Structural and Magnetic Properties of the Oxides AB₂O₆ (A²⁺ = Mn, Fe, Co, Ni, Cu; and B⁺⁵ = As or Sb)

Abstract

<p>The structural and magnetic properties of a number of transition metal oxides of the general formula AB₂O₆, where A is Mn, Fe, Co, Ni, and Cu, and B is Sb⁵⁺ or As⁵⁺, have been investigated.</p> <p>CuSb₂O₆ crystallizes in a monoclinically distorted trirutile structure in the space<br />group P2₁/n. Magnetic suscepnbility is characterized by a broad maximum at 60 K<br />indicative of the presence of significant short range correlations in this system, and an<br />abrupt decrease in the susceptibility suggesting the presence of a magnetically ordered<br />state. Further analysis of the susceptibility provided that the magnetic correlations in this system are dominant along the chains; this is the first trirutile compound that gives the<br />clearest evidence for 1-d magnetic correlations. Long range magnetic order was further<br />studied by low temperature neutron diffraction. This reflection was indexed with a<br />propagation vector k =(½, 0, ½) in accordance with some of the trirutile phases such<br />as CoSb₂O₆ and FeTa₂O₆. The magnetic moment of Cu²⁺ was estimated at about 0.5 μʙ. The presence of long range order was further supported by the registration of the spin<br />flop transition on a single crystal of CuSb₂O₆.</p> <p>CoAs₂O₆, NiAs₂O₆, and MnAs₂O₆ are isostructural and adopt the lead antimonate structure and crystallize in the space group P-3lm. Magnetic susceptibility data were<br />characterized by the absence of short range correlations. The long range order in these materials was studied in detail using low temperature neutron diffraction.</p> <p>For NiAs₂O₆ and CaAs₂O₆, a number of superlattice reflections appears in their<br />respective low temperature neutron diffraction profiles which were indexed with the<br />propagation vector k =(0, 0, ½). The profiles of both materials were refined using the<br />Rietveld method in the space group P-3lm. The refinement was handled by the Rietan<br />program in which the refined magnetic moment of Ni²⁺ was found to be 2.11(1)μʙ, and<br />for Co²⁺ was 2.66(30).</p> <p>The low temperature neutron diffraction profile of MnAs₂O₆ was much more<br />complicated than those of NiAs₂O₆ and CoAs₂O₆. An exhaustive list of propagation<br />vectors were used in order to fit the pattern without much success.</p> <p>Lastly, in preparing the iron member of this series, Fe₂As₄O₁₂ was obtained instead of the isostruetural member of the lead antimonate structure which has a markedly different crystal structure. Although the crystal structure contains Fe₂O₉ dimers, the magnetic susceptibility did not fit the predictions of the S = 5/2 - 5/2 dimer model.<br />This peculiarity is attnbuted to the details ofthe crystal structure. As for the magnetically<br />ordered state, the superlattice reflections were indexed with the wavevector k =(0,0,0)<br />and the Rietveld refinement was carried out in the space group P3.</p>