Structural, Magnetic and Catalytic Properties of a New Vacancy Ordered Perovskite Type Barium Cobaltate BaCoO2.67.

A new vacancy ordered, anion deficient perovskite modification with composition of BaCoO2.67 (Ba3 Co3 O8 □1 ) has been prepared via a two-step heating process. Combined Rietveld analysis of neutron and X-ray powder diffraction data shows a novel ordering of oxygen vacancies not known before for barium cobaltates. A combination of neutron powder diffraction, magnetic measurements, and density functional theory (DFT) studies confirms G-type antiferromagnetic ordering. From impedance measurements, the electronic conductivity of the order of 10-4  S cm-1 is determined. Remarkably, the bifunctional catalytic activity for oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) is found to be comparable to that of Ba0.5 Sr0.5 Co0.8 Fe0.2 O3-y , confirming that charge-ordered anion deficient non-cubic perovskites can be highly efficient catalysts.

On the crystal structures and phase transitions of hydrates in the binary dimethyl sulfoxide-water system.

Neutron powder diffraction data have been collected from a series of flash-frozen aqueous solutions of dimethyl sulfoxide (DMSO) with concentrations between 25 and 66.7 mol% DMSO. These reveal the existence of three stoichiometric hydrates, which crystallize on warming between 175 and 195 K. DMSO trihydrate crystallizes in the monoclinic space group P21/c, with unit-cell parameters at 195 K of a = 10.26619 (3), b = 7.01113 (2), c = 10.06897 (3) Å, ß = 101.5030 (2)° and V = 710.183 (3) Å3 (Z = 4). Two of the symmetry-inequivalent water molecules form a sheet of tiled four- and eight-sided rings; the DMSO molecules are sandwiched between these sheets and linked along the b axis by the third water molecule to generate water-DMSO-water tapes. Two different polymorphs of DMSO dihydrate have been identified. The α phase is monoclinic (space group P21/c), with unit-cell parameters at 175 K of a = 6.30304 (4), b = 9.05700 (5), c = 11.22013 (7) Å, ß = 105.9691 (4)° and V = 615.802 (4) Å3 (Z = 4). Its structure contains water-DMSO-water chains, but these are polymerized in such a manner as to form sheets of reniform eight-sided rings, with the methyl groups extending on either side of the sheet. On warming above 198 K, α-DMSO·2H2O undergoes a solid-state transformation to a mixture of DMSO·3H2O + anhydrous DMSO, and there is then a stable eutectic between these two phases at ∼203 K. The ß-phase of DMSO dihydrate has been observed in a rapidly frozen eutectic melt and in very DMSO-rich mixtures. It is observed to be unstable with respect to the α-phase; above ∼180 K, ß-DMSO·2H2O converts irreversibly to α-DMSO·2H2O. At 175 K, the lattice parameters of ß-DMSO·2H2O are a = 6.17448 (10), b = 11.61635 (16), c = 8.66530 (12) Å, ß = 101.663 (1)° and V = 608.684 (10) Å3 (Z = 4), hence this polymorph is just 1.16% denser than the α-phase under identical conditions. Like the other two hydrates, the space group appears likely, on the basis of systematic absences, to be P21/c, but the structure has not yet been determined. Our results reconcile 60 years of contradictory interpretations of the phase relations in the binary DMSO-water system, particularly between mole fractions of 0.25-0.50, and confirm empirical and theoretical studies of the liquid structure around the eutectic composition (33.33 mol% DMSO).

New Li-Mg phosphates with a 3D framework: experimental and ab initio calculations.

Two new lithium-magnesium phosphates LiMg6(PO4)3(P2O7) and Li(Mg5.62Sc0.19Li0.19)(PO4)3(P2O7) were synthesized by a solid-phase method. Using high-resolution time-of-flight neutron powder diffraction (TOF NPD) and X-ray powder diffraction (XRPD), we established that these phosphates have a Pnma orthorhombic structure with the cell parameters a = 9.14664(5) Å, b = 18.83773(8) Å, c = 8.27450(4) Å, and V = 1425.71(1) Å3 and a = 9.14516(5) Å, b = 18.84222(9) Å, c = 8.28204(4) Å, and V = 1427.12(1) Å3, respectively. The crystal structures can be described by stacking of the [Mg6O18]∞ or [Mg5.62Sc0.19Li0.19O18]∞ wavy layers, which are parallel to the (100) direction and interconnected through PO4 tetrahedra and P2O7 groups to form a 3D-framework. The Li atoms are located in large tunnels formed in a 3D lattice, which contributes to lithium diffusion. AC impedance spectroscopy analysis shows that LiMg6(PO4)3(P2O7) and Li(Mg5.62Sc0.19Li0.19)(PO4)3(P2O7) have a Li ion conductivity of 3.6 × 10-4 S cm-1 and 1.7 × 10-4 S cm-1 at 950 °C, with an activation energy of 1.28 eV and 1.55 eV, respectively. NMR MAS studies confirmed the coexistence of pyro- and orthogroups in the structure of both phases and two lithium positions in Li(Mg5.62Sc0.19Li0.19)(PO4)3(P2O7). The first-principles method was used to study the electronic structure and stability of the two phases. The calculated formation enthalpies demonstrated that Sc is a stabilizing impurity in LiMg6(PO4)3(P2O7), while a strong destabilization of olivine LiMgPO4 is observed upon doping with Sc. This explains the failure to synthesize Sc-doped olivine. The new phosphate LiMg6(PO4)3(P2O7) is a dielectric with a band gap of 5.35 eV, which decreases to 4.85 eV due to the appearance of a localized Sc 3d peak upon doping with Sc. These findings are consistent with the results obtained by UV-Vis spectroscopy. The new phase may be a good optical matrix similar to LiMgPO4.

Structural manifestation of partial proton ordering and defect mobility in ice Ih.

Water ice is one of the most fundamental and well-studied molecular solids, which continues to provide surprising and useful insights into its emergent complexity as ever more precise experimental techniques are applied. Using one of the highest resolution neutron powder diffraction instruments in the world, I report a small but systematic distribution in the c/a axial ratios of D2O ice Ih below 160 K that depends upon both the preparation method and the thermal history of the sample. The general decrease in c/a on cooling is interpreted as a consequence of short-range partial ordering in the hydrogen-atom substructure: the synthesis-dependent variation then follows from there being a spectrum of relaxation times between samples formed by slow natural freezing, samples formed by very rapid quench-freezing and samples formed with a small amount of alkali hydroxide dopant, probably due to differences in the relative abundance of orientational versus ionic point defects.

BaCoO2+δ: a new highly oxygen deficient perovskite-related phase with unusual Co coordination obtained by high temperature reaction with short reaction times.

A new highly oxygen deficient metastable modification of perovskite-related BaCoO2+δ (δ ∼ 0.01-0.02) has been prepared using high temperature reactions with short heating times. This defect rich compound has at least partially square planar coordination of the Co2+ ions, a highly unusual coordination environment for Co. Low temperature neutron powder diffraction showed a G-type antiferromagnetic ordering, confirmed by SQUID magnetic measurements, which indicate a high Néel temperature of 220 K. This work shows how novel defective phases can be synthesized by exploiting short reaction times in solid state synthesis, thus offering an alternative route for new materials synthesis.

Synthesis, structure and electrical conductivity of a new perovskite type barium cobaltate BaCoO1.80(OH)0.86.

Perovskite oxides exhibiting mixed protonic and electronic conductivities have interesting applications in protonic ceramic fuel cells. In this work, we report on a hydrated phase of BaCoO1.80(OH)0.86 synthesized using nebulized spray pyrolysis. Structural analysis based on X-ray and neutron powder diffraction data showed that the compound is isotypic to BaFeO2.33(OH)0.33. The water loss behaviour was studied using simultaneous thermal analysis and high temperature X-ray diffraction, indicating that protons (respectively water) can be stabilized within the compound up to temperatures significantly above 673 K, confirmed by ex situ Fourier transform infrared spectroscopy studies. Impedance spectroscopy was used to determine the conductivity characteristics of BaCoO1.80(OH)0.86, finding and a total electrical conductivity in the order of 10-4 S cm-1 at ambient temperature with an activation energy of 0.28 eV.

H/D isotope effect on the molar volume and thermal expansion of benzene.

Time-of-flight neutron powder diffraction data have been collected from C6H6 and C6D6, as a function of temperature between 10 and 276 K using the High Resolution Powder Diffractometer at the ISIS Neutron and Muon Source. In order to achieve high accuracy and precision both in the determination of lattice parameters and in the control of the sample temperature, data were acquired using the same instrumental conditions for each isotopologue with an internal NIST silicon standard and in situ-heated sample holders. In contrast with the work from J. D. Dunitz and R. M. Ibberson, Angew. Chem. Int. Ed., 2008, 47, 4208-4210, we find that the difference in molar volume between the two isotopologues is almost invariant with temperature and that the anisotropic linear expansivities of the crystallographic axes are nearly identical in C6H6 and C6D6.

Neutron powder diffraction studies of sulfuric acid hydrates. II. The structure, thermal expansion, incompressibility, and polymorphism of sulfuric acid tetrahydrate (D2SO4.4D2O).

We report results of the first neutron powder diffraction study of sulfuric acid tetrahydrate (SAT); D(2)SO(4)4D(2)O is tetragonal, space group P42(1)c, with two formula units per unit cell. At 1.7 K the unit-cell dimensions are a=b=7.475 12(6) A, c=6.324 66(5) A and V=353.405(5) A(3). At 225 K the unit-cell dimensions are a=b=7.4833(1) A, c=6.4103(1) A, and V=358.98(1) A(3). The deuteron positions refined from the neutron data are in excellent agreement with the single crystal x-ray analysis of Kjallman and Olovsson [Acta Crystallogr., Sect. B: Struct. Crystallogr. Cryst. Chem. B28, 1692 (1972)]; the structure consists of SO(4) (2-) and D(5)O(2) (+) ions hydrogen bonded to form a three dimensional network. Although no structural change is observed between 2 K and the melting point at approximately 232 K, the thermal expansion and incompressibility of the crystal are highly anisotropic. The bulk modulus of SAT at 200 K is 9.2(2) GPa, ( partial differentialK partial differentialP)(T)=7.9(8), and -( partial differentialK partial differentialT)(P)=10.6(5) MPa K(-1), values which are very similar to D(2)O ice Ih. A new polymorph of SAT has been discovered above 235 K at 5.5 kbars. The structure of this phase could not be determined, but we have indexed the diffraction pattern with a monoclinic unit cell of likely space-group P2(1)a (Z=2). SAT-II has a lower density than SAT-I under the same PT conditions; the refined unit-cell parameters at 235 K, 5.435 kbars are a=6.1902(3) A, b=11.1234(5) A, c=5.6446(3) A, beta=110.287(4) degrees , and V=364.56(2) A(3). This phase has been quenched to low pressures and temperatures, and we have obtained estimates of the thermal expansivity and incompressibility which reveal SAT-II to be significantly stiffer and more isotropic than SAT-I.

Neutron powder diffraction studies of sulfuric acid hydrates. I. The structure of sulfuric acid hemitriskaidekahydrate D2SO4.6(1/2)D2O.

We report the first neutron diffraction data from D2SO4.6(1/2)D2O. The crystal is monoclinic, space group Cm, with four formula units per unit cell. At 4.2 K the unit cell dimensions are a = 6.253 26(4) A, b = 26.813 62(10) A, c = 5.908 45(2) A, and beta = 112.1939(3) degrees [V = 917.286(6) A3 and rho(deuterated) = 1664.14(2) kg m(-3)]. The deuteron positions refined from the neutron data are in agreement with those established by single crystal x-ray analysis [D. Mootz and A. Merschenz-Quack, Z. Naturforsch. B 42, 1231 (1987)], but not with those found from the ab initio simulation of Hirsch and Ojamae [Acta Crystallogr, Sect. B: Struct. Sci. 60, 179 (2004)]. The crystal consists of SO4(2-), D3O+ ions, and D2O molecules hydrogen bonded to form a layered structure in which sheets of "icelike" D3O+ and D2O are separated by layers of opposing SO4(2-) tetrahedra.

No evidence for large-scale proton ordering in Antarctic ice from powder neutron diffraction.

We have examined a sample of 3000 year old Antarctic ice, collected at the Kohnen Station, by time-of-flight powder neutron diffraction to test the hypothesis of Fukazawa et al. [e.g., Ann. Glaciol. 31, 247 (2000)] that such ice may be partially proton ordered. Great care was taken to keep our sample below the proposed ordering temperature (237 K) at all times, but we did not observe any evidence of proton ordering.