Crystal growth, structural characterization and magnetic properties of Ca3CuRhO6, Ca3Co1.34Rh0.66O6 and Ca3FeRhO6

Single crystals of Ca₃CuRhO₆, Ca₃Co_1.34Rh_0.66O₆ and Ca₃FeRhO₆ were synthesized by high temperature flux growth in molten K₂CO₃ and structurally characterized by single crystal X-ray diffraction. While Ca₃Co_1.34Rh_0.66O₆ and Ca₃FeRhO₆ crystallize with trigonal (rhombohedral) symmetry in the space group , Z=6: Ca₃Co_1.34Rh_0.66O₆a=9.161(1) Å, c=10.601(2) Å; Ca₃FeRhO₆a=9.1884(3) Å, c=10.7750(4) Å; Ca₃CuRhO₆ adopts a monoclinic distortion of the K₄CdCl₆ structure in the space group C2/c, Z=4: a=9.004(2) Å, b=9.218(2) Å, c=6.453(1) Å, β=91.672(5). All crystals of Ca₃CuRhO₆ examined were twinned by pseudo-merohedry. Ca₃CuRhO₆, Ca₃Co_1.34Rh_0.66O₆, and Ca₃FeRhO₆ are structurally related and contain infinite one-dimensional chains of alternating face-sharing RhO₆ octahedra and MO₆ trigonal prisms. In the monoclinic modification, the copper atoms are displaced from the center of the trigonal prism toward one of the rectangular faces adopting a pseudo-square planar configuration. The magnetic properties of Ca₃CuRhO₆, Ca₃Co_1.34Rh_0.66O₆, and Ca₃FeRhO₆ are discussed.     

Crystal structure and magnetic properties of Co2TeO3Cl2 and Co2TeO3Br2

The crystal structures of the two new synthetic compounds Co₂TeO₃Cl₂ and Co₂TeO₃Br₂ are described together with their magnetic properties. Co₂TeO₃Cl₂ crystallize in the monoclinic space group P2₁/m with unit cell parameters a=5.0472(6) Å, b=6.6325(9) Å, c=8.3452(10) Å, β=105.43(1)°, Z=2. Co₂TeO₃Br₂ crystallize in the orthorhombic space group Pccn with unit cell parameters a=10.5180(7) Å, b=15.8629(9) Å, c=7.7732(5) Å, Z=8. The crystal structures were solved from single crystal data, R=0.0328 and 0.0412, respectively. Both compounds are layered with only weak interactions in between the layers. The compound Co₂TeO₃Cl₂ has [CoO₄Cl₂] and [CoO₃Cl₃] octahedra while Co₂TeO₃Br₂ has [CoO₂Br₂] tetrahedra and [CoO₄Br₂] octahedra. The Te(IV) atoms are tetrahedrally [TeO₃E] coordinated in both compounds taking the 5s² lone electron pair E into account. The magnetic properties of the compounds are characterized predominantly by long-range antiferromagnetic ordering below 30 K.     

La doping of the Sr2CoWO6 double perovskite: A structural and magnetic study

La-doped Sr₂CoWO₆ double perovskites have been prepared in air in polycrystalline form by solid-state reaction. These materials have been studied by X-ray powder diffraction (XRPD), neutron powder diffraction (NPD) and magnetic susceptibility. The structural refinement was performed from combined XRPD and NPD data (D2B instrument, λ=1.594 Å). At room temperature, the replacement of Sr²⁺ by La³⁺ induces a change of the tetragonal structure, space group I4/m of the undoped Sr₂CoWO₆ into the distorted monoclinic crystal structure, space group P2₁/n, Z=2. The structure of La-doped phases contains alternating CoO₆ and (Co/W)O₆ octahedra, almost fully ordered. On the other hand, the replacement of Sr²⁺ by La³⁺ induces a partial replacement of W⁶⁺ by Co²⁺ into the B sites, i.e. Sr_2−xLa_xCoW_1−yCo_yO₆ (y=x/4) with segregation of SrWO₄. Magnetic and neutron diffraction measurements indicate an antiferromagnetic ordering below T_N=24 K independently of the La-substitution.     

Complex three-dimensional platinum-indium networks in the ternary indides Dy2Pt7In16 and Tb6Pt12In23

Well crystallized samples of Dy₂Pt₇In₁₆ and Tb₆Pt₁₂In₂₃ were synthesized by an indium flux technique. Arc-melted precursor alloys with the starting compositions ∼DyPt₃In₆ and ∼TbPtIn₄ were annealed with a slight excess of indium at 1200 K followed by slow cooling (5 K/h) to 870 K. Both indides were investigated by X-ray diffraction on powders and single crystals: Cmmm, a=1211.1(2), b=1997.8(3), c=439.50(6) pm, wR2=0.0518, 1138 F² values, 45 variable parameters for Dy₂Pt₇In₁₆ and C2/ma=2834.6(4), b=440.05(7), c=1477.1(3) pm, β=112.37(1)°, wR2=0.0753, 2543 F² values, 126 variable parameters for Tb₆Pt₁₂In₂₃. The platinum atoms in the terbium compound have a distorted trigonal prismatic coordination. In Dy₂Pt₇In₁₆, trigonal and square prismatic coordination occur. The shortest interatomic distances are observed for Pt-In followed by In-In contacts. Considering these strong interactions, both structures can be described by complex three-dimensional [Pt₇In₁₆] and [Pt₁₂In₂₃] networks. The networks leave distorted pentagonal channels in Dy₂Pt₇In₁₆, while pentagonal and hexagonal channels occur in Tb₆Pt₁₂In₂₃. The crystal chemistry and chemical bonding of the two indides are briefly discussed.     

High-pressure transitions in MgAl2O4 and a new high-pressure phase of Mg2Al2O5

Phase transitions in MgAl₂O₄ were examined at 21-27 GPa and 1400-2500 °C using a multianvil apparatus. A mixture of MgO and Al₂O₃ corundum that are high-pressure dissociation products of MgAl₂O₄ spinel combines into calcium-ferrite type MgAl₂O₄ at 26-27 GPa and 1400-2000 °C. At temperature above 2000 °C at pressure below 25.5 GPa, a mixture of Al₂O₃ corundum and a new phase with Mg₂Al₂O₅ composition is stable. The transition boundary between the two fields has a strongly negative pressure-temperature slope. Structure analysis and Rietveld refinement on the basis of the powder X-ray diffraction profile of the Mg₂Al₂O₅ phase indicated that the phase represented a new structure type with orthorhombic symmetry (Pbam), and the lattice parameters were determined as a=9.3710(6) Å, b=12.1952(6) Å, c=2.7916(2) Å, V=319.03(3) Å³, Z=4. The structure consists of edge-sharing and corner-sharing (Mg, Al)O₆ octahedra, and contains chains of edge-sharing octahedra running along the c-axis. A part of Mg atoms are accommodated in six-coordinated trigonal prism sites in tunnels surrounded by the chains of edge-sharing (Mg, Al)O₆ octahedra. The structure is related with that of ludwigite (Mg, Fe²⁺)₂(Fe³⁺, Al)(BO₃)O₂. The molar volume of the Mg₂Al₂O₅ phase is smaller by 0.18% than sum of molar volumes of 2MgO and Al₂O₃ corundum. High-pressure dissociation to the mixture of corundum-type phase and the phase with ludwigite-related structure has been found only in MgAl₂O₄ among various A²⁺B³⁺₂O₄ compounds.     

Crystal and magnetic structures of Sr4MMn2O9 (M=Cu or Zn)

The crystal and magnetic structures of Sr₄MMn₂O₉ (M=Cu, Zn) have been refined from neutron powder diffraction data. These trigonal compounds (space group P321, a=9.5918(1), c=7.8114(1) Å (Cu); a=9.5894(1), c=7.5039(1) Å (Zn)) are n=3 members of the series A_3n+3M_nB_n+3O_6n+9, with each unit cell containing three offset [001] polyhedral chains, each of which ideally contains a 1:1 ratio of B₂O₉ units and MO₆ trigonal prisms. In fact anti-site disorder between Mn and M is observed, and for M=Cu the cations are disordered off the center of the prism towards a rectangular face. Both compositions show 3D anti-ferromagnetic order at 1.6 K, with an ordered magnetic moment of 1.91(6) (M=Cu) or 1.8(1) (M=Zn) μ_B per Mn. No ordered magnetic moment was detected on the trigonal prismatic site in either compound, consistent with the observed temperature dependence of the magnetic susceptibility.     

Synthesis, structural characterization and Li ion conductivity of a new vanado-molybdate phase, LiMg3VMo2O12

A new vanado-molybdate LiMg₃VMo₂O₁₂ has been synthesized, the crystal structure determined an ionic conductivity measured. The solid solution Li_2−zMg_2+zV_zMo_3−zO₁₂ was investigated and the structures of the z=0.5 and 1.0 compositions were refined by Rietveld analysis of powder X-ray (XRD) and powder neutron diffraction (ND) data. The structures were refined in the orthorhombic space group Pnma with a∼5.10, b∼10.4 and c∼17.6 Å, and are isostructural with the previously reported double molybdates Li₂M₂(MoO₄)₃ (M=M²⁺, z=0). The structures comprise of two unique (Li/Mg)O₆ octahedra, (Li/Mg)O₆ trigonal prisms and two unique (Mo/V)O₄ tetrahedra. A well-defined 1:3 ratio of Li⁺:Mg²⁺ is observed in octahedral chains for LiMg₃VMo₂O₁₂. Li⁺ preferentially occupies trigonal prisms and Mg²⁺ favours octahedral sheets. Excess V⁵⁺ adjacent to the octahedral sheets may indicate short-range order. Ionic conductivity measured by impedance spectroscopy (IS) and differential scanning calorimetry (DSC) measurements show the presence of a phase transition, at 500-600 °C, depending on x. A decrease in activation energy for Li⁺ ion conductivity occurs at the phase transition and the high temperature structure is a good Li⁺ ion conductor, with σ=1×10⁻³-4×10⁻² S cm⁻¹ and E_a=0.6 to 0.8 eV.     

Contrasting oxide crystal chemistry of Nb and Ta:: The structures of the hexagonal bronzes BaTa2O6 and Ba0.93Nb2.03O6

The high-temperature hexagonal forms of BaTa₂O₆ and Ba_0.93Nb_2.03O₆ have P6/mmm symmetry with unit-cell parameters a=21.116(1) Å, c=3.9157(2) Å and a=21.0174(3) Å, c=3.9732(1) Å, respectively. Single crystal X-ray structure refinements for both phases are generally consistent with a previously proposed model, except for displacements of some Ba atoms from high-symmetry positions. The structures are based on a framework of corner- and edge-connected Nb/Ta-centred octahedra, with barium atoms occupying sites in four different types of [0 0 1] channels with hexagonal, triangular, rectangular and pentagonal cross-sections. The refinements showed that the non-stoichiometry in the niobate phase is due to barium atom vacancies in the pentagonal channels and to extra niobium atoms occupying interstitial sites with tri-capped trigonal prismatic coordination. The origin of the non-stoichiometry is attributed to minimisation of non-bonded Ba-Ba repulsions. The hexagonal structure is related to the structures of the low-temperature forms of BaNb₂O₆ and BaTa₂O₆, through a 30° rotation of the hexagonal rings of octahedra centred at the origin.     

Crystal structure and magnetic properties of the solid-solution phase Ca3Co2−vScvO6

The two crystallographically non-equivalent Co atoms of the quasi-one-dimensional crystal structure of Ca₃Co₂O₆ form chains with alternating, face-sharing polyhedra of Co2O₆ trigonal prisms and Co1O₆ octahedra. This compound forms a substitutional solid-solution phase with Sc, in which the Sc atoms enter the Co2 sublattice exclusively. The homogeneity range of Ca₃Co_2−vSc_vO₆ (more specifically Ca₃Co1Co2_1−vSc_vO₆) extends up to v≈0.55. The crystal structure belongs to space group R3¯c with lattice parameters (in hexagonal setting): 9.0846(3)?a?9.1300(2) Å and 10.3885(4)?c?10.4677(4) Å. The magnetic moment decreases rapidly with increasing amount of the non-magnetic Sc solute in the lattice.     

Syntheses, crystal structures and Raman spectra of Ba(BF4)(PF6), Ba(BF4)(AsF6) and Ba2(BF4)2(AsF6)(H3F4); the first examples of metal salts containing simultaneously tetrahedral BF4 and octahedral AF6 anions

In the system BaF₂/BF₃/PF₅/anhydrous hydrogen fluoride (aHF) a compound Ba(BF₄)(PF₆) was isolated and characterized by Raman spectroscopy and X-ray diffraction on the single crystal. Ba(BF₄)(PF₆) crystallizes in a hexagonal space group with a=10.2251(4) Å, c=6.1535(4) Å, V=557.17(5) Å³ at 200 K, and Z=3. Both crystallographically independent Ba atoms possess coordination polyhedra in the shape of tri-capped trigonal prisms, which include F atoms from BF₄⁻ and PF₆⁻ anions. In the analogous system with AsF₅ instead of PF₅ the compound Ba(BF₄)(AsF₆) was isolated and characterized. It crystallizes in an orthorhombic Pnma space group with a=10.415(2) Å, b=6.325(3) Å, c=11.8297(17) Å, V=779.3(4) Å³ at 200 K, and Z=4. The coordination around Ba atom is in the shape of slightly distorted tri-capped trigonal prism which includes five F atoms from AsF₆⁻ and four F atoms from BF₄⁻ anions. When the system BaF₂/BF₃/AsF₅/aHF is made basic with an extra addition of BaF₂, the compound Ba₂(BF₄)₂(AsF₆)(H₃F₄) was obtained. It crystallizes in a hexagonal P6₃/mmc space group with a=6.8709(9) Å, c=17.327(8) Å, V=708.4(4) Å³ at 200 K, and Z=2. The barium environment in the shape of tetra-capped distorted trigonal prism involves 10 F atoms from four BF₄⁻, three AsF₆⁻ and three H₃F₄⁻ anions. All F atoms, except the central atom in H₃F₄ moiety, act as μ₂-bridges yielding a complex 3-D structural network.     

High-pressure synthesis and structures of novel chromium sulfides, Ba3CrS5 and Ba3Cr2S6 with one-dimensional chain structures

Two new ternary chromium sulfides, Ba₃CrS₅, and Ba₃Cr₂S₆ were synthesized by the reaction of sulfur, barium sulfide, and chromium metal under a high pressure of 5 GPa at 1200°C. Ba₃CrS₅ crystallized in the hexagonal space group P6₃cm (No. 185) with a=9.1208(3) Å, c=6.1930(3) Å, V=446.17(3) Å³, and Z=6. It had a column structure with one-dimensional chains of [CrS₃]_∞ composed of face-sharing CrS₆ octahedra surrounded with Ba²⁺ ions. Additional S columns surrounded with Ba ions were running along with the CrS₆ columns. Ba₃Cr₂S₆ crystallized in the trigonal space group R-3c (No. 167) with a=11.8179(7) Å, c=12.796(1) Å, V=1547.7(2) Å³, and Z=6. The structure of Ba₃Cr₂S₆ also contains [CrS₃]_∞ chains but the chains are composed of octahedral and trigonal prismatic CrS₆ units, which are alternately stacked in a face-sharing manner. The formal charges of Cr ions in Ba₃CrS₅ and Ba₃Cr₂S₆ are 4+ and 3+, respectively.     

Synthesis, structure and magnetic properties of Na6Co2O6

Na₆Co₂O₆ was synthesized via the azide/nitrate route by reaction between NaN₃, NaNO₃ and Co₃O₄. Stoichiometric mixtures of the starting materials were heated in a special regime up to 500°C and annealed at this temperature for 50 h in silver crucibles. Single crystals have been grown by subsequent annealing of the reaction product at 500°C for 500 h in silver crucibles, which were sealed in glass ampoules under dried Ar. According to the X-ray analysis of the crystal structure (, Z=1, a=5.7345(3), b=5.8903(3), c=6.3503(3) Å, α=64.538(2), β=89.279(2), γ=85.233(2)°, 1006 independent reflections, R₁=8.34% (all data)), cobalt is tetrahedrally coordinated by oxygen. Each two CoO₄ tetrahedra are linked through a common edge forming Co₂O₆^6- anions. Cobalt ions within the dimers, being in a high spin state (S=2), are ferromagnetically coupled (J=17 cm^-1). An intercluster spin exchange (zJ′=−4.8 cm^-1) plays a significant role below 150 K and leads to an antiferromagnetically ordered state below 30 K. Heat capacity exhibits a λ-type anomaly at this temperature and yields a value of 19.5 J/mol K for the transition entropy, which is in good agreement with the theoretical value calculated for the ordering of the ferromagnetic-coupled dimers. In order to construct a model for the spin interactions in Na₆Co₂O₆, the magnetic properties of Na₅CoO₄ have been measured. This compound features isolated CoO₄ tetrahedra and shows a Curie-Weiss behavior (μ=5.14 μ_B, Θ=−20 K) down to 15 K. An antiferromagmetic ordering is observed in this compound below 10 K.     

Crystal structure and magnetic properties of Sr4Mn2NiO9

The crystal structure of Sr₄Mn₂NiO₉ has been refined on single crystal. This phase belongs to the series A_1+x(A^′_xB_1–x)O₃ (x=1/3) related to the 2H-hexagonal perovskite. The structure contains transition metals in chains of oxide polyhedra (trigonal prisms and octahedra); neighboring chains are separated from each other by the Sr atoms. The sequence of the face sharing polyhedra along the chains is two octahedra + one trigonal prism. Mn occupies the octahedra and Ni is disordered in the trigonal prism with ≈80% in the pseudo square faces of the prism and ≈20% at the centre. This result has been confirmed by XANES experiments at Mn K and Ni K edges, respectively. Sr₄Mn₂NiO₉ is antiferromagnetic with a Néel temperature at T=3 K. The Curie constant measured at high temperature is in good agreement with ≈80% of the Ni²⁺ ions in the spin state configuration S=0.     

Synthesis and structural characterization of novel tin and titanium potassium silicates K4M2Si6O18

The synthesis of a new potassium titanosilicate, K₄Ti₂Si₆O₁₈ (Ti-AV-11), possessing the crystal structure of potassium stannosilicate AV-11, has been reported. The unit cell of this material is trigonal, space group R3 (no. 146), Z=3, a=10.012, c=14.8413 Å, γ=120°, V=1289 Å³. The structure of AV-11 is built up of MO₆ (M=Sn, Ti) octahedra and SiO₄ tetrahedra by sharing corners. The SiO₄ tetrahedra form helix chains, periodically repeating every six tetrahedra. These chains extend along the [001] direction and are linked by isolated MO₆ octahedra, thus producing a mixed octahedral-tetrahedral oxide framework. AV-11 materials have been further characterized by bulk chemical analysis, powder X-ray diffraction (XRD), scanning electron microscopy (SEM), ²⁹Si and ¹¹⁹Sn magic-angle spinning (MAS) NMR spectroscopy.     

Crystal Structure of SrAl2B2O7 and Eu Luminescence

The crystal structure of strontium dialuminodiborate SrAl₂B₂O₇ has been established by single-crystal X-ray diffraction methods. The compound crystallizes in the trigonal system (space group R c, Z=6) with cell parameters a=4.893(1) Å and c=47.78(1) Å. Aluminium and boron atoms are, respectively, in tetrahedral and triangular oxygen coordination. The assembly of Al₂O₇ units and BO₃ triangles forms double layers between which Sr²⁺ ions are located. The Eu²⁺-doped crystalline powder exhibits a luminescence band with maximum at 415 nm. Luminescence characteristics are compared to those of other strontium borates.     

Single-crystal synthesis and structure refinement of the LiCoO2-LiAlO2 solid-solution compounds: LiAl0.32Co0.68O2 and LiAl0.71Co0.29O2

Single crystals of the LiCoO₂-LiAlO₂ solid solution compounds LiAl_0.32Co_0.68O₂ and LiAl_0.71Co_0.29O₂ were synthesized by a flux method using alumina crucibles. A single-crystal X-ray diffraction study confirmed the trigonal space group and the lattice parameters a=2.8056(11) Å, c=14.1079(15) Å, and c/a=5.028 for LiAl_0.32Co_0.68O₂, and a=2.8023(7) Å, c=14.184(4) Å, and c/a=5.061 for LiAl_0.71Co_0.29O₂. The crystal structures have been refined to the conventional values R=3.2% and wR=2.4% for LiAl_0.32Co_0.68O₂, and R=3.6% and wR=3.5% for LiAl_0.71Co_0.29O₂. The evidence of the location of Al atoms in the pseudotetragonal coordination (6c site), reported previously in LiAl_0.2Co_0.8O₂, could not be observed in the present electron density distribution maps in both LiAl_0.32Co_0.68O₂ and LiAl_0.71Co_0.29O₂. The octahedral distortion analysis indicated that the Al-substitution strongly affected the distortion of the LiO₆ octahedron in this solid-solution compound system, but hardly affected that of the (Al.Co)O₆ octahedron.     

Er17Ru6Te3: A highly condensed metal-rich ternary telluride

Er₁₇Ru₆Te₃ is obtained from high-temperature solid-state reactions in tantalum ampoules. The structure according to single-crystal X-ray diffraction is monoclinic, C2/m (no. 12), Z=4, a=40.185(8) Å, b=3.9969(8) Å, c=16.037(3) Å, β=95.12(3)°, V=2565.5(9) Å³. The condensed structure consists of a complex intermetallic network of intergrown sheets of edge-sharing tetrakaidecahedra (tricapped trigonal prisms, TCTP), and pairs of rectangular-face-sharing bicapped trigonal prisms (BCTP) built of erbium and centered by ruthenium. This array also contains isolated columns of TCTP erbium normal to these sheets that contain tellurium. Basal face sharing of all Er polyhedra along the short b-axis gives rise to the three-dimensional network. Synthesis and the crystal structure of the compound are discussed.     

Synthesis and characterization of Sr0.75Y0.25Co1−xMxO2.625+δ (M=Ga, 0.125?x?0.500 and M=Fe, 0.125?x?0.875)

The effect of replacing Co³⁺ by Ga³⁺ and Fe³⁺ in the perovskite-related tetragonal phase Sr_0.75Y_0.25CoO_2.625 with unit cell parameters: a=2a_p, and c=4a_p (314 phase) has been investigated. The 314 phase is formed by Sr_0.75Y_0.25Co_1−xM_xO_2.625+δ, with x?0.375 for M=Ga and x?0.625 for M=Fe. High-resolution transmission electron microscopy and electron diffraction revealed frequent microtwinning in the iron-containing compounds, in contrast to the Ga-substituted 314 phases. Diffraction experiments and electron microscope images indicated that at higher Fe contents, 0.75?x?0.875, a disordered cubic perovskite structure forms. The crystal structures of Sr_0.75Y_0.25Co_0.75Ga_0.25O_2.625 and Sr_0.75Y_0.25Co_0.5Fe_0.5O_2.625+δ were refined using neutron powder diffraction data. It was found that the oxygen content of Sr_0.75Y_0.25Co_0.5Fe_0.5O_2.625+δ is higher than in Fe-free 314 phase, so that δ corresponds to 0.076, whereas δ=0 in Sr_0.75Y_0.25Co_0.75Ga_0.25O_2.625+δ. Magnetization measurements on the unsubstituted Sr_0.7Y_0.3CoO_2.62 and Ga-substituted Sr_0.75Y_0.25Co_0.75Ga_0.25O_2.625 compounds indicate the presence of a ferromagnetic-like contribution to the measured magnetization at 320 and 225 K, respectively, while replacing Co by Fe leads to the suppression of this contribution. A neutron diffraction study shows that the Sr_0.75Y_0.25Co_0.5Fe_0.5O_2.625+δ compound is G-type antiferromagnetic at room temperature, whereas Sr_0.75Y_0.25Co_0.75Ga_0.25O_2.625 does not exhibit magnetic ordering at room temperature.     

Reduction properties of phases in the system La0.5Sr0.5MO3 (M=Fe, Co)

Phases formed by the reduction of compounds of the type La_0.5Sr_0.5MO₃ (M=Fe, Co) have been characterized by means of temperature programmed reduction, X-ray powder diffraction, ⁵⁷Fe Mössbauer spectroscopy and Fe K-, Co K-, Sr K-, and La L_III-edge X-ray absorption spectroscopy. The results show that treatment of the material of composition La_0.5Sr_0.5FeO₃ (which contains 50% Fe⁴⁺ and 50% Fe³⁺) at 650 °C in a flowing 90% hydrogen/10% nitrogen atmosphere results in the formation of an oxygen-deficient perovskite-related phase containing only trivalent iron. Further heating in the gaseous reducing environment at 1150 °C results in the formation of the Fe³⁺-containing phase SrLaFeO₄, which has a K₂NiF₄-type structure, and metallic iron. The material of composition La_0.5Sr_0.5CoO₃ is more susceptible to reduction than the compound La_0.5Sr_0.5FeO₃ since, after heating at 520 °C in the hydrogen/nitrogen mixture, all the Co⁴⁺ and Co³⁺ are reduced to metallic cobalt with the concomitant formation of strontium- and lanthanum-oxides.     

Crystal chemistry and crystallography of the SrR2CuO5 (R=lanthanides) phases

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.