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1.
The complex perovskite BiMn7O12 occurs with two polymorphic structures, cubic and monoclinic. Currently their crystal structures are investigated with high-resolution synchrotron powder X-ray diffraction at room temperature. Rietveld analysis reveals unusual behavior for, respectively, the oxygen and bismuth atoms in the monoclinic and cubic phases. Bond valence calculations indicate that all the Mn atoms in both the phases are in trivalent state. Possible roles of the 6s2 lone-pair electrons of Bi3+ in BiMn7O12 are discussed in comparison with the LaMn7O12 phase that is isomorphic to monoclinic BiMn7O12. Multiple roles of the lone-pair electrons are revealed, causing (i) A-site cation deficiency, (ii) octahedral tilting, (iii) A-site cation displacement, and (iv) Mn3+ Jahn-Teller (JT) distortion. Relationships between the monoclinic and cubic phases are discussed with emphasis on the MnO2 and MnO6 local structural aspects. All Mn atoms in the monoclinic polymorph have distorted coordination consistent with JT-active Mn(III) high spin, whereas for the cubic polymorph, the B-site Mn atoms show regular octahedral coordination.  相似文献   

2.
Mn3+ ion (3d4, t32geg1) is liable to induce, by a cooperative Jahn-Teller effect, a macroscopic distortion of the cubic spinel structure; it is so in haussmanite Mn3O4, a tetragonal structure. The effect of chemical composition on “tetragonal-cubic” spinel transformations in the system Mn3O4Mn2SnO4 has been studied by X-ray diffraction; the c and a′ unit-cell dimensions show an abrupt change at a critical composition beyond which the system has the cubic spinel structure. The cation distribution has been worked out from an analysis of the X-ray diffraction intensities, and a correlation between the number of Mn3+ ions in octahedral sites and the degree of distortion has been obtained; the values of “cation-anion” bond distances, in six coordination, show that, in this system, the oxygen octahedral distortion and the macroscopic cell distortion are in a direct relationship. The paramagnetic study always attributes the “high-spin” configuration t32geg1 to manganese.  相似文献   

3.
Comparison between synthesis in air and in vacuum of the solid solution tMn3O4 + (1 ? t)Zn2SnO4, and cristallographic study of the nonoxidized compounds allowed us to establish the distribution and the electronic configuration of cations in tetrahedral (A) and octahedral (B) sites. The competitive aspect of Zn2+ and Mn2+ ions to occupy tetrahedral site is discussed. In air, the non-oxidizable character of Mn2+ on an A-site is clearly borne out, whereas the B-site displaced manganese oxidizes to Mn3+. In vacuum, the critical concentration of Mn3+ ion at the octahedral site, involving a cooperative Jahn-Teller effect, is about 50%.An important fact has also been put forward: the microscopic distortion of the oxygen octahedra, which the ratio of long and short anion-cation distances expresses, is equal to the unit-cell macroscopic deformation that the ratio ca√2 represents.  相似文献   

4.
Pyrochlore oxides of the type Mn2Sb2O7 and (Mn1?xCdx)2Sb2O7 have been synthesized by high-temperature solid state reactions and characterized by X-ray diffraction and chemical analysis. X-Ray diffraction studies showed that the compound Mn2Sb2O7 has a rhombohedrally distorted pyrochlore structure. In the solid solutions (Mn1?xCdx)2Sb2O7, the phases with x ≥ 0.6 are cubic. Magnetic and 121Sb Mössbauer studies indicate that all the Mn and Sb are present in the +2 and +5 state occupying A and B sites, respectively, in the pyrochlore structure. Electrical measurements indicate that the compounds are insulators or semiconductors exhibiting p-type behavior. The stoichiometry and probable cause of the rhombohedral distortion in Mn2Sb2O7 and solid solutions are discussed.  相似文献   

5.
Single crystals of [ThCu3](Mn3+2Mn4+2]O12, a ferrimagnetic perovskite-like compound, have been synthesized by hydrothermal conditions at 600°C and 2 kbar. They have been found to be cubic, of space group Im3, with a = 7.359 Å, and isostructural with [NaMn3](Mn3+2Mn4+2)O12. The crystal structure has been refined by single-crystal X-ray diffraction data. The Th4+ cations are surrounded by slightly distorted icosahedra; the ThO distance is 2.556 Å. The Cu2+ cations are also surrounded by 12 oxygens, which are arranged as three mutually perpendicular rectangles of different size, the smallest and the largest of which are almost squares. The three sets of CuO distances are 1.973, 2.800, and 3.238 Å. The octahedral MnO distance is 1.950 Å. A test based on neutron diffraction powder data indicated that the square sites are occupied by only the Cu2+ cations.  相似文献   

6.
The structural properties of the YMexMn1−xO3 (Me=Cu, Ni, Co) pseudobinary oxides have been studied by X-ray diffraction and electrical measurements. The powders were prepared by solid state reaction between the corresponding oxides. The incorporation in solid solution of small divalent cations, Cu2+, Ni2+, and Co2+, substituting for Mn in the hexagonal YMnO3 compound, leads to a phase transition in which a perovskite-type structure is formed. The amount of substituting cation necessary for such a transition depends on the cation nature and, to a small extent, on the ionic radius. The phase transition depends strongly on the progressive substitution of the Jahn-Teller Mn3+ cation and therefore of the cooperative Jahn-Teller interaction weakness. The steric influence plays a secondary role, as is shown by the very small variation of the tolerance factor, t, as a function of the cation content. The solid solutions with perovskite-type structure show semiconducting behavior. The conductivity mechanism is of a thermally activated small polaron hopping.  相似文献   

7.
A new Cd-containing transition metal Zintl phase, Sr11Cd6Sb12, was obtained from a direct element combination reaction using the Sn flux method. Its structure was determined using single-crystal X-ray diffraction methods. It crystallizes in the monoclinic space group C2/m with a=32.903(3) Å, b=4.7666(5) Å, c=12.6057(13) Å, β=109.752(2)°, and Z=2. Sr11Cd6Sb12 has a one-dimensional infinite chain structure consisting of double pentagonal tubes, where Sr2+ cations reside both within two tubes and between the infinite chains of tubes. The anionic framework [Cd6Sb12]22− has features similar to those of Eu10Mn6Sb13. The difference in Eu10Mn6Sb13 is that its double pentagonal tubes are further condensed to form two-dimensional layers.  相似文献   

8.
The cation ordering in the fluorite-like transparent conductors In4+xSn3−2xSbxO12 and In6TeO12, was investigated by Time of Flight Neutron Powder Diffraction and X-ray Powder Diffraction (tellurate). The structural results including atomic positions, cation distributions, metal-oxygen distances and metal-oxygen-metal angles point to a progressive cation ordering on both sites of the Tb7O12-type structure with a strong preference of the smaller 4d10 cations (Sn4+, Sb5+, Te6+) for the octahedral sites. The corresponding increase of the overall structure-bonding anisotropy is analyzed in terms of the crystal chemical properties of the OM4 tetrahedral network of the antistructure. The relationships between the M7O12 and the M2O3 bixbyite-type structures are explored. Within the whole series of compositions In4+xM3−xO12 (M=Sn, Sb, Te) there exists an increase of the symmetry gap between the more symmetrical bixbyite structure and the M7O12 type. This is tentatively correlated with the progressive weakening of thermal stability of these compositions from Sn to Te via Sb.  相似文献   

9.
The crystal structure of the A-site deficient perovskite Ln1/3NbO3 (Ln=Nd, Pr) at room temperature has been determined, for the first time, as orthorhombic in space group Cmmm using high-resolution neutron powder diffraction. Pertinent features are the alternation of unoccupied layers of A-sites and layers partly occupied by Ln cations, as well as out-of-phase tilting of the NbO6 octahedra around an axis perpendicular to the direction of the cation/vacancy ordering. The phase transition behaviour of Nd1/3NbO3 has also been studied in situ. This compound undergoes a continuous phase transition at around 650 °C to a tetragonal structure in space group P4/mmm due to the disappearance of the octahedral tilting. The analysis of spontaneous strains shows that this phase transition is tricritical in nature.  相似文献   

10.
Single crystals of CaCu3Mn4O12, a new ferromagnetic perovskite-like compound (Tc ? 160°C), have been synthesized at 50 kbar and 1000°C. By X-ray analysis it was found to be cubic (a = 7.241Å), space groupIm3 with two molecules per unit cell. The two Ca2+ and six Cu2+ cations occupy theA sites of the ideal perovskite structure, while the eight Mn4+ cations occupy theB sites. In theIm3 space group the sites occupied by the calcium and copper cations have different point symmetry and therefore the 12-oxygen polyhedra have different distortions. The Ca cations are surrounded by slightly distorted icosahedra, the CaO distance is 2.562Å. The twelve oxygens around the copper cations are arranged as three mutually perpendicular rectangles of different size, the smallest and the largest of which are almost squares. The three sets of Cu0 distances are 1.942, 2.707, and 3.181Å, respectively. The octahedral Mn0 distance is 1.915Å. This arrangement is similar to that found in NaMn3Mn4O12. A comparison between the two structures and a discussion of their thermal data are given.  相似文献   

11.
By using neutron diffraction together with anomalous dispersion X-ray diffraction, it has been possible to ascertain the distribution of close atomic numbered cations in CoMnxFe2?xO4 system spinels.At 950°C, these compounds have a cubic structure in the range 0 ? x ? 1.25 and exhibit a macroscopic tetragonal distortion as soon as 60% of the Mn3+ ions occupy octahedral sites.The great mobility of cobalt between both types of sites has been pointed out; it can be related to oxidation and reduction phenomena. In these compounds, Fe3+ iron remains neutral towards four or six coordinences.  相似文献   

12.
The structural and magnetic properties of Ta-doped Ca4Mn3−xTaxO10 (0≤x≤0.3) compounds have been investigated. Structural refinement indicates that the Ta doping maintains the orthorhombic layered perovskite structure with space group Pbca as Ca4Mn3O10 but induces an increase in both unit cell volume and octahedral distortion. The magnetization measurements reveal that the magnetization first increases and reaches to maximum for the x=0.1 sample and then gradually decreases with the increase of Ta content. There appear short-range ferromagnetic (FM) clusters in all the doped samples, which are caused by the double-exchange interaction between Mn4+ and Mn3+ that is induced by the charge compensation effect. As x is higher than 0.1, the overall results show evidence for the gradual appearance of a cluster glass behavior. When x increases to 0.3, the long-range antiferromagnetic (AFM) ground state is melted into the short-range magnetically ordered regions due to the increase of Ta5+ and Mn3+ at the expense of Mn4+. The competition between AFM regions and FM clusters makes the short-range magnetic components frustrate when the temperature falls to a frustrating point, and thus cluster glass transition occurs.  相似文献   

13.
Zn7Sb2O12 forms a full range of Co-containing α solid solutions, Zn7−xCoxSb2O12, with an inverse-spinel structure at high temperature. At low temperatures for x<2, the solid solutions transform into the low temperature β-polymorph. For x=0, the βα transition occurs at 1225±25 °C; the transition temperature decreases with increasing x. At high x and low temperatures, α solid solutions are formed but are non-stoichiometric; the (Zn+Co):Sb ratio is >7:2 and the compensation for the deficiency in Sb is attributed to the partial oxidation of Co2+ to Co3+. From Rietveld refinements using ND data, Co occupies both octahedral and tetrahedral sites at intermediate values of x, but an octahedral preference attributed to crystal field stabilisation, causes the lattice parameter plot to deviate negatively from the Vegard's law. Sub-solidus compatibility relations in the ternary system ZnO-Sb2O5-CoO have been determined at 1100 °C for the compositions containing ?50% Sb2O5.  相似文献   

14.
The crystal structure of the deficient perovskite Nd1/3NbO3 has been determined at room temperature, using electron microscopy observations and X-ray powder diffraction data. Electron diffraction study and HRTEM images evidence the doubling of a and b parameters and confirm one of the c-parameter with respect to perovskite unit cell. The structure refinement has been carried out using the orthorhombic Cmmm space group. The Nd3+ cations occupy randomly and alternatively the (00l) planes and Nb5+ cations distorted octahedral sites. This structure is characterized by the presence of empty Nd3+ sites in every second layer and the tilting of the octahedral sites around the b-axis. In addition, electron diffraction patterns exhibit weak additional diffuse reflections, which supposes some ordering of the Nd vacancies.  相似文献   

15.
At a temperature above 1173 K the spinel-type structure of CdHo2S4 becomes unstable and gradually transforms into a rocksalt-type structure. During transition both Cd2+ and Ho3+ cations shift. Electron diffraction patterns obtained during transition show that the Cd, which shifted from a tetrahedral to an octahedral site, distorts the sulfur lattice. This distortion becomes visible as a modulation of the sulfur matrix. The modulation originally occurs in many crystallographic directions, however, upon annealing the 〈111〉 direction becomes preferred. Small domains of the high-temperature structure of CdHo2S4 appear to occur also in samples that were annealed well below 1173 K for a long period.  相似文献   

16.
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   

17.
The electric and magnetic properties of the perovskites Nd0.8Na0.2Mn(1−x)CoxO3 (0x0.2) prepared by the usual ceramic procedure were investigated. The insulator-to-metal-like (IM) transition, closely related to a ferromagnetic arrangement, was revealed for the composition of x=0.04 and a similar tendency was detected for x=0. The insulating behavior persists down to low temperatures for higher contents of cobalt ions in spite of the transition to the bulk ferromagnetism. The properties are interpreted in terms of the steric distortion, tilting of the Mn(Co)O6 octahedra and the double-exchange interactions of the type Mn3+–O2−–Mn4+and Mn3.5+δ–O2−–Co2+, respectively. Presence of antiferromagnetic domains in the ferromagnetic matrix for the most of cobalt-substituted samples is supposed.  相似文献   

18.
The new oxy-chloro-sulfide (Mn1−xPbx)Pb10+ySb12−yS26−yCl4+yO (x ∈ [0.2-0.3]; y ∈ [0.3-1.6]) was synthesized by dry way at 500-600 °C. A single crystal ∼Mn0.7Pb11.0Sb11.3S25.3Cl4.7O indicates a monoclinic symmetry, space group C2/m, with a = 37.480(8), b = 4.1178(8), c = 18.167(4) Å, β = 106.37(3)°, V = 2690.2(9) Å3, Z = 2. Its crystal structure was determined by X-ray single crystal diffraction, with a final R = 5.11%. Modular analysis of the crystal structure reveals a “waffle” architecture, where complex rods with lozenge section delimitate an internal channel filled by a single chain of (Mn0.7Pb0.3)Cl6 octahedra connected by opposite edges. Minimal inter-chain distances are close to 18 Å. The rod wall, two-atom thick, presents, in alternation with S atoms, Pb or (Pb,Sb) cations with prismatic coordination in the internal atom layer, while the external atom layer is constituted exclusively by Sb cations with dissymmetric square pyramidal coordination. A (Pb,Sb)2S2 fragment connects two successive rods along (2 0 1) to form a waffle-type palissadic layer. The unique O position, half filled, presents the same environment than the isolated O positions in the oxy-sulfide Pb14Sb30S54O5, or oxy-chloro-sulfides Pb18Sb20S46Cl2O and (Cu,Ag)2Pb21Sb23S55ClO. This compound belongs to a pseudo-homologous series of chalcogenides with waffle structure, ordered according to the size of their lozenge shape channel. Such a complex senary compound of the oxy-chloro-sulfide type illustrates the structural competition between three cations, on one hand, and, on the other hand, three anions. This compound is of special interest regarding the 1D distribution of magnetic Mn2+ atoms at the ∼2 nm scale.  相似文献   

19.
A single crystal of ordered LiAl5O8 has been prepared by image furnace melting and its structure has been determined; 400 X-ray reflections were collected on an automatic counter diffractometer (Mo radiation). The structure is of the spinel type. The space group is P4332 (or P4132); the lattice parameter is a = 7.908(2), Å, Z = 4. The value of R is 0.022. The distribution of cations shows the absence of Li+ ions in tetrahedral cation sites and 1–3 imperfect ordering of Li+ and Al3+ in octahedral cation sites of spinel.  相似文献   

20.
A new family of complex antimonates MII4MIIISbVO8 has been synthesized and investigated by X-ray diffractometry and ir and Mössbauer spectroscopy. Many of these compounds possess a spinel structure, with a cation distribution which depends on the nature of MII and MIII. When all the tetrahedral sites are occupied by bivalent cations, these spinels MII2[MII2MIIISb]O8 are cubic, with either a disordered or a 3 : 1 ordered distribution of the octahedral cations. When the trivalent cation is able to go on the tetrahedral sites, the X-ray powder diagram may be either simple cubic (disordered cation distribution) or more complex, with a number of superstructure lines which can be indexed by considering an orthorhombic cell with a ? a0spinel212; b ? 3a; c ? a0spinel, possible space group Imma. The cation distribution of this type of phase has not been fully elucidated, but the combined X-ray, ir, and Mössbauer evidence suggests the presence of the trivalent cation on both tetrahedral and octahedral sites, and a 2 : 1 order on both tetrahedral and octahedral sites, namely, (MII4 : MIII2)[MII8 : MIIISb3]O24.  相似文献   

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