Neutron Diffraction Study of Magnetic and Structural Transitions in Complex Nb-Doped Cobalt Oxides
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eutron Diffraction Study of Magnetic and Structural Transitions in Complex Nb-Doped Cobalt Oxides V. V. Sikolenkoa, b, *, V. V. Efimova, D. V. Karpinskyc, M. V. Bushinskyc, S. I. Tiutiunnikova, and S. Schorrd, e aJoint
Institute for Nuclear Research, Dubna, Moscow oblast, 141980 Russia Research and Educational Centre “Functional nanomaterials,” Immanuel Kant Baltic Federal University, Kaliningrad, 236041 Russia cScientific-Practical Material Research Centre, National Academy of Sciences of Belarus, Minsk, 220072 Belarus dHelmholtz Zentrum Berlin, Berlin, 14109 Germany eFreie University Berlin, Berlin, 14195 Germany *e-mail: [email protected] b
Received July 25, 2019; revised August 30, 2019; accepted September 3, 2019
Abstract—The magnetic and structural transitions in rhombohedral La0.7Sr0.3Co1 – xNbxO3 and cubic La0.3Sr0.7Co1 – xNbxO3 have been studied as a function of Nb content and temperature by using of neutron and X-ray diffraction. The increase in the Nb concentration leads to the gradual increase in the lattice volume and significant decrease in the magnetic moment which completely disappears at the niobium doping level higher than 0.1. No essential structure changes and rhombohedral distortions have been found. Keywords: cobaltite, neutron diffraction DOI: 10.1134/S1027451020070447
INTRODUCTION The discovery of the “colossal” magnetoresistance in the manganites with perovskite structure has stimulated the research of the compounds exhibiting large magnetoresistance, in particular, in complex lanthanum-based cobalt oxides [1–6]. The substitution of La3+ ions for Sr2+ in LaCoO3 leads to a formation of tetravalent Co4+ [4, 5]. This fact modifies both the crystal and magnetic structure of rhombohedral nonmagnetic parent compound LaCoO3, and the arising interaction Co3+–O–Co4+ establishes a long-range ferromagnetic ordering. Another end-member of these series, SrCoO3, is a cubic ferromagnetic compound with metallic type conductivity. The Sr2+ ionic radius is significantly greater than that of the La3+ ion. So, it is possible to expect stabilization of the intermediate spin state of cobalt ions by substituting Sr2+ ions for La3+ ones. The origin of the ferromagnetic state in metallic cobaltites and manganites has been a subject of discussion for a long time [7, 8]. Short range ferromagnetism and a weak antiferromagnetic correlation have been observed in LaCoO3 with a nominal oxidation state of cobalt closed to 3+. To prevent the Co4+ ion appearance, it is possible to introduce simultaneously Nb ions, which at the presence of Co3+ ions will be in oxidation state of 5+. With the simultaneous
introduction of Sr2+ and Nb5+, the cobalt ions keep their oxidation state, and the electroconductivity of La1 – xSrxCo1 – x/2Nbx/2O3 solid solutions decreases with dopant concentration enhancement [9]. In this work, we present the results of neutron diffraction experiment on rhombohedral La0.7Sr0.3Co1 – xNbxO3 and cubic La0.3Sr0.7Co1 – xNbxO3. EXPERIMENTAL TECHNIQUES The synthesis of powder samples is described in [1
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