Neutron and X-ray diffraction studies of the SrCr8Ga4O19 kagomè compound synthesised by the citrate route

The SrCrxGa12-xO19 (SCGO) has the magnetoplumbite structure with part of the Cr3+ (S=3/2) moments lying on the kagomè layers (12k sites) separated by Cr3+ triangular layers (4f and 2a sites). The strong geometric frustrations of the magnetic sublattices gives a high degeneracy of the ground state that is the origin of particular low temperature properties. The magnetic sublattice is diluted by a partial interchange of the Cr and Ga atoms that has been observed in all structural refinements published up to now. This dilution affects the magnetic properties and therefore it is important to investigate the Cr/Ga distribution in this system, as well as the synthesis conditions that control the structural properties. We synthesized different samples of SCGO by solid state reaction and by citrate route. The last one is a wet method that is known to give homogeneous powders with high purity and controlled stoichiometry. For the last method we followed, by X-ray diffraction patterns, the formation of the crystalline phase starting from the amorphous precursor and calcining at different temperatures. The analysis of the X-ray spectra reveals the formation of the Cr2O3 at 600°C, and due to the high stability of this oxide, it is necessary to reach very high temperatures (like in the solid state reaction) to synthesize the SCGO. The complete crystallization of the compound is reached at 1150°C, but we obtained a very pure sample only at 1350°C. Comparing the spectra for the samples prepared by the different methods, we observed a better crystallization in the sample from the citrate route, but no other differences are evident from the XRD spectra. We performed also high resolution neutron powder diffraction analysis on the D2B two axis diffractometer at the Institute Laue Langevin to investigate the differences in the Cr/Ga distributions on the 12k and 4f sites in order to relate them with the synthesis route.