Magnetic properties of some RCo2B2 and RCo4B4 compounds

RCo₂B₂ (R=Pr, Nd, Sm, Gd) and RCo₄B₄ (R=Pr, Nd, Sm) compounds have been investigated by X-ray and magnetometry techniques in the temperature range 4.2-300 K. These compounds crystallize in tetragonal crystal structures of the types CeAl₂Ga₂ and CeCo₄B₄, respectively, with lattice parameters decreasing for increasing atomic weight of the rare earth ion. All compounds order magnetically with the Curie temperatures much below room temperature. Anomalous magnetic behavior is observed for RCo₄B₄ alloys.     

IR-Spectra of M2UO2F4.H2O Complexes

This work is devoted to the study of and Rb₂UO₂F₄.H₂O and Cs₂UO₂F₄. H₂O IR-spectra with the aim of obtaining a set of vibration frequencies characterising M₂U0₂F₄.H₂O complexe elucidating the role and nature of water bonds in the structures of the above-mentioned compounds; and receiving preliminary information on the structure of M₂UO₂F₄.H₂O. The investigated compounds were synthesized in accord with our previous paper¹. M₂UO₂D₂O and M₂UO₂F₄.HDO were obtained by recrystallizing M₂UO₂F₄.H₂O from D₂O and HDO respectively     

Heat capacity and thermal expansion of CdCr2Se4 and CdCr2S4

The thermodynamic properties of the spinel ferromagnetic compounds CdCr₂Se₄ and CdCr₂S₄ have been investigated by making heat capacity and thermal expansion measurements on single crystals. For both compounds, the ferromagnetic transition is marked by λ-type thermal anomalies, and the results provide a pressure dependence of the transition temperatures that is in agreement with direct measurements. Below the transition, CdCr₂S₄ shows an anomalous heat-capacity contribution and negative thermal expansion, which are in contrast to the conventional behavior found in CdCr₂Se₄.     

Proton magnetic relaxation in (TMA)2HgBr4 and (TMA)2HgI4

Internal motions of the protonic groups have been studied in polycrystalline [(CH₃)₄N]₂HgBr₄ and [(CH₃)₄N]₂HgI₄ from the temperature dependence of proton spin relaxation time (T ₁) and the data analysed according to the spin lattice relaxation model due to Albert and coworkers. The temperature dependence ofT ₁ in the above compounds is compared with that in (TMA)₂HgCl₄ and (TMA)₂ZnCl₄.     

Calorimetric and dielectric studies of the (NH4)2MoO2F4 oxyfluoride

The temperature dependences of the heat capacity, dielectric properties, and response to an external pressure and an electric field for the (NH₄)₂MoO₂F₄ oxyfluoride (space group Cmcm, Z = 4) have been studied. A comparative analysis of the data on the entropy of phase transitions, p-T phase diagrams, permittivity, and anomalous heat capacity in combination with the results of previous studies of the related compounds (NH₄)₂WO₂F₄ and (ND₄)₂WO₂F₄ has made it possible to establish that both [MO₂F₄]²⁻ anions and ammonium groups play a substantial role in the mechanism and nature of the structural transformations.     

Ab initio calculations of the electron structure of the compounds AgGaS2, CdGa2S4, InPS4

Crystalline structures of the compounds in the row AgGaS₂–CdGa₂S₄–InPS₄ originate from the sphalerite structure with the gradual growth of the deficiency of metallic atoms from defect-free Ag₄^IGa₄^IIIS₈ to □₂^IICd₂^IIGa₄^IIIS₈ and further to □₂^IIIIn₂^III□₂^VP₂^VS₈, where the symbol □ denotes a pseudovacancy. With the FEFF8 full multiple-scattering code, the local partial electron densities states of all atoms of all compounds investigated have been calculated. A comparison of the calculated electron densities with the experimental X-ray K- and L_2,3-spectra of emission and absorption of sulfur and phosphorus shows good correspondence between the form and energy positions of the fine structure elements. For the compounds investigated there is no significant influence of the defectiveness of the crystalline lattice on the electron density of states.     

μ + SR studies on Sr2CuO2Cl2, La2CuO4 and Nd2CuO4: 2-d magnetism,local fields and muon sitesSR studies on Sr2CuO2Cl2, La2CuO4 and Nd2CuO4: 2-d magnetism,local fields and muon sites

We compare the temperature dependence of the zero-field muon spin precession frequency observed in Sr₂CuO₂Cl₂ and La₂CuO₄ below the Néel temperature with a 2-d Heisenberg model with an additional small anisotropy. Good agreement of the observed results with this model reflects the highly 2 dimensional magnetic properties of these cuprates. The measured local fields are compared with a simple calculation of dipolar fields in the crystal to search for muon site locations. Our results provide evidence that the muon sites are related to the oxygen atoms in these compounds.     

Trends in the band-gap pressure coefficients and bulk moduli in different structures of ZnGa2S4, ZnGa2Se4 and ZnGa2Te4

We have performed a first-principles investigation for the family of compounds ZnGa2X4 (X = S, Se, Te). The properties of two possible structures, defect chalcopyrite and defect famatinite are both calculated. We reveal that ZnGa2S4 and ZnGa2Se4 have direct band gaps, while ZnGa2Te4 has an indirect band gap. The local density approximation band gaps are found to be very different in two structures, while the lattice parameters and bulk moduli are similar. We extend Cohen’s empirical formula for zinc-blende compounds to this family of compounds. The pressure coefficients are calculated and metallization pressures are discussed. We find that agi remains fairly constant when the group-Ⅵ element X is varied in ZnGa2X4 (Ⅱ-Ⅲ2 -Ⅵ4 ).     

Magnetic domain behaviour of MgFe2O4-CoFe2O4 system

Ferrites with the composition Mg_1−x CO _x Fe₂O₄(0<x≤0·33) were synthesized by solid state reaction between MgO,α-Fe₂O₃ and CoFe₂O₄ at 1300°C. The compounds were analyzed from the results of x-ray diffraction, magnetic hysteresis and initial susceptibility (χ) studies. All the compounds of the series were found to be monophasic with no appreciable change in their lattice parameter. The saturation magnetization and coercive field (H _c ) values increased with increasing cobalt ferrite content. Similarly, the shapes of theχ-T curves and temperature variations ofH _c values indicated that the magnetic behaviour changes from multi-domain to single domain with increasing cobalt content. The results are similar to those observed for the (MgFe₂O₄)_1−x (Co₃O₄) system.     

Mössbauer and magnetic studies of mixed-valence linear chain compounds: Na3Fe2S4 and Na3Fe2Se4

Mössbauer and magnetic data establish the two compounds Na ₃ Fe ₂ X ₄ (X=S, Se) as mixed-valence linear chain compounds with rapid valence fluctuation. The compounds exhibit 1-dimensional antiferromagnetic coupling within the chains, and below a critical temperature a 3-dimensional ferrimagnetic ordering between the chains.     

Cation ordering and crystal structures in AGa2X4 compounds (CoGa2S4, CdGa2S4, CdGa2Se4, HgGa2Se4, HgGa2Te4)

Single crystals of some AGa₂X₄ compounds (CoGa₂S₄, CdGa₂S₄, CdGa₂Se₄, HgGa₂Se₄, HgGa₂Te₄) were prepared by chemical vapour deposition and flux method.The X-ray structural investigations indicated blende or defect chalcopyrite structures.A simple relationship is suggested between the c/a ratio and the cationic sublattice ordering.     

PAC study of CdFe2O4 and ZnFe2O4

The time-differential perturbed angular correlation technique has been used to study the temperature dependence of the electric-field gradient at ¹¹¹Cd nuclei in the spinels CdFe₂O₄ and ZnFe₂O₄. The samples were doped with radioactive ¹¹¹In tracers which decay to ¹¹¹Cd by electron capture. For both compounds the results of the measurements show the presence of two electric quadrupole interactions. One is well defined and corresponds to a fairly symmetrical electric field gradient. The other is broadly distributed and corresponds to electric field gradients with lower symmetry. The fractions of probes associated with these interactions are temperature dependent. The interactions are analysed taking into account the possible sites in the spinel structure for In occupation. The results are compared with those obtained with other spinels. The electric field gradient associated with the octahedral site is compared with the predictions of point charge model and with Mössbauer spectroscopy results at iron site.

     

High-temperature heat capacity of Sm2CuO4 and Ho2Cu2O5

The heat capacities of Sm₂CuO₄ and Ho₂Cu₂O₅ have been measured in the temperature ranges 329–839 and 359–751 K, respectively. The experimental data have been used to determine the thermodynamic properties of the oxide compounds.     

Hydrogen-bonded ferroelectrics: Evidence for Slater configurations from E.P.R. studies of X-irradiated KH2AsO4 and NH4H2AsO4

The temperature dependence of the powder E.P.R. spectrum of the AsO₄ ^4- centre in x-irradiated samples of KH₂AsO₄ and NH₄H₂AsO₄ yields results which can be understood as arising from exchange processes taking place between the six configurations in the Slater model for this type of hydrogen-bonded ferroelectric and antiferroelectric. Knowing that the ⁷⁵As hyperfine parameters have rhombic symmetry in the compounds studied, it is possible to interpret certain specific features in the spectra as being associated with the Slater configurations of higher energy.     

Electronic properties of MgIn2S4 and CdIn2S4 by the self-consistent pseudopotential method

Summary The electronic properties of CdIn₂S₄ and MgIn₂S₄ are studied in the self-consistent local-density pseudopotential scheme based on hard-core atomic potentials. Our self-consistent calculation confirms the assumptions of previous empirical studies and provides for the lowestenergy gap values which agree with experimental data. The electron charge densities indicate that both sulphospinels are highly ionic. The two compounds have similar ground-state properties but different conduction bands. Paper presented at the ?V International Conference on Ternary and Multinary Compounds?, held in Cagliari, September 14–16, 1982.     

Structural and electrical characterization of the NASICON-type Li2FeZr(PO4)3 and Li2FeTi(PO4)3 compounds

In order to develop new electrolytes for all-solid-state rocking chair lithium batteries, the NASICON-type compounds Li₂FeZr(PO₄)₃ and Li₂FeTi(PO₄)₃ were investigated by powder X-ray diffraction technique and impedance spectroscopy. Li₂FeZr(PO₄)₃ is orthorhombic Pbna (a=8.706(3), b=8.786(2), c=12.220(5) Å) and Li₂FeTi(PO₄)₃ is orthorhombic Pbca (a=8.557(3), b=8.624(3), c=23.919(6) Å). They show no phase transitions from RT to 800 °C. In the same temperature range logσT vs. 1/T show no slope variations. The activation energies for the ionic conductivity were 0.62 and 0.64 eV for Li₂FeTi(PO₄)₃ and Li₂FeTi(PO₄)₃, respectively. In order to better evaluate the present results they were compared with those of α and β-LiZr₂(PO₄)₃ phases, which were also prepared and characterised. A change of activation energy from 0.47 eV to 1.03 eV was observed in the case of β phase, at about 300 °C; attributed to the β (orthorhombic) ? β′ (monoclinic) phase transition. In the α phase the activation energy 0.47 eV in the temperature range 150 – 850 °C. The Li₂FeZr(PO₄)₃ and Li₂FeTi(PO₄)₃ compounds can be interesting for applications as solid electrolytes in high temperature (>300 °C) lithium batteries.     

Mössbauer study of tin-doped 1-2-3 and 2-1-4 highT c superconductors

Samples of YBa₂(Cu_0.85Sn_0.15)₃O_7–y (1-2-3), La₂(Cu_0.95Sn_0.05)O4_4–x, and La_1.85Ba_0.15(Cu_0.95Sn_0.05)O_4–x (2-1-4), with different oxygen stoichiometry, have been studied by Mössbauer spectroscopy. These measurements reveal the existence of two inequivalent sites for Sn in the (1-2-3) compounds. The (1-2-3) spectra display two quadrupole doublets which we associate with Sn in Cu(1) and Cu(2) sites, respectively. La compounds show a single quadrupole doublet.     

Effective ionic charges in CdGa2Se4 and CdGa2S4

Available experimental data (Raman and Infrared) on the optic phonon spectra in defect chalcopyrite crystals CdGa₂Se₄ and CdGa₂S₄ are used to determine the dynamical ionic effective charges. The results are compared with those found in other ternary compounds as spinels and chalcopyrites. An emperical correlation between the normalized Szigeti charges and the optical dielectric constant is discussed.     

Vibrational Spectra of Arsenates and Phosphates of the Type Kcaln(XO4)2

The infrared and Raman spectra of polycrystal-line samples of different compounds of the types KCaLn(AsO₄)₂ and KCaLn(PO₄)₂, belonging to the hexagonal LaPO₄, structural modification, were recorded and discussed. A complete vibrational assignment, based on a factor group analysis, is proposed. The influence of the different lanthanide cations on the internal vibrations of the XO₄ groups is briefly discussed and some comparisons with related compounds are also made.

It is well known that simple lanthanide arsenates, phosphates, vanadates and cnromates (V) of the type LnXO₄ belong to two different structural types.     

Interpretation of neutron magnetic scattering in the Kondo lattices YbPd2Si2 and YbAgCu4

We present an interpretation of published neutron inelastic scattering spectra in the Kondo lattices YbPd₂Si₂ and YbAgCu₄ obtained in terms of the Anderson impurity model, describing the hybridisation of the 4f Yb electrons with the band electrons, and also including the crystal electric field interaction. In YbPd₂Si₂, the tetragonal crystal field parameters were determined. In YbAgCu₄ the crystal field interaction was taken to exist by analogy with the isoelectronic compound YbAuCu₄ where it has been identified. Both compounds can be described by a Kondo temperature,T ₀=60 K and a Yb valency very close to 3.