Synthesis, Structure, and Compositional Tuning of the Layered Oxide Tellurides Sr2MnO2Cu2- xTe2 and Sr2CoO2Cu2Te2.

The synthesis and structure of two new transition metal oxide tellurides, Sr2MnO2Cu1.82(2)Te2 and Sr2CoO2Cu2Te2, are reported. Sr2CoO2Cu2Te2 with the purely divalent Co2+ ion in the oxide layers has magnetic ordering based on antiferromagnetic interactions between nearest neighbors and appears to be inert to attempted topotactic oxidation by partial removal of the Cu ions. In contrast, the Mn analogue with the more oxidizable transition metal ion has a 9(1)% Cu deficiency in the telluride layer when synthesized at high temperatures, corresponding to a Mn oxidation state of +2.18(2), and neutron powder diffraction revealed the presence of a sole highly asymmetric Warren-type magnetic peak, characteristic of magnetic ordering that is highly two-dimensional and not fully developed over a long range. Topotactic oxidation by the chemical deintercalation of further copper using a solution of I2 in acetonitrile offers control over the Mn oxidation state and, hence, the magnetic ordering: oxidation yielded Sr2MnO2Cu1.58(2)Te2 (Mn oxidation state of +2.42(2)) in which ferromagnetic interactions between Mn ions result from Mn2+/3+ mixed valence, resulting in a long-range-ordered A-type antiferromagnet with ferromagnetic MnO2 layers coupled antiferromagnetically.

High-spin cobalt(II) ions in square planar coordination: structures and magnetism of the oxysulfides Sr2CoO2Cu2S2 and Ba2CoO2Cu2S2 and their solid solution.

The antiferromagnetic structures of the layered oxychalcogenides (Sr(1-x)Ba(x))(2)CoO(2)Cu(2)S(2) (0 ≤ x ≤ 1) have been determined by powder neutron diffraction. In these compounds Co(2+) is coordinated by four oxide ions in a square plane and two sulfide ions at the apexes of an extremely tetragonally elongated octahedron; the polyhedra share oxide vertexes. The magnetic reflections present in the diffraction patterns can in all cases be indexed using a √2a × âˆš2a × c expansion of the nuclear cell, and nearest-neighbor Co(2+) moments couple antiferromagnetically within the CoO(2) planes. The ordered magnetic moment of Co(2+) in Sr(2)CoO(2)Cu(2)S(2) (x = 0) is 3.8(1) µ(B) at 5 K, consistent with high-spin Co(2+) ions carrying three unpaired electrons and with an additional significant unquenched orbital component. Exposure of this compound to moist air is shown to result in copper deficiency and a decrease in the size of the ordered moment to about 2.5 µ(B); there is a strong correlation between the size of the long-range ordered moment and the occupancy of the Cu site. Both the tetragonal elongation of the CoO(4)S(2) polyhedron and the ordered moment in (Sr(1-x)Ba(x))(2)CoO(2)Cu(2)S(2) increase with increasing Ba content, and in Ba(2)CoO(2)Cu(2)S(2), which has Co(2+) in an environment that is close to purely square planar, the ordered moment of 4.5(1) µ(B) at 5 K is over 0.7 µ(B) larger than that in Sr(2)CoO(2)Cu(2)S(2), so the unquenched orbital component in this case is even larger than that observed in octahedral Co(2+) systems such as CoO. The experimental observations of antiferromagnetic ground states and the changes in properties resulting from replacement of Sr by Ba are supported by ab initio calculations on Sr(2)CoO(2)Cu(2)S(2) and Ba(2)CoO(2)Cu(2)S(2). The large orbital moments in these systems apparently result from spin-orbit mixing of the unequally populated d(xz), d(yz), and d(z(2)) orbitals, which are reckoned to be almost degenerate when the CoO(4)S(2) polyhedron reaches its maximum elongation. The magnitudes of the ordered moments in high-spin Co(2+) oxide, oxychalcogenide, and oxyhalide systems are shown to correlate well with the tetragonal elongation of the coordination environment. The large orbital moments lead to an apparently magnetostrictive distortion of the crystal structures below the Neél temperature, with the symmetry lowered from tetragonal I4/mmm to orthorhombic Immm and the size of the distortion correlating well with the size of the long-range ordered moment for all compositions and for temperature-dependent data gathered on Ba(2)CoO(2)Cu(2)S(2).

Compositional control of the superconducting properties of LiFeAs.

The response of the superconducting state and crystal structure of LiFeAs to chemical substitutions on both the Li and the Fe sites has been probed using high-resolution X-ray and neutron diffraction measurements, magnetometry, and muon-spin rotation spectroscopy. The superconductivity is extremely sensitive to composition: Li-deficient materials (Li(1-y)Fe(1+y)As with Fe substituting for Li) show a very rapid suppression of the superconducting state, which is destroyed when y exceeds 0.02, echoing the behavior of the Fe(1+y)Se system. Substitution of Fe by small amounts of Co or Ni results in monotonic lowering of the superconducting transition temperature, T(c), and the superfluid stiffness, rho(s), as the electron count increases. T(c) is lowered monotonically at a rate of 10 K per 0.1 electrons added per formula unit irrespective of whether the dopant is Co and Ni, and at higher doping levels superconductivity is completely suppressed. These results and the demonstration that the superfluid stiffness in these LiFeAs-derived compounds is higher than in all of the iron pnictide materials underlines the unique position that LiFeAs occupies in this class.

Control of the competition between a magnetic phase and a superconducting phase in cobalt-doped and nickel-doped NaFeAs using electron count.

Using a combination of neutron, muon, and synchrotron techniques we show how the magnetic state in NaFeAs can be tuned into superconductivity by replacing Fe by either Co or Ni. The electron count is the dominant factor, since Ni doping has double the effect of Co doping for the same doping level. We follow the structural, magnetic, and superconducting properties as a function of doping to show how the superconducting state evolves, concluding that the addition of 0.1 electrons per Fe atom is sufficient to traverse the superconducting domain, and that magnetic order coexists with superconductivity at doping levels less than 0.025 electrons per Fe atom.

Response of superconductivity and crystal structure of LiFeAs to hydrostatic pressure.

On the application of hydrostatic pressures of up to 1.3 GPa, the superconducting transition temperatures (T(c)) of samples of LiFeAs are lowered approximately monotonically at approximately -2 K GPa(-1). Measurements of the X-ray powder diffraction pattern at hydrostatic pressures of up to 17 GPa applied by a He gas pressure medium in a diamond anvil cell reveal a bulk modulus for LiFeAs of 57.3(6) GPa which is much smaller than that of other layered arsenide and oxyarsenide superconductors. LiFeAs also exhibits much more isotropic compression than other layered iron arsenide superconductors. The higher and more isotropic compressibility is presumably a consequence of the small size of the lithium ion. At ambient pressure the FeAs(4) tetrahedra are the most compressed in the basal plane of those in any of the superconducting iron arsenides. On increasing the pressure the Fe-Fe distance contracts more rapidly than the Fe-As distance so that the FeAs(4) tetrahedra become even more distorted from the ideal tetrahedral shape. The decrease in T(c) with applied pressure is therefore consistent with the observations that in the iron arsenides and related materials investigated thus far, T(c) is maximized for a particular electron count when the FeAs(4) tetrahedra are close to regular.

Structure, antiferromagnetism and superconductivity of the layered iron arsenide NaFeAs.

A new layered iron arsenide NaFeAs isostructural with the superconducting lithium analogue displays evidence for the coexistence of superconductivity and magnetic ordering.

Structure and superconductivity of LiFeAs.

Lithium iron arsenide phases with compositions close to LiFeAs exhibit superconductivity at temperatures at least as high as 16 K, demonstrating that superconducting [FeAs](-) anionic layers with the anti-PbO structure type occur in at least three different structure types and with a wide range of As-Fe-As bond angles.

Structures, physical properties, and chemistry of layered oxychalcogenides and oxypnictides.

A series of layered oxychalcogenide and oxypnictide solids is described that contain oxide layers separated by distinct layers, which contain the softer chalcogenide (S, Se, Te) or pnictide (P, As, Sb, Bi) anions. The relationships between the crystal structures adopted by these compounds are described, and the physical and chemical properties of these materials are related to the structures and the properties of the elements. The properties exhibited by the oxychalcogenide materials include semiconductor properties, for example, in LaOCuCh (Ch = chalcogenide) and derivatives, unusual magnetic properties exhibited by the class Sr 2MO 2Cu 2-deltaS 2 (M = Mn, Co, Ni), and redox properties exhibited by the materials Sr 2MnO 2Cu 2 m-0.5 S m+1 ( m = 1-3) and Sr 4Mn 3O 7.5Cu 2Ch 2 (Ch = S, Se). Recent results in the oxychalcogenide area are reviewed, and some new results on the intriguing series of compounds Sr 2MO 2Cu 2-deltaS 2 (M = Mn, Co, Ni) are reported. Oxypnictides have received less recent attention, but this is changing: a new frenzy of research is underway following the discovery of high-temperature superconductivity (>40 K) in derivatives of the layered oxyarsenide LaOFeAs. The early results in this exciting new area will be reviewed.

Crossover from positive to negative magnetoresistance in a spinel.

We present a new class of colossal magnetoresistance materials based on a series of frustrated spinels. The spin glass-like compound Zn0.95Cu0.05Cr2Se4, shows a field-induced transition to a ferromagnetic, which is associated with a highly unusual negative magnetoresistance effect (MR > 80%) in low magnetic field. At higher temperatures there is an unprecedented crossover to positive magnetoresistance (MR > 50%).