Layered Oxychalcogenides Research Articles

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 × √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).

Synthesis, crystal structure, and properties of novel perovskite oxychalcogenides, Ca2CuFeO3Ch (Ch=S, Se)

Two new perovskite oxychalcogenides, Ca2CuFeO3S and Ca2CuFeO3Se, have been synthesized in evacuated quartz tubes. They crystallize in P4/nmm space group with lattice parameters a = 3.8271(1), c = 14.9485(2) Å and a = 3.8605(1), c = 15.3030(2) Å for Ca2CuFeO3S and Ca2CuFeO3Se, respectively. They appear to be the first layered chalcogenide perovskites involving calcium and are structural analogs of the corresponding Sr and Ba compounds. The new compounds exhibit semiconducting properties with energy gap decreasing from the oxysulfide to the oxyselenide. Possibility of introducing Ca2+ into structures of known layered oxychalcogenides and oxypnictides is discussed.

Heteroepitaxial film growth of layered compounds with the ZrCuSiAs-type and ThCr 2Si 2-type structures: From Cu-based semiconductors to Fe-based superconductors

FeAs-based layered superconductors such as F-doped LaFeAsO have recently been investigated intensively because of their high superconducting transition temperatures. Epitaxial films of these compounds are important to examine their intrinsic materials properties as well as to transfer them to device applications. In this review, we first present our research route from transparent p-type oxides semiconductors to the Fe-based superconductors. Then we review growth of epitaxial thin films for the layered oxychalcogenides and oxypnictides. Reactive solid-phase epitaxy technique was inevitable to prepare epitaxial thin films of the oxychalcogenides and Zn-based oxypnictides. On the other hand, epitaxial thin films of Mn-based oxypnictides were grown by standard pulsed laser deposition. These techniques, however, did not grow epitaxial thin films for LaFeAsO. Thus, we developed a modified pulsed laser deposition process and succeeded in obtaining epitaxial thin films of FeAs-based superconductors, LaFeAsO and cobalt-doped SrFe 2As 2.

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.

Layered mixed-anion compounds: Epitaxial growth, active function exploration, and device application

Optoelectronic properties and device applications of layered mixed-anion compounds such as oxychalcogenide LaCuO Ch ( Ch = chalcogen) and oxypnictide La T MO Pn ( T M = 3d transition metal, Pn = pnicogen) are reviewed. Several distinctive functions have been found in these materials based on our original material exploration concept. Fabrication of high-quality epitaxial films of LaCuO Ch leads to clarifying the excellent electrical and optical properties such as high hole mobility of 8 cm 2/(V s) and heavy hole doping at >10 21 cm −3 in LaCuOSe, and sharp and tunable-wavelength photoluminescence in the solid–solution systems in LaCuO Ch. In addition, a room temperature operation of a light-emitting diode is demonstrated using LaCuOSe as a light-emitting layer. These results suggest that the layered oxychalcogenides have potential for light-emitting layers as well as transparent hole-injection layers in organic/inorganic light-emitting diodes. Furthermore, by extending the material system from the copper-based oxychalcogenides to isostructural compounds, transition metal-based oxypnictides La T MOP ( T M = Fe, Ni), we have found novel superconductors, LaFeOP and LaNiOP.

Crystal Structures, Optoelectronic Properties, and Electronic Structures of Layered Oxychalcogenides MCuOCh (M = Bi, La; Ch = S, Se, Te): Effects of Electronic Configurations of M3+ Ions

Crystal structures, optoelectronic properties, and electronic structures of layered oxychalcogenides BiCuOCh (Ch = S, Se, Te) have been compared to those of LaCuOCh, with an emphasis on the electronic configurations of Bi3+ (5d106s2) and La3+ (5d06s0). The BiCuOCh series were expected to exhibit better hole-transport properties than the LaCuOCh series because the pseudo-closed-shell 6s2 electronic configuration of the Bi3+ cation was expected to form valence band maxima (VBM) by admixing with the p orbitals of the Ch anions. However, the two series of compounds exhibited similar electrical properties, suggesting that the contribution of the Bi 6s orbitals to the VBM is small in BiCuOCh. The crystal structures and optical properties showed distinct differences; for example, the band gaps of BiCuOCh were smaller than those of LaCuOCh. These findings can be understood on the basis of the electronic structures obtained by photoelectron spectroscopy and density functional theory calculations. The Bi 6s orbital...

Topotactic Oxidative and Reductive Control of the Structures and Properties of Layered Manganese Oxychalcogenides.

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Topotactic Oxidative and Reductive Control of the Structures and Properties of Layered Manganese Oxychalcogenides

Topotactic modification, by both oxidation and reduction, of the composition, structures, and magnetic properties of the layered oxychalcogenides Sr4Mn3O7.5Cu2Ch2 (Ch=S, Se) is described. These Mn3+ compounds are composed of alternating perovskite-type strontium manganese oxide slabs separated by anti-fluorite-type copper chalcogenide layers and are intrinsically oxide deficient in the central layer of the perovskite slabs. The systems are unusual examples of perovskite-related compounds that may topotactically be both oxidized by fluorination and reduced by deintercalation of oxygen from the oxide-deficient part of the structure. The compounds exhibit antiferromagnetic ordering of the manganese magnetic moments in the outer layers of the perovskite slabs, while the other moments, in the central layers, exhibit spin-glass-like behavior. Fluorination has the effect of increasing the antiferromagnetic ordering temperature and the size of the ordered moment, whereas reduction destroys magnetic long-range order by introducing chemical disorder which leads to both further disorder and frustration of the magnetic interactions in the manganese oxide slab.

Recent progress in transparent oxide semiconductors: Materials and device application

This paper reviews our recent research progress on new transparent conductive oxide (TCO) materials and electronic and optoelectronic devices based on these materials. First, described are the materials including p-type materials, deep-UV transparent TCO(β-Ga 2O 3), epitaxially grown ITO with atomically flat surface, transparent electrochromic oxide (NbO 2F), amorphous TCOs, and nanoporous semiconductor 12CaO · 7Al 2O 3. Second, presented are TCO-based electronic/optoelectronic devices realized to date, UV/blue LED and UV-sensors based on transparent pn junction and high performance transparent TFT using n-type TCO as an n-channel. Finally, unique optoelectronic properties (p-type degenerate conduction, transfer doping of carriers, RT-stable exciton, and large optical nonlinearity) originating from 2D-electronic nature in p-type layered oxychalcogenides are summarized along with the fabrication method of epitaxial thin films of these materials.

Device applications of transparent oxide semiconductors: Excitonic blue LED and transparent flexible TFT

Recent progress in oxide-based transparent optoelectronic devices is reviewed. It is important to understand electronic structures inherent to oxides in order to develop new materials and to find suitable device applications that oxide materials can have distinct advantages over conventional semiconductors. Two new transparent oxide semiconductors, (i) p-type layered oxychalcogenides LaCuOCh (Ch = chalcogen), and (ii) large-mobility amorphous oxide semiconductors (AOSs), are taken as examples. Their peculiar properties are discussed in comparison with conventional semiconductors based on consideration of electronic structures. Two associated devices, an excitonic light-emitting diode using LaCuOCh and transparent flexible thin film transistors using AOSs, are also shown.

Opto‐electronic properties and light‐emitting device application of widegap layered oxychalcogenides: LaCuOCh (Ch = chalcogen) and La2CdO2Se2

AbstractElectronic and optical properties of widegap oxychalcogenides, LaCuOCh (Ch = chalcogen) and La2CdO2Se2, are reviewed with a focus on those relevant to their layered crystal structures, including high hole mobility, degenerate p‐type conduction, room temperature exciton, and large third order optical nonlinearity. In particular, the widegap p‐type metallic conduction was realized in Mg‐doped LaCuOSe: the first demonstration among any class of widegap materials including GaN:Mg. Furthermore, we demonstrate the room temperature operation of a blue light‐emitting diode using a pn hetero‐junction composed of a LaCuOSe epilayer and an n‐type amorphous InGaZn5O8. Those results strongly suggest that a series of the layered oxychalcogenides are applicable to the light‐emitting layers in opto‐electronic devices that operate in the ultraviolet–blue region as well as to transparent p‐type conductors. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Synthesis, structure and physical properties of layered semiconductors MCuF Ch ( M=Sr, Eu, Ch=S, Se)

Effects of open shell cations on magnetic, optical and carrier transport properties were examined for layered wide bandgap semiconductors MCuF Ch ( M=Sr, Eu, Ch=S, Se). Single-phase MCuF Ch powder and ceramic samples were synthesized by solid-state reactions. The crystal structures refined by the Rietveld analyzes revealed that all the materials have the space group P 4 / nmm , indicating that the samples have the same crystal structure as that of layered oxychalcogenides LaCuO Ch, and cation vacancies of several percent were present for Cu + and Eu 2+ sites in EuCuF Ch. Thermopower measurements revealed that both SrCuF Ch and EuCuF Ch were p-type semiconductors. Degenerate conduction was observed for EuCuF Ch with conductivities >1 S cm −1, whereas SrCuF Ch exhibited thermally activated behavior. The optical band gaps of SrCuFS and SrCuFSe estimated were approximately 3.0 and 2.7 eV, respectively, and those of EuCuF Ch were ∼2 eV. Temperature dependence of magnetic susceptibilities of EuCuF Ch followed the Curie–Weiss law down to 5 K and the samples did not show any transition to a magnetic ordering phase.

Preparation, magnetic and optical properties of layered oxychalcogenides SmCuOCh (Ch = S or Se)

This paper reports on the synthesis and the electrical, magnetic and optical properties of SmCuOS and SmCuOSe. The magnetic properties reveal that Sm is in its 3+ oxidation state (μtheo=g√J(J+1)=0.85μB; g=2/7) with a large Van Vleck contribution, and exclude the possibility of a divalent oxidation state for samarium (Sm2+; 7F0 state, g=J=0, μeff=0).Optical properties were studied by means of diffuse reflectance and photoluminescence spectra in the UV–vis range. The electrical measurements show that the two samarium copper oxychalcogenides, SmCuOSe and SmCuOS are semiconductors with optical band gap (Eg) values of 2.60 and 2.90eV, respectively.

Wide-gap layered oxychalcogenide semiconductors: Materials, electronic structures and optoelectronic properties

Applying the concept of materials design for transparent conductive oxides to layered oxychalcogenides, several p-type and n-type layered oxychalcogenides were proposed as wide-gap semiconductors and their basic optical and electrical properties were examined. The layered oxychalcogenides are composed of ionic oxide layers and covalent chalcogenide layers, which bring wide-gap and conductive properties to these materials, respectively. The electronic structures of the materials were examined by normal/inverse photoemission spectroscopy and energy band calculations. The results of the examinations suggested that these materials possess unique features more than simple wide-gap semiconductors. Namely, the layered oxychalcogenides are considered to be extremely thin quantum wells composed of the oxide and chalcogenide layers or 2D chalcogenide crystals/molecules embedded in an oxide matrix. Observation of step-like absorption edges, large band gap energy and large exciton binding energy demonstrated these features originating from 2D density of states and quantum size effects in these layered materials.

Valence-band structures of layered oxychalcogenides, LaCuOCh (Ch=S, Se, and Te), studied by ultraviolet photoemission spectroscopy and energy-band calculations

To examine the electronic structure of the valence band, ultraviolet photoemission spectra of a series of layered oxychalcogenides, LaCuOCh (Ch=S, Se, and Te), were measured. The measurements were conducted using HeII, HeI, and NeI excitation lines to observe the excitation energy dependence of the spectral shape. Energy-band calculations based on a full-potential linearized augmented plain-wave method were performed. The calculated density of states and partial density of states were compared to the observed photoemission spectra. Five bands were observed in the valence band of LaCuOCh, and NeI radiation remarkably enhanced two of them. The energy dependence of the photoionization cross section of atomic orbitals indicated that the two enhanced bands were due to the Ch p states. Energy calculations were used to assign the remaining bands. The electronic structure of LaCuOCh was further discussed using molecular-orbital diagrams to visualize the (La2O2)2+ and (Cu2Ch2)2− layers as large donor-acceptor pairs. The energy-band calculation and molecular-orbital diagram analyses suggested that the main difference among the valence-band structures of LaCuOCh (Ch=S, Se, and Te) originates from the variations in the energy position of the Ch p bands. The observed spectra are consistent with the results of the band calculations and clearly show the energy variations in the Ch p bands with respect to spectral shape and excitation energy dependence.

Two-dimensional electronic structure and multiple excitonic states in layered oxychalcogenide semiconductors, LaCuOCh (Ch=S, Se, Te): Optical properties and relativistic ab initio study

Electronic structures of layered oxychalcogenides LaCuOCh (Ch=S, Se, Te) were studied using relativistic ab initio band calculations to understand their optical and electronic properties. Step-like structures terminated with one or two sharp peaks were observed in low-temperature (10 K) optical absorption spectra. Third optical nonlinearity measurements supported that the sharp peaks came from split excitonic levels. The ab initio calculations reproduced well these characteristic structures in the spectra and proved that the step-like optical absorption structures originated from two-dimensional nature of the electronic structures associated with the layered crystal structure of LaCuOCh. The split energies of the excitonic levels were quantitatively explained by spin–orbit interaction in the chalcogen ions.

Optical Properties and Two‐Dimensional Electronic Structure in Wide‐Gap Layered Oxychalcogenide: La2CdO2Se2.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Excitonic properties related to valence band levels split by spin–orbit interaction in layered oxychalcogenide LaCuOCh(Ch=S,Se)

The energy splitting of valence band in oxychalcogenides LaCuOCh (Ch=S, Se) was studied through degenerative four-wave mixing (DFWM) signals excited by femtosecond laser pulses and high-resolution absorption spectra at 4 K. The DFWM signals for LaCuOS exhibited a beat structure with a period of 480 fs just at the exciton peak energy, indicating that the lowest exciton states were split by 9 meV, while the corresponding splitting in LaCuOSe was 125 meV. The spin–orbit interaction of Ch ion accompanied by the hybridization of Cu 3d orbital causes the splitting of the exciton levels.

Optical Properties and Two-Dimensional Electronic Structure in Wide-Gap Layered Oxychalcogenide: La2CdO2Se2

The optical and electronic properties of a wide-gap (∼3.3 eV) layered oxychalcogenide, La2CdO2Se2, were examined using epitaxial thin films prepared by a reactive solid-phase epitaxy. Two optical absorption peaks due to exciton split by the spin−orbit interaction were observed at 3.43 and 3.61 eV at 10 K near the absorption edge. A sharp ultraviolet photoluminescence was observed even at room temperature, indicating that the free exciton had a large binding energy (estimated value = ∼40 meV) similar to Cu-based, layered oxychalcogenides LnCuOCh (Ln = lanthanide, Ch = chalcogen). The optical properties were compared to those of the Cu-based oxychalcogenides and low-dimensional CdSe-based materials. It is concluded that the two-dimensional crystal structure, which remarkably reduces the bandwidth of the conduction band, is a major origin for the wide band gap. Energy band calculations indicate that the holes are confined in the two-dimensional (CdSe2)2- layer, which is most likely responsible for the large ...

Degenerate electrical conductive and excitonic photoluminescence properties of epitaxial films of wide gap p-type layered oxychalcogenides, LnCuOCh (Ln=La, Pr and Nd; Ch=S or Se)

Electrical and photoluminescence properties were investigated for epitaxial films of layered oxychalcogenides, LnCuOCh (Ln = La, Pr, and Nd, Ch = S or Se). Epitaxial films of Mg 10 at. % doped LaCuOS1-xSex are the first demonstration of degenerate conduction with high hole concentration >1020 cm-3 in wide gap p-type semiconductors. Ion substitution varied the excitonic emission energy from 3.21 eV to 2.89 eV while lanthanide and chalcogenide ion substitutions displayed the opposite tendency against cell volume. These unique properties are discussed with respect to the electronic structure originating from the layered crystal structure.