An organic–inorganic hybrid combining a semiconducting BEDT-TTF layer and a Jahn–Teller distorted oxalato-bridged honeycomb antiferromagnetic layer [Cu₂(C₂O₄)₃ ²⁻]_n was obtained and characterized.

Crystal-to-crystal transformation is a path to obtain crystals with different crystal structures and physical properties. K₂[Co(C₂O₄)₂(H₂O)₂]·4H₂O (1) is obtained from K₂C₂O₄·2H₂O, CoCl₂·6H₂O in H₂O with a yield of 60%. It is crystallized in the triclinic with space group P1 and cell parameters: a = 7.684(1) Å, b = 9.011(1) Å, c = 10.874(1) Å, α = 72.151(2)°, β = 70.278(2)°, γ = 80.430(2)°, V = 670.0(1) ų, Z = 2 at 100 K. 1 is composed of K⁺, mononuclear anion [Co(C₂O₄)₂(H₂O)₂²⁻] and H₂O. Co²⁺ is coordinated by two bidentated oxalate anion and two H₂O in an octahedron environment. There is a hydrogen bond between mononuclear anion [Co(C₂O₄)₂(H₂O)₂²⁻] and H₂O. K₂[Co(μ-C₂O₄)(C₂O₄)] (2) is obtained from 1 by dehydration. The cell parameters of 2 are a = 8.460(5) Å, b = 6.906 (4) Å, c = 14.657(8) Å, β = 93.11(1)°, V = 855.0(8) ų at 100 K, with space group in P2/c. It is composed of K⁺ and zigzag [Co(μ-C₂O₄)(C₂O₄²⁻]_n chain. Co²⁺ is coordinated by two bisbendentate oxalate and one bidentated oxalate anion in trigonal-prism. 1 is an antiferromagnetic molecular crystal. The antiferromagnetic ordering at 8.2 K is observed in 2.

Two one-dimensional oxalate-bridged Cu(II) ammonium salts, [(CH₃)₃NH]₂[Cu(μ-C₂O₄)(C₂O₄)]·2.5H₂O (I) and [(C₂H₅)₃NH]₂[Cu(μ-C₂O₄)(C₂O₄)]·H₂O (II) were obtained and characterized. They were composed of ammonium: (CH₃)₃NH⁺ in (I), (C₂H₅)₃NH⁺ in (II), [Cu(μ-C₂O₄)(C₂O₄)²⁻]_n and H₂O. The Jahn–Teller-distorted Cu(II) is octahedrally coordinated by six O atoms from three oxalates and forms a one-dimensional zigzag chain. The hydrogen bonds between ammonium, the anion and H₂O form a three-dimensional network. There is no hydrogen bond between the anion chains. They were insulated at 20 °C with a relative humidity of 40%. Ferromagnetic and weak-ferromagnetic behaviors were observed in I and II, separately. No long-range ordering was observed above 2 K.

MgB₂ exhibits distinct advantages over both high-temperature and low-temperature superconductors, endowing it with extensive application prospects. Nevertheless, the high sintering temperature of MgB₂ (exceeding 800 °C) and its weak intrinsic pinning severely restrict its practical applications. Utilizing Mg(BH₄)₂ as a precursor represents an innovative approach for fabricating MgB₂ at relatively low temperatures. This method can reduce the sintering temperature by approximately 200 °C–300 °C and refine the grain structure. However, the intermediate process of Mg(BH₄)₂ decomposition into MgB₂ remains unclear, and there is currently no direct evidence elucidating the underlying cause of the low temperature preparation. Additionally, directly employing Mg(BH₄)₂ as a precursor to fabricate MgB₂ often leads to an increased number of pores and a low density. In this study, MgB₂ bulks were fabricated through a two-step reaction method involving precursor decomposition and hot pressing. Firstly, Mg(BH₄)₂ powder was subjected to heat treatment at different temperatures. The products were comprehensively characterized, and the intermediate process and mechanism of Mg(BH₄)₂ decomposition into MgB₂ were analyzed. Subsequently, the powder heat-treated at 520 °C containing nanoscale Mg, B, and MgB₂ was deliberately selected to prepare MgB₂ bulks through hot pressing. The resultant MgB₂ bulks demonstrated a nanoscale grain structure, with a density exceeding 70% of the theoretical value. The sample hot-pressed at 650 °C for 3 h showed the superconducting transition temperature (T _c) of 37.4 K, and the critical current density (J _c) value of 4.21 × 10⁵ A cm⁻² at 10 K in self-field, which is comparable to that of traditional MgB₂ sintered at 800 °C. The nanoscale crystallite size of MgB₂ generates abundant grain boundary pinning centers, effectively mitigating J _c degradation. The MgB₂ bulk achieves a J _c of 5.07 × 10⁴ A cm⁻² at 10 K, 3 T, surpassing most pure MgB₂ bulk materials reported to date. Compared to other methods, this low-cost approach achieves efficient synthesis through milder conditions and shorter processing times, demonstrating significant potential for fabricating MgB₂ superconductors in high-field applications.