Layered ruthenium hexagonal perovskites: The new series [Ba2Br2−2x(CO3)x][Ban+1RunO3n+3] with n=2, 3, 4, 5

Single crystals of the title compounds were prepared by solid state reactions from barium carbonate and ruthenium metal using a BaBr₂ flux and investigated by X-ray diffraction method using Mo(Kα) radiation and a Charge Coupled Device (CCD) detector. A structural model for the term n=2, Ba₅Ru₂Br₂O₉ (1) was established in the hexagonal symmetry, space group P6₃/mmc, a=5.8344(2) Å, c=25.637(2) Å, Z=2. Combined refinement and maximum-entropy method (MEM) unambiguously show the presence of CO₃²⁻ ions in the three other compounds (2, 3, 4). Their crystal structures were solved and refined in the trigonal symmetry, space group , a=5.8381(1) Å, c=15.3083(6) Å for the term n=3, Ba₆Ru₃Br_1.54(CO₃)_0.23O₁₂ (2), and space group , a=5.7992(1) Å, c=52.866(2) Å and a=5.7900(1) Å, c=59.819(2) Å for the terms n=4, Ba₇Ru₄Br_1.46(CO₃)_0.27O₁₅ (3), and n=5, Ba₈Ru₅Br_1.64(CO₃)_0.18O₁₈ (4), respectively. The structures are formed by the periodic stacking along [0 0 1] of (n+1) hexagonal close-packed [BaO₃] layers separated by a double layer of composition [Ba₂Br_2−2x(CO₃)_x]. The ruthenium atoms occupy the n octahedral interstices created in the hexagonal perovskite slabs and constitute isolated dimers Ru₂O₉ of face-shared octahedra (FSO) in 1 and isolated trimers Ru₃O₁₂ of FSO in 2. In 3 and 4, the Ru₂O₉ units are connected by corners either directly (3) or through a slab of isolated RuO₆ octahedra (4) to form a bidimensional arrangement of RuO₆ octahedra. These four oxybromocarbonates belong to the family of compounds formulated [Ba₂Br_2−2x(CO₃)_x][Ba_n+1Ru_nO_3n+3] where n represents the thickness of the octahedral string in hexagonal perovskite slabs. These compounds are compared to the oxychloride series.     

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.     

A new 2212-type stair like structure: Bi14Sr21Fe12O61, m=5 member of the generic [Bi2Sr3Fe2O9]m[Bi4Sr6Fe2O16] family

A new oxide, Bi₁₄Sr₂₁Fe₁₂O_61, with a layered structure derived from the 2212 modulated type structure Bi₂Sr₃Fe₂O₉, was isolated. It crystallizes in the I2 space group, with the following parameters: a=16.58(3) Å, b=5.496(1) Å, c=35.27(2) Å and β=90.62°. The single crystal X-ray structure determination, coupled with electron microscopy, shows that this ferrite is the m=5 member of the [Bi₂Sr₃Fe₂O₉]_m[Bi₄Sr₆Fe₂O₁₆] collapsed family. This new collapsed structure can be described as slices of 2212 structure of five bismuth polyhedra thick along , shifted with respect to each other and interconnected by means of [Bi₄Sr₆Fe₂O₁₆] slices. The latter are the place of numerous defects like iron or strontium for bismuth substitution; they can be correlated to intergrowth defects with other members of the family.     

Layered perovskite-related ruthenium oxychlorides: crystal structure of two new compounds Ba5Ru2Cl2O9 and Ba6Ru3Cl2O12

Single crystals of the title compounds were prepared using a BaCl₂ flux and investigated by X-ray diffraction methods using MoKα radiation and a charge coupled device (CCD) detector. The crystal structures of these two new compounds were solved and refined in the hexagonal symmetry with space group P6₃/mmc, a=5.851(1) Å, c=25.009(5) Å, ρ_cal=4.94 g cm⁻³, Z=2 to a final R₁=0.069 for 20 parameters with 312 reflections for Ba₅Ru₂Cl₂O₉ and space group , a=5.815(1) Å, c=14.915(3) Å, ρ_cal=5.28 g cm⁻³, Z=1 to a final R₁=0.039 for 24 parameters with 300 reflections for Ba₆Ru₃Cl₂O₁₂. The structure of Ba₅Ru₂Cl₂O₉ is formed by the periodic stacking along [001] of three hexagonal close-packed BaO₃ layers separated by a double layer of composition Ba₂Cl₂. The BaO₃ stacking creates binuclear face-sharing octahedra units Ru₂O₉ containing Ru(V). The structure of Ba₆Ru₃Cl₂O₁₂ is built up by the periodic stacking along [001] of four hexagonal close-packed BaO₃ layers separated by a double layer of composition Ba₂Cl₂. The ruthenium ions with a mean oxidation degree +4.67 occupy the octahedral interstices formed by the four layers hexagonal perovskite slab and then constitute isolated trinuclear Ru₃O₁₂ units. These two new oxychlorides belong to the family of compounds formulated as [Ba₂Cl₂][Ba_n+1Ru_nO_3n+3], where n represents the thickness of the octahedral string in hexagonal perovskite slabs.     

Synthesis and single crystal structures of ternary phosphides Li4SrP2 and AAeP (A=Li, Na; Ae=Sr, Ba)

Two new (NaSrP, Li₄SrP₂) and two known (LiSrP, LiBaP) ternary phosphides have been synthesized and characterized using single crystal X-ray diffraction studies. NaSrP crystallizes in the non-centrosymmetric hexagonal space group (#189, a=7.6357(3) Å, c=4.4698(3) Å, V=225.69(2) Å³, Z=3, and R/wR=0.0173/0.0268). NaSrP adopts an ordered Fe₂P structure type. PSr₆ trigonal prisms share trigonal (pinacoid) faces to form 1D chains. Those chains define large channels along the [001] direction through edge-sharing. The channels are filled by chains of PNa₆ face-sharing trigonal prisms. Li₄SrP₂ crystallizes in the rhombohedral space group (#166, a=4.2813(2) Å, c=23.437(2) Å, V=372.04(4) Å³, Z=3, and R/wR=0.0142/0.0222). In contrast to previous reports, LiSrP and LiBaP crystallize in the centrosymmetric hexagonal space group P6₃/mmc (#194, a=4.3674(3) Å, c=7.9802(11) Å, V=131.82(2) Å³, Z=2, and R/wR=0.0099/0.0217 for LiSrP; a=4.5003(2) Å, c=8.6049(7) Å, V=150.92(2) Å³, Z=2, and R/wR=0.0098/0.0210 for LiBaP). Li₄SrP₂, LiSrP, and LiBaP can be described as Li₃P derivatives. Li atoms and P atoms make a graphite-like hexagonal layer, . In LiSrP and LiBaP, Sr or Ba atoms reside between layers to substitute for two Li atoms of Li₃P, while in Li₄SrP₂, Sr substitutes only between every other layer.     

Some A6B5O18 cation-deficient perovskites in the BaO-La2O3-TiO2-Nb2O5 system

Some dielectric oxides have been synthesized and characterized in the BaO-La₂O₃-TiO₂-Nb₂O₅ system. Through Rietveld refinement of X-ray powder diffraction data, Ba₅LaTi₂Nb₃O₁₈ and Ba₄La₂Ti₃Nb₂O₁₈ are identified as the A_nB_n−1O_3n (n=6) type cation-deficient perovskites with space group and lattice constants , and for Ba₅LaTi₂Nb₃O₁₈; , and for Ba₄La₂Ti₃Nb₂O₁₈, respectively. Their ceramics exhibit high dielectric constant up to 57 and high quality factors (Qf) up to 21,273 GHz. The temperature coefficient of resonant frequency (τ_f) of these ceramics is decreased with the increase of B-site bond valence.     

Synthesis, crystal structure, infrared and Raman spectra of Sr5(As2O7)2(AsO3OH)

The new compound Sr₅(As₂O₇)₂(AsO₃OH) was synthesized under hydrothermal conditions. It represents a previously unknown structure type and belongs to a group of a few compounds in the system SrO-As₂O₅-H₂O; (As₂O₇)⁴⁻ besides (AsO₃OH)²⁻ groups have not been described yet. The crystal structure of Sr₅(As₂O₇)₂(AsO₃OH) was determined by single-crystal X-ray diffraction (space group P2₁/n, a=7.146(1), b=7.142(1), , β=93.67(3)°, , Z=4). One of the five symmetrically unique Sr atoms is in a trigonal antiprismatic (Inorg. Chem. 35 (1996) 4708)—coordination, whereas the other Sr atoms adopt the commonly observed (“Collect” data collection software, Delft, The Netherlands, 1999; Methods Enzymol. 276 (1997) 307)—coordination. The position of the hydrogen atom was located in a difference Fourier map and subsequently refined with an isotropic displacement parameter. Worth mentioning is the very short hydrogen bond length O_h-H?O(1) of 2.494(4) Å; it belongs to the shortest known examples where the donor and acceptor atoms are crystallographically different. This hydrogen bond was confirmed by IR spectroscopy. In addition, Raman spectra were collected in order to study the arsenate groups.     

Order-disorder in In perovskites: The example of A(In2/3B″1/3)O3 (A=Ba, Sr; B″=W, U)

We describe the preparation and structural characterization of four In-containing perovskites from neutron powder diffraction (NPD) and X-ray powder diffraction (XRPD) data. Sr₃In₂B″O₉ and Ba(In_2/3B″_1/3)O₃ (B″=W, U) were synthesized by standard ceramic procedures. The crystal structure of the W-containing perovskites and Ba(In_2/3U_1/3)O₃ have been revisited based on our high-resolution NPD and XRPD data, while for the new U-containing perovskite Sr₃In₂UO₉ the structural refinement was carried out from high-resolution XRPD data. At room temperature, the crystal structure for the two Sr phases is monoclinic, space group P2₁/n, where the In atoms occupy two different sites Sr₂[In]_2d[In_1/3B″_2/3]_2cO₆, with a=5.7548(2) Å, b=5.7706(2) Å, c=8.1432(3) Å, β=90.01(1)° for B″=W and a=5.861(1) Å, b=5.908(1) Å, c=8.315(2) Å, β=89.98(1)° for B″=U. The two phases with A=Ba should be described in a simple cubic perovskite unit cell (S.G. Pm3¯m) with In and B″ distributed at random at the octahedral sites, with a=4.16111(1) Å and 4.24941(1) Å for W and U compounds, respectively.     

Synthesis, crystal structure, and magnetic properties of the manganate La2Ca2MnO6(O2) related to the hexagonal perovskite-type structure

The compound La₂Ca₂MnO₆(O₂) has been synthesized from La₂Ca₂MnO₇ heated at 1123 K under high pressure (4 GPa) with KClO₃ as oxygen source. The crystal structure has been refined from X-ray powder data in the space group. The unit-cell parameters are a=5.6335(2) Å and c=17.4879(8) Å. Perpendicular to the c-axis, the structure is built up by the periodic stacking of two close packed [LaO₃] layers separated by a layer of composition [Ca₂O₂] containing (O₂)²⁻ peroxide ions. This oxide belongs to the family of compounds formulated as [A′₂O_2−δ][A_nB_n−1O_3n] for n=2 and δ=0. It is the first member of the series where the thickness of the perovskite slab corresponds to one [BO₆] (B=Mn) octahedron. The structural relationships with La₂Ca₂MnO₇ are discussed and the magnetic properties show that in both phases manganese is tetravalent.     

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.     

Two crystalline modifications of RuO4

RuO₄ was prepared by oxidation of elemental ruthenium. Two different modifications were obtained and investigated by X-ray single crystal diffraction. RuO₄-I has cubic symmetry , and two independent tetrahedral molecules are present in the unit cell. Within the standard uncertainties in both molecules the distances Ru-O are 1.695 Å. The second modification, RuO₄-II, is monoclinic and isotypic with OsO₄. There is one independent molecule in the unit cell, which shows distances Ru-O of 1.697 and 1.701 Å, respectively.     

Ordered perovskites in the A(Li1/4Nb3/4)O3-A(Li2/5W3/5)O3 (A=Sr, Ca) systems

Single-phase 1:2 B-site ordered perovskites are formed in the (1−x)A²⁺(Li_1/4Nb_3/4)O₃-(x)A²⁺(Li_2/5W_3/5)O₃ systems, A²⁺=Sr and Ca, within the range 0.238?x?0.333. The X-ray and electron diffraction patterns are consistent with a P2₁/c monoclinic supercell, , , , β≈125°, where the 1:2 order is combined with b⁻b⁻c⁺ octahedral tilting. Rietveld refinements of the ordered A(B^I_1/3B^II_2/3)O₃ structures give a good fit to a model with B^I occupied by Li and Nb, B^II by W and Nb, and a general stoichiometry (Sr,Ca)(Li_3/4+y/2Nb_1/4−y/2)_1/3(Nb_1−yW_y)_2/3O₃, y=0.9x=0.21-0.30. The Sr system also includes regions of stability of a 1:3 ordered phase for 0.0?x?0.111, and a 1:1 ordered double perovskite for 0.833?x?1.0. The formation of the non-stoichiometric 1:2 ordered phases is associated with the large site charge/size differences that can be accessed in these systems, and restricted by local charge imbalances at the A-sites for W-rich compositions. These concepts are used to generate stability maps to rationalize the formation of the known 1:2 ordered oxide perovskites.     

Structure and magnetism of NaRu2O4 and Na2.7Ru4O9

The structures of NaRu₂O₄ and Na_2.7Ru₄O₉ are refined using neutron diffraction. NaRu₂O₄ is a stoichiometric compound consisting of double chains of edge sharing RuO₆ octahedra. Na_2.7Ru₄O₉ is a non-stoichiometric compound with partial occupancy of the Na sublattice. The structure is a mixture of single, double and triple chains of edge-shared RuO₆ octahedra. NaRu₂O₄ displays temperature independent paramagnetism with . Na_2.7Ru₄O₉ is paramagnetic, χ₀= with and a Curie constant of 0.0119 emu/mol Oe K. Specific heat measurements reveal a small upturn at low temperatures, similar to the upturn observed in La₄Ru₆O₁₉. The electronic contribution to the specific heat (γ) for Na_2.7Ru₄O₉ was determined to be15 mJ/mole_Ru K².     

Sr18Ru1.9Bi4.1O33: crystallization of a Ru(V)/Bi(V) oxide from molten hydroxide

Single crystals of a new complex oxide, Sr₁₈Ru_1.9Bi_4.1O₃₃, were precipitated from a mixture of molten alkali and alkaline earth metal hydroxides at 750°C. The structure was determined from a ruby-red crystal using single-crystal X-ray diffraction. Sr₁₈Ru_1.88Bi_4.12O₃₃ crystallizes in the space group C2/c (monoclinic) and has unit-cell dimensions: a=10.2102(11) Å, b=17.882(2) Å, c=19.579(2) Å, and β=102.043(2)°. The structure (refined to R₁=4.5%, wR₂=9.2%) is an unusual ABO₃ defect perovskite, with th of the oxygen positions vacant. All the A sites and half of the B sites are occupied by Sr²⁺, while the remaining B sites are occupied by Bi⁵⁺ or Ru⁵⁺. The oxygen atom vacancies are located within the Bi coordination sphere exclusively. The bonding in the BO₃ sublattice is less covalent than that in the perovskite archetype from which it is derived due to the presence of 50% Sr²⁺ on the B sites. Thus, the structure of Sr₁₈Ru_1.9Bi_4.1O₃₃ can also be viewed as being made up of isolated bismuthate anions (BiO₅⁵⁻, BiO_5.5⁶⁻ and BiO₆⁷⁻) and ruthenate anions (RuO₆⁷⁻) separated by strontium cations.     

A structure and phase analysis investigation of the “1:1” ordered A2InNbO6 perovskites (A=Ca, Sr, Ba)

The structures of the Ba₂InNbO₆, Sr₂InNbO₆ and Ca₂InNbO₆ “1:1” complex perovskites have been refined from neutron powder diffraction data. Both the A=Ca and Sr compounds occur at room temperature in P12₁/n1 (a=a_p+b_p, b=-a_p+b_p, c=2c_p) perovskite-related superstructures while the A=Ba compound occurs in the , a=2a_p, elpasolite structure type. In the case of the A=Ca compound, an extensive Ca₂[(Ca_2x/3In_1−xNb_x/3)Nb]O₆ ‘solid solution’ field spanning compositions between Ca₄Nb₂O₉ and Ca₂InNbO₆ in the CaO-InO_3/2-NbO_5/2 ternary phase diagram is shown to exist. Under the conditions of synthesis used, the ‘solid solution’ field stops just short of the ideal 1:1 Ca₂InNbO₆ composition.     

Synthesis and characterization of the novel Nb3O5F5 niobium oxyfluoride: the term n=3 of the NbnO2n−1Fn+2 series

The solid-state synthesis of the oxyfluoride Nb₃O₅F₅, its crystal structure determined from X-ray powder diffraction data as well as some physical characterizations, are reported. Nb₃O₅F₅ constitutes the term n=3 of the Nb_nO_2n−1F_n+2 series related to the Dion-Jacobson phases. It crystallizes, at room temperature, in the tetragonal system (space group I4/mmm (no. 139); Z=4; a=3.9135(1) Å, c=24.2111(2) Å, and V=370.80(3) Å³). The crystal structure appears to be an in-between of the three-dimensional network of NbO₂F and the two-dimensional packing of NbOF₃ (term n=1 of the Nb_nO_2n−1F_n+2 series). This layered structure consists of slabs made of three Nb(O,F)₆ corner-linked octahedra in thickness (n=3) shifted one from another by a ()/translation. Oxygen and fluorine atoms are randomly distributed over all the ligand sites.