The software program SPuDS has been developed to predict the crystal structures of perovskites, including those distorted by tilting of the octahedra. The user inputs the composition and SPuDS calculates the optimal structure in ten different Glazer tilt systems. This is performed by distorting the structure to minimize the global instability index, while maintaining rigid octahedra. The location of the A-site cation is chosen so as to maximize the symmetry of its coordination environment. In its current form SPuDS can handle up to four different A-site cations in the same structure, but only one octahedral ion. Structures predicted by SPuDS are compared with a number of previously determined structures to illustrate the accuracy of this approach. SPuDS is also used to examine the prospects for synthesizing new compounds in tilt systems with multiple A-site coordination geometries (a(+)a(+)a(+), a(0)b(+)b(+), a(0)b(-)c(+)).

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Prediction of the crystal structures of perovskites using the software program SPuDS.

We synthesized layered cuprates where the intermediate layer between the CuO 2 blocks consists of vertex sharing RuO 6 octahedra. The (idealized) stoichiometries of these compounds are RuSr 2 LnCu 2 O 8 (Ru-1212) and RuSr 2 (Ln 1+ x Ce 1− x ) Cu 2 O 10 (Ru-1222), where Ln is one of the three neighboring lanthanides Sm, Eu and Gd. The analogy to the isostructural Nb based compounds, as well as simple valence counting, suggest that Ru is pentavalent in these compounds. Both 1212 and 1222 compounds are semiconducting when prepared in air and rapidly cooled to room temperature. Metallic behavior and superconductivity can be achieved for various compositions upon appropriate annealing.

Ruthenium-based layered cuprates RuSr2LnCu2O8 and RuSr2(Ln1+xCe1−x)Cu2O10 (LnSm, Eu and Gd)

Solid oxide fuel cells (SOFC) are highly efficient energy conversion devices with the advantage of directly utilizing hydrocarbon fuels. Starting with a short introduction about the fuel challenges and early achievements in this field, this review paper focuses on advances in oxygen-ion conducting electrolyte-based SOFC during the last 15 years. Robust anodes immune to carbon deposition are a prerequisite for direct hydrocarbon SOFC. In this paper, direct hydrocarbon SOFC anode materials are classified into three general categories: Ni-cermet, Cu-cermet, and oxide-based anodes. Oxide anodes are further classified in terms of their crystalline structures, namely fluorite, rutile, tungsten bronze, pyrochlore, perovskite, and double perovskite. Achievements and recent advances on these SOFC anodes are reviewed and discussed. The concluding remarks summarize the pros and cons of direct hydrocarbon SOFC anode materials along with the perspective of future research trends.

Solid Oxide Fuel Cell Anode Materials for Direct Hydrocarbon Utilization

In the series of 3d(t2g)1 perovskites, SrVO3–CaVO3–LaTiO3–YTiO3, the transition-metal d electron becomes increasingly localized and undergoes a Mott transition between CaVO3 and LaTiO3. By defining a low-energy Hubbard Hamiltonian in the basis of Wannier functions for the t2g LDA band and solving it in the single-site dynamical mean-field (DMFT) approximation, it was recently shown (Pavarini et al (2004) Phys. Rev. Lett. 92 176403) that simultaneously with the Mott transition there occurs a strong suppression of orbital fluctuations due to splitting of the t2g levels. The present paper reviews and expands this work, in particular in the direction of exposing the underlying chemical mechanisms by means of ab initio LDA Wannier functions generated with the Nth order muffin-tin orbital (NMTO) method. The Wannier functions for the occupied oxygen-p band illustrate the importance of oxygen-p to large cation-d covalency for the progressive GdFeO3-type distortion along the series. The oxygen-p orbitals which pdσ-bond to the cations are the same as those which pdπ-bond to the transition-metal t2g orbitals. As a consequence, the Wannier functions for the t2g band exhibit residual covalency between the transition-metal t2g, the large cation-d, and the oxygen-p states. This residual covalency, which increases along the series, turns out to be responsible not only for the splittings, Δ, of the t2g levels, but also for non-cubic perturbations of the hopping integrals, both of which are decisive for the Mott transition. We find good agreement with the optical and photoemission spectra for all four materials, with the crystal-field splittings and orbital polarizations recently measured for the titanates, and with the metallization volume (pressure) for LaTiO3. The metallization volume for YTiO3 is predicted and the role of the Jahn–Teller (JT) distortion is discussed. For use in future many-body calculations, we tabulate the t2g on-site and hopping matrix elements for all four materials and give an analytical expression for the orthorhombic Hamiltonian in the k + Q representation. Using conventional super-exchange theory, our on-site and hopping matrix elements reproduce the observed magnetic orders in LaTiO3 and YTiO3, but the results are sensitive to detail, in particular for YTiO3 where, without the JT distortion, the magnetic order would be antiferromagnetic C- or A-type, rather than ferromagnetic. It is decisive that upon increasing the GdFeO3-type distortion, the nearest-neighbour hopping between the lowest and the upper-level Wannier functions becomes stronger than the hopping between the lowest-level Wannier functions. Finally, we show that the non-cubic perturbations responsible for this behaviour make it possible to unfold the orthorhombic t2g LDA bandstructure to a pseudo-cubic zone. In this zone, the lowest band is separated from the two others by a direct gap and has a width, WI, which is significantly smaller than that, W, of the entire t2g band. The progressive GdFeO3-type distortion thus favours electron localization by decreasing W, by increasing Δ/W, and by decreasing WI/W. Our conclusions concerning the roles of GdFeO3-type and JT distortions agree with those of Mochizuki and Imada (2003 Phys. Rev. Lett. 91 167203).

https://iopscience.iop.org/article/10.1088/1367-2630/7/1/188/pdf

How chemistry controls electron localization in 3d 1 perovskites: a Wannier-function study

We apply machine learning (ML) methods to a database of 390 experimentally reported $AB{\mathrm{O}}_{3}$ compounds to construct two statistical models that predict possible new perovskite materials and possible new cubic perovskites. The first ML model classified the 390 compounds into 254 perovskites and 136 that are not perovskites with a 90% average cross-validation (CV) accuracy; the second ML model further classified the perovskites into 22 known cubic perovskites and 232 known noncubic perovskites with a 94% average CV accuracy. We find that the most effective chemical descriptors affecting our classification include largely geometric constructs such as the $A$ and $B$ Shannon ionic radii, the tolerance and octahedral factors, the $A$-O and $B$-O bond length, and the $A$ and $B$ Villars' Mendeleev numbers. We then construct an additional list of $625AB{\mathrm{O}}_{3}$ compounds assembled from charge conserving combinations of $A$ and $B$ atoms absent from our list of known compounds. Then, using the two ML models constructed on the known compounds, we predict that 235 of the 625 exist in a perovskite structure with a confidence greater than 50% and among them that 20 exist in the cubic structure (albeit, the latter with only $\ensuremath{\sim}50%$ confidence). We find that the new perovskites are most likely to occur when the $A$ and $B$ atoms are a lanthanide or actinide, when the $A$ atom is an alkali, alkali earth, or late transition metal atom, or when the $B$ atom is a $p$-block atom. We also compare the ML findings with the density functional theory calculations and convex hull analyses in the Open Quantum Materials Database (OQMD), which predicts the $T=0$ K ground-state stability of all the $AB{\mathrm{O}}_{3}$ compounds. We find that OQMD predicts 186 of 254 of the perovskites in the experimental database to be thermodynamically stable within 100 meV/atom of the convex hull and predicts 87 of the 235 ML-predicted perovskite compounds to be thermodynamically stable within 100 meV/atom of the convex hull, including 6 of these to be in cubic structures. We suggest these 87 as the most promising candidates for future experimental synthesis of novel perovskites.

https://link.aps.org/accepted/10.1103/PhysRevMaterials.2.043802

Predictions of new AB O 3 perovskite compounds by combining machine learning and density functional theory

Preparation and structure of the compounds SrVO3 and Sr2VO4

Properties of a new perovskite oxyde Sr2VO4

A low temperature structural transition in the 123-superconductor YBa 2 Cu 3 O 7−δ has been proposed involving zig-zag folding of the CuO-chains, but it has not been possible to demonstrate the resulting superlattice reflections because the CuO-chains are broken after relatively short distances unless δ=0. In the 123-isomorph LaB 2 (Cu 2 Nb)O 8 , which is not a superconductor, Y is replaced by La, and the CuO-chains by planes of NbO-octahedra. We find by direct observation of superlattice reflections that zig-zag folding of the NbO-octahedra can indeed occur in this structure type. The tilt pattern is similar to that of the low temperature phase of SrTiO 3 , with the same I4/mcm space group.

M.J. Rey

Papers

Preparation and structure of the compounds SrVO3 and Sr2VO4

Properties of a new perovskite oxyde Sr2VO4

A structural distortion in the 123-superconductor isomorph LaBa2(Cu2Nb)O8