Magnetic ordering of divalent europium in double perovskites Eu2LnTaO6 (Ln=rare earths): Magnetic interactions of Eu ions determined by magnetic susceptibility, specific heat, and Eu Mössbauer spectrum measurements

Structures and magnetic properties of double perovskite-type oxides Eu₂LnTaO₆ (Ln=Eu, Dy-Lu) were investigated. These compounds adopt a distorted double perovskite structure with space group P2₁/n. Magnetic susceptibility, specific heat, and ¹⁵¹Eu Mössbauer spectrum measurements show that the Eu²⁺ ions at the 12-coordinate sites of the perovskite structure are antiferromagnetically ordered at ∼4 K, and that Ln³⁺ ions at the 6-coordinate site are in the paramagnetic state down to 1.8 K.     

Role of doping and CuO segregation in improving the giant permittivity of CaCu3Ti4O12

The dopant role on the electric and dielectric properties of the perovskite-type CaCu₃Ti₄O₁₂ (CCTO) compound is evidenced. Impedance spectroscopy measurements show that the relevant permittivity value attributed to sintered CCTO is due to grain boundary (g.b.) effects. The g.b. permittivity value of the pure CCTO can be increased of 1-2 orders of magnitude by cation substitution on Ti site and/or segregation of CuO phase, while the bulk permittivity keeps values 90<ε_r<180. Bulk and g.b. conductivity contributions are discussed: electrons are responsible for the charge transport and a mean bulk activation energy of 0.07 eV is obtained at room temperature for all the examined samples. The g.b. activation energy ranges between 0.54 and 0.76 eV. Defect models related to the transport properties are proposed, supported by electron paramagnetic resonance measurements.     

Dielectric properties of 1,1,1-trifluoroethane (HFC-143a) in the liquid phase

The relative permittivity (?_r) data of 1,1,1-trifluoroethane (HFC-143a), (CAS N# 420-46-2), a hydrofluorocarbon (HFC) developed as a refrigerant that has zero ozone depletion potential, is reported. The relative permittivity of HFC-143a in the liquid phase was measured using a direct capacitance method at temperatures from T = 218 to 294 K and at pressures up to P = 15 MPa, for a frequency of 10 kHz. The uncertainty of the ?_r measurements is estimated to be better than ±1.2 × 10⁻². A complete set of tables of experimental data as a function of temperature, pressure and density, is presented that covers the dielectric property needs for most engineering applications. To study the dependence of ?_r on density and temperature on a molecular basis, the theory developed by Vedam et al. and adapted by Diguet was applied to analyse the data. The Kirkwood modification of the Onsager equation was used to obtain the value of its dipole moment in the liquid phase (μ*). The apparent dipole moment obtained was μ* = 3.293 D. The effective dipole in the liquid state predicted by the Kirkwood–Frölich theory is 2.530 D. The measured values are compared with density functional and density functional self-consistent calculations (SCIPCM) of the electronic distribution and of the dipole moment of HFC-143a. Finally, the values of the isobaric thermal expansion and isothermal compressibility were estimated from the reported measurements.     

Structural evolution of Na0.5K0.5NbO3 at high temperatures

Changes in structure and dielectric properties at elevated temperatures have been investigated on single-crystals of sodium potassium niobate, Na_0.5K_0.5NbO₃, grown by the flux method. Single-crystal X-ray diffraction studies revealed that the crystals underwent orthorhombic-tetragonal and tetragonal-cubic phase transitions at 465 and 671 K during heating and 446 and 666 K during cooling, respectively. Both transitions were accompanied by volumetric discontinuities of collapse upon heating and expansion upon cooling, suggesting that the transitions were of the first order. The coordination numbers of an Nb showed a decreasing tendency with decreasing temperature, i.e., 6 in cubic, 5+1 in tetragonal and 4+2 in orthorhombic. An Na atom occupied a slightly different position from the K atom in 12-fold coordination, resulting in fewer coordination numbers of 8+4 in cubic and tetragonal and 7+5 in orthorhombic. The spontaneous polarisation (P_s) estimated from the atom positions and formal charges were approximately 0.29 C m⁻² in orthorhombic and 0.18 C m⁻² in tetragonal. The contribution of the alkaline oxide components to P_s was estimated to be approximately 15% in both ferroelectric forms. The temperature-induced transitions were also confirmed through the dielectric constant and dielectric loss at various frequencies and the differential scanning calorimetry.     

Synthesis, crystal structure and magnetic properties of a new pillared perovskite La5Mo2.75V1.25O16

A new pillared perovskite compound La₅Mo_2.76(4)V_1.25(4)O₁₆, has been synthesized by solid-state reaction and its crystal structure has been characterized using powder X-ray and neutron diffraction. The magnetic properties of this compound have been investigated using SQUID magnetometry, and the magnetic structure has been studied using neutron diffraction data. A theoretical calculation of relative strengths of spin interactions among different magnetic ions and through different pathways has been performed using extended Hückel, spin dimer analysis. The crystal structure of this material contains perovskite-type layers that are connected through edge-sharing dimeric units of octahedra. The structure is described in space group C2/m with unit cell parameters a=7.931(2) Å, b=7.913(2) Å, c=10.346(5) Å and β=95.096(5)°. The material shows both short-range ferrimagnetic correlations from ∼200 to 110 K and long-range antiferromagnetic order below T_c∼100 K. The magnetic structure was investigated by neutron diffraction and is described by k=(0 0 ) as for other pillared perovskites. It consists of a ferrimagnetic arrangement of Mo and V within the layers that are coupled antiferromagnetically between layers. This is the first magnetic structure determination for any Mo-based pillared perovskite.     

Thermal, structural and solution studies of a new lead(II) coordination polymer with η Pb-C interactions

A new Pb(II) one-dimensional coordination polymer {[Pb(PAA)₂]_n (1), PAA⁻ = phenylacetate} was synthesized by the reaction of Pb(CH₃COO)₂ · 3H₂O and ligand phenyl acetic acid. Compound 1 was structurally characterized by single-crystal X-ray diffraction. The crystal structure of this compound consists of one-dimensional polymeric units of [Pb(PAA)₂] and the coordination number of Pb^II ions is six. The lead atoms have irregular coordination sphere containing stereo-chemically active lone pair and tetra-hapto (η⁴) interactions, thus attaining a total hapticity of 10 with environment C₄O₆Pb. The thermal stability of compound 1 was studied by thermal gravimetric (TG) and differential thermal analyses (DTA). The results of studies of the stoichiometry and formation of complex 1 in methanol, ethanol and acetonitrile solutions were found to be in support of their solid state stoichiometry.     

Studies on magnetic and calorimetric properties of double perovskites Ba2HoRuO6 and Ba2HoIrO6

Magnetic properties of double perovskite compounds Ba₂HoRuO₆ and Ba₂HoIrO₆ have been reported. Powder X-ray and neutron diffraction measurements show that these compounds have a cubic perovskite-type structure with the space group and the 1:1 ordered arrangement of Ho³⁺ and Ru⁵⁺ (or Ir⁵⁺) over the 6-coordinate B sites. Results of the magnetic susceptibility and specific heat measurements show that Ba₂HoRuO₆ exhibits two magnetic anomalies at 22 and 50 K. Analysis of the temperature dependence of magnetic specific heat indicates that the anomaly at 50 K is due to the antiferromagnetic ordering of Ru⁵⁺ ions and that the anomaly at 22 K is ascribable to the magnetic interaction between Ho³⁺ ions. Neutron diffraction data collected at 10 and 35 K show that the Ba₂HoRuO₆ has a long range antiferromagnetic ordering involving both Ho³⁺ and Ru⁵⁺ ions. Each of their magnetic moments orders in a Type I arrangement and these magnetic moments are anti-parallel in the ab-plane with each other. The magnetic moments are aligned along the c-direction. On the other hand, Ba₂HoIrO₆ is paramagnetic down to 1.8 K.     

Low temperature phase transition studies on Pb(Mg0.5W0.5)O3 ceramic

We present here the results of X-ray diffraction (XRD), dielectric and calorimetric studies on lead magnesium tungustate, Pb(Mg_0.5W_0.5)O₃ (PMW) ceramic. It is shown that the low temperature antiferroelectric phase of PMW having orthorhombic structure (space group Pmcn) transforms to paraelectric cubic (space group Fm3m) phase at 281 K. The phase transition is of first order character as confirmed by coexistence of Pmcn and Fm3m phases over wide temperature range ∼50 K. The first order character of phase transition is also revealed by the observation of thermal hysteresis in the real part of dielectric permittivity and calorimetric studies. We do not find any evidence for the additional intermediate phase between antiferroelectric (Pmcn) and paraelectric (Fm3m) phases as reported in the literature. Anomalies in the heat flow and dielectric measurements support the finding of our XRD results and reveals that the phase transition temperature (T_c) is 281 K instead of 312 K reported in the literature.     

Magneto-thermal and dielectric properties of biferroic YCrO3 prepared by combustion synthesis

Microstructural, magnetothermal and dielectric properties of YCrO₃ powders prepared by combustion and solid state methods have been studied by a combination of XRD, specific heat, magnetization and permittivity measurements. The TEM and XRD characterization confirm that the combustion powders are amorphous plate-like agglomerates of nano-sized crystalline particles. A more uniform grain size along with an increase of the relative density is observed by SEM in the sintered samples prepared by combustion route with respect to those produced by solid state reaction. Similar to the material obtained through solid state synthesis, the material prepared by the combustion method also shows spin canted antiferromagnetic ordering of Cr⁺³ (S=3/2) at ∼140 K, which is shown by magnetization as well as λ-type anomaly in the total specific heat. Furthermore, the magnetic contribution to the total specific heat reveals spin fluctuations above T_N and a spin reorientation transition at about 60 K. Both YCrO₃ compounds show a diffuse phase transition at about 450 K, typical of a relaxor ferroelectric, which is characterized by a broad peak in the real part of the dielectric permittivity as a function of temperature, with the peak decreasing in magnitude and shifting to higher temperature as the frequency increases. The relaxor dipoles are due to the local non-centrosymmetric structure. Furthermore, the high loss tangent in a broad range of temperature as well as conductivity analysis indicates a hopping mechanism for the electronic conductivity as we believe it is a consequence of the outer d³-shell, which have detrimental effects on the polarization and the pooling process in the YCrO₃ bulk material. The more uniform particle size and higher density material synthesized through the combustion process leads to an improvement in the dielectric Properties.     

Ferroelectric perovskite-type barium copper niobate: BaCu1/3Nb2/3O3

A perovskite-type BaCu_1/3Nb_2/3O₃ was prepared by high temperature reaction using BaCO₃, CuO and Nb₂O₅. The X-ray powder diffraction pattern of this compound was indexed with the tetragonal cell with the lattice parameters of a=4.0464(4) and c=4.1807(4) Å (c/a=1.033). This compound had the tetragonal perovskite-type structure in which the B site was occupied statistically by Nb and Cu atoms. From high temperature X-ray powder diffraction patterns this compound had a phase transition from the tetragonal to cubic symmetry in the temperature range of 500-600 °C. The P-E and S-E hysteresis loops occurred at room temperature and the apparent maximum in the temperature dependence of the dielectric constant was observed at 520 °C. The temperature dependence of the inverse of magnetic susceptibility exhibited paramagnetic behavior.     

Phase transition of pyridinium tetrachloroiodate(III), PyHICl4, studied by a single crystal X-ray analysis and dielectric and heat capacity measurements

Change of a local environment of a polar pyridinium ion, which is associated with the phase transition of crystalline pyridinium tetrachloroiodate(III) at T_c = 217 K, was investigated by a single crystal X-ray analysis and dielectric and heat capacity measurements. The site symmetry 2/m of the ion at T > T_c indicates an orientational disorder in the high-temperature phase (HTP). The energy difference ΔE between the stable and meta-stable orientations of the pyridinium ion at the 2/m site was estimated to be ΔE/R ? 560 K at 280 K in the HTP. Below the T_c, an antiferroelectric ordering of the ions was revealed.     

The linear thermal expansion and the thermal diffusivity measurements for near-stoichiometric (U, Ce)O2 solid solutions

The thermal diffusivities of near-stoichiometric (U, Ce)O₂ solid solutions containing CeO₂ up to 22 mol% were investigated in the temperature range of 298-1273 K using the laser flash method. Also, linear thermal expansion measurements were performed in the temperature range of 298-1673 K using a thermomechanical analysis. The thermal conductivities were determined by a calculation of the thermal diffusivity, the density and the specific heat. The thermal conductivities of the tested samples could be expressed as a function of the temperature by the phonon conduction equation k = (A + BT)⁻¹. The thermal conductivity decreased gradually with an increasing Ce content. This was attributable to the increasing lattice defect thermal resistance caused by the U⁴⁺, Ce⁴⁺ and O²⁻ ions as phonon scattering centers.     

On permittivity and density of the systems (tetraglyme + dimethyl or diethyl carbonate) and the formulation of Δε in terms of volume or mole fraction

Relative permittivity and density on mixing from T = 288.15 K to 328.15 K and atmospheric pressure have been measured over the whole composition range for CH₃O(CH₂CH₂O)_mCH₃ polyoxyethyleneglycol dimethyl ether with m = 4 (also called 2,5,8,11,14-pentaoxapentadecane or tetraglyme) + (dimethyl or diethyl carbonate). For these systems the deviation of permittivity, Δε, changes sign depending on whether it is defined on the basis of mole fractions of volume fractions. Arguments are put forward that support the choice of the definition in terms of mole fraction. The Redlich-Kister equation has been used to estimate the binary fitting parameters and standard deviations from the regression lines were calculated. The density and excess molar volumes were fitted as a function of temperature and mole fraction to a polynomial equation. The temperature dependence of derived magnitudes, such as the isobaric thermal expansion coefficient, α, , and were computed, due to its importance in the study of specific molecular interactions. Different traditional mixing rules have been applied to predict the permittivity of these mixtures.     

Thermal expansion behaviour in the oxygen deficient perovskites Sr2BSbO5.5 (B=Ca, Sr, Ba). Competing effects of water and oxygen ordering

Neutron diffractions studies reveal the presence of oxygen disorder in the oxygen deficient perovskites Sr₂BSbO_5.5 (B=Ca, Sr, Ba). Synchrotron X-ray studies demonstrate that these oxides have a double perovskite-type structure with the cell size increasing as the size of the B cation increases from 8.2114(2) Å for B=Ca to 8.4408(1) Å for B=Ba. It is postulated that a combination of local clustering of the anions and vacancies together with water-water and water-host hydrogen bonds plays a role in defining the volume of the encapsulated water clusters and that changes in the local structure upon heating result in anomalous thermal expansion observed in variable temperature diffraction measurements.     

Thermoelectric properties of polycrystalline La1−xSrxCoO3

Thermoelectric properties of polycrystalline La_1−xSr_xCoO₃, where Sr²⁺ is substituted in La³⁺ site in perovskite-type LaCoO₃, have been investigated. Sr-doping increases the electrical conductivity (σ) of La_1−xSr_xCoO₃, and also decreases the Seebeck coefficient (S) for 0.01?x?0.40. A Hall coefficient measurement reveals that the increase in electrical conductivity arises from increases in both carrier concentration and the Hall mobility. The decrease in the Seebeck coefficient is caused by a decrease in carrier effective mass as well as increase in carrier concentration. The highest power factor (σS²) is 3.7×10⁻⁴ W m⁻¹ K⁻² at 250 K for x=0.10. The thermal conductivity (κ) is about 2 W m⁻¹ K⁻¹ at 300 K for 0?x?0.04, and increases for x?0.05 because of an increase in heat transport by conductive carrier. The thermoelectric properties of La_1−xSr_xCoO₃ are improved by Sr-doping, and the figure of merit (Z=σS² κ⁻¹) reaches 1.6×10⁻⁴ K⁻¹ for x=0.06 at 300 K (ZT=0.048). For heavily Sr-doped samples, the thermoelectric properties diminish mainly because of the decrease in the Seebeck coefficient and the increase in thermal conductivity.     

Perovskite-type SrTi1−xNbx(O,N)3 compounds: Synthesis, crystal structure and optical properties

The synthesis, crystal structure, thermal stability and absorbance spectra of perovskite-type oxynitrides with the general formula SrTi_1−xNb_x(O,N)₃ (x=0.05, 0.10, 0.20, 0.50, 0.80, 0.90, 0.95) have been investigated. Oxide samples were prepared by a polymerized complex synthesis route and post-treated under ammonia at 850 °C for 24 h to substitute nitrogen for oxygen. Synchrotron X-ray powder diffraction (XRD) evidenced that the mixed oxide phases were all transformed into oxynitrides with perovskite-type structure during a thermal ammonolysis. SrTi_1−xNb_x(O,N)₃ with compositions x≤0.80 crystallized in a cubic and samples with x≥0.90 in a tetragonal structure. The Rietveld refinement indicated a continuous enlargement of the lattice parameters towards higher niobium content of the samples. Thermogravimetric analysis (TGA) and hotgas extraction revealed the dependence of the nitrogen incorporation upon the degree of niobium substitution. It showed that more nitrogen was detected in the samples with higher niobium content. Furthermore, TGA disclosed stability for all oxynitrides at T≤400 °C. Diffuse reflectance spectroscopy indicated a continuous decrease of the band gap’s width from 3.24 eV (SrTi_0.95Nb_0.05 (O,N)₃) to 1.82 eV (SrTi_0.05Nb_0.95(O,N)₃) caused by the increasing amount of nitrogen towards the latter composition.     

K2B12F12: A rare A2X structure for an ionic compound at ambient conditions

The anhydrous salt K₂B₁₂F₁₂ crystallized from aqueous solution and its structure was determined by single crystal X-ray diffraction. The Ni₂In-type structure it exhibits is rare for an A₂X ionic compound at 25 °C and 1 atm., consisting of an expanded hexagonal close-packed array of B₁₂F₁₂²⁻ centroids (cent?cent distances: 7.204-8.236 Å) with half of the K⁺ ions filling all of the O_h holes and half of the K⁺ ions filling all of the D_3h trigonal holes in the close-packed layers that are midway between two “empty” T_d holes. The structure is also unusual in that the bond-valence sum for the K⁺ ions in O_h holes is less than or equal to 0.73 (the bond-valence sum for the other type of K⁺ ion is 1.16). A variation of the Ni₂In structure is exhibited by the previously published monohydrate Cs₂(H₂O)B₁₂F₁₂, for which an improved structure is also reported here. For K₂B₁₂F₁₂: monoclinic, C2/c, a = 8.2072(8), b = 14.2818(7), c = 11.3441(9) Å, β = 92.832(5)°, Z = 4, T = 120(2) K. For Cs₂(H₂O)B₁₂F₁₂: orthorhombic, P2₁2₁2₁, a = 9.7475(4), b = 10.2579(4), c = 15.0549(5) Å, Z = 4, T = 110(1) K.     

New ferromagnets of Sr8ARe3Cu4O24 (A=Sr, Ca) with an ordered perovskite structure

New phases Sr₈ARe₃Cu₄O₂₄ (A=Sr,Ca) were discovered under high-pressure/high-temperature condition. X-ray powder diffraction and electron diffraction studies for these phases indicated that they have an ordered perovskite-type structure with cubic lattices of ∼8 Å. They showed ferromagnetism at room temperature when they were synthesized under high-oxygen-pressure condition. The Ca-containing phase has a very high T_c of 440 K with a spontaneous magnetization of ∼1 μ_B/f.u.     

Synthesis and magnetic properties of 12L-perovskites Ba4LnIr3O12 (Ln=lanthanides)

New quadruple perovskite oxides Ba₄LnIr₃O₁₂ (Ln=lanthanides) were prepared and their magnetic properties were investigated. They crystallize in the monoclinic 12L-perovskite-type structure with space group C2/m. The Ir₃O₁₂ trimers and LnO₆ octahedra are alternately linked by corner-sharing and form the perovskite-type structure with 12 layers. The Ln and Ir ions are both in the tetravalent state for Ln=Ce, Pr, and Tb compounds , and for other compounds (Ln=La, Nd, Sm-Gd, Dy-Lu), Ln ions are in the trivalent state and the mean oxidation state of Ir ions is . An antiferromagnetic transition has been observed for Ln=Ce, Pr, and Tb compounds at 10.5, 35, and 16 K, respectively, while the other compounds are paramagnetic down to 1.8 K.     

Synthesis and crystal structure of Bi6.4Pb0.6P2O15.2: A new polymorph in the series Bi6+xM1−xP2O15+y

Bi_6.4Pb_0.6P₂O_15.2 is a polymorph of structures with the general stoichiometry Bi_6+xM_1−xP₂O_15+y. However, unlike previously published structures that consist of layers formed by edge sharing OBi₄ tetrahedra bridged by PO₄ and TO₆ (T=transition metal) tetrahedra and octahedra the title compound's structure is more complex. It is monoclinic, C2, a=19.4698(4) Å, b=11.3692(3) Å, c=16.3809(5) Å, β=101.167(1)°, Z=10. Single-crystal X-ray diffraction data were refined by least squares on F² converging to R₁=0.0387, wR₂=0.0836 for 7023 intensities. The crystal twins by mirror reflection across (001) as the twin plane and twin component 1 equals 0.74(1). Oxygen ions are in tetrahedral coordination to four metal ions and the O(BiPb)₄ units share corners to form layers that are part of the three-dimensional framework. Eight oxygen ions form a cube around the two crystallographically independent Pb ions. Pb-O bond lengths vary from 2.265(14) to 2.869(14) Å. Pairs of such cubes share an edge to form a Pb₃O₂₀ unit. The two oxygen ions from the unshared edges are part of irregular Bi polyhedra. Other oxygen ions of Bi polyhedra are part only of O(BiPb)₄ units, and some oxygen ions of the polyhedra are also part of PO₄ tetrahedra. One, two, three and or four PO₄ moieties are connected to the Bi polyhedra. Bi-O bond lengths ?3.1 Å vary from 2.090(12) to 3.07(3) Å. The articulations of Pb cubes, Bi polyhedra and PO₄ tetrahedra link into the three-dimensional structure.