The influence of cation ordering, oxygen vacancy distribution and proton siting on observed properties in ceramic electrolytes: the case of scandium substituted barium titanate.

The origin of the 2-order of magnitude difference in the proton conductivity of the hydrated forms of hexagonal and cubic oxygen deficient BaScxTi1-xO3-δ (x = 0.2 and x = 0.7) was probed using a combination of neutron diffraction and density functional theory techniques to support published X-ray diffraction, conductivity, thermogravimetric and differential scanning calorimetry studies. Cation ordering is found in the 6H structure type (space group P63/mmc) adopted by BaSc0.2Ti0.8O3-δ with scandium preferentially substituting in the vertex sharing octahedra (2a crystallographic site) and avoiding the face-sharing octahedra (4f site). This is coupled with oxygen vacancy ordering in the central plane of the face-sharing octahedra (O1 site). In BaSc0.7Ti0.3O3-δ a simple cubic perovskite (space group Pm3[combining macron]m) best represents the average structure from Rietveld analysis with no evidence of either cation ordering or oxygen vacancy ordering. Significant diffuse scattering is observed, indicative of local order. Hydration in both cases leads to complete filling of the available oxygen vacancies and permits definition of the proton sites. We suggest that the more localised nature of the proton sites in the 6H structure is responsible for the significantly lower proton conduction observed in the literature. Within the 6H structure type final model, proton diffusion requires a 3-step process via higher energy proton sites that are unoccupied at room temperature and is also likely to be anisotropic whereas the highly disordered cubic perovskite proton position allows 3-dimensional diffusion by well-described modes. Finally, we propose how this knowledge can be used to further materials design for ceramic electrolytes for proton conducting fuel cells.

C-type related order in the defective fluorites La2Ce2O7 and Nd2Ce2O7 studied by neutron scattering and ab initio MD simulations.

This work presents a structural investigation of La2-xNdxCe2O7 (x = 0.0, 0.5, 1.0, 1.5, 2.0) using X-ray powder diffraction and total scattering neutron powder diffraction, analysed using Rietveld and the reverse Monte Carlo method (RMC). Ab initio molecular dynamics (MD) modelling is also performed for further investigations of the local order. The main intensities in the neutron diffraction data for the La2-xNdxCe2O7 series correspond to the fluorite structure. However, additional C-type superlattice peaks are visible for x > 0 and increase in intensity with increasing x. The Nd-containing compositions (x > 0) are best fitted with Rietveld analysis by using a combination of oxygen deficient fluorite and oxygen excess C-type structures. No indications of cation order are found in the RMC or Rietveld analysis, and the absence of cation order is supported by the MD modelling. We argue that the superlattice peaks originate from oxygen vacancy ordering and associated shift in the cation position away from the ideal fluorite site similar to that in the C-type structure, which is seen from the Rietveld refinements and the observed ordering in the MD modelling. The vacancies favour alignments in the 〈110〉, 〈111〉 and especially the 〈210〉 direction. Moreover, we find that such ordering might also be found to a small extent in La2Ce2O7, explaining the discernible modulated background between the fluorite peaks. The observed overlap of the main Bragg peaks between the fluorite and C-type phase supports the co-existence of vacancy ordered and more disordered domains. This is further supported by the observed similarity of the radial distribution functions as modelled with MD. The increase in long range oxygen vacancy order with increasing Nd-content in La2-xNdxCe2O7 corresponds well with the lower oxide ion conductivity in Nd2Ce2O7 compared to La2Ce2O7 reported earlier.

Crystal structure and proton conductivity of BaSn0.6Sc0.4O3-δ : insights from neutron powder diffraction and solid-state NMR spectroscopy.

The solid-state synthesis and structural characterisation of perovskite BaSn1-x Sc x O3-δ (x = 0.0, 0.1, 0.2, 0.3, 0.4) and its corresponding hydrated ceramics are reported. Powder and neutron X-ray diffractions reveal the presence of cubic perovskites (space group Pm3m) with an increasing cell parameter as a function of scandium concentration along with some indication of phase segregation. 119Sn and 45Sc solid-state NMR spectroscopy data highlight the existence of oxygen vacancies in the dry materials, and their filling upon hydrothermal treatment with D2O. It also indicates that the Sn4+ and Sc3+ local distribution at the B-site of the perovskite is inhomogeneous and suggests that the oxygen vacancies are located in the scandium dopant coordination shell at low concentrations (x ≤ 0.2) and in the tin coordination shell at high concentrations (x ≥ 0.3). 17O NMR spectra on 17O enriched BaSn1-x Sc x O3-δ materials show the existence of Sn-O-Sn, Sn-O-Sc and Sc-O-Sc bridging oxygen environments. A further room temperature neutron powder diffraction study on deuterated BaSn0.6Sc0.4O3-δ refines the deuteron position at the 24k crystallographic site (x, y, 0) with x = 0.579(3) and y = 0.217(3) which leads to an O-D bond distance of 0.96(1) Å and suggests tilting of the proton towards the next nearest oxygen. Proton conduction was found to dominate in wet argon below 700 °C with total conductivity values in the range 1.8 × 10-4 to 1.1 × 10-3 S cm-1 between 300 and 600 °C. Electron holes govern the conduction process in dry oxidizing conditions, whilst in wet oxygen they compete with protonic defects leading to a wide mixed conduction region in the 200 to 600 °C temperature region, and a suppression of the conductivity at higher temperature.

Thermal evolution of the crystal structure of proton conducting BaCe0.8Y0.2O3-δ from high-resolution neutron diffraction in dry and humid atmosphere.

The crystal structure of the proton conducting perovskite BaCe(0.8)Y(0.2)O(3-δ) (BCY20) has been studied via high-resolution in situ neutron diffraction performed in controlled dry and humid (heavy water) oxygen flow. Two phase transitions, cubic Pm3[combining macron]m→R3[combining macron]c (775 °C)→Imma (250 °C) were observed on cooling from 1000 °C in dry O(2). A significant shift of the phase stability fields was observed on cooling in wet oxygen (pD(2)O ≈ 0.2 atm) with the R3[combining macron]c structure stabilised at 900 °C, and the R3[combining macron]c→Imma transition occurring at 675 °C. On cooling below 400 °C a monoclinic, I2/m, phase started to appear. The structural dependence on hydration level is primarily due to the de-stabilisation of the correlated, octahedra tilts as a consequence of structural relaxation around the oxygen vacancies present in the non-hydrated phase. The tendency of hydrated BaCe(0.8)Y(0.2)O(3-δ) to show octahedral tilting is also found to be enhanced, indicating that the deuteronic (protonic) defects influence the crystal structure, possibly via hydrogen bonding. Stabilisation of the monoclinic I2/m phase is attributed to the structural effect of deuterons that is inferred to increase on cooling as deuterons localise to a greater extent. Changing from wet oxidising (O(2) + D2O(g)) to wet reducing (5% H2 in Ar + D2O(g)) atmosphere did not influence the structure or the phase stability, indicating that Ce(4+) was not reduced under the present conditions. Based on the observed cell volume expansion protonic defects are present in the material at 900 °C at a D(2)O partial pressure of ∼0.2 atm. The origin of the chemical expansion is explained by the effective size of the oxygen vacancy being significantly smaller than the [OD] defect. Rietveld analysis has been used to locate possible sites for the deuterons in the high temperature, R3[combining macron]c and Imma, phases that are most relevant for proton transport.

Hydration thermodynamics of the proton conducting oxygen-deficient perovskite series BaTi1-xMxO3-x/2 with M = In or Sc.

This article establishes the effect of structure and composition on water uptake and the hydration and proton transport properties of the oxygen-deficient perovskite series BaTi1-x(In,Sc)xO3-x/2, with 0.2 ≤ x ≤ 0.7. The equilibrium water uptake is determined by thermogravimetry, while combining thermogravimetry with differential scanning calorimetry allows for direct determination of the materials' hydration thermodynamics. Proton and oxide ion transport properties are characterized by means of ac impedance measurements up to 1000 °C. In general, the hydration thermodynamics of the scandates are more favorable than that of the indates and are also affected by changes in crystal structure throughout the series. The more favorable hydration thermodynamics of cubic scandates increase their proton conductivity at higher temperatures compared to their indate counterparts. In contrast to the BaTi1-xInxO3-x/2 series, the BaTi1-xScxO3-x/2 (0.5 ≤ x ≤ 0.7) materials retain their cubic structures upon full saturation by protons and show no signs of chemical instability upon exposure to 1 atm H2O(g) down to 100 °C. The BaTi1-xScxO3-x/2 materials with 0.5 ≤ x ≤ 0.7 may therefore find application in, for instance, steam electrolysis or similar processes involving high water vapor pressures.

Proton conductivity of hexagonal and cubic BaTi1-xScxO3-δ (0.1 ≤x≤ 0.8).

BaTi1-xScxO3-δ (x = 0.1-0.8) was prepared via solid state reaction. High resolution X-ray powder diffraction was used to characterise the synthesised materials. It was found that low substitution (x = 0.1 and 0.2) of Ti(4+) for Sc(3+) gives a hexagonal perovskite structure, whereas high substitution (x = 0.5-0.7) results in a cubic perovskite structure. Thermogravimetric analysis revealed significant levels of protons in both as-prepared and hydrated samples. Electrical conductivity was measured by AC impedance methods under oxygen, argon and under dry and humid, both H2O and D2O, conditions for BaTi1-xScxO3-δ (x = 0.2, 0.6 and 0.7). In the temperature range of 150-600 °C, under humid conditions, the conductivity is significantly higher than that under the dry conditions. The increase in conductivity is especially prominent for the cubic phases, indicating that protons are the dominant charge carriers. The proton conductivity of hexagonal BaTi0.8Sc0.2O3-δ is approx. two orders of magnitude lower than that of the more heavily substituted cubic phases. Conductivity is also found to be higher in dry O2 than in Ar in the whole temperature range of 150-1000 °C, characteristic of a significant contribution from p-type charge carriers under oxidising atmospheres. Greater Sc(3+) substitution leads to a higher proton concentration and the highest proton conductivity (σ∼ 2 × 10(-3) S cm(-1) at 600 °C) is found for the BaTi0.3Sc0.7O3-δ composition.

The proton conducting electrolyte BaTi0.5In0.5O2.75: determination of the deuteron site and its local environment.

Deuterated BaTi0.5In0.5O2.75 has been studied with neutron total (Bragg plus diffuse) scattering data, using both the Rietveld refinement method and the reverse Monte Carlo (RMC) modelling technique, to investigate the preferred proton site and its local structural environment. The Rietveld analysis shows an excellent fit between experimental data and a long-range cubic description of the BaTi0.5In0.5O2.53(OD)0.44 perovskite structure containing a statistical distribution of Ti and In ions at the centre of regular (Ti/In)O6 octahedra. However, an RMC analysis of the data reveals substantial local structural features that reflect limitations of the Rietveld method for studies of this type. The Ti-O and In-O pair distribution functions given by the RMC analysis are markedly different from each other, with average Ti-O and In-O bond distances of 2.035 Å and 2.159 Å, respectively. The InO6 octahedra are regular in shape whereas the TiO6 octahedra are distorted. The average O-D bond distance is roughly 0.96 Å, and the preferred deuteron sites have a second nearest oxygen distance of 2.13 Å, which confirms localized tilting of the deuteron and indicates a substantial degree of hydrogen bonding. The impact of octahedral distortion and hydrogen bonding on the proton conduction mechanism is discussed.

Role of B-site ion on proton conduction in acceptor-doped Sm2B2O(7-δ) (B = Ti, Sn, Zr and Ce) pyrochlores and C-type compounds.

Proton conduction in three pyrochlores, Sm(1.92)Ca(0.08)B(2)O(7-δ), B = Ti, Sn, Zr and one phase with a related C-type fluorite superstructure, B = Ce, has been investigated. The samples were prepared by solid state reaction. Infrared spectroscopy measurements and thermogravimetric analysis were carried out to study the extent of proton dissolution and determine its dependence on the B-site ion. Electrochemical impedance spectroscopy, performed on heating and cooling pre-hydrated samples, confirmed significant levels of proton conduction for Sm(1.92)Ca(0.08)Ti(2)O(7-δ) and Sm(1.92)Ca(0.08)Sn(2)O(7-δ) up to T∼ 500 °C. In comparison the B = Zr and Ce samples revealed lower levels of proton conductivity, confined to temperatures below ∼ 400 °C. Proton diffusion coefficients of 3.36 × 10(-8), 1.73 × 10(-9), 5.53 × 10(-10) and 2.78 × 10(-11) cm(2) s(-1) were determined at 300 °C for samples with B = Ti, Sn, Zr and Ce respectively. The proton mobility of Sm(1.92)Ca(0.08)Ti(2)O(7-δ) is therefore approximately one order of magnitude lower than that found in yttrium-doped perovskite phases such as BaZrO(3) and BaCeO(3).

Crystal structure of Bi(0.9)Sm(0.1)Fe(1-x)Mn(x)O3 multiferroics.

Bi(0.9)Sm(0.1)Fe(1-x)Mn(x)O(3), with x=0.00, 0.15, 0.30 have been synthesised by solid-state reaction. The structures of the materials, characterised via Rietveld analysis of high resolution powder neutron diffraction data, reveal a structural transition from R3c to orthorhombic Imma symmetry is complete for the x=0.30 sample. The antiferromagnetic ordering temperature, magnitude of the ordered magnetic moment at the B-site, and the dielectric constant all decrease as a function of increasing Mn content.

The pyrochlore family -- a potential panacea for the frustrated perovskite chemist.

Many known complex oxides of general formula A(2)B(2)X(7) adopt the pyrochlore structure, a key structure-type that has been shown to demonstrate a vast range of useful physical properties. Areas currently of much interest with respect to pyrochlores, include metal-insulator transitions, magnetic frustration/spin ices, magnetoresistance, superconductivity, ferroelectrics, O/F ionic conductivity, mixed conductivity, pigments and catalysis. We present some recent results on three types of pyrochlore materials that show unusual magnetic, optical and electronic behaviours associated with subtle structural and compositional changes. High-resolution powder neutron diffraction studies of the superconducting Cd(2)Re(2)O(7) and the ferroelectric Cd(2)Nb(2)O(7) have been undertaken on material cooled below room temperature. Both Cd(2)Re(2)O(7) and Cd(2)Nb(2)O(7) exhibit small structure distortions, in each case involving a distortion from a cubic unit cell, on cooling below approximately 180 K and possible models that can be used to describe the low-temperature structures and associated atomic displacements are developed and described in this article. A range of materials of the general formula Ca(1-x)Ln(x)TaO(2-x)N(1+x), x= 0.5 and x= 1, Ln = La-Yb have been synthesised and shown to adopt pyrochlore and/or perovskite structures. The absorption spectra of these materials are discussed in terms of their structures and compositions.

New layered manganese oxide halides.

The first layered manganese(III) oxide chlorides, Sr2MnO3Cl and Sr4Mn3O8-yCl2, have been synthesised; Sr2MnO3Cl adopts a K2NiF4 type structure with sheets of MnO5 square based pyramids linked through oxygen and separated by SrCl layers; it is the end member of a new family of Ruddlesden-Popper type manganese oxide halides which includes the three-layer member Sr4Mn3O8-yCl2 also reported herein.