Copper Sublattice Research Articles

Description of CuInP<sub>2</sub>S<sub>6</sub> ferrielectrics in a mixed Ising model

The appearance of spontaneous polarization in CuInP2S6 ferrielectrics is related to the second order Jahn-Teller effect for copper cations located in a double-well local potential, the stereoactivity of indium cations located in a three-well local potential, as well as the valence fluctuations of phosphorus cations. The paraelectric to ferrielectric phase transition is primarily determined by the coupling of indium cations with their surroundings. This transition can be analyzed using the mixed Ising model with spins s = 1/2 and S = 1. The spectrum of pseudospin fluctuations at different temperatures was calculated using a mean-field approach for a set of quantum anharmonic oscillators. The results were then compared with Raman spectroscopy data for CuInP2S6 crystal. The analysis indicates that the lattice anharmonicity below 150 K, is mainly determined by the indium sublattice, leading to the coexistence of the glassy state and ferrielectric phase. Above 150 K, the anharmonicity of the copper sublattice activates the ionic conductivity and results in the existence of a long-ranged fluctuated cluster of spontaneous polarization in a temperature interval of the paraelectric phase above TC.

Effect of fluorine on terrace-ledge-kink morphology and valence state of copper and titanium ions in CaCu3Ti4O12

Ceramic CaCu3Ti4O12 (ССТО), Ca0.98Cu3Ti4O11.96F0.04 (CΔCTOF), and CaCu3Ti4O11.92F0.08 (CCTOF) were synthesized by the solid-state reaction technique. Fluorine stimulates the formation of Cu-depleted grains, Cu3+ ions, and Cu-rich composites CuO-xCCTO-yTiO2-zSiO2-wСaF2 (w < z < y < x < 1), which include parts of grinding bodies. The observed structures are distinct from simple grain boundaries of the perovskite phase. They exhibit a terrace-ledge-kink (TLK) morphology and, in some cases, the presence of twinning planes, both independent of fluorine content. They are responsible for the nanoscale barrier layer capacitance (NBLC) component of the dielectric response of both CCTO and CuO ceramics. Changes in the unit cell parameter a, and titanium and copper valences indicate that Ca2+ ions occupy part of Cu vacancies in the grains of CCTO and CCTOF. In CΔCTOF, Ti3+ ions in the copper sublattice were found for the first time using NMR. The maximum ε′1kHz = 6.9 × 104 demonstrates CΔCTOF and the minimum tan δ = 0.045 is characteristic of CCTOF.

Ordering/displacive ferrielectricity in 2D CuInP2S6

Using DFT-based molecular dynamics simulation of Cu+ cations flipping and In3+ cations displacive dynamics, we clarify the dipole ordering of CuInP2S6 ferrielectrics through the second order Jahn-Teller effect which determines the double-well potential for copper cations as well as three-well potential for indium cations inside the structural layers. The temperature dependence of the spatial distribution of Cu+ and In3+ cations is also described within the quantum anharmonic oscillator model. In3+ cations play a decisive role in the character of the polar ordering: at ambient or for positive pressures, the first order transition occurs; with the rise of negative compression, the ferrielectric transition evolves to the second order. The peculiarities of spontaneous polarization switching are related to the contributions of copper and indium cationic sublattices.

Involvement of cationic sublattices in multiple polarization states in CuInP2S6 ferrielectrics

The contribution of copper and indium cationic sublattices on polarization hysteresis loops in CuInP2S6 van der Waals ferrielectrics has been studied within the quantum anharmonic oscillator model that considers the ordering dynamics of Cu+ cations in the double-well local potential and displacive dynamics of In3+ cations in the three-well shaped potential. The multiple polarization states are manifested as single, double, and triple P(E) hysteresis loops. The triple loops originate from a field-induced ferrielectric to ferroelectric phase transition within a certain range of the phase diagram with continuous increase in the spontaneous polarization below the second-order transition from the paraelectric phase to the ferrielectric state.

Equilibrium magnetic properties of the Fe-doped HTSC YBaCuO

For the polycrystalline high-temperature superconductor Y1 –xFexBa2Cu3Oy (0 ≤ x ≤ 0.05), where a significant increase in the intragrain critical current density has recently been found, the effect of iron doping on the equilibrium magnetic characteristics is studied. The field dependences of the magnetization are measured in the temperature range of 40–85 K and in the field range up to 60 kOe with increasing and decreasing fields. The temperature dependences of the London penetration depth λab, the superconducting carrier density ns, and the thermodynamic critical field Hc are determined from an analysis of the middle lines of the magnetic hysteresis loop. It is shown that the thermodynamic properties of Fe substituted samples do not change at x ≤ 0.03. At a higher Fe content, a slight decrease in Hc and the superconducting transition temperature is observed. However, changes in the condensation energy turn out to be much smaller than values expected due to an increase in the density of effective pinning centres created by substitutional atoms in both the yttrium and copper sublattices. It is concluded that the pinning regions in chain CuO planes near substituted Cu sites are two-dimensional in nature and do not affect the superconducting pairing in CuO2 planes. Another possible pinning mechanism, which does not affect Hc, is the interaction of vortices with the magnetic moments of Fe3+ ions.

Electron, phonon and thermoelectric properties of Cu7PS6 crystal calculated at DFT level

The promising class of the environment-friendly thermoelectrics is the copper-based argyrodite-type ion-conducting crystals exhibiting just extraordinary low thermal conductivity below the glass limit associated with the molten copper sublattice leading to a softening of phonon modes. To explain why the argyrodite structure containing copper ions favors the low thermal conductivity, we have utilized the ab initio calculations of the electron, phonon, and thermoelectric properties of Cu7PS6 crystal in the framework of the density functional and Boltzmann transport theories. To obtain the reliable thermoelectric properties of Cu7PS6, we take into account the dependence of the electron effective mass m* on the redundant carrier concentration n. We propose to use the Burstein–Moss effect for the calculation of the electron effective mass m* of a semiconductor. We have found the strong nonlinear character of copper atom vibrations in Cu7PS6 which exceeds substantially the similar values for phosphorous and sulfur atoms. The large vibration nonlinearity of the copper atoms found in Cu7PS6 explains the diffusion-like heat transfer and the relatively low coefficient of the lattice thermal conductivity (κ = 0.7 W/(m K)), which is favorable to achieve the large thermoelectric figure of merit.

Crystallographic and Luminescence Characteristics of the Cu2MgSnSe4 Quaternary Compound and Cu2 – xMgSnSe4 (0 < x ≤ 0.15) Copper-Deficient Solid Solutions

Crystallographic characteristics of the Cu2MgSnSe4 quaternary compound and Cu2 – xMgSnSe4 (0 < x ≤ 0.15) copper-deficient solid solutions have been determined for the first time. It has been shown that the band peaking at 1.39 eV in the 78-K cathodoluminescence spectrum of Cu2MgSnSe4 is most likely due to CuMg and MgCu antisite defects resulting from exchange of atoms between the copper and magnesium sublattices in the kesterite structure and that the spectra of Cu2 – xMgSnSe4 contain, in addition to the 1.39-eV band, a band peaking at 1.34 eV, which is due to Cu2+ · VCu defect associates.

Vanadium doped layered copper-chromium sulfides: The correlation between the magnetic properties and XES data

X-ray emission spectroscopy and static magnetochemistry study of CuCr1-xVxS2 (x = 0 ÷ 0.40) was carried out. The atom charge were estimated using the core-level S, V, Cr and Cu(Kα)-lines energy. The additional data on the electron density distribution on the vanadium and copper atoms were studied analyzing the V(Lα) and Cu(Lα,β) spectra fine structure features. The charge distribution in the CuCrS2 matrix was found to be Cu+Cr3+(S2−)2. In the case of low vanadium concentrations x < 0.15 both Cu+Cr3+(S2−)2 and Cu0+δCr(4−δ)+1-xV(4−δ)+x(S2−)2 (δ→0) charge/magnetic configurations were found. The charge distribution for solid solutions doped with concentration x = 0.15 ÷ 0.40 was described as Cu+Cr3+1-xV3+x(S2−)2. The obtained data on the charge/magnetic configurations is in good agreement with the magnetochemical measurements results. The oxidation state changes could be due to the defectness caused by intercalation of the vanadium atoms into the copper sublattice.

PROBLEMS OF COEXISTENCE OF SUPERCONDUCTIVITY AND MAGNETIC ORDERING OF COPPER SUBLATTICES IN YBa2Cu3-XFeXO7-X CERAMICS

The article studies the problems of coexistence of superconductivity and magnetic ordering of copper sublattices in YBa2Cu3-xFexO7-x ceramics. It is known that in superconducting ceramics YBa2Cu3O7-z with orthorhombic lattice as the oxygen content decreases, the transition temperature in superconducting state Tc decreases, and at z&gt;0.6 superconductivity disappears, the lattice becomes tetragonal and at the same time, antiferromagnetic ordering of sublattices Cu (2) appears. The substitution in ceramics of YBa2Cu3O7 part of copper atoms by iron atoms (i.e., the formation of a solid solution of YBa2Cu3-xFexO7+y) is accompanied by similar effects: as x increases, Tc decreases, at x&gt;0.05 the orthorhombic lattice becomes tetragonal, at x&gt;0.45 the superconductivity disappears. The most significant moment is the fact of coexistence in ceramics YBa2Cu3-xFexO7+y in the region of compositions 0.03 &lt; x &lt; 0.45 of superconductivity and magnetic ordering of iron atoms in copper nodes (the latter is established by Mossbauer spectroscopy on isotope 57Fe in a large number of works. However, it remains unclear whether the magnetic ordering of iron atoms in the YBa2Cu3-xFexO7+y lattice is related to the magnetic ordering of copper atoms

Chemical aspect of the structural disorder in CuCrS2 and CuCr1–x V x S2 solid solutions

A gradient chemical structure of micron-sized crystals of CuCrS2 and CuCr1–x V x S2 powders is established by the differential dissolution technique. It is shown that their surface region coherently conjugated with planar faces of CuCrS2 crystals is enriched with copper whereas the bulk is depleted of copper and the copper sublattice is disordered. A possible mechanism of the formation of surface regions due to a high bulk mobility of copper atoms in the defect copper sublattice is proposed. It is shown that for solid solutions the disordering process involves copper and vanadium atoms and vacancy complexes. The phenomena found are discussed together with the structural and magnetic data.

Thermoelectric transport in Cu7PSe6 with high copper ionic mobility.

Building on the good thermoelectric performances of binary superionic compounds like Cu2Se, Ag2Se and Cu2S, a better and more detailed understanding of phonon-liquid electron-crystal (PLEC) thermoelectric materials is desirable. In this work we present the thermoelectric transport properties of the compound Cu7PSe6 as the first representative of the class of argyrodite-type ion conducting thermoelectrics. With a huge variety of possible compositions and high ionic conductivity even at room temperature, the argyrodites represent a very good model system to study structure-property relationships for PLEC thermoelectric materials. We particularly highlight the extraordinary low thermal conductivity of Cu7PSe6 below the glass limit, which can be associated with the molten copper sublattice leading to a softening of phonon modes.

First-principles study of the effect of iron on the crystal structure, stability and chemical bonding in the β-based AlCu ordered η2-phase and the pretransition state of a solid solution

First-principles calculations showed that the thermodynamic stability of β-based ordered η2-AlCu phase doped with Fe is due to iron substitution in the copper sublattice (FeCu), which corresponds to the maximum number of Fe–Al bonds in the first cubic coordination polyhedron. This iron localisation leads to stable ω-like atomic displacements and pentagonal Al-nets in the (010) plane of η2-AlCu(Fe). This phase with iron substituting copper (e/a = 1.925) is an energetically preferred η-based non-canonical approximant of the icosahedral phase (e/a = 1.86). The energy gain for the FeCu position is determined by strong covalent Fe3d–Al3p bonding, while there is a weak Fe3d–Cu4s3d hybridisation for the FeAl substitution. Using a composite cluster model, we demonstrate that short-range order in the pretransition state of the β-Al–Cu–Fe solid solution observed prior to the precipitation of η-phase is stabilised due to formation of Fe–Al bonds in the first cubic coordination polyhedron of the composite cluster.

Ab initio calculations of non-stoichiometric copper nitride, pure and with palladium

We present first principles calculations of copper nitride by using periodic density functional theory within a plane-wave ultrasoft pseudopotential scheme. The insertions of extra Cu and/or Pd atoms in the empty sites, vacancy reorganization, and substitution of Cu by Pd atoms were studied. We have used an equivalent reduced-symmetry 2 × 2 × 2 Cu 3N-like cubic super-cell. Small Cu and/or Pd concentrations and vacancy rearrangements in the copper sub-lattice were conveniently calculated in these low-symmetry cells. We cover probable situations like: the occupation of the initially empty copper sites by (1) copper atoms, and by (2) palladium; (3) the relocation of vacancies in the copper sub-lattice; and (4) the substitution of small quantities of copper by palladium atoms in the copper sub-lattice. The equilibrium volumes and energies after relaxing the atomic positions are compared to those of intrinsic copper nitride. We found that the most stable arrangement corresponds to the ideal stoichiometric Cu 3N. We also found that any deviation from this ideal configuration shift the semiconductor state to a metallic or semi-metallic one.

Phase transformations of CuCrS2: Structural and chemical study

The structure and composition of the CuCrS2 powder synthesized by sulfidation of a mixture of oxides Cu2O:Cr2O3 = 1:1 at 850°C and cooled to room temperature at a rate of 60°C/min were studied by X-ray powder diffraction and differentiating solution. A rhombohedral CuCrS2 phase (space group R3m) was found, which was stoichiometric in composition and had disordering in the copper sublattice because copper was arranged at the tetrahedral and octahedral sites with occupancy 10% at the latter. The structure of CuCrS2, in which the octahedra were occupied by copper atoms at room temperature, was found for the first time; in known structures, the copper atoms occupied only the tetrahedral sites, while the probability of octahedral occupation appeared around 400°C (order-disorder transition). The partially disordered CuCrS2 phase is intermediate on the route to complete ordering. The quickly cooled CuCrS2 powder is unstable; after the second heating to 500°C with prolonged annealing at 390°C→180°C→80°C→25°C, its transition to the stable state was accompanied by liberation of 2–4 wt.% Cu9S5. The real composition of ternary sulfide after isolation of the Cu9S5 phase is discussed using the data of the structural method, differential dissolution, and magnetic measurements.

Experimental determination of the spatial distribution of electron defects in La2 − x Sr x CuO4 and Nd2 − x Ce x CuO4 crystal lattices

The data of Mossbauer emission spectroscopy on 67Cu(67Zn) and 67Ga(67Zn) isotopes show that holes appearing as a result of the Sr2+ substitution for La3+ in the La2 − xSrxCuO4 crystal lattice are localized predominantly at oxygen atoms occurring in the same atomic plane as the copper atoms. In contrast, electrons appearing as a result of the Ce4+ substitution for Nd3+ in the Nd2 − xCexCuO4 crystal lattice are localized in the copper sublattice. These results are consistent with the model assuming that a mechanism responsible for the high-temperature superconductivity in La2 − xSrxCuO4 and Nd2 − xCexCuO4 crystal lattices is based on the interaction of electrons with two-site two-electron centers possessing negative correlation energies (negative-U centers).

Diffusion paths formation for Cu + ions in superionic Cu 6PS 5I single crystals studied in terms of structural phase transition

In order to understand the structural transformations leading to high ionic conductivity of Cu + ions in Cu 6PS 5I argyrodite compound, the detailed structure analysis based on single-crystal X-ray diffraction has been performed. Below the phase transition at T c = ( 144 – 169 ) K Cu 6PS 5I belongs to monoclinic, ferroelastic phase (space group Cc) with ordered copper sublattice. Above T c delocalization of copper ions begins and crystal changes the symmetry to cubic superstructure with space group F-43 c ( a ′ = 19.528 Å , z = 32 ). Finally, above T 1 = 274 K increasing disordering of the Cu + ions heightens the symmetry to F-43 m ( a = 9.794 Å , z = 4 ). In this work, the final structural model of two cubic phases is presented including the detailed temperature evolution of positions and site occupation factors of copper ions ( R 1 = 0.0397 for F-43 c phase, and 0.0245 for F-43 m phase). Possible diffusion paths for the copper ions are represented by means of the atomic displacement factors and split model. The structural results coincide well with the previously reported non-Arrhenius behavior of conductivity and indicate significant change in conduction mechanism.

Spin-freezing in the two-dimensional spin-gap systems SrCu 2− xMg x(BO 3) 2 ( x=0, 0.04, 0.12)

The magnetic properties of the two-dimensional dimer spin-gap system SrCu 2(BO 3) 2 were investigated by the μ +SR technique. The relatively slow fluctuations of spin-dimers slow down with decreasing temperature and an unusual spin-freezing process is unraveled at T f<3.75 K, well within the spin-gap temperature range ( T SG≈20 K). This quasi-static phase displays a Gaussian field distribution with a remarkable stability with applied longitudinal fields. In support of the criticality of the SrCu 2(BO 3) 2 spin-gap ground state towards an antiferromagnetic transition, Knight-shift measurements suggest that implanted muons may liberate spin density at T< T SG that undergoes spin-freezing at very low temperatures. On the other hand, non-magnetic impurity-doping of the copper sublattice does not suppress the spin-gap ground state and does not lead to magnetic ordering effects of static nature.

Experimental observation of localization-delocalization of Cooper pairs in Nd1.85Ce0.15CuO4

Localization—delocalization of cooper pairs on 67Zn impurity centers in the copper sublattice in the high-temperature superconductor Nd1.85Ce0.15CuO4 was observed by emission Mossbauer spectroscopy on 67Cu (67Zn).

Oxygen permeability of La2Cu(Co)O4+δ solid solutions

Formation of the La2Cu1−xCoxO4+δ solid solutions with orthorhombic K2NiF4-type structure was found to be in the range of 0≤x≤0.30 at temperatures above 1270 K. Incorporating cobalt into the copper sublattice of lanthanum cuprate leads to increasing oxygen hyperstoichiometry and decreasing electrical conductivity. Thermal expansion coefficients of the La2Cu1−xCoxO4+δ (x=0.02–0.30) ceramics at 470–1100 K were calculated from the dilatometric data to vary in the range (12.2–13.2)×10−6 K−1. Studying the dependence of oxygen permeation fluxes through La2Cu(Co)O4+δ on the membrane thickness demonstrated that the oxygen transport at the thickness values below 1 mm is limited by both surface exchange rate and bulk ionic conductivity. Oxygen permeability of the La2Cu1−xCoxO4+δ solid solutions was ascertained to increase with cobalt concentration at x=0.02–0.10 and to decrease with further dopant additions, indicating a participation of interstitial oxygen in the ionic transport.

Growth and chemical analysis of bulk Nd 2− xCe xCuO y single crystals

Single crystals of Nd 2− x Ce x CuO y (0< x<0.17) from a family of the electron-type superconductors have been grown using platinum crucibles by the top seeded solution growth technique. The structural quality of the crystals was examined by X-ray diffraction. The FWHM of the Bragg reflections for the best samples varies from 8 to 20 angular seconds. Full neutron activation analysis was carried out to determine the chemical composition of the as-grown crystals. Small cation stoichiometry deviations from ideal (NdCe)/Cu ratio are found as well as Pt substitution on the copper sublattice at a level up to 2 at% exists depending on growth conditions. The results of thermogravimetric measurements and further annealing of samples show that the problem of an oxygen reduction of large Nd 2− x Ce x CuO y crystals is closely connected with inhomogeneity of anion distribution in the lattice possibly caused by non-optimal parameters for the reduction process and the level of impurities contamination. A three step reduction process which produces superconductivity with T c∼19 K in single crystals thicker than 1 mm has been developed.