Crystal structure, magnetic and thermal properties of LaFeTeO6

LaFeTeO₆ was prepared by solid state reaction of La₂O₃, Fe₂O₃ and TeO₂ in 1:1:2 molar ratios and characterized by powder X-ray diffraction, thermogravimetry and magnetometry. The detailed crystal structure analysis was carried out by Rietveld refinement. LaFeTeO₆ crystallizes in a trigonal lattice with unit cell parameters: a = 5.2049(1) Å and c = 10.3457(2) Å, V = 242.73(2) Å³. The crystal structure is built from sheets of the edge shared FeO₆ and TeO₆ octahedra stacked along the c-axis. These sheets are connected together by La³⁺ ions. Thermogravimetric analysis of the compound showed it to be thermally stable up to 1323 K and continuous loss of TeO₂ was observed above 1323 K leading to the formation of LaFeO₃. High temperature XRD studies revealed a normal expansion behavior of the compound. Temperature and field dependent magnetization of LaFeTeO₆ showed paramagnetic behavior in the temperature range of 3-300 K. The effective magnetic moment per Fe³⁺ ion (5.14 μB) indicates the high spin d⁵ state of Fe³⁺ ion.     

Role of chromium ion valence states in ZnO-As2O3-Sb2O3 glass system by means of spectroscopic and dielectric studies

ZnO-Sb₂O₃-As₂O₃ transparent glasses containing small concentrations of chromium ions (introduced as Cr₂O₃) ranging from 0 to 0.2 mol% is prepared. A number of studies viz., XRD, SEM, DTA, optical absorption, FT-IR, Raman, ESR spectra, magnetic susceptibility and dielectric properties (constant ?′, loss tan δ, ac. conductivity σ_ac over a wide range of frequency and temperature as well as dielectric breakdown strength at room temperature) of these glasses have been carried out as a function of chromium ion concentration. The results have been analysed in the light of different oxidation states of chromium ions. The analyses indicates that when the concentration of chromium ions is low, these ions mostly exist in Cr⁶⁺ and Cr⁵⁺ states, occupy network forming positions with CrO₄²⁻ and CrO₄³⁻ structural units respectively and increase the rigidity of the glass network. When the concentration of chromium ions is gradually increased, these ions seem to be existing mostly in Cr³⁺ state.     

Crystal structure and spectroscopic properties of a new oxyarsenate Li0.5Ni0.25TiOAsO4

The new oxyarsenate Li_0.5Ni_0.25TiOAsO₄ has been synthesized and studied by a combination of X-ray powder diffraction, neutrons powder diffraction and vibrational spectroscopy. Li_0.5Ni_0.25TiOAsO₄ crystallizes in the monoclinic P2₁/c space group with the unit cell parameters: a = 6.5854(3) Å, b = 7.4665(4) Å, c = 7.4969(4) Å, β = 89.884(6)°, V = 368.62(1) Å³ and Z = 4. The structure has been determined at room temperature from neutrons diffraction by the Rietveld method analysis. It is formed by a 3D network of TiO₆ octahedra and AsO₄ tetrahedra sharing corners. Structural refinement shows a partial and a statistical occupancy of 2a and 2b sites by Li⁺ and Ni²⁺ ions. TiO₆ octahedra are linked together by corners and form infinite chains along c-axis. Raman and infrared studies confirm the existence of -TiOTi- chains. Diffuse reflectance spectrum indicates the presence of octahedrally coordinated Ni²⁺ ions.     

Ti-Ca-Al-doped YCrO3 pigments: XRD and UV-vis investigation

Compounds based on the YCrO₃ perovskite structure, doped with Al, Ti and Ca, were studied by XRD and DRS, aiming at assessing their potential as green ceramic pigments. Nine samples were synthesized by the ceramic route according to the Y_1−yCa_yCr_1−x−yAl_xTi_yO₃ stoichiometry, where 0 < x < 0.5 and 0 < y < 0.2. Doping affected the phase composition (minor amounts of Y₂O₃ and Y-Ti oxides were found) and induced a series of structural rearrangements in perovskite without symmetry changes, involving unit cell parameters, metal-oxygen distances and distortion of cation sites. Different trends were observed varying x and y. Optical properties were influenced by changes in perovskite crystal structure and stoichiometry. In particular, the oxidation of Cr³⁺ to Cr⁴⁺ had a deleterious effect on its green colour, turning to gray-brown. The occurrence of Cr⁴⁺ in the perovskite lattice was required to balance the replacement of Y³⁺ by Ca²⁺ ions not fully compensated by the designed coupled substitution of Ti⁴⁺ for Cr³⁺.     

Controllable synthesis and magnetic property of BiMn2O5 crystals

Uniform submicron BiMn₂O₅ particles were prepared via a facile one-step hydrothermal route at low temperature. Bi(NO₃)₃, MnCl₂·4H₂O and KMnO₄ were used as starting materials; KOH as a pH adjustor and also as a mineralizer. Single-crystalline orthorhombic BiMn₂O₅ sample with controllable morphology was obtained. The microstructure strongly depends on the molar ratio of the starting materials, KOH concentration and reaction temperature. X-ray photoelectron spectroscopy shows the existence of Mn⁴⁺ state. Magnetic measurement indicates Néel temperature T_N at 44 K. The susceptibility above T_N obeys the Curie-Weiss law, χ = C/(T − θ), with θ = −350 K. The effective paramagnetic moment μ_eff = 4.66 μ_B/Mn, demonstrating the coexistence of mixed Mn³⁺ and Mn⁴⁺ valences.     

Luminescent properties of Na3YSi3O9 doped with Eu under UV-VUV excitation

The luminescent properties of Na₃Y_1−xSi₃O₉:xEu³⁺ (0.05 ≦ x ≦ 0.80) powder crystals were investigated in UV-VUV region. The Eu³⁺-O²⁻ charge transfer band (CTB) was observed to be located at around 233 nm and the environmental parameter (h_e) was estimated to be about 0.730. The excitation spectrum monitoring the 613 nm red emission from Eu³⁺ ions reveals the host absorption band (HAB) to be around 145 nm. The calculated Commission Internationale de l’Eclairage (CIE) chromaticity coordinates indicate the emission by 233 nm rather than by 147 nm excitation has the better color purity and the possible mechanisms have been proposed. The Eu³⁺-emission showed high quenching concentration due to the isolated YO₆ octahedra in the host and the small h_e for the Eu³⁺ ions and the optimum concentration was determined to be as high as x = 0.65 and 0.30 with 233 and 147 nm excitation, respectively.     

The effect of Cu content on the structure of Ni1−xCuxFe2O4 spinels

A series of Ni_1−xCu_xFe₂O₄ (0 ≤ x ≤ 0.5) spinels were synthesized employing sol-gel combustion method at 400 °C. The decomposition process was monitored by thermal analysis, and the synthesized nanocrystallites were characterized by X-ray diffraction, transmission electron microscopy, infra-red and X-ray photoelectron spectroscopy. The decomposition process and ferritization occur simultaneously over the temperature range from 280 °C to 350 °C. TEM indicates the increase of lattice parameter and particle size with the increase of copper content in accordance with the XRD analysis. Cu²⁺ can enter the cubic spinel phase and occupy preferentially the B-sites within x = 0.3, and redundant copper forms CuO phase separately. A broadening of the O 1s region increases with the increment of copper content compared to pure NiFe₂O₄, showing different surface oxygen species from the spinel and CuO. Cu²⁺ substitution favors the occupancy of A-sites by Fe³⁺.     

Crystal structure and ionic conductivity of ruthenium diphosphate ARu2(P2O7)2, A=Li, Na, and Ag, with a tunnel structure

Crystal structure and ionic conductivity of ruthenium diphosphates, ARu₂(P₂O₇)₂ A=Li, Na, and Ag, were investigated. The structure of the Ag compound was determined by single crystal X-ray diffraction techniques. It crystallized in the triclinic space group P−1 with a=4.759(2) Å, b=6.843(2) Å, c=8.063(1) Å, α=90.44(2)°, β=92.80(2)°, γ=104.88(2)°, V=253.4(1) Å³. The host structure of it was composed of RuO₆ and P₂O₇ groups and formed tunnels running along the a-axis, in which Ag⁺ ions were situated. The ionic conductivities have been measured on pellets of the polycrystalline powders. The Li and Ag compounds showed the conductivities of 1.0×10⁻⁴ and 3.5×10⁻⁵ S cm⁻¹ at 150 °C, respectively. Magnetic susceptibility measurement of the Ag compound showed that it did not obey the Curie-Weiss law and the effective magnetic moment decreased as temperature decreased due to the large spin-orbital coupling effect of Ru⁴⁺ ions.     

Characterization and magnetic properties of Sm- and Gd-substituted CoFe2O4 nanoparticles prepared by forced hydrolysis in polyol

Pure nanoparticles of the CoFe_2−xRE_xO₄ (RE = Gd, Sm; x = 0.0, 0.1) system have been prepared by forced hydrolysis in polyol. The insertion of Sm³⁺ and Gd³⁺ cations into the cobalt ferrite structure has been investigated. X-ray micro-analysis (EDX) shows that the RE contents are close to the nominal ones. X-ray diffraction (XRD) evidences a cell size increase with slight distortions in the spinel-like lattice indicating the entrance of RE³⁺ ions. Micro-Raman spectroscopy confirms the cubic inverse-spinel structure and rules out the existence of impurities like hematite. Magnetic measurements (SQUID) show important differences in the magnetic properties of the unsubstituted and substituted particles. All the particles are superparamagnetic at room temperature and ferrimagnetic at low temperature. However, their main magnetic characteristics appear to be directly dependent on the RE content.     

Crystal structure and magnetic properties of double perovskite Mn2FeSbO6

The double perovskite Mn₂FeSbO₆ has been synthesized under pressure 6 GPa and temperature 1000 °C. The crystal structure refinement of Mn₂FeSbO₆ was carried out with the GSAS program suite using X-ray diffraction data. XRD pattern of Mn₂FeSbO₆ was indexed with a monoclinic unit cell (space group P2₁/n) with parameters: a = 5.2431(3) Å, b = 5.3935(3) Å, c = 7.6358(5) Å, β = 89.693(2)°, V = 215.927 Å³, Z = 2. It found that Fe and Sb atoms are completely ordered in 2d and 2c positions of double perovskite structure respectively. According to XPS measurements, manganese in this compound is present as Mn²⁺, whiles the iron - as Fe³⁺. Magnetization measurements revealed the presence about 3 mass% of ferromagnetic impurity in the sample. Dependence of AC susceptibility χ″ from temperature showed that magnetic properties compound are determined probably by transformation in antiferromagnetic state below 19.5 K.     

Specific heat, electrical resistivity and thermoelectric power of YbNi4Si

The studies of the specific heat, electrical resistivity and thermoelectric power of YbNi₄Si are reported. These studies are supported by magnetic susceptibility and X-ray photoemission spectroscopy (XPS) measurements. YbNi₄Si does not order magnetically down to 4 K. Nearly in the whole temperature range studied the magnetic susceptibility follows a Curie law with μ_eff = 4.15 μ_B/f.u. This effective magnetic moment is close to the value expected for the 4f¹³ configuration (4.54 μ_B). The Yb²⁺ and Yb³⁺ peaks observed by XPS in the valence band region confirm the domination of the Yb³⁺ valence state. Based on the specific heat measurements, the electronic specific heat coefficient γ = 25 mJ/mol/K² and the Debye temperature θ_D = 320 K were derived. A quadratic dependence of electrical resistivity at low temperatures has been observed. The Kadowaki-Woods ratio has been discussed. The thermoelectric power has been analyzed in the framework of the two band model.     

Crystal structure and properties of the new complex vanadium oxide K2SrV3O9

The new complex vanadium oxide K₂SrV₃O₉ has been synthesized and investigated by means of X-ray powder diffraction (XPD), electron microscopy and magnetic susceptibility measurements. The oxide has an orthorhombic unit cell with lattice parameters a = 10.1922(2) Å, b = 5.4171(1) Å, c = 16.1425(3) Å, space group Pnma and Z = 4. The crystal structure of K₂SrV₃O₉ has been refined by Rietveld method using X-ray powder diffraction data. The structure contains infinite chains built by V⁴⁺O₅ square pyramids linked to each other via VO₄ tetrahedra. The chains form layers and potassium and strontium cations orderly occupy structural interstices between these layers. Electron diffraction as well as high resolution electron microscopy confirmed the structure solution. Magnetic susceptibility measurements revealed an antiferromagnetic interaction with J of the order of 100 K inside the chains and no long-range magnetic order above 2 K. The origin of the magnetic exchange is likely a result of super-exchange interaction through the two VO₄ tetrahedra linking the polyhedra with the magnetic V⁴⁺ cations.     

Synthesis of spherical nanostructured LiMxMn2−xO4 (M = Ni, Co, and Ti; 0 ≤ x ≤ 0.2) via a single-step ultrasonic spray pyrolysis method and their high rate charge-discharge performances

LiM_xMn_2−xO₄ (M = Ni²⁺, Co³⁺, and Ti⁴⁺; 0 ≤ x ≤ 0.2) spinels were prepared via a single-step ultrasonic spray pyrolysis method. Comparative studies on powder properties and high rate charge-discharge electrochemical performances (from 1 to 15 C) were performed. XRD identified that pure spinel phase was obtained and M was successfully substituted for Mn in spinel lattice. SEM and TEM studies confirmed that powders had a feature of ‘spherical nanostructural’, that is, powders consisted of spherical secondary particles with the size of about 1 μm, which were developed from close-packed primary particles with several tens of nanometers. Substitutions enhanced density of second particles to different extents, depending on M and its content. Charge-discharge tests showed that as-prepared LiMn₂O₄ could deliver excellent rate performance (around 100 mAh/g at 10 C). Ni substitution contributed to improving electrochemical performances. In the voltage range of 4.95-3.5 V, the materials showed much better electrochemical performances than LiMn₂O₄ in terms of capacity, cycleability and rate capability.     

Dielectric properties and substitution mechanism of samarium-doped Ba0.68Sr0.32TiO3 ceramics

Ba_0.68Sr_0.32TiO₃ ceramics of perovskite structure are prepared by solid state reaction method with addition of x mol% Sm₂O₃, and their dielectric properties are investigated. It is found that, integrating with the lattice parameters and tolerance factor t, there is an alternation of substitution preference of Sm³⁺ for the host cations in perovskite lattice. Owing to the replacement of Sm³⁺ ions for Ba²⁺ ions in the A site, T_c rises with the increase of Sm₂O₃ doping when the doping content is below 0.1 mol%; meanwhile, when the content is more than 0.1 mol%, Sm³⁺ ions tend to occupy the B-site, causing a drop of T_c. Owing to the modifications of Sm³⁺ doping, dielectric constant, dissipation factor and temperature stability of dissipation factor are influenced remarkably, making it a superior candidate for environment-friendly applications. Moreover, the creation of oxygen vacancies controls the dielectric constant when the addition is above 0.1 mol%, so the dielectric constant decreases with increasing of samarium.     

Effect of doping with Nd ions on the structural and ferroelectric properties of Ca0.28Ba0.72Nb2O6 single crystal

The crystal structure of Ca_0.28Ba_0.72Nb₂O₆ (CBN-28) crystal with Nd-doping has been determined from X-ray single crystal diffraction data, in the tetragonal system with space group P4bm and the following parameters: a = b = 12.458 Å, c = 3.954 Å, V = 613.688 Å³, and Z = 5. X-ray diffraction results on a Nd-doped CBN-28 single crystal also have demonstrated that Nd³⁺ and Ca²⁺ occupy the same site in the crystal structure. Dielectric and ferroelectric measurements have been performed. Transition from ferroelectric to paraelectric at around 223 °C has been observed. The Nd-doped crystal has a lower Curie temperature (T_m) than that of undoped CBN-28 crystal. The spontaneous polarization (P_s) and coercive electric field (E_c) also decrease compared with their values in the undoped CBN-28 crystal.     

Crystal structure and Eu/Tb doped luminescent properties of a new borate Ba3BiB9O18

Undoped and doped either by Eu³⁺ or Tb³⁺ bismuth borate Ba₃BiB₉O₁₈ was structurally characterized and analyzed by fluorescence spectroscopy. Belonging to synthetic borate member of the family Ba₃XB₉O₁₈, layers of planar triangular B₃O₆ groups connecting with deformed BaO₆ hexagons are interleaved by 9-coordinate Ba atoms, and 6-coordinate Bi atoms. Its crystal structure was determined and refined from powder X-ray diffraction data by the Rietveld method and the results showed that Ba₃BiB₉O₁₈ belongs to space group P6₃/m with unit cell dimensions of a = 7.1999(2) Å, c = 17.3567(6) Å, and z = 2. Curves of differential thermal analysis and thermogravimetric analysis showed that Ba₃BiB₉O₁₈ is a congruent melting compound and chemically stable above 728 °C. Ba₃Bi_1−xEu_xB₉O₁₈ and Ba₃Bi_1−xTb_xB₉O₁₈ form a continuous solid solution from x = 0.01 to x = 0.9. The ultraviolet excited photoluminescence intensity increased with both Eu³⁺ and Tb³⁺ concentration in the matrix of Ba₃BiB₉O₁₈. There may be an interesting correlation between spectroscopic properties and lattice structural features of doped Ba₃BiB₉O₁₈.     

Crystal and magnetic study of the disordered perovskites Ca(Mn0.5Sb0.5)O3 and Ca(Fe0.5Sb0.5)O3

We have investigated the double perovskites Ca₂MSbO₆ (M = Mn, Fe) that have been prepared by solid-state reaction (M = Fe) and wet chemistry procedures (M = Mn). The crystal and magnetic structures have been studied from X-ray (XRD) and neutron powder diffraction (NPD) data. Rietveld refinements show that the crystal structures are orthorhombic (space group Pbnm) with complete disorder of M and Sb cations, so the formula should be rewritten as Ca(M_0.5Sb_0.5)O₃. Due to this disorder no evidences of Jahn-Teller distortion can be observed in the MnO₆ octahedra of Ca(Mn_0.5Sb_0.5)O₃, in contrast with the ordered double perovskite Sr₂MnSbO₆. Ca(Fe_0.5Sb_0.5)O₃ behaves as an antiferromagnet with an ordered magnetic moment for Fe³⁺ of 1.53(4)μ_B and a propagation vector k = 0, as investigated by low-temperature NPD. The antiferromagnetic ordering is a result of the high degree of Fe/Sb anti-site disorder of the sample, which originates the spontaneous formation of Fe-rich islands, characterized by the presence of strong Fe-O-Fe antiferromagnetic couplings with enough long-range coherence to produce a magnetic contribution perceptible by NPD. By contrast, the magnetic structure of Ca(Mn_0.5Sb_0.5)O₃ cannot be observed by low-temperature NPD because the magnitude of the ordered magnetic moments is below the detection threshold for neutrons.     

Preparation and crystal structure of a new tin titanate containing Sn; Sn2TiO4

Single crystals of a new tin titanate containing Sn²⁺, Sn₂TiO₄ was prepared by high temperature reaction in an evacuated quartz tube and its crystal structure was determined by single crystal X-ray diffraction data. The tin titanate crystallizes in the tetragonal space group P4₂/mbc with a = 8.490(2) and c = 5.923(3) Å, Z = 4 and the final R factors are R = 0.0497 and R_w = 0.0676 for 354 unique reflections. This tin titanate is isostructural with the low temperature form of Pb₃O₄(Pb₂²⁺Pb⁴⁺O₄). This compound was oxidized above 600 °C accompanying the mass gain and finally changed to rutile-type solid solution (Sn,Ti)O₂.     

Effects of Cr2O3 doping on the electrical properties and the temperature stabilities of PNW-PMN-PZT ceramics

The ceramics with 0.90Pb(Zr_0.50Ti_0.50)O₃-0.07Pb(Mn_1/3Nb_2/3)O₃-0.03Pb(Ni_1/2W_1/2)O₃ were prepared by adding Cr₂O₃. The effects of Cr₂O₃ doping on the phase structure, the microstructure and the electrical properties of ceramics were investigated. Meanwhile, the temperature stabilities of the resonant frequency (f_r) and the electromechanical coupling factor (K_p) were studied. The results showed that the better temperature stability could be obtained at x = 0.2 wt.% when the calcining temperature was 800 °C and the sintering temperature was 1150 °C. The parameters were Δf_r/f_r25 °C = −0.17% and ΔK_p/K_p25 °C = −1.39%. Moreover, the optimized electrical properties were also achieved, which were K_P = 0.54, Q_m = 1730, d₃₃ = 330 pC/N, ?_r = 2078 and tan δ = 0.0052. The optimized properties make the ceramics with this composition to be a good candidate for high power piezoelectric transformers applications.     

Materials with layered structures XIII: electrical and optical properties of layered chalcogenides in the system CoIn2S4-CoIn2Se4

The system CoIn₂S_4xSe_4(1−x) has been investigated by X-ray powder methods on samples quenched at 700 °C. The spinel type phase has a phase width of 1≥x>0.9. A new layered compound is formed for 0.9>x>0.45 which crystallizes with the α-FeGa₂S₄-type with a=392.6 pm and c=1270.3 pm (x=0.5) for the hexagonal cell. Platelike crystals of the layered phase are obtained by transport reactions with iodine in a temperature gradient 750→700 °C. The band gaps of these crystals measured by optical absorption vary from 1.2 to 1.4 eV. The electrical conductivities of the crystals are found in the order of 10⁻⁵ Ω⁻¹ cm⁻¹.