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1.
B-site disordered RFe0.5V0.5O3 compounds, with R=La, Nd, Eu and Y, have been prepared by solid-state reaction technique and their structures and magnetic properties have been investigated through X-ray powder diffraction, time-of-flight neutron powder diffraction and magnetization measurements at temperatures ranging from 5 to 700 K. The four compounds can be described as distorted perovskites with space group symmetry Pbnm and a+bb tilt system. The studied compounds also show antiferromagnetic ordering with Neel temperatures of 299, 304, 304, and 335 K respectively. The magnetic structures of R=La, Nd and Y compounds were determined from the neutron powder diffraction as Gz with observed magnetic moments of 2.55, 2.54 and 2.69μB at 30, 40 and 40 K, respectively.  相似文献   

2.
The title compound has been first synthesized by a citrate technique followed by thermal treatments under moderate oxygen pressure conditions, and characterized by X-ray and neutron powder diffraction (NPD) and magnetization measurements. The crystal structure of DyCrMnO5 has been refined from NPD data in the space group Pbam; a=7.2617(6) Å, b=8.5161(6) Å, and c=5.7126(5) Å at 295 K. This oxide is isostructural with RMn2O5 oxides (R=rare earths) and it contains infinite chains of (Cr, Mn)4+O6 octahedra-sharing edges, linked together by (Mn, Cr)3+O5 pyramids and DyO8 units. The high degree of antisite disordering exhibited by DyCrMnO5 is noteworthy. The octahedral positions are occupied by roughly 50% of Mn and Cr cations, and the pyramidal groups contain two thirds of Mn and one third of Cr cations. We assume that Mn and Cr cations at the octahedral positions exhibit a tetravalent oxidation state, whereas the metals at the pyramidal positions are trivalent, in order to preserve the electroneutrality of this oxide. The susceptibility vs temperature curve of DyCrMnO5 does not suggest the establishment of a long-range magnetic structure even at low temperatures; the NPD technique does not provide any signal of magnetic ordering, since the reflections do not show any magnetic contribution.  相似文献   

3.
The title compounds have been synthesized by a citrate technique followed by thermal treatments in air (BiFe0.5Mn1.5O5) or under high oxygen pressure conditions (BiFeMnO5), and characterized by X-ray diffraction (XRD), neutron powder diffraction (NPD) and magnetization measurements. The crystal structures have been refined from NPD data in the space group Pbam at 295 K. These phases are isostructural with RMn2O5 oxides (R=rare earths) and contain infinite chains of Mn4+O6 octahedra sharing edges, linked together by (Fe,Mn)3+O5 pyramids and BiO8 units. These units are strongly distorted with respect to those observed in other RFeMnO5 compounds, due to the presence of the electronic lone pair on Bi3+. It is noteworthy the certain level of antisite disorder exhibited in both samples, where the octahedral positions are partially occupied by Fe cations, and vice versa. BiFexMn2−xO5 (x=0.5, 1.0) are short-range magnetically ordered below 20 K for x=0.5 and at 40 K for x=1.0. The main magnetic interactions seem to be antiferromagnetic (AFM); however, the presence of a small hysteresis in the magnetization cycles indicates the presence of some weak ferromagnetic (FM) interactions.  相似文献   

4.
Polycrystalline samples and single crystals of the R4Mo4O11 compounds (R=Yb and Lu) were synthesized by solid-state reactions at high temperature in sealed Mo crucibles. The structure of Lu4Mo4O11 (a=10.5611(1), b=5.61930(5), c=15.6877 (2), β=99.5131(4) and Z=4) was determined by single crystal X-ray diffraction and refined by least squares on F2 converging to R1=0.0425, wR2=0.0980 for 3508 intensities. Contrary to the R4Mo4O11 compounds with lighter rare earths, which crystallize in the orthorhombic space group Pbam, the Yb and Lu compounds crystallize in the monoclinic space group P2/m. The R4Mo4O11 compounds contain distorted infinite oxide-molybdenum chains of trans-edge-sharing Mo6 octahedra diluted with the rare earths. Magnetic susceptibility measurements indicate that the oxidation state of the Yb atoms is +3, affording 14 metallic valence electrons per Mo4 fragment and, the absence of localized moments on the Mo network. Resistivity measurements on single crystals show that the Yb4Mo4O11 and Lu4Mo4O11 compounds are small band-gap semi-conductors.  相似文献   

5.
Powder samples of the Cr6+-containing compound Bi6Cr2O15 were prepared by solid state reaction of Bi2O3 and Cr2O3 in air at 650°C. The structure was solved and refined using high-resolution neutron powder diffraction data in space group Ccc2, with anisotropic thermal displacement parameters a=12.30184(5), b=19.87492(7), and c=5.88162(2) Å, V=1438.0 Å3, and 126 variables to RF=1.8%. Bi6Cr2O15 exhibits a new structure type that contains (Bi12O14)8n+n columns, of the kind previously found only for phases isotypic with Bi13Mo4VO34. Each column is surrounded by eight CrO2−4 tetrahedra. The ionic conductivity of Bi6Cr2O15 was determined by impedance measurements to be 3.5×10−5 (Ω cm)−1 at 600°C.  相似文献   

6.
The subsolidus phase relations of R2O3-CaO-CuO ternary systems (R=Nd, Sm, Gd, Tm) have been investigated by X-ray powder diffraction. All samples were synthesized at about 950° in air. There exists a ternary compound Ca14−xRxCu24O41 (x = 4 for R=Nd, Gd and x = 5 for R = Sm) and a ternary solid solution Ca2+xR2−xCu5O10 (R=Nd, Sm, Gd, Tm) with a wide composition range Δx of about 0.6. The compound Ca14−xRxCu24O41 possesses a layered orthorhombic structure and is isostructural to Sr14−xCaxCu24O41. The lattice parameters a and c of the compound are basically independent of the ionic radius of R, while the lattice parameter b and unit-cell volume V decrease substantially with the decrease of the ionic radii of R. The Ca2+xR2−xCu5O10 solid solution is isostructural to Ca2+xY2−xCu5O10, the structure of which is based on an orthorhombic “NaCuO2-type” subcell containing infinite one-dimensional chains of edge-shared square planar cuprate groups crosslinked by the layered cations Ca and R that locate in the inter-chain tunnels.  相似文献   

7.
We have prepared 14 new AABB′O6 perovskites which possess a rock salt ordering of the B-site cations and a layered ordering of the A-site cations. The compositions obtained are NaLnMnWO6 (Ln=Ce, Pr, Sm, Gd, Dy, and Ho) and NaLnMgWO6 (Ln=Ce, Pr, Sm, Eu, Gd, Tb, Dy, and Ho). The samples were structurally characterized by powder X-ray diffraction which has revealed metrically tetragonal lattice parameters for compositions with Ln=Ce, Pr and monoclinic symmetry for compositions with smaller lanthanides. Magnetic susceptibility vs. temperature measurements have found that all six NaLnMnWO6 compounds undergo antiferromagnetic ordering at temperatures between 10 and 13 K. Several compounds show signs of a second magnetic phase transition. One sample, NaPrMnWO6, appears to pass through at least three magnetic phase transitions within a narrow temperature range. All eight NaLnMgWO6 compounds remain paramagnetic down to 2 K revealing that the ordering of the Ln3+ cations in the NaLnMnWO6 compounds is induced by the ordering of the Mn2+ sub-lattice.  相似文献   

8.
The new Pb5Sb2MnO11 compound was synthesized using a solid-state reaction in an evacuated sealed silica tube at 650°C. The crystal structure was determined ab initio using a combination of X-ray powder diffraction, electron diffraction and high-resolution electron microscopy (a=9.0660(8)Å, b=11.489(1)Å, c=10.9426(9)Å, S.G. Cmcm, RI=0.045, RP=0.059). The Pb5Sb2MnO11 crystal structure represents a new structure type and it can be considered as quasi-one-dimensional, built up of chains running along the c-axis and consisting of alternating Mn+2O7 capped trigonal prisms and Sb2O10 pairs of edge sharing Sb+5O6 octahedra. The chains are joined together by Pb atoms located between the chains. The Pb+2 cations have virtually identical coordination environments with a clear influence of the lone electron pair occupying one vertex of the PbO5E octahedra. Electronic structure calculations and electron localization function distribution analysis were performed to define the nature of the structural peculiarities. Pb5Sb2MnO11 exhibits paramagnetic behavior down to T=5 K with Weiss constant being nearly equal to zero that implies lack of cooperative magnetic interactions.  相似文献   

9.
Although R2O3:MoO3=1:6 (R=rare earth) compounds are known in the R2O3-MoO3 phase diagrams since a long time, no structural characterization has been achieved because a conventional solid-state reaction yields powder samples. We obtained single crystals of R2Mo6O21·H2O (R=Pr, Nd, Sm, and Eu) by thermal decomposition of [R2(H2O)12Mo8O27nH2O at around 685-715 °C for 2 h, and determined their crystal structures. The simulated XRD patterns of R2Mo6O21·H2O were consistent with those of previously reported R2O3:MoO3=1:6 compounds. All R2Mo6O21·H2O compounds crystallize isostructurally in tetragonal, P4/ncc (No. 130), a=8.9962(5), 8.9689(6), 8.9207(4), and 8.875(2) Å; c=26.521(2), 26.519(2), 26.304(2), and 26.15(1) Å; Z=4; R1=0.026, 0.024, 0.024, and 0.021, for R=Pr, Nd, Sm, and Eu, respectively. The crystal structure of R2Mo6O21·H2O consists of two [Mo2O7]2−-containing layers (A and B layers) and two interstitial R(1)3+ and R(2)3+ cations. Each [Mo2O7]2− group is composed of two corner-sharing [MoO4] tetrahedra. The [Mo2O7]2− in the B layer exhibits a disorder to form a pseudo-[Mo4O9] group, in which four Mo and four O sites are half occupied. R(1)3+ achieves 8-fold coordination by O2− to form a [R(1)O8] square antiprism, while R(2)3+ achieves 9-fold coordination by O2− and H2O to form a [R(2)(H2O)O8] monocapped square antiprism. The disorder of the [Mo2O7]2− group in the B layer induces a large displacement of the O atoms in another [Mo2O7]2− group (in the A layer) and in the [R(1)O8] and [R(2)(H2O)O8] polyhedra. A remarkable broadening of the photoluminescence spectrum of Eu2Mo6O21·H2O supported the large displacement of O ligands coordinating Eu(1) and Eu(2).  相似文献   

10.
Twenty-one R2R2N+X -type (R=methyl or ethyl, R=alkyl, X=Br or I) quaternary ammonium (QA) halides have been prepared by using a novel one-pot synthetic route in which a formamide (dimethyl-, diethylformamide, etc.) is treated with alkyl halide in the presence of sodium or potassium carbonate. The formation of QA halides was verified with 1H-NMR, 13C-NMR, MS and elemental analysis. The crystal structures of four QA halides (two bromide and two iodide) were determined using X-ray single crystal diffraction, and the powder diffraction method was used to study the structural similarities between the single crystal and microcrystalline bulk material. The thermal properties of all compounds were studied using TG/DTA and DSC methods. The smallest compounds decomposed during or before melting. The decreasing trend of melting points was observed when the alkyl chain length was increased. The liquid ranges of 120-180 °C were observed for compounds with 5-6 carbon atoms in the alkyl chain. The low melting points and wide liquid ranges suggest potential applicability of these compounds for example as ionic liquids precursors.  相似文献   

11.
The magnetic and transport properties of ternary rare-earth chromium germanides RCr0.3Ge2 (R=Y and Tb-Er) have been determined. X-ray and neutron diffraction studies indicate that these compounds have the CeNiSi2-type structure (space group Cmcm) [1]. Magnetic measurements reveal the antiferromagnetic ordering below TN equal to 18.5 K (R=Tb), 11.8 K (Dy), 5.8 K (Ho) and 3.4 K (Er). From the neutron diffraction data the magnetic structures have been determined. For TbCr0.3Ge2 and DyCr0.3Ge2 at low temperatures the magnetic ordering can be described by two vectors k1=(,0,0) and k2=(,0,), and k1=(,0,0) and k2=(,0,), respectively. In HoCr0.3Ge2 and ErCr0.3Ge2 the ordering can be described by one propagation vector equal to (,,0) and (0,0,0.4187(2)), respectively. In DyCr0.3Ge2 some change in the magnetic ordering is observed at Tt=5.1 K. In temperature range from Tt to TN the magnetic ordering is given by one propagation vector k=(,0,0). YCr0.3Ge2 is a Pauli paramagnet down to 1.72 K which suggests that in the entire RCr0.3Ge2 series the Cr atoms do not carry magnetic moments. All compounds studied exhibit metallic character of the electrical conductivity. The temperature dependencies of the lattice parameters reveal strong magnetostriction effect at the respective Nèel temperatures.  相似文献   

12.
The title compounds have been prepared as polycrystalline powders by thermal treatments of stoichiometric mixtures of R2O3 and MoO3 in air. The room-temperature crystal structure for all the series has been refined from high-resolution neutron powder diffraction data. All the phases are isostructural (space group C2/c, Z=8) with the polymorph α-R2MoO6, typified by Sm2MoO6. The structure contains four zigzag, one-dimensional MoO5 polyhedral rows per unit cell, running through the RO8 polyhedral framework along the [001] direction. MoO5 form discrete units (i.e. do not share common oxygen), with Mo-O distances ranging from 1.77 to 2.24 Å, although the oxygen coordination can be extended to distances of about 3.1 Å, giving rise to strongly distorted MoO8 scalenohedra. Thus, MoO8 and RO8 polyhedra are fully ordered in R2MoO6 compounds, which in fact can be considered as superstructures of fluorite (M3O6), containing 24 MO2 fluorite units per unit cell, with unit-cell parameters related to that of cubic fluorite ( Å). A bond valence study demonstrates that the present crystal structure is especially stable for small rare-earth cations, and becomes more unstable when the R3+ size increases, thus explaining the observed preference of the large rare-earth molybdates for polymorphs β and γ with the same stoichiometry.  相似文献   

13.
The magnetic structures of RSn1+xGe1−x (R=Tb, Dy, Ho and Er, x≈0.1) compounds have been determined by neutron diffraction studies on polycrystalline samples. The data recorded in a paramagnetic state confirmed the orthorhombic crystal structure described by the space group Cmcm. These compounds are antiferromagnets at low temperatures. The magnetic ordering in TbSn1.12Ge0.88 is sine-modulated described by the propagation vector k=(0.4257(2), 0, 0.5880(3)). Tb magnetic moment equals 9.0(1) μB at 1.62 K. It lies in the b-c plane and form an angle θ=17.4(2)° with the c-axis. This structure is stable up to the Nèel temperature equal to 31 K. The magnetic structures of RSn1+xGe1−x, where R are Dy, Ho and Er at low temperatures are described by the propagation vector k=(1/2, 1/2, 0) with the sequence (++−+) of magnetic moments in the crystal unit cell. In DySn1.09Ge0.91 and HoSn1.1Ge0.9 magnetic moments equal 7.25(15) and 8.60(6) μB at 1.55 K, respectively. The moments are parallel to the c-axis. For Ho-compound this ordering is stable up to TN=10.7 K. For ErSn1.08Ge0.92, the Er magnetic moment equals 7.76(7) μB at T=1.5 K and it is parallel to the b-axis. At Tt=3.5 K it tunes into the modulated structure described by the k=(0.496(1), 0.446(4), 0). With the increase of temperature there is a slow decrease of kx component and a quick decrease of ky component. The Er magnetic moment is parallel to the b-axis up to 3.9 K while at 4 K and above it lies in the b-c plane and form an angle 48(3)° with the c-axis. In compounds with R=Tb, Ho and Er the magnetostriction effect at the Nèel temperature is observed.  相似文献   

14.
The crystal chemistry and crystallography of the compounds SrR2CuO5 (Sr-121, R=lanthanides) were investigated using the powder X-ray Rietveld refinement technique. Among the 11 compositions studied, only R=Dy and Ho formed the stable SrR2CuO5 phase. SrR2CuO5 was found to be isostructural with the “green phase”, BaR2CuO5. The basic structure is orthorhombic with space group Pnma. The lattice parameters for SrDyCuO5 are a=12.08080(6) Å, b=5.60421(2) Å, c=7.12971(3) Å, V=482.705(4) Å3, and Z=8; and for the Ho analog are a=12.03727(12) Å, b=5.58947(7) Å, c=7.10169(7) Å, V=477.816(9) Å3, and Z=8. In the SrR2CuO5 structure, each R is surrounded by seven oxygen atoms, forming a monocapped trigonal prism (RO7). The isolated CuO5 group forms a distorted square pyramid. Consecutive layers of prisms are stacked in the b-direction. Bond valence calculations imply that residual strain is largely responsible for the narrow stability of the SrR2CuO5 phases with R=Dy and Ho only. X-ray powder reference diffraction patterns for SrDy2CuO5 and SrHo2CuO5 were determined.  相似文献   

15.
The ternary rare-earth chromium germanides RECrxGe2 (RE=Sm, Gd-Er) have been obtained by reactions of the elements, either in the presence of tin or indium flux, or through arc-melting followed by annealing at 800 °C. The homogeneity range is limited to 0.25?x?0.50 for DyCrxGe2. Single-crystal and powder X-ray diffraction studies on the RECr0.3Ge2 members revealed that they adopt the CeNiSi2-type structure (space group Cmcm, Z=4, a=4.1939(5)-4.016(2) Å, b=16.291(2)-15.6579(6) Å, c=4.0598(5)-3.9876(2) Å in the progression for RE=Sm to Er), which can be considered to be built up by stuffing transition-metal atoms into the square pyramidal sites of a “REGe2” host with the ZrSi2-type structure. (The existence of YbCr0.3Ge2 is also implicated.) Only the average structure was determined here, because unusually short Cr-Ge distances imply the development of a superstructure involving distortions of the square Ge net. Magnetic measurements on RECr0.3Ge2 (RE=Gd-Er) indicated that antiferromagnetic ordering sets in below TN (ranging from 3 to 17 K), with additional transitions observed at lower temperatures for the Tb and Dy members.  相似文献   

16.
17.
Cr2V4O13, a tetravanadate of Cr3+ has been prepared by repeated heating of stoichiometric amounts of Cr2O3 and V2O5 and its crystal structure is refined by Rietveld refinement of the powder XRD data. This compound crystallizes in a monoclinic lattice with unit cell parameters: a=8.2651(3), b=9.2997(3), c=14.5215(5) Å, β=102.618(3)°, V=1089.21(6) Å3 and Z=4 (Space group: P21/c). The U shaped (V4O13)6− formed by corner connected VO4 tetrahedra links the Cr2O10 (dimers of two edge shared CrO6 octahedra) forming a three dimensional network structure of Cr2V4O13. This compound is stable up to 635 °C and peritectically decomposes to orthorhombic CrVO4 and V2O5 above this temperature. A possible long range antiferromagnetic ordering below 10 K is suggested from the squid magnetometry and electron paramagnetic resonance (epr) spectroscopic studies of Cr2V4O13.  相似文献   

18.
RMn2O5 (R=La, Pr, Nd, Tb, Bi) crystallites were prepared by a mild hydrothermal method and characterized by powder X-ray diffraction, scanning electron microscopy, X-ray photoelectron spectroscopy (XPS) and magnetic measurement. The formation of manganates was sensitive to the alkalinities and Mn-containing precursors of the reaction mixtures. This family of manganates is isostructural and has a space group of Pbam. The magnetic measurements for RMn2O5 showed an antiferromagnetic transition. The strong irreversibility between the ZFC and FC curves indicated a helicoidally magnetic structure below 40 K. The max d.c. susceptibilities of LaMn2O5+δ (δ=0.01, 0.06, 0.08, 0.16, 0.17) were found to be variable and the excess oxygen (δ) in the compounds was influenced by the alkalinity used in the hydrothermal synthesis.  相似文献   

19.
The new monoclinic cerium borogermanate Ce6(BO4)2Ge9O22 was synthesized under high-pressure and high-temperature conditions in a Walker-type multianvil apparatus at 10.5 GPa and 1200 °C. Ce6(BO4)2Ge9O22 crystallizes with two formula units in the space group P21/n with lattice parameters a=877.0(2), b=1079.4(2), c=1079.1(2) pm, and β=95.94(3)°. As the parameter pressure favours the formation of compounds with cations possessing high coordination numbers, it was possible to produce simultaneously BO4-tetrahedra and GeO6-octahedra in one and the same borogermanate for the first time. Furthermore, the cerium atoms show high coordination numbers (C.N.: 9 and 11), and one oxygen site bridges one boron and two germanium atoms (O[3]), which is observed here for the first time. Besides a structural discussion, temperature-dependent X-ray powder diffraction data are presented, demonstrating the metastable character of this high-pressure phase.  相似文献   

20.
Pyrolysis of rare earth (R) polyoxomolybdate, [R2(H2O)12Mo8O27xH2O (R=La, Nd and Sm), at 750°C for 2-8 h results in crystallization of R2Mo4O15 compounds. β-La2Mo4O15 crystallizes together with an α-form in monoclinic P21/a (No. 14), a=13.8893(5), b=13.0757(4), c=20.0927(8) Å, β=95.199(2)°, V=3634.1(2) Å3, Z=12, R1(I>2σ(I))=0.048, Rw (all data)=0.116. The structure is built up with {LaOn} (n=9, 10) and {MoOn′} (n′=4-6) polyhedral units. The {LaOn} units are polymerized into a linear {La6O39} chain, while the {MoOn} are connected together to form {Mo4O15} and {Mo7O26} groups. The structure can be related to the α-form by partial rearrangement of O atoms and small shifts of La and Mo atoms. The R2Mo4O15 (R=Nd and Sm) compounds are isomorphous with the previously reported R=Eu and Gd analogs, crystallizing in triclinic, (No. 2), a=9.4989(5) and 9.4076(7), b=11.0088(7) and 10.9583(8), c=11.5665(6) and 11.5234(8) Å, α=104.141(3) and 104.225(3), β=109.838(3) and 109.603(3), γ=108.912(3) and 108.999(3)°, V=987.3(1) and 970.5(1) Å3, Z=3, R1(I>2σ(I))=0.028 and 0.030, Rw (all data)= 0.079 and 0.094, respectively. The crystal structure is composed of {RO8} and {MoOn′} (n′=4-6) polyhedral units. The molybdate units are condensed to give a corrugated {Mo4O17} chain. The square-antiprismatic {RO8} units share their trigonal and square faces, forming {R2O13} and {R2O12} groups, respectively. A very short R?R distance (3.557(6) Å for R=Nd; 3.4956(6) Å for R=Sm) is achieved in the latter unusual {R2O12} group. A common cationic arrangement was found in all the structures in the R2Mo4O15 family: a R-R pair with the shortest separation and surrounding 12 Mo atoms. The symmetry of the cationic arrangement was reduced with an increase of atomic number of R, viz. La>Ce, Pr>Nd-Gd≈Tb, Ho.  相似文献   

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