Magnetic studies of a mixed antiferromagnetic system Fe1?xNixPS3

The magnetic properties of the layered compounds FePS3, NiPS3 and the mixed system Fe1-xNixPS3 have been investigated. All the compounds show a maximum in chi at a temperature T(max) followed by the onset of antiferromagnetic order at low temperatures at T(N). As the composition is changed there is a continuous change in T(N) and in the ratio T(N)/T(max). Our results , show that in the mixed antiferromagnetic system Fe1-xNixPS3, the strongly antiisotropic Fe component imposes its anisotropy on the weakly anisotropic Ni component. We find also no frustration or spin-glass-like ordering at any composition.

AILaNb2O7:A new series of layered perovskites exhibiting ion exchange and intercalation behaviour

A new series of layered perovskite oxides, AILaNb2O7 (A = Li, Na, K, Rb, Cs, NH4) constituting n = 2 members of the family A A′n−1BnO3n+1, has been prepared. Their structure consists of double perovskite slabs interleaved by A atoms. Hydrated HLaNb2O7 is formed by topotactic proton exchange of the A atoms in ALaNb2O7 (A = K, Rb, Cs). The hydrate readily loses water to give anhydrous HLaNb2O7 which is isostructural with RbLaNb2O7. HLaNb2O7 exhibits Bronsted acidity forming intercalation compounds with bases such as n-octylamine and pyridine.

HNbWO6 and HTaWO6: Novel layered oxides related to the rutile structure. Synthesis and investigation of ion-exchange and intercalation behaviour

New protonated layered oxides, HMWO6·1.5H2O (M=Nb or Ta), have been synthesized by topotactic exchange of lithium in trirutile LiMWO6 with protons by treatment with dilute HNO3. The tetragonal cell constants are a=4.71 (2) and c=25.70 (8)Å for HNbWO6·1.5H2O and a=4.70 (2) and c=25.75 (9) Å for HTaWO6·1.5H2O. Partially hydrated compounds, HMWO6·0.5H2O and anhydrous compounds, HMWO6 retain the layered structure. The structure of these oxides consists of MWO6 sheets built up of M/W-oxygen octahedra with rutile type corner- and edge-sharing. Interlayer protons in HMWO6 are exchanged with Li+, Na+, K+ and Tl+. HMWO6 exhibit Brønsted acidity intercalating n-alkylamines and pyridine.

Influence of Geometrical Factors on the Physico-Chemical Properties of Perovskite and Layered Perovskite Oxides

A brief qualitative comparison is made of perovskite ABO sub 3 and layered perovskite ABO sub 3 and layered perovskite A sub 2 BO sub 4 oxides with special emphasis on the influence of geometrical factors on certain physico-chemical properties. The layered perovskite oxides are distinguished from three-dimensional oxides by a looser packing, frustration in three-dimensional interactions, more internal pressure on B--O bonds for small tolerance factors, and by different values of site-percolation thresholds. Their influence on electronic configurations of metal ions, stabilities and syntheses of compounds is discussed. The influence of increased anisotropy in layered oxides on localisation of charge carriers and in suppressing the onset of long-range ferromagnetic ordering is also discussed.

Effect of High Pressure on the Electrical Conductivity of TlInX2 (X = Se, Te) Layered Semiconductors

The dc electrical conductivity of TlInX2 (X = Se, Te) single crystals, parallel and perpendicular to the (001) c-axis is studied under high quasi-hydrostatic pressure up to 7.0 GPa, at room temperature. Conductivity measurements parallel to the c-axis are carried out at high pressures and down to liquid nitrogen temperatures. These materials show continuous metallization under pressure. Both compounds have almost the same pressure coefficient of the electrical activation energy parallel to the c-axis, d(ΔE∥)/dP = −2.9 × 10−10 eV/Pa, which results from the narrowing of the band gap under pressure. The results are discussed in the light of the band structure of these compounds.

K1-xLaxCa2-xNb3O10, a Layered Perovskite Series with Variable Interlayer Cation Density, and LaCaNb3O10, a Novel Layered Perovskite Oxide with No Interlayer Cations

A series of layered perovskite oxides of the formula K1-xLaxCa2-xNb3O10 for 0 < x ≤ 1.0 have been prepared. All the members are isostructural, possessing the structure of KCa2Nb3O10. The interlayer potassium ions in the new series can be ion-exchanged with protons to give H1-xLaxCa2-xNb3O10. The latter readily forms intercalation compounds of the formula (CnH2n+1NH3)1-x LaxCa2-xNb3O10, just as the parent solid acid HCa2Nb3O10. The end member LaCaNb3O10 containing no interlayer cations is a novel layered perovskite oxide, being a n = 3 member of the series An-1BnX3n+1.

A[Bi3Ti4O13] and A[Bi3PbTi5O16] (A = K, Cs): New n=4 and n=5 members of the layered perovskite series, A[A ' n-1B nO3n+1], and their hydrates

We describe the synthesis and structural characterization of new layered bismuth titanates, A[Bi3Ti4O13] and A[Bi3PbTi5O16]for A = K, Cs, corresponding to n = 4 and 5 members of the Dion-Jacobson series of layered perovskites of the general formula, A[A'n-1BnO3n+1]. These materials have been prepared by solid state reaction of the constituents containing excess alkali, which is required to suppress the formation of competitive Aurivillius phases. Unlike the isostructural niobates and niobium titanates of the same series, the new phases reported here are spontaneously hydrated-a feature which could make them potentially useful as photocatalysts for water splitting reaction. On hydration of the potassium compounds, the c axis expands by ca. 2 Angstrom and loses its doubling [for example, the tetragonal lattice parameters of K[Bi3Ti4O13] and its dihydrate are respectively a = 3900(1)Angstrom c 37.57(2) Angstrom; a 3.885(1) Angstrom, c = 20.82(4) Angstrom]; surprisingly, the cesium analogues do not show a similar change on hydration.

Magnetism in MnPSe3: a layered 3d(5) antiferromagnet with unusually large XY anisotropy

The anisotropic magnetic susceptibilities of single crystals of the isostructural layered antiferromagnets, MnPS3 (T-N = 78 K) and MnPSe3 (T-N = 74 K), have been measured as functions of temperature. In both compounds, divalent manganese is present in the high-spin S = 5/2 state. The anisotropies in the susceptibilities of the two are, however, very different; while the susceptibility of MnPS3 is isotropic, that of MnPSe3 shows a large XY anisotropy, unusual for a manganese compound. The anisotropic susceptibilities are described by the zero-field spin Hamiltonian: H = DSiz2 - Sigma J(ij).(S) over right arrow (S) over right arrow(j) with the quadratic single-ion anisotropy term introducing anisotropy in an otherwise isotropic situation. The exchange J and the single-ion zero-field-splitting (ZFS) parameter D were evaluated using the correlated effective-field theory of Lines. For MnPSe3, J/k = -5.29 K and D/k = 26.6 K, while for isotropic MnPS3, J/k = -8.1 K. It is suggested that the large value of the ZFS parameter for MnPSe3 as compared to MnPS3 could be due to the large ligand spin-orbit contribution of the heavier selenium.

Conformation and orientation of alkyl chains in the layered organic-inorganic hybrids: (CnH2n+1NH3)(2)PbI4 (n=12,16,18)

Layered organic inorganic hybrids based on perovskite-derived alkylammonium lead halides have been demonstrated as important new materials in the construction of molecular electronic devices. Typical of this class of materials are the single-perovskite slab lead iodides of the general formula (CnH2n+1NH3)(2)PbI4. While for small n, these compounds are amenable to single-crystal structure determination, the increasing degree of disorder in the long chain (n = 12,14...) compounds makes such an analysis difficult. In this study, we use powder X-ray diffraction, and vibrational and C-13 NMR spectroscopies to establish the conformation, orientation and organization of hydrocarbon chains in the series of layered alkylammonium lead iodides (CnH2n+1NH3)(2)PbI4 (n = 12,16,18). We find that the alkyl chains adopt a tilted bilayer arrangement, while the structure of the inorganic layer remains invariant with respect to the value of n. Conformation-sensitive methylene stretching modes in the infrared and Raman spectra, as well as the C-13 NMR spectra indicate that bonds in the methylene chain are in trans configuration. The skeletal modes of the alkyl chain in the Raman spectra establish that there is a high degree of all-trans conformational registry for the values of n studied here. From the orientation dependence of the infrared spectra of crystals of (CnH2n+1NH3)(2)PbI4 ( n = 12,16), we find that the molecular axis of the all-trans alkyl chains are tilted away from the interlayer normal by an angle of 55degrees. This value of this tilt angle is consistent with the dependence of the c lattice expansion as a function of n, as determined from powder X-ray diffraction.

A layered chlorophosphate, Na-3[Cd4Cl3(HPO4)(2)(H2PO4)(4)], containing Na+ ions in the interlamellar space

Reaction between CdCl2.H2O and NaH2PO4.H2O Under hydrothermal conditions gives rise to a new cadmium chlorophosphate of the formula Na-3[Cd4Cl3(HPO4)(2)(H2PO4)(4)] I. This material crystallizes in the orthorhombic system with space group Fmm2(no. 42). I has macroanionic layers of [Cd4Cl3(HPO4)(2)(H2PO4)(4)](3-) with Na+ ions in the interlamellar space. The discovery of such compounds suggests that metathetic reactions carried out under hydrothermal conditions may provide a novel route for the synthesis of new open-framework structures.

Spontaneous generation of optical activity in urea inclusion compounds vis-a-vis current theories

A novel test of recent theories of the origin of optical activity has been designed based on the inclusion of certain alkyl 2-methylhexanoates into urea channels.

Structure and magnetotransport properties of the layered manganites REl.2Srl.8Mn,07(R E = La, Pr, Nd)

We have examined the magnetotransport properties and the structure, by Rietveld refinement of powder X-ray data, of the phases RE(1.2)Sr(1.8)Mn(2)O(7) (RE = La, Pr, Nd). We find that on cooling, La1.2Sr1.8Mn2O7 undergoes a transition to a nearly perfect ferromagnet with 90% magnetization at 1.45 T, as reported by earlier workers, but the Pr and Nd phases show only a small magnetization that grows gradually as the temperature is decreased. There seems to be significant correlation between electrical transport and the Jahn-Teller elongation of the apical Mn-O bonds in these systems. The elongation of the apical Mn-O bonds forces the nine-coordinate rock-salt site to be occupied preferentially by the smaller rare-earth-metal cations. This preferential occupation is reliably obtained from the X-ray refinement. All three title phases show a magnetoresistance ratio of about 4(corresponding to a magnetoresistance, [R(0)-R(H)]/R(0), of about 75%) at a field of 7 T and temperatures around 100 K.

A Synthetic Model for the Inhibition of Glutathione Peroxidase by Antiarthritic Gold Compounds

In this paper, inhibition of the glutathione peroxidase activity of two synthetic organoselenium compounds, bis[2-(N,N-dimethylamino)benzyl]diselenide (5) and bis[2-(N,N-dimethylamino)benzyl]selenide (9), by gold(I) thioglucose (1), chloro(triethylphosphine)gold(I), chloro(trimethylphosphine)gold(I), and chloro(triphenylphosphine)gold(I) is described. The inhibition is found to be competitive with respect to a peroxide (H2O2) substrate and noncompetitive with respect to a thiol (PhSH) cosubstrate. The diselenide 5 reacts with PhSH to produce the corresponding selenol (6), which upon treatment with 1 equiv of gold(I) chlorides produces the corresponding gold selenolate complexes 11−13. However, the addition of 1 equiv of selenol 6 to complexes 11−13 leads to the formation of bis-selenolate complex 14 by ligand displacement reactions involving the elimination of phosphine ligands. The phosphine ligands eliminated from these reactions are further converted to the corresponding phosphine oxides (R3PO) and selenides (R3PSe). In addition to the replacement of the phosphine ligand by selenol 6, an interchange between two different phosphine ligands is also observed. For example, the reaction of complex 11 having a trimethylphosphine ligand with triphenylphosphine produces complex 13 by phosphine interchange reactions via the formation of intermediates 15 and 16. The reactivity of selenol 6 toward gold(I) phosphines is found to be similar to that of selenocysteine.

Relative Stabilities of Layered Perovskite and Pyrochlore Structures in Transition Metal-Oxides Containing Trivalent Bismuth

In order to investigate the factors determining the relative stabilities of layered perovskite and pyrochlore structures of transition metal oxides containing trivalent bismuth, several ternary and quaternary oxides have been investigated. While d0 cations stabilize the layered perovskite structure, cations containing partially-filled d orbitals (which suppress ferroelectric distortion of MO6 octahedra) seem to favor pyrochlore-related structures. Thus, the vanadium analogue of the layered perovskite Bi4Ti3O12 cannot be prepared; instead the composition consists of a mixture of pyrochlore-type Bi1.33V2O6, Bi2O3, and Bi metal. The distortion of Bi1.33V2O6 to orthorhombic symmetry is probably due to an ordering of anion vacancies in the pyrochlore structure. None of the other pyrochlores investigated, Bi2NbCrO7, Bi2NbFeO7, TlBiM2O7 (M = Nb, Ta), shows evidence for cation ordering in the X-Ray diffraction patterns, as indeed established by structure refinement of TlBiNb2O7.

XANES and EXAFS of copper compounds: Studies of copper carboxylates with metal-metal bonds and of the complex formed byPseudomonas aeruginosa

X-ray absorpion near edge structure (xanes) of copper compounds with copper in 1+, 2+ and 3+ states has been studied. Extended x-ray absorption fine structure (exafs) has been employed to determine bond distances and coordination numbers in several model copper compounds. Employing bothxanes andexafs, the structure of the copper complex formed by the micro-organismPseudomonas aeruginosa has been shown to be square-planar with the Cu-O distance close to that in cupric glucuronates and cupric acetylacetonate.exafs has been shown to be useful for studying metal-metal bonds in copper carboxylates.