The oxides with composition ${\mathrm{Y}}_{1\mathrm{-}\mathit{x}}$ ${\mathrm{Pr}}_{\mathit{x}}$ ${\mathrm{Ba}}_2$(${\mathrm{Cu}}_3$ ${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$)(0≤x≤1) 7-δ (0≤x≤1) are characterized by x-ray diffraction, resistivity (ρ) and thermoelectric power (S) measurements. X-ray diffraction shows an orthorhombically distorted perovskite structure for all compositions, the pseudotetragonality increasing with x. The substitution of the tetravalent Pr for the trivalent Y atom increases the band filling in these materials. As a consequence, the superconducting transition temperature is progressively decreased with increasing x, and S significantly increases. Thermopower and resistivity measurements indicate that these oxides have relatively narrow bands and that the electronic correlations dominate their properties.

Recent approaches to improve performance of bulk thermoelectric (TE) materials show that they should have complex structures, include inclusions and impurities, possess mass fluctuations, disorder and be based on heavy elements. Glasses can possess these properties. In order to identify glasses with interesting TE potential, attention should be focused on small gap semiconducting or semimetallic glasses. Chalcogenide glasses with Cu_x+yGe_20−xTe_80−y (0 ≤ x ≤ 20; 0 ≤ y ≤ 10) compositions were prepared by melt spinning. Their powder X-ray diffraction analyses point to a short-range order analogous to Ge₂₀Te₈₀ , with Cu atoms most likely replacing Ge atoms in the GeTe₄ structural unit. It also indicates, together with the differential scanning calorimetry results, a reduction in the glass stability with the increase in Cu concentration. The enhancement of Cu content dramatically reduces (five orders of magnitude) the electrical resistivity, while keeping the Seebeck coefficients at large values (∼400 μV K⁻¹ ). As a consequence, a huge increase in the power factor is observed, up to a maximum value of 60 μW K⁻² m⁻¹ for the Cu_27.5Ge_2.5Te₇₀ glass at T = 300 K. Ge₂₀Te₈₀ has extremely low lattice thermal conductivity values (∼0.1 W K⁻¹ m⁻¹ at 300 K), which points to relatively high values for the figure of merit ZT for this family of glasses, and indicates Cu_x+yGe_20−xTe_80−y based glasses as good candidates for obtaining high performance thermoelectric materials.

Racemic and enantiopure ethylenedithio-tetrathiafulvalene-thiomethyl-oxazoline (EDT-TTF-(SMe)-OX) derivatives have been synthesized. Single-crystal X-ray diffraction analyses reveal the establishment of O⋯S nonbonded interactions, unprecedented in the TTF series, characterized by short S⋯O distances and linear O⋯SMe motifs. Theoretical calculations at the DFT/B3LYP level on a model molecule demonstrate that both possible planar s-trans (O⋯S interaction) and s-cis (N⋯S interaction) conformations are energy minima, separated by an extremely weak energy gap. The energy barrier corresponding to the equilibrium between the two forms has been also estimated from DFT calculations, as well as their relative stability in the radical cation state of the TTF. According to the latter, there is a slight tendency towards the enhancement of the N⋯S interactions. Following the predictions of the theoretical calculations, the coexistence of both O⋯S and N⋯S nonbonded interactions is observed in two mixed valence radical cation salts of the racemic TTF with the dianionic cluster Mo₆Cl₁₄, prepared upon electrocrystallization. Interestingly, the ratio TTF : dianion is finely tuned by the choice of the electrocrystallization solvent. Physical measurements such as electrical conductivity and thermoelectric power on single crystals combined with magnetic susceptibility data in one case and extended Hückel tight-binding calculations demonstrate and rationalize the semiconducting behavior of both mixed valence salts. This study demonstrates that intramolecular N⋯S and O⋯S interactions can efficiently modulate and direct in the TTF series the occurrence of original solid-state structures provided with physical properties.

We report a thorough experimental study on the microstructure, thermal behavior and thermoelectric properties of the amorphous composition Cu₁₅As₃₀Te₅₅ and the glass–ceramics related-compounds synthesized by using the Spark Plasma Sintering (SPS) technique. Varying the conditions of the SPS process enables the synthesis of composite glassy-crystalline samples with different crystal/glass ratios. Such treatments result in complex microstructures composed of large glassy domains where nanocrystals of the metastable β-As₂Te₃ phase are embedded. These domains are separated by regions of the dendritic crystalline phase surrounded by a Cu-rich glassy matrix. The presence of β-As₂Te₃, confirmed by both powder X-ray diffraction and scanning electron microscopy, suggests that pressure and/or internal stresses play an important role in stabilizing this phase. This conclusion is further supported by neutron thermodiffraction experiments revealing a sharp crossover from the β-As₂Te₃ to the stable α-As₂Te₃ phase at temperatures below that of the SPS treatment. Transport properties measurements show that the presence of a crystalline fraction significantly lowers the electrical resistivity by four orders of magnitude. However, the probable intrinsic n-type behavior of β-As₂Te₃ has a detrimental influence on the thermopower values. Even though the partial crystallization of the glassy matrix leads to an increase in the thermal conductivity, the measured values remain on the order of 1 W m⁻¹ K⁻¹ at 300 K. Besides an overall increase in the dimensionless figure of merit ZT, our results demonstrate that the partial crystallization of an amorphous matrix is an efficient tool to tune the electrical resistivity over several orders of magnitude while maintaining low thermal conductivity values.