High pressure synthesis and magnetic properties of CaFe₂O₄-type NaMn₂O₄ and LiMn₂O₄

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 Mn^J+/Mn⁴⁺ at different Mn sites caused for NaMn₂O₄ samples, whereas Mn³⁺ and Mn⁴⁺ ions are seem to distribute randomly for LiMn_2O4 samples. The temperature dependence of static magnetic susceptibility of NaMn₂O₄ shows antiferromagnetic behaviour with Neel temperature(T_N)=13K while the spin-glass-like behaviour is observed in both LiMn₂O₄ compounds in the low temperature region.