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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.

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