High-Spin Cobalt(II) Ions in Square Planar Coordination: Structures and Magnetism of the Oxysulfides Sr₂CoO₂Cu₂S₂ and Ba₂CoO₂Cu₂S₂ and Their Solid Solution

Abstract

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 × √2a × 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).

Density functional theory study of hypotheticalPbTiO3-based oxysulfides

“…Band Gap (eV) Polarization (C/m 2 ) Ba) solid solutions, Smura et al 22 have found c/a ratios ranging between 1.52 and 1.66 for CoO 4 S 2 octahedra. Similarly, Ishikawa et al 16 have found an average c/a ratio of 1.6 for Ln 2 Ti 2 O 5 S 2 (Ln = Pr, Nd, Sm, Gd, Tb, Dy, Ho, Er).…”

“…[19][20][21][22] Thus the current work is novel in that it explores the feasibility of synthesizing a purely corner-sharing perovskite phase.…”

Physical properties and electronic structure ofSr2Cr3As2O2containingCrO2Jiang1, Bao2, Zhai3 et al. 2015Phys. Rev. B287510View full textAdd to dashboardCite

“…Later in 1990s, a series of 2322-type materials were synthesized and characterized [11][12][13][14][15][16][17][18] , presumably due to the structural similarity with cuprate superconductors. Since the birth of high-temperature iron-based superconductors in 2008, there has been a renewed interest on studying the related systems 7,8,[19][20][21][22][23] . Among them, the Crcontaining iron arsenide Sr 2 CrO 2 Fe 2 As 2 23 seems to be very close to superconductivity, because it comprises of the superconductively active Fe 2 As 2 layers and, a self doping may be realized through charge transfer between the distinct layers, like the cases in Sr 2 VFeAsO 3 24 and Ba 2 Ti 2 Fe 2 As 4 O 25 .…”

Introduction to the density functional formalism and some illustrative applications to magnetism

“…Their antiferromagnetic (AF) structures have been determined by powder neutron diffraction [121]. The magnetic reflections present in the diffraction patterns can in all cases be indexed using a √ a × √ a × c expansion of the nuclear cell ( Figure 8).…”

“…Thus, J was kept fixed at 0 eV, whereas different values were tested for the U parameter. All results were obtained using the refined experimental magnetic structures [121] and The effect of inclusion of the on-site electron-electron repulsion parameter U on the electronic density of states for Sr 2 CoO 2 Cu 2 S 2 (U n means U = n eV). With increasing n, the insulating behavior is well recovered through the opening of the band-gap.…”