Decoding ionic conductivity and reordering in cation-disordered pyrochlores

The ordered structure A 2 B 2 O 6 O’ in pyrochlores engenders twin rows of inequivalent anion sublattices each centred on alternating cations. While it is known that cation antisite disorder augments the ionic conductivity by several orders of magnitude, the local cation environment around the anions and the dynamic anion reordering during the cation disordering are not well-elucidated. Using atomistic simulations on Gd 2 Zr 2 O 7 , we first show that the anions engage in concerted hops to the neighbouring tetrahedral sites mostly along with the 〈1 0 0〉 direction while completely avoiding the octahedral sites. While the initially vacant 8 a sites start accommodating oxygen ions with increasing cation disorder, they show noticeable reluctance even at significant levels of disorder. We have also tracked both the distribution of available oxygen sites following random cation disorder, which is dependent only on cation disordering, and the probability of occupation of these sites. Interestingly, the available oxygen sites show a non-monotonic dependence on the number of B ions in the nearest neighbouring shell while the occupation probability of all the available oxygen sites increases monotonically. A tetrahedral oxygen site thus has a better probability of being occupied when it has a greater number of second neighbour B ions. This article is part of the Theo Murphy meeting issue 'Understanding fast-ion conduction in solid electrolytes'.

Stabilization of ferromagnetism in carbon doped Mgn−2Mn2On clusters

We present density functional theory-based first principles study of magnetic properties of Mn-doped Mg[Formula: see text]Mn2On clusters. Mn-doped MgnOn clusters are found to exhibit antiferromagnetic order. We show that ferromagnetic state can be stabilized by codoping with carbon at oxygen site. Our study suggests that the holes created due to C doping are responsible for the stabilization of ferromagnetism.

How Li diffusion in spinel Li[Ni1/2Mn3/2]O4 is seen with μ ±SR

The diffusive behavior in a spinel-type Li+ ion battery material, Li[Ni1/2Mn3/2]O4, has been studied with positive and negative muon spin rotation and relaxation (μ ±SR) measurements in the temperature range between 200 and 400 K using a powder sample. The implanted μ + locates at an interstitial site near O2− ion so as to form a O–H like bond, while the implanted μ − is mainly captured by an oxygen nucleus, resulting in the formation of muonic oxygen. This means that local magnetic environments in Li[Ni1/2Mn3/2]O4 were investigated from the two different sites in the lattice, i.e., one is an interstitial site for μ +SR and the other is an oxygen site for μ −SR. Since both μ +SR and μ −SR detected an increase in the fluctuation rate of a nuclear magnetic field for temperatures above 200 K, the origin of this increase is clearly confirmed as Li diffusion. Assuming a random walk process with the hopping of thermally activated Li+ between a regular Li site and the nearest neighboring vacant octahedral sites, a self-diffusion coefficient of Li+ was found to range above 10−11 cm2/s at temperatures above 250 K with an activation energy of about 0.06 eV.

High oxide-ion conductivity through the interstitial oxygen site in Ba7Nb4MoO20-based hexagonal perovskite related oxides

Oxide-ion conductors are important in various applications such as solid-oxide fuel cells. Although zirconia-based materials are widely utilized, there remains a strong motivation to discover electrolyte materials with higher conductivity that lowers the working temperature of fuel cells, reducing cost. Oxide-ion conductors with hexagonal perovskite related structures are rare. Herein, we report oxide-ion conductors based on a hexagonal perovskite-related oxide Ba7Nb4MoO20. Ba7Nb3.9Mo1.1O20.05 shows a wide stability range and predominantly oxide-ion conduction in an oxygen partial pressure range from 2 × 10−26 to 1 atm at 600 °C. Surprisingly, bulk conductivity of Ba7Nb3.9Mo1.1O20.05, 5.8 × 10−4 S cm−1, is remarkably high at 310 °C, and higher than Bi2O3- and zirconia-based materials. The high conductivity of Ba7Nb3.9Mo1.1O20.05 is attributable to the interstitial-O5 oxygen site, providing two-dimensional oxide-ion O1−O5 interstitialcy diffusion through lattice-O1 and interstitial-O5 sites in the oxygen-deficient layer, and low activation energy for oxide-ion conductivity. Present findings demonstrate the ability of hexagonal perovskite related oxides as superior oxide-ion conductors.

Oxygen migration and optical properties of coronene oxides and their persulfurated derivatives: insight into the electric field effect and the oxygen-site dependence

Oxygen migration on the surface of coronene (C24) epoxides and their persulfurated derivatives (PSCs) can be easily tuned by an external electric field and their low-lying states show a remarkable oxygen-site dependence.

Crystal structure of Ti8Bi9O0.25 containing interstitial oxygen atoms

Single crystals of Ti8Bi9O0.25, titanium bismuth oxide (8/9/0.25), were obtained from a sample prepared by heating a mixture of Ti, TiO2 and Bi powders in an Ar atmosphere. Single-crystal X-ray analysis revealed that the introduction of O atoms into the structure of Ti8Bi9 retains the space-group type P4/nmm. The oxygen site is located within a Ti4 tetrahedron (point group symmetry \overline{4}m2) that is vacant in the Ti8Bi9 crystal structure. The occupancy of this site is 0.25 (4), and the O—Ti distance is 1.8824 (11) Å.

Ab initio calculation of magnetic properties in B, Al, C, Si, N, P and As-doped rutile TiO2

The ferromagnetic (FM) ordering in rutile TiO2 has been theoretically studied by substituting different [Formula: see text]-block elements (B, Al, C, Si, N, P and As) doped at oxygen site (B[Formula: see text], Al[Formula: see text], C[Formula: see text], Si[Formula: see text], N[Formula: see text], P[Formula: see text] and As[Formula: see text]) as well as at titanium site. Ab initio calculations in the frame work of density functional theory indicates that the [Formula: see text]-block elements (B, C, Al, Si, N, P and As) when substituting the oxygen site give significant amount of magnetic moment, but induce zero magnetic moment in case of substitution at Ti site. Spin–spin interaction for (Ti[Formula: see text]O[Formula: see text]X)2 with X = B, Al, C, Si, N, P and As system has also been studied with two different doping distances. Among all the possibilities, carbon substitution at oxygen site (C[Formula: see text]) results the most stable FM ordering in rutile TiO2.