Stability and oxide ion conductivity in rare-earth aluminium cuspidines

RE₄(Al_2−xGe_xO_7+x/2□_1−x/2)O₂ (RE=Gd³⁺ and Nd³⁺) oxy-cuspidine series have been prepared by ceramic method (RE=Gd³⁺) and freeze-dried precursor method (RE=Nd³⁺). The compositional ranges and the high temperature stability have been determined for both series. Gadolinium aluminium cuspidines are stable at very high temperatures but the analogous neodymium compounds are only stable below 1273 K. Joint Rietveld analyses of neutron powder diffraction (NPD) and laboratory X-ray powder diffraction (LXRPD) have been carried out for Nd₄(Al₂O₇□₁)O₂ and Nd₄(Al_1.5Ge_0.5O_7.25□_0.75)O₂ compositions. Furthermore, Rietveld refinement of synchrotron X-ray powder diffraction (SXRPD) data were carried out for Gd₄(Al_1.0Ge_1.0O_7.5□_0.5)O₂ composition. The refinements have confirmed the known structural features of the cuspidine framework. These cuspidines series are oxide ion conductors with negligible electronic contribution as determined from impedance spectroscopy at variable oxygen partial pressures. The enhancement in the overall oxide conductivity along the two oxy-cuspidine series is two orders of magnitude. Typical ionic conductivity values for doped samples are around 4×10⁻⁵ S cm⁻¹ at 973 K.     

La6Mo8O33: a new ordered defect scheelite superstructure

The structure of La₆Mo₈O₃₃ has been determined from a triple pattern powder diffraction analysis. Two high-resolution neutron diffraction patterns collected at 1.594 and 2.398 Å and one X-rays were used. This molybdate crystallizes in a non-centrosymmetric monoclinic space group P2₁(N°4), Z=2,a=10.7411(3) Å, b=11.9678(3) Å, c=11.7722(3) Å, β=116.062 (1)°. La₆Mo₈O₃₃ is an unusual ordered defect Scheelite. Hence, it should be described with cation vacancies and an extra oxygen atom following the formula: La₆□₂Mo₈O₃₂₊₁. This extra oxygen atom leads to a pyramidal environment, whereas the other molybdenum atoms present tetrahedral environment. A molybdenum tetrahedral is connecting to the pyramid, forming an [Mo₂O₉] unit.     

The ionic conductivity and local environment of cations in Bi9ReO17

The influence of temperature on the structure of Bi₉ReO₁₇ has been investigated using differential thermal analysis, variable temperature X-ray diffraction and neutron powder diffraction. The material undergoes an order-disorder transition at ∼1000 K on heating, to form a fluorite-related phase. The local environments of the cations in fully ordered Bi₉ReO₁₇ have been investigated by Bi L_III- and Re L_III-edge extended X-ray absorption fine structure (EXAFS) measurements to complement the neutron powder diffraction information. Whereas rhenium displays regular tetrahedral coordination, all bismuth sites show coordination geometries which reflect the importance of a stereochemically active lone pair of electrons. Because of the wide range of Bi-O distances, EXAFS data are similar to those observed for disordered structures, and are dominated by the shorter Bi-O bonds. Ionic conductivity measurements indicate that ordered Bi₉ReO₁₇ exhibits reasonably high oxide ion conductivity, corresponding to 2.9×10⁻⁵ Ω⁻¹ cm^-1 at 673 K, whereas the disordered form shows higher oxide ion conductivity (9.1×10⁻⁴ Ω⁻¹ cm⁻¹ at 673 K).     

Magnetic and thermodynamic properties of NdT2Ge2 (T=Pd, Ag) compounds

Physical properties of NdPd₂Ge₂ and NdAg₂Ge₂, crystallizing with the tetragonal ThCr₂Si₂-type crystal structure, were investigated by means of magnetic, calorimetric, electrical transport as well as by neutron diffraction measurements. The specific heat studies and neutron diffraction measurements were performed down to 0.30 K and 0.47 K, respectively. Both compounds exhibit antiferromagnetic ordering below T_N equal to 1.5 K for NdPd₂Ge₂ and 1.8 K for NdAg₂Ge₂. Neutron diffraction data for the latter germanide indicate antiferromagnetic collinear structure described by the propagation vector k=(0.5, 0, 0.5). The Nd magnetic moments equal to 2.24(5) μ_B at 0.47 K are aligned along the a-axis and have the +− sequence within the crystal unit cell. For NdPd₂Ge₂ only very small Bragg peaks of magnetic origin were observed in the neutron diffraction patterns measured below T_N, thus hampering determination of the magnetic structure. Both compounds exhibit metallic-like electrical conduction. From the specific heat data the crystal electric field (CEF) levels schemes were determined. Difference between the overall CEF splitting in the two compounds is correlated with their structural parameters.     

Formation of Co octahedra and tetrahedra in YBaCo4O8.1

High-resolution neutron and synchrotron X-ray powder diffraction experiments were performed, at 300 and 10 K, for the determination of the structure of YBaCo₄O_8.1, which was prepared by controlled oxidation of the Kagomé lattice compound YBaCo₄O₇. Our diffraction data demonstrate that YBaCo₄O_8.1 crystallizes in the orthorhombic Pbc2₁ space group with the formation of a large superstructure (a=12.790 Å, b=10.845 Å, c=10.149 Å), with respect to the parent trigonal YBaCo₄O₇ material. The Co ions occupy both corner-sharing tetrahedral and edge-sharing octahedral sites, in contrast to YBaCo₄O₇, which has only corner-sharing tetrahedra. The octahedral sites form by the addition of two extra oxygen atoms and the drastic displacements of some of the original O atoms relative to the parent. The edge-sharing octahedra form isolated zigzag chains parallel to the c-axis linked to one another via tetrahedra. While found in a few phosphates, silicates and germanates, this motif appears unique to YBaCo₄O_8.1 among mixed-metal oxides. No structural phase transition or long range antiferromagnetic ordering are observed at 10 K.     

Neutron powder diffraction study of the layer organic-inorganic hybrid iron(II) methylphosphonate-hydrate, Fe[(CD3PO3)(D2O)]

The crystal and magnetic structures of the hybrid organic-inorganic layer compound Fe[(CD₃PO₃)(D₂O)] have been studied by neutron powder diffraction as a function of temperature down to 1.5 K. The neutron diffraction pattern recorded at 200 K shows that the fully deuterated compound crystallizes in one of the two known forms of the undeuterated Fe[(CH₃PO₃)(H₂O)]. The crystal structure is orthorhombic, space group Pmn2₁, with the following unit-cell parameters: a=5.7095(1) Å, b=8.8053(3) Å and c=4.7987(1) Å; Z=2. The crystal structure remains unchanged on cooling from 200 to 1.5 K. Moreover, at low temperature, Fe[(CD₃PO₃)(D₂O)] shows a commensurate magnetic structure (k=(0,0,0)). As revealed by bulk susceptibility measurements on Fe[(CH₃PO₃)(H₂O)], the magnetic structure corresponds to a canted antiferromagnet with a critical temperature T_N=25 K. Neutron powder diffraction reveals that below T_N=23.5 K the iron magnetic moments in Fe[(CD₃PO₃)(D₂O)] are antiferromagnetically coupled and oriented along the b-axis, perpendicular to the inorganic layers. No ferromagnetic component is observable in the neutron powder diffraction experiment, due to its too small value (<0.1μ_B).     

Thermal expansion and cation disorder in Bi2InNbO7

The structure of the pyrochlore-type oxide Bi₂InNbO₇ has been investigated between room temperature and 700 °C using electron and synchrotron X-ray powder diffraction and at room temperature and 10 K using neutron diffraction methods. Bi₂InNbO₇ exhibits an A₂B₂O₇ cubic pyrochlore-type average structure at all temperatures that is characterized by an apparently random mixing of the In³⁺ and Nb⁵⁺ cations on the octahedral B sites. The Bi cations on the eight-coordinate pyrochlore A sites are displacively disordered, presumably as a consequence of their lone pair electron configuration. Heating the sample does not alter this disorder.     

Crystal and magnetic structures of the brownmillerite compound Ca2Fe1.039(8)Mn0.962(8)O5

The crystal and magnetic structures of the brownmillerite material, Ca₂Fe_1.039(8)Mn_0.962(8)O₅ were investigated using powder X-ray and neutron diffraction methods, the latter from 3.8 to 700 K. The compound crystallizes in Pnma space group with unit cell parameters of a=5.3055(5) Å, b=15.322(2) Å, c=5.4587(6) Å at 300 K. The neutron diffraction study revealed the occupancies of Fe³⁺ and Mn³⁺ ions in both octahedral and tetrahedral sites and showed some intersite mixing and a small, ∼4%, Fe excess. While bulk magnetization data were inconclusive, variable temperature neutron diffraction measurements showed the magnetic transition temperature to be 407(2) K below which a long range antiferromagnetic ordering of spins occurs with ordering wave vector k=(000). The spins of each ion are coupled antiferromagnetically with the nearest neighbors within the same layer and coupled antiparallel to the closest ions from the neighboring layer. This combination of intra- and inter-layer antiparallel arrangement of spins forms a G-type magnetic structure. The ordered moments on the octahedral and tetrahedral sites at 3.8 K are 3.64(16) and 4.23(16) μ_B, respectively.     

Structure and thermal conductivity of Na1−xGe3+z

Structural analyses as well as low temperature thermal conductivity is reported for the binary phase Na_1−xGe_3+z. Specimens were characterized by thermal analysis, conventional and synchrotron powder X-ray diffraction, neutron powder diffraction, ²³Na nuclear magnetic resonance spectroscopy, and electrical and thermal transport measurements. With structural characteristics qualitatively analogous to some aluminum-silicate zeolites, the crystal structure of this phase exhibits an unconventional covalently bonded tunnel-like Ge framework, accommodating Na in channels of two different sizes. Observed to be non-stochiometric, Na_1−xGe_3+z concurrently exhibits substantial structural disorder in the large channels and a low lattice thermal conductivity, of interest in the context of identifying novel low thermal conductivity intermetallics for thermoelectric applications.     

High-temperature neutron diffraction study of the cation ordered perovskites TbBaMn2O5+x and TbBaMn2O5.5−y

The miscibility of TbBaMn₂O_5+x and TbBaMn₂O_5.5−y has been investigated at 100-600 °C using in situ powder neutron diffraction. No miscibility is observed, and the two phases remain oxygen stoichiometric (x,y=0) at 600 °C. Structure refinement results show that neither material undergoes a phase transition in this temperature range. TbBaMn₂O₅ is Mn²⁺/Mn³⁺ charge ordered and any charge melting transition is >600 °C. This symmetry-broken charge ordering is remarkably robust in comparison to that in other oxides.     

Crystalline Structure and Electroconductivity of Solid Electrolytes Li3.75Ge0.75V0.25O4 and Li3.70Ge0.85W0.15O4

Crystalline structure of low- and high-temperature modifications of solid lithium-conducting electrolytes Li_3.75Ge_0.75V_0.25O₄ and Li_3.70Ge_0.85W_0.15O₄ are studied by powder neutron diffraction analysis with use made of Rietveld full-profile analysis. The structure of either composition is derivative from -Li₃PO₄. The high conductivity of the electrolytes is connected with the presence of interstitial lithium cations, whose positions are occupied but partially. Phase transitions in the electrolytes consist of the leveling of the occupation degrees of these positions and make practically no impact on the rigid cage of the lattice.     

The ability of RP-type cobaltites to accommodate carbonate groups: A new layered oxide Sr4Co2(CO3)O5.86

The possibility to introduce carbonate groups inside a pure cobalt based Ruddlesden Popper type matrix is demonstrated. This paper reports the synthesis as a single phase of a new layered oxycarbonate Sr₄Co₂(CO₃)O_5.86 and its structural characterizations at room temperature combining TEM observations and powder neutron diffraction data. Close structural relationships are observed with the homologous Sr₄Fe_3−x(CO₃)_xO_10−4x series and deal with the presence of two specific carbonate configurations, namely coat hanger and flag. Magnetization measurements reveal a complex antiferromagnetic behavior below 50 K with three others transitions, at 160, 210 and 250 K. Such behavior can be ascribed to the latent reactivity in air of this largely oxygen deficient phase.     

High‐Pressure Synthesis and Physical Properties of the Europium‐Substituted Barium Clathrate Ba8−xEuxGe43□3 (x ≤ 0.6)

The new clathrate Ba_8–xEuxGe₄₃□₃ (x = 0.6) was synthesized at a pressure of 1 GPa and temperatures of up to 1220 K by means of a multi‐anvil device (Walker module) and a hydraulic 1000 ton press. X‐ray powder diffraction data indicate that the crystal structure of Ba_8–xEuxGe₄₃□₃ (x = 0.6, space group , a = 21.2588(3) Å) corresponds to that of Ba₈Ge₄₃□₃. Measurements of the magnetic susceptibility of Ba_8–xEuxGe₄₃□₃ reveal Curie‐paramagnetic behaviour and prove that the electronic state of europium corresponds to 4f⁷, i.e., Eu²⁺. Electrical resistivity shows a metal‐like temperature dependence and ρ(300) ≈ 2mΩ cm for x = 0.6. Heat capacity measurements reveal an upturn of c_p/T(T) below 7 K that is attributed to magnetic interaction of the europium ions.     

Oxygen vacancy ordering within anion-deficient Ceria

The structural properties of anion deficient ceria, CeO_2−δ, have been studied as a function of oxygen partial pressure, p(O₂), over the range 0≥log₁₀ p(O₂)≥−18.9 at 1273(2) K using the neutron powder diffraction technique. Rietveld refinement of the diffraction data collected on decreasing p(O₂) showed increases in the cubic lattice parameter, a, the oxygen nonstoichiometry, δ, and the isotropic thermal vibration parameters, u_Ce and u_O, starting at log₁₀ p(O₂)~−11. The increases are continuous, but show a distinct kink at log₁₀ p(O₂)~−14.5. Analysis of the total scattering (Bragg plus diffuse components) using reverse Monte Carlo (RMC) modelling indicates that the O²⁻ vacancies preferentially align as pairs in the 〈111〉 cubic directions as the degree of nonstoichiometry increases. This behaviour is discussed with reference to the chemical crystallography of the CeO₂-Ce₂O₃ system at ambient temperature and, in particular, to the nature of the long-range ordering of O²⁻ vacancies within the crystal structure of Ce₇O₁₂.     

Magnetic ordering and spin structure in Ca-bearing clinopyroxenes CaM(Si, Ge)2O6, M=Fe, Ni, Co, Mn

The compounds CaFeSi₂O₆ (hedenbergite), CaNiGe₂O₆, CaCoGe₂O₆ and CaMnGe₂O₆ have been synthesized by hydrothermal or ceramic sintering techniques and were subsequently characterized by SQUID magnetometry and powder neutron diffraction in order to determine the magnetic properties and the spin structure at low temperature. All four compounds reveal the well-known clinopyroxene structure-type with monoclinic symmetry, space group C2/c, Z=4 at all temperatures investigated. Below 35 K hedenbergite shows a ferromagnetic (FM) coupling of spins within the infinite M1 chains of edge-sharing octahedra. This FM coupling dominates an antiferromagnetic (AFM) coupling between neighbouring chains. The magnetic moments lie within the a-c plane and form an angle of 43° with the crystallographic a-axis. Magnetic ordering in CaFeSi₂O₆ causes significant discontinuities in lattice parameters, Fe-O bond lengths and interatomic Fe-Fe distances through the magnetic phase transition, which could be detected from the Rietveld refinements of powder neutron diffraction data. CaCoGe₂O₆ and CaNiGe₂O₆ show magnetic ordering below 18 K, the spin structures are similar to the one in hedenbergite with an FM coupling within and an AFM coupling of spins between the M1 chains. The moments lie within the a-c plane. The paramagnetic Curie temperature, however, decreases from CaFeSi₂O₆ (+40.2 K) to CaCoGe₂O₆ (+20.1 K) and CaNiGe₂O₆ (−13.4 K), suggesting an altered interplay between the concurring AFM and FM interaction in and between the M1 chains. CaMnGe₂O₆ finally shows an AFM ordering below 11 K. Here the magnetic moments are mainly oriented along the a-axis with a small tilt out from the a-c plane.     

Nano size crystals of goethite, α-FeOOH: Synthesis and thermal transformation

An aqueous suspension of amorphous iron(III) hydroxide was kept at room temperature (298 K) for 23 years. During this period of time the pH of the liquid phase changed from 4.3 to 2.85, and nano size crystals of goethite, α-FeOOH crystallised from the amorphous iron(III) hydroxide. Transmission electron microscopy (TEM) investigations, Mössbauer spectra, and powder X-ray diffraction using Co Kα radiation showed that the only iron containing crystalline phase present in the recovered product was α-FeOOH. The size of these nano particles range from 10 to 100 nm measured by TEM. The thermal decomposition of α-FeOOH was investigated by time-resolved in situ synchrotron radiation powder X-ray diffraction and the data showed that the sample of α-FeOOH transformed to α-Fe₂O₃ in the temperature range 444-584 K. A quantitative phase analysis shows the increase in scattered X-ray intensity from α-Fe₂O₃ to follow the decrease of intensity from α-FeOOH in agreement with the topotactic phase transition.     

The hydrothermal decomposition of calcium monosulfoaluminate 14-hydrate to katoite hydrogarnet and β-anhydrite: An in-situ synchrotron X-ray diffraction study

We apply in-situ synchrotron X-ray diffraction to study the transformation of calcium monosulfoaluminate 14-hydrate Ca₄Al₂O₆(SO₄)·14H₂O [monosulfate-14] to hydrogarnet Ca₃Al₂(OH)₁₂ on the saturated water vapor pressure curve up to 250 °C. We use an aqueous slurry of synthetic ettringite Ca₆Al₂(SO₄)₃(OH)₁₂·26H₂O as the starting material; on heating, this decomposes at about 115 °C to form monosulfate-14 and bassanite CaSO₄·0.5H₂O. Above 170 °C monosulfate-14 diffraction peaks slowly diminish in intensity, perhaps as a result of loss of crystallinity and the formation of an X-ray amorphous meta-monosulfate. Hydrogarnet nucleates only at temperatures above 210 °C. Bassanite transforms to β-anhydrite (insoluble anhydrite) at about 230 °C and this transformation is accompanied by a second burst of hydrogarnet growth. The transformation pathway is more complex than previously thought. The mapping of the transformation pathway shows the value of rapid in-situ time-resolved synchrotron diffraction.     

Synthesis, crystal structure and magnetic properties of a new pillared perovskite La5Mo2.75V1.25O16

A new pillared perovskite compound La₅Mo_2.76(4)V_1.25(4)O₁₆, has been synthesized by solid-state reaction and its crystal structure has been characterized using powder X-ray and neutron diffraction. The magnetic properties of this compound have been investigated using SQUID magnetometry, and the magnetic structure has been studied using neutron diffraction data. A theoretical calculation of relative strengths of spin interactions among different magnetic ions and through different pathways has been performed using extended Hückel, spin dimer analysis. The crystal structure of this material contains perovskite-type layers that are connected through edge-sharing dimeric units of octahedra. The structure is described in space group C2/m with unit cell parameters a=7.931(2) Å, b=7.913(2) Å, c=10.346(5) Å and β=95.096(5)°. The material shows both short-range ferrimagnetic correlations from ∼200 to 110 K and long-range antiferromagnetic order below T_c∼100 K. The magnetic structure was investigated by neutron diffraction and is described by k=(0 0 ) as for other pillared perovskites. It consists of a ferrimagnetic arrangement of Mo and V within the layers that are coupled antiferromagnetically between layers. This is the first magnetic structure determination for any Mo-based pillared perovskite.     

Structural and X-ray powder diffraction studies of nano-structured lead(II) coordination polymer with η Pb?C interactions

A new nano-sized Pb(II) one-dimensional coordination polymer with η² Pb-C interactions, [Pb₂(μ₃-ba)₂(μ₂-ba)₂]_n (1) [ba⁻ = benzylacetylacetonate] has been synthesized and characterized by SEM, X-ray powder diffraction, IR spectroscopy and elemental analyses. Single-crystal X-ray diffraction shows the coordination number of Pb(II) ions is seven and the lead atoms have hemidirected coordination sphere containing involving Pb?C interactions, C₂O₇Pb. PbO nanoparticles were obtained by calcinations of the nano-sized compound 1 at 600 °C.     

Structural and magnetic study of RFe0.5V0.5O3 (R=Y, Eu, Nd, La) perovskite compounds

B-site disordered RFe_0.5V_0.5O₃ compounds, with R=La, Nd, Eu and Y, have been prepared by solid-state reaction technique and their structures and magnetic properties have been investigated through X-ray powder diffraction, time-of-flight neutron powder diffraction and magnetization measurements at temperatures ranging from 5 to 700 K. The four compounds can be described as distorted perovskites with space group symmetry Pbnm and a⁺b⁻b⁻ tilt system. The studied compounds also show antiferromagnetic ordering with Neel temperatures of 299, 304, 304, and 335 K respectively. The magnetic structures of R=La, Nd and Y compounds were determined from the neutron powder diffraction as G_z with observed magnetic moments of 2.55, 2.54 and 2.69μ_B at 30, 40 and 40 K, respectively.