Hexagonal P63cm Research Articles

Growth and Structural Characterization of h-LuMnO3 Thin Films Deposited by Direct MOCVD.

In this work, we investigated the MOCVD conditions to synthesize thin films with the hexagonal P63cm h-LuMnO3 phase as a potential low-band gap ferroelectric material. The main parameters investigated were the ratio of organometallic starting materials, substrate temperature, and annealing effect. Two different substrates were used in the study: fused silica (SiO2) glass and platinized silicon (Pt\Ti\SiO2\Si(100)). In order to investigate the thermodynamic stability and quality of the developed phases, a detailed analysis of the crystal structure, microstructure, morphology, and roughness of the films was performed by X-ray diffractometer, scanning electron microscopy (SEM), energy dispersive spectrometry (EDS), Raman spectroscopy, and piezoelectric force microscopy (PFM). Molar compositions in the film within 0.93 < |Lu|/|Mn| < 1.33 were found to be suitable for obtaining a single-phase h-LuMnO3. The best films were obtained by depositions at 700 °C, followed by thermal treatments at 800 °C for long periods of up to 12 h. These films exhibited a highly crystalline hexagonal single phase with a relatively narrow direct band gap, around 1.5 eV, which is within the expected values for the h-LuMnO3 system.

Temperature-driven magnetic and structural transitions in multiferroic Lu(1-x)ScxFeO3

Novel metastable hexagonal Lu(1-x)SxFeO3 compounds were prepared by an ethylene glycol-based small molecule sol-gel synthesis route. Samples were investigated by means of XRD, Raman spectroscopy, XPS, SEM and magnetometry techniques. Based on the obtained data several different phase composition regions were discovered at room temperature depending on the Sc content. Three dominant phases were observed and denoted as the orthorhombic Pnma, hexagonal P63cm, and cubic Ia-3 with the hexagonal phase being stable without any additional phases when 0.40 ≤ x < 0.65. Temperature dependent structural analysis allowed for the construction of the phase diagram which revealed that upon increasing Sc content the reversible polar to non-polar phase transition temperature can be decreased from above 1200 °C (when x = 0.15) to around 800 °C (x = 0.75). SEM analysis indicated a rapid reduction in particle size from ∼2.726 µm when x = 0 down to ∼0.962 µm when x = 0.75, however in the case when x = 1, an increase in particle size was observed. XPS analysis confirmed the elemental composition and its changes as well as the stability of the 3 + oxidation state of the iron ions. Magnetization measurements confirmed a non-zero room temperature magnetization as well as the spin reorientation transitions whose critical temperatures TN increases from 156 K (x = 0.25) to 170 K (x = 0.75).

Room‐temperature multiferroic characteristics and unique vortex domain structures of h‐Yb1−xInxFeO3 solid solutions

AbstractHexagonal rare‐earth ferrites (h‐RFeO3) have attracted much scientific attention due to their room‐temperature multiferroicity. However, it is still a hard job to obtain h‐RFeO3 bulk materials because of the meta‐stability of such hexagonal phase, and the evaluation of room‐temperature ferroelectric and magnetoelectric characteristics in such materials is also a challengeable issue. In the present work, Yb1−xInxFeO3 ceramics with the stable hexagonal structure were obtained by introducing chemical pressure, where the unique ferroelectric domain structures of sixfold vortex combined with tenfold vortex with a typical size of ~400 nm were determined. Symmetry of the present system evolved from centrosymmetric orthorhombic Pbnm (x = 0–0.4) to non‐centrosymmetric hexagonal P63cm (x = 0.5 and 0.6) with a ferroelectric polarization up to 3.2 μC/cm2, and finally to centrosymmetric hexagonal P63/mmc (x = 0.7 and 0.8). The Curie point decreased monotonically from 723 K to a temperature below room temperature with increasing x, and the antiferromagnetic phase transition above room temperature was determined for all compositions. Meanwhile, a large linear magnetoelectric coefficient (αME) up to 0.96 mV/cm Oe was obtained at room temperature, and this indicated the great application potential for magnetoelectric devices.

Multiferroic behaviour in B-site Cr-doped hexagonal YInO3 perovskites: Synthesis, structure and properties

New phase pure multiferroic oxides, YIn1-xCrxO3 (x = 0.1, 0.3, 0.5), are synthesized by glycine–aided sol-gel combustion technique using binary oxides and nitrates. YIn0.9Cr0.1O3 (I), YIn0.7Cr0.3O3 (II) and YIn0.5Cr0.5O3 (III) are investigated through powder X-ray diffraction (PXRD), elemental analysis (EDAX), magnetization and polarisation studies. Crystal structure analysis reveals that the sample with 10% Cr3+ (compound I) crystallizes in hexagonal P63cm space group with a = 6.28035 and c = 12.533 Å. The samples with higher concentration of Cr3+ (compounds II and III) crystallize with a mixture of hexagonal and orthorhombic phases. From the detailed magnetic measurements, the compounds are found to exhibit dominating antiferromagnetic behaviour with Neel temperatures of 140, 130 and 125 K for I, II and III, respectively. The Curie-Weiss fittings provide different θ values and the effective magnetic moment, μeff (2.60 μB, to 4.23μB), gradually increases with increasing Cr3+ concentration. The observed values indicate that the spins are not perfectly aligned antiparallel, rather the system exhibits canted antiferromagnetic (AFM) behaviour with weak ferromagnetic interactions. The existence of magnetic ordering is also confirmed by the presence of hysteresis loop below the Neel temperature. The room temperature ferroelectric polarisation loops are indicative of the multiferroic nature of the synthesized compounds. The work reveals the significant role of Cr3+ substitution for structural evolution and enhanced multiferroic behaviour in YIn1-xCrxO3.

New Insights into the Crystal Structures of Plutonium Hydrides from First-Principles Calculations

One of the important research contents on hydrogen corrosion of plutonium is the determination of the complex crystal structures of plutonium hydrides and the bonding interactions between plutonium and hydrogen. However, it is very difficult to carry out the structural characterization of plutonium hydrides because of their high activity, high toxicity, and radioactivity. In this work, the crystal structures, lattice vibrations, and bonding properties of plutonium hydrides under ambient pressure are investigated by means of the density functional theory + U approach. Results show that PuH3 exhibits many competition phase structures. After considering spin polarization, strong correlation (U), and spin–orbit coupling effects on the total energy and lattice dynamics stability, it is found that PuH3 at ambient pressure is more likely to be hexagonal P63cm or trigonal P3c1 structure, instead of the usual supposed structures of hexagonal P63/mmc (LaF3-type) and face-centered cubic (BiF3-type). The calculated e...

Magnetic characterization and charge state of nonmagnetic ion-substituted YMn0.8Fe0.2O3

The structure, magnetic properties and charge state of Al/Bi-substituted YMn0.8Fe0.2O3 were investigated. The Y(Mn0.8Fe0.2)0.95Al0.05O3 and Y0.95Bi0.05Mn0.8Fe0.2O3 solid solutions all exhibited the hexagonal P63cm space group, similar to YMnO3. The magnetic structures of Al-substituted YMn0.8Fe0.2O3 were not uniform, and were predominantly antiferromagnetic with weak ferromagnetic properties at low temperatures. The ratios Mn2+:Mn3+ and Fe2+:Fe3+ noticeably decreased upon Al substitution in YMn0.8Fe0.2O3, and the magnetic transition temperature was reduced dramatically. The cluster glass state behavior observed in Y(Mn0.8Fe0.2)0.95Al0.05O3 was absent in the case of Y0.95Bi0.05Mn0.8Fe0.2O3, which indicated weakened ferromagnetic interactions in the latter. This may be related to the decreased oxygen content in Y0.95Bi0.05Mn0.8Fe0.2O3. Bismuth substitution can strengthen the antiferromagnetic interactions in YMn0.8Fe0.2O3.

Enhanced room temperature multiferroic characteristics in hexagonal LuFe1−xNixO3 (x = 0 − 0.3) nanoparticles

Single phase polycrystalline LuFe1−xNixO3 (x=0−0.3) (LFNO) nanoparticles are synthesized using the sol-gel method. X-ray diffraction measurements revealed that the crystal structure of Ni-doped samples is isomorphic to hexagonal LuFeO3 (LFO). The phase pure hexagonal P63cm symmetry exists for 0≤x≤0.3, and the secondary phases appear for x≥0.4. Raman spectra show a shift in the mode frequency corresponding to the changes in Lu-O and Fe-O bond lengths with Ni doping. An enhancement in the magnetization is observed for LFNO throughout the temperature range (400–5K) compared to LFO. The antiferromagnetic state of LFO becomes ferrimagnetic at low temperatures, and a net magnetization is observed at room temperature with Ni doping. As Ni concentration increases, a systematic increment in the ferroelectric polarization is observed. This enhancement in polarization is believed to be due to the distortion in FeO5 cage, while the improvement in magnetic properties is due to the induced magnetic interactions, caused by the Fe-Ni interactions on the triangular lattice with Ni doping in LuFeO3.

Investigation of structural, electronic and optical properties of hexagonal LuFeO3 using first principles LDA + U

Extensive research of a new multiferroic material has been conducted due to its possibility of existing in more than one ferroic order—for example ferroelectricity, (anti-)ferromagnetism and ferroelasticity—which gives additional functionalities to a material. In this work, the structural, electronic and optical properties of LuFeO3 in the antiferromagnetic (AFM) hexagonal P63cm phase have been investigated using first-principles calculations based on the local density approximation with Hubbard U (LDA + U) method in the frame of plane-wave pseudopotential density functional theory (DFT). The LDA + U method is implemented to improve the accuracy of the calculated properties of materials which contain strong electronic localized d/f orbitals. The Coulomb repulsion U of 3 eV was chosen. The G-type AFM arrangement of hexagonal LuFeO3 is reported to stabilize more than another type of spin AFM and ferromagnetic arrangement. The direct band gap of 0.54 eV is calculated from the band structure. Other than that, the dielectric constant is shown to be 4.6. The theoretical data from this work could be used as a reference for experimental works and provides important information for future applications.

The effect of Dy–Fe co-doping on the structural and magnetic properties of h-YMnO3

The polycrystalline samples of YMn1−xFexO3 (YMFO) and co-doped Y1−yDyyMn0.8Fe0.2O3 (YDMFO) with x ranging from 0 to 0.7 and y from 0 to 0.2 are synthesized by solid state reaction method and comparatively investigated. Structural analysis by X-Ray diffraction and Raman scattering shows that sample crystallizes in a pure hexagonal P63 cm phase for YMFO with x ≤ 0.2 and all the co-doped compounds. The tilting angle α of MnO5 bipyramids decreases, while the Mn–OT bond lengths are extended with increasing Fe3+ concentration. These parameters are almost not changed in co-doped samples. The structural results indicate that the distortion of MnO5 is weakened by Fe3+ substitution at Mn3+ site, while those are not altered in YDMFO. Magnetic measurements indicate that the antiferromagnetic interaction is suppressed by Fe3+ doping, which is ascribed to the weakened distortion of MnO5 bipyramids. Weak ferromagnetic ordering is found in Dy–Fe co-doped YMnO3 samples at low temperature, which is resulted from the Dy3+−Fe3+/Mn3+ exchange interaction.

Characterization of Oxygen Storage and Structural Properties of Oxygen-Loaded Hexagonal RMnO3+δ (R = Ho, Er, and Y)

Single-phase polycrystalline samples of stoichiometric RMnO3+δ (R = Er, Y, and Ho) were achieved in the hexagonal P63cm structure through solid state reaction at ∼1300 °C. Thermogravimetric measurements in oxygen atmospheres demonstrated that samples with the larger Ho and Y show rapid and reversible incorporation of large amounts of excess oxygen (0.3 > δ > 0) at an unusually low temperature range of ∼190–325 °C, indicating the industrial usefulness of RMnO3+δ materials for lower cost thermal swing adsorption processes for oxygen separation from air. Further increase of the excess oxygen intake to δ ∼ 0.38 was achieved for all the investigated materials when annealed under high pressures of oxygen. The formation of three oxygen stable phases with δ = 0, 0.28, and 0.38 was confirmed by thermogravimetric measurements, synchrotron X-rays, and neutron diffraction. In situ synchrotron diffraction proved the thermal stability of these single phases and the regions of their creation and coexistence, and demonst...

Effect of atmosphere annealing on the magnetic and dielectric properties of YMn0.8Fe0.2O3 multiferroic ceramics

YMn0.8Fe0.2O3 ceramics were annealed in oxygen and nitrogen atmosphere, respectively, and the magnetic and dielectric properties were investigated. The structures of the annealed ceramics were maintained in hexagonal P63cm space group. The magnetic structures of the annealed ceramics were not uniform, and predominantly antiferromagnetic with weak ferromagnetic properties at low temperature was detected. Cluster glass state behavior was observed in the O2-annealed ceramics and absent in the N2-annealed ceramics, which indicated the weakened ferromagnetic interaction in the latter. This was related to the decreased oxygen content in the N2-annealed ceramics. Because of the varied valence distribution of Mn and Fe ions in the ceramics annealed in different atmosphere, the dielectric properties were obviously modified. The dielectric permittivity and dielectric loss were smaller in the O2-annealed ceramics, which should be related to the lower content of Mn2+ ions in the mixed-valence structure of Mn ions. These results indicated that the magnetic and dielectric properties of YMn0.8Fe0.2O3 ceramics were strongly dependent on the oxygen vacancy content and the mixed-valence structure of magnetic ions.

Room temperature structure vibrational and dielectric properties of Ho modified YMnO3

The structural, vibrational, and dielectric properties of bulk Ho-doped Y1−xHoxMnO3 (x = 0, 0.03, 0.05) solids prepared by standard solid-state reaction route were investigated. X-ray diffraction (XRD) patterns confirmed the hexagonal P63cm structure of Y1−xHoxMnO3 (x = 0.0, 0.03, 0.05) ceramics. Rietveld refinements of XRD data revealed that the doping ions led to unit cell contraction in three directions due to nearly equal ionic radii of Ho3+ ion (0.901 Å) substituted at the Y-site ion. The grain size of Ho-doped solids varied from 5 to 10 μm. For pristine h-YMnO3, the experimentally observed Raman scattering lines at around 151, 305, 460, and 682 cm−1 are of A1 symmetry, those at 410 cm−1 are of E1 symmetry, and the lines at 139 and 219 cm−1 are of E2 symmetry. Another interesting observation is the existence of an A1 line at 682 cm−1 and an E1 line at about 410 cm−1 which are much stronger than the remaining lines of A1 and E1 symmetries, respectively. The high value of dielectric constant and dielectric loss tangent at low frequency is explained by space charge polarization and the saturation in the high-frequency region is due to the electric dipoles not being in pace with the frequency of the applied electric field.

Investigation of magnetic, dielectric and electrical properties of Ce-substituted YMn0.8Fe0.2O3 multiferroic ceramics

Y1−xCexMn0.8Fe0.2O3 (x = 0, 0.05 and 0.1) solid solution ceramics were prepared and the magnetic, dielectric and electrical properties were investigated. The structures of the present ceramics were maintained in hexagonal P63cm space group as the ferroelectric YMnO3. The magnetic structures of the solid solution ceramics were not uniform, and all the compositions were predominantly antiferromagnetic with weak ferromagnetic properties at low temperature. The antiferromagnetic transition temperatures of the present ceramics were decreased upon Ce substitution, originating in the weakened antiferromagnetic exchange interactions of Mn3+ ions. Varied valence distributions of Fe and Mn ions led to the effective paramagnetic moment decreasing with Ce introduction. Compared with YMn0.8Fe0.2O3, the dielectric loss of Y0.95Ce0.05Mn0.8Fe0.2O3 ceramics was reduced and the resistivity was enhanced, which should be related to the decreased Mn2+ content in the mixed structure of Mn ions. Whereas, the presence of Mn3+–Mn4+ ions pair led to the increased conductivity and dielectric loss in the Y0.9Ce0.1Mn0.8Fe0.2O3 ceramics.

Structural, magnetic, and oxygen storage properties of hexagonal Dy1−xYxMnO3+δ

Single-phase polycrystalline samples of hexagonal Dy1−xYxMnO3+δ were synthesized over the full solubility range under reducing oxygen partial pressure conditions. Rietveld refinements using neutron powder diffraction data (NPD) confirmed the persistence of the parent hexagonal P63cm (Space group #185) structure for all the stoichiometric (δ=0) samples. The (Mn–O) bond lengths derived from NPD data were found to be consistent with non-degenerate Mn+3 high-spin state e“2e′2 in trigonal–bipyramidal coordination. Detailed thermogravimetric measurements show large reversible oxygen storage/release capabilities (within the range of oxygen content 3.0–3.3) at unusually low temperatures of 200–400°C in air or oxygen atmospheres. A phase transition to a new hexagonal phase (Hex1) was observed to take place around δ~0.29 for the oxygen-loaded Dy-rich samples (x<0.5). Analysis of x-ray and neutron diffraction data using Rietveld refinements demonstrate that the superstructure is formed by tripling the c-axis of the original P63cm unit cell. Modeling of the Hex1 superstructure converged to a structural symmetry consistent with the noncentrosymmetric space group R3c (#161). In magnetization measurements, a ferrimagnetic order for DyMnO3.0 was observed below 8K. The dilution of Dy magnetism with nonmagnetic yttrium results in reducing both the transition temperature and the effective paramagnetic moment.

Influence of growth ambience and doping on the structural properties of multiferroic DyMnO3

We report on the single crystal growth of 50% Sr and Y doped multiferroic DyMnO3 using optical floating zone technique. A comparison of the effect of growth ambience and of chemical substitution on the crystal structure of DyMnO3 is attempted. It is observed that DyMnO3 adopts Pm3m cubic structure with 50% Sr doping whereas with 50% Y doping, the crystal structure is hexagonal P63cm. Orthorhombic Pnma structure is adopted by DyMnO3 when grown in air, whereas hexagonal P63cm structure is obtained when grown under the ambience of argon. The structural polymorphism is discussed in terms of difference in ionic sizes of Sr, Y and Dy, comparable Gibbs free energies and coordination schemes of surrounding oxygens for hexagonal and orthorhombic structures of DyMnO3.

Synthesis and Oxygen Storage Capacities of Hexagonal Dy1–xYxMnO3+δ

Polycrystalline samples of hexagonal P63cm (space group No. 185) Dy1–xYxMnO3+δ were synthesized over complete solubility ranges under reducing conditions with oxygen partial pressures of 5 ppm to air at high temperatures (1200–1400 °C). All samples displayed unusually large, reversible oxygen content excess (0 ≤ δ ≤ 0.35) at low temperatures of ∼200–400 °C under air and oxygen atmospheres in thermogravimetric measurements. During heating, stoichiometric samples showed rapid uptakes of oxygen at 200–300 °C and equally fast releases of oxygen at 275–375 °C when materials transformed back to the stoichiometric P63cm phase. Upon slow cooling from the stoichiometric P63cm phase, all samples displayed rapid uptakes of oxygen at 200–350 °C. Similar changes were observed as a function of oxygen partial pressure at constant temperature. Increased, reversible changes in oxygen content were also achieved by high-pressure annealings in oxygen and hydrogen reduction at 400 °C. Thermogravimetric and X-ray diffraction m...

High-pressure structural stability of multiferroic hexagonalRMnO3(R=Y, Ho, Lu)

Structural changes in REMnO3 (RE= Y, Ho, Lu) under high pressure were examined by synchrotron x-ray diffraction methods at room temperature. Compression occurs more readily in the ab plane than along the c-axis. Under hydrostatic pressure (~11 GPa), the atoms hold their approximate ambient fractional positions in the unit cell and the spontaneous polarization shows no significant change. With increased pressure, a pressure-induced hexagonal to orthorhombic phase transition was observed starting at ~ 22GPa for Lu(Y)MnO3. A small volume fraction of Lu(Y)MnO3 is converted to the orthorhombic phase when the pressure is increased to 35 GPa and the orthorhombic phase is maintained on pressure release. High pressure IR absorption spectroscopy and Mn K-edge near edge x-ray absorption spectroscopy confirm that the hexagonal P63cm structure is stable below ~20 GPa and the environment around Mn ion is not changed. Shifts in the unoccupied p-band density of states with pressure are observed in the Mn K-Edge spectra. A schematic pressure-temperature phase diagram is given for the small ion REMnO3 system.

Effect of Oxygen Partial Pressure on the Formation of Metastable Phases from an Undercooled YbFeO3 Melt Using an Aerodynamic Levitator

The Yb2O3–Fe2O3 system was studied to investigate the effect of oxygen partial pressure on the formation of metastable phases over a wide range of oxygen partial pressures from 105 to 10−1 Pa. Two kinds of metastable phases, with space groups of P63cm and P63/mmc, were found through rapid solidification of an undercooled YbFeO3 melt in an atmosphere with reduced Po2. The crystal structure of the as‐solidified samples changed from orthorhombic Pbnm to hexagonal P63cm and P63/mmc with decreasing Po2. X‐ray diffractometric and scanning electron microscopic results confirmed the existence of various phases in the as‐solidified samples. The stabilities of each phase were studied by annealing the bulk sample in the thermogravimetric–differential thermal analysis (TG‐DTA) furnace up to 1673 K, and the equilibrium phase diagram was constructed for the Yb–Fe–O system at 1473 K. TG analysis showed an increase of the sample mass during annealing and revealed that the existence of Fe2+, which has an ionic radius larger than that of Fe3+, decreases the tolerance factor and therefore destabilizes the perovskite structure.

Structural and magnetic properties of the solid solution ([formula omitted]) YMn1−x(Cu3/4Mo1/4)xO3

Recently, the ferroelectromagnet YMnO3 has been the focus of interest because it exhibits both antiferromagnetism (Néel temperature ∼80 K) and ferroelectricity (Curie temperature ∼914 K). There have been no reports of complete YMn1−xMxO3 solid solutions in which substitution of the foreign M cation preserves the hexagonal P63cm structure. In contrast there exist several homeotypic phases with the general formula, Ln1+nCunMO3+3n (n=1 (M=Ti), 2 (M=V) and 3 (M=Mo); Ln: lanthanide). Several YMn1−x(Cu3/4Mo1/4)xO3 compounds have been synthesized. The solid solution, from YMnO3 (x=0) to YCu3/4Mo1/4O3 (x=1) has been characterized by X-ray diffraction and transmission electron microscopy study. For 0<x<0.9, the compounds are found to crystallize in the non-centrosymmetric structure, space group P63cm, of YMnO3. The Mn-free end member, x=1, crystallizes in a complex multiple cell, the superstructure being associated to Cu3+/Mo6+ cationic ordering. Dilution of the Mn3+ magnetic array by the paramagnetic (Cu2+) and diamagnetic (Mo6+) cations is found to decrease the antiferromagnetic ordering temperature and it becomes undetectable for x⩾0.5 compositions.