Effects of pressure and distortion on superconductivity in Tl₂Ba₂CaCu₂O(8+δ)

Investigating the influence of Cu and Ti substitution on the structural, optical, and dielectric properties of BiFeO3

The environmentally friendly BiFe1-x(Ti1/2Cu1/2)xO3 system with various substitution rates, including x = 0 (BFO), x = 0.02 (BFTCO2) and x = 0.04 (BFTCO4), has been synthesized using the solid-state reaction technique. All compositions exhibited a distorted rhombohedral structure with R3c space, as observed from the results of XRD and Raman spectroscopy. A significant impurity phase (Bi25FeO40) appears in pure and doped BFO, with a percentage ranging between 6 and 9%. This impurity was also detected using Mössbauer spectroscopy. UV-vis spectroscopy revealed a decrease in optical band energy with the substitution, suggesting the potential applications of doped BFO within the visible range of the spectrum, making it suitable for photocatalytic and solar cell applications. The smallest bandgap was observed for BFTCO2 with Eg = 1.93 eV. The origin of this reduction is discussed from a scientific point of view. Furthermore, Cu2+ and Ti4+ co-doped BFO display an improvement in dielectric properties due to the reduction in the value of tan δ. Dielectric measurements revealed an anomaly below TN with diffusive and dispersive behavior, suggesting a relaxor-like behavior for all compositions. The relaxor character was quantified by using the Vogel-Fulcher relationship which yielded activation energy of 0.359-0.614 eV. In our system, the relaxor behavior showed an enhancement with the heterogeneity created by the substitution rate, reaching its maximum for BFTCO4, characterized by the empirical parameters which are: ΔTrelax = 96 K and γ = 1.96. Finally, co-doped BFO ceramics not only present promising materials for optical applications due to the narrow bandgap, but their relaxor behavior can also be tailored for promising applications in high-energy storage devices.

Defect Trapping and Phase Separation in Chemically Doped Bulk AgF2

We report a computational survey of chemical doping of silver(II) fluoride, which has recently attracted attention as an analogue of La₂CuO₄—a known precursor of high-temperature superconductors. By introducing fluorine defects (vacancies or interstitial adatoms) into the crystal structure, we obtain nonstoichiometric, electron- and hole-doped polymorphs of AgF_2±x. We find that the ground-state solutions show a strong tendency for localization of defects and of the associated electronic states, and the resulting doped phases exhibit insulating or semiconducting properties. Furthermore, the distribution of Ag(I)/Ag(III) sites which appear in the crystal structure points to the propensity of the AgF₂ system for phase separation upon chemical doping, which is in line with observations from previous experimental attempts. Overall, our results indicate that chemical modification may not be a feasible way to achieve doping in bulk silver(II) fluoride, which is considered essential for the emergence of high-T_c superconductivity.

A theoretical study of the 1/32, 1/16, and 1/8 electron- and hole-doped AgF₂ reveals that vacancies and adatoms have a tendency for localization, that is, formation of mixed-, rather than intermediate-valence, fluoride systems, which results in a persistent band gap at the Fermi level.