Phase and electrical behaviour in the Bi14W1 − xLaxO24 − 3x/2 system

The compositional and thermal dependencies of phase and electrical behaviour of compositions in the system Bi₁₄W_1 − xLa_xO_{24 − 3x/2} (0.00 < x < 1.00) have been studied by X-ray powder diffraction, differential thermal analysis and a.c. impedance spectroscopy. The system exhibits polymorphism and phase separation, which shows both compositional and thermal dependence. Compositions with x = 0.25 and x = 0.50 exhibit a single phase tetragonal structure at room temperature. In contrast, the x = 0.75 composition at room temperature shows a mixture of a cubic phase and a secondary β-Bi₂O₃ related tetragonal phase. A full solid solution is observed at high temperatures, corresponding to the occurrence of a δ-Bi₂O₃ type phase. The appearance of the various phases correlates well with the observed electrical behaviour. The x = 0.75 composition exhibits exceptionally high conductivity at high temperatures (σ₈₀₀ = 1.34 S cm^− 1), but also shows significant phase separation at lower temperatures.     

Electrical properties of SrBi2(Nb0.5Ta0.5)2O9 ceramics

Aurivillius SrBi₂(Nb_0.5Ta_0.5)₂O₉ (SBNT 50/50) ceramics were prepared using the conventional solid-state reaction method. Scanning electron microscopy was applied to investigate the grain structure. The XRD studies revealed an orthorhombic structure in the SBNT 50/50 with lattice parameters a=5.522 Å, b=5.511 Å and c=25.114 Å. The dielectric properties were determined by impedance spectroscopy measurements. A strong low frequency dielectric dispersion was found to exist in this material. Its occurrence was ascribed to the presence of ionized space charge carriers such as oxygen vacancies. The dielectric relaxation was defined on the basis of an equivalent circuit. The temperature dependence of various electrical properties was determined and discussed. The thermal activation energy for the grain electric conductivity was lower in the high temperature region (T>303.6 °C, E_a−ht=0.47 eV) and higher in the low temperature region (T<303.6 °C, E_a−lt=1.18 eV).     

Ordered fluorite phases in the Bi2O3-Ta2O5 system: A structural and electrical investigation

Structure and electrical behaviour are reported for the system Bi_1-xTa_xO_1.5 + x (0.167 ≤ x ≤ 0.250). In the compositional range 0.200 < x ≤ 0.250 an incommensurately modulated pseudo-cubic phase (type II) is observed, with the appearance of a larger pseudo-cubic phase in the region 0.167 ≤ x ≤ 0.200. Structural analysis of the type II phases by neutron diffraction reveals subtle changes in the oxide ion distribution with temperature, associated with changes in the incommensurate modulation parameter. Analysis of the defect structure of the type II phase reveals chains of tantalate octahedra as a likely structural motif. It is proposed that these chains facilitate an electronic contribution to total conductivity at low temperatures through electron hopping along the chains. Changes in oxide ion vacancy ordering may explain the observed non-linear behaviour in the thermal expansion of lattice parameter and Arrhenius plots of total conductivity.     

Synthesis, structure and magnetic properties of the new double perovskite Ca2CrSbO6

The title double perovskite has been synthesized by solid-state reaction in air. The crystal structure has been studied from powder X-ray diffraction data. Rietveld fits to the pattern show that this compound has a monoclinic symmetry [a=5.4932(3) Å, b=5.4081(3) Å, c=7.6901(5) Å, β=90.0022(1)°, at 300 K] defined in the space group P2₁/n, where the Cr and Sb cations are almost completely ordered in the B-sublattice of the perovskite structure. Magnetic susceptibility and magnetization measurements show that this compound behaves as a Curie-Weiss paramagnet at high temperatures with μ_eff=3.53(1) μ_B and θ_P=8 K, and exhibits a robust ferromagnetic component below the ordering temperature of T_C=13 K, with a saturation magnetization of 2.36 μ_B/f.u. at 5 K. To our knowledge, this is the first example of a ferromagnetic double perovskite containing a non-magnetic element, such as Sb, occupying one half of the B positions of the perovskite structure.     

Synthesis and characterization of Co7(OH)12(C2H4S2O6)(H2O)2—a single crystal structural study of a ferrimagnetic layered cobalt hydroxide

A novel layered hydrotalcite-like material, Co₇(H₂O)₂(OH)₁₂(C₂H₄S₂O₆), has been prepared hydrothermally and the structure determined using single crystal X-ray diffraction (a=6.2752(19) Å, b=8.361(3) Å, c=9.642(3) Å, α=96.613(5)°, β=98.230(5)°, γ=100.673(5)°, R1=0.0551). The structure consists of brucite-like sheets where 1/6 of the octahedral sites are replaced by two tetrahedrally coordinated Co(II) above and below the plane of the layer. Ethanedisulfonate anions occupy the space between layers and provide charge balance for the positively charged layers. The compound is ferrimagnetic, with a Curie temperature of 33 K, Curie-Weiss θ of −31 K, and a coercive field of 881 Oe at 5 K.     

Structural and ac electrical properties of a newly synthesized single phase rare earth double perovskite oxide: Ba2CeNbO6

A single phase rare earth double perovskite oxide Ba₂CeNbO₆ (BCN) is synthesized by solid-state reaction technique for the first time. The X-ray diffraction pattern of the sample at room temperature shows monoclinic structure, with the lattice parameters, a=5.9763 Å, b=5.975 Å and c=8.48 Å and β=90.04°. Impedance spectroscopy is used to study the ac electrical behavior of this material as a function of frequency (10²-10⁶ Hz) at various temperatures (30-450 °C). A relaxation is observed in the entire temperature range. Conduction mechanism is investigated by fitting the complex impedance data to Cole-Cole equation. Complex impedance plane plots show only one semicircular arc, indicating only the grain contribution of dielectric relaxation. The scaling behavior of imaginary part of electric modulus (M″) and imaginary part of electrical impedance (Z″) suggests that the relaxation describes the same mechanism at various temperatures. The frequency dependence of conductivity is interpreted in terms of the jump relaxation model and is fitted to Jonscher's power law. The values of dc conductivities extracted from the Jonscher power law varies from 2.79×10⁻⁷ to 3.5×10⁻⁵ Sm⁻¹ with the increase in temperature from 100 to 450 °C. The activation energies (0.37 eV) extracted from M″(ω) and Z″(ω) peaks are found to follow the Arrhenius law.     

Formation of a cubic Sr2MnWO6 phase at elevated temperature; a neutron powder diffraction study

In a temperature dependent neutron powder diffraction (NPD) study we observed the high temperature cubic phase at 973 K in the polycrystalline double perovskite Sr₂MnWO₆. Rietveld analysis of the NPD data shows that the room temperature tetragonal phase exists up to 573 K (space group P4₂/n, a=8.0119 (4) Å, c=8.0141(8) Å). At 773 K, the primitive tetragonal symmetry change to body-centred tetragonal (space group I4/m, a=5.6935(5) Å, c=8.077(1) Å) and finally at 973 K it becomes face-centred cubic (space group Fm-3m, a=8.0864(8) Å). The changes in the structural symmetry are connected to the small distortion of the B-site octahedra, which are insensitive to the Differential Thermal Analysis (DTA) signal.     

Structural phase transitions at high-temperature in double perovskite Sr2GdRuO6

The crystal structure evolution of the Sr₂GdRuO₆ complex perovskite at high-temperature has been investigated over a wide temperature range between 298 K≤T≤1273 K. Powder X-ray diffraction measurements at room temperature and Rietveld analysis show that this compounds crystallizes in a monoclinic perovskite-type structure with P2₁/n (#14) space group and the 1:1 ordered arrangement of Ru⁵⁺ and Gd³⁺ cations over the six-coordinate M sites, with lattice parameters a=5.81032(8) Å, b=5.82341(4) Å, c=8.21939(7) Å, V=278.11(6) Å³ and angle β=90.311(2)^o. The high-temperature analysis shows that this material suffers two-phase transitions. At 373 K it adopts a monoclinic perovskite structure with I2/m space group, and lattice parameters a=5.81383(2) Å, b=5.82526(4) Å, c=8.22486(1) Å, V=278.56(2) Å³ and angle β=90.28(2)^o. Above of 773 K, it suffers a phase transition from monoclinic I2/m to tetragonal I4/m, with lattice parameters a=5.84779(1) Å, c=8.27261(1) Å, V=282.89(5) Å³ and angle β=90.02(9)^o. The high-temperature phase transition from monoclinic I2/m to tetragonal I4/m is characterized by strongly anisotropic displacements of the anions.     

Crystal structure,ionic conductivity and dielectric relaxation studies in the (C5H10N)2BiBr5 compound

The synthesis and crystal structure of the bis (3-dimethylammonium-1-propyne) pentabromobismuthate(III) salt are given in the present paper. After an X-ray investigation, it has been shown that the title compound crystallizes at 298 K in a centrosymmetric monoclinic system, in the space group C₂/c with the following lattice parameters a=12.9034(3) Å, b=19.4505(6) Å, c=8.5188(2) Å, β=102.449(2). Not only were the impedance spectroscopy measurements of (C₅H₁₀N)₂BiBr₅ carried out from 209 Hz to 5 MHz over the temperature range of 318 K–373 K, but also its ac conductivity evaluated. Besides, the dielectric relaxation was examined using the modulus formalism. Actually, the near values of activation energies obtained from the impedance and modulus spectra confirms that the transport is of an ion hopping mechanism, dominated by the motion of the H⁺ ions in the structure of the investigated material.     

Intermediate-temperature polymorphic phase transition in KH2PO4: A synchrotron X-ray diffraction study

We have used synchrotron X-ray diffraction to investigate the structural and chemical changes undergone by polycrystalline KH₂PO₄ (KDP) upon heating within the 30-250 °C temperature interval. Our data show evidence of a polymorphic transition at T∼190 °C from the room-temperature tetragonal KDP phase to a new intermediate-temperature monoclinic KDP modification (spacegroup P2₁/m and lattice parameters a=7.590, b=6.209, c=4.530 Å, and β=107.36°). The monoclinic RDP polymorph remains stable upon further heating to 235 °C, and is isomorphic to its RbH₂PO₄ and CsH₂PO₄ counterparts.     

Low-frequency alternative-current magnetic susceptibility of amorphous and nanocrystalline Co60Fe20B20 films

This investigation experimentally studies the low-frequency alternating-current magnetic susceptibility (χ_ac) of amorphous and nanocrystalline CoFeB films by measuring the magnetic field established by passing currents of various frequencies through such films of various thicknesses (t_f). A CoFeB film is sputtered onto a glass substrate with t_f from 100 Å to 500 Å under the following conditions: (a) As-deposited films were maintained at room temperature (RT) and (b) films were post-annealed at T_A=150 °C for 1 h. The samples thus obtained are analyzed in a magnetic field that was generated by an alternating current (AC) at various frequencies from 10 Hz to 25,000 Hz. The experimental results demonstrate that the χ_ac declines as the thickness of the as-deposited sample and the post-annealed sample (T_A=150 °C) increases because the lower coercivity (H_c) of thinner CoFeB films is similar to a soft magnetic characteristic and is associated with a higher χ_ac value. The best χ_ac value is obtained at a thickness of 100 Å under both conditions. The χ_ac value of the post-annealed sample exceeds that of the RT sample at thicknesses from 100 Å to 500 Å because the magneto crystalline anisotropy of the post-annealed sample yields the highest χ_ac value at the optimal resonance frequency (f_res), at which the spin sensitivity is maximal. The X-ray diffraction patterns (XRD) of as-deposited CoFeB films reveal their amorphous structure. The XRD results for the post-annealed films include a main peak at 2θ=44.7° from the body-centered cubic (BCC) nanocrystalline CoFe that indicated a (110) textured structure. Post-annealing treatment caused that the amorphous structure to become more crystalline by a thermally driven process, such that the χ_ac value of the post-annealed sample exceeded that of the RT sample. This experimental result demonstrates that the χ_ac value decreased as the thickness of the thin film increased. Finally, the CoFeB thin films had the best χ_ac at low frequency (<50 Hz) following post-annealing treatment. The results obtained under the two conditions indicate that the maximum χ_ac value and the optimal f_res of a 100 Å-thick CoFeB thin film were 1.6 and 30 Hz, respectively, following post-annealing at T_A=150 °C for 1 h, suggesting that a 100 Å-thick CoFeB thin film that has been post-annealed at T_A=150 °C can be utilized as a gage sensor and in transformer applications at low frequencies.     

Crystal and magnetic structures of Y2CrS4

Chromium(II) sulfide, Y₂CrS₄, prepared by a solid-state reaction of Y₂S₃ and CrS, showed an antiferromagnetic transition at 65 K. The neutron diffraction patterns at 10 and 90 K were both well refined with the space group Pca2₁. At 90 K, cell parameters were a=12.5518(13) Å, b=7.5245(8) Å, and c=12.4918(13) Å. At 10 K, magnetic peaks were observed, which could be indexed on the same unit cell. Magnetic moments of chromium ions were parallel to the b-axis and antiferromagnetically ordered in each set of the 4a sites.     

Immiscibility in the NiFe2O4-NiCr2O4 spinel binary

The solid solution behavior of the Ni(Fe_1−nCr_n)₂O₄ spinel binary is investigated in the temperature range 400-1200 °C. Non-ideal solution behavior, as exhibited by non-linear changes in lattice parameter with changes in n, is observed in a series of single-phase solids air-cooled from 1200 °C. Air-annealing for 1 year at 600 °C resulted in partial phase separation in a spinel binary having n=0.5. Spinel crystals grown from NiO, Fe₂O₃ and Cr₂O₃ reactants, mixed to give NiCrFeO₄, by Ostwald ripening in a molten salt solvent, exhibited single-phase stability down to about 750 °C (the estimated consolute solution temperature, T_cs). A solvus exists below T_cs. The solvus becomes increasingly asymmetric at lower temperatures and extrapolates to n values of 0.2 and 0.7 at 300 °C. The extrapolated solvus is shown to be consistent with that predicted using a primitive regular solution model in which free energies of mixing are determined entirely from changes in configurational entropy at room temperature.     

Crystal structure and dielectric properties of ordered perovskites Ba2BiSbO6 and BaSrBiSbO6

Neutron powder diffraction studies showed that the ordered perovskites Ba₂BiSbO₆ (BBS) and BaSrBiSbO₆ (BSBS) crystallize in a rhombohedral structure with the space group R3¯. The room-temperature lattice parameters are a=6.0351(2) Å; α=60.202(1)° and a=5.9809(2) Å; α=60.045(2)°, respectively. BBS exhibits a dielectric anomaly near room temperature which may be related to structural transition from the R3¯ to low-temperature monoclinic I2/m symmetry. BSBS shows a dielectric anomaly near 723 K which coincides with a phase transition from the rhombohedral to cubic (Fm3¯m) structure. In contrast to BBS, BSBS does not undergo structural transition below room temperature.     

Crystallographic study of the phase transition in (Me4P)2ZnBr4

The compound (Me₄P)₂ZnBr₄, a member of the β-K₂SO₄ structure class, undergoes a phase transition at 84°C from the room temperature space group P12₁/c1 to the parent Pmcn structure. The room temperature structure corresponds to a ferrodistortive transition of B_1g symmetry at the zone center. At room temperature, the compound has lattice constants a=9.501(1), b=16.055(2), c=13.127(2) Å and β=90.43(1)°. For the high temperature phase, the orthorhombic cell has dimensions a=9.466(2), b=16.351(3) and c=13.284(2) Å. The structures consist of two crystallographically independent Me₄P⁺ cations and the ZnBr₄²⁻ anions. In the room temperature phase, all three ionic species show substantial displacement from the mirror plane perpendicular to the a-axis that exists in the high temperature phase, as well as rotations out of that plane. The thermal parameters of the cations are indicative of substantial librational motion. Measurements of lattice parameters have been made at 2-5°C intervals over the temperature range 40-140°C. The changes in the lattice constants appear continuous at T_c (within experimental limits) indicating that the phase transition is likely second-order. The a lattice constant shows an anomalous shortening as T_c is approached. Thermal expansion coefficients are calculated from this data. An application of Landau theory is used to derive the temperature dependencies of spontaneous shear strain and corresponding elastic stiffness constants associated with the primary order parameter.     

Crystal chemistry of layered carbide, Ti3(Si0.43Ge0.57)C2

 The crystal structure of a layered ternary carbide, Ti₃(Si_0.43Ge_0.57)C₂, was studied with single-crystal X-ray diffraction. The compound has a hexagonal symmetry with space group P6₃/mmc and unit-cell parameters a=3.0823(1) Å, c=17.7702(6) Å, and V=146.21(1) Å³. The Si and Ge atoms in the structure occupy the same crystallographic site surrounded by six Ti atoms at an average distance of 2.7219 Å, and the C atoms are octahedrally coordinated by two types of symmetrically distinct Ti atoms, with an average C-Ti distance of 2.1429 Å. The atomic displacement parameters for C and Ti are relatively isotropic, whereas those for A (=0.43Si+0.57Ge) are appreciably anisotropic, with U₁₁ (=U₂₂) being about three times greater than U₃₃. Compared to Ti₃SiC₂, the substitution of Ge for Si results in an increase in both A-Ti and C-Ti bond distances. An electron density analysis based on the refined structure shows that each A atom is bonded to 6Ti atoms as well as to its 6 nearest neighbor A site atoms, whether the site is occupied by Si or Ge, suggesting that these bond paths may be significantly involved with electron transport properties.     

Transmission electron microscopy studies of HfO2 thin films grown by chloride-based atomic layer deposition

Detailed transmission electron microscopy characterization of HfO₂ films deposited on Si(1 0 0) using atomic layer deposition has been carried out. The influence of deposition temperature has been investigated. At 226 °C, a predominantly quasi-amorphous film containing large grains of cubic HfO₂ (a₀ = 5.08 Å) was formed. Grain morphology enabled the nucleation sites to be determined. Hot stage microscopy showed that both the cubic phase and the quasi-amorphous phase were very resistant to thermal modification up to 500 °C. These observations suggest that nucleation sites for the growth of the crystalline cubic phase form at the growing surface of the film, rather homogeneously within the film. The films grown at higher temperatures (300-750 °C) are crystalline and monoclinic. The principal effects of deposition temperature were on: grain size, which coarsens at the highest temperature; roughness with increases at the higher temperatures due to the prismatic faceting, and texture, with texturing being strongest at intermediate temperatures. Detailed interfacial characterization shows that interfacial layers of SiO₂ form at low and high temperatures. However, at intermediate temperatures, interfaces devoid of SiO₂ were formed.     

Magnetic properties and structural characterization of Sr2RuHoO6 complex perovskite

We report an experimental study of the crystallographic lattice, morphologic characteristics and magnetic feature of Sr₂RuHoO₆ complex perovskite, which is used as a precursor in the fabrication process of the superconducting ruthenocuprate RuSr₂HoCu₂O₈. The samples were produced through the standard solid state reaction. A Rietveld refinement of experimental X-ray diffraction patterns shows that the material crystallizes in a monoclinic structure, which belongs to the P21/n (#14) space group, with lattice parameters a=5.7719(6) Å, b=5.8784(5) Å, c=8.1651(9) Å, and tilt angle β=90.200°. Magnetic susceptibility measurements reveal the occurrence of an antiferromagnetic ordering for a Néel temperature T_N=10.1 K. From the Curie-Weiss fitting of the paramagnetic regime we obtain an effective magnetic moment of 11.31 μ_B.     

Phase transition of Ta2NiO6 with the trirutile-type structure under high pressure and high temperature

High-pressure phase transition of Ta₂NiO₆ with the trirutile-type structure was investigated from the viewpoint of crystal chemistry. A new quenchable high-pressure phase was found in the pressure range higher than 7 GPa and 900°C. The high-pressure phase has an orthorhombic cell (a=4.797(1) Å, b=5.153(2) Å and c=14.85(1) Å and space group; Abm2), and it is more dense by 9.6% than the trirutile-structured phase. Infrared spectra of the trirutile-type phase and the high-pressure phase show that Ni²⁺ ions in the high-pressure phase are still in octahedral sites. The crystal structure of the high-pressure phase is considered as a cation-ordering trifluorite-type structure, which can be stabilized by a crystal field effect of Ni²⁺ ions.