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.     

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.     

Influence of the structure on electric and magnetic properties of La0.8Na0.2Mn1−xCoxO3 perovskites

The influence of the cobalt substitution for manganese ions in the mixed valence perovskites La_0.8Na_0.2Mn_1−xCo_xO₃ (0?x?0.2) was investigated by X-ray, electric transport and magnetic measurements. The study carried out on sintered polycrystalline samples revealed the rhombohedral () structure and the insulator-metal transition connected with a ferromagnetic arrangement in the whole concentration range. Increasing concentration of cobalt ions leads to a gradual decrease of PM-FM and I-M transition temperatures. An influence of the cobalt ions on the observed behavior is attributed to charge compensation Mn³⁺→Mn⁴⁺ leading to the formation of stable couples Mn⁴⁺-Co²⁺. Therefore the double-exchange interactions Mn³⁺-O²⁻-Mn⁴⁺ partly vanish and they are replaced by positive superexchange interactions Mn⁴⁺-O²⁻-Co²⁺, but of a semiconducting character.     

“Unusual” phase transitions in CeAlO3

High-resolution time-of-flight neutron powder diffraction was carried out to investigate the crystal structures of CeAlO₃ over a wide temperature range between 4.2 and 1423 K. Confirming the recent result of X-ray powder diffraction, the room temperature structure is tetragonal with the space group I4/mcm (tilt system (a⁰a⁰c⁻)). The tetragonal structure persists down to 4.2 K. However, above room temperature CeAlO₃ undergoes three phase transitions: first to the orthorhombic Imma structure (tilt system (a⁰b⁻b⁻)) at, e.g., 373 K, then to the rhombohedral structure (tilt system (a⁻a⁻a⁻)) at, e.g., 473 K, and finally, to the primitive cubic structure which exists above 1373 K. The sequence of phases, , which occurs in CeAlO₃ is a rare one in oxide perovskites.     

Ca6.3Mn3Ga4.4Al1.3O18—A novel complex oxide with 3D tetrahedral framework

A new Ca_6.3Mn₃Ga_4.4Al_1.3O₁₈ compound has been prepared by solid state reaction in a dynamic vacuum of 5×10⁻⁶ mbar at 1200 °C. The crystal structure of Ca_6.3Mn₃Ga_4.4Al_1.3O₁₈ was studied using X-ray powder diffraction (, SG F432, Z=8, R_I=0.031, R_P=0.068), electron diffraction and high resolution electron microscopy. The Ca_6.3Mn₃Ga_4.4Al_1.3O₁₈ structure can be described as a tetrahedral [(Ga_0.59Mn_0.24Al_0.17)₁₅O₃₀]^18.24− framework stabilized with embedded [(Ca_0.9Mn_0.1)₁₄MnO₆]^18.24+ polycations, which consists of an isolated MnO₆ octahedron surrounded by a capped cube of (Ca_0.9Mn_0.1) atoms. The Ca_6.3Mn₃Ga_4.4Al_1.3O₁₈ structure is related to the structure of Ca₇Zn₃Al₅O_17.5, but appears to be significantly disordered due to the presence of two orientations of oxygen tetrahedra around the cationic 0,0,0 and x,x,x () positions in a random way according to the F432 space symmetry. The analogy between the Ca_6.3Mn₃Ga_4.4Al_1.3O₁₈ crystal structure and the structure of the “fullerenoid” Sr₃₃Bi_24+δAl₄₈O_141+3δ/2 oxide is discussed. Ca_6.3Mn₃Ga_4.4Al_1.3O₁₈ adopts a Curie-Weiss behavior of χ(T) above with a Weiss temperature and per formula unit. At lower temperatures, the χ(T) deviates from the Curie-Weiss law indicating a strengthening of the ferromagnetic component of the exchange interaction.     

Crystal structure, electronic structure and physical properties of the new low-valent thallium silicon telluride Tl6Si2Te6 in comparison to Tl6Ge2Te6

The title compounds were prepared from the elements in the stoichiometric ratio at 800 °C under exclusion of air. Tl₆Si₂Te₆ crystallizes in the space group P1¯, isostructural with Tl₆Ge₂Te₆, with , , , α=89.158(2)°, β=96.544(2)°, γ=100.685(2)°, (Z=2). Its structure is composed of dimeric [Si₂Te₆]⁶⁻ units with a Si-Si single bond, while the Tl atoms are irregularly coordinated by five to six Te atoms. Numerous weakly bonding Tl-Tl contacts exist. Both title compounds are black semiconductors with small band gaps, calculated to be 0.9 eV for Tl₆Si₂Te₆ and 0.5 eV for Tl₆Ge₂Te₆. The Seebeck coefficients are +65 μV K⁻¹ in case of Tl₆Si₂Te₆ and +150 μV K⁻¹ in case of Tl₆Ge₂Te₆ at 300 K, and the electrical conductivities are 5.5 and 3 Ω⁻¹ cm⁻¹, respectively.     

Synthesis, crystal structure and magnetic properties of an Os-containing pillared perovskite La5Os3MnO16

A new Os-containing, pillared perovskite, La₅Os₃MnO₁₆, has been synthesized by solid state reaction in sealed quartz tubes. This extends the crystal chemistry of these materials which had been known only for Mo and Re, previously. The crystal structure has been characterized by X-ray and neutron powder diffraction and is described in space group C-1 with parameters a=7.9648(9) Å; b=8.062(1) Å; c=10.156(2) Å, α=90.25(1)°, β=95.5(1)°; γ=89.95(2)°, for La₅Os₃MnO₁₆. The compound is isostructural with the corresponding La₅Re₃MnO₁₆ phase. A very short Os-Os distance of 2.50(1) Å was found in the dimeric pillaring unit, Os₂O₁₀, suggestive of a triple bond as demanded by electron counting. Nearly spin only values for the effective moment for Os⁵⁺ () and Mn²⁺ () were derived from magnetic susceptibility data. Evidence for magnetic transitions was seen near ∼180 and 80 K. Neutron diffraction data indicate that T_c is 170(5) K. The magnetic structure of La₅Os₃MnO₁₆ at 7 K was solved revealing that Os⁵⁺ and Mn²⁺ form ferrimagnetically coupled layers with antiferromagnetic interlayer ordering. The ordered moments are for Mn²⁺ and for Os⁵⁺, which are reduced from the respective spin only values of 5.0 and . The observation of net ferrimagnetic (antiparallel) intraplanar coupling between Os⁵⁺(t_2g³) and Mn²⁺(t_2g³e_g²) is interesting as it appears to contradict the Goodenough-Kanamori rules for 180° superexchange.     

Thermodynamic studies on SrFe12O19(s), SrFe2O4(s), Sr2Fe2O5(s) and Sr3Fe2O6(s)

The citrate-nitrate gel combustion route was used to prepare SrFe₂O₄(s), Sr₂Fe₂O₅(s) and Sr₃Fe₂O₆(s) powders and the compounds were characterized by X-ray diffraction analysis. Different solid-state electrochemical cells were used for the measurement of emf as a function of temperature from 970 to 1151 K. The standard molar Gibbs energies of formation of these ternary oxides were calculated as a function of temperature from the emf data and are represented as (SrFe₂O₄, s, T)/kJ mol⁻¹ (±1.7)=−1494.8+0.3754 (T/K) (970?T/K?1151). (Sr₂Fe₂O₅, s, T)/kJ mol⁻¹ (±3.0)=−2119.3+0.4461 (T/K) (970?T/K?1149). (Sr₃Fe₂O₆, s, T)/kJ mol⁻¹ (±7.3)=−2719.8+0.4974 (T/K) (969?T/K?1150).Standard molar heat capacities of these ternary oxides were determined from 310 to 820 K using a heat flux type differential scanning calorimeter (DSC). Based on second law analysis and using the thermodynamic database FactSage software, thermodynamic functions such as Δ_fH°(298.15 K), S°(298.15 K) S°(T), C_p°(T), H°(T), {H°(T)-H°(298.15 K)}, G°(T), free energy function (fef), Δ_fH°(T) and Δ_fG°(T) for these ternary oxides were also calculated from 298 to 1000 K.     

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.     

Synthesis, characterization and standard molar enthalpy of formation of La(C7H5O3)2·(C9H6NO)

The product from reaction of lanthanum chloride seven-hydrate with salicylic acid and 8-hydroxyquinoline, La(C₇H₅O₃)₂·(C₉H₆NO), was characterized by IR, elemental analysis, molar conductance, and thermogravimetric analysis. The standard molar enthalpies of solution of [LaCl₃·7H₂O (s)], [2C₇H₆O₃ (s)], [C₉H₇NO (s)] and [La(C₇H₅O₃)₂·(C₉H₆NO) (s)] in a mixed solvent of absolute ethyl alcohol, dimethyl formamide (DMF) and perchloric acid were determined by calorimetry to be [LaCl₃·7H₂O (s), 298.15 K] = −96.45 ± 0.18 kJ mol⁻¹, [2C₇H₆O₃ (s), 298.15 K] = 14.99 ± 0.17 kJ mol⁻¹, [C₉H₇NO (s), 298.15 K] = −3.86 ± 0.06 kJ mol⁻¹ and [La(C₇H₅O₃)₂·(C₉H₆NO) (s), 298.15 K] = −117.78 ± 0.11 kJ mol⁻¹. The enthalpy change of the reaction

(1)     

Structure and magnetic properties of Ca14MnP11

The compound Ca₁₄MnP₁₁ crystallizes in the Ca₁₄AlSb₁₁ structure type with the tetragonal space group I4₁/acd (Z=8) and lattice parameters of , c=20.7565(9) at 90 K. The structure consists of MnP₄⁹⁻ tetrahedron, P₃⁷⁻ trimer, 4 P³⁻ isolated anions and 14 Ca²⁺ cations. Similar to other compounds of this structure type containing phosphorous, the P₃⁷⁻ trimer has a central P atom that is best modeled in the structure as being equally split between two sites. In addition, there is no additional distortion of the manganese-containing tetrahedron compared with the main group analog, Ca₁₄GaP₁₁, suggesting that the Mn oxidation state is Mn²⁺. Temperature-dependent magnetic susceptibility shows that the compound is paramagnetic over the entire temperature range measured (2-300 K). The data can be fit with a modified Curie-Weiss law and provide an effective magnetic moment of 5.80 (2) B.M. with a Weiss constant of −2.13(2) K and . This moment is significantly higher than those measured for any of the Mn-containing analogs and is consistent with Mn²⁺. This result will be discussed in light of the electron counting scheme for Mn compounds of the Ca₁₄AlSb₁₁ structure-type.     

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.     

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.     

Magnetic and vibrational properties and crystal structure of Sr9.2Co1.3(PO4)7 with disordered arrangements of some strontium, cobalt, and phosphate ions

Solid solutions of Sr_9+xCo_1.5−x(PO₄)₇ were found in the compositional range of 0.05?x?0.30. The structure of Sr_9.2Co_1.3(PO₄)₇ (x=0.2) was determined from single crystal X-ray diffraction (space group (No. 166); Z=3; and ; ; ; ) and refined to R₁=0.0343 and wR₂=0.0633 for 586 reflections with I>2σ(I). Sr_9.2Co_1.3(PO₄)₇ is structurally related to β-Ca₃(PO₄)₂ and Sr₃(PO₄)₂ and has disordered arrangements of some Sr²⁺, Co²⁺, and PO₄³⁻ ions. Sr²⁺ ions at a 9e site are statistically disordered among four positions near the center of symmetry. Co²⁺ and Sr²⁺ ions are split along the c-axis to occupy a 6c site that is 75% vacant. The P1O₄ tetrahedra are orientationally disordered. Sr²⁺ ions at an 8-fold coordinated 18h site, Co²⁺ ions at an octahedral 3a site, and the P2O₄ tetrahedra are ordered in the structure of Sr_9.2Co_1.3(PO₄)₇. Features of Raman spectra are discussed in relation to the crystallographic structure of Sr_9.2Co_1.3(PO₄)₇ and in comparison with Raman spectra of β-Ca₃(PO₄)₂-type and Sr₃(PO₄)₂-type compounds. Sr_9.2Co_1.3(PO₄)₇ is paramagnetic between 2 and 300 K with an effective magnetic moment of 4.98μ_B per Co²⁺ ion.     

Synthesis and NMR spectroscopic investigation of salts containing the novel [Au(CF3)nX4−n] (n=4-1, X=F, CN, Cl) anions

First examples for the syntheses of trifluoromethyl transition metal complexes by conversion of a cyano into a trifluoromethyl ligand are described. The fluorination of [][Au(CN)₄] with ClF in CH₂Cl₂ leads to the formation of a mixture of gold complexes of the type [AuF_xCl_y(CF₃)_4−x−y]⁻ (x=0-4, y=0-2). Ligand exchange reactions of [AuF_xCl_y(CF₃)_4−x−y]⁻ (x=0-4, y=0-2) with (CH₃)₃SiY (Y=Cl, CN) are performed resulting in anions of the type [AuY_x(CF₃)_4−x]⁻ (x=0-4). All products are characterised by - and NMR spectroscopy.     

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.