Abstract
The crystal structures, lattice dynamics and magnetic properties of synthetic Co-cordierite, Co2Al4Si5O18, and Mn-cordierite, Mn2Al4Si5O18 have been studied by neutron powder diffraction, infrared spectroscopy and magnetisation measurements. Due to different synthesis conditions, the Co-cordierite used here crystallised in the hexagonal α-cordierite structure with a disordered Si/Al distribution in the framework, while for the Mn-cordierite the orthorhombic β-structure has been determined. The experimentally determined paramagnetic moments, exp (Mn) = 5.47(6) B and exp (Co) = 3.88(4) B , are in good agreement with theoretical predictions for octahedrally coordinated Mn2+ and Co2+, respectively. In both compounds there is no magnetic 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 measurements. This short-range magnetic correlation becomes evident from diffuse scattering observed at 2 K. The diffuse scattering has been interpreted in terms of a Blech-Averbach function.
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