Mario Bieringer

In Situ Powder X-ray Diffraction, Synthesis, and Magnetic Properties of the Defect Zircon Structure ScVO4−x

We report the formation pathway of ScVO(4) zircon from ScVO(3) bixbyite with emphasis on the synthesis and stability of the novel intermediate defect zircon phase ScVO(4-x) (0.0 < x <or= 0.1). The formation pathway has been investigated by means of thermogravimetric/differential thermal analysis and in situ powder X-ray diffraction. The oxidation of ScVO(3) to ScVO(4) involves two intermediates of composition ScVO(3.5+y) (0.00 <or= y <or= 0.22) and the novel phase ScVO(4-x). ScVO(4-x) crystallizes in the defect zircon structure in space group I4(1)/amd (141) with a = 6.77761(5) A and c = 6.14045(8) A. Oxygen defect concentrations in bulk ScVO(4-x) samples range from 0.0 < x <or= 0.1. ScVO(4-x) is compared with the fully oxidized zircon structure ScVO(4) using powder X-ray diffraction, neutron diffraction, and bulk magnetic susceptibility data as well as (45)Sc and (51)V solid state NMR spectroscopy. ScVO(4-x) can only be obtained by oxidation of ScVO(3) or ScVO(3.5+y) while the reduction of ScVO(4) does not yield the novel defect structure. Mechanistic insights into the oxidative formation of ScVO(4) via the defect structure are presented.

Modulation of atomic positions in CaCuxMn7−xO12 (x ≤ 0.1)

The modulation of atomic positions in CaCu(x)Mn(7-x)O12 (x = 0 and 0.1) was studied using synchrotron radiation powder diffraction below 250 and 220 K, respectively. The copper-rich member CaCu(x)Mn(7-x)O12 (x = 0.23) does not show any modulation of the atomic positions at temperatures as low as 10 K. Using low-temperature neutron powder diffraction the modulation of the magnetic moments of Mn ions in CaCu(x)Mn(7-x)O12 (x = 0, 0.1 and 0.23) has been investigated. Long-range modulated magnetic ordering in CaCu(x)Mn(7-x)O12 (x = 0, 0.1 and 0.23) is observed below 90.4, 89.2 and 78.1 K. (0,0,q(p)) and (0,0,q(m)) are the propagation vectors describing the modulations of the atomic positions and the magnetic moments. For CaCu(x)Mn(7-x)O12 (x = 0 and 0.1) the magnetic modulation and atomic modulation lengths are related by a factor of 2 satisfying the relation (1-q(p)) = 2(1-q(m)).

The hydrothermal synthesis of the new manganese and vanadium oxides, NiMnO3H, MAV3O7 and MA0.75V4O10·0.67H2O (MA=CH3NH3)

The hydrothermal reaction of manganese or vanadium oxides in the presence of organic cations leads to the formation of new layered structures. When nickel salts are hydrothermally reacted with tetramethylammonium permanganate a new orthorhombic form of nickel manganese oxide, NiMnO 3 H x (where x≤1) is formed. It readily chemically intercalates lithium; it has space group Cmc2 1 , a=2.861(1) A, b=14.650(1) A, and c=5.270(1) A. The magnetic properties of this compound are quite different from the ilmenite NiMnO 3 phase, showing paramagnetism above 390 K and more complex behavior below that temperature. The hydrothermal reaction of vanadium pentoxide with methylamine leads to a series of new layered vanadium oxides, which differ in structure from the corresponding ones prepared in the presence of the tetramethylammonium ion because of the existence of hydrogen bonding. Methylamine is the first organic to form a double sheet vanadium oxide, (CH 3 NH 3 ) 0.75 V 4 O 10 ·0.67H 2 O, with the δ-Ag x V 2 O 5 structure. (CH 3 NH 3 ) 0.75 V 4 O 10 ·0.67H 2 O is monoclinic, space group C2/m with a=11.673(1) A, b=3.668(1) A, c=11.095(1) A and β=99.865(5)°. (CH 3 NH 3 )V 3 O 7 shows significant buckling of the layers compared with N(CH 3 ) 4 V 3 O 7 , and has a monoclinic unit cell, space group P2 1 /c with a=11.834(8) A, b=6.663(4) A, c=15.193(9) A and β=138.104(1)°.

In-situ powder X-ray diffraction investigation of reaction pathways for the BaCO(3)-CeO(2)-In(2)O(3) and CeO(2)-In(2)O(3) systems.

We report the first in-situ powder X-ray diffraction (PXRD) study of the BaCO(3)-CeO(2)-In(2)O(3) and CeO(2)-In(2)O(3) systems in air over a wide range of temperature between 25 and 1200 degrees C. Herein, we are investigating the formation pathway and chemical stability of perovskite-type BaCe(1-x)In(x)O(3-delta) (x = 0.1, 0.2, and 0.3) and corresponding fluorite-type Ce(1-x)In(x)O(2-delta) phases. The potential direct solid state reaction between CeO(2) and In(2)O(3) for the formation of indium-doped fluorite-type phase is not observed even up to 1200 degrees C in air. The formation of the BaCe(1-x)In(x)O(3-delta) perovskite structures was investigated and rationalized using in-situ PXRD. Furthermore the decomposition of the indium-doped perovskites in CO(2) is followed using high temperature diffraction and provides insights into the reaction pathway as well as the thermal stability of the Ce(1-x)In(x)O(3-delta) system. In CO(2) flow, BaCe(1-x)In(x)O(3-delta) decomposes above T = 600 degrees C into BaCO(3) and Ce(1-x)In(x)O(2-delta). Furthermore, for the first time, the in-situ PXRD confirmed that Ce(1-x)In(x)O(2-delta) decomposes above 800 degrees C and supported the previously claimed metastability. The maximum In-doping level for CeO(2) has been determined using PXRD. The lattice constant of the fluorite-type structure Ce(1-x)In(x)O(2-delta) follows the Shannon ionic radii trend, and crystalline domain sizes were found to be dependent on indium concentration.

Studies on polymorphic sequence during the formation of the 1:1 ordered perovskite-type BaCa(0.335)M(0.165)Nb(0.5)O(3-δ) (M = Mn, Fe, Co) using in situ and ex situ powder X-ray diffraction.

Here, we report a synthetic strategy to control the B-site ordering of the transition metal-doped perovskite-type oxides with the nominal formula of BaCa(0.335)M(0.165)Nb(0.5)O(3-δ) (M = Mn, Fe, Co). Variable temperature (in situ) and ex situ powder X-ray diffraction (PXRD), selected area electron diffraction (SAED), energy dispersive X-ray spectroscopy (EDX), Fourier transform infrared spectroscopy (FTIR), scanning/transmission electron microscopy (SEM/TEM), and thermogravimetic analysis (TGA) were used to understand the B-site ordering as a function of temperature. The present study shows that BaCa(0.335)M(0.165)Nb(0.5)O(3-δ) crystallizes in the B-site disordered primitive perovskite (space group s.g. Pm3̅m) at 900 °C in air, which can be converted into the B-site 1:2 ordered perovskite (s.g. P3̅m1) at 1200 °C and the B-site 1:1 ordered perovskite phase (s.g. Fm3̅m) at 1300 °C. However, the reverse reaction is not feasible when the temperature is reduced. FTIR revealed that no carbonate species were present in all three polymorphs. The chemical stability of the investigated perovskites in CO2 and H2 highly depends on the B-site cation ordering. For example, TGA confirmed that the B-site disordered primitive perovskite phase is more readily reduced in dry and wet 10% H2/90% N2 and is less stable in pure CO2 at elevated temperature, compared to the B-site 1:1 ordered perovskite-type phase of the same nominal composition.

Topotactic Oxidation Pathway of ScTiO3 and High-Temperature Structure Evolution of ScTiO3.5 and Sc4Ti3O12-Type Phases

The novel oxide defect fluorite phase ScTiO(3.5) is formed during the topotactic oxidation of ScTiO(3) bixbyite. We report the oxidation pathway of ScTiO(3) and structure evolution of ScTiO(3.5), Sc(4)Ti(3)O(12), and related scandium-deficient phases as well as high-temperature phase transitions between room temperature and 1300 °Cusing in-situ X-ray diffraction. We provide the first detailed powder neutron diffraction study for ScTiO(3). ScTiO(3) crystallizes in the cubic bixbyite structure in space group Ia3 (206) with a = 9.7099(4) Å. The topotactic oxidation product ScTiO(3.5) crystallizes in an oxide defect fluorite structure in space group Fm3m (225) with a = 4.89199(5) Å. Thermogravimetric and differential thermal analysis experiments combined with in-situ X-ray powder diffraction studies illustrate a complex sequence of a topotactic oxidation pathway, phase segregation, and ion ordering at high temperatures. The optimized bulk synthesis for phase pure ScTiO(3.5) is presented. In contrast to the vanadium-based defect fluorite phases AVO(3.5+x) (A = Sc, In) the novel titanium analogue ScTiO(3.5) is stable over a wide temperature range. Above 950 °C ScTiO(3.5) undergoes decomposition with the final products being Sc(4)Ti(3)O(12) and TiO(2). Simultaneous Rietveld refinements against powder X-ray and neutron diffraction data showed that Sc(4)Ti(3)O(12) also exists in the defect fluorite structure in space group Fm3m (225) with a = 4.90077(4) Å. Sc(4)Ti(3)O(12) undergoes partial reduction in CO/Ar atmosphere to form Sc(4)Ti(3)O(11.69(2)).

Topotactic Solid-State Metal Hydride Reductions of Sr2MnO4.

We report novel details regarding the reactivity and mechanism of the solid-state topotactic reduction of Sr2MnO4 using a series of solid-state metal hydrides. Comprehensive details describing the active reducing species are reported and comments on the reductive mechanism are provided, where it is shown that more than one electron is being donated by H(-). Commonly used solid-state hydrides LiH, NaH, and CaH2, were characterized in terms of reducing power. In addition the unexplored solid-state hydrides MgH2, SrH2, and BaH2 are evaluated as potential solid-state reductants and characterized in terms of their reductive reactivities. These 6 group I and II metal hydrides show the following trend in terms of reactivity: MgH2 < SrH2 < LiH ≈ CaH2 ≈ BaH2 < NaH. The order of the reductants are discussed in terms of metal electronegativity and bond strengths. NaH and the novel use of SrH2 allowed for targeted synthesis of reduced Sr2MnO(4-x) (0 ≤ x ≤ 0.37) phases. The enhanced control during synthesis demonstrated by this soft chemistry approach has allowed for a more comprehensive and systematic evaluation of Sr2MnO(4-x) phases than previously reported phases prepared by high temperature methods. Sr2MnO3.63(1) has for the first time been shown to be monoclinic by powder X-ray diffraction and the oxidative monoclinic to tetragonal transition occurs at 450 °C.

Dilemma on the crystal structure of CaCu3Ti4O12

The crystal structure of CaCu3Ti4O12 has been studied using high resolution synchrotron radiation based x-ray powder diffraction. The observed x-ray diffraction patterns show pronounced Bragg peak asymmetries which should not be present assuming the commonly accepted cubic crystal structure of CaCu3Ti4O12 described by the space group Im-3. Several structural models are discussed. The first model assumes a coexistence of two phases with the cubic symmetry (both space group Im-3) and different lattice constants. The next models are based on subgroups of the cubic space group Im-3. The best agreement with the experimental data is found for the two-phase cubic model.

Platinum uptake and Ba2CePtO6 formation during in situ BaCe(1-x)M(x)O(3-δ) (M = La, In) formation.

We report the formation, structures, temperature-dependent phase transitions, and high-temperature reactivity of the potential proton and oxide ion conductors BaCe(1-x)M(x)O3 (M(3+) = In(3+), La(3+)). The present in situ diffraction studies show oxidative platinum uptake at temperatures as low as 950 °C into BaCeO3, forming the cubic Ba2CePtO6 double perovskite. The transient B-site double perovskite expels platinum at around 1200-1250 °C. Platinum oxidation via BaCeO3 is investigated by in situ powder X-ray and neutron diffraction experiments in various atmospheres. Doped BaCe(1-x)M(x)O3 phases show the formation of Ba2CePtO6 without incorporating the M(3+) dopant. Oxidative platinum uptake is also observed during the synthesis of BaCeO3 on platinum metal. We report the reaction pathway for the low-temperature oxidative formation of Ba2CePtO6 and the subsequent liberation of platinum for the barium cerate system. The findings are supported by ambient-temperature X-ray diffraction, in situ powder X-ray, and powder neutron diffraction as well as XPS.

Charge ordering in CaCuxMn7−xO12 (x = 0.0 and 0.1) compounds

The crystal structure of CaMn7O12 and CaCu0.1Mn6.9O12 has been studied by synchrotron radiation (SR) based powder x-ray diffraction and neutron powder diffraction in the temperature range from 10 K up to 290 K. The lattice parameter a exhibits a minimum at 45 K in CaMn7O12. The c lattice parameter in CaMn7O12 and CaCu0.1Mn6.9O12 has a maximum at the same temperature. Additional Bragg peaks have been found in the SR diffraction patterns in CaMn7O12 and CaCu0.1Mn6.9O12 below 250 K and 220 K, respectively. All diffraction peaks have been indexed as (h,k,l ± κ), where κ was equal to 0.079(15) for CaMn7O12 and 0.093(15) for CaCu0.1Mn6.9O12. The incommensurate low-temperature diffraction peaks are not observed in neutron diffraction patterns. This leads to the conclusion that the phase transition to the incommensurate structure is due to charge ordering rather than atomic position modulation. The charge ordering temperature coincides with dielectric constant changes of four orders of magnitude for CaMn7O12.