Electrical and magnetic properties of new rhodium perovskites: La2MRhO6, M = Cr, Fe, Cu

The compositions La₂FeRhO₆, La₂CrRhO₆, La₂CuRhO₆ were prepared for the first time, and their electrical and magnetic properties were determined. Known La₂MnRhO₆, La₂CoRhO₆, and La₂NiRhO₆ were also prepared and their properties determined for comparison. All six phases have the orthorhombic perovskite structure. Electrical resistivity measurements as a function of temperature indicate that all phases are semiconducting. Seebeck measurements show that all are p-type conductors. Magnetic measurements indicate general antiferromagnetic interactions for all compositions, although evidence for weak ferromagnetism is found in the case of La₂CrRhO₆ and La₂FeRhO₆. The oxidation state of M cation appears to be close to 3+ for Fe and Cr, and a mixture of 2+/3+ for Cu. A favorable combination of electrical conductivity and Seebeck coefficient results in a relatively high thermoelectric power (1.11 W/m-K² at ∼575 K) factor for La₂CuRhO₆.     

Crystal growth and luminescence of Sb3+-Doped Cs2NaMCl6 (M = Sc,Y, La)

Crystal growth and the luminescence properties of Cs₂NaMCl₆-Sb³⁺ (M = Sc, Y, La) are described. The emission consists of one band which shifts to lower energies for an increasing ionic radius of M³⁺ (i.e. from Sc via Y to La). The excitation bands of the luminescence of all three compounds are at the same energy and show a structure which can be explained by assuming a dynamical Jahn-Teller effect to be operative in the excited state of the Sb³⁺-ion.     

Crystal and electronic band structures of homologous compounds ZnkIn2Ok+3 by Rietveld analysis and first-principle calculation

Crystal structure of homologous compounds, Zn₃In₂O_6, Zn₄In₂O₇, Zn₅In₂O₈, Zn₇In₂O₁₀, and In₂O₃ were refined by X-ray Rietveld analysis. Band structures of the homologous compounds were evaluated by first-principle calculation (Cambridge Serial Total Energy Package, CASTEP), using the structural data obtained from the Rietveld analysis. According to the results of CASTEP calculations, a sharp cut-off at the Fermi level could be observed when In³⁺ preferentially occupies the tetrahedral site (Zn₃In₂O₆(4)) or the trigonal bipyramid site (Zn₃In₂O₆(5)) in the (InZn_k)O_k+1⁺ layers. The cut-off at the Fermi level could not be observed when In³⁺ and Zn²⁺ are totally disordered at these sites. Electronic structure calculation suggested that Zn₃In₂O₆(4) is a good conductor and that Zn₃In₂O₆(5) is a poor conductor. Results of geometry optimization indicate that the formation enthalpy of Zn₃In₂O₆(4) was lower than that of Zn₃In₂O₆(5). Considering the electronic structure and the formation enthalpy, Zn₃In₂O₆(4) in which In³⁺ in the (InZn_k)O_k+1⁺ layer occupies the tetrahedral site preferentially, is likely to be the favored structure.     

An investigation of structural, magnetic and dielectric properties of R2NiMnO6 (R = rare earth, Y)

We have investigated the structure, magnetic and dielectric properties of the double perovskite oxides, R₂NiMnO₆ (R = Pr, Nd, Sm, Gd, Tb, Dy, Ho and Y). We could refine powder X-ray diffraction patterns of all the phases on the basis of monoclinic (P2₁/n) double perovskite structure where Ni and Mn atoms are ordered at 2c and 2d sites, respectively. All the phases are ferromagnetic insulators exhibiting relatively low dielectric loss and dielectric constants in the range 15-25. The ferromagnetic ordering temperature of the R₂NiMnO₆ series seems to correlate better with the radius of R³⁺ atoms than with the average Ni-O-Mn angle (φ) in the double perovskite structure. These results are consistent with all samples having Mn⁴⁺ and Ni²⁺ with minimal antisite disorder.     

Synthesis, structural and magnetic studies of the CuCr1−xRhxO2 delafossite solid solution with 0 ≤ x ≤ 0.2

The CuCr_1−xRh_xO₂ series is investigated by X-ray diffraction, magnetization measurements and Raman spectroscopy on ceramic samples. It is found that a delafossite solid solution is maintained up to x = 0.2 in CuCr_1−xRh_xO₂. The small observed variation in cell parameters is consistent with the small difference between the ionic radii of Cr³⁺ and Rh³⁺. A significant broadening of X-ray reflections is observed and when analyzed using the Williamson-Hall relationship showed that the strain generated by Rh substitution is strongly anisotropic, affecting mainly (Cr,Rh)-O bonds in the ab plane. Room temperature Raman spectra displayed three main Raman active modes. All modes shift to lower frequency and undergo significant changes in intensity with increasing Rh content, showing the effect of Rh atoms on the M³⁺-O bond strength. The magnetic behavior of CuCr_1−xRh_xO₂ samples was investigated as a function of temperature and applied field. At high temperature paramagnetic behavior, and at low temperature, evidence for weak ferromagnetism, reinforced by a hysteresis loop at 4 K is observed. The magnetic behavior of CuCr_1−xRh_xO₂ is attributed to the disorder of Cr and Rh in octahedral sites resulting in short-range Cr-O-Cr and Cr-O-Rh interactions, which give rise to short-range weak ferromagnetism.     

Synthesis and properties of liquid luminophores POCl3-SbCl5-Nd3+ and POCl3-SbCl5-235UO 22+

Luminophores POCl₃-SbCl₅-Nd³⁺ with Nd concentration of up to 0.8 M and Nd³⁺ luminescence lifetime of up to 220 μs and POCl₃-SbCl₅-²³⁵UO ₂ ²⁺ with uranyl concentration of up to 0.16 M were prepared. It was found that stable liquids POCl₃-SbCl₅-Nd³⁺ and POCl₃-SbCl₅-²³⁵UO ₂ ²⁺ can be obtained when salt crystal hydrates are used or water is added to anhydrous Nd and U(VI) compounds. The SbCl₅ content should be 1 ≤ [SbCl₅]/[Nd³⁺] ≤ 2 for the POCl₃-SbCl₅-Nd³⁺ system and [SbCl₅]/[UO ₂ ²⁺ ] > 3 for the POCl₃-SbCl₅-²³⁵UO ₂ ²⁺ system. Based on the solubility of Nd and U(VI) compounds in the POCl₃-SbCl₅ solvent and spectral-luminescence properties of Nd³⁺ and UO ₂ ²⁺ in the luminophores obtained, the following compositions of the complexes formed were suggested: Nd(PO₂Cl₂)_3−y (SbCl_y)_y · xPOCl₃, where y = 1, 2, and 3, and UO₂(PO₂Cl₂)(SbCl₆) · xPOCl₃. Original Russian Text ? G.V. Tikhonov, S.V. Kiselev, 2007, published in Radiokhimiya, 2007, Vol. 49, No. 6, pp. 524–528.     

Near-ultraviolet excitable Ca4Y6(SiO4)6O: Ce,Tb white phosphors for light-emitting diodes

The present investigation aims to demonstrate the potentiality of Tb³⁺ and Ce³⁺ co-doped Ca₄Y₆(SiO₄)₆O phosphors. By incorporation of Ce³⁺ into Ca₄Y₆(SiO₄)₆O: Tb³⁺, the excitation band was extended from short-ultraviolet to near-ultraviolet region. The energy transfer from Ce³⁺ to Tb³⁺ in Ca₄Y₆(SiO₄)₆O host was investigated and demonstrated to be a resonant type via a dipole–dipole mechanism with the critical distance of 10.2 Å. When excited by 352 nm, Ca₄Y₆(SiO₄)₆O: Ce³⁺, Tb³⁺ exhibited a brighter and broader violet-blue emission (421 nm) from the Ce³⁺ and an intense green emission (542 nm) from the Tb³⁺. Combining the two emissions whose intensities were adjusted by changing the doping levels of the co-activator, an optimized white light with chromaticity coordinates of (0.278, 0.353) is generated in Ca₄Y₆(SiO₄)₆O: 2% Ce³⁺, 8% Tb³⁺, and this phosphor could be potentially used in near-ultraviolet light-emitting diodes.     

Synthesis and photoluminescence properties of SrLu2O4:Eu superfine phosphor

Superfine powder SrLu₂O₄:Eu³⁺ was synthesized with a precursor prepared by an EDTA - sol-gel method at relatively low temperature using metal nitrate and EDTA as starting materials. The heat decomposition mechanism of the precursor, formation process of SrLu₂O₄:Eu³⁺and the properties of the particles were investigated by thermo-gravimetric (TG) - differential thermal analysis (DTA), X-ray diffraction (XRD), transmission electron microscopy (TEM) and photoluminescence (PL) analyses. The results show that pure SrLu₂O₄:Eu³⁺ superfine powder has been produced after the precursor was calcinated at 900 °C for 2 h and has an elliptical shape and an average diameter of 80-100 nm. Upon excitation with 250 nm light, all the SrLu₂O₄:Eu³⁺ powders show red and orange emissions due to the 4f-4f transitions of Eu³⁺ ions. The highest photoluminescence intensity at 610 nm was found at a content of about 6 mol% Eu³⁺. Splitting of the ⁵D₀-⁷F₁ emission transition revealed that the Eu³⁺ ions occupied two nonequivalent sites in the crystallite by substituting Lu³⁺ ions.     

Structure, infrared and Raman spectroscopic studies of new Sr0.50SbFe(PO4)3 and SrSb0.50Fe1.50(PO4)3 Nasicon phases

Two newly synthesised Sr_0.50SbFe(PO₄)₃ [Sr_0.5.] and SrSb_0.50Fe_1.50(PO₄)₃ [Sr.] phases were obtained by conventional solid-state reaction techniques at 1000 °C in air atmosphere. Their crystallographic structures were determined at room temperature from X-ray powder diffraction (XRPD) data using the Rietveld analysis. Both compounds belong to the Nasicon structural family. [Sr_0.5.] and [Sr.] crystallise in rhombohedral system with \textR[`3] {\text{R}}\overline{3} and \textR[`3] \textc {\text{R}}\overline{3} {\text{c}} space group, respectively. Hexagonal cell parameters for [Sr_0.5.] and [Sr.] are: a = 8.227(1) ?, c = 22.767(2) ? and a = 8.339(1) ?, c = 22.704(2) ?, respectively. Sr²⁺ and vacancies in {[Sr_0.50]_3a[□_0.50]_3b}_M1SbFe(PO₄)₃ are practically ordered within the two positions, 3a and 3b, of M1 sites. Structure refinements show also an ordered distribution of Sb⁵⁺ and Fe³⁺ ions within the Nasicon framework. Within the structure, each Sr_(3a)O₆ octahedron shares two faces with two Fe³⁺O₆ octahedra and each vacancy (□_(3b)O₆) site is located between two Sb⁵⁺O₆ octahedra. In [Sr]_M1Sb_0.50Fe_1.50(PO₄)₃ compound, all M1 sites are occupied by Sr²⁺ and the Sb⁵⁺ and Fe³⁺ ions are randomly distributed within the Nasicon framework. A Raman and infrared spectroscopic study was used to obtain further structural information about the nature of bonding in both selected compositions.     

GGA and GGA+U Description of Structural, Magnetic, and Elastic Properties of Rh2MnZ (Z=Ge, Sn, and Pb)

Structural, magnetic, electronic, and elastic properties of Rh₂MnGe, Rh₂MnSn, and Rh₂MnPb Heusler compounds have been calculated using full potential linearized augmented-plane wave plus local orbitals (FP-L/APW+lo) method based on the spin density functional theory, within the generalized gradient approximation (GGA) and (GGA+U) (U is the Hubbard correction). Results are given for the lattice parameters, bulk moduli, spin magnetic moments and elastic constants. We have derived the bulk and the shear moduli, Young’s and Poisson’s ratio for Rh₂MnZ (Z=Ge, Sn, and Pb). The elastic modulus of Rh₂MnGe is predicted to be the highest. Also, we have estimated the Debye temperatures from the average sound velocity. We discuss the electronic structures, total and partial densities of states and local moments and we investigate the pressure effect on the elastic properties by calculating the elastic constants at various volumes.     

The magnetic properties of Bi0.9Ba0.1Fe0.81M0.09Ti0.1O3 solid solutions (M = Co, Mn, Sc, Al)

Solid solutions with general formula Bi_0.9Ba_0.1Fe_0.81M_0.09Ti_0.1O₃ (M = Co, Mn, Sc, Al) together with parental Bi_0.9Ba_0.1Fe_0.9Ti_0.1O₃ were prepared by the traditional solid state reaction method. Their structural, room temperature magnetic, and dielectric properties were investigated. X-ray diffraction analysis indicated that all samples maintained original R3c space group. M–H hysteresis loop of Co³⁺ doped sample saturated at an applied field of 1 T with spontaneous magnetization of 1.735 emu/g, while Mn⁴⁺ substitution enhanced the magnetization of Bi_0.9Ba_0.1Fe_0.9Ti_0.1O₃ less strongly; addition of Sc³⁺ helped decrease magnetic coercive field while Al³⁺ modified sample exhibited paramagnetic M–H hysteresis loop. Differential scanning calorimetry was applied to determine the Neel temperature (T_N) and the T_N for undoped, Co³⁺, Mn⁴⁺, Sc³⁺, Al³⁺ doped solid solutions were 318.1, 324.3, 335.7, 293.9 and 295.8 respectively. Sc³⁺ substitution had little influence on the dielectric properties of Bi_0.9Ba_0.1Fe_0.9Ti_0.1O₃ while Al³⁺ doping improved its dielectric constant. In contrast, Co³⁺, Mn⁴⁺ doped samples showed decreased permittivity but inhibited tan δ at frequencies larger than 30 kHz.     

Novel Dy-doped Ca2Gd8(SiO4)6O2 white light phosphors for Hg-free lamps application

The luminescent properties of Ca₂Gd_8(1−x)(SiO₄)₆O₂:xDy³⁺ (1% ≤ x ≤ 5%) powder crystals with oxyapatite structure were investigated under vacuum ultraviolet excitation. In the excitation spectrum, the peaks at 166 nm and 191 nm of the vacuum ultraviolet region can be assigned to the O²⁻ → Gd³⁺, and O²⁻ → Dy³⁺ charge transfer band respectively, which is consistent with the theoretical calculated value using Jφrgensen's empirical formula. While the peaks at 183 nm and 289 nm are attributed to the f-d spin-allowed transitions and the f-d spin-forbidden transitions of Dy³⁺ in the host lattice with Dorenbos's expression. According to the emission spectra, all the samples exhibited excellent white emission under 172 nm excitation and the best calculated chromaticity coordinate was 0.335, 0.338, which indicates that the Ca₂Gd₈(SiO₄)₆O₂:Dy³⁺ phosphor could be considered as a potential candidate for Hg-free lamps application.     

Synthesis and magnetic properties of (Ba,Bi)1.54Rh8O16 hollandite

The synthesis, structure and electrical and magnetic properties of (Ba,Bi)_1.54Rh₈O₁₆ hollandite are discussed. The addition of Bi stabilizes the Ba-Rh hollandite, otherwise not found under our synthesis conditions. The compound crystallizes in the monoclinic space group I2/m, a = 9.425(5) Å, b = 10.425(4) Å, c = 3.160(1) Å, γ = 94.11(5). Electron diffraction and structure imaging show that the (Ba,Bi) columns along the tunnels are incommensurate with the Rh₈O₁₆ framework, and analysis of the electron diffraction and elemental analysis allow the formula to be determined as Ba_1.21Bi_0.33Rh₈O₁₆. The compound shows Curie-Weiss paramagnetism, and we compare its susceptibility to that of Rh₂O₃. Polycrystalline samples show a negative dρ/dT between 0.3 and 300 K. No superconductivity is observed down to 0.3 K.     

Transformation des composes Ba2M2+F6 (M2+ = Mn,Fe, Co,Ni, Zn,Mg) sous haute pression et haute temperature

New quenchable high pressure phases have been found for the Ba₂M²⁺F₆ compounds (M²⁺ = Mn, Fe, Co, Ni, Zn, Mg). They are isotypic with K₂GeF₆. The synthesis has been performed by subjecting a mixture of 2BaF₂ + MF₂ to 40 kbar and 800°C in a belt-type apparatus. The cation coordination is the same in the high pressure phase as in the low pressure one whereas the fluorine-baryum packing changes from cubic to hexagonal. The previously unknown compounds Ba₂MnF₆ and Ba₂MgF₆ with the normal-pressure structure have been synthesized at 10 kbar and 700°C.     

Optical, magnetic and electronic properties of Ln2O2Te (Ln = La, Sm and Gd)

The synthesis, the optical and magnetic properties and the electronic structure of the rare-earth oxytellurides of formula Ln₂O₂Te (Ln = La, Sm and Gd) are reported. The magnetic measurements show that La₂O₂Te exhibit a diamagnetic behavior. The value of the magnetization M of Gd₂TeO₂ at our highest available field (50 kOe) is about 5.5μ_B, well below the expected value for two independent Gd ions (2gJ = 14μ_B) confirming the antiferromagnetic character of this compound. The magnetic properties of Sm₂TeO₂ reveal that Sm is in its 3+ oxidation state with a large Van Vleck contribution. The electronic structure calculations, studied by means of a first principles DFT approach, are consistent with the optical measurements and suggest an indirect band gap semiconductor behavior.     

Synthesis and crystal structures of Ba6Mg11F34 and the solid solutions Ba6Mg11−xMxF34 (M = Mn, Fe) and luminescence of Ba6Mg11F34: Eu

Ba₆Mg₁₁F₃₄, a new compound of the pseudobinary BaF₂-MgF₂ system, has been synthesized by solid state techniques from stoichiometric amounts of BaF₂ and MgF₂ and its crystal structure determined by single crystal X-ray diffraction (space group P, a=7.5084(6), b=9.9192(8), c=10.0354(8) Å, α=81.563(2), β=72.402(2), γ=71.198(1)°, 3899 structure factors, 233 parameters, R(F²>2σ(F²))=0.018, wR(F² all) = 0.046). It is isotypic with the copper(II) analogue, Ba₆Cu₁₁F₃₄. The main features of the structure are a network of [MgF₆] octahedra and three different [BaF_x] polyhedra with x=12, 11+1 and 13. Ba₆Mg_11−xFe_xF₃₄ and Ba₆Mg_11−xMn_xF₃₄ solid solutions were prepared and their composition determined by single crystal structure analyses. The luminescence properties of Ba₆Mg₁₁F₃₄ doped with Eu²⁺ were studied using fluorescence spectroscopy. The observed luminescence was pale blue with a maximum at 465 nm.     

Crystallographic phase transition, electrical and magnetic properties of stoichiometric La(Mn,Rh)O3

Temperature-induced phase transitions, electrical conductivity, and some magnetic properties of stoichiometric LaMn_1−xRh_xO₃ (0.1≤x≤0.5) have been studied. The phase transitions of LaMn_0.9Rh_0.1O₃ were investigated by high-temperature X-ray powder diffractometry and orthorhombic-pseudocubic-rhombohedral transitions were observed. The electrical conductivity measurement shows semiconducting property above room temperature. Breaking of slope in the temperature dependence of the conductivity bring the evidence in support of the phase transition for LaMn_0.9Rh_0.1O₃. At low temperature, the compounds show ferromagnetic behavior but the ferromagnetic intensity increases with the increase of the oxygen nonstoichiometry.     

Two new barium indium phosphates with intersecting tunnel structures: BaIn2P4O14, and Ba3In2P4O16

Two new barium indium phosphates BaIn₂P₄O₁₄ and Ba₃In₂P₄O₁₆ were synthesized by high temperature solution growth method and structurally characterized by single crystal X-ray diffraction analysis. They represent first compounds in the Ba-In-P-O systems. BaIn₂P₄O₁₄ and Ba₃In₂P₄O₁₆ display different types of 3D architectures. The projections of both structures concern about the existence of the intersecting tunnels. In compound BaIn₂P₄O₁₄, the tunnels are built up of the corner-sharing InO₆ octahedra and P₂O₇ groups, whereas in compound Ba₃In₂P₄O₁₆, they are formed by corner sharing of PO₄ tetrahedra and InO₆ octahedra. The electronic band structure calculations of two compounds have been performed with the density functional theory method. The study of calculations and optical diffuse reflectance experimental results show both compounds are insulators with direct band-gap.     

Luminescent properties of Sr2P2O7:Eu,Mn phosphor under near UV excitation

The phosphors in the system Sr_2−x−yP₂O₇:xEu²⁺,yMn²⁺ were synthesized by solid-state reactions and their photoluminescence properties were investigated. These phosphors have strong absorption in the near UV region, which is suitable for excitation of ultraviolet light emitting diodes (UVLEDs). The orange-reddish emission of Mn²⁺ in these phosphors can be used as a red component in the tri-color system and may be enhanced by adjusting the Mn²⁺/Eu²⁺ ratio. The energy transfer from Eu²⁺ to Mn²⁺ is observed with a transfer efficiency of ∼0.45 and a critical distance of ∼10 Å. The results reveal that Sr_2−x−yP₂O₇:xEu²⁺,yMn²⁺ phosphors could be used in white light UVLEDs.     

Crystal structure and physical properties of Mg6Cu16Si7-type M6Ni16Si7, for M = Mg, Sc, Ti, Nb, and Ta

Five compounds were investigated for magnetic character and superconductivity, all with non-magnetic nickel and band structures containing flat bands and steep bands. The syntheses and crystal structures, refined by powder X-ray diffraction, are reported for M₆Ni₁₆Si₇, where M = Mg, Sc, Ti, Nb, and Ta. All compounds form in the Mg₆Cu₁₆Si₇ structure type. Resistance measurements are also reported on M₆Ni₁₆Si₇ (M = Mg, Sc, Ti, and Nb) down to 0.3 K, with all four showing metallic conductivity. No superconductivity is observed. Magnetization measurements for all compounds reveal essentially temperature independent paramagnetism, with a tendency toward more enhanced low temperature paramagnetism for the cases of Mg₆Ni₁₆Si₇ and Sc₆Ni₁₆Si₇.