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Here we report a new versatile approach for the calculation of phonon modes which is applicable to anisotropic, dilute alloys with allowance for a large variety of alloying elements. This approach has significant advantages over previously reported methods, especially for the lattice dynamics of such complex alloys. We use this approach to model the effects of Fe-doping on the vibrational modes in dilute alloys of CuAl1–xFexO2(x= 0, 0.01, 0.05, and 0.10) delafossite powders. These samples were structurally characterized with X-ray diffraction (XRD) combined with Rietveld refinement to measure their lattice parameters and Raman and FTIR spectroscopies to measure optical phonon frequencies. To compare experimental results from XRD with calculations for lattice parameters, we use a disordered supercell for xin the range 0–0.1. Both results agree well with Vegard’s law. For the phonon calculations, an approach using a disordered supercell is not feasible because it is too computationally expensive. Instead, we developed our weighted dynamical matrix (WDM) approach that uses a straightforward ordered supercell for force-constant calculations of the CuAlO2and CuFeO2parent end points, and combines them using a WDM approach leading to an effective medium for vibrational mode calculation in random alloys. Computationally, when Fe is substituted for Al (increasing x), an increase in the bond length is observed leading to a red-shift in the peak positions in all of the phonon modes vs x, in agreement with the experimentally observed trend.

The 10% Co/aerogel catalyst used in Fischer−Tropsch (FT) synthesis recently by Dunn et al. (Dunn, B. C.; Cole, P.; Covington, D.; Webster, M. C.; Pugmire, R. J.; Ernst, R. D.; Eyring, E. M.; Shah, N.; Huffman, G. P. Appl. Catal., A 2005, 278, 233) is investigated using the complimentary techniques of X-ray diffraction (XRD), transmission electron microscopy (TEM), and detailed magnetic measurements. TEM studies show that the catalyst contains Co nanoneedles with diameter d ≈ 1 nm, whereas the XRD studies show the presence of CoO and cobalt silicate hydroxide Co3(Si2O5)(OH)2, in addition to Co. Analysis of the temperature and magnetic field variation of the magnetization from 5 to 350 K shows that the measured magnetic properties are primarily due to the Co nanoneedles oriented randomly, with concentration ≈8%. The presence of the unreduced CoO and the Co silicate explains why the FT activity of this catalyst is about equal to that of the 6% Co/aerogel catalyst. Some interesting magnetic properties of the Co nanoneedles are also presented.

The electronic and magnetic properties of MnO and other transition-metal oxides have remained the focus of major attention for many years [1]. In this work, we report the discovery of an anomaly in the low-frequency (10 kc) dielectric constnat K' of MnO near its Neel temperature TN ≃ 118 K. The measurements were made in a single crystal of MnO, stressed along [111], with the electric field E ‖ [111] axis, using a three-terminal capacitance bridge. In the temperture range of 6K to 250K, both K' and K'' (magnitude ∼ 10-2) were frequency-independent up to 100 kc. The data for K' near TN for stress P = 20 bars (a stress sufficient to eliminate T domains [1]) are shown in the figure. For P = 0, the observed decrease in K' was about ½ of the value for P ≃ 20 bars. It is evident that the anomaly in K' near TN is associated with magnetic ordering. This anomaly is not due to a magnetoelectric effect since no significant changes in K' were observed near TN with an applied magnetic field of up to 8 kOe along [111]. A magnon-phonon coupling usually results in an increase in K' [2] rather than the decrease observed here. It is suggested that considerable lattice contraction along [111] below TN observed in MnO [4] may be the source of the observed anomaly in K' since energy for the corresponding long-wavelength phonon mode is expected to increase due to contraction along [111], thus resulting in a decrease in K'. The first order nature of the transition is evident from the behavior of dK'/dT near TN.