Electrocatalytic Site Activity Enhancement via Orbital Overlap in A ₂MnRuO ₇(A = Dy ³⁺, Ho ³⁺, and Er ³⁺) Pyrochlore Nanostructures

Oxygen electrocatalysis at transition metal oxides is one of the key challenges underpinning electrochemical energy conversion systems, involving a delicate interplay of the bulk electronic structure and surface coordination of the active sites. In this work, we investigate for the first time the structure-activity relationship of A2RuMnO7 (A = Dy3+, Ho3+, and Er3+) nanoparticles, demonstrating how orbital mixing of Ru, Mn, and O promotes high density of states at the appropriate energy range for oxygen electrocatalysis. The bulk structure and surface composition of these multicomponent pyrochlores are investigated by high-resolution transmission electron microscopy, X-ray diffraction, X-ray absorption spectroscopy, X-ray emission spectroscopy (XES), and X-ray photoemission spectroscopy (XPS). The materials exhibit high phase purity (cubic fcc with a space group Fd3\uaf m) in which variations in M-O bonds length are less than 1% upon replacing the A-site lanthanide. XES and XPS show that the mean oxidation state at the Mn-site as well as the nanoparticle surface composition was slightly affected by the lanthanide. The pyrochlore nanoparticles are significantly more active than the binary RuO2 and MnO2 toward the 4-electron oxygen reduction reaction in alkaline solutions. Interestingly, normalization of kinetic parameters by the number density of electroactive sites concludes that Dy2RuMnO7 shows twice higher activity than benchmark materials such as LaMnO3. Analysis of the electrochemical profiles supported by density functional theory calculations reveals that the origin of the enhanced catalytic activity is linked to the mixing of Ru and Mn d-orbitals and O p-orbitals at the conduction band which strongly overlap with the formal redox energy of O2 in solution. The activity enhancement strongly manifests in the case of Dy2RuMnO7 where the Ru/Mn ratio is closer to 1 in comparison with the Ho3+ and Er3+ analogs. These electronic effects are discussed in the context of the Gerischer formalism for electron transfer at the semiconductor/electrolyte junctions

Raman and infrared spectroscopy of Sr2B′UO6 (B′ = Ni; Co) double perovskites

Temperature dependent normal modes and lattice thermal expansion of Sr 2B′UO6 (B′ = Ni, Co) double perovskites were investigated by Raman/infrared spectroscopies and synchrotron X-ray diffraction, respectively. Monoclinic crystal structures with space group P21/n were confirmed for both compounds, with no clear structural phase transition between 10 and 400 K. As predicted for this structure, the first-order Raman and infrared spectra show a plethora of active modes. In addition, the Raman spectra reveal an enhancement of the integrated area of an oxygen stretching mode, which is also observed in higher-order Raman modes, and an anomalous softening of ∼1 cm-1 upon cooling below T* ∼ 300 K. In contrast, the infrared spectra show conventional temperature dependence. The band profile phonon anomalies are possibly related to an unspecified electronic property of Sr2B′UO6 (B′ = Ni, Co).Centro de Química Inorgánic

Raman and infrared spectroscopy of Sr2B′UO6 (B′ = Ni; Co) double perovskites

Temperature dependent normal modes and lattice thermal expansion of Sr 2B′UO6 (B′ = Ni, Co) double perovskites were investigated by Raman/infrared spectroscopies and synchrotron X-ray diffraction, respectively. Monoclinic crystal structures with space group P21/n were confirmed for both compounds, with no clear structural phase transition between 10 and 400 K. As predicted for this structure, the first-order Raman and infrared spectra show a plethora of active modes. In addition, the Raman spectra reveal an enhancement of the integrated area of an oxygen stretching mode, which is also observed in higher-order Raman modes, and an anomalous softening of ∼1 cm-1 upon cooling below T* ∼ 300 K. In contrast, the infrared spectra show conventional temperature dependence. The band profile phonon anomalies are possibly related to an unspecified electronic property of Sr2B′UO6 (B′ = Ni, Co).Centro de Química Inorgánic

Structural evolution of the double perovskites Sr2B'UO6 (B' = Mn, Fe, Co, Ni, Zn) upon reduction: Magnetic behavior of the uranium cations

We describe the preparation of five perovskite oxides obtained upon reduction of Sr2B′UO6 (B′ = Mn, Fe, Co, Ni, Zn) with H2/N2 (5%/95%) at 900 °C during 8 h, and their structural characterization by X-ray powder diffraction (XRPD). During the reduction process there is a partial segregation of the elemental metal when B′ = Co, Ni, Fe, and the corresponding B′O oxide when B′ = Mn, Zn. Whereas the parent, oxygen stoichiometric double perovskites Sr2B′UO6 are long-range ordered concerning B′ and U cations. The crystal structures of the reduced phases, SrB′0.5−xU0.5+xO3 with 0.37 < x < 0.27, correspond to simple, disordered perovskites; they are orthorhombic, space group Pnma (No. 62), with a full cationic disorder at the B site. Magnetic measurements performed on the phase with B′ = Zn, indicate uncompensated antiferromagnetic ordering of the U5+/U4+ sublattice below 30 K.Fil: Pinacca, Ruben Miguel. Universidad Nacional de San Luis. Facultad de Química, Bioquímica y Farmacia. Área Química General e Inorgánica; ArgentinaFil: Viola, Maria del Carmen. Universidad Nacional de San Luis. Facultad de Química, Bioquímica y Farmacia. Área Química General e Inorgánica; ArgentinaFil: Pedregosa, Jose Carmelo. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - San Luis. Instituto de Investigaciones en Tecnología Química. Universidad Nacional de San Luis. Facultad de Química, Bioquímica y Farmacia. Instituto de Investigaciones en Tecnología Química; Argentina. Universidad Nacional de San Luis. Facultad de Química, Bioquímica y Farmacia. Área Química General e Inorgánica; ArgentinaFil: Carbonio, Raul Ernesto. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Córdoba. Instituto de Investigaciones en Físico-química de Córdoba. Universidad Nacional de Córdoba. Facultad de Ciencias Químicas. Instituto de Investigaciones en Físico-química de Córdoba; ArgentinaFil: Martínez Lope, M. J.. Instituto de Ciencia de Materiales de Madrid; España. Consejo Superior de Investigaciones Científicas; EspañaFil: Alonso, J. A.. Instituto de Ciencia de Materiales de Madrid; España. Consejo Superior de Investigaciones Científicas; Españ

Cationic ordering and role of the B-site lanthanide(III) and molybdenum(V) cations on the structure and magnetism of double perovskites Sr2LnMoO6

We describe the preparation, crystal structure determination and magnetic properties of a new series of ordered double perovskite oxides Sr2LnMoO6 (Ln = Eu, Gd, Dy, Ho, Er, Yb) with Mo5+ and Ln3+ electronic configurations. These compounds have been obtained by solid state reaction under reducing conditions in order to stabilize Mo5+ cations. Structural characterization by XRPD and NPD was performed when Ln = Ho, Er, Yb and just XRPD for absorbing Ln = Eu, Gd, Dy. At room temperature, an excellent Rietveld fit was obtained for all the samples in a monoclinic symmetry, space group P21/n, with long-range ordering of Ln and Mo atoms. Magnetic susceptibility measurements show that some of these materials present magnetic ordering below 25 K and the determined effective magnetic moments are consistent with those expected for the pair Ln3+-Mo5+. All the phases have negative dominance of the Weiss temperature indicating dominance of antiferromagnetic interactions.S.A.L. and C.A.L. thank CONICET fellowships. J.C.P. and RDS thanks the CONICET (Projects PIP 01360/08, PIP 00912/12 and PIP 00450/11) and SECyT-UNSL (Projects PROICO 7707 and PROICO 2-1612). J.C.P. and R.D.S are members of CONICET. J.A.A. acknowledges the financial support of the Spanish Ministry of Science and Innovation to the project MAT2010-16404

Correlation between Crystal Structure and Thermoelectric Properties of Sr1−xTi0.9Nb0.1O3−δ Ceramics

Polycrystalline Sr1−xTi0.9Nb0.1O3−δ (x = 0, 0.1, 0.2) ceramics have been prepared by the solid state method and their structural and thermoelectric properties have been studied by neutron powder diffraction (NPD), thermal, and transport measurements. The structural analysis of Sr1-xTi0.9Nb0.1O3−δ (x = 0.1, 0.2) confirms the presence of a significant amount of oxygen vacancies, associated with the Sr-deficiency of the materials. The analysis of the anisotropic displacement parameters (ADPs) indicates a strong softening of the overall phonon modes for these samples, which is confirmed by the extremely low thermal conductivity value (κ ≈ 1.6 W m-1 K−1 at 823 K) found for Sr1−xTi0.9Nb0.1O3−δ (x = 0.1, 0.2). This approach of introducing A-site cation vacancies for decreasing the thermal conductivity seems more effective than the classical substitution of strontium by rare-earth elements in SrTiO3 and opens a new optimization scheme for the thermoelectric properties of oxides

Raman And Infrared Spectroscopy Of Sr2b′uo6 (b′ = Ni; Co) Double Perovskites

Temperature dependent normal modes and lattice thermal expansion of Sr 2B′UO6 (B′ = Ni, Co) double perovskites were investigated by Raman/infrared spectroscopies and synchrotron X-ray diffraction, respectively. Monoclinic crystal structures with space group P21/n were confirmed for both compounds, with no clear structural phase transition between 10 and 400 K. As predicted for this structure, the first-order Raman and infrared spectra show a plethora of active modes. In addition, the Raman spectra reveal an enhancement of the integrated area of an oxygen stretching mode, which is also observed in higher-order Raman modes, and an anomalous softening of ∼1 cm-1 upon cooling below T* ∼ 300 K. In contrast, the infrared spectra show conventional temperature dependence. The band profile phonon anomalies are possibly related to an unspecified electronic property of Sr2B′UO6 (B′ = Ni, Co). © 2010 Elsevier B.V. All rights reserved.542142147Serrate, D., Serrate, D., De Teresa, J.M., Ibarra, M.R., (2007) J. Phys.: Condens. Matter, 19, p. 023201Kobayashi, K.-I., Kimura, T., Sawada, H., Terakura, K., Tokura, Y., (1998) Nature (London), 395, p. 677Kobayashi, K.I., Kimura, T., Tomioka, Y., Sawada, H., Terakura, K., (1999) Phys. Rev. B, 59, p. 11159Prellier, W., Smolyaninova, V., Biswas, A., Galley, C., Greene, R.L., Ramesha, K., Gopalakrishnan, J., (2000) J. Phys. C, 12, p. 965Gopalakrishnan, J., Chattopadhyay, A., Ogale, S.B., Venkatesan, T., Greene, R.L., Millis, A.J., Ramesha, K., Marest, G., (2000) Phys. Rev. B, 62, p. 9538Maignan, A., Raveau, B., Martin, C., Hervieu, M., (1999) J. Solid State Chem., 144, p. 224Dai, J.M., Song, W.H., Wang, S.G., Ye, S.L., Wang, K.Y., Du, J.J., Sun, Y.P., Gao, B.J., (2001) Mat. Sci. Eng. B, 83, p. 217Granado, E., Hung, Q., Lynn, J.W., Gopalakrishnan, J., Greene, R.L., Ramesha, K., (2002) Phys. Rev. B, 66, p. 064409Azimonte, C., Cezar, J.C., Granado, E., Huang, Q., Lynn, J.W., Campoy, J.C.P., Gopalakrishnan, J., Ramesha, K., (2007) Phys. Rev. Lett., 98, p. 017204Azimonte, C., Granado, E., Cezar, J.C., Gopalakrishnan, J., Ramesha, K., (2007) J. Appl. Phys., 101, pp. 09H115Serrate, D., De Teresa, J.M., Algarabel, P.A., Galibert, J., Ritter, C., Blasco, J., Ibarra, M.R., (2007) Phys. Rev. B, 75, p. 165109Sikora, M., Mathon, O., Van Der Linden, P., Michalik, J.M., De Teresa, J.M., Kapusta, C., Pascarelli, S., (2009) Phys. Rev. B, 79, p. 220402Pinacca, R.M., Viola, M.C., Pedregosa, J.C., Muñoz, A., Alonso, J.A., Martínez-Lope, M.J., Carbonio, R.E., (2005) Dalton Trans., p. 447Pinacca, R.M., Viola, M.C., Pedregosa, J.C., Martínez-Lope, M.J., Carbonio, R.E., Alonso, J.A., (2007) J. Solid State Chem., 180, p. 1582Ferreira, F.F., Granado, E., Carvalho Jr., W., Kycia, S.W., Bruno, D., Droppa Jr., R., (2006) J. Synchrotron Rad., 13, p. 46Larson, A.C., Von Dreele, R.B., (2000) Los Alamos National Laboratory Report LAUR 86-748Toby, B.H., (2001) J. Appl. Cryst., 34, pp. 210-213Prosandeev, S.A., Waghmare, U., Levin, I., Maslar, J., (2005) Phys. Rev. B, 71, p. 214307Iliev, M.N., Abrashev, M.V., Litvinchuk, A.P., Hadjiev, V.G., Guo, H., Gupta, A., (2007) Phys. Rev. B, 75, p. 104118Balkanski, M., Wallis, R.F., Haro, E., (1983) Phys. Rev. B, 28, p. 1928Andreasson, J., Holmlund, J., Knee, C.S., Käll, M., Börjesson, L., Naler, S., Bäckström, J., Eriksson, S.-G., (2007) Phys. Rev. B, 75, p. 104302Fujioka, Y., Frantti, J., Kakihana, M., (2004) J. Phys. Chem. B, 108, p. 17012Fujioka, Y., Frantti, J., Kakihana, M., (2006) J. Phys. Chem. B, 110, p. 777Kurosawa, T., (1961) J. Phys. Soc. Jpn., 16, p. 1208Siny, I.G., Katiyar, R.S., Bhalla, A.S., (2000) Ferroelectr. Rev., 2, p. 51Granado, E., García, A., Sanjurjo, J.A., Rettori, C., Torriani, I., Prado, F., Sánchez, R.D., Oseroff, S.B., (1999) Phys. Rev. B, 60, p. 11879Iliev, M.N., Guo, H., Gupta, A., (2007) Appl. Phys. Lett., 90, p. 15191

Electrocatalytic site activity enhancement via orbital overlap in A₂MnRuO₇(A = Dy³⁺, Ho³⁺, and Er³⁺) pyrochlore nanostructures

Oxygen electrocatalysis at transition metal oxides is one of the key challenges underpinning electrochemical energy conversion systems, involving a delicate interplay of the bulk electronic structure and surface coordination of the active sites. In this work, we investigate for the first time the structure-activity relationship of A2RuMnO7 (A = Dy3+, Ho3+, and Er3+) nanoparticles, demonstrating how orbital mixing of Ru, Mn, and O promotes high density of states at the appropriate energy range for oxygen electrocatalysis. The bulk structure and surface composition of these multicomponent pyrochlores are investigated by high-resolution transmission electron microscopy, X-ray diffraction, X-ray absorption spectroscopy, X-ray emission spectroscopy (XES), and X-ray photoemission spectroscopy (XPS). The materials exhibit high phase purity (cubic fcc with a space group Fd3¯ m) in which variations in M-O bonds length are less than 1% upon replacing the A-site lanthanide. XES and XPS show that the mean oxidation state at the Mn-site as well as the nanoparticle surface composition was slightly affected by the lanthanide. The pyrochlore nanoparticles are significantly more active than the binary RuO2 and MnO2 toward the 4-electron oxygen reduction reaction in alkaline solutions. Interestingly, normalization of kinetic parameters by the number density of electroactive sites concludes that Dy2RuMnO7 shows twice higher activity than benchmark materials such as LaMnO3. Analysis of the electrochemical profiles supported by density functional theory calculations reveals that the origin of the enhanced catalytic activity is linked to the mixing of Ru and Mn d-orbitals and O p-orbitals at the conduction band which strongly overlap with the formal redox energy of O2 in solution. The activity enhancement strongly manifests in the case of Dy2RuMnO7 where the Ru/Mn ratio is closer to 1 in comparison with the Ho3+ and Er3+ analogs. These electronic effects are discussed in the context of the Gerischer formalism for electron transfer at the semiconductor/electrolyte junctions.</p