Water-containing derivative phases of the Srn+1FenO3n+1 series

The n=1, 2, 3 and ∞ members of the homologous series Sr_n+1Fe_nO_3n+1 of layered iron oxides are investigated for their tendency to accept additional layers of water in their crystals. The phases possess a Ruddlesden-Popper-type SrO-(SrO-FeO₂)_n crystal structure, where the n=∞ limit is nothing but the perovskite structure. It is revealed that the n=1, 2 and 3 phases readily accommodate one or two layers of water between adjacent SrO layers, whereas the n=∞ member which lacks the SrO-SrO double-layer unit remains intact in the presence of water. The speed of the water intercalation process is found to decrease with increasing n. Among the layered water derivatives, the n=2 phase with two water molecules per formula unit, i.e. Sr₃Fe₂O₇·2H₂O, was found to be most stable.     

Synthesis of homologously pure bacteriochlorophyll-e and f analogues from BChls-c/d via transformation of the 7-methyl to formyl group and self-aggregation of synthetic zinc methyl bacteriopheophorbides-c/d/e/f in non-polar organic solvent

Homologously pure methyl bacteriopheophorbides-e and f (BPhes-e/f_M) were prepared from modification of naturally occurring bacteriochlorophylls-c and d (BChls-c/d), respectively, by transformation of the methyl to formyl group at the 7-position. The absolute configuration of the 1-hydroxyethyl group at the 3-position of (Zn-)BPhes-e/f_M was determined from comparison with structurally known BChl-c/d epimers. Visible spectra of synthetic (Zn-)BPhe-c/d/e/f_M showed that the 7¹-oxidation and the 8²/12¹/20-methylation affected Soret, Q_x and Q_y bands of both the monomeric (in a polar organic solvent) and oligomeric species (in a non-polar solvent).     

A structural study of the perovskite series Ca1−xNaxTi1−xTaxO3

Compounds in the solid solution series Ca_1−xNa_xTi_1−xTa_xO₃ were synthesized at 1300 °C, followed by annealing at 850 °C or 800 °C with quenching and/or slow cooling to room temperature. Rietveld refinement of their powder X-ray diffraction patterns show that all compounds are single-phase ternary perovskites which adopt the space group Pbnm (a≈b≈√2a_p; c≈2a_p; Z=4) at ambient conditions. The unit cell parameters and cell volumes of the compounds increase regularly with increasing values of x. The coordination of the A-site cations changes throughout the series from eight for CaTiO₃ to nine for NaTaO₃. Compounds with 0?x ?0.4 have A-site cations in eight fold coordination, whereas the coordination of those with 0.4<x<0.9 is ambiguous. Analysis of the crystal chemistry of the compounds shows that the change in coordination at x=0.4 is related to the departure of the B-site cations from the second coordination sphere of the A-site cations, as in compounds with x>0.4 the A-^IIO distances become less than the A-B intercation distances. Contemporaneous with these coordination changes, the tilt angles of the BO₆ polyhedra decrease with increasing values of x. This solid solution series is unusual in that these structural and coordination changes occur regardless that Goldschmidt tolerance factors remain essentially constant at approximately 0.89, and observed tolerance factors, assuming eight fold coordination of the A-site cations, range only from 0.91 to 0.93 (0?x?0.8).     

Structural investigation of KxBa1−xGa2−xGe2+xO8 solid solutions using the X-ray Rietveld method

The K_xBa_1−xGa_2−xGe_2+xO₈ (x=0.6−1.0) solid solutions undergo a structural phase transition that has a significant effect on their sintering behavior and their microwave dielectric properties. The crystal structures of both phases within the solid-solution region were determined by the Rietveld method using powder X-ray diffraction data. We found that the low-temperature-stable phase is isostructural with the pseudo-orthorhombic KGaGe₃O₈ (space group P2₁/a), while the high-temperature-stable phase has a typical monoclinic feldspar structure (space group C2/m). Due to the topological differences between the two structures, the T-O bonds within the tetrahedra must be partially recombined to make a new framework, which causes an endothermic effect during the P2₁/a to C2/m phase transition. The correlation between the crystal structures, the microwave dielectric properties and the phase-transition behaviors were discussed in terms of the crystallographic features, the lattice parameters, and the strain-induced anisotropic peak-broadening.     

High Lithium Capacity MxV2O5Ay·nH2O for Rechargeable Batteries

The aqueous synthesis and electrochemical properties of nanocrystalline M_xV₂O₅A_y·nH₂O are described. It is easily and quickly prepared by precipitation from acidified vanadate solutions. M_xV₂O₅A_y·nH₂O has been characterized by X-ray powder diffraction, electron microscopy, TGA, chemical analyses, and electrochemical studies. The atomic structure is related to that of xerogel-derived V₂O₅·nH₂O. In M_xV₂O₅A_y·nH₂O, M is a cation from the starting vanadate salt and A is an anion from the mineral acid. This material exhibits high, reversible Li capacity and may be considered for use in a cathode in primary and secondary batteries. The lithium capacity of an electrode composed of M_xV₂O₅A_y·nH₂O/EPDM/carbon (88/4/8) is ∼380(mA h)/g (C/80 rate) and the energy density is ∼1000(W h)/kg (120-μm-thick cathode, 4-1.5 V, versus Li metal anode). Critical parameters identified in the synthesis of M_xV₂O₅A_y·nH₂O, with respect to achieving high Li-ion insertion capacity, are acid/vanadium ratio, starting vanadate salt, and temperature. Inclusion of carbon black in the synthesis yields a composite that maintains the high Li capacity, lowers the electrochemical-cell polarization, and preserves the lithium capacity at higher discharge rates. Li-ion coin cells, using pre-lithiated graphite anodes, exhibit electrochemical performance comparable to that of Li-metal coin cells.     

Magnetic and structural properties of Cr1−tTitAs

The pseudobinary CrAsTiAs system has been investigated by X-ray and neutron diffraction and magnetic susceptibility measurements at temperatures between 10 and 1000 K. The phase diagram includes paramagnetic regions with the MnP-, NiAs-, and TiP-type structures and a low-temperature helimagnetic, MnP-type state for0.00 ≦ t ≦ 0.10 of Cr_1?tTi_tAs. The first-order para- to helimagnetic transition in Cr_1?tTi_tAs is accompanied by a hysteresis of 10–15 K. The results are discussed in relation to the findings for other Cr_1?tT_tAs(T =V, Mn, Fe, Co, Ni) andCrAs_1?xX_x (X = P, Sb, Se) phases.     

Structural characterisation of the perovskite series SrxCa1−x−yNdyMnO3: Influence of the Jahn-Teller effect

The crystal structures of the perovskite manganites Sr_xCa_1−x−yNd_yMnO₃ with y=0.1 or 0.2 have been investigated using synchrotron X-ray powder diffraction. At room temperature the structures change from depending on the cation distribution, the different structures exhibiting different tilts of the MnO₆ octahedra. High temperature diffraction measurements demonstrate the presence of, an apparently continuous, isosymmetric I4/mcm to I4/mcm phase transition associated with the removal of long range orbital ordering. Heating the manganites to still higher temperatures results in a continuous transition to the cubic structure. A feature of such transitions is the continuous evolution of the octahedral tilt angle through the I4/mcm to I4/mcm phase transition. The orthorhombic structures do not exhibit orbital ordering and although a first order transition to the tetragonal structure is observed in Sr_0.4Ca_0.5Nd_0.1MnO₃, this high temperature tetragonal structure does not exhibit orbital ordering.     

Synthesis and properties of the Co7Se8−xSx and Ni7Se8−xSx solid solutions

We report the synthesis and elementary properties of the Co₇Se_8−xS_x (x=0-8) and Ni₇Se_8−xS_x (x=0-7) solid solutions. Both systems form a NiAs-type structure with metal vacancies. In general, the lattice parameters decrease with increasing x, but in the Ni₇Se_8−xS_x system c increases on going from x=5 to 7. Magnetic susceptibility measurements show that all samples exhibit temperature-independent paramagnetism from 25-250 K. Samples within the Co₇Se_8−xS_x system, as well as Ni₇Se₈ and Ni₇SeS₇, were found to be poor metals with resistivities of ∼0.20 and ∼0.06 mΩ cm at 300 K, respectively. The Sommerfeld constant (γ) was determined from specific heat measurements to be ∼13 mJ/mol_CoK² and ∼7 mJ/mol_NiK² for Co₇Se_8−xS_x and Ni₇Se_8−xS_x, respectively.     

Coupled In/Te and Ni/vacancy ordering and the modulated crystal structure of a B8 type, Ni3±xIn1−yTe2+y solid solution phase

The commensurate superstructures of a NiAs/Ni₂In type parent structure, Ni_3.32InTe₂ and Ni_3.12In_0.86Te_2.14 (q=γ[0 0 1]^*, γ=2/3) as well as one dimensionally incommensurate structure of Ni₃InTe₂ (γ=0.71) were refined from neutron powder diffraction data (R_wp=4.77%, 4.53% and 4.91% for the three structures, respectively, at 298 K). The commensurate structures were refined in the P6₃/mmc space group (c=3c_NiAs). The stacking sequence at the hcp array is -In/Te/Te/- and the trigonal bipyramidal site within the In layer, Ni(2), is partially occupied while it is empty in the Te layers. The octahedral position in between the In and Te layers, Ni(1a), is fully occupied while the octahedral position in between two adjacent Te layers, Ni(1b), is partially occupied. With decreasing In and Ni content, the modulation wave vector, γ, was found to increase continuously until γ=1. From this, crenel functions to describe the whole homogeneity range of the solid solution were constructed with the length of the atomic domains Δ^Te=γ (and hence Δ^In=Δ^Ni=1−γ) and Δ^Ni(1b)=γ/2 (and hence Δ^Ni(1a)=1−γ/2) which were then used for the refinement of the incommensurate structure of Ni₃InTe₂. The corresponding effect in real space is that the single In layers separating double layers of Te occur less frequent when γ in increasing until at γ=1 the CdI₂ type structure of Ni_1+xTe₂ is reached.     

Mixed-metal flux synthesis of quaternary RMn2TrxZn20−x compounds with Tr=Al, In

Eighteen new intermetallic compounds RMn₂Tr_xZn_20−x (2<x<7; R=rare-earth metal; Tr=Al, In) were synthesized using low-melting mixtures of (Tr/Zn) as a solvent. Structural refinement using single-crystal X-ray diffraction data shows that the compounds are substituted variants of the cubic CeCr₂Al₂₀-type structure (Fd-3m, Z=8; unit cell parameters vary from a=14.1152(3)Å for YbMn₂Al_5.3Zn_14.7 to a=14.8125(4)Å for SmMn₂In_5.9Zn_14.1). The Zn and Tr elements show site preferences in the indium compounds, but not in the aluminum analogs. The substitution of trielide element for zinc modifies the valence electron count of the compounds to allow for the incorporation of Mn into the structure. Magnetic susceptibility data show no evidence of magnetic ordering down to 3 K.     

Atomistic simulation on the site preference and mechanical properties of Th3Co4+xAl12−x and U3Co4+xAl12−x

An atomistic study is presented on the phase stability, site preference and lattice constants of the actinide intermetallic compounds Th₃Co_4+xAl_12−x and U₃Co_4+xAl_12−x. Calculations are based on a series of interatomic pair potentials related to the actinides and transition metals, which are obtained by a strict lattice inversion method. The lattice constants of Th₃Co_4+xAl_12−x and U₃Co_4+xAl_12−x are calculated for different values of x. The site preference of Co atoms at Al sites is also evaluated and the order is given as 6h, 4f, 2b and 12k for Th₃Co_4+xAl_12−x, and 6h, 4f, 12k and 2b for U₃Co_4+xAl_12−x. In addition, some simple mechanical properties such as the elastic constants and bulk modulus are investigated for the actinide compounds with complex structures.     

Long-range ordering in the Bi1−xAexFeO3−x/2 perovskites: Bi1/3Sr2/3FeO2.67 and Bi1/2Ca1/2FeO2.75

Two-ordered perovskites, Bi_1/3Sr_2/3FeO_2.67 and Bi_1/2Ca_1/2FeO_2.75, have been stabilized and characterized by transmission electron microscopy, Mössbauer spectroscopy and X-ray powder diffraction techniques. They both exhibit orthorhombic superstructures, one with a≈b≈2a_p and c≈3a_p (S.G.: Pb2n or Pbmn) for the Sr-based compound and one with a≈b≈2a_p and c≈8a_p (S.G.: B222, Bmm2, B2mm or Bmmm) for the Ca-based one. The high-resolution transmission electron microscopy (HRTEM) images evidence the existence of one deficient [FeO_x]_∞ layer, suggesting that Bi_1/3Sr_2/3FeO_2.67 and Bi_1/2Ca_1/2FeO_2.75 behave differently compared to their Ln-based homolog. The HAADF-STEM images allow to propose a model of cation ordering on the A sites of the perovskite. The Mössbauer analyses confirm the trivalent state of iron and its complex environment with three types of coordination. Both compounds exhibit a high value of resistivity and the inverse molar susceptibility versus temperature curves evidence a magnetic transition at about 730 K for the Bi_1/3Sr_2/3FeO_2.67 and a smooth reversible transition between 590 and 650 K for Bi_1/2Ca_1/2FeO_2.75.     

Crystal structure of Bi1−xTbxFeO3 from high-resolution neutron diffraction

Samples of Bi_1−xTb_xFeO₃, with x=0.05, 0.10, 0.15, 0.20 and 0.25, have been synthesised by solid state reaction. The crystal structures of the perovskite phases, characterised via Rietveld analysis of high resolution powder neutron diffraction data, reveal a structural transition from the R3c symmetry of the parent phase BiFeO₃ to orthorhombic Pnma symmetry, which is complete for x=0.20. The x=0.10 and 0.15 samples are bi-phasic. The transition from a rhombohedral to orthorhombic unit cell is suggested to be driven by the dilution of the stereochemistry of the Bi³⁺ lone pair at the A-site. The G-type antiferromagnetic spin structure, the size of the ordered magnetic moment (∼3.8 μ_B) and the T_N (∼375 °C) are relatively insensitive to increasing Tb concentrations at the A-site.     

Preparation and thermoelectric properties of AgPbmSbSem+2 materials

Hydrothermally synthesized AgPb_mSbSe_m+2 (m=10, 12, 16, 18) nanoparticles with diameters of 20-50 nm were compacted by pressureless sintering. The Seebeck coefficient and electrical conductivity of the samples were measured from room temperature up to ∼750 K. The samples show large and positive values of the Seebeck coefficient and moderate electrical conductivity. The thermoelectric properties of Ag_xPb₁₈SbSe₂₀ (x=0.8, 0.85) and AgPb₁₈SbSe_20−yTe_y (y=1, 3) samples have also been studied. It has been found that Ag_0.85Pb₁₈SbSe₂₀ sample has a higher thermoelectric power factor. A significant difference in thermoelectric properties has also been observed for the AgPb₁₈SbSe₂₀ samples prepared with pressureless sintering and spark plasma sintering.     

Effects of the A-site cation number on the properties of Ln5/8M3/8MnO3 manganites

The properties of manganites can be tuned by changing the doping level x in Ln_1−xM_xMnO₃. A second mechanism allows tuning of magnetic and electronic properties, for fixed x values, by varying the average A-cation radius, 〈r_A〉. Moreover, for fixed x and 〈r_A〉 values, the changes in the A-cation size variance, σ², also modify the ferromagnetic and metal-insulator transition temperatures. Here, we investigate the influence of the number of A-site cations on Ln_5/8M_3/8MnO₃ manganites, where x, 〈r_A〉 and σ² values are kept constant, and in the absence of phase separation phenomena. We have found that the number of cation species at the A site (N_A) has a strong influence on the width of the ferromagnetic and metal-insulator transitions, and a small influence on the average transition temperature. This behavior is opposite to that observed for increasing values of the variance σ² in manganites, with the same x and 〈r_A〉 values, where average transition temperatures are strongly reduced.     

Isothermal decomposition of ternary oxide AxByOz on an isobar—Stability of perovskite ABO3 (A = La,Sm, Dy; B = Mn,Fe) in a reducing atmosphere

The isothermal decomposition of any ternary oxide A_xB_yO_z on liberation of n moles of oxygen at a constant pressure is found to be driven by the mixing entropy ΔS_m = ?nRln P_O2 of the total entropy change ΔS = ΔS° + ΔS_m. The stability of A_xB_yO_z towards isothermal decomposition into a biphasic solid mixture is derived from the equilibrium condition $\text{ΔG1 = 0}$ as functions of standard changes ΔH° and ΔS°. Assuming ΔS° = 44n and calculating ΔH° in terms of lattice energies U(ABO₃) and U(A₂O₃), the stability of perovskites is given as a function of the ionic radius of the A³⁺ ion. The calculated stability agrees well with that observed. The effect of electronic entropy change ΔS_e on ΔS° is demonstrated for AFeO₃ (A = La, Sm, Dy).     

Synthesis, Structures, and Thermal Expansion of the La2W2−xMoxO9 Series

The La₂W_2−xMo_xO₉ series has been synthesized by the ceramic method. An alternative synthesis using microwave radiation is also reported. La₂W₂O₉ has two polymorphs and the low-temperature phase (α) transforms to the high-temperature form (β) at 1077°C. The influence of the W/Mo substitution in this phase transition has been investigated by DTA. The β structure for x≥0.7 compositions can be prepared as single phase at any cooling rate. The β phase for 0.3≤x≤0.7 compounds can be prepared as single phase by quenching, whereas a mixture of α and β phases is obtained by slow cooling. The W/Mo ratio in both coexisting phases is different with the β-phase having a higher Mo content. The x=0.1 and 0.2 compounds have been prepared as mixtures of phases. The room temperature structure of β-La₂W_1.7Mo_0.3O₉ has been analyzed by the Rietveld method in P2₁3 space group. The final R-factors were R_WP=9.0% and R_F=5.6% with a structure similar to that of β-La₂Mo₂O₉. Finally, the thermal expansion of both types of structures has been determined from a thermodiffractometric study. The thermal expansion coefficients were 2.9×10⁻⁶ and 9.7×10⁻⁶°C⁻¹ for α-La₂W₂O₉ and β-La₂W_1.2Mo_0.8O₉, respectively.     

Synthesis and aromatic nucleophilic N-N, N-S and N-O exchange reactions of N,N-dimethyl-2-trifluoroacetyl-1-naphthylamine

We succeeded in the synthesis of N,N-dimethyl-2-trifluoroacetyl-1-naphthylamine (10) by the regioselective deacylation of N,N-dimethyl-2,4-bis(trifluoroacetyl)-1-naphthylamine with trifluoroacetic acid and water. The aromatic nucleophilic substitutions of 10 with various amines, thiols and alcohols proceeded cleanly to give the corresponding N-N, N-S and N-O exchanged products in moderate to excellent yields.     

Crystal structure, microstructure and reducibility of LaNixCo1−xO3 and LaFexCo1−xO3 Perovskites (0<x≤0.5)

Nickel and iron substituted LaCoO₃ with rhombohedrally distorted perovskite structure were obtained in the temperature range of 600-900 °C by thermal decomposition of freeze-dried citrates and by the Pechini method. The crystal structure, morphology and defective structure of LaCo_1−xNi_xO₃ and LaCo_1−xFe_xO₃ were characterized by X-ray diffraction and neutron powder diffraction, TEM and SEM analyses and electron paramagnetic resonance spectroscopy. The reducibility was tested by temperature programmed reduction with hydrogen. The products of the partial and complete reduction were determined by ex-situ XRD experiments. The replacement of Co by Ni and Fe led to lattice expansion of the perovskite structure. For perovskites annealed at 900 °C, there was a random Ni, Fe and Co distribution. The morphology of the perovskites does not depend on the Ni and Fe content, nor does it depend on the type of the precursor used. LaCo_1−xNi_xO₃ perovskites (x>0.1) annealed at 900 °C are reduced to Co/Ni transition metal and La₂O₃ via the formation of oxygen deficient Brownmillerite-type compositions. For LaCo_1−xNi_xO₃ annealed at 600 °C, Co/Ni metal, in addition to oxygen-deficient perovskites, was formed as an intermediate product at the initial stage of the reduction. The interaction of LaCo_1−xFe_xO₃ with H₂ occurs by reduction of Co³⁺ to Co²⁺ prior to the Fe³⁺ ions. The reducibility of Fe-substituted perovskites is less sensitive towards the synthesis procedure in comparison with that of Ni substituted perovskites.     

Study of Pr1−xMn1+xO3 perovskites

The structural and magnetic properties of the Pr_1?xMn_1+xO₃ perovskites were studied. The increase of x (i.e., $\text{Pr}\text{Mn}\text{< 1}$) leads to the decrease of the orthorhombic deformation and of the Néel temperature and, simultaneously, to an increase of the ferromagnetic contribution. The latter effect is explained from the suggested distribution of the cations (Pr³⁺_1?xMn²⁺_x)_A(Mn³⁺_1?xMn⁴⁺_x)O^2?₃ by the double exchange of Mn³⁺Mn⁴⁺ pairs at the B—sublattice.