Enhanced Thermal Stability in Cu1234 Superconductor with Oxygen Annealing

Abstract

Recently, CuBa₂Ca₃Cu₄O_10+δ (Cu1234) has garnered significant interest owing to its distinctive triple-high superconducting properties (118K high T_c, combined with high J_c and high H_irr at liquid nitrogen temperature at ambient pressure) and potential for practical applications. The Cu1234 is initially synthesized at high pressures and is stable at a room temperature range but tends to decompose upon heating above 300 °C at ambient. In this study, we investigate the thermal stability of Cu1234 through annealing at various temperatures and oxygen pressures. It is found that Cu1234 starts to decompose at approximately 350 °C, 550 °C, and 600 °C when annealed at 1 bar, 100 bar, and 150 bar oxygen pressure, respectively. Prior to decomposition, however, the superconducting properties remain largely unchanged. The decrease in oxygen occupancy within the BaO layer of the BaCuO_3−δ charge reservoir block is proposed to be the primary cause of the structural instability of Cu1234, while higher oxygen pressures retard oxygen loss from this block. Our result suggests that the decomposition temperature of Cu1234 will further increase with higher oxygen pressure, e.g., possibly to 800 °C at 260 bar if a linear extrapolation is adopted. This study offers important insights for fabricating Cu1234 tapes via the powder-in-tube method.

1. Introduction

CuBa₂Ca₃Cu₄O_10+δ (Cu1234), discovered as a high-temperature superconductor with a T_c of ~118 K, has recently attracted renewed interest because of its promising practical applications owing to its triple-high properties [1,2]. It has been revealed that Cu1234 has high T_c, high critical current density J_c (~5 × 10⁵ A/cm² at self-field) and high irreversible field H_irr (>20 Tesla) in the liquid nitrogen temperature range [3,4,5], exhibiting overall performance superior to that of the established superconducting tapes of Bi₂Sr₂Ca₂Cu₃O_x (Bi2223) and YBa₂Cu₃O_7−δ (YBCO) [6,7]. The triple-high properties are attributed to the small anisotropy factor of Cu1234 similar to YBCO [8,9]. Following the method of manufacture of the YBCO tape in the form of epitaxial thin-films, the Cu1234 and its related phase CuBa₂Ca₂Cu₃O_8+δ (Cu1223) films have been successfully grown on different single-crystal substrates and flexible Hastelloy tapes [10,11]. It was found that a partially incorporated carbon element at the [CuO₂] deficient charge reservoir layer resembling the tetragonal phase of YBCO is relatively easier to synthesize the Cu1234 structure at high pressure. The superconductor is consequently sometime labeled as (Cu,C)1234 [12,13]. To fabricate a Cu1234 tape, the epitaxial thin-film approach seems feasible [14,15,16], but the powder-in-tube (PIT) method poses a significant challenge. The PIT process, used for Bi2223/Ag tapes, involves intermediate rolling and requires high-temperature healing of cracks at ambient pressure to enhance superconducting connectivity [17,18]. However, bulk polycrystalline Cu1234 can only be synthesized under high-pressure conditions. Therefore, if the PIT method is adopted for Cu1234 tapes manufacture, a critical issue must be addressed: how to heat-treat the samples to heal the cracks without degrading their superconducting properties at ambient pressure or available pressures.

In this work, we systematically investigate the thermal stability and the evolution of the superconducting properties of Cu1234 by annealing the sample at varying temperature and different atmospheres: 1 bar O₂, 100 bar O₂ and 150 bar O₂. It is found that the structure of Cu1234 begins to decompose at about 350 °C in 1 bar O₂ due to the oxygen loss from the BaCuO_3−δ charge reservoir block during the annealing process, and high oxygen pressure can efficiently slow down the oxygen loss and enhance the decomposition temperature. Our result suggests that the decomposition temperature of Cu1234 will further increase with higher oxygen pressure, e.g., possibly to 800 °C at 260 bar if a linear extrapolation is adopted. At this temperature, cracks and grain boundaries could be effectively healed, thereby enhancing the superconducting connectivity—a process similar to observations in Bi₂Sr₂Ca₂Cu₃O_x tapes. This study therefore provides important insights for fabricating Cu1234 tapes via the PIT method.

2. Materials and Methods

The polycrystalline Cu1234 sample was synthesized by a solid-state reaction method under high pressure and high temperature conditions. The synthesis procedure followed established methods described in the literature [1,2]. Cu1234 samples were annealed in a high-pressure tube furnace for 12 h at various temperatures under three different atmospheres: 1 bar O₂, 100 bar O₂ or 150 bar O₂ air. Samples were heated at 5 °C/min to the target temperature, held for 12 h, and then cooled at 5 °C/min. DC magnetic susceptibility measurements were performed using a Magnetic Property Measurement System (Quantum Design MPMS3 ) at the Institute of Physics, Chinese Academy of Siences (IoPCAS) in Beijing, China with an applied magnetic field of 30 Oe under both zero-field-cooled (ZFC) and field-cooled (FC) modes. All samples were cut into identical rectangular dimensions of approximately 3 × 2 × 2 mm³ to ensure consistent demagnetizing factors, allowing direct comparison of relative SVF values without explicit demagnetization corrections. The superconducting volume fraction was estimated using the relation SVF = −4πχ, where χ is the DC magnetic susceptibility in emu/cm³·Oe at 5 K under ZFC conditions. T_c is defined as the temperature at which the magnetic susceptibility becomes negative, i.e., the onset of the diamagnetic signal corresponding to the superconducting transition. Phase composition and structural changes after annealing were examined by X-ray diffraction (XRD) with a Philips X’pert diffractometer using a Huber diffractometer with Cu K_α1 radiation (λ = 1.5406 Å, 40 kV, 30 mA) at the IoPCAS in Beijing China. Diffraction patterns were collected over the 2θ range of 10–100° with a step size of 0.005°. Rietveld refinement of the XRD data was carried out using the GSAS program with version of v1.80 [19].

3. Results and Discussions

Figure 1 shows the crystal structure of Cu1234, which consists of superconducting layers [Ca₃Cu₄O₈] (SCL) with four CuO₂ planes and a charge-reservoir block (CRB) BaCuO_3−δ. The CuO₆ octahedra within the CRB are compressed. This compression lifts the energy of the Cu-3dz² orbital, thereby enhancing conductivity along the c-axis and strengthening the coupling between SCL. Consequently, the system exhibits a small superconducting anisotropy [9]. Furthermore, the oxygen sites in the CRB are only partially occupied, with the occupancy determining the hole carrier density. A superstructure with a × 2b × 2c modulation has been observed due to the oxygen vacancy ordering [20]. The resulting 90° microdomains, where the c-axes are parallel while the a-axes are mutually perpendicular, can act as natural flux-pinning centers and contribute to the high J_c and high H_irr in Cu1234 [20].

Figure 2a and Figure S1 present the XRD patterns of the Cu1234 sample annealed at different temperatures under an oxygen pressure of 1 bar. At this pressure, no additional diffraction peaks appear below 350 °C. Above this temperature, however, weak peaks corresponding to BaCuO_2+x (marked with *) emerge, indicating the onset of Cu1234 decomposition. The observed decomposition products, primarily BaCuO_2+x and CuO, suggest that the initial stage of decomposition involves collapse of the charge reservoir block, releasing barium and copper to form binary oxides. A possible decomposition pathway is: CuBa₂Ca₃Cu₄O_10+δ → BaCuO_2+x + CuO + … Interestingly, even after heating to 450 °C, Cu1234 remains the dominant phase, suggesting relatively slow decomposition kinetics. The annealing behavior of Cu1234 was also investigated by M. Hirai et al. [21], who used a shorter heat treatment time (3 h above 400 °C) compared to ours (12 h). Their reported onset decomposition temperature (~450 °C) is higher than our observed value, likely due to the slow kinetics of the decomposition process [21]. The dependence of the apparent decomposition temperature on annealing duration suggests that the observed onset is at least partially kinetically controlled.

The lattice parameters (a and c) and unit-cell volume (V) of Cu1234, obtained from Rietveld refinement of the XRD data (see Figure S2), are plotted in Figure 2d as a function of annealing temperature. While the a-axis expands only slightly with increasing temperature, both the c-axis and the unit-cell volume increase noticeably above 300 °C. This expansion is attributed to oxygen loss during annealing. The reduction in oxygen content lowers the average valence of Cu ions, increasing their ionic radius and thus causing an overall lattice expansion. A similar relationship between lattice parameters and oxygen content has been reported for the YBCO system [22,23].

To investigate the effect of annealing on superconductivity, magnetic susceptibility measurements were performed. Figure 3a displays the zero-field-cooled (ZFC) magnetic susceptibility versus temperature for samples annealed at 1 bar O₂ and different temperatures. Both the T_c and the SVF vary systematically with increasing annealing temperature. Their trends are summarized in Figure 4a,b. For samples annealed below 300 °C in 1 bar O₂, T_c remains nearly constant, while the SVF decreases only slightly. The SVF stays above 70%, confirming that bulk superconductivity is preserved in Cu1234 under these conditions. As the annealing temperature is raised further, a step-like drop in T_c of more than 3 K occurs between 350 and 400 °C, accompanied by a substantial reduction in SVF to about 30%. At 450 °C, the SVF further decreases to approximately 5%. In cuprate superconductors, oxygen content is a key parameter controlling T_c [24]. A well-known example is Cu1223, in which controlled deoxygenation via annealing in argon gradually increases T_c from 67 K to 118 K [25]. In contrast, the compound CuBa₂Ca₄Cu₅O_12+δ (Cu1245) shows a more complex evolution: with progressive oxygen loss, T_c first decreases slightly from 94.7 K to 93.3 K before rising to 99.0 K [26]. In the present case, the variation in T_c with annealing temperature at 1 bar O₂ is consistent with reported data from annealing under nitrogen flow and is attributed to the loss of lattice oxygen.

Given the close relationship between structural stability, superconductivity, and oxygen loss, higher oxygen pressure is expected to retard oxygen loss during annealing. Accordingly, annealing experiments were performed under elevated oxygen pressures. Figure 2b,c show the XRD patterns of samples annealed at different temperatures under 100 bar and 150 bar O₂, respectively. Compared with annealing at 1 bar O₂, the onset decomposition temperature increases to approximately 550 °C at 100 bar O₂ and further rises to about 600 °C at 150 bar O₂. The enhanced thermal stability under high oxygen pressure is also reflected in the superconducting properties [27]. Figure 3b and Figure 4a,b also present magnetic susceptibility and T_c as well as the SVF for the case of 150 bar O₂, respectively. Both the T_c and SVF curves obtained at 150 bar O₂ shift toward higher temperatures by roughly 200 °C relative to the results obtained at 1 bar O₂. These findings strongly indicate that high oxygen pressure effectively suppresses lattice oxygen loss during annealing, inhibits the decomposition of Cu1234, and thereby significantly improves the thermal stability of its superconducting state. In addition to suppressing oxygen loss from the charge reservoir block, high-pressure oxygen annealing may also induce microstructural changes such as grain growth, phase purification, and reduction in intergranular material. These effects are known to enhance superconducting connectivity in cuprate ceramics and may partially contribute to the retained superconducting volume fraction and stable T_c observed in samples annealed under elevated oxygen pressure. Further studies using electron microscopy are needed to decouple the contributions of oxygenation and microstructural evolution.

Figure 5 shows the relationship between oxygen pressure and decomposition temperature (T_d). Based on the limited data available, a linear fit of the three measured pressure points yields a slope of approximately 1.73 (±0.25) °C/bar. This trend suggests that the decomposition temperature of Cu1234 will further increase with higher oxygen pressure, e.g., possibly to 800 °C at 260 bar if a linear extrapolation is adopted.

A further notable finding is that oxygen loss during annealing primarily originates from a reduction in oxygen occupancy within the BaO layers of the BaCuO_3−δ CRB. The oxygen occupancy as a function of annealing temperature was examined through Rietveld refinement of XRD data from samples annealed at 1 bar O₂ with the refined results shown in Tables S1–S6. In the Rietveld refinement of the as-prepared Cu1234 sample, structural parameters were refined sequentially in the order of lattice parameters, peak profile parameters, atomic sites, thermal parameters, and occupancies. During occupancy refinement, the occupancies of metal sites were fixed to 1, while those of Cu1, O1, and O2 in the CRB were allowed to vary, based on previous reports indicating the presence of vacancies at these sites [28]. This yielded the initial structural model. For samples annealed at different temperatures (all XRD data collected at room temperature), this model was used as the starting point. Only lattice parameters, atomic sites, and the occupancies of O1 and O2 were refined. The occupancies of all metal sites (including Cu1) and thermal parameters were fixed to the values obtained from the unannealed sample, under the assumption that cation stoichiometry and thermal vibration behavior remain unchanged during low-temperature annealing. This refinement strategy minimized correlations between occupancy, scale factors, and thermal parameters, ensuring that the observed variations in oxygen occupancy reliably reflect annealing-induced oxygen loss. Figure 6 displays oxygen occupancies in the BaCuO_3−δ CRB as a function of annealing temperature. O1 corresponds to oxygen in the CuO layers, and O2 corresponds to oxygen in the BaO layers. For the O2 site (BaO layer), the refined occupancy decreases from 0.81(9) (See Table S1) in the unannealed sample to 0.62(0) (See Table S6) after annealing at 450 °C under 1 bar O₂. The change of 0.19 is far larger than error values (~10⁻³), demonstrating that this reduction is statistically significant and cannot be attributed to random refinement uncertainty. In contrast, the O1 site (CuO layer) shows occupancies from 0.27(6) (See Table S1) to 0.28(0) (See Table S6) across the same temperature range, with variations near to the error values. This confirms that oxygen loss occurs predominantly from the BaO layer, while the CuO layer remains stable. This indicates that oxygen in the BaO layers plays a decisive role in both superconductivity and thermal stability of Cu1234. Consequently, two approaches can be considered to control oxygen occupancy and enhance thermal stability: applying high oxygen pressure during annealing or doping Cu or Ba sites with higher-valence ions to intrinsically increase oxygen content. These strategies may also be combined, allowing effective superconducting connectivity to be achieved at lower oxygen pressures through higher annealing temperatures.

4. Conclusions

In summary, we have systematically examined the thermal stability of the Cu1234 superconductor through annealing under varying oxygen pressures. The onset decomposition temperature is found to be approximately 350 °C at 1 bar O₂ but increases to 600 °C under 150 bar O₂ while largely preserving superconducting properties. A clear linear correlation is observed between oxygen pressure and decomposition temperature, with a slope of about 1.73 (±0.25) °C/bar. Furthermore, refinement analysis suggests that the structural decomposition of Cu1234 is closely related to the reduction of oxygen occupancy within the BaO layers of the BaCuO_3−δ CRB. High oxygen pressure effectively suppresses this oxygen loss, thereby enhancing thermal stability. We note that this site-specific assignment currently relies on diffraction data alone and independent validation from complementary techniques such as TEM is desirable in future work. These findings offer valuable guidance for improving superconducting connectivity through annealing, particularly in the context of fabricating Cu1234 tapes using the PIT method.