Columbite-type Fe x Mn 1x Nb 2 O 6 solid solution: structural and magnetic characterization Cristina Tealdi, a Maria Cristina Mozzati, b Lorenzo Malavasi,* a Tiziana Ciabattoni, c Roberta Amantea a and Carlo B . Azzoni b a Dipartimento di Chimica Fisica ‘‘M. Rolla’’, INSTM, IENI/CNR Unita ` di Pavia of Universita ` di Pavia, V.le Taramelli 16, Pavia I-27100, Italy. E-mail: lorenzo.malavasi@unipv.it b INFM, Unita ` di Pavia and Dipartimento di Fisica ‘‘A. Volta’’, Universita ` di Pavia, Via Bassi 6, Pavia I-27100, Italy c Centro di Calcolo of Universita ` di Pavia, via Ferrata 1, Pavia I-27100, Italy Received 16th April 2004, Accepted 21st May 2004 First published as an Advance Article on the web 8th June 2004 This paper deals with the columbite Fe x Mn 1x Nb 2 O 6 (x ¼ 0, 0.25, 0.50, 0.75 and 1) solid solution. After synthesis under vacuum, structural and magnetic properties were studied by means of X-ray diffraction, Rietveld refinement, electron paramagnetic resonance and magnetization measurements. This investigation showed that (i) the homogeneity range of the solid solution is complete and the A (Fe and/or Mn) cations are randomly distributed in the A-octahedral sites of the structure; (ii) the oxidation state of Fe and Mn is always þ2; (iii) there is no cation disorder between the A and Nb sites; (iv) an antiferromagnetic transition is present only for the two end-members (FeNb 2 O 6 and MnNb 2 O 6 ). Introduction Dielectric oxides have been of particular interest during the last decades due to their possible technological applications. In this context, ternary oxides of general formula AB 2 O 6 (A ¼ M 21 , B ¼ Nb, Ta), crystallizing in the columbite or trirutile struc- ture, have attracted attention because of their dielectric properties. 1–5 Indeed, the high dielectric constant and the low dielectric losses at microwave frequency make them suitable as dielectric resonator with applications in satellite communication. Moreover, AB 2 O 6 compounds are used as important intermedi- ates during the synthesis of perovskite related structures through the synthetic route known as columbite precursor method. 6–8 Among AB 2 O 6 compounds with columbite structure the solid solution Fe x Mn 1x Nb 2 O 6 is one of the most relevant for its technological role. In the natural state, the columbite is found in granitic pegmatites. It is the most widespread niobium containing mineral and primary source of this industrially attractive metal. In the mineralogical field, the columbite-group minerals, and in particular the ironcolumbite-manganocolumbite solid solu- tion, show variable degrees of cation disorder, i.e. divalent cations on the B site and, consequently, a correspondent amount of pentavalent cations on the A site. For this reason, most of the literature is related to the natural mineral compounds, with particular reference to the influence of impurities on the struc- tural parameters and the cation ordering. 9–11 Few works are found that are concerned with their magnetic properties and only about the two end-members of the solid solution. 12–14 Among the current literature, anyhow, no work about a systematic study of the synthetic solid solution properties can be found. In this work we report the study of the structural and magnetic properties of the synthetic Fe x Mn 1x Nb 2 O 6 (x ¼ 0, 0.25, 0.50, 0.75 and 1) solid solution. The structural para- meters, derived from X-ray diffraction studies coupled with Rietveld refinement, and the electron paramagnetic resonance (EPR) and static magnetization results are presented as a function of x. Particular attention is paid to the oxidation state for A-cations. Experimental Fe x Mn 1x Nb 2 O 6 samples were synthesised by solid state reac- tion of stoichiometric amounts of high purity Nb 2 O 5 , MnO, Fe, and Fe 2 O 3 (Aldrich 499.9%). The powdered reagents were mixed, ground, pelletized, encapsulated in evacuated closed quartz tubes and heated at 950 1C for 2 weeks. After this first thermal treatment the samples have been taken out from the silica tubes, reground thoroughly, re-pelletized and heated for another two weeks in evacuated quartz tubes at 950 1C. X-ray powder diffraction (XRPD) and electron microprobe analysis (EMPA) were performed to check the phase purity and cation stoichiometry of the obtained materials. XRPD patterns were acquired in y–2y geometry on a Bruker D8 Advance diffractometer. Measurements were performed from 101 to 1101 with a scan step of 0.021, with a fixed counting time of 5 s for each step. EMPA measurements were carried out using a ARL SEMQ scanning electron microscope, performing at least 10 measurements in different regions of each sample. The samples were made of powders embedded in a resin which underwent a common metallographic preparation. According to EMPA and XRPD, the above synthetic procedure gave single phase materials. The prepared samples were found to be homogeneous in chemical composition with deviations from the nominal one less than 4%. All the XRPD patterns were analyzed by the Rietveld method using the GSAS program. 15 The atomic positions of the mineral columbite, (Fe,Mn)Nb 2 O 6 16 were used as starting point in the Rietveld procedure. No regions of the diffraction patterns were excluded in the refinement. The following refin- able parameters were considered: zero point and background, scale factor, pseudo-Voigt and asymmetry parameters for the peak shape, atomic positions, lattice constants and cationic thermal factors. Oxygen thermal factors were derived from previously reported refinements based on neutron diffraction data relative to the solid solution end-members 17 and kept fixed to those values. In particular, for the x ¼ 0.74 and 0.52 samples they were fixed to the FeNb 2 O 6 values while RESEARCH PAPER PCCP www.rsc.org/pccp DOI: 10.1039/b405666c 4056 Phys. Chem. Chem. Phys., 2004, 6 , 4056–4061 This journal is & The Owner Societies 2004