We have investigated the spin orientation in antiferromagnetic polycrystalline LiFe1-xZnxPO4 using Mössbauer spectroscopy. The temperature-dependent magnetic susceptibility curves show antiferromagnetic behavior with ordering temperature. The experimentally determined effective moment of LiFe1-xZnxPO4 is larger than the theoretical value, which can be explained as incomplete absence of orbital contribution by the crystalline field around distorted octahedra. The value of the Néel temperature (TN) and the spin reorientation temperature (TS) of LiFe1-xZnxPO4 decreased with the increasing Zn concentrations from 48 and 14 K for x = 0.1 to 36 and 8 K for x = 0.5, resulting in weak antiferromagnetic interaction. Below TN, Mössbauer spectra of LiFe1-xZnxPO4 showed asymmetric eight-line shape due to the strong crystalline field in the distorted octahedral structure. A change in both the magnetic hyperfine field and electric quadrupole splitting below TS suggests that magnetic phase transition is related to the spin rotation and the superexchange interaction.

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 LiFe1-xZnxPO4 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.

LiFe1-xZnxPO4 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 LiFe1-xZnxPO4 samples was determined to be orthorhombic with Pnma space group from the Rietveld refinement analysis. The lattice constants and the volume of the LiFe1-xZnxPO4 decreased from a0 = 10.3293 Å, b0 = 6.0060 Å, c0 = 4.6939 Å, and V = 291.479 Å3 for x = 0.1 to a0 = 10.2741 Å, b0 = 5.9846 Å, c0 = 4.6985 Å, and V = 288.738 Å3 for x = 0.5, as shown Fig. 1. The unit cell parameters of a0 and b0 for LiFe1-xZnxPO4 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 LiFe1-xZnxPO4. 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 LiFe0.5Zn0.5PO4.

The temperature dependence of magnetic susceptibility χ(T) curves of LiFe1-xZnxPO4 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 (TN) and the spin reorientation temperature (TS) of LiFe1-xZnxPO4 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+-O2--Zn2+ link is smaller relative to that of the Fe2+-O2--Fe2+ link, resulting in weak antiferromagnetic interaction. The TN and TS values of LiFePO4 were reported in our previous publication as 51 and 23 K.16–18 This trend is consistent with the trend in TN and TS 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 (θCW), which is obtained from the fitted inverse magnetic susceptibility curves. The values of the θCW are shown in Table I. Above TN, 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 LiFe1-xZnxPO4 is larger than the theoretical value, which is given by μtheo = [(1-x)μFe2+2+ xμZn2+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 LiFe1-xZnxPO4 decreased with increasing Zn concentration from 5.30 μB for x = 0.1 to 4.08 μB for x = 0.5.

To investigate the microscopic magnetic structure of LiFe1-xZnxPO4, we obtained Mössbauer spectra at temperatures from 4.2 to 295 K. Some representative LiFe0.5Zn0.5PO4 spectra are shown in Fig. 3. The spectra show an asymmetric eight-line shape from 4.2 K to TN due to the strong CF in the distorted octahedral structure of the Fe2+ ion and six O2- ions. Also, in the previously reported LiFePO4, Mössbauer spectra in the antiferromagnetic region below TN exhibited asymmetric eight-line shape.16 At temperatures below TN, 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 LiFe0.5Zn0.5PO4 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: Peij×(Si×Sj), where eij is the unit vector between two magnetic ions, and Si and Sj 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 (Hhf). 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 Hhf of LiFe1-xZnxPO4. The reduction in Hhf 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 Hhf. In addition, the slope of Hhf at TS changes due to not only the spin-rotation but also the superexchange interaction. Figure 5 shows the temperature dependence of the electric quadrupole splitting (ΔEQ) of LiFe1-xZnxPO4. The value of ΔEQ increases as the temperature decreases down to TS, and then decreases again as the temperature decreases below TS due to spin-orbit coupling. The value of ΔEQ gradually increased with increasing Zn concentration. We consider that the large values of ΔEQ can be attributed to the distorted octahedra oxygen environment around Fe ions by the CF and ion valance state contributions.

LiFe1-xZnxPO4 (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 TN, TS, and antiferromagnetic behavior. The reduction of antiferromagnetic order in LiFe1-xZnxPO4 can be explained by a weakened superexchange interaction. The Mössbauer spectra below TN showed asymmetric eight-line shape due to large electric quadrupole interaction caused by the EFG. The magnetic properties of LiFe1-xZnxPO4 affected the quenched orbital angular momentum with a strong CF from the distorted FeO6 octahedra. The slope of the Hhf curve and the value of ΔEQ at TS 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).

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Supplementary Material