Investigation of spin-orientation in antiferromagnetic ordering for LiFe_1-xZn_xPO₄ with Mössbauer spectroscopy

Modern electrochemical systems are considered for critical environmental, technical, and commercial use, with wide application in green energy storage device.^1–3 Lithium-ion batteries have high energy, power density, and excellent cycling performances.^4,5 Among these compounds, olivine phosphate materials have attracted attention as promising cathode candidates. In particular, LiFePO₄ is low-cost, electrochemically and thermally stable, and eco-friendly. However, there are major drawbacks, such as low lithium-ion mobility and poor electronic conductivity. To overcome these drawbacks, the effects of Zn-doped LiFePO₄ have been studied. It was reported that the divalent cations could sustain the lattice after lithium ion extraction from cathodes.^6,7 Many research groups reported improved electron conductivity and increased charge/discharge capacity with Zn-doping.^8,9 The magnetic properties of electrode materials can help the understanding of electrochemical behavior for rechargeable battery.¹⁰ The close relation between magnetism and chemical bonding in solids can be explained by local molecular field theory of antiferromagnetism and superexchange interactions.^11,12 Goodenough’s group reported that chemical bonding and electron structure in the covalent compounds obtained over the course of the research on magnetic interactions.¹³ The magnetic properties of electrode materials have been studied that they have an antiferromagnetic ordering at low temperature.^14,15 A study of antiferromagnetic ordering of LiFe_1-xZn_xPO₄ has not been previously reported in the literature.

To obtain understanding of the Zn doping effects LiFePO₄, we studied the effect of Zn doping on the structure and magnetic properties of LiFePO₄. Detailed magnetic analyses have been performed via vibrating sample magnetometer (VSM) measurement and Mössbauer spectroscopy.

LiFe_1-xZn_xPO₄ samples were prepared by solid-state reaction method. The starting materials Li₂CO₃, FeC₂O₄·2H₂O, NH₄H₂PO₄, and ZnO were mixed, followed by calcination at 300 °C for 4 h in Ar flow. Next, the powder was pressed into a pellet and sintered at 700 °C for 10 h in Ar flow. The X-ray diffraction (XRD) was performed with diffractometer using Cu-Kα radiation (λ = 1.5406 Å). The crystalline parameters were refined by the Rietveld method using the FullProf program. The magnetic susceptibility curves were carried out using a VSM. Mössbauer spectra were obtained in the various temperature using a ⁵⁷Co source in Rh matrix. The measured spectra were fit using the least-squares method assuming Lorentzian shape.

The crystal structure of LiFe_1-xZn_xPO₄ samples was determined to be orthorhombic with Pnma space group from the Rietveld refinement analysis. The lattice constants and the volume of the LiFe_1-xZn_xPO₄ decreased from a₀ = 10.3293 Å, b₀ = 6.0060 Å, c₀ = 4.6939 Å, and V = 291.479 ų for x = 0.1 to a₀ = 10.2741 Å, b₀ = 5.9846 Å, c₀ = 4.6985 Å, and V = 288.738 ų for x = 0.5, as shown Fig. 1. The unit cell parameters of a₀ and b₀ for LiFe_1-xZn_xPO₄ decreased with increasing Zn concentrations because the Zn²⁺ ion (r = 0.74 Å) has smaller ionic radius than that of the Fe²⁺ ion (r = 0.78 Å) in the Fe(Zn)O₆ octahedral site. Table S-1 shows the distances between the Fe-ion and each of the six O-ions in the FeO₆ sites of LiFe_1-xZn_xPO₄. We confirmed that the Fe(Zn)O₆ octahedral sites had an asymmetric structure. Fig. S1 exhibits Fe-O bond lengths with distortion of the FeO₆ octahedron of LiFe_0.5Zn_0.5PO₄.

The temperature dependence of magnetic susceptibility χ(T) curves of LiFe_1-xZn_xPO₄ were measured by a VSM from 4.2 to 295 K with an applied field of 1000 Oe, as shown in Fig. 2(a). The value of the Néel temperature (T_N) and the spin reorientation temperature (T_S) of LiFe_1-xZn_xPO₄ decreased with increasing Zn concentrations from 48 and 14 K for x = 0.1 to 36 and 8 K for x = 0.5, because the superexchange interaction in the Zn²⁺-O^2--Zn²⁺ link is smaller relative to that of the Fe²⁺-O^2--Fe²⁺ link, resulting in weak antiferromagnetic interaction. The T_N and T_S values of LiFePO₄ were reported in our previous publication as 51 and 23 K.^16–18 This trend is consistent with the trend in T_N and T_S temperature with Zn concentrations. The magnetic properties for LiZnPO₄ have not been reported, but we can predict that Zn ions are non-magnetic. The superexchange interaction can be deduced from the Curie-Weiss temperature (θ_CW), which is obtained from the fitted inverse magnetic susceptibility curves. The values of the θ_CW are shown in Table I. Above T_N, the inverse magnetic susceptibility χ^-1(T) curves were fitted to the Curie-Weiss law, 1/χ = (T-θ/C), as shown in Fig. 2(b). The experimental effective moment of LiFe_1-xZn_xPO₄ is larger than the theoretical value, which is given by μ_theo = [(1-x)$μFe2+2$+ x$μZn2+2$]^1/2. This can be explained as incomplete absence of orbital contribution by the crystalline field (CF) around distorted octahedron.¹⁹ The experimental value of the effective moment of LiFe_1-xZn_xPO₄ decreased with increasing Zn concentration from 5.30 μ_B for x = 0.1 to 4.08 μ_B for x = 0.5.

To investigate the microscopic magnetic structure of LiFe_1-xZn_xPO₄, we obtained Mössbauer spectra at temperatures from 4.2 to 295 K. Some representative LiFe_0.5Zn_0.5PO₄ spectra are shown in Fig. 3. The spectra show an asymmetric eight-line shape from 4.2 K to T_N due to the strong CF in the distorted octahedral structure of the Fe²⁺ ion and six O^2- ions. Also, in the previously reported LiFePO₄, Mössbauer spectra in the antiferromagnetic region below T_N exhibited asymmetric eight-line shape.¹⁶ At temperatures below T_N, the spectra were fitted by using the full Hamiltonian for the ⁵⁷Fe nucleus, including both the magnetic dipole and electric quadrupole interaction. We determined the polar (θ) and azimuthal (φ) angle of the direction of the hyperfine field at the ⁵⁷Fe nuclei with respect to the principal axes of the electric field gradient (EFG) tensor as well as the asymmetric parameter (η) and the ratio (R) of the electric quadrupole interaction to the magnetic dipole interaction. The Mössbauer parameters of LiFe_0.5Zn_0.5PO₄ at various temperatures are listed in Table II. The EFG tensor is heavily influenced by spin configuration, which induces the electric polarization (P). The P can be expressed as: P ∝ e_ij×(S_i×S_j), where e_ij is the unit vector between two magnetic ions, and S_i and S_j are magnetic moments.²⁰ The electric polarization caused by spin configuration gives rise to an asymmetric EFG around Fe nuclei. The value of η gradually increases above 8 K; however, it is not likely that the electric polarization will increase continuously due to decreasing magnetic hyperfine field (H_hf). From the value of the isomer shift (δ) at all temperatures, we concluded that the Fe ion is ferrous (Fe²⁺). We observed a large value of R, around 3.4-3.9, which indicates that the quadrupole interaction is larger than the magnetic dipole interaction.

Figure 4 shows the temperature dependence of the H_hf of LiFe_1-xZn_xPO₄. The reduction in H_hf with increasing temperature can be explained in terms of the temperature dependence of the cancellation effect between the orbital current field term and the Fermi contact term in H_hf. In addition, the slope of H_hf at T_S changes due to not only the spin-rotation but also the superexchange interaction. Figure 5 shows the temperature dependence of the electric quadrupole splitting (ΔE_Q) of LiFe_1-xZn_xPO₄. The value of ΔE_Q increases as the temperature decreases down to T_S, and then decreases again as the temperature decreases below T_S due to spin-orbit coupling. The value of ΔE_Q gradually increased with increasing Zn concentration. We consider that the large values of ΔE_Q can be attributed to the distorted octahedra oxygen environment around Fe ions by the CF and ion valance state contributions.

LiFe_1-xZn_xPO₄ (x = 0.1, 0.2, 0.3, and 0.5) was determined to have orthorhombic structure with space group Pnma. The temperature dependence of the magnetic susceptibility curves from the VSM measurement showed T_N, T_S, and antiferromagnetic behavior. The reduction of antiferromagnetic order in LiFe_1-xZn_xPO₄ can be explained by a weakened superexchange interaction. The Mössbauer spectra below T_N showed asymmetric eight-line shape due to large electric quadrupole interaction caused by the EFG. The magnetic properties of LiFe_1-xZn_xPO₄ affected the quenched orbital angular momentum with a strong CF from the distorted FeO₆ octahedra. The slope of the H_hf curve and the value of ΔE_Q at T_S show the change, which is caused by spin-orbit coupling.

See supplementary material for the sketch of distorted FeO₆ octahedron, and the bond length between Fe-ion and six O-ions for samples.

This work was supported by Mid-Career Researcher Program, through the National Research Foundation of Korea (NRF), with a grant funded by the Ministry of Education, Science and Technology (MEST) (NRF-2017R1A2B2012241).