Lithium iron silicate compounds of Li2FeSiO4 and partially delithiated Li2−xFeSiO4 (x = 0.66) were synthesized by vacuum-sealed solid-state and chemical delithiation reactions, and their magnetic properties were characterized based on Mössbauer analysis. Crystal structures of both Li2FeSiO4 and Li2−xFeSiO4 (x = 0.66) compounds are found to be γs-type (P21/n) monoclinic structures with difference in the lattice parameters due to lithium delithiation. Mössbauer spectrum of Li2FeSiO4 below TN1 = 20 K exhibits eight Lorentzians of Fe2+ with antiferromagnetic ordering. However, the spectrum of intermediate Li2−xFeSiO4 (x = 0.66) compound shows the appearance of magnetically ordered Fe3+ sextet below TN2 = 28 K. The temperature-dependent isomer shift of Li2−xFeSiO4 indicates the coexistence of nonequivalent Fe2+/Fe3+ valence states with the partial oxidation of FeO4, enhanced by the lithium ion deficiency. Also, we have observed a considerable change in electric quadrupole interaction between Fe2+/Fe3+ ions in Li2−xFeSiO4, when compared to that of Li2FeSiO4, due to the different lattice and valence electron contributions, being originated from crystalline and valence transitions caused by the lithiation/delithiation process.

The structural and magnetic phase transitions of LixFe1–yNiyPO4 were investigated by x-ray diffraction (XRD), superconducting quantum interference device magnetometry, and Mössbauer spectroscopy. Rietveld refinement of XRD patterns of LiFe1–yNiyPO4 (0.0 ≤ y ≤ 0.6) revealed that the lattice parameters a0 = 10.328, b0 = 6.007, and c0 = 4.692 Å for LiFePO4 changed linearly to a0 = 10.154, b0 = 5.923, and c0 = 4.687 Å for LiFe0.4Ni0.6PO4 with the substitution of Ni ions. Also, the fully lithium-deintercalated Fe1–yNiyPO4 (0.0 ≤ y ≤ 0.6) series had enhanced lattice distortions along the c axis compared to LiFe1-yNiyPO4 because the Jahn–Teller distortion changed as the unit cell volume decreased due to lithium ion deintercalation. LiFe1–yNiyPO4 has an antiferromagnetic order; the magnetic Nèel temperature (TN) decreased from 51 K for LiFePO4 to 36 K for LiFe0.4Ni0.6PO4. Fully deintercalated Fe1–yNiyPO4 has strong antiferromagnetic order; TN decreased from 114 K for FePO4 to 62 K for Fe0.4Ni0.6PO4 due to the charge transition of Fe2+/Fe3+ and Ni2+/Ni3+, mediated by lithium ion vacancies in an olivine structure. The Mössbauer spectra below the TN of LixFe1–yNiyPO4 (x = 0, 1, 0.0 ≤ y ≤ 0.6) were fitted with eight asymmetrical Lorentzian functions. The electric quadrupole splitting value (ΔEQ) of LiFe1–yNiyPO4 is larger than that of Fe1–yNiyPO4 due to more asymmetric charge distributions around Fe2+ (3d6) than Fe3+ (3d5) in FeO6 sites.

The effect of proton irradiation on magnetic properties of lithium ferrites has been investigated with x-ray diffraction ͑XRD͒, magnetization, and Mössbauer spectroscopy measurements. Li 0.5 Fe 2.5 O 4 powders have been fabricated by the sol-gel method. Following the annealing at 700°C, these samples have been proton irradiated with 1, 5, and 10 pC/ m 2. The analysis of XRD patterns by Rietveld refinement method shows that these samples have ordered cubic spinel structures with space group of P4 3 32. We have observed that the corresponding lattice constant a 0 linearly increases from 8.3301 to 8.3314Ϯ 0.0001 Å with increasing proton irradiation. Compared to nonirradiated sample, which has the saturation magnetization ͑M s ͒ of 66.4 emu/g and oxygen occupancy of 3.9980 at room temperature, the values of magnetization and oxygen occupancy at room temperature are 66.0, 62.6, and 60.8 emu/g and 3.9840, 3.9452, and 3.9272, respectively, for 1, 5, and 10 pC/ m 2 irradiated powders. Also, the coercivity ͑H c ͒ decreases from 175.6 to 154.0 Oe with increasing proton irradiation. The Mössbauer spectra taken at room temperature show that the values of isomer shift ͑␦͒ for the tetrahedral ͑A͒ and octahedral ͑B͒ sites are consistent with the Fe 3+ valence state. The results suggest that the proton irradiation induces the oxygen vacancy defects, which in turn leads to the changes in magnetic properties.