Experimental validation of high electrical conductivity in Ni-rich LaNi1−xFexO3 solid solutions (x ≤ 0.4) in high-temperature oxidizing atmospheres

Accurate electrical conductivity measurements of LaNi1−xFexO3 using fully dense polycrystalline samples revealed that LaNiO3 (x = 0) has the highest electrical conductivity among precious-metal-free oxides in high-temperature oxidizing atmospheres.

In this study, we have prepared dense polycrystalline samples of single-phase LaNi1−xFexO3 with high Ni contents (0 ≤ x ≤ 0.4) by virtue of the post-sintering oxidation process 46 . In this process, pre-sintered dense tablets of mixed oxides were converted to single-phase LaNi1−xFexO3 under high oxygen partial pressures. Then their accurate electrical conductivity has been evaluated to reveal the composition dependence. Dense samples of LaNi1−xFexO3 with x = 0.4 and 0.36 were readily prepared by conventional solid-state reaction of La2O3, NiO, and Fe2O3 and sintering under ordinary pressure, while that with x = 0.2 could not be obtained in the same way. Figure 1(a) shows powder X-ray diffraction patterns and SEM images of LaNi1−xFexO3 tablets with compositions x = 0.4, 0.36, and 0.2 after sintered at 1300 C in air, oxygen gas, and air, respectively. The sintered tablets with x = 0.4 and 0.36 were almost singlephase perovskite, while the tablet with x = 0.2 contained impurity phases La4Ni3O10 and NiO in addition to the perovskite phase. § The upper temperature limit of the stability of the perovskite phase LaNi1−xFexO3 has been reported to become lower as x or oxygen partial pressure (pO2) decreases 38,44 . The present results follow this trend and reconfirm the difficulty in sintering LaNi1−xFexO3 with high Ni contents (x ≤ 0.2) under ordinary pressure. The post-sintering oxidation realized dense samples of LaNi1−xFexO3 with x = 0.2 and 0. The above mixed-phase tablet with x = 0.2 was subsequently oxidized at 1250 C under pO2 = 392 bar using hot isostatic pressing (HIP). As evident from its powder X-ray diffraction pattern in Figure 1 (b), the sample with x = 0.2 was converted to almost single-phase perovskite owing to this high-pressure oxidation treatment. A dense sample of single-phase LaNiO3 (x = 0) was also obtained similarly by  There are few pores in their fracture surfaces after the final heat treatments, and their relative densities finally exceeded 98%.
Electrical conductivity measurements using the dense LaNiO3 sample revealed that the electrical conductivity of LaNiO3 has been much underestimated in the literature due to insufficient sintering. The electrical conductivity measurements of LaNi1−xFexO3 were carried out by the four-probe technique using bar-like tablets with silver electrodes (illustrated in Figure 2) in air or a mixture of O2 and Ar gases. Figure 3 shows the electrical conductivity values of LaNiO3 either obtained in this study using the dense sample or reported in the literature . Temperature dependence of the electrical conductivity (σ) of LaNiO3 measured using a dense sample in this study or published in the literature. 5,12,14,18,28,49,50,51,52,53,54 Atmospheres where measurements were carried out are indicated in the figure. Preparation conditions and relative densities of the specimen are summarized in the legend. The sintering condition of Świerczek et al. 28 was given in their paper as "1000-1200 °C range, depending on the sample chemical composition". As LaNiO3 is considered to be thermally most unstable composition in their paper, we guess the sintering temperature of LaNiO3 was approximately 1000 °C.    25 , Basu et al. 26 , Zhou et al. 19 , Iwasaki et al. 20 , and Zhang et al. 53 are also shown. The figure shows only the electrical conductivities of the samples whose relative densities were over 80% or that of a single crystal. 53  5, 12,14,18,28,49,50,51,52,53 54 . While the temperature dependences (or the slopes) of the present and the reported data are consistent, significant discrepancies are seen in the electrical conductivity values. The present data and those obtained from a singlecrystal LaNiO3 53 are almost one order of magnitude higher than the other literature values. In general, the apparent electrical conductivity of a pressed/sintered material is heavily dependent on its relative density: 32,55 The apparent electrical conductivity varies from the inherent value of the material to almost an order of magnitude lower values as its relative density changes from 100% to 50%. The LaNiO3 samples used in the cited studies in Figure 3 except the single crystal were only pressed or sintered at around 1000 °C or lower 5,12,14,18,28,49,50,51,52 54 due to the poor thermal stability of LaNiO3 under an ordinary partial pressure of oxygen (1 bar O2 or less). Based on previous studies 12, 49,51,56 and our recent study on the La-Ni-O system 46 , it is thought to be difficult to obtain fully dense samples (relative density over 95%) under such conditions. Therefore, the literature values of the electrical conductivity of LaNiO3 are likely to be much underestimated due to the lower relative densities. It is worth mentioning that this large discrepancy cannot be attributed to the difference in the atmosphere where the electrical conductivity measurements were carried out, as the variation of the electrical conductivity of LaNiO3 is less than 12 % in the pO2 range of 10 -3 -1 bar at 800 °C (see Figure S1). The composition dependence of the electrical conductivity of LaNi1−xFexO3 is proved to be monotonic.  Please do not adjust margins Please do not adjust margins the underestimation of the conductivity of Ni-rich samples in the past studies due to the lower relative densities as discussed in the previous paragraph or the existence of secondary phases. 24,32 The increasing trend of the electrical conductivity with Ni content could be attributed to the previously proposed picture that the metal-insulator transition is driven by disorder effects arising from the substitution for Ni in metallic LaNiO3 with Fe. However, further experimental and theoretical studies using appropriately synthesized samples are desirable to elucidate the nature of M-I transition in this system. Referring also to literature data on Ni-poor compositions, we can see that the electrical conductivity of LaNi1−xFexO3 increases monotonically with decreasing x in the whole composition range. In Figure 5 (a), the electrical conductivity of LaNi1−xFexO3 is compared with those of simple perovskite or perovskite-related (layered-perovskite) oxides based on 3d transition metals Cr, Mn, Fe, Co, Ni, and/or Cu. These oxides are stable in air and at high temperatures (~500-800 °C), and have potential applications as the cathodes and interconnects of SOFCs. The electrical conductivity of LaNiO3 (LaNi1−xFexO3 with x = 0) is found to be the highest among the above oxides including popular cathode materials such as La0.6Sr0.4Co0.2Fe0.8O3-d (LSCF), Ba0.5Sr0.5Co0.8Fe0.2O3-d (BSCF), La0.5Sr0.5MnO3-d (LSM), and La0.6Sr0.4CoO3-d (LSC). More broadly, Figure 5 (b) compares the electrical conductivities of LaNi1−xFexO3 and other representative electrically conducting oxides out of the above category, based on 3d, 4d, or 5d transition metals. ReO3 exhibits 1-2 orders of magnitude higher conductivity than that of LaNiO3. Also, RuO2 with rutile structure, TiOx (x = ~1) with NaCl structure, Ti2O3 with corundum structure, and NaxWO3, La1-xSrxVO3, SrMoO3, and SrRuO3 with perovskite structure exhibit equal or higher conductivity than LaNiO3. However, many of them (ReO3, 60 NaCl-type TiOx, 61 Ti2O3, 61 NaxWO3, 62,63 La1-xSrxVO3, 64,65 and SrMoO3 66 ) are unstable in high-temperature air and the rest of them (SrRuO3 and RuO2) are based on precious metal, both can be drawbacks for practical use. Therefore LaNiO3 is now considered as the precious-metal-free oxide with the highest electrical conductivity in high-temperature air.

Conclusions
In conclusion, the electrical conductivity of Ni-rich LaNi1−xFexO3 (0 ≤ x ≤ 0.4) is found to monotonically increase with decreasing x (increasing Ni content) at both room temperature and 800 °C by use of the fully dense single-phase polycrystalline samples prepared by the post-sintering oxidation process. The confirmed trend suggests that LaNi1−xFexO3 with smaller x (higher Ni content) might be more suitable for the cathodes of SOFCs than the widely-studied composition LaNi0.6Fe0.4O3 (x = 0.4). It should be noted, however, that a higher conductivity does not necessarily lead to a better electrode performance. Therefore, it is desirable to investigate the electrochemical properties of Ni-rich LaNi1−xFexO3 (0 ≤ x ≤ 0.4). One concern with Ni-rich LaNi1−xFexO3 is the thermal instability during operation at high temperatures, 35 but it may be less problematic for the applications in low-temperature SOFCs, which are under active development. In addition, Ni-rich LaNi1−xFexO3 can be more readily applicable as oxygen electrodes in solid oxide electrolyzer cells (SOECs) because the electrodes are operated in highly oxidative conditions, where LaNi1−xFexO3 is more stable. 38,46 (See Figure S2) Furthermore, in a broader context, LaNiO3 is now considered as the precious-metal-free oxide with the highest electrical conductivity in high-temperature air (2.5  10 3 S cm −1 at 800 °C in 0.2 bar O2) and can find other applications as an oxidation-resistant, high-temperature electrical conductor.

Conflicts of interest
There are no conflicts to declare.

Acknowledgment
This study was supported by Grant-in-Aid for JSPS Research Fellow Grant Number 16J09427. We are grateful to Shin-Etsu Chemical Co., Ltd for providing lanthanum oxide.
Notes and references § Fe may partially occupy the Ni-sites in La4Ni3O10 and NiO 44 .
§ § Based on XRD patterns (Figure 1), the amount of secondary phases (NiO and an unknown phase) is estimated to be ~1 wt% or less. Therefore the secondary phases would not constitute a long-range conduction network and the high electrical conductivity observed in this study can be attributed to the main phase, LaNi1-xFexO3. Also, the fact that the conductivity value of the LaNiO3 polycrystal obtained in this study are almost identical to that of a single crystal at room temperature ( Figure  3(a)) suggests that the influence of secondary phases is not significant. § § § In LaNi1-xFexO3 (0 ≤ x ≤ 0.4) solid solutions, electronic conduction should dominate over ionic conduction. Although they contain oxygen vacancies 9,35,54 and can conduct oxide ions, the oxide ion conductivity at 800 °C is assumed to be less than