The solid-solution Zintl compounds with the mixed cations of Ca 2+ and Yb 2+ in the Ca 5−x Yb x Al 2 Sb 6 (1.0 ≤ x ≤ 5.0) system have been synthesized by high-temperature solid-state reactions. Two slightly different crystal structures of the Ba 5 Al 2 Bi 6 -type and Ca 5 Ga 2 Sb 6 -type phases have been characterized for seven compounds with 2.5 ≤ x ≤ 5.0 and three compounds with 1.0 ≤ x ≤ 2.0, respectively, by both powder and single-crystal X-ray diffraction analyses. The two title phases adopt the orthorhombic space group Pbam (Z = 2, oP26) with seven independent asymmetric atomic sites and share certain structural similarities, including infinite one-dimensional [Al 2 Sb 8 ] double chains and isolated space-filling Ca 2+ /Yb 2+ cations. Interestingly, we reveal the crystal-to-crystal solid-state structural transformation of the Yb-rich compound Ca 1.5 Yb 3.5 Al 2 Sb 6 from the Ba 5 Al 2 Bi 6 -type to the Ca 5 Ga 2 Sb 6 -type phase through the postannealing process, which can be rationalized as the phase transition from the kinetically more stable structure to the thermodynamically more stable crystal structure on the basis of theoretical calculations. Discrepancies of the local coordination geometries of the anionic [Al 2 Sb 8 ] units and the geometrical arrangements of structural building moieties in the two distinct phases provoke the different electrical properties of metallic and semiconducting conduction, respectively, for the Ba 5 Al 2 Bi 6 -type and Ca 5 Ga 2 Sb 6 -type phases. Density of states and crystal orbital Hamilton population analyses based on tight-binding linear muffin-tin orbital calculations prove that the band-gap opening in the Ca 5 Ga 2 Sb 6 -type phase should mainly be attributed to an extended bond distance of the bridging Sb−Sb in the [Al 2 Sb 8 ] unit. A series of thermoelectric (TE) property measurements indicates that the phase transition via the postannealing process eventually results in an enhancement of the TE performance of Yb-rich Ca 1.5 Yb 3.5 Al 2 Sb 6 .

The Zintl phase solid-solution CaYbSbGe (0 ≤ x ≤ 9; 0 ≤ y ≤ 3; 0 ≤ z ≤ 3) system with the cationic/anionic multisubstitution has been synthesized by molten Sn metal flux and arc-melting methods. The crystal structure of the nine title compounds were characterized by both powder and single-crystal X-ray diffractions and adopted the HoGe-type structure with the tetragonal space group I4/mmm (Z = 4, Pearson Code tI84). The overall isotypic structure of the nine title compounds can be illustrated as an assembly of three different types of cationic polyhedra sharing faces with their neighboring polyhedra and the three-dimensional cage-shaped anionic frameworks consisting of the dumbbell-shaped Sb units and the square-shaped Sb or (Sb/Ge) units. During the multisubstitution trials, interestingly, we observed a metal-to-semiconductor transition as the Ca and Ge contents increased in the title system from YbSb to CaYbSbGe (nominal compositions) on the basis of a series of thermoelectric property measurements. This phenomenon can be elucidated by the suppression of a bipolar conduction of holes and electrons via an extra hole-carrier doping. The tight-binding linear muffin-tin orbital calculations using four hypothetical structural models nicely proved that the size of a pseudogap and the magnitude of the density of states at the Fermi level are significantly influenced by substituting elements as well as their atomic sites in a unit cell. The observed particular cationic/anionic site preferences, the historically known abnormalities of atomic displacement parameters, and the occupation deficiencies of particular atomic sites are further rationalized by the QVAL value criterion on the basis of the theoretical calculations. The results of SEM, EDS, and TGA analyses are also provided.

Single crystals of CsBi2Ti2TaO10 (I) are prepared by solid state reaction of Cs2CO3, Bi2O3, TiO2, and Ta2O5 (Pt crucible, 1000 °C, 24 h, 30% yield).

Two Zintl phase thermoelectric compounds of EuKBiSn (x = 0, 0.26(1); y = 0.86(2), 1.93(2)) have been synthesized by a high-temperature solid-state reaction and arc-melting methods. The two isotypic crystal structures are characterized by both single-crystal and powder X-ray diffractions, and adopt a tetragonal HoGe-type structure (space group I4/mmm, Z = 2, Pearson code tI84) containing nine crystallographically independent asymmetric atomic sites in a unit cell. The chemical compositions are confirmed by EDS analysis. The complex crystal structure of the two title compounds can be described as an assembly of three different types of co-facial polyhedra formed by cations and 3-dimensional anionic frameworks surrounding these polyhedra. A quaternary title compound, EuKBiSn, which simultaneously contains both cationic and anionic p-dopants in a single compound, was successfully crystallized for the first time in the AM (A = alkaline-earth metals, rare-earth metals; M = triels, tetrels, pnictogens) series. In particular, two different types of p-dopants K and Sn show particular site-preferences, respectively, where K and Sn prefer to occupy the cationic Wyckoff 4e site and the anionic Wyckoff 8h site. These noticeable site preferences can be elucidated by either a size-factor criterion for the K-doping case or by an electronic-factor criterion for the Sn-doping case. The tight-binding linear muffin-tin orbital calculations show that as the double p-doping is applied to the EuKBiSn system, some extra holes are generated on the electronic structures according to the density of states curves. However, a series of thermoelectric property measurements prove that this extra hole-carrier doping is hardly effective enough to completely suppress a bipolar conduction of holes and electrons due to the rigid metallic band structure of the title system.

No abstract

The effects of S doping at the oxygen site on the thermoelectric properties of BiOCuSe have been investigated.

Single crystals of CsBi2Ti2TaO10 (I) are prepared by solid state reaction of Cs2CO3, Bi2O3, TiO2, and Ta2O5 (Pt crucible, 1000 °C, 24 h, 30% yield).

No abstract