Orbital fluctuations and orbital order in FeCr2S4

Heat-capacity investigations on the ferrimagnetic spinel FeCr₂S₄ poly- and single crystals provide experimental evidence of orbital liquid and orbital glass states. The low-temperature transition in the polycrystals at 10 K arises from orbital order and is very sensitive to the sample stoichiometry. In the single crystals the orbital order is fully suppressed resulting in an orbital glass state with the heat capacity following a strict T² dependence towards zero temperature. At elevated temperatures, FeCr₂S₄ exhibits an unexpected large linear term of about 100 mJ mol⁻¹ K⁻² as the fingerprint of the orbital liquid.     

Photoluminescence spectra of rare earth doped CaGa2S4 single crystals

Six kind CaGa₂S₄ single crystals doped with different rare earth (RE) elements are grown by the horizontal Bridgman method, and their photoluminescence (PL) spectra are measured in the temperature range from 10 to 300 K. The PL spectra of Ce or Eu doped crystals have broad line shapes due to the phonon assisted 4f-5d transitions. On the other hand, those of Pr³⁺, Tb³⁺, Er³⁺ or Tm³⁺ doped samples show narrow ones owing to the 4f-4f transitions. The assignments of the electronic levels are made in reference to the reported data of RE 4f multiplets observed in same materials.     

Structure determination, magnetic and optical properties of a new chromium(II) thioantimonate, [Cr((NH2CH2CH2)3N)]Sb4S7

The chromium(II) antimony(III) sulphide, [Cr((NH₂CH₂CH₂)₃N)]Sb₄S₇, was synthesised under solvothermal conditions from the reaction of Sb₂S₃, Cr and S dissolved in tris(2-aminoethyl)amine (tren) at 438 K. The products were characterised by single-crystal X-ray diffraction, elemental analysis, SQUID magnetometry and diffuse reflectance spectroscopy. The compound crystallises in the monoclinic space group P2₁/n with a=7.9756(7), b=10.5191(9), c=25.880(2) Å and β=90.864(5)°. Alternating SbS₃³⁻ trigonal pyramids and Sb₃S₆³⁻ semi-cubes generate Sb₄S₇²⁻ chains which are directly bonded to Cr(tren)²⁺ pendant units. The effective magnetic moment of 4.94(6)μ_B shows a negligible orbital contribution, in agreement with expectations for Cr(II):d⁴ in a ⁵A ground state. The measured band gap of 2.14(3) eV is consistent with a correlation between optical band gap and framework density that is established from analysis of a wide range of antimony sulphides.     

Spin glass like behavior in Cd0.42Mn0.58In2S4

The magnetic behavior of the diluted magnetic semiconductor Cd_0.42Mn_0.58In₂S₄ has been study by dc magnetization and ac susceptibility experiments. Zero field cooled and field cooled measurements reveal irreversibility below T_irr=2.60±0.15 K. Ac susceptibility data, performed as a function of the temperature and the frequency, confirm the spin-glass like behavior of the material with T_f=2.75±0.15 K. High temperature susceptibility data follow a typical Curie-Weiss law with θ=−74±1 K which suggests predominant antiferromagnetic interactions. The randomness of the magnetic ions, necessary to explain the magnetic behavior of the material, has been determined by X-ray powder diffraction experiments.     

Crystal field investigation on the magnetic properties of Yb in Yb(CF3SO3)3·9H2O

Ytterbium tri-fluoromethanesulfonate (YbTFMS) single crystals are prepared from the slow evaporation of the aqueous solution of YbTFMS and the principal magnetic susceptibility perpendicular to the c-axis of the hexagonal crystal (χ_⊥) is measured from 300 K down to 13 K. Principal magnetic anisotropy Δχ(=χ_∥−χ_⊥) is measured from 300 K down to 80 K which provides principal magnetic susceptibility parallel to the c-axis (χ_∥) down to 80 K. Very good theoretical simulation of the observed magnetic properties of YbTFMS has been obtained using one electron crystal field (CF) analysis having C_3h site symmetry. No signature of ordering effect in the observed magnetic data is noticed down to the lowest temperature (13 K) attained, indicating the inter-ionic interaction to be of predominantly dipolar type. The calculated g-values are found to be g_∥=2.67 and g_⊥=2.51, respectively. CF analysis provides the electronic specific heat which gives two Schottky anomalies in its thermal variation down to ∼13 K. The temperature dependences of quadrupole splitting and hyperfine heat capacity are studied from the necessary information obtained from the CF analysis.     

Superconducting properties in Rh17S15 under magnetic field and pressure

We have studied superconducting properties by measuring the electrical resistivity and magnetization for a single crystal of Rh₁₇S₁₅ with a superconducting transition temperature T_c=5.4 K. The upper critical field H_c2(0) and the lower critical field H_c1(0) were obtained as 20.5 and 0.0033 T, respectively. Correspondingly, the coherence length and the penetration depth were estimated to be 40 and 4900 Å, respectively, indicating that Rh₁₇S₁₅ is a typical type-II superconductor with strong correlations of conduction electrons with a 4d-electron character of Rh atoms. The present electron correlations are formed to be enhanced with increasing pressure.     

Spectroscopic properties of Pr:KY(MoO4)2 crystal as a visible laser gain medium

A Pr³⁺-doped KY(MoO₄)₂ single crystal was grown by the Czochralski method. The polarized absorption and fluorescence spectra of the Pr³⁺:KY(MoO₄)₂ crystal were measured at room temperature. The stimulated emission cross-sections for the transitions from the ³P₀ multiplet were estimated from the fluorescence spectra. The fluorescence lifetime of the ³P₀ multiplet was estimated from the fluorescence decay curve at room temperature. The analysis of spectral properties shows that the Pr³⁺:KY(MoO₄)₂ crystal is a promising gain medium for visible lasers.     

Optical properties of Er:SrMoO4 single crystal

Er³⁺:SrMoO₄ crystal of high optical quality was grown by the Czochralski method. The room temperature polarized absorption and emission spectra together with the lifetime decay curve were measured. Based on the Judd-Ofelt theory, three intensity parameters, radiative transition rates, radiative lifetimes and fluorescent branching ratios, were obtained. Emission cross-section and gain cross-section around 1.54 μm were also obtained.     

Preparation of Ca(1−x)EuxGa2S4 crystals and their photoluminescence, absorption and excitation spectra

The single crystal of CaGa₂S₄:Eu is expected as a useful laser material with a high quantum efficiency of light emission. However, as far as our knowledge is concerned, the systematic study of the mixed compounds of Ca_(1−x)Eu_xGa₂S₄ as a function of x has not been reported up to now. Here, we have first constructed the phase diagram of the CaGa₂S₄ and EuGa₂S₄ pseudo binary system, and show that it forms the solid solution. Then we have grown single crystals of these compounds. The maximum photoluminescence efficiency is achieved at x=0.25. From the three peak energies observed in the photoluminescence excitation (PLE) and absorption spectra, the 5d excited states are suggested to consist of three levels arising from the multiplets of Eu²⁺ ions.     

Band structure calculations on the layered compounds FeGa2S4 and NiGa2S4

For the compounds FeGa₂S₄ and NiGa₂S₄ band structure calculations have been performed by the ab initio plane wave pseudo-potential method. The valence charge density distribution points to an ionic type of chemical bonding between the transition metal atoms and the ligand atoms. Two models for the pseudo-potentials are used to calculate the band structures: (a) only s and p electrons and (b) also the d-shells of the transition metal atoms are included in the pseudo-potentials. The differences between these two cases of band structures are discussed. Energy gap formation peculiarities are analysed for both crystals. Zak's elementary energy band concept is demonstrated for the energy spectra of the considered crystals.     

Growth and optical properties of Eu/Li-co-doped SrB4O7 single crystals

This paper reports the growth and optical properties of Eu²⁺/Li⁺-co-doped SrB₄O₇ single crystals. High-quality Eu,Li:SrB₄O₇ crystals without macro-defects or cracks were grown using the top-seeded solution growth (TSSG) method. The absorption and luminescent properties were measured and different spectra were observed in the as-grown crystals. As the doping amount of lithium increases, the absorption peak at 300 nm becomes stronger and the emission peak shifts to a longer wavelength. This phenomenon could be attributed to the doping lithium ions, which might affect the electric field distribution in the lattice structure.     

Synthesis of goethite from Fe(OH)2 precipitates: Influence of Fe(II) concentration and stirring speed

Magnetic methods are efficient tools in soil and environmental science. But in such natural environments, several magnetic minerals are generally present. So, synthetic standard samples are necessary for calibration of laboratory techniques. The aim of this study was to synthesise goethite free of magnetic impurities (concentration <∼1 μg kg⁻¹) with different crystal sizes. Goethite was prepared by oxidation of aqueous suspensions of Fe(OH)₂ precipitates. Final products were characterised by X-ray diffraction, infrared spectroscopy, scanning and transmission electron microscopy and magnetic methods. Goethite could be obtained in the absence of any trace of strong magnetic minerals using FeSO₄·7H₂O and NaOH as reactants with the following experimental conditions: temperature=45 °C, [FeSO₄·7H₂O]=0.50 mol L⁻¹, [NaOH]=0.20 mol L⁻¹, stirring speed=760 rpm. The Fe(II) concentration and the stirring speed were varied. It proved possible to modify the size of the goethite crystals by varying the Fe(II) concentration and the stirring speed, but important changes of these parameters induced the formation of other phases, lepidocrocite when the oxidation reaction was drastically accelerated and Fe₃O₄ when the reaction was slowed down. In any case, for weak magnetic fields, a low-coercivity magnetic mineral saturating at weak magnetic fields was observed. It may correspond to traces of δ-FeOOH or to domains structurally similar to δ-FeOOH inside the multidomainic crystals of δ-FeOOH.     

The unusual magnetic and electronic properties of Gd0.5Ba0.5CoO2.9

Magnetic and electrical properties of well-characterized Gd_0.5Ba_0.5CoO_2.9 have been studied carefully in order to compare them with those of other analogous cobaltates of the type Ln_0.5A_0.5CoO₃ (Ln=La, Nd and A=Sr, Ba) which are ferromagnetic. The results show that Gd_0.5Ba_0.5CoO_2.9, which has A-site cation ordering at room temperature, does not become a genuine ferromagnet at low temperatures, but the ferromagnetic interactions observed at 280 K give over to an antiferromagnetic (AFM) state on cooling to 230 K. The AFM state is rendered ferromagnetic on the application of high magnetic fields. The properties can be understood on the basis of phase separation induced by the large A-site cation-disorder, arising from the size mismatch.     

High sensitivity of magnetism of the “114” ferrimagnet CaBaCo4O7 to stoichiometry deviation

The effect of oxygen/cobalt off-stoichiometry upon magnetism in CaBaCo₄O₇ has been investigated. It is shown that the oxides CaBaCo₄O_7+δ and CaBaCo_4−xO_7−δ (0≤x≤0.20) synthesized below 1100 °C in air exhibit phase separation, where ferrimagnetic regions with T_C~56 K to 64 K coexist with regions of magnetic clusters. The latter are detected from ac-susceptibility measurements, which show various frequency dependent peaks at ~14–20 K, 37 K, and 45 K, depending on the stoichiometry. The origin of this phenomenon is attributed to the great sensitivity of the material to oxidation as the synthesis of temperature is lowered, leading to the introduction of additional Co³⁺ cations, with respect to the ideal formula CaBaCo₂²⁺Co₂³⁺O_7. This excess Co³⁺ tends to destroy the ferromagnetic zig-zag chains of the ferrimagnetic structure and creates various cobalt spin clusters, leading to the inherent phase separation in the samples.     

Crystal and magnetic structures of Y2CrS4

Chromium(II) sulfide, Y₂CrS₄, prepared by a solid-state reaction of Y₂S₃ and CrS, showed an antiferromagnetic transition at 65 K. The neutron diffraction patterns at 10 and 90 K were both well refined with the space group Pca2₁. At 90 K, cell parameters were a=12.5518(13) Å, b=7.5245(8) Å, and c=12.4918(13) Å. At 10 K, magnetic peaks were observed, which could be indexed on the same unit cell. Magnetic moments of chromium ions were parallel to the b-axis and antiferromagnetically ordered in each set of the 4a sites.     

Polymorphism in Cu-substituted GaV4S8: structural and electronic properties

The hexagonal and cubic phases of Ga_1−xCu_xV₄S₈ are obtained by different methods of preparation. The reaction of elements above 900 °C gives hexagonal phases for large range of x=0.02-0.5. These are metallic and show enhanced paramagnetism. The reduction of oxides by H₂S at a lower temperature of 700 °C gives non-stoichiomertic compositions of cubic-V₄ cluster compound GaV₄S₈. The solubility of Cu-atoms in cubic phase is less than 10% and above x=0.2 the samples contain a mixture of phases, Cu_xVS₂, Ga_xVS₂ and CuGaS₂. The cubic phases are insulating and show Mott's Variable Range Hopping conduction. The non-stoichiomerty and the Cu-substitution reduce the resistivity and thermopower. For x=0.15 and 0.20, the additional peaks are observed in X-ray patterns. These compositions showed a sharp metal to insulator transition on cooling below 180 K. The transitional behaviour is very similar to that previously reported intercalated VS₂ compound Al_xVS₂. The transport and magnetic properties of these phases are discussed in terms of the clustering interactions among V-atoms and the localisation of carriers on the metallic clusters frequently found in V-chalcogenides.     

Local order and structural transitions in the molecule-based layer ferrimagnet (n-C4H9)4N FeFe(C2O4)3

Local structural order and temperature-dependent structural variation have been studied in the molecular-based layer ferrimagnet (n-C₄H₉)₄N Fe^IIFe^III(C₂O₄)₃ by EXAFS and high resolution X-ray powder diffraction. The EXAFS spectra measured at the Fe K-edge are successfully modelled by successive O, C, O and metal shells, showing that even when there is extensive structural disorder due to stacking faults, the local structural order in this class of ferrimagnets is fully retained. In this salt, which shows remarkable negative magnetisation at low temperature (Néel class Q), the EXAFS Debye-Waller factor has a discontinuity at 40 K, corresponding to one found in the magnetisation. At the same temperature there is also a change in the expansion of the lattice as evidenced by the high resolution X-ray powder diffraction.     

Facile synthesis and electrochemical properties of hierarchical MnO2 submicrospheres and LiMn2O4 microspheres

Hierarchical MnO₂ submicrospheres have been successfully synthesized by a wet chemical method. The as-prepared products were characterized by means of XRD, SEM, FTIR, TG, and TEM. With the as-prepared MnO₂ submicrospheres as precursors, LiMn₂O₄ microspheres were conveniently prepared by a simple solid-state reaction between MnO₂ and LiOH at a temperature as low as 600 °C. Electrochemical properties of the as-prepared MnO₂ submicrospheres and LiMn₂O₄ microspheres as cathode materials in lithium ion cells were investigated by galvanostatic charge/discharge tests.     

Structure and magnetic properties of perovskite Sr2CuNbO6−δ

Single phase perovskite Sr₂CuNbO_6−δ with a high proportion of Cu¹⁺ ions and oxygen vacancies was synthesized by solid-state reaction. The structure was determined by Rietveld method with space group Pm3m. Isotropic g value was evaluated from electron spin resonance (ESR) measurements. The ESR result is consistent with that of magnetic susceptibility.     

Optical spectroscopy of Yb/Er codoped NaY(WO4)2 crystal

Erbium and ytterbium codoped double tungstates NaY(WO₄)₂ crystals were prepared by using Czochralski (CZ) pulling method. The absorption spectra in the region 290-2000 nm have been recorded at room temperature. The Judd-Ofelt theory was applied to the measured values of absorption line strengths to evaluate the spontaneous emission probabilities and stimulated emission cross sections of Er³⁺ ions in NaY(WO₄)₂ crystals. Intensive green and red lights were measured when the sample were pumped by a 974 nm laser diode (LD), especially, the intensities of green upconversion luminescence are very strong. The mechanism of energy transfer from Yb³⁺ to Er³⁺ ions was analyzed. Energy transfer and nonradiative relaxation played an important role in the upconversion process. Photoexcited luminescence experiments are also fulfilled to help analyzing the transit processes of the energy levels.