RIETVELD ANALYSIS OF La(III)-DOPED Sr2CoWO6 DOUBLE PEROVSKITES

C.A. LOPEZ; M. DEL C. VIOLA; J.C. PEDREGOSA

Puerto Madryn, Chubut, Argentina.

Workshop; II Meeting of the AACr, Asociación Argentina de Cristalografía; 2006

AACr, Asociación Argentina de Cristalografía.

The discovery of CMR has stimulated the enthusiasm of solid-state chemists and physicists since this effect is of technological interest for the detection of magnetic fields in magnetic memory devices. Materials with substantially higher TC must be developed to operate in a useful temperature range around room temperature (RT). Recently, some members of the family of double perovskites of composition A2B?B??O6 where A is an alkali earth, B' and B'' are transition metals have been proposed as half-metallic ferromagnets (best described as ferrimagnets), with TC's well above RT, as an alternative to perovskite manganites. The revival of interest in this family was triggered by a report on Sr2FeMoO6, demonstrating that in the electronic structure only minority spins are present at the Fermi level: this material was shown to exhibit intrinsic tunneling-type magnetoresistance (TMR) at RT. In a simple picture, FeO6 and MoO6 octahedral alternate along the three directions of the crystal structure of Sr2FeMoO6. Further studies on other members of the A2B'B''O6 family seem to indicate that the occurrence of CMR and/or TMR properties is a common feature in some of them. We recently reported the induction of CMR in Sr2CoMoO6 upon chemical reduction, via topotactical removal of oxygen atoms. Sr2CoWO6 perovskite has also been prepared in polycrystalline form by solid-state reaction at 1150 ºC with important magnetic behavior. This material has been studied by high-resolution synchrotron X-ray and neutron powder diffraction (NPD), magnetic measurements, and differential scanning calorimetry (DSC). At room temperature, the crystal structure is tetragonal, space group I4/m, with a = 5.58277(1) Å and c = 7.97740(1) Å. The structure contains alternating CoO6 and WO6 octahedral, tilted in anti-phase by 7.24 in the basal ab plane along the [001] direction of the pseudocubic cell. This corresponds to the a0a0c- Glazer´s notation as derived by Woodward for 1:1 ordering of double perovskites, consistent with space group I4/m. Magnetic and neutron diffraction measurements indicate an antiferromagnetic ordering below TN = 24 K. As prepared, the sample is an electrical insulator. Magnetic and electrical properties and bond valence sums are consistent with the electronic configuration Co2+(3d7)-W6+(5d0). The doping of the sites A with an ion with higher charge is an alternative to the chemical reduction method via topotactical removal of oxygen atoms to reduce B´´ cation, since it provokes an electronic transference on the site B. J. A. Alonso et al. have examined such electron-doping effect on Ba2CoMoO6. In this system, the electron configuration of Co2+ (3d7) and Mo6+ (4d0) is proposed. By substituting La3+ for Ba2+; electrons are expected to enter the 4d-site and behave as carriers. In the present work we describe the synthesis of the La(III)-doped Sr2-xLaxCoWO6 (x = 0.1, 0.15 and 0.25) prepared by a solid-state reaction as well as the results of Rietveld analysis of powder X-ray diffraction (XRPD) data on well-crystallized samples. The complete structural data for these La-doped double perovskites is reported. Distortion of the primitive tetragonal perovskite towards the monoclinic phase P21/n occurs on these three substitutions as it also has been observed when Sr2CoWO6 is cooled below 260 K. The contraction in the unit cell volume values when x increases is also analyzed. The effect observed on partial substitution of Sr(II) by La(III) is the transfer of Co2+ cation to B'' site with gradual elimination of W6+ and formation of SrWO4.