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NdCrSb3, a compound containing alternating layers of Nd and Cr atoms in its structure, undergoes two ferromagnetic ordering transitions. At 107.8 K, ferromagnetic ordering of the Cr moments occurs; at 12.7 K, ferromagnetic ordering of the Nd and Cr moments occurs. Neutron diffraction studies place the magnetization axis for the ordered Cr moments along [010], and for the coupled Nd and Cr moments along [100]. These transitions are also observed in the temperature dependence of the dc magnetization, and as maxima in the χ‘ac and χ‘ ‘ac magnetic susceptibilities. Hysteresis loops at 2 and 40 K revealed saturation magnetization values of 3.70 and 2.37 μB/f.u. and coercive fields of 2100 and 400 Oe, respectively. The saturation magnetization of 3.7 μB at 2 K is the same as that determined from neutron diffraction studies. Both ferromagnetic ordering transitions are manifested as kinks in the temperature dependence of the electrical resistivity. Field-dependent resistivity measurements made between 2 and 130 K...

[Co(2,2'-thiodiethanol)2Cl2] (1) and [Mn(2,2'-thiodiethanol)Cl2]n (2), have been synthesized and characterized structurally. Compound 1 is a six-coordinate, mononuclear Co(II) complex. The mononuclear units in the crystal lattice of 1 are linked together through intermolecular hydrogen bonds between hydroxy and chloride groups to form a three-dimensional array. Compound 2 consists of six-coordinate Mn(II) units that are linked by covalently bound 2,2'-thiodiethanol and chloride ligands to form an alternating one-dimensional chain, which is further linked together by inter-chain hydrogen bonds to form a two-dimensional sheet. Antiferromagnetic exchange is present in compound 2.Key words: cobalt, manganese, 2,2'-thiodiethanol, structure, magnetism.

NMR measurements on ^63Cu in underdoped Pb2Sr2(Y,Ca)Cu3O8+δxa crystals are reported. A pseudogap is observed in the Knight shift and spin-lattice relaxation rate for the planar ^63Cu. In contrast to other underdoped compounds, the pseudogap observed in the Knight shift is weak and occurs at a significantly lower temperature. On the other hand, the effect the pseudogap has on spin-lattice relaxation is quite similar to that in other compounds. The manifestation of the pseudogap at q = 0 is quite different than at q = Q.

The syntheses and structures of copper(II) and cobalt(II) complexes employing tetraacetylethane as the bridging ligand have been investigated. The reaction of Cu(O2CCF3)2 with tetraacetylethane (tae) and 2,2'-dipyridylamine (dpa) yielded a dinuclear complex CuII2(dpa)2(tae)(O2CCF3)2 (1), which has an extended hydrogen-bonded chain structure in the solid state. The reaction of 4,4'-dipyridyl (4,4'-bipy) with compound 1 resulted in the formation of a polymeric compound {[CuII2(dpa)2(tae)(4,4'-bipy)](O2CCF3)2}n (2), where the dinuclear unit is cross-linked by the 4,4'-dipyridyl ligand. The reaction of Co(O2CCH3)2 with tetraacetylethane and 2,2'-dipyridylamine yielded a helical dinuclear complex [CoII2(dpa)4(tae)](O2CCH3)2(H2O)2 (3), which forms a hydrogen-bonded band architecture in the solid state. The three-dimensional structure of 3 has molecular sieve-like channels that host methanol molecules reversibly. While only either negligible or weak magnetic exchanges appear to be present in compounds 1 and 2, there appears to be a significant ferromagnetic exchange in 3, which is likely caused by orbital orthogonality of the Co(II) ions, as supported by the crystal structure and EHMO calculations.Key words: cobalt, copper, polynuclear, tetraacetylethane complex, magnetism.

The syntheses of heterometallic compounds involving praseodymium, barium and copper ions by using 2,2'-thiodiethanol (H(2)tde) as the bridging ligand have been investigated. Three new polynuclear compounds, Pr2Cu4(tde)(3)(Htde)(2)(hfacac)(4)(mu(6)-O) 1, Ba2Cu2(Htde)(2)(hfacac)(4) 2 and Cu-4(tde)(2)(hfacac)(4) 3 have been obtained and characterized structurally. The metal atoms in 1 have an octahedral arrangement with the two Pr-III occupying two opposite vertices. In the center of the octahedron is a mu(6)-oxygen atom. The metal atoms in 2 have a rhombic arrangement with a long Cu ... Cu separation distance (6.082 Angstrom). The metal ions in 3 also have a rhombic arrangement with the copper atoms in close proximity to each other. The sulfur atoms of the Htde(-) or tde(2-) ligand are bound exclusively to copper centers in all three compounds. The Htde(-) and tde(2-) ligands display versatile bonding modes in the three compounds. Antiferromagnetic exchanges dominate in these compounds.

This paper presents a bird's eye view of the structural, electrical and magnetic properties of A 2 Mn 2 O 7 (A = rare earth, Y, Sc, In, Tl) phases crystallizing in pyrochlore-related structure. Electrical measurements indicate the compounds containing rare earth, Y, Sc or In are insulators whereas Tl 2 Mn 2 O 7 is highly conducting. All the compounds show spontaneous magnetization at low temperatures indicating ferromagnetic transitions. Detailed ac and dc susceptibility as well as neutron diffraction studies do not provide any evidence for long-range order in A 2 Mn 2 O 7 phases where A is a rare earth, Y or Sc, but suggest a spin-glass like behavior. However, powder neutron diffraction and small-angle neutron scattering investigations substantiate the long-range magnetic ordering in Tl 2 Mn 2 O 7 , which is also now known to exhibit colossal magnetoresistance (CMR) behavior. Structural analysis of this phase shows no deviations from ideal stoichiometry and gives an Mn-O distance of ∼1.90 A, significantly shorter than the Mn-O distances in the range 1.94 to 2.00 A observed in phases based on LaMnO 3 perovskites which show CMR. The above observation indicates that oxidation state of Mn in TI 2 Mn 2 O 7 should be very close to 4+ and has neither mixed valency for a double-exchange magnetic interaction nor a Jahn-Teller cation such as Mn 3+ . Both were thought to play an essential role in CMR materials. We propose that CMR in Tl 2 Mn 2 O 7 is based on magnetic ordering driven by superexchange and strong spin fluctuation scattering above T c .

We report on NMR measurements in underdoped Pb2Sr2(Y,Ca)Cu3O8+d crystals. A pseudogap is observed in the Knight shift and spin-lattice relaxation rate. In contrast to other underdoped compounds, the pseudogap observed in the Knight shift is weak and occurs at a significantly lower temperature. On the other hand, the effect the pseudogap has on spin-lattice relaxation is quite similar to that in other compounds. The contrast between weak and strong pseudogaps is discussed.

The reactions of the singly deprotonated di-2-pyridylmethanediol ligand (dpmdH(-)) with copper(II) and bismuth(III) have been investigated. A new dinuclear bismuth(III) complex Bi(2)(dpmdH)(2)(O(2)CCF(3))(4)(THF)(2), 1, has been obtained by the reaction of BiPh(3) with di-2-pyridyl ketone in the presence of HO(2)CCF(3) in tetrahydrofuran (THF). The reaction of Cu(OCH(3))(2) with di-2-pyridyl ketone, H(2)O, and acetic acid in a 1:2:2:2 ratio yielded a mononuclear complex Cu[(2-Py)(2)CO(OH)](2)(HO(2)CCH(3))(2), 2, while the reaction of Cu(OAC)(2)(H(2)O) with di-2-pyridyl ketone and acetic acid in a 2:1:1 ratio yielded a tetranuclear complex Cu(4)[(2-Py)(2)CO(OH)](2)(O(2)CCH(3))(6)(H(2)O)(2), 3. The structures of these complexes were determined by single-crystal X-ray diffraction analyses. Three different bonding modes of the dpmdH(-) ligand were observed in compounds 1-3. In 2, the dpmdH(-) ligand functions as a tridentate chelate to the copper center and forms a hydrogen bond between the OH group and the noncoordinating HO(2)CCH(3) molecule. In 1 and 3, the dpmdH(-) ligand functions as a bridging ligand to two metal centers through the oxygen atom. The two pyridyl groups of the dpmdH(-) ligand are bound to one bismuth(III) center in 1, while in 3 they are bound two copper(II) centers, respectively. Compound 3 has an unusual one dimensional hydrogen bonded extended structure. The intramolecular magnetic interaction in 3 has been found to be dominated by ferromagnetism. Crystal data: 1, C(38)H(34)N(4)O(14)F(12)Bi(2), triclinic P&onemacr;, a = 11.764(3) Å, b = 11.949(3) Å, c = 9.737(1) Å, alpha =101.36(2) degrees, beta = 105.64(2) degrees, gamma = 63.79(2) degrees, Z = 1; 2, C(26)H(26)N(4)O(8)Cu/CH(2)Cl(2), monoclinic C2/c, a = 25.51(3) Å, b = 7.861(7) Å, c = 16.24(2) Å, beta = 113.08(9) degrees, Z = 4; 3, C(34)H(40)N(4)O(18)Cu(4)/CH(2)Cl(2), triclinic P&onemacr;, a = 10.494(2) Å, b = 13.885(2) Å, c = 7.900(4) Å, alpha =106.52(2) degrees, beta = 90.85(3) degrees, gamma = 94.12(1) degrees, Z = 1.

We report on recent inelastic neutron scattering measurements of the magnetic response close to the metal–insulator transition in La1−xSrxTiO3. Specific heat and susceptibility data from Tokura et al. give evidence for a divergent effective mass at the critical concentration xc=0.05 in agreement with recent mean-field theories of the transition. The mass enhancement is believed to arise from the formation of a d-electron resonance at the Fermi energy close to the transition. The aim of this investigation is to look for evidence of this resonance in the dynamic magnetic susceptibility. We studied samples with x=0, 0.05, and 0.2 using incident energies between 25 and 200 meV. After correction for the phonon scattering, we observe a broad response above a threshold of 20–30 meV extending to over 100 meV. In addition, the Mott insulating antiferromagnet (x=0) has a peak at 40 meV, consistent with the estimated activation energy derived from resistivity measurements. This feature becomes washed out with temperature and doping. Possible origins for this peak are discussed.

The single crystal growth of the transition metal antimonates with the general formula AB(2)O(6) (A = Mn, Co, Ni, and Cu) is reported. V2O5 was used as a flux. Additives such as B2O3 and Na2O had a pronounced effect on the crystal growth, yielding larger and better faceted crystals than with pure V2O5.

Ba[sub 8]V[sub 7]O[sub 22], the first vanadium oxide in which both VO[sup 4[minus]][sub 4] and VO[sup 8[minus]][sub 6] coordinations and the coexistence and ordering of all the three common oxidation states of vanadium (+3, +4, and +4) are observed, has been synthesized by reducing Ba[sub 2]V[sub 2]O[sub 7] in hydrogen at 1350[degrees]C. Its crystal structure has been solved by single crystal X-ray diffraction, and magnetic and electrical properties have been examined. Ba[sub 8]V[sub 7]O[sub 22] crystallizes in the trigonal system, space group R[bar 3]m (No. 166). The structure consists of face-shared VO[sup 8[minus]][sub 6] and VO[sup 9[minus]][sub 6] octahedra that form V[sub 3]O[sup 14[minus]][sub 12] trimers, corner-shared VO[sup 4[minus]][sub 4] tetrahedra that are connected to the trimers such that they together form pseudo-two-dimensional magnetic layers perpendicular to the c-axis, and isolated VO[sup 3[minus]][sub 4] tetrahedra that are arranged in planes separating the magnetic layers. Vanadium atoms occupy the tetrahedral sites and 3/4 of the octahedral sites in an ordered fashion. The 12-layer structure with (cchh)[sub 3] stacking, previously proposed for BaFeO[sub 3[minus]x] but improperly described in space group R3m, is redescribed in R[bar 3]m. The 24R structure of Ba[sub 8]V[sub 7]O[sub 22] is closely related to the 12R structure. Ba[submore » 8]V[sub 7]O[sub 22] is a semiconductor with three activation energies ranging from 0.31 to 0.48 eV in the temperature range investigated (145-295 K), and a room temperature resistivity of about 12 [Omega] [center dot] cm. Its magnetic behavior is very complicated. Single crystal samples show a broad susceptibility maximum at 280 K, followed by an increasing susceptibility at lower temperatures and additional anomalies at 50 and 20 K.« less

Materials with the pyrochlore structure are subject to extreme frustration due to the topology of the magnetic sublattice which consists of a three-dimensional array of perfect corner sharing tetrahedra. The pyrochlore Y2Mn2O7 has been studied recently.1 The compound is crystalline and appears to have negligible crystallographic disorder, yet its magnetic properties resemble those of a re-entrant spin glass with a ferromagetic-like anomaly in the susceptibility at 17 K followed by a maximum at 7 K. In order to characterize this behavior dc and ac susceptibility measurements have been carried out along with small angle neutron scattering (SANS). Analysis of the spin dynamics through the frequency and temperature dependence of χ′ and χ″ show evidence for spin-freezing below 4 K. The SANS data can be fit by a cross section with both Lorentzian (L) and Lorentzian-squared (L2) terms. Both the magnitude and temperature dependence of the correlation lengths, ξ, of the L and L2 terms are different with ξ(L) growing to a finite size ∼25 Å at 15 K but ξ(L2) becomes resolution limited at the same temperature. In many respects the behavior of Y2Mn2O7 parallels that seen in magnetic metallic glasses.2

The reflectance of a La1.84Sr0.16Ni4+δ crystal has been measured between 115 cm−1 and 5500 cm−1 with the electric-field vector in the ab-plane for several temperatures between 80 and 300 K. On the basis of the Kramers-Kronig derived optical conductivity data we show that it is unlikely that small-polaron absorption can account for the mid-infrared absorption, as had been previously suggested. A doped-semiconductor model is also considered.

The rhenium-rich phase Re2Si crystallizes in a monoclinic unit cell, a = 6.4444(2) angstrom, b = 5.3898(2) angstrom, c = 9.6019(4) angstrom, gamma = 94.215(5)-degrees (unique axis c, Pearson symbol mP24, Z = 8 and space group P2(1)/b. The structure is based on a hexagonal close packing of rhenium and silicon atoms ordered on distinct sites and represents a new structure type. Re2Si becomes superconducting below 3.8 K.

A new family of high temperature superconductors based on Pb2Sr2YCu3O9−δ has recently been reported. One method of improving Tc has been to replace Y partially with Ca. Although the basic structure of this type of superconductors is known, the detailed structure is still unclear, and various space groups has been proposed. In our work, crystals of Pb2Sr2YCu3O9−δ with dimensions up to 1 × 1 × 0.25.mm and with Tc of 84 K have been grown and their superconducting properties described. The defects and crystal symmetry have been investigated using electron microscopy performed on crushed crystals supported on a holey carbon film.Electron diffraction confirmed x-ray diffraction results which showed that the crystals are primitive orthorhombic with a=0.5383, b=0.5423 and c=1.5765 nm. Convergent Beam Electron Diffraction (CBED) patterns for the and axes are shown in Figs. 1 and 2 respectively.