Influence of the Ba2+/Sr2+ content and oxygen vacancies on the stability of cubic Ba(x)Sr(1-x)Co(0.75)Fe(0.25)O(3-δ).

We present a theoretical and experimental study on the influence of the Ba/Sr and Co/Fe ratios as well as the oxygen-non-stoichiometry on the stability of Ba0.5Sr0.5Co0.8Fe0.2O3-δ (BSCF). Thin-layer depositions are analysed by looking at TEM images and EDX spectra. Bond-analytical calculations are performed to explain the stability difference between hexagonal and cubic BSCF. Finally, annealing experiments analysed using XRD give an insight into the differences of phase-fraction growth with respect to the Ba/Sr ratio.

Site-preferential design of itinerant ferromagnetic borides: experimental and theoretical investigation of MRh6B3 (M = Fe, Co).

Single-phase polycrystalline samples of the compounds MRh(6)B(3) (M = Fe, Co) as well as single crystals of CoRh(6)B(3) have been synthesized by arc-melting the elements under a purified argon atmosphere in a water-cooled copper crucible. The characterization of the new phases was achieved by using single-crystal and powder X-ray diffraction as well as EDX measurements. The two phases are isotypic and crystallize in the hexagonal Th(7)Fe(3) structure type (space group P6(3)mc, no. 186, Z = 2). In this structure, the magnetically active atoms (Fe, Co) are preferentially found on only one of the three available rhodium sites, and together with rhodium they build a three-dimensional network of interconnected (Rh/M)(3) triangles. Magnetic properties investigations show that both phases order ferromagnetically below Curie temperatures of 240 K (for FeRh(6)B(3)) and 150 K (for CoRh(6)B(3)). First-principles DFT calculations correctly reproduce not only the lattice parameters but also the ground state magnetic ordering in the two phases. These calculations also show that the long-range magnetic ordering in both phases occurs via indirect ferromagnetic coupling between the iron atoms mediated by rhodium. This magnetic structural model also predicts the saturation magnetizations to be 4.02 µ(B) for FeRh(6)B(3) (3.60 µ(B) found experimentally) and 2.75 µ(B) for CoRh(6)B(3). Furthermore, both phases are predicted to be metallic conductors as expected for these intermetallic borides.

A combinatorial study of inverse Heusler alloys by first-principles computational methods.

In continuation of our recent combinatorial work on 810 X(2)YZ full Heusler alloys, a computational study of the same class of materials but with the inverse (XY)XZ crystal structure has been performed on the basis of first-principles (GGA) total-energy calculations using pseudopotentials and plane waves. The predicted enthalpies of formation evidence 27 phases to be thermochemically stable against the elements and the regular X(2)YZ type. A chemical-bonding study yields an inherent tendency for structural distortion in a majority of these alloys, and we predict the existence of the new tetragonal phase Fe(2)CuGa (P4(2)/ncm; a = 5.072 A, c = 7.634 A; c/a approximately 1.51) with a saturation moment of mu = 4.69 micro(B) per formula unit. Thirteen more likewise new, isotypical phases are predicted to show essentially the same behavior. Six phases turn out to be the most stable in the inverse tetragonal arrangement. The course of the magnetic properties as a function of the valence-electron concentration is analyzed using a Slater-Pauling approach.

A combinatorial study of full Heusler alloys by first-principles computational methods.

A combinatorial scan of a total of 810 full Heusler alloys is performed on the basis of first-principles (GGA) total-energy calculations using pseudopotentials and plane waves to predict their lattice parameters and magnetic moments. About 60% of the investigated intermetallics turn out as being thermochemically stable with respect to the constituting elements. The presentation of the calculated magnetic moments in a periodic system of full Heusler phases is accomplished and yields periodic trends for the physical properties as a function of their compositions and as a function of the valence-electron concentration within a modified Slater-Pauling scheme. In addition, hot synthetic spots with respect to magnetically interesting stable and also presumably metastable phases are identified to propose new and economically lucrative synthetic targets, and a series of new rhodium-containing phases is analyzed in depth with respect to their electronic structures.

The role of vacancies and local distortions in the design of new phase-change materials.

Phase-change materials are of tremendous technological importance ranging from optical data storage to electronic memories. Despite this interest, many fundamental properties of phase-change materials, such as the role of vacancies, remain poorly understood. 'GeSbTe'-based phase-change materials contain vacancy concentrations around 10% in their metastable crystalline structure. By using density-functional theory, the origin of these vacancies has been clarified and we show that the most stable crystalline phases with rocksalt-like structures are characterized by large vacancy concentrations and local distortions. The ease by which vacancies are formed is explained by the need to annihilate energetically unfavourable antibonding Ge-Te and Sb-Te interactions in the highest occupied bands. Understanding how the interplay between vacancies and local distortions lowers the total energy helps to design novel phase-change materials as evidenced by new experimental data.

Unusual Mn-Mn spin coupling in the polar intermetallic compounds CaMn2Sb2 and SrMn2Sb2.

Large single-crystals of two polar intermetallic phases, CaMn2Sb2 and SrMn2Sb2, have been grown using In or Sn as metal fluxes and characterized by single-crystal X-ray diffraction. The two compounds are isostructural and crystallize with the CaAl2Si2 structure (space group P3m1, No. 164) with unit cell parameters determined at 120(2) K of a = 4.5204(6) angstroms, c = 7.456(2) angstroms and a = 4.5802(17) angstroms, c = 7.730(5) angstroms for CaMn2Sb2 and SrMn2Sb2, respectively. Temperature- and field-dependent dc- and ac-magnetization measurements suggest complex magnetic ordering of the Mn moments below ca. 250 and 35 K for CaMn2Sb2 and below ca. 265 K for SrMn2Sb2. Resistivity measurements reveal metallic-like temperature dependence with rho(290) = 40 m omega cm for CaMn2Sb2 and rho290 = 100 m omega cm for SrMn2Sb2 with negligible magnetoresistance at 5 K in applied magnetic fields up to 10 kOe. Spin-polarized DFT electronic structure calculations confirm the metallic-like properties and provide further evidence for a magnetic structure where Mn atoms form two magnetic sublattices with ferromagnetic coupling within them and strong antiferromagnetic coupling between them.