Exceptional Layered Ordering of Cobalt and Iron in Perovskites

Exceptional Layered Ordering of Cobalt and Iron in Perovskite

Copper Hyper-Stoichiometry: The Key for the Optimization of Thermoelectric Properties in Stannoidite Cu_8+<i>x</i>Fe_3–<i>x</i>Sn₂S₁₂

A univalent copper hyper-stoichiometric stannoidite Cu_8+<i>x</i>Fe_3–<i>x</i>Sn₂S₁₂ with 0 ≤ <i>x</i> ≤ 0.5 has been synthesized using mechanical alloying followed by spark plasma sintering. The X-ray diffraction analysis combined with ⁵⁷Fe and ¹¹⁹Sn Mössbauer investigations has allowed the charge distribution of the cationic species on the various sites to be established and suggests the possibility of a small tin deficiency. The transport properties show a remarkable crossover from a semiconducting to a metal-like behavior as the copper content increases from <i>x</i> = 0 to <i>x</i> = 0.5, whereas correlatively the Seebeck coefficient decreases moderately, with <i>S</i> values ranging from 310 to 100 μV/K. The thermal conductivity decreases as the temperature increases showing low values at high temperature, far below those reported in related stannite materials. The investigation of the thermoelectric properties shows that the <i>ZT</i> figure of merit is dramatically enhanced by the copper hyper-stoichiometry by a factor of 5 going from 0.07 for <i>x</i> = 0 to 0.35 for <i>x</i> = 0.5 at 630 K. This thermoelectric behavior is interpreted on the basis of a model involving the Cu–S framework as the conducting electronic network where the Fe²⁺/Fe³⁺ species play the role of hole reservoir

Lone-Pair-Driven Structure Dimensionality: the Way to Ultralow Thermal Conductivity in Pb_<i>m</i>Bi₂S_3+<i>m</i> Sulfides

Understanding the mechanisms that connect heat transport with crystal structures is fundamental to develop materials with optimized electrical and thermal properties for thermoelectric applications. In this work, we synthesized a series of bulk Cl-doped PbBi2S4 by mechanical alloying combined with spark plasma sintering. A detailed structural analysis of PbBi2S4 (m = 1 member of the series PbmBi2S3+m) and of the compounds Bi2S3 (m = 0) and Pb3Bi2S6 (m = 3) shows that the low dimensionality of their frameworks is induced by the stereochemical activity of Bi3+ and Pb2+ 6s2 lone pairs (L) and is mainly governed by the presence of BiS3L chains of tetrahedrons. By combining experiments with the ab initio band structure and phonon calculations, we discuss the structure-thermoelectric property relationships and clarify the interesting crystal chemistry in this system. We demonstrate that the ultralow thermal conductivity of these sulfides originates from the prominent 1D character induced by the bismuth chains in these frameworks, leading to weak interchain interactions compared to their strong intrachain bonds

Designing a Thermoelectric Copper-Rich Sulfide from a Natural Mineral: Synthetic Germanite Cu₂₂Fe₈Ge₄S₃₂

This study shows that the design of copper-rich sulfides by mimicking natural minerals allows a new germanite-type sulfide Cu₂₂Fe₈Ge₄S₃₂ with promising thermoelectric properties to be synthesized. The Mössbauer spectroscopy and X-ray diffraction analyses provide evidence that the structure of our synthetic compound differs from that of the natural germanite mineral Cu₂₆Fe₄Ge₄S₃₂ by its much higher Cu⁺/Cu²⁺ ratio and different cationic occupancies. The coupled substitution Cu/Fe in the Cu_{26–<i>x</i>}Fe_4+<i>x</i>­Ge₄S₃₂ series also appears as a promising approach to optimize the thermoelectric properties. The electrical resistivity, which decreases slightly as the temperature increases, shows that these materials exhibit a semiconducting behavior, but are at the border of a metallic state. The magnitudes of the electrical resistivity and Seebeck coefficient increase with <i>x</i>, which suggests that Fe for Cu substitution decreases the hole concentration. The thermal conductivity decreases as the temperature increases leading to a moderately low value of 1.2 W m^–1 K^–1 and a maximum ZT value of 0.17 at 575 K