Correlation between structure and magnetic properties of Cd-substituted La0.7(Ca0.3−xCdx)MnO3 CMR manganites

Structural, electrical and magnetic properties of Cd-doped La_0.7(Ca_0.3−xCd_x)MnO₃ (0?x?0.3) manganites are presented. All compositions were indexed in the orthorhombic (Pnma) space group, except the Cd_0.3 sample, indexed as a combination of trigonal and orthorhombic (Pnma) space groups. Substitution of Ca by Cd has a strong influence on the magnetic and magnetoresistive properties of these compounds, continuously decreasing both the magnetic moment and the Curie temperature (from 3.5 μ_B and 270 K for the x=0 composition to 1.59 μ_B and 90 K for the fully doped x=0.3 one). Samples corresponding to x=0 and 0.1 show a semiconductor-metal transition at temperatures close to the Curie ones. The measured magnetoresistance change is about 49% at 270 K and 95% at 165 K for those samples, respectively. However, the x=0.2 and 0.3 compositions show insulating behaviour in the whole temperature range studied, with values of the magnetoresistance about 85% at 105 K and 74% at 90 K, respectively. The observed weakening of the double-exchange mechanism as the Cd doping level in these samples increases is discussed in terms of structural properties, cationic disorder and Mn³⁺/Mn⁴⁺content ratio.     

Magnetic properties of the chemically delithiated LixMn2O4 with 0.07≤x≤1

Magnetism for the Li_xMn₂O₄ samples with 0.07≤x≤1, which are prepared by a chemical reaction in HNO₃ solution, is investigated by direct current susceptibility (χ) and muon-spin rotation/relaxation (μSR) measurements. The effective magnetic moment (μ_eff) of Mn ions decreases monotonically with decreasing x, indicating that Mn³⁺ ions with S=2 () are oxidized to Mn⁴⁺ ions with S=3/2 () with decreasing x. On the other hand, as x decreases from 1 to 0.6, the Curie-Weiss temperature (Θ_p) increases monotonically from ∼−260 to −100 K, and then levels off to −100 K with further decreasing x. This indicates that the antiferromagnetic interaction is dominant in the whole x range. For the x=0.48 sample, the temperature dependence of χ in field-cooling mode clearly deviates from that in zero-field-cooling mode below ∼63 K (=T_m). Furthermore, the hysteresis loop is observed in the magnetization vs. field curve at 5 K. Since the zero-field μSR spectrum is well fitted by a strongly damped oscillation function, the Mn moments for the x=0.48 sample are in a highly disordered fashion down to the lowest temperature measured.     

From ordered antiferromagnet to spin glass: A new phase Mn4Ir7−xMnxGe6

A new ternary phase, Mn₄Ir_7−xMn_xGe₆ (0?x?1.3), was studied by X-ray and neutron powder diffraction and SQUID magnetometry. The crystal structure is cubic, of the U₄Re₇Si₆ type, space group , Z=2, with the lattice parameter at 295 K. Within the limited range of homogeneity small variations of the composition yield dramatic changes of the magnetic structure. For x=0 long-range antiferromagnetic order is formed below the transition temperature 228 K, with large magnetic moments on Mn, 4.11(9) μ_B at 10 K, in a magnetic unit cell , c_M=2a_C. In contrast, for x=1.3 spin glass behavior is observed below 90 K. The Mn atoms form an ideal cubic framework, on which geometric frustration of competing nearest and next nearest neighbor antiferromagnetic interactions is suggested to explain the composition sensitive magnetic properties. A TiNiSi-type phase, IrMnGe, is found in samples of 1:1:1 composition quenched from the melt.     

Synthesis and magnetic properties of the quasi-one-dimensional compound Ca0.83(Cu1−xCox)O2

A new solid solution of the quasi-one-dimensional composite crystal, , has been synthesized under of O₂ at 830°C. The non-doped compound Ca_0.83CuO₂ consists of two interpenetrating monoclinic subsystems of the [Ca] atoms and the edge-shared square planar [CuO₂] chains. Upon increasing x, both the subsystems undergo a phase change from monoclinic to orthorhombic (M-O). The M-O change occurs at x∼0.04 for the [(Cu,Co)O₂] subsystem, while such a change occurs at x∼0.17 for the [Ca] subsystem. Magnetic susceptibility measurements show an evolution from a short-range ordered state near x=0 to a long-range antiferromagnetic state for the samples with x?0.15. The effective magnetic moment μ_eff is found to increase with increasing x from for x=0.10 to for x=0.30, suggesting that the solid solution can be regarded as Ca_0.83[Cu_0.66²⁺Cu_0.34−x³⁺Co_x³⁺]O₂, in which a mixed state of Cu²⁺(S=1/2), Cu³⁺(S=0) and high-spin Co³⁺(S=2) ions is realized.     

Synthesis and crystal structure of CaxCo4Sb12 skutterudites

New skutterudite compounds Ca_xCo₄Sb₁₂ (0<x?0.2) have been prepared by traditional metallurgical synthesis. The compounds have been characterized by X-ray powder diffraction (XRD), electron probe microanalysis (EPMA) and neutron powder diffraction. Rietveld refinement of the structures against neutron powder diffraction data (on Ca_0.1Co₄Sb₁₂, , a=9.0429 Å, χ²=1.55; wR_p=1.52) enabled the location of Ca in the voids of the skutterudite structure to be verified. The large displacement ellipsoid for Ca is consistent with “rattling” in the cage of the crystalline structure. XRD combined with EPMA analyses showed that the maximum occupancy of Ca atoms is about 0.2.     

Long-range magnetic ordering of S=1/2 linear trimers in A3Cu3(PO4)4 (A=Ca, Sr, and Pb)

Magnetic properties of S=1/2 linear trimer cluster compounds A₃Cu₃(PO₄)₄ (A=Ca, Sr, and Pb) were investigated. Magnetic susceptibility data for the three compounds showed that paramagnetic copper spins form trimers with the total spin of 1/2 below about 45 K. Specific heat and magnetization measurements indicated that the trimer clusters undergo ferromagnetic long-range ordering at for A=Ca and antiferromagnetic long-range ordering at for A=Sr and for A=Pb. A₃Cu₃(PO₄)₄ exhibited 1/3-magnetization plateau at least up to magnetic field of 55 T at 1.3 and 4.2 K. A₃Cu₃(PO₄)₄ with A=Sr and Pb showed a spin-flop transition near 0.03 T in the antiferromagnetic state at 0.08 K. Specific heat data at magnetic fields clearly showed broad maxima at low temperatures due to the finite intra-chain interaction in one-dimensional arrays of the trimers.     

Synthesis, structure and electric studies for La0.7A0.3Mn0.96(InxAl(1−x))0.04O3; A=Ca and Sr perovskites

Two polycrystalline series of samples in the form La_0.7A_0.3Mn_0.96(In_xAl_(1−x))_0.04O₃; A=Ca and Sr with (0.0?x?1.0) were synthesized using solid-state reaction. Rietveld analysis of the X-ray powder diffraction showed that, the Ca-series has an orthorhombic unit cell (space group Pnma) and the Sr-series is rhombohedral with (space group ). Structural parameters (refined lattice parameters, atom positions, bond distance, bond angles, valence sum, valence bond, bond lengths and bond angles) are reported and discussed as a function of In content for the two series. Resistivity measurements in zero field are carried out in the range 50-300 K for the Ca-series and in the range 300-400 K for the Sr-series. The obtained transition temperature T_ρ at which the metal-semiconducting (MS) occurred, is discussed as a function of the compositional parameter x.     

The composite structure of Cu2.33−xV4O11

The copper vanadium oxide bronze Cu_2.33−xV₄O₁₁ exhibits a three part composite structure refined on the basis of XRD low-temperature studies. It crystallizes in the triclinic system with the non-centric superspace group X1 and cell parameters ; ; ; α=90.0°; β=101.95(3)°; γ=90.0° with a modulation q-vector equal to (0,0.11,0). The three different parts of this composite structure differ by their b-unit cell repeat defined as b₁ ; () and (). These parts are respectively associated to the V₄O₁₁ substructure and to each of the two different copper sites. Such refinement allows us to describe the structure using only one and fully occupied crystallographic site for each of the Cu ions. The maximum composition (x=0) is then achieved. Bond valence sum calculations on the basis of such composite structure is in agreement with electronic structure calculation made using the average one and allows us to attribute the proper valence state to each Cu ions. Then, the calculated ratio appears, contrary to the average structure, in prefect agreement with the one deduced from XPS experiment.     

Extension of the La7Mo7O30 structural type with La7Nb3W4O30 and La7Ta3W4O30 compounds

Two compounds of formula La₇A₃W₄O₃₀ (with A=Nb and Ta) were prepared by solid-state reaction at 1450 and 1490 °C. They crystallize in the rhombohedric space group R-3 (No. 148), with the hexagonal parameters: , and , . The structure of the materials was analyzed from X-ray, neutron and electronic diffraction. These oxides are isostructural of the reduced molybdenum compound La₇Mo₇O₃₀, which are formed of perovskite rod along [111]. An order between (Nb, Ta) and W is observed.     

REAuAl4Ge2 and REAuAl4(AuxGe1−x)2 (RE=rare earth element): Quaternary intermetallics grown in liquid aluminum

The two families of intermetallic phases REAuAl₄Ge₂ (1) (RE=Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Er, Tm and Yb) and REAuAl₄(Au_xGe_1−x)₂ (2) (x=0.4) (RE=Ce and Eu) were obtained by the reactive combination of RE, Au and Ge in liquid aluminum. The structure of (1) adopts the space group R-3m (CeAuAl₄Ge₂, , ; NdAuAl₄Ge₂, , ; GdAuAl₄Ge₂, , ; ErAuAl₄Ge₂, , ). The structure of (2) adopts the tetragonal space group P4/mmm with lattice parameters: , for EuAuAl₄(Au_xGe_1−x)₂ (x=0.4). Both structure types present slabs of “AuAl₄Ge₂” or “AuAl₄(Au_xGe_1−x)₂” stacking along the c-axis with layers of RE atoms in between. Magnetic susceptibility measurements indicate that the RE atoms (except for Ce and Eu) possess magnetic moments consistent with +3 species. The Ce atoms in CeAuAl₄Ge₂ and CeAuAl₄(Au_xGe_1−x)₂ (x=0.4) appear to be in a mixed +3/+4 valence state; DyAuAl₄Ge₂ undergoes an antiferromagnetic transition at 11 K and below this temperature exhibits metamagnetic behavior. The Eu atoms in EuAuAl₄(Au_xGe_1−x)₂ (x=0.4) appear to be in a 2+ oxidation state.     

Independent structural and valence state transitions in the cation-ordered double perovskites Ba2−xSrxTbIrO6

Synchrotron X-ray and neutron powder diffraction were used to investigate the formation, structure and bonding in the double perovskite Ba_2−xSr_xTbIrO₆ solid solutions. The results showed that these oxides all exhibit ordering of the Tb and Ir cations in a double perovskite-type structure. Three distinct structural types differing in symmetry and/or valence states were formed depending on the precise Ba:Sr ratio on the perovskite A site; x?0.3 cubic () with Tb⁴⁺ and Ir⁴⁺; 0.4?x?1.0 cubic () with Tb³⁺ and Ir⁵⁺ and x?1.2 monoclinic (P2₁/n) with Tb³⁺ and Ir⁵⁺. The transitions between these appear to be first order in nature.     

The disordered structures and low temperature dielectric relaxation properties of two misplaced-displacive cubic pyrochlores found in the Bi2O3-MO-Nb2O5 (M=Mg, Ni) systems

The disordered structures and low temperature dielectric relaxation properties of Bi_1.667Mg_0.70Nb_1.52O₇ (BMN) and Bi_1.67Ni_0.75Nb_1.50O₇ (BNN) misplaced-displacive cubic pyrochlores found in the Bi₂O₃-M^IIO-Nb₂O₅ (M=Mg, Ni) systems are reported. As for other recently reported Bi-pyrochlores, the metal ion vacancies are found to be confined to the pyrochlore A site. The B₂O₆ octahedral sub-structure is found to be fully occupied and well-ordered. Considerable displacive disorder, however, is found associated with the O′A₂ tetrahedral sub-structure in both cases. The A-site ions were displaced from Wyckoff position 16d (, , ) to 96 h (, , ) while the O′ oxygen was shifted from position 8b (, , ) to Wyckoff position 32e (, , ). The refined displacement magnitudes off the 16d and 8b sites for the A and O′ sites were 0.408 Å/0.423 Å and 0.350 Å/0.369 Å for BMN/BNN, respectively.     

Vapour pressure and excess Gibbs free energy of the ternary system {x1CH3F + x2HCl + x3N2O} at temperature of 182.33 K

The equilibrium pressure of ternary mixtures of {x₁CH₃F + x₂HCl + x₃N₂O} covering the entire composition range has been measured at temperature of 182.33 K by the static method. The system exhibits a minimum pressure for the binary {x₁CH₃F + x₂HCl}. The molar excess Gibbs free energy has been calculated from the experimental equilibrium pressure. For the equimolar mixture . The (p, x, y) surface for the ternary system and the corresponding curves for the three constituent binary mixtures obtained from the Peng-Robinson equation of state are in agreement with the experimental data.     

A TEM study of Ni ordering in the Ni6Se5−xTex, 0<x<∼1.7, system

The Ni₆Se_5−xTe_x, 0<x<∼1.7, system has been carefully investigated via electron diffraction and TEM imaging. They reveal a somewhat disordered modulated superstructure phase arising from Ni ion ordering within an essentially well-defined chalcogen sub-structure. As x, and the underlying parent substructure cell dimensions increase, the incommensurate primary modulation wave-vector q characteristic of this Ni ion ordering quickly swings from close to for x=0 towards for x?0.5. A lock-in to would formally transform the underlying parent Bmmb (a_p, b_p, c_p) structure into a P1a1 (a_s=2a_p, b_s=b_p, c_s=a_p+c_p) superstructure phase.     

A computational study of SbnF5n (n = 1-4): Implications for the fluoride ion affinity of nSbF5

This contributions shows with a series of ab initio MP2 and DFT (BP86 and B3-LYP) computations with large basis sets up to cc-pVQZ quality that the literature value of the standard enthalpy of depolymerization of Sb₄F_20(g) to give SbF_5(g) (+18.5 kJ mol⁻¹) [J. Fawcett, J.H. Holloway, R.D. Peacock, D.R. Russell, J. Fluorine Chem. 20 (1982) 9] is by about 50 kJ mol⁻¹ in error and that the correct value of (Sb₄F_20(g)) is +68 ± 10 kJ mol⁻¹. We assign , , and values for Sb_nF_5n with n = 2-4 and compare the results to available experimental gas phase data. Especially the MP2/TZVPP values obtained in an indirect procedure that rely on isodesmic reactions or the highly accurate compound methods G2 and CBS-Q are in excellent agreement with the experimental data, and reproduce also the fine experimental details at temperatures of 423 and 498 K. With these data and the additional calculation of [Sb_nF_5n+1]⁻ (n = 1-4), we then assessed the fluoride ion affinities (FIAs) of Sb_nF_5n(g), nSbF_5(g), nSbF_5(l) and the standard enthalpies of formation of Sb_nF_5n(g) and [Sb_nF_5n+1]⁻_(g): FIA(Sb_nF_5n(g)) = 514 (n = 1), 559 (n = 2), 572 (n = 3) and 580 (n = 4) kJ mol⁻¹; FIA(nSbF_5(g)) = 667 (n = 2), 767 (n = 3) and 855 (n = 4) kJ mol⁻¹; FIA(nSbF_5(l)) = 434 (n = 1), 506 (n = 2), 528 (n = 3) and 534 (n = 4) kJ mol⁻¹. Error bars are approximately ±10 kJ mol⁻¹. Also the related Gibbs energies were derived. Δ_fH°([Sb_nF_5n+1]⁻_(g)) = −2064 ± 18 (n = 1), −3516 ± 25 (n = 2), −4919 ± 31 (n = 3) and −6305 ± 36 (n = 4) kJ mol⁻¹.     

Polymorphism of the iron doped strontium aluminate SrAl1.5Fe0.5O4

The partial substitution of Al by Fe atoms in SrAl₂O₄ allowed to stabilize four stuffed tridymite derivative structures SrAl_1.5Fe_0.5O₄. The different phases have been characterized by TEM and XRPD techniques. Two are isotypes of those observed for the undoped oxides, namely the hexagonal phase with A×A×C (with A and C being the tridymite unit cell parameters) and space group P6₃ and the monoclinic one with A×A×C, β≈93° and space group P2₁, with a synthesis temperature lower than the one required for SrAl₂O₄. By annealing, two original phases, denoted O₁ and O₂, are obtained; they are metrically similar (3A×A×C and β≈90°) and only differ by their space groups. The TEM study showed that the transitions between the different phases follow topotactic mechanisms, through the formation of twinning boundaries, antiphase boundaries and planar defects. The annealed sample exhibits a mesomorph state and a reversible transition from this semi-ordered state to a crystalline phase. This dynamic transition takes place over a very wide temperature range from 620 to 1120 °C. The reversibility of the transition has been studied by DSC measurements. The crystallization energy of the orthorhombic phases is of the order of 10 J/g, at T≈622 °C as T decreases. The variation of the peak height observed as the annealing temperature increases is explained by the complex microstructures, which create an ill-defined energy barrier. Structural models related to the stuffed tridymite derivative structures are proposed for the new forms of the ferri-aluminate.     

Crystal structure and phase transformations of calcium yttrium orthophosphate, Ca3Y(PO4)3

Crystal structure and phase transformations of calcium yttrium orthophosphate Ca₃Y(PO₄)₃ were investigated by X-ray powder diffraction, selected-area electron diffraction, transmission electron microscopy and optical microscopy. The high-temperature phase is isostructural with eulytite, cubic (space group ) with a=0.983320(5) nm, V=0.950790(8) nm³, Z=4 and D_x=3.45 Mg m⁻³. The crystal structure was refined with a split-atom model, in which the oxygen atoms are distributed over two partially occupied sites. Below the stable temperature range of eulytite, the crystal underwent a martensitic transformation, which is accompanied by the formation of platelike surface reliefs. The inverted crystal is triclinic (space group P1) with a=1.5726(1) nm, b=0.84267(9) nm, c=0.81244(8) nm, α=109.739(4)°, β=90.119(5)°, γ=89.908(7)°, V=1.0134(1) nm³, Z=4 and D_x=3.24 Mg m⁻³. The crystal grains were composed of pseudo-merohedral twins. The adjacent twin domains were related by the pseudo-symmetry mirror planes parallel to with the composition surface . When the eulytite was cooled relatively slowly from the stable temperature range, the decomposition reaction of Ca₃Y(PO₄)₃→β-Ca₃(PO₄)₂+YPO₄ occurred.     

Growth of high quality and large-sized Rb0.3MoO3 single crystals by molten salt electrolysis method

High quality and large-sized Rb_0.3MoO₃ single crystals were synthesized by molten salt electrolysis method. X-ray diffraction (XRD) patterns and rocking curves, as well as the white beam Laue diffraction of X-ray images show the crystals grown by this method have high quality. The lattice constants evaluated from XRD patterns are a₀=1.87 nm, b₀=0.75 nm, c₀=1.00 nm, β=118.83^°. The in situ selected area electron diffraction (SAED) patterns along the , and zone axes at room temperature indicate that the Rb_0.3MoO₃ crystal possess perfect C-centered symmetry. Temperature dependence of the resistivity shows this compound undergoes a metal to semiconductor transition at 183 K.     

The Al stabilized phase Li3−3xAlxBO3

The structure of an Al³⁺ stabilized phase Li_3−3xAl_xBO₃ (x≈0.18) was determined by means of single crystal X-ray diffraction. This phase crystallizes in space group P6₁22 or P6₅22, with lattice constants , and Z=6. The unit cell consists of six layers of BO₃ groups with Li⁺ cations distributing statistically on five crystallographic sites, none of which is fully occupied. The Li sites are close to each other and a three-dimensional network results when Li sites only within 1.65 Å are connected. Significant ionic conductivity was observed for this phase.