Structural and magnetic properties of Co2Al4Si5O18 and Mn2Al4Si5O18 cordierite

The crystal structures, lattice dynamics and magnetic properties of synthet ic Co-cordierite, Co2Al4Si5O18, and Mn-cordierite, Mn2Al4Si5O18 have been s tudied by neutron powder diffraction, infrared spectroscopy and magnetisati on measurements. Due to different synthesis conditions, the Co-cordierite u sed here crystallised in the hexagonal cl-cordierite structure with a disor dered Si/Al distribution in the framework, while for the Mn-cordierite the orthorhombic p-structure has been determined. The experimentally determined paramagnetic moments, mu(exp)(Mn) = 5.47(6)mu(B) and mu(exp)(Co) = 3.88(4) mu(B), are in good agreement with theoretical predictions for octahedrally coordinated Mn2+ and Co2+, respectively. In both compounds there is no magn etic long-range order down to at least 1.5 K. However, the onset of an anti -ferromagnetic short-range correlation of magnetic moments along [001] has been observed for Mn-cordierite by magnetisation and neutron diffraction me asurements. This short-range magnetic correlation becomes evident from diff use scattering observed at 2 K. The diffuse scattering has been interpreted in terms of a Blech-Averbach function.