Effect of Ni substitution on the antiferromagnetic ordering in olivine LiFe1-xNixPO4

Ni-doped LiFe1-xNixPO4 polycrystalline samples were prepared by using the solid-state reaction method in order to investigate the effect of Ni substitution in antiferromagnetic LiFe1-xNixPO4 system by using a superconducting quantum interference device (SQUID) magnetometer and Moessbauer spectroscopy. The crystal structure was determined to be orthorhombic with the space group Pnma for all LiFe1-xNixPO4 samples. The magnetic order for all LiFe1-xNixPO4 samples was antiferromagnetic with different superexchange interactions. From the analyzed Moessbauer spectra, the value of the hyperfine field for LiFe1-xNixPO4 decreased with increasing Ni substitution at temperatures ranging from 4.2 K to below the Neel temperature. This result suggests that the Fe-O-Fe superexchange interaction is larger than that of the Fe-O-Ni link. Moreover, we found that Ni ions in LiFe1-xNixPO4 could induce an asymmetric charge density in the FeO6 octahedral site due to the charge on distant Ni ions.