and the distortion effectively improves the physical properties of the I4/mmm structure. The thermal properties at finite temperatures show that the trigonal P3¯m1 structure is more suitable for high-temperature applications compared to other structures. The results of linear thermal expansion coefficient indicate that anisotropy order of thermal expansion is Immm > I4/mmm > Cmcm > P3¯m1. More interesting is the discovery of the Invar effect along the y-axis direction in orthorhombic Immm structure.

Phase Transitions and Electric Properties of PbBr2 under High Pressure: A First-Principles Study.

PbBr2 has recently attracted considerable attention as a precursor for lead halide perovskite-based devices because of its attractive properties. It is known that pressure can modify the chemical and physical properties of materials by altering the distance between atoms in the lattice. Here, a global structure-searching scheme was used to explore the high-pressure structures of PbBr2, whose structures and properties at high pressure are still far from clear. Three new phases of PbBr2 were predicted in the pressure range of 0-200 GPa, and the pressure-driven phase transition sequence of orthorhombic Pnma (0-52 GPa) → tetragonal I4/mmm (52-80 GPa) → orthorhombic Cmca (80-153.5 GPa) → orthorhombic Immm (153.5-200 GPa) is proposed. Electronic calculations indicate a semiconductor-to-metallic transition of PbBr2 in the Cmca phase at ~120 GPa. Our present results could be helpful in improving the understanding of fundamental physical properties and provide insights to modulate the structural and related photoelectric properties of PbBr2.

Crystal structure thermal evolution and novel orthorhombic phase of methylammonium lead bromide, CH3NH3PbBr3

Methylammonium (MA) lead trihalide perovskites, CH3NH3PbX3 (X = I, Br, Cl), have emerged as a new class of light-absorbing materials for photovoltaic applications, reaching efficiencies of 23% when implemented in solar cell heterojunctions. In particular, MAPbBr3 is a promising member with a large bandgap that gives rise to a high open circuit voltage. Here we present a structural study from neutron diffraction (ND) data of an undeuterated MAPbBr3 specimen, carried out to follow its crystallographic behaviour in the 2–298 K temperature range. Besides the known crystallographic phases, i.e. the high-temperature Pmoverline{3}m cubic structure, the intermediate I4/mcm tetragonal symmetry and the low-temperature Pnma orthorhombic phase, we additionally identified, from a detailed sequential ND analysis, a novel intermediate phase within the 148.5–154.0 K temperature range as an orthorhombic Imma structure, early associated with a coexistence of phases. Moreover, our ND data allowed us to unveil the configuration of the organic MA units and their complete localization within the mentioned temperature range, thus improving the crystallographic description of this compound. The evolution with temperature of the H-bonds between the organic molecule and the inorganic cage is also followed. A deep knowledge of the crystal structure and, in particular, the MA conformation inside the perovskite cage seems essential to establish structure–property correlations that may drive further improvements.

Prediction of new stable phases of FePd2 crystal alloy

The stable crystal structures of FePd2 under ambient pressure are explored using the particle swarm optimization technique. In addition to the previously reported tetragonal I4/mmm, orthorhombic Immm and trigonal P3¯m1 phases, three as-yet-unreported phases with R3¯m, Cmcm, and P4/nmm space group are uncovered. Subsequent first principles calculations reveals that all phases have negative enthalpy of formation and the enthalpy of the tetragonal I4/mmm phase is the lowest. Also, these phases are mechanically and dynamically stable. Their ferromagnetism and metal ductility are confirmed by the electronic structure and Pugh's ratio. The bulk modulus of these phases are almost equal, and the I4/mmm structure has the relatively higher Young's modulus and shear modulus. The dimensionless magnetic hardness parameter demonstrated that the I4/mmm and P4/nmm structures are semihard magnets, and the other structures are permanent/hard magnetic materials, in which the Immm, Cmcm, and P3¯m1 structures are the potential candidates for ultra-high density magnetic storage. The results of the bulk modulus and thermal expansion coefficient at finite temperature show that the P3¯m1 structure as a promising high-temperature structural material.

Metastable phase diagram of the Gd2O3–SrO–CoO x ternary system

Abstract The metastable phase diagram of the Gd2O3–SrO–CoO x system in air was established based on the powder X-ray diffraction results of the 1100 °C-synthesized and then furnace-cooled or slowly-cooled (1 K min−1) samples. It consists of two solid solutions, Gd1−x Sr x CoO3−δ (0.6 ≤ x ≤ 0.9) with a tetragonal I4/mmm superstructure and Gd x Sr2−x CoO4−δ (0.5 ≤ x ≤ 1.2) with a layered tetragonal I4/mmm K2NiF4-type structure, and one ternary compound Gd2SrCo2O7 with a tetragonal P42/mnm structure. The existence of six binary oxide compounds Gd2SrO4, GdCoO3, Sr2Co2O5 (R), Sr6Co5O15, Sr5Co4O12 and Sr14Co11O33 was confirmed. This metastable phase diagram is of technological interest in the controlled preparation of single-phase complex oxides. New phases Gd0.375Sr2.625Co2O7−δ and τ4 with an orthorhombic Immm structure were found in the quenched samples. Differences between the present metastable phase diagram and the reported 1100 °C equilibrium one are discussed.

Guest-Induced Multistep-to-One-Step Reversible Spin Transition with Enhanced Hysteresis in a 2D Hofmann Framework.

The interplay of host-guest interactions and controlled modulation of spin-crossover (SCO) behavior is one of the most exploited topics regarding data storage, molecular sensing, and optical technologies. In this work, we demonstrate the experimental approach of tuning the SCO behavior via controlled modulation of the spin-state cooperativity in a 2D Hofmann coordination polymer, [FeIIPd(CN)4(L)2]·1.3MeOH (1·1.3MeOH; L = methyl isonicotinate). Removal of the solvent changes the four-step transition to a complete one-step spin transition with an enhanced hysteresis width (∼20 K). Structural analysis of solvated (1·1.3MeOH) and partially desolvated (1·0.3MeOH) compounds reveals that the crystal system changes from a monoclinic C2/c space group to an orthorhombic Imma space group, where the FeII sites are present in a more symmetrically equivalent environment. Consequently, the axial ligand-field (LF) strength and face-to-face interactions of the ligands were increased by removing the guest, which is reflected in the highly cooperative SCO in 1 (desolvated compound). Also, the shift of the CN bond stretching frequencies and decrease of their relative intensities from the variable-temperature Raman spectroscopic measurements further corroborate the SCO behavior. Besides, theoretical calculations reveal that the singlet (1Γ) LF terms decrease by removing guest molecules, enhancing the molecular population in the low-spin state at low temperature, as experimentally observed for 1. Notably, fine tuning of the SCO behavior via host-guests interactions provides a great opportunity to design potential chemosensors.

Chukochenite, (Li0.5Al0.5)Al2O4, a new lithium oxyspinel mineral from the Xianghualing skarn, Hunan Province, China

Abstract Chukochenite, (Li0.5Al0.5)Al2O4, is a new mineral species from the Xianghualing skarn, Hunan Province, southern China. It occurs as subhedral to euhedral crystals up to 200 μm across in the green rock of Xianghualing skarn, closely associated with fluorite, phlogopite, chrysoberyl, margarite, chlorite, ferronigerite-2N1S, and zinconigerite-2N1S. The crystals are colorless and transparent with a vitreous luster. Chukochenite is brittle with irregular fracture, has a Mohs hardness of 8, and shows light red fluorescence under 253.7 nm UV radiation and light green fluorescence under 365 nm UV radiation. The calculated density is 3.771 g/cm3. Chukochenite is optically biaxial (–) with α = 1.79(2), β = 1.82(2), and γ = 1.83(2) (589 nm). The calculated 2V is 60°, with the optical orientations X, Y, and Z parallel to the crystallographic a, b, and c, respectively. Electron microprobe analysis (Li by LA-ICP-MS) yielded in wt% Al2O3 80.70, Fe2O3 8.16, Li2O 3.68, ZnO 3.25, MnO 2.49, MgO 1.70, Na2O 0.11, CaO 0.08, TiO2 0.02, K2O 0.01, and Cr2O3 0.01 (total 100.24 wt%), giving an empirical formula [(Li0.355Al0.138Na0.005Ca0.002)Σ0.5(Al0.145Fe0.147+3Mg0.061Zn0.058Mn0.051Si0.001)Σ0.463]Al2O4 on a basis of 4 O atoms per formula unit. Chukochenite is orthorhombic, Imma, a = 5.659 (1), b = 16.898 (1), c = 7.994 (1) Å, V = 764.46 (8) Å3, and Z = 12. The nine strongest lines of powder XRD [d in Å (I) (hkl)] are: 2.405 (53) (231); 1.996 (29) (260); 1.535 (77) (303); 1.413 (100) (264); 1.260 (52) (2 12 0); 1.068 (36) (1 13 4); 1.039 (61) (503); 0.999 (59) (008); and 0.942 (35) (3 13 4). Chukochenite has a framework structure of spinel with low symmetry (orthorhombic Imma) due to the ordering of Li cations over octahedrally coordinated sites, which has not been previously reported for synthetic (Li0.5Al0.5) Al2O4. This structure type is based on a framework of AlO4 tetrahedra, AlO6, and LiO6 octahedra. AlO6 edge-sharing octahedra form chains along the a axis. AlO6 octahedra and LiO6 octahedra in a 2:1 ratio share edges, forming octahedral chains along b. These octahedral chains are connected by AlO4 tetrahedra and each corner of an AlO4 tetrahedron shares with three AlO6 octahedra or two AlO6 + one LiO6 octahedra. The discovery of chukochenite adds a new perspective on the cation ordering and the mechanism of luminescence and magnetism in (Li0.5Al0.5)Al2O4.

Crystal structures and mechanical properties of osmium diboride at high pressure

We have investigated the crystal structures and mechanical properties of osmium diboride (OsB2) based on the density functional theory. The structures of OsB2 from 0 to 400 GPa were predicted using the particle swarm optimization algorithm structure prediction technique. The orthorhombic Pmmn structure of OsB2 (oP6-OsB2) was found to be the most stable phase under zero pressure and it will transfer to the hexagonal P63/mmc structure (hP6-OsB2) around 12.4 GPa. Meanwhile, we have discovered a new stable orthorhombic Immm structure (oI12-OsB2) above 379.6 GPa. After that, a thorough and comprehensive investigation on mechanical properties of different OsB2 phases is performed in this work. Further studies showed that the hardness of oP6-OsB2 and hP6-OsB2 at zero pressure is 15.6 and 20.1 GPa, while that for oI12-OsB2 under 400 GPa is 15.4 GPa, indicating that these three phases should be potentially hard materials rather than superhard materials. Finally, the pressure–temperature phase diagram of OsB2 is constructed for the first time by using the quasi-harmonic approximation method. Our results showed that the transition pressures of oP6-OsB2 → hP6-OsB2 and hP6-OsB2 → oI12-OsB2 all decreases appreciably with the increase of temperature.

Probing the Local Site Disorder and Distortion in Pyrochlore High-Entropy Oxides.

High-entropy oxides (HEOs) have attracted great interest in diverse fields because of their inherent opportunities to tailor and combine materials functionalities. The control of local order/disorder in the class is by extension a grand challenge toward realizing their vast potential. Here we report the first examples of pyrochlore HEOs with five M-site cations, for Nd2M2O7, in which the local structure has been investigated by neutron diffraction and pair distribution function (PDF) analysis. The average structure of the pyrochlores is found to be orthorhombic Imma, in agreement with radius-ratio rules governing the structural archetype. The computed PDFs from density functional theory relaxed special quasirandom structure models are compared with real space PDFs in this work to evaluate M-site order/disorder. Reverse Monte Carlo combined with ab initio molecular dynamics and Metropolis Monte Carlo simulations demonstrates that Nd2(Ta0.2Sc0.2Sn0.2Hf0.2Zr0.2)2O7 is synthesized with its M-site local to nanoscale order highly randomized/disordered, while Nd2(Ti0.2Nb0.2Sn0.2Hf0.2Zr0.2)2O7+x exhibits a strong distortion of the TiO6 octahedron and small degree of Ti chemical short-range order (SRO) on the subnanometer scale. Calculations suggest that this may be intrinsic, energetically favored SRO rather than due to sample processing. These results offer an important demonstration that the engineered variation of participating ions in HEOs, even among those with very similar radii, provides richly diverse opportunities to control local order/disorder motifs-and therefore materials properties for future designs. This work also hints at the exquisite level of detail that may be needed in computational and experimental data analysis to guide structure-property tuning in the emerging HEO materials class.

Exploring the Different Degrees of Magnetic Disorder in TbxR1-xCu2 Nanoparticle Alloys.

Recently, potential technological interest has been revealed for the production of magnetocaloric alloys using Rare-Earth intermetallics. In this work, three series of TbRCu (R ≡ Gd, La, Y) alloys have been produced in bulk and nanoparticle sizes via arc melting and high energy ball milling. Rietveld refinements of the X-ray and Neutron diffraction patterns indicate that the crystalline structure in all alloys is consistent with TbCu orthorhombic bulk crystalline structure. The analyses of the DC-magnetisation () and AC-susceptibility () show that three distinct degrees of disorder have been achieved by the combination of both the Tb replacement (dilution) and the nanoscaling. These disordered states are characterised by transitions which are evident to , and specific heat. There exists an evolution from the most ordered Superantiferromagnetic arrangement of the TbLaCu NPs with Néel temperature, 27 K, and freezing temperature, 7 K, to the less ordered weakly interacting Superparamagnetism of the TbYCu nanoparticles ( absent, and T 3 K). The Super Spin Glass TbGdCu nanoparticles ( absent, and 20 K) are considered an intermediate disposition in between those two extremes, according to their enhanced random-bond contribution to frustration.

Observation of structural phase transition in Nd0.5Sr0.5MnO3 from high real-space-resolution X-ray diffraction

ABSTRACTThe real-space total scattering atomic Pair Distribution Function (PDF) method has been used to study the effect of temperature (T) on the structural features of the manganite . The powder X-ray diffraction data were collected as a function of T ranging from 92 to 300 K. The PDF data were analyzed at a value of and a room temperature model was used for the refinement of the data at two different refinement ranges (r-ranges): 1.7–10 and 1.7–30 for all the temperatures. Good agreements between the model and the PDF data were obtained with good agreement factor () values and a possible evidence for a structural phase transition (mainly from an orthorhombic Imma to a monoclinic phase) was observed at T = 145 K through the variation of the lattice parameters a, b, and c, and of as T decreases.

Raman spectra of Si nanocrystals under high pressure: Metallization and solid state amorphization

We have observed and identified two crystalline peaks at ∼270 cm−1 and ∼400 cm−1 in nanocrystalline silicon during both compression and decompression. We attribute the first peak to the orthorhombic Imma phase (Si-XI) and the second one to the TO mode of the silicon metallic β-Sn phase (Si-II). Also, in the process of decompression, we observed a first-order-like transition from a highly coordinated metallic crystalline β-Sn phase to the normal tetrahedral LDA phase of the amorphous semiconductor. When the pressure is completely relieved, the powder regains its initial crystalline structure, which suggests that the amorphous phase has retained some structural features of the parent crystalline phase.

First-principle calculations of ternary compounds: Immm-BxTi3−xN2

Boron nitride (BN) and Titanium nitrides (TiNs) have been successfully researched recently. In order to analyze the relationship among the Boron, Nitrogen and Titanium, the ternary compounds with an orthorhombic structure Immm- are studied. We further researched on the mechanical, electronic and optical properties of new Immm-B[Formula: see text]Ti[Formula: see text]N2 ([Formula: see text]). The structures of BTi2N2 and B2TiN2 are mechanically stable at 0, 50 and 100 GPa. The BTi2N2 has the higher cutting resistance and better ductility than the B2TiN2. The higher Young’s modulus of B2TiN2 indicates the B2TiN2 is stiffer than BTi2N2. The BTi2N2 is harder to compress in the [Formula: see text] direction and the B2TiN2 is harder in [Formula: see text] direction. Immm-BTi2N2 and B2TiN2 have good metallicity at 0 and 100 GPa. Immm-BTi2N2 has the higher dielectric function than B2TiN2 and the plasma frequency of B2TiN2 is bigger than that of BTi2N2. We hope our work will provide some help to the experimental work about the technology of the material.

Structural evolution of BaCe0.65Zr0.20Y0.15O3-δ-Ce0.85Gd0.15O2-δ composite MPEC membrane by in-situ synchrotron XRD analyses

Nowadays, dense ceramic membranes based on mixed ionic and electronic conductors are considered very promising materials for H2 separation at T > 600 °C. Among these, BaCe0.65Zr0.2Y0.15O3-δ-Ce0.85Gd0.15O2-δ (BCZ20Y15-GDC15) composite combine an acceptable H2 flux and good chemical stability under CO2- and H2S-containing atmospheres. However, a clear understanding of its crystal structure, phase stability and mechanical stability under real working conditions could not yet be obtained. In this work, its structural evolution was investigated from room temperature to 800 °C by in-situ synchrotron XRD analyses under dry and wet H2. No chemical interaction between the BCZ20Y15 and GDC15 phases occurred in the composite, thus demontrating its excellent chemical stability under operating conditions. However, some phase transitions were observed for the BCZ20Y15 phase, under both dry and wet H2: i.e., it showed an orthorhombic Imma structure from room temperature to 100 °C, trigonal R-3c up to 700 °C and cubic Pm-3m up to 800 °C. On the other hand, the GDC15 phase did not display any phase transition, remaining in a cubic Fm-3m structure under all tested conditions. Moreover, a synergistic effect of the BCZ20Y15 and GDC15 phases in the volume expansion of the composite was revealed: indeed, BCZ20Y15 and GDC15 lattice expansion rates tend to approach each other in the composite under reducing conditions. This synergistic effect is very important for the mechanical performances of BCZ20Y15-GDC15 composite. The similar expansion rate observed for BCZ20Y15 and GDC15 may reduce the strain and prevent failure of this ceramic membrane under operating conditions.

Order-disorder phenomena and octahedral tilting in SrTi1-XSnXO3 perovskites – A structural and spectroscopic study

Oxides of the type SrTi1-xSnxO3 have been prepared using both a solid state and polymeric precursor method and characterized using high-resolution Synchrotron X-ray Diffraction and X-ray Absorption and Raman spectroscopies. The diffraction measurements demonstrate that a continuous pseudo-solid solution exists between the cubic SrTiO3 and orthorhombic SrSnO3 end-members with the same sequence of structures cubic Pm3̅m → tetragonal I4/mcm → orthorhombic Imma → orthorhombic Pnma being observed irrespective of the preparation method. The Raman and XAS measurements revealed the presence of extensive disorder in samples treated at low temperatures, and that this disorder is reduced, but not totally eliminated, by annealing above 1300 °C. It is likely the local structure evolves similarly to that established by S-XRD for samples annealed above 1300 °C. This postulate is supported by the Ti K-edge XANES measurements that showed the local symmetry of the Ti cation evolves systematically as Sn4+ ions replace the Ti4+cations in the samples, irrespective of the temperature to which the sample had been heated.

Crystal Chemistry and Photocatalytic Properties of RE4S4Te3 (RE = Gd, Ho, Er, Tm): Experimental and Theoretical Investigations.

Reported are the synthesis and structural characterization of a new series of ternary rare-earth mix-chalcogenides RE4S4Te3 (RE = Gd, Ho, Er, Tm) that have been obtained from high-temperature solid state reactions. These compounds crystallize in Ho4S4Te2.68 structure types with monoclinic C2/ m and/or orthorhombic Immm space groups. The space group variation within this series is due to the position disorder along the Te plane (Te to TeA and TeB). The structural relationship and change between these two space groups are analyzed. It is realized that these compounds are all photocatalytic active under simulated sunlight. The trend of their photocatalytic activities and photocurrent responses is well-explained by using theoretical calculation as well as dipole moment analysis.

Structure and Phase Transformation in the Giant Magnetostriction Laves-Phase SmFe2.

As one class of the most important intermetallic compounds, the binary Laves-phase is well-known for its abundant magnetic properties. Samarium-iron alloy system SmFe2 is a prototypical Laves compound that shows strong negative magnetostriction but relatively weak magnetocrystalline anisotropy. SmFe2 has been identified as a cubic Fd3̅m structure at room temperature; however, the cubic symmetry, in principle, does not match the spontaneous magnetization along the [111]cubic direction. Here we studied the crystal structure of SmFe2 by high-resolution synchrotron X-ray powder diffraction, X-ray total scattering, and selected-area electron diffraction methods. SmFe2 is found to adopt a centrosymmetric trigonal R3̅m structure at room temperature, which transforms to an orthorhombic Imma structure at 200 K. This transition is in agreement with the changes of easy magnetization direction from [111]cubic to [110]cubic direction and is further evidenced by the inflection of thermal expansion behavior, the sharp decline of the magnetic susceptibility in the field-cooling-zero field-cooling curve, and the anomaly in the specific heat capacity measurement. The revised structure and phase transformation of SmFe2 could be useful to understand the magnetostriction and related physical properties of other RM2-type pseudocubic Laves-phase intermetallic compounds.

Observation of Vacancies, Faults, and Superstructures in Ln5Mo2O12 (Ln = La, Y, and Lu) Compounds with Direct Mo-Mo Bonding.

Among oxide compounds with direct metal-metal bonding, the Y5Mo2O12 (A5B2O12) structural family of compounds has a particularly intriguing low-dimensional structure due to the presence of bioctahedral B2O10 dimers arranged in one-dimensional edge-sharing chains along the direction of the metal-metal bonds. Furthermore, these compounds can have a local magnetic moment due to the noninteger oxidation state (+4.5) of the transition metal, in contrast to the conspicuous lack of a local moment that is commonly observed when oxide compounds with direct metal-metal bonding have integer oxidation states resulting from the lifting of orbital degeneracy typically induced by the metal-metal bonding. Although a monoclinic C2/m structure has been previously proposed for Ln5Mo2O12 (Ln = La-Lu and Y) members of this family based on prior single crystal diffraction data, it is found that this structural model misses many important structural features. On the basis of synchrotron powder diffraction data, it is shown that the C2/m monoclinic unit cell represents a superstructure relative to a previously unrecognized orthorhombic Immm subcell and that the superstructure derives from the ordering of interchangeable Mo2O10 and LaO6 building blocks. The superstructure for this reason is typically highly faulted, as evidenced by the increased breadth of superstructure diffraction peaks associated with a coherence length of 1-2 nm in the c* direction. Finally, it is shown that oxygen vacancies can occur when Ln = La, producing an oxygen deficient stoichiometry of La5Mo2O11.55 and an approximately 10-fold reduction in the number of unpaired electrons due to the reduction of the average Mo valence from +4.5 to +4.05, a result confirmed by magnetic susceptibility measurements. This represents the first observation of oxygen vacancies in this family of compounds and provides an important means of continuously tuning the magnetic interactions within the one-dimensional octahedral chains of this system.