Daniela Menegon Trichês

The high-pressure structural behavior of nanostructured defect-fluorite Sm₃NbO₇ was investigated up to 47 GPa by synchrotron X-ray diffraction combined with Rietveld refinement and equation-of-state analysis. Upon compression, the parent cubic Fm-3m phase exhibits progressive anisotropic line broadening and anomalous compressional behavior preceding the structural transformation. Around 20 GPa, the (111) Bragg reflection develops a pronounced asymmetric profile, which evolves into peak splitting between 34 and 36 GPa, indicating the onset of a pressure-induced phase transition. Above this pressure range, substantial diffraction broadening and phase coexistence are observed, suggesting a gradual transformation mechanism mediated by local structural rearrangements within the defect-rich fluorite framework. The high-pressure polymorph was successfully described using an orthorhombic Pnma structural model derived from a defect-cotunnite arrangement, preserving both the cationic disorder and oxygen deficiency characteristic of Sm₃NbO₇. A third-order Birch–Murnaghan equation of state fitted below the transition region yielded B0 = 129(4) GPa and B′ = 2.9(4) for the Fm-3m phase, whereas the high-pressure Pnma polymorph was described using a second-order Birch–Murnaghan equation with B0 = 93(2) GPa. The results demonstrate that the high-pressure response of nanostructured Sm₃NbO₇ is governed by the interplay between structural disorder, nanometric effects, and pressure-induced local rearrangements, leading to an unconventional compressional behavior prior to stabilization of the orthorhombic high-pressure polymorph.

A nanostructured NiBi₃/α-Bi₂O₃ superconductor composite has been synthesized by the process of mechanical alloying through 19 h of high energy ball milling. The kinetics of formation is analyzed in detail by x-ray diffraction and the Rietveld method. A simple method is proposed to determine the crystallinity of the resulting sample by using the background Rietveld fitting that permits a quantitative analysis of the diffuse x-ray profile taking into account the air and Compton scattering. The diffuse scattering is attributed to the formation of a nanocluster of α-Bi₂O₃ which is confirmed by high-resolution transmission electron microscopy and Raman measurements. The magnetic measurements show superconducting transition at around 4 K and the existence of NiBi₃/α-Bi₂O₃ nanophases with a low irreversibility field at around 1.01 T at 2 K. Analysis of magnetic susceptibility and isothermal magnetic hysteresis of the NiBi₃/α-Bi₂O₃ nanocomposite indicates the simultaneous existence of bulk superconductivity and ferromagnetism below 4 K.