Evolution of structural distortions in solid solutions between BiMnO3 and BiScO3

Crystal structures of solid solutions of BiMn_1−xSc_xO₃ with x=0.05, 0.1, 0.2, 0.3, 0.4, 0.5, and 0.7 were studied with synchrotron X-ray powder diffraction. The strong Jahn-Teller distortion, observed in BiMnO₃ at 300 K and associated with orbital order, disappeared already in BiMn_0.95Sc_0.05O₃. The orbital-ordered phase did not appear in BiMn_0.95Sc_0.05O₃ down to 90 K. Almost the same octahedral distortions were observed in BiMn_1−xSc_xO₃ with 0.05?x?0.7 at room temperature and in BiMnO₃ at 550 K above the orbital ordering temperature T_OO=473 K. These results allowed us to conclude that the remaining octahedral distortions observed in BiMnO₃ above T_OO are the structural feature originated from the highly distorted monoclinic structure.     

Stability of lithium in α-rhombohedral boron

The stability of lithium atoms in α-rhombohedral boron was studied by the density functional theory and Car-Parrinello molecular dynamics (MD) simulations. At a low Li concentration (1.03 at%), a Li atom at the center of the icosahedral B₁₂ site (the I-site) was found to be metastable, and the potential barrier was estimated at 775±25 K (=67±25 meV). Over 800 K, Li atoms began to escape from the B₁₂ cage and settled at the tetrahedral site (the T-site) or at the octahedral site (the O-site). Li at the T-site was also metastable below 1400 K, and Li at the O-site was energetically the most favorable. At a high Li concentration (7.69 at%), the I-site changed to an unstable saddle point. The T-site was still metastable, and the O-site was the most stable. Regardless of concentration, MD simulations showed that Li atoms at the O-site never jumped to other sites below 1400 K. The migration of Li would be very slow below this temperature.     

MnO2上氧气第一个电子转移步骤的从头计算研究

采用裸露簇和嵌入簇模型, 对β-MnO₂ (001), (110), (111)三个晶面以及O₂在(110)晶面的单址吸附模式(Pauling和Griffths模式), 进行从头计算. 从β-MnO₂ (001), (110), (111)三个晶面的电子结构差异以及O₂在(110)晶面吸附的吸附能、几何结构、集居数以及净电荷数分析得到: (001), (110), (111)三个晶面中(110)晶面的催化活性最高, 其活性顺序为(110)＞(111)＞(001). 氧气在(110)晶面的吸附, Pauling和Griffths两种吸附模式均存在, 属于化学吸附中的离子吸附. 氧气与MnO₂固体间发生了单电子转移, 氧气得到电子被还原成O₂^-, 转移电子属于整个体系, 具有离域性.     

Synthesis and Structure of New BiMn2MO6 Compounds Where M=P, As, or V

The new compounds BiMn₂PO₆, BiMn₂AsO₆, and BiMn₂ VO₆ have been prepared and shown to be structurally related to several other BiA₂MO₆ compounds. The structure of BiMn₂PO₆ was refined from neutron powder diffraction data in space group Pnma with a=12.04 Å, b=5.37 Å, c=8.13 Å, and Z=4. It contains (BiO₂)¹⁻ chains and (PO₄)³⁻ tetrahedra. The observed fivefold coordination for the Mn²⁺ cations is unusual for Mn in this oxidation state.     

New manganese pyrophosphates: The syntheses, crystallographic characterizations and magnetic properties of BaMnP2O7 and CaMnP2O7

The new compound BaMnP₂O₇ was obtained in two crystallographic modifications, (monoclinic-1) and (triclinic-2), by heating mixtures of BaCO₃, P₂O₅ and MnO₂ to 1100°C and to 1000°C for 72 h, respectively. CaMnP₂O₇ (3) was obtained by heating a mixture of CaCO₃, P₂O₅ and MnO₂ to 1050°C for 48 h. Both crystallographic forms of BaMnP₂O₇ and CaMnP₂O₇ (3) were investigated by single-crystal X-ray diffraction analysis. The high-temperature monoclinic form of BaMnP₂O₇ could not be obtained free of the low-temperature triclinic form in the bulk form. In monoclinic-1 the manganese ions exist in a distorted MnO₆ octahedron surrounded by five closely and one remotely positioned oxygen atoms. In triclinic-2 and -3 forms the manganese ions are associated in pairs by the formation of Mn₂O₁₀ units that share one edge of two adjacent octahedra. The magnetic properties of the triclinic-2 and -3 forms were also investigated. The effective magnetic moments, μ_eff, are 5.7 B.M. and 5.8 B.M./Mn atom for triclinic-2 and -3, respectively, and are consistent with a high-spin Mn²⁺ ion in an octahedral environment with five unpaired electrons. The temperature-dependent magnetic measurements of 2 and 3 have revealed a combination of short-range antiferromagnetic coupling, J, between the two Mn ions within the Mn₂O₁₀ units and a longer range weaker antiferromagnetic coupling, J′, between the neighbouring Mn₂O₁₀ units, |J′/J| = 0.18 and 0.074 for 2 and 3, respectively.     

Crystal structure of vanadium suboxide V2O1±x

A monoclinic structure with the unit cell content 2V₇O₃ and its derivative structure designated as V₇O_3+x have been determined by X-ray and electron diffraction study. In both the structures, the oxygen atoms occupy regularly special octahedral interstitial sites in the body-centered monoclinic (or pseudo-tetragonal) metal lattice with the axial ratio $\text{c}\text{a}\text{≈ 1.2}$. The ordered distribution of the oxygen atoms is interpreted from the condition of minimization of the elastic strain in the vanadium lattice.     

A-site cation ordering in AA′BB′O6 perovskites

Unlike ordering of the octahedral B-site cations, ordering of the larger A-site cations in stoichiometric perovskites is rare. Herein the A- and B-site ordering characteristics of several double perovskites with AA′BB′O₆ stoichiometry have been investigated. The compounds investigated include NaLaMgWO₆, NaLaMgTeO₆, NaLaScNbO₆, NaLaScSbO₆, NaLaTi₂O₆, and NaLaZr₂O₆. Group theoretical methods are used to enumerate the possible structures of AA′BB′X₆ double perovskites that result from the combination of rock salt ordering of the B-site cations, layered ordering of the A-site cations, and octahedral tilting distortions. This combination results in 12 possible structures in addition to the aristotype. Among the compounds investigated only NaLaMgWO₆ and NaLaScNbO₆ show significant long-range ordering of the A-site cations, Na⁺ and La³⁺. A complete structural characterization is presented for NaLaMgWO₆. This compound possesses monoclinic C2/m (#12) space group symmetry, with unit cell dimensions of , , , β=90.136(1)° at room temperature. The results presented here show that in AA′BB′O₆ perovskites layered ordering of A-site cations creates a bonding instability that is compensated for by a second-order Jahn-Teller distortion of the B′ cation. These two distortions are synergistic and the removal of one leads to the disappearance of the other.     

Novel rare-earth-containing manganites Ba4REMn3O12 (RE=Ce, Pr) with 12R structure

Novel rare-earth-containing manganites, Ba₄REMn₃O₁₂ (RE=Ce, Pr), with 12R structure, have been prepared by solid-state reaction. Although the phases are formed at 950°C, to obtain single-phase samples high temperatures (up to 1300°C) and long synthesis periods are needed.Their structure is built up from chains of BO₆ face-sharing and corner-sharing octahedra running along the c-axis giving a quasi-one-dimensional oxide. Every polyhedral column consists of (Mn₃O₁₂) units of three face-sharing octahedra, both ends connected by the three terminal oxygen atoms to three different (REO₆) octahedra. Mixed occupation of the three octahedral positions in the structure, (Mn(1), Mn(2) and Re), was not found. Vacancies are not observed, neither in the cationic sublattice nor in the oxygen one. Thus, as in all the other 1-D manganites, the oxidation state of manganese ions seems to be four, as the rare-earth valence is. High-resolution electron microscopy suggests the eventual existence of ordered polytypes for different compositions, which could be stabilized by adjusting the thermodynamic conditions.     

Electron-doping through La-for-Sr substitution in (Sr1−xLax)2FeTaO6: Effects on the valences and ordering of the B-site cations, Fe and Ta

We have employed aliovalent A-site cation substitution, La^III-for-Sr^II, to dope the Sr(Fe_0.5Ta_0.5)O₃ perovskite oxide with electrons. Essentially single-phase samples of (Sr_1−xLa_x)(Fe_0.5Ta_0.5)O₃ were successfully synthesized up to x≈0.3 in a vacuum furnace at 1400 °C. The samples were found to crystallize (rather than with orthorhombic symmetry) in monoclinic space group P2₁/n that accounts for the partial ordering of the B-site cations, Fe and Ta. With increasing La-substitution level, x, the degree of Fe/Ta order was found to increase such that the La-richest compositions are best described by the B-site ordered double-perovskite formula, (Sr,La)₂FeTaO₆. From Fe L₃ and Ta L₃ XANES spectra it was revealed that upon electron doping the two B-site cations, Fe^III and Ta^V, are both prone to reduction. Magnetic susceptibility measurements showed spin-glass type behaviour for all the samples with a transition temperature slightly increasing with increasing x.     

Charge and structural ordering in the brownmillerite phases: La1−xSrxMnO2.5 (0.2<x<0.4)

The topotactic reduction of La_1−xSr_xMnO₃ (0.2<x<0.4) perovskite phases to the corresponding La_1−xSr_xMnO_2.5 brownmillerite phases with NaH is described. Neutron and electron diffraction data show the x=0.25 and 0.2 phases adopt structures with an unusual ordered L-R-L-R alternation of twisted chains of Mn(II) tetrahedra within each anion-deficient layer. This is accompanied by Mn(II)/(III) charge ordering within the remaining MnO₆ octahedral layers. In contrast, the x=0.4 phase adopts a structure in which the twisted chains of tetrahedra are disordered.     

Neutron powder diffraction study of tetragonal Li7La3Hf2O12 with the garnet-related type structure

We have successfully synthesized a polycrystalline sample of tetragonal garnet-related Li-ion conductor Li₇La₃Hf₂O₁₂ by solid state reaction. The crystal structure is analyzed by the Rietveld method using neutron powder diffraction data. The structure analysis identifies that tetragonal Li₇La₃Hf₂O₁₂ has the garnet-related type structure with a space group of I4₁/acd (no. 142). The lattice constants are a=13.106(2) Å and c=12.630(2) Å with a cell ratio of c/a=0.9637. The crystal structure of tetragonal Li₇La₃Hf₂O₁₂ has the garnet-type framework structure composed of dodecahedral La(1)O₈, La(2)O₈ and octahedral HfO₆. Li atoms occupy three types of crystallographic site in the interstices of this framework structure, where Li(1) atom is located at the tetrahedral 8a site, and Li(2) and Li(3) atoms are located at the distorted octahedral 16f and 32g sites, respectively. These Li sites are filled with the Li atom. The present tetragonal Li₇La₃Hf₂O₁₂ sample exhibits bulk Li-ion conductivity of σ_b=9.85×10⁻⁷ S cm⁻¹ and grain-boundary Li-ion conductivity of σ_gb=4.45×10⁻⁷ S cm⁻¹ at 300 K. The activation energy is estimated to be E_a=0.53 eV in the temperature range of 300-580 K.     

Composition- and temperature-dependent phase transitions in 1:3 ordered perovskites Ba4−xSrxNaSb3O12

A series of 25 members of the 1:3 ordered perovskite family of the type Ba_4−xSr_xNaSb₃O₁₂ has been synthesized and their structures determined using synchrotron X-ray and neutron powder diffraction techniques. At room temperature the sample Ba₄NaSb₃O₁₂ has a cubic structure in space group with a=8.2821(1) Å, where the Na and Sb cations are ordered in the octahedral sites but there is no tilting of the (Na/Sb)O₆ octahedra. As the average size of the A-site cation decreases, through the progressive replacement of Ba by Sr, tilting of the octahedra is introduced firstly lowering the symmetry to tetragonal in P4/mnc then to orthorhombic in Cmca and ultimately a monoclinic structure in P2₁/n as seen for Sr₄NaSb₃O₁₂ with a=8.0960(2) Å, b=8.0926(2) Å, c=8.1003(1) Å and β=90.016(2)°. The powder neutron diffraction studies show that the orthorhombic and tetragonal phases in Cmca and P4/mnc co-exist at room temperature for samples with x between 1.5 and 2.     

Hydrothermal synthesis, thermal, structural, spectroscopic and magnetic studies of the Mn5−xCox(HPO4)2(PO4)2(H2O)4 (x=1.25, 2, 2.5 and 3) finite solid solution

The Mn_5−xCo_x(HPO₄)₂(PO₄)₂(H₂O)₄ (x=1.25, 2, 2.5, 3) finite solid solution has been synthesized by mild hydrothermal conditions under autogeneous pressure. The phases crystallize in the C2/c space group with Z=4, belonging to the monoclinic system. The unit-cell parameters obtained from single crystal X-ray diffraction are: a=17.525(1), b=9.0535(6), c=9.4517(7) Å, β=96.633(5) ° being R1=0.0436, wR2=0.0454 for Mn75Co25; a=17.444(2), b=9.0093(9), c=9.400(1) Å, β=96.76(1) ° being R1=0.0381, wR2=0.0490 for Mn60Co40; a=17.433(2), b=8.9989(9), c=9.405(1) Å, β=96.662(9) ° being R1=0.0438, wR2=0.0515 for Mn50Co50 and a=17.4257(9), b=8.9869(5), c=9.3935(5) Å, β=96.685(4) ° being R1=0.0296, wR2=0.0460 for Mn40Co60. The structure consists of a three dimensional network formed by octahedral pentameric entities (Mn,Co)₅O₁₆(H₂O)₆ sharing vertices with the (PO₄)³⁻ and (HPO₄)²⁻ tetrahedra. The limit of thermal stability of these compounds is, approximately, 165 °C, near to this mean temperature the phases loose their water content in two successive steps. IR spectra show the characteristic bands of the water molecules and the phosphate and hydrogen-phosphate oxoanions. The diffuse reflectance spectra are consistent with the presence of MO₆ octahedra environments in slightly distorted octahedral geometry, except for the M(3)O₆ octahedron which presents a remarkable distortion and so a higher Dq parameter. The mean value for the Dq and B-Racah parameter for the M(1),(2)O₆ octahedra is 685 and 850 cm⁻¹, respectively. These parameters for the most distorted M(3)O₆ polyhedron are 825 and 880 cm⁻¹, respectively. The four phases exhibit antiferromagnetic couplings as the major magnetic interactions. However, a small spin canting phenomenon is observed at low temperatures for the two phases with major content in the anisotropic-Co(II) cation.     

Synthesis and phase transition of Li-Mn-O spinels with high Li/Mn ratio by thermo-decomposition of LiMnC2O4(Ac)

LiMnC₂O₄(Ac) precursor in which Li⁺ and Mn²⁺ were amalgamated in one molecule was prepared by solid-state reaction at room-temperature using manganese acetate, lithium hydroxide and oxalic acid as raw materials. By thermo-decomposition of LiMnC₂O₄(Ac) at various temperatures, a series of Li_1+y[Mn_2−xLi_x]_16dO₄ spinels were prepared with Li₂MnO₃ as impurities. The structure and phase transition of these spinels were investigated by XRD, TG/DTA, average oxidation state of Mn and cyclic voltammeric techniques. Results revealed that the Li-Mn-O spinels with high Li/Mn ratio were unstable at high temperature, and the phase transition was associated with the transfer of Li⁺ from octahedral 16c sites to 16d sites. With the sintering temperature increasing from 450 to 850 °C, the phase structure varied from lithiated-spinel Li₂Mn₂O₄ to Li₄Mn₅O₁₂-like to LiMn₂O₄-like and finally to rock-salt LiMnO₂-like. A way of determining x with average oxidation state of Mn and the content of Li₂MnO₃ was also demonstrated.     

The crystal structure of Li2WO4II: A structure related to spinel

Li₂WO₄II, synthesized at 3 kbar and 630°C, has tetragonal symmetry, $\text{I4}{}_1\text{amd}\text{, a = 11.954(2)}$ and c = 8.410(1)Å, Z = 16, D_calc = 5.78 g cm^?3. The structure was determined by countermeasuring 469 independent reflections from a single crystal and was refined up to R = 0.032 by the full-matrix least-squares method. It is based on cubic closest packing of oxygen atoms and is closely related to the β-phase structure of Mg₂SiO₄. W and Li(2) are in octahedral sites and Li(1), in tetrahedral sites. Four Li(1)O₄ tetrahedra form a Li₄O₁₂ group, WO₆ and Li(2)O₆ construct a octahedral double chain along the a axis, and four WO₆ octahedra build a W₄O₁₆ group by sharing their octahedral edges.     

Synthesis, structure and magnetic properties of A2MnB′O6 (A=Ca, Sr; B′=Sb, Ta) double perovskites

A₂MnB′O₆ (A=Ca, Sr; B=Sb, Ta) double perovskites have been synthesized and their structural and magnetic properties have been investigated. Rietveld refinement of the powder X-ray diffraction data for Sr₂MnSbO₆ indicated significant ordering of Mn and Sb at the B-site while all other phases showed mostly a random distribution of the B-site cations. X-ray absorption spectroscopic data established the presence of Mn in the 3+ and Sb/Ta in the 5+ oxidation states in all the phases. Magnetic susceptibility data indicated ferromagnetic correlations for all the A₂MnB′O₆ phases with Weiss temperatures varying from 64 to 107 K.     

Low-temperature synthesis of SrAl2O4 by a modified sol-gel route: XRD and Raman characterization

Among other alkaline-earth aluminates, the monoclinic (M) polymorph of SrAl₂O₄ can be used as host material for Eu²⁺ luminescence based phosphors. With the aim of reducing the synthesis temperature of this polymorph, we have produced and characterized by XRD and Raman scattering solid solutions of the SrAl_2−xB_xO₄ system (x?0.3) obtained by two different methods, a ceramic route and a modified sol-gel synthesis. Though the addition of boron lowers the temperature of obtention of the M polymorph in both type of samples, lower B contents are needed to stabilize the M form as single phase for samples prepared by the sol-gel method than through the ceramic route. In the sol-gel method, the M polymorph can be obtained at temperatures as low as 1200 °C, with a Boron content of just 1%. Rietveld profile analysis allows us to conclude that coexistence of the monoclinic and hexagonal polymorphs of SrAl₂O₄ occurs for samples synthesized below an onset temperature of about 1000-1100 °C, that depends on the sample composition. Above those temperatures, only the monoclinic phase is formed.     

High-pressure structure of Li2CO3

The high-pressure behavior of Li₂CO₃ is studied up to 25 GPa with synchrotron angle-dispersive powder X-ray diffraction in diamond anvil cells and synthesis using a multi-anvil apparatus. A new non-quenchable hexagonal polymorph (P6₃/mcm, Z=2) occurs above 10 GPa with carbonate groups in a staggered configuration along the c-axis—a=4.4568(2) Å and c=5.1254(6) Å at 10 GPa. Two columns of face-shared distorted octahedra around the Li atoms are linked through octahedral edges. The oxygen atoms are coordinated to one carbon atom and four lithium atoms to form a distorted square pyramid. Splittings of X-ray reflections for the new polymorph observed above about 22 GPa under non-hydrostatic conditions arise from orthorhombic or monoclinic distortions of the hexagonal lattice. The results of this study are discussed in relation to the structural features found in other Me₂CO₃ carbonates (Me: Na, K, Rb, Cs) at atmospheric conditions.     

Hydroxide flux synthesis and crystal structure of the ordered palladate, LuNaPd6O8

We report the single crystal structure of LuNaPd₆O₈ grown from a sodium hydroxide flux. The utilization of a hydroxide flux has led to the preparation of the first ordered substitution of a lanthanide metal and an alkali metal on the A-site in a platinum group oxide and the first palladate to contain both a lanthanide metal and an alkali metal. The 1:1 ordered substitution of Lu³⁺ and Na⁺ in place of the commonly observed divalent cation leads to slabs of LuO₈ and NaO₈ cubes bridged together by PdO₄ square planes. The compound crystallizes in the cubic space group Pm-3 (#200) with a=5.72500(10) Å and is structurally related to other cubic palladium oxides.