The review considers bulk MgB2-based materials in terms of their structure, superconducting and mechanical properties. Superconducting transition temperatures of 34.5-39.4 K, critical current densities of 1.8-1.0 x E6 A/sq.cm in self field and 103 in 8 T field at 20 K, 3-1.5xE5 A/sq. cm in self field at 35 K, HC2 15 T at 22 K and Hirr 13 T at 20 K have been registered for polycrystalline materials. As TEM and SEM study show, dispersed higher borides and rather big amount (5-14 percents) of oxygen (bonded simultaneously with Mg and B) can be present in the structure even if X-ray pattern contains only reflexes of well crystallized MgB2 with traces of MgO. Materials with such a rather high oxygen content demonstrated high superconducting characteristics. At present it is established that nanosized MgB12 grains provide effective pinning in polycrystalline material. Besides, additions can introduce the MgB2 structure inducing disorder in lattice sites (for example, C substitution for B). The disorder increases the normal state resistivity, magnetic penetration depth, and the upper critical field, but reduces the transition temperature and anisotropy. It is highly probable that the additives (Ti, Ta, Zr, SiC) together with synthesis or sintering temperature can affect the distribution of oxygen and hydrogen in the material structure as well as the formation of grains of higher borides, thus influencing superconducting properties. The superconductivity of materials with matrix close to MgB12 in stoichiometry (Tc=37 K) has been defined.
Stabilizing and Tuning Superconductivity in La$_3$Ni$_2$O$_{7-δ}$ Films: Oxygen Recycling Protocol Reveals Hole-Doping Analogue
Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences（中国科学院物理研究所凝聚态物理国家实验室）；University of Chinese Academy of Sciences（中国科学院大学）；Shanghai Synchrotron Radiation Facility, Shanghai Advanced Research Institute, Chinese Academy of Sciences（中国科学院上海高等研究院上海同步辐射光源）；Beijing Academy of Quantum Information Sciences（北京量子信息科学研究院）；Songshan Lake Materials Laboratory（松山湖材料实验室）
Interstitial oxygen order and its competition with superconductivity in La$_2$PrNi$_2$O$_{7+δ}$
Tsinghua University（清华大学）；Institute of Physics, Chinese Academy of Sciences（中国科学院物理研究所）；University of Chinese Academy of Sciences（中国科学院大学）；New Cornerstone Science Laboratory（新 cornerstones 科学实验室）；Hefei National Laboratory（合肥国家实验室）
