共查询到20条相似文献,搜索用时 46 毫秒
1.
The Gd5Si2.75P1.25 phase with all interslab Si/P–Si/P dimers broken (Sm5Ge4-type structure) undergoes a ferromagnetic transition at 184 K. For this phase, the magnetocaloric effect in terms of the magnetic entropy change, ΔS, reaches the maximum value of −7.8 J/kg K at 177 K. Absence of a temperature-dependant structural transition, as confirmed by the low-temperature single crystal diffraction studies, together with the moderate value of ΔS points to the presence of a conventional magnetocaloric effect. Gd5Si3.5P0.5 and Gd5Si3.25P0.75, which are composites of the Gd5Si4- (all Si/P–Si/P dimers intact) and Sm5Ge4-type phases, possess two magnetic transitions associated with the two-phases. Introduction of P into Gd5Si4 lowers the Curie temperature from 336 K to 332 K in Gd5Si3.25P0.75. 相似文献
2.
Sumohan Misra 《Journal of solid state chemistry》2006,179(8):2290-2297
A crystallographic study of the Si/Ge site preferences in the Si-rich regime of Gd5(SixGe1−x)4 and a crystal chemical analysis of these site preferences for the entire range is presented. The room temperature crystal structure of Gd5Si4 as well as four pseudobinary phases, Gd5(SixGe1−x)4 for x?0.6, is reported. All structures are orthorhombic (space group Pnma), Gd5Si4-type and show decreasing volume as the Si concentration increases. Refinements of the site occupancies for the three crystallographic sites for Si/Ge atoms in the asymmetric unit reveal a nonrandom, but still incompletely ordered arrangement of Si and Ge atoms. The distribution of Si and Ge atoms at each site impacts the fractions of possible homonuclear and heteronuclear Si-Si, Si-Ge and Ge-Ge dimers in the various structures. This distribution correlates with the observed room temperature crystal structures for the entire series of Gd5(SixGe1−x)4. 相似文献
3.
Tae-Soo You Jing-Tai Zhao Rainer Pttgen Walter Schnelle Ulrich Burkhardt Yuri Grin Gordon J. Miller 《Journal of solid state chemistry》2009,182(9):2430-2442
Two isoelectronic series, Eu(Ga1−xTtx)2 (Tt=Si, Ge, 0≤x≤1), have been synthesized and characterized by powder and single-crystal X-ray diffraction, physical property measurements, and electronic structure calculations. In Eu(Ga1−xSix)2, crystal structures vary from the KHg2-type to the AlB2-type, and, finally, the ThSi2-type structure as x increases. The hexagonal AlB2-type structure is identified for compositions 0.18(2)≤x<0.70(2) with Ga and Si atoms statistically distributed in the polyanionic 63 nets. As smaller Si atoms replace Ga atoms while the number of valence electrons increases, the lattice parameters, unit cell volumes, and Ga–Si distances in this phase region decrease significantly. Although aspects of X-ray diffraction results suggest puckering of the 63 nets for the Si-richest example of the AlB2-type Eu(Ga1−xSix)2, the complete experimental evidence remains inconclusive. On the other hand, in Eu(Ga1−xGex)2, six different structural types were observed as x varies. In addition to EuGa2 (KHg2-type; space group Imma) and EuGe2 (own structure type, space group P3¯m1), the ternary phases studied show four different structures: the AlB2-type for Ga-rich compositions; the YPtAs-type structure for EuGaGe; and two new structures, which are intergrowths of the YPtAs-type EuGaGe and EuGe2, for Ge-rich compositions. These two Ge-rich phases include: (1) Eu(Ga0.45(2)Ge0.55(2))2 containing two YPtAs-type motifs of EuGaGe plus one EuGe2 motif; and (2) Eu(Ga0.40(2)Ge0.60(2))2 containing one YPtAs-type motif alternating with a split site at and z=0.4798(2) with ca. 50% site occupancy by Ga and Ge along the c-axis. Magnetic susceptibilities of three Eu(Ga1−xGex)2 compounds display Curie–Weiss behavior above ca. 100 K, and show effective magnetic moments indicative of divalent Eu with a 4f7 electronic configuration, consistent with. X-ray absorption spectra (XAS). Density of states (DOS) and crystal orbital Hamilton population (COHP) analyses, based on first principles electronic structure calculations, rationalize the observed homogeneity ranges of the AlB2-type phases in both systems and the structural variations as a function of Tt content. 相似文献
4.
The structure of Ni0.85Mo6Te8 was refined from single-crystal X-ray diffraction data at room temperature. It is triclinic, space group
; 1619 reflections, 75 refined parameters, R = 0.031. The Mo atoms form distorted octahedral clusters (2.69 Å ≤ dintra[Mo---Mo] ≤ 2.81 Å; 3.58 Å < dinter[Mo---Mo]). The Ni atoms are disordered (site occupancy: 0.423(7); d[Ni---Ni] = 2.586(6) Å), and interact strongly with one Mo6 cluster (d[Ni---Mo] = 2.603(3) and 2.958(3) Å), and weakly with another (d[Ni---Mo] = 2.985(3) Å). The structure transforms at 1057(5) K into a rhombohedral modification (ahex = 10.457(2) Å, chex = 11.866(3) Å at 1073 K). Measurements on powders suggest metallic conductivity (5.1 × 10−4 Ω-cm at 293 K) and weakly temperature-dependent paramagnetism (110 × 10−6 emu/g at 100 K). 相似文献
Full-size image
5.
Long-standing uncertainty on the structure type of Na2Ge4O9has been resolved. Sodium tetragermanate has been grown by crystallization from a supercooled melt and its single-crystal X-ray structure has been determined (R=0.022). Sodium tetragermanate is trigonal witha=11.3234(12),c=9.6817(9) Å, space groupP
c1,Z=6, andDx=4.451 g cm−3. The structure is comprised of a mixed tetrahedral–octahedral framework with three-membered [Ge3O9] rings of GeO4tetrahedra interconnected by isolated GeO6octahedra via shared corners and isA2Ge4O9-type. Bond distances and angles for GeO4tetrahedra and GeO6octahedra are very similar to the corresponding values in the type structure of K2Ge4O9, the two structures differing mainly in the accommodation of the smaller (medium–large-sized) Na cation, which is now in a 5+2 coordination. The structure–composition relationships of wadeite-type,A2Ge4O9-type, and Na2Si4O9-type structures of germanates and silicates depend largely on theT–O distance and the size of the monovalent cation. We confirm that sodium tetragermanate is a metastable phase at all pressures up to 2 kbar, the stable assemblage for the Na2Ge4O9composition being sodium enneagermanate (Na4Ge9O20) plus a more sodic phase. 相似文献
6.
C. Ma J.-Q. Yan K.W. Dennis R.W. McCallum X. Tan 《Journal of solid state chemistry》2009,182(11):3013-3020
Polycrystalline samples of the Lu1−xLaxMn2O5 solid solution system were synthesized under moderate conditions for compositions with x up to 0.815. Due to the large difference in ionic size between Lu3+ and La3+, significant changes in lattice parameters and severe lattice strains are present in the solid solution. This in turn leads to the composition dependent thermal stability and magnetic properties. It is found that the solid solution samples with x≤0.487 decompose at a single well defined temperature, while those with x≥0.634 decompose over a temperature range with the formation of intermediate phases. For the samples with x≤0.487, the primary magnetic transition occurs below 40 K, similar to LuMn2O5 and other individual RMn2O5 (R=Bi, Y, and rare earth) compounds. In contrast, a magnetic phase with a 200 K onset transition temperature is dominant in the samples with x≥0.634. 相似文献
7.
Crystal structure, redox, and magnetic properties for the Pr1−xSrxFeO3−δ solid-solution phase have been studied. Oxidized samples (prepared in air at 900°C) crystallize in the GdFeO3-type structure for 0≤x≤0.80, and probably in the Sr8Fe8O23-type (unpublished) structure for x=0.90. Reduced samples (containing virtually only Fe3+) crystallize as the perovskite aristotype for x=0.50 and 0.67 with randomly distributed vacancies. The Fe4+ content increases linearly in the oxidized samples up to x≈0.70, whereupon it stabilizes at around 55%. Antiferromagnetic ordering of the G type is observed for oxidized samples (0≤x≤0.90) which show decreasing Néel temperature and ordered magnetic moment with increasing x, while the Néel temperature is nearly constant at 700 K for reduced samples. Electronic transitions for iron from an average-valence state via charge-separated to disproportionated states are proposed from anomalies in magnetic susceptibility curves in the temperature ranges 500–600 K and 150–185 K. 相似文献
8.
Alexey Lipatov Alexander Gribanov Andriy Grytsiv Peter Rogl Elena Murashova Yurii Seropegin Gerald Giester Konstantin Kalmykov 《Journal of solid state chemistry》2009,182(9):2497-2509
Phase relations in the ternary system Ce–Pd–Si have been established for the isothermal section at 800 °C based on X-ray powder diffraction and EMPA techniques on about 130 alloys, which were prepared by arc-melting under argon or powder reaction sintering. Eighteen ternary compounds have been observed to participate in the phase equilibria at 800 °C. Atom order was determined by direct methods from X-ray single-crystal counter data for the crystal structures of τ8—Ce3Pd4Si4 (U3Ni4Si4-type, Immm; a=0.41618(1), b=0.42640(1), c=2.45744(7) nm), τ16—Ce2Pd14Si (own structure type, P4/nmm; a=0.88832(2), c=0.69600(2) nm) and also for τ18—CePd1−xSix (x=0.07; FeB-type, Pnma; a=0.74422(5), b=0.45548(3), c=0.58569(4) nm). Rietveld refinements established the atom arrangement in the structures of τ5—Ce3PdSi3 (Ba3Al2Ge2-type, Immm; a=0.41207(1), b=0.43026(1), c=1.84069(4) nm) and τ13—Ce3−xPd20+xSi6 (0≤x≤1, Co20Al3B6-type, Fm3¯m; a=1.21527(2) nm). The ternary compound Ce2Pd3Si3 (structure-type Ce2Rh1.35Ge4.65, Pmmn; a=0.42040(1), b=0.42247(1), c=1.72444(3) nm) was detected as a high-temperature compound, however, does not participate in the equilibria at 800 °C. Phase equilibria in Ce–Pd–Si are characterized by the absence of cerium solubility in palladium silicides. Mutual solubility among cerium silicides and cerium–palladium compounds are significant whereby random substitution of the almost equally sized atom species palladium and silicon is reflected in extended homogeneous regions at constant Ce-content such as for τ2—Ce(PdxSi1−x)2 (AlB2-derivative type), τ6—Ce(PdxSi1−x)2 (ThSi2-type) and τ7—CePd2−xSi2+x. The crystal structures of compounds τ4—Ce~8Pd~46Si~46, τ12—Ce~29Pd~49Si~22, τ15—Ce~22Pd~67Si~11, τ17—Ce~5Pd~77Si~18 and τ18—CePd1−xSix (x~0.1) are still unknown. 相似文献
9.
Hiroshi Fukuoka Kazuya Baba Mayumi Yoshikawa Fumiko Ohtsu Shoji Yamanaka 《Journal of solid state chemistry》2009,182(8):2024-2029
A series of lanthanide penta-germanides LnGe5 (Ln=Ce, Pr, Nd and Sm) has been prepared by high-pressure (5–13 GPa) and high-temperature (500–1200 °C) reaction. CeGe5 crystallizes in an orthorhombic unit cell (S.G. Immm (71)) with a=4.000(5) Å, b=6.192(5) Å, c=9.86(1) Å, and V=244.1(5) Å3. The new germanides are isotypic with LaGe5 consisting of a Ge covalent network with tunnels where guest ions Ln3+ are situated. The network is composed of sublayers with edge-sharing Ge six-membered rings with only boat conformation. The sublayers are connected by rare eight-coordinated Ge atoms. The cell volume of the compounds systematically decreases from La to Sm compounds, except for CeGe5, owing to the lanthanide contraction. The lattice constants of CeGe5 are smaller than those of the Pr compound because it contains Ce4+ ions. CeGe5 is paramagnetic above 2 K, but does not obey the Curie–Weiss law. PrGe5 and NdGe5 are Curie–Weiss type paramagnets with Weiss temperatures of –3.3 and –18.4 K. SmGe5 shows an antiferromagnetic transition at 10.4 K. 相似文献
10.
Dy5Ni0.66Bi2.34 and Lu5Ni0.56Sb2.44 were synthesized by arc-melting and were found to adopt an orthorhombic Yb5Sb3-type structure. Cell parameters are a = 12.075(2), b = 9.165(2), c = 8.072(1) Å for Dy5Ni0.66Bi2.34 and a = 11.6187(9), b = 8.933(1) and c = 7.8377(6) Å for Lu5Ni0.56Sb2.44. Dy5Ni0.66Bi2.34 undergoes a step-like ferromagnetic transition around 66 K. Magnetocaloric effect in terms of the magnetic entropy change, ΔS, reaches −3.73 J/kg K at 75 K for Dy5Ni0.66Bi2.34. 相似文献
11.
Li13Cu6Ga21 crystallizes in a cubic structure, space group Im3, with a = 13.568(2) Å, Z = 4. Diffraction data were collected on a NONIUS CAD 4 diffractometer in the range 4 ≤ 2θ ≤ 50° (MoKα radiation). The structure was solved by direct methods and refined by full-matrix least-squares to a final R(F) = 0.033 for 346 independent reflections with I> 3σ(I). Li13Cu6Ga21 presents an interesting structure composed of Samson's polyhedral clusters (104 atoms) linked to each other through smaller junction polyhedral clusters (truncated tetrahedra and hypho-13-vertex polyhedra) containing lithium atoms in their centers. 相似文献
12.
S.N. Achary 《Journal of solid state chemistry》2007,180(1):84-91
The results of variable temperature powder X-ray diffraction and differential thermal analysis (DTA) studies on the orthorhombic (α) low-cristobalite to cubic (β) high-cristobalite phase transition for Al1−xGaxPO4, (0.00?x?1.00) are presented. These studies reveal that all these compositions undergo reversible phase transitions from orthorhombic to cubic form at higher temperature. The high-temperature behavior of GaPO4 is observed to have a different behavior compared to all other compositions in this series. Orthorhombic low-cristobalite-type GaPO4 transforms to cubic high-cristobalite form at ∼605 °C. Above ∼700 °C, the cubic high-cristobalite-type GaPO4 slowly transforms to trigonal quartz type structure. At about 960 °C, the quartz type GaPO4 transforms back to the cubic high-cristobalite form. During cooling cycles the cubic phase of GaPO4 reverts to trigonal quartz type phase. However, annealing of GaPO4 at higher temperatures for longer duration can stabilize the orthorhombic low cristobalite phase. The phase transition temperatures and associated enthalpies are related to the change in unit cell volume and the orthorhombicity of the respective low cristobalite lattice. 相似文献
13.
Artem M. Abakumov Joke Hadermann Alexander A. Tsirlin Haiyan Tan Jo Verbeeck Haitao Zhang Evgeny V. Dikarev Roman V. Shpanchenko Evgeny V. Antipov 《Journal of solid state chemistry》2009,182(8):2231-2238
The crystal structure of the Pb4Mn9O20 compound (previously known as “Pb0.43MnO2.18”) was solved from powder X-ray diffraction, electron diffraction, and high resolution electron microscopy data (S.G. Pnma, a=13.8888(2) Å, b=11.2665(2) Å, c=9.9867(1) Å, RI=0.016, RP=0.047). The structure is based on a 6H (cch)2 close packing of pure oxygen “h”-type (O16) layers alternating with mixed “c”-type (Pb4O12) layers. The Mn atoms occupy octahedral interstices formed by the oxygen atoms of the close-packed layers. The MnO6 octahedra share edges within the layers, whereas the octahedra in neighboring layers are linked through corner sharing. The relationship with the closely related Pb3Mn7O15 structure is discussed. Magnetization measurements reveal a peculiar magnetic behavior with a phase transition at 52 K, a small net magnetization below the transition temperature, and a tendency towards spin freezing. 相似文献
14.
Volodymyr Svitlyk 《Journal of solid state chemistry》2008,181(5):1080-1086
Nickel was successfully introduced into the Gd5Sb3 and Gd5Bi3 binaries to yield the Gd5Ni0.96(1)Sb2.04(1) and Gd5Ni0.71(1)Bi2.29(1) phases. Both Ni-substituted compounds adopt the orthorhombic Yb5Sb3-type structure. While the Gd5Ni0.71Bi2.29 phase is thermodynamically stable at 800 °C and decomposes at lower temperatures upon annealing, it can be easily quenched to room temperature by rapid cooling from 800 °C. The Gd5Ni0.96Sb2.04 phase is found to be thermodynamically stable till room temperature. Through annealing at different temperatures, Gd5Bi3 was proven to undergo the Mn5Si3-type (LT)↔Yb5Sb3-type (HT) transformation reversibly, whereas Gd5Sb3 was found to adopt only the hexagonal Mn5Si3-type structure. Orthorhombic Gd5Ni0.96Sb2.04 and Gd5Ni0.71Bi2.29 and low-temperature hexagonal Gd5Bi3 order ferromagnetically at 115, 162 and 112 K, respectively. In Gd5Bi3, the ferromagnetic ordering is followed by spin reorientation below 64 K. Magnetocaloric effect in terms of ΔS was evaluated from the magnetization data and found to reach the maximum values of −7.7 J/kgK for Gd5Ni0.96Sb2.04 and −5.6 J/kgK for Gd5Ni0.71Bi2.29 around their Curie temperatures. 相似文献
15.
The new ternary phases Zr4−xTa1+xGe4 (0.1<x<0.4) and Zr2+xTa3−xGe4 (0.1<x<1.1) were prepared from the elements by arc melting and subsequent induction heating at 1400–1450°C. Single-crystal X-ray diffraction was used to determine their structures and to refine mixed site occupancies. Zr4−xTa1+xGe4 was found to crystallize in the monoclinic space group P21/c (structure type: U2Mo3Si4) and the compound Zr2−xTa3−xGe4 shows orthorhombic symmetry (space group Pnma, structure type: Sm5Ge4). The close structural relationship between the two structures is discussed. Both phases exhibit pronounced differential fractional site occupancy of Ta and Zr on the metal sites and considerable composition ranges. Extended Hückel calculations were performed for various site occupancy models and Mulliken overlap populations for the different lattice sites of each structure were calculated for these models. The correlation of the cumulated Mulliken overlap populations and the atomic orbital populations with the actual site occupancies is discussed. 相似文献
16.
Ce6−xDyxMoO15−δ (0.0 ≤ x ≤ 1.8) were synthesized by modified sol–gel method. Structural and electrical properties were investigated by means of X-ray diffraction (XRD), Raman, X-ray photoelectron spectroscopy (XPS) and electrochemical impedance spectroscopy (EIS). The XRD patterns showed that the materials were single phase with a cubic fluorite structure. Impedance spectroscopy measurement in the temperature range between 350 °C and 800 °C indicated a sharp increase in conductivity for the system containing small amount of Dy2O3. The Ce5.6Dy0.4MoO15−δ detected to be the best conducting phase with the highest conductivity (σt = 8.93 × 10−3 S cm−1) is higher than that of Ce5.6Sm0.4MoO15−δ (σt = 2.93 × 10−3 S cm−1) at 800 °C, and the corresponding activation energy of Ce5.6Dy0.4MoO15−δ (0.994 eV) is lower than that of Ce5.6Sm0.4MoO15−δ (1.002 eV). 相似文献
17.
D. M. De Leeuw C. A. H. A. Mutsaers G. P. J. Geelen C. Langereis 《Journal of solid state chemistry》1989,80(2)
The subsolidus phase diagram of the system La2O3---SrO---CuO at 950°C under 1 bar of pure oxygen has been investigated and a new ternary compound, La1+xSr2−xCu2O5.5+δ with 0.05 ≤ x ≤ 0.15, was isolated. This compound crystallizes in an orthorhombic unit cell with lattice constants related to the lattice constant of the perovskite cubic unit cell, ap, by a = 3.80 Å ap, B = 11.48 Å 3ap, and c = 20.23 Å 5ap. The structure is isotypical to that of LnSr2Cu2O5.5+δ with Ln = Sm, Eu, or Gd. Reported data on the crystal chemistry of the equilibrium compounds in the system La2O3---SrO---CuO have been summarized and compared with the present data. The structure of all compounds is built up of a La---O rock-salt layer separated by a number of LaCuO3 perovskite layers. The general formula is (La---O)(LaCuO3)n where La can be replaced either partly or completely by Sr. Compounds are found for n = 1, 2, and ∞. The structures of the compounds show different types of oxygen vacancy ordering. 相似文献
18.
Two new series of ternary gallides with the chemical formulas (La, Ce, Pr, Nd, Sm)AuxGa4−x and (La, Ce, Pr, Nd, Sm)Au1.5Ga2.5 were synthesized from the elements by arc melting. From X-ray powder diffraction analysis the REAuxGa4−x series of compounds was found to be isotypic and crystallize with BaAl4 type of structure; the homogeneity range at 600°C of each of the REAuxGa4−x phases was established, revealing remarkable deviations from Vegard's rule. At 600°C the REAuxGa4−x phases of the BaAl4 type were observed to be in thermodynamic equilibrium with a structure variant crystallizing at the composition REAu1.5Ga2.5 and with a narrow homogeneous range. In case of PrAu1.5Ga2.5 the structure type was refined from X-ray single-crystal counter data (CaBe2Ge2 type, space group P4/nmm, Rw = 0.046). Gold and gallium atoms were generally found on separate crystallographic sites; however, a statistical distribution of 51% Au + 49% Ga was derived for the 2b sites. 197Au Mössbauer spectroscopy confirmed the occupational mode of the gold atoms in CeAu1.5Ga2.5. For the BaAl4-type phases, X-ray powder and Mössbauer data revealed preferential occupation of the 4e sites of I4/mmm by gold atoms; practically no Au was observed on the 4d sites. Magnetic susceptibilities were determined over a temperature range extending from 2 to 1100 K. Above liquid nitrogen temperatures the paramagnetic behavior of the (Ce, Pr, Nd)AuxGa4−x and the (Ce, Pr, Nd)Au1.5Ga2.5 compounds is characterized by magnetic moments close to the ideal trivalent rare earth values. Lanthanum compounds are diamagnetic, whereas SmAuxGa4−x alloys are characterized by a typical Van Vleck-type paramagnetism of closely spaced multiplets. At very low temperatures onset of ferromagnetic ordering is observed for the BaAl4-type series of compounds. No superconductivity was encountered down to 2 K. 相似文献
19.
The structural and magnetic properties of a new ternary Ir-Mn-Ge phase, Mn3IrGe, as well as the solid solution Mn3Ir(Si1−xGex), 0?x?1, have been investigated by means of X-ray and neutron powder diffraction, magnetization measurements and first principles calculations. The crystal structure is cubic, of the AlAu4-type (an ordered form of the β-Mn structure), Z=4, space group P213, and the unit-cell dimension varies linearly with the silicon content. For all compositions, antiferromagnetic ordering is found below a critical temperature of about 225 K. The magnetic structure is noncollinear, as a result of frustrated magnetic interactions on a triangular network of Mn atoms, on which the moments rotate 120° around the triangle axes. The magnitude of the magnetic moment at 10 K is 3.39(4) μB for Mn3IrGe. The theoretical calculations reproduce with very good accuracy the magnitudes as well as the directions of the experimentally observed magnetic moments. 相似文献
20.
S. Vilminot M. Richard-Plouet G. Andr D. Swierczynski M. Guillot F. Boure-Vigneron M. Drillon 《Journal of solid state chemistry》2003,170(2):194-264
Cu3(OH)4SO4, obtained by hydrothermal synthesis from copper sulfate and soda in aqueous medium, is isostructural with the corresponding antlerite mineral, orthorhombic, space group Pnma (62), with a=8.289(1) b=6.079(1) and c=12.057(1) Å, V=607.5(2) Å3, Z=4. Its crystalline structure has been refined from X-ray single crystal and powder neutron diffraction data at room temperature. It consists of copper (II) triple chains, running in the b-axis direction and connected to each other by sulfate groups. The magnetic structure, solved from powder neutron diffraction data at 1.4 K below the transition at 5 K evidenced by susceptibility and specific measurements, reveals that, inside a triple chain, the magnetic moments of the copper ions (μB=0.88(5) at 1.4 K) belonging to outer chains are oriented along the c-axis of the nuclear cell, with ferromagnetic order inside a chain and antiferromagnetic order between the two outer chains. No long-range magnetic order is obtained along the central chain with an idle spin behavior. 相似文献