Structural and magnetic characterization of A3Cr2ReO9 (A= Sr, Ba) double Perovskites

PANNUNZIO-MINER, E. V.; DE PAOLI, J. M.; SÁNCHEZ, R. D.; ALONSO, J. A.; FERNÁNDEZ-DÍAZ, M. T.; CARBONIO, R. E.

Praga. República Checa.

Conferencia; 9th Conference on Solid State Chemistry; 2010

In the search for new colossal magnetoresistant materials, some members of the family of double Perovskites of composition A2BB’O6 (A= alkali earths and B, B’= transition metals) have been proposed as half-metallic ferromagnets, with TC’s well above room temperature, as an alternative to Perovskite manganites [1]. These transition metal double Perovskite oxides have a double crystalline structure because they exhibit, in an ideal order; the M ions alternated in two different B sites. Also, double Perovskites containing transition metals other than Mo, such as A2FeReO6 (A= Ca, Sr, Ba), present a half-metallic ground state and show magnetoresistance at room temperature. Despite the large number of double Perovskites informed up to now, only few reports exist about compounds with stoichiometry A3B2B’O9, whose crystallographic formula can be re-written as A2[B2a(B1/3B’2/3)2c]O6, where 2a and 2c denote the crystallographic sites in the corresponding space group [2, 3]. This composition gives the possibility of changing the oxidation states of the B and B’ ions, and consequently changing the electrical and magnetic properties.   For the previously mentioned reasons, we focus on preparing and characterizing double Perovskites as potential candidates for colossal magnetoresistant materials. In particular, we report on new double Perovskites: A3Cr2ReO9 (A= Sr, Ba). These compounds were synthesized as polycrystalline powders under high-pressure conditions, 1000 ºC and 20 kbar, in a piston cylinder system.   We refined their crystal structure and relative Fe/Re occupancies of B sites using synchrotron X-ray and neutron powder diffraction. For Sr3Cr2ReO9 we have found an orthorhombic crystal symmetry (SG: Immm), the same as Sr3Fe2ReO9 [3]. For the compound with A= Ba, the crystal structure obtained has a hexagonal symmetry (SG: P63/mmc) with a 6-H packing.   Dc magnetization was measured with a commercial superconducting quantum interference device magnetometer on powdered samples, in a 10-300 K temperature range and in a -5 to 5 T magnetic field range. Vibrating sample magnetometer was used to take the magnetization from RT to 700 K. The observed magnetic behavior for both samples was complex, however antiferromagnetic interactions were predominant. This is in agreement with an antiparallel coupling between spins at 2a and 2c sites.   [1] Y. Kobayashi et al., Nature 395 (1998) 677. [2] M. C. Viola et al., Chem. Mater. 14 (2002) 812. [3] E. V. Pannunzio-Miner et al., Phys. B 398(2) (2007) 397.