The oxygen-deficient perovskite Sr₃YCo₄O_10.5+δ (314-SYCO) with an ordered tetragonal structure has potential applications in microwave absorbing materials, solid oxide fuel cells, and other fields owing to its room-temperature ferromagnetism (Golosova et al., 2009; Marik et al., 2018; Istomin et al., 2003), orbital and charge ordering (Khalyavin et al., 2011; Sheptyakov et al., 2009; Kishida et al., 2016), high electronic conductivity, and excellent activity for the oxygen-reduction reaction (Li et al., 2011; Lalan et al., 2019). The A-site ordered (AO) and oxygen vacancy ordered (OO) tetragonal superstructure of 314-SYCO is composed of alternating octahedral CoO₆ layers and tetrahedral CoO_4.25+δ layers stacked along the c-axis. The A-site cations are arranged as –Sr–Y–Y–Sr– units along the c-axis, and the Sr²⁺: Y³⁺ ratio is 3:1 in the ab-plane. The oxygen vacancies of the CoO_4.25+δ layer are arranged in a zigzag pattern in the bc-plane (Khalyavin et al., 2011). For δ = −0.26–0.3, 314-SYCO usually has an AO/OO tetragonal superstructure, as illustrated in Figure 1A (Istomin et al., 2003; Fukushima et al., 2009). Moreover, the basic magnetic structure of 314-SYCO is a G-type antiferromagnet, as illustrated in Figure 1B (Sheptyakov et al., 2009). The magnetic properties are mainly derived from oxygen ordering at oxygen-vacancy sites (Kobayashi et al., 2005; Fukushima et al., 2008; Fukushima et al., 2009).

The 314-SYCO material has a complex and interesting phase-transition process. Hu et al. (Hu et al., 2017) observed an exothermic peak at 963°C in differential scanning calorimetry (DSC) curves, corresponding to the crystallisation of 314-SYCO. In addition, a weight loss at 400°C in the thermogravimetric (TG) curves of 314-SYCO samples in oxygen, air, and helium atmospheres were observed due to the removal of O4 (the Co1–O4 distance is the longest, as illustrated in Figure 1). At approximately 1,000°C in a helium atmosphere, the oxygen content in polycrystals that had lost most of their O4 was close to 10 (Sr₃YCo₄O₁₀), resulting in a brownmillerite-type structure (Istomin et al., 2003). However, there are limited reports on the phase transition of 314-SYCO during its synthesis.

The phase structure, ordering, and physical properties of 314-SYCO could be modulated by Co-site doping. With increased Fe doping, Sr_0.75Y_0.25Co_1−x Fe _x O_2.625+δ changes from an ordered tetrahedral structure to a disordered cubic structure, while Sr_0.75Y_0.25Co_1−x Ga _x O_2.625+δ has a tetragonal superstructure with decreased magnetic order at x = 0.25 (Lindberg et al., 2006). In the CoO₆ layer and/or antiferromagnetic CoO_4.25 layer, Ga³⁺ ions both replace the high-spin-state Co³⁺ ions and increase the saturation magnetisation of Co ions. Further, it was shown that both the thermal expansion coefficient and conductivity of Sr₃YCo_4−x Fe _x O_10.5+y decrease at 1173 K (Istomin et al., 2008). According to previous studies, intermediate-spin state Co³⁺ ions get preferentially replaced by Al³⁺ ions in the ferrimagnetic CoO₆ layer, which lowers the Sr_3.1Y_0.9Co₄O_10.5 saturation magnetisation (Tsuruta et al., 2020). The addition of Al³⁺ disrupts the oxygen-ordering superstructure, which further inhibits the ferromagnetism of 314-SYCO at room temperature (Rajan and Subodh, 2020). As Cu has a similar ionic radius to Co, Cu doping at the Co site can directly increase carrier concentration. Therefore, the electrical properties of 314-SYCO can be significantly improved by substituting Co with Cu. In addition, CuO, as a sintering agent, increases the content of the liquid phase during sintering, thus improving the sintering quality. Our research group first proposed that Cu doping can reduce the resistivity of 314-SYCO (Du et al., 2014). However, a thorough investigation of the effects of Cu doping on the electromagnetic characteristics of polycrystalline 314-SYCO has not yet been conducted. The oxygen-deficient perovskite 314-SYCO has been proven to be useful as a microwave absorber or as a cathode for solid oxide fuel cells due to its high electronic conductivity and excellent activity for oxygen reduction reactions (Li et al., 2011; Lalan et al., 2019). Moreover, low electrical resistivity is an important physical parameter for microwave absorbers or cathodes for solid oxide fuel cells.

Here, the ordered phase transformation of 314-SYCO is reported for the first time. The microstructure, electrical transport, and magnetic properties of Sr₃YCo_4−x Cu _x O_10.5+δ (x = 0, 0.2, 0.4) were investigated in detail. For x = 0–0.4, the ordered tetragonal Sr₃YCo_4−x Cu _x O_10.5+δ phase was formed. The key step in the formation of the ordered tetragonal phase is an exothermic reaction at ∼1,042°C and an oxygen mass gain of ∼0.8%. With increasing Cu content, the electrical resistivity decreases greatly, and the polycrystalline material shows the three-dimensional Mott variable-range hopping mechanism typical of a semiconductor. Additionally, a G-type antiferromagnetic-to-ferromagnetic phase transition occurs due to a reduction in the spin state of some Co³⁺ ions.

Figure 2 shows the XRD patterns of the polycrystalline Sr₃YCo_4−x Cu _x O_10.5+δ (x = 0–0.8) sintered at 1,100°C for 24 h. For x = 0–0.4, all diffraction peaks were indexed to the tetragonal superstructure (PDF#54–0,234), i.e., I4/mmm. As the Cu doping content increased, the diffraction peaks of the samples shifted toward a lower angle, indicating an increase in the lattice constant due to the partial replacement of Co^3+/4+ (0.61/0.53 Å) by Cu²⁺ (0.73 Å) (Hsieh and Fung, 2008; Zhao et al., 2010). Figures 2A,B show the presence of (008)(400), (228)(424), and (408)(440) split peaks. Both sets of spectra indicate a = b ≠ 1/2c, which is characteristic of a tetragonal structure (Hu et al., 2017). The peaks of (103) and (215) in Figures 2C,D imply that Sr₃YCo_4−x Cu _x O_10.5+δ polycrystals had an ordered lattice, consistent with the ordered tetragonal phase extinction law (h, k, l ≠ 2n) (Kobayashi et al., 2005). Figures 2E–G show that the experimentally measured XRD diffraction peaks exhibit a high degree of overlap with the feasibility of Rietveld refinement, indicating that the samples from x = 0 to 0.4 are pure ordered tetragonal phase. The final reliability factors for the recorded patterns are R _wp = 3.346% and χ ² = 1.69 for x = 0, R _wp = 3.503% and χ ² = 1.77 for x = 0.2, and R _wp = 6.719% and χ ² = 3.30 for x = 0.4.

The average grain size in the direction of the vertical grain plane (204), D ₍₂₀₄₎, was calculated using Scherrer equation D D 204 = K λ / β ⁡ cos ⁡ θ (Vinila and Isac, 2022). Here, β is the full width at half maximum of the peak, and K = 0.89. Table 1 summarises the lattice constant, grain size, linear shrinkage, and density of Sr₃YCo_4−x Cu _x O_10.5+δ (x = 0, 0.2, and 0.4) samples. The linear shrinkage (ΔL/L ₀, where ΔL is the shortening value after sintering and L ₀ is the diameter of the sample before sintering) and bulk density (P) significantly increased with increasing x. In Figures 2A,C,D, the (103) and (215) diffraction peaks are very small for x = 0.6–0.8, suggesting that the ordered phase was either minimal or not present.

Figure 3A displays the HRTEM pictures together with their accompanying FFT diffraction spots, and Figure 3B illustrates how the crystal structure of the Sr₃YCo_4−x Cu _x O_10.5+δ (x = 0.2) samples correspond to the diffraction spots. The d-spacings of 0.539 and 0.424 nm are indexed to the 1 1 ¯ 0 and 1 ¯ 0 3 ¯ lattice planes, respectively. The FFT pattern (inset in Figure 3A) and Figure 3B show the diffraction spots corresponding to the 1 1 ¯ 0 , 0 1 ¯ 3 ¯ , and 1 ¯ 0 3 ¯ planes along the 33 1 ¯ orientation. The superlattice (103) diffraction surface confirms that the tetragonal Sr₃YCo_4−x Cu _x O_10.5+δ polycrystalline structure has an ordered superstructure.

TG-DSC curves were measured to simulate the sintering of the raw ingredients to produce Sr₃YCo_4−x Cu _x O_10.5+δ (x = 0–0.8). Figures 4A–E show three stages in all TG curves, i.e., approximately 25–680°C for stage I, 680–969°C for stage II, and 969–1,100°C for stage III. The curves were similar for all samples in stages I and II.

In stage Ⅰ, the mass loss of all samples was in the range of 0.5–2.5% due to the dehydration of the raw materials. The mass loss was in the range of 12–15.6% in stage II. The endothermic peak at ∼910°C corresponds to the phase transition from orthorhombic to trigonal SrCO₃ and the decomposition of Co₃O₄ into CoO and O₂ (Delorme et al., 2015). Trigonal SrCO₃ is broken into SrO and CO₂, which produce the endothermic peak at ∼933°C (Ptáček et al., 2015).

As the temperature increased above 946°C, all TG curves showed further weight loss, indicating that SrCO₃ continued to decompose. In the DSC curves, the exothermic peak at ∼968°C corresponds to the crystallisation of Sr₃YCo_4–x Cu _x O_10.5, because one of the four adjacent oxygen vacancies (Figure 1A) can be occupied easily (O₁₀→O_10.5) (Istomin et al., 2003; Rupasov et al., 2009), as indicated by Eq. 1. 4 – x C o O + 3 S r O + 1 2 Y 2 O 3 + x C u O + O 2   → 968 ° C   T e t r a g o n a l − S r 3 Y C o 4 – x C u x O 10.5 (1)

For x = 0.2–0.8, the weak endothermic peak observed at 746–811°C may be related to a trace eutectic mixture formed by CuO and SrCO₃–Y₂O₃–Co₃O₄ (Kingery and Narasimhan, 1959).

In stage Ⅲ, the mass variation and thermal behaviour are significantly dependent on x. As shown in Figure 4F, the mass gain is 0.8–0.9% for x = 0–0.4, while the DSC curves show a weak exothermic peak at approximately 1,042°C, corresponding to the uptake of oxygen (δ) (Eq. 2). T e t r a g o n a l − S r 3 Y C o 4 – x C u x O 10.5 + δ 2 O 2 → 1042 ° C   O r d e r e d   t e t r a g o n a l − S r 3 Y C o 4 – x C u x O 10.5 + δ (2)

For x = 0.6–0.8, the weak endothermic peak at 978°C may correspond to an impurity formed by heating. Despite a mass gain of 0.7–1%, the exothermic peaks related to the ordered phase were not observed at ∼1,042°C. This observation is consistent with the XRD patterns in Figure 2, where the (103) and (215) peaks were not observed for x = 0.6. The lack of the ordered phase may be caused by excessive Cu doping, implying that the solid solubility of Cu is x = 0.4–0.6.

Figure 5A shows photographs of polycrystalline Sr₃YCo_4−x Cu _x O_10.5+δ sintered at 1,100°C for 24 h. For x = 0–0.4, the surfaces are smooth and flat, and the volume drastically reduced with increasing x (i.e., enhanced sintering). For x = 0.6–0.8, the corrosion of the samples with the crucible became increasingly severe as the dissolution exceeded the solid limit, and low-melting-point heterogeneous phases were produced. Cross-sectional SEM images of polycrystalline Sr₃YCo_4−x Cu _x O_10.5+δ (x = 0–0.4) (Figures 5B–D) indicate the porous structures of these samples. With increasing x, an increased density is observed as a result of sintering, i.e., the pore volume reduced to form close grain connections (Figure 5E). These trends also align with the findings in Table 1. Figure 5F shows a surface SEM image of Sr₃YCo_4−x Cu _x O_10.5+δ (x = 0.6) polycrystalline material. The grain clusters are highlighted in blue and the lighter lines are the grain boundaries. According to EDS analysis, spot 2 (Figure 5H) has significantly higher Cu content than spot 1 (Figure 5G). Therefore, the grain clusters are thought to be a Cu-rich phase.

The XPS survey spectra of Sr₃YCo_4−x Cu _x O_10.5+δ samples (x = 0, 0.2, and 0.4) are shown in Figure 6A, confirming the presence of elements in these samples; no other impurity elements were detected, demonstrating the high purity of the experimentally-produced polycrystals. The XPS Co elemental spectra for the samples (x = 0, 0.2, and 0.4) are presented in Figures 6B–D. In contrast with the Co³⁺ ion, Co⁴⁺ contains a more positive electrical charge and lower electron cloud density, thus the binding energy of 2p electrons should also be higher (LU et al., 2013). As a result, Co³⁺ ions have a lower binding energy than Co⁴⁺ ions. Therefore, the intensity peaks appearing at 779.77/794.74 eV for x = 0, 780.5/795.02 eV for x = 0.2 and 779.68/794.65 eV for x = 0.4 can be assigned to Co³⁺, while the peaks of 782.18/797.15 eV for x = 0, 782.36/797.33 eV for x = 0.2 and 781.81/796.78 eV for x = 0.4 can be assigned to Co⁴⁺ ion. For each composition, the Co³⁺/Co⁴⁺ ratio is determined by fitting the area under the curve. The Co³⁺/Co⁴⁺ ratio for each composition is presented in Table 2. It was observed that the Co³⁺ content decreased significantly with the increase in the Cu doping amount. The XPS results confirm that the Cu²⁺ ions predominantly occupy Co³⁺ sites. In Figure 6E, the Cu²⁺ signal (Cu2p_3/2 and Cu2p_1/2 are 934.07 and 953.87 eV for x = 0.2, 933.49 and 953.29 eV for x = 0.4, respectively) and its satellite peak (936.0–946.0 eV) in the Cu 2p spectral range are obvious (Wu et al., 2018; Li et al., 2022). The results show that in Sr₃YCo_4−x Cu _x O_10.5+δ , Cu ions exist in the oxidation state of +2 valence. Compared to the element Co, Cu 2p is relatively weak. This phenomenon is consistent with its low element content.

As shown in Figure 7A, the resistivity-temperature (ρ-T) curves of polycrystalline Sr₃YCo_4−x Cu _x O_10.5+δ (x = 0–0.6) showed electrical transport characteristics typical of a semiconductor. The resistivity decreased significantly from x = 0 to 0.4 and stabilised from x = 0.4 to 0.6. Figure 7B shows that the room temperature resistivity at 300 K for x = 0.4 and x = 0.6 is 0.0277203 Ω cm and 0.0040672 Ω cm, respectively. Compared with the room temperature resistivity at x = 0 of 0.3403219 Ω cm, the resistivity decreased by 92–99% with Cu doping.

The lnρ-T ^−1/(n+1) curve was fitted using the Mott variable-range hopping model expressed in Eq. 3 (Mott, 1969): ρ = ρ 0 · ⁡ exp ⁡ T m / T 1 / n + 1 (3)

Here, ρ ₀ is the initial electrical resistivity (constant), T _m is the characteristic temperature of the variable-range hopping mechanism, and n = 2 or n = 3 is the dimension of the system. Figure 7C shows that when n = 3, lnρ-T ^−1/4 is almost linear for x = 0–0.04, indicating that a small number of Co⁴⁺ ions provide hole carriers (Terasaki et al., 2010), and hopping conduction occurs between Co⁴⁺ ions via a three-dimensional variable-range hopping mechanism. A deviation was found between the experimental and fitted curves for the x = 0.6 sample, which may be caused by changes in the resistivity induced by impurity phases produced beyond the limit of solid solubility of Cu in the perovskite structure. When Co^3+/4+ was replaced with Cu²⁺, the concentration of hole carriers increased. In addition, the mobility of the hole carriers increased as the grain size and density of Sr₃YCo_4−x Cu _x O_10.5+δ increased due to sintering.

The S-T curves of Sr₃YCo_4−x Cu _x O_10.5+δ (x = 0–0.4) in Figure 7D indicate that the thermopower decreased with increasing temperature. The curves for x = 0 and 0.2 are very similar, and the thermopower of the x = 0.4 material was significantly lower due to the higher carrier concentration. In contrast, increasing the spin entropy of the Co³⁺ ions increases the thermopower (Yoshida et al., 2009).

The M-T and dM/dT-T curves of polycrystalline Sr₃YCo_4−x Cu _x O_10.5+δ (x = 0–0.4) are shown in Figures 8A,B. For x = 0, the Hopkinson peak is observed at 319 K in the zero-field-cooling (ZFC) curve. Magnetic moments in the magnetic domains of materials are randomly distributed and frozen at low temperatures, and the net magnetic moments tend to be zero in G-type antiferromagnetic phases (Sheptyakov et al., 2009; Troyanchuk et al., 2009; Troyanchuk et al., 2019). As the temperature increases, the magnetic moments rotate to align along the direction of the external magnetic field, while the magnetic domain wall moves and the magnetic domain grows. When all magnetic moments are aligned in the same direction, the magnetic domain walls disappear, and the magnetisation reaches its maximum at 319 K. At T_c , thermal agitation causes magnetic moments to misalign, resulting in a ferromagnetic-paramagnetic second-order transition and a decrease in magnetisation. There is no resistance between the magnetic domain walls when the field-cooling (FC) curve is recorded, and the magnetic moments are aligned in the same direction under an external magnetic field. Therefore, the maximum magnetisation obtained for the FC curve was higher than that obtained for the ZFC curve. The FC curves had the highest peak at 267 K. As the temperature continues to decrease, a ferromagnetic-antiferromagnetic transition occurs, and a G-type antiferromagnetic phase is generated (Lindberg et al., 2006). The separation of the ZFC and FC curves at low temperature indicates that spin-glass state-like components may be present (Motohashi et al., 2005; Sutjahja et al., 2015; Guo et al., 2018; Srivastava et al., 2020).

For x = 0.2, the maximum magnetisation of the ZFC curve was found at 4 K, attributed to the chemical compressive stress induced by lattice distortion due to Cu doping. It allows most Co³⁺ ions to transit from a high/intermediate spin (HS/IS, t _2g ⁴ e _g ²/t _2g ⁵ e _g ¹, S = 2/1) to an intermediate/low spin (IS/LS, t _2g ⁵ e _g ¹/t _2g ⁶ e _g ⁰, S = 1/0), resulting in a decrease in magnetisation (Tsuruta et al., 2020). The Co³⁺ ions in the LS state are nonmagnetic. At this point, the spin magnetic moments of the Co³⁺ ions in most regions are aligned in the same direction, as shown in Figure 8C, which is similar to the ferromagnetic structure. The T_c from the ZFC curves decreased from 320 K (x = 0) to 307 K (x = 0.2) and 288 K (x = 0.4), which is related to the decrease in the spin state of Co³⁺ ions, leading to a decrease in magnetisation and weakening of the magnetic interaction. Moreover, in conjunction with the XPS results, the doping of Cu ions instead of Co ions led to a reduction in Co³⁺ ion content, which has a direct effect on the reduction of the magnetisation intensity of the sample.

A conventional solid-state reaction method was used to synthesise polycrystalline Sr₃YCo_4−x Cu _x O_10.5+δ (x = 0–0.8). An ordered tetragonal phase and ordering phase transformations due to oxygen uptake above 1,000°C were observed for x = 0–0.4 for the first time. An ordered tetragonal phase and ordering phase transformation due to oxygen uptake above 1,000°C were observed for x = 0–0.4 for the first time. A study of the phase-transformations process during heating provides a research reference for optimising the conditions of the material synthesis process, such as sintering temperature and holding time. Cu doping was observed to significantly reduce the electrical resistivity, and the three-dimensional Mott variable-range hopping conduction mechanism of polycrystalline Sr₃YCo_4−x Cu _x O_10.5+δ was explored. We also found that Cu doping reduced both the spin state and content of Co³⁺ ions, thereby inhibiting the room temperature ferromagnetism of 314-SYCO and providing a reference for doping to modulate the properties of oxygen-deficient perovskites.