High pressure synthesis and magnetic properties of CaFe₂O₄-type NaMn₂O₄and LiMn₂O₄

Abstract

Ploycrystalline samples of NaMn204 and LiMn2O4 with the calcium ferrite-type structure were synthesized using high pressure technique. For synthesis of LiMn2O4 samples, we applied for two different synthesis routes; one is directly obtained from the starting mixture of LiMn2O4 composition under high pressure of 4.5 GPa and the other is obtained by alkali ion exchange from high pressure synthesized NaMn2O4. From the powder X-ray diffraction measurements, these samples showed almost single phases. The crystal structures were determined from the results of neutron diffraction measurements. The data were refined with the space group orthorhombic Pnam. The valence states of Mn are determined by the Mn-0 distance using the bond valence sum analyses. Ordering of MnJ+/Mn4+ at different Mn sites caused for NaMn2O4 samples, whereas Mn3+ and Mn4+ ions are seem to distribute randomly for LiMn2O4 samples. The temperature dependence of static magnetic susceptibility of NaMn2O4 shows antiferromagnetic behaviour with Neel temperature(TN)=13K while the spin-glass-like behaviour is observed in both LiMn2O4 compounds in the low temperature region.