Making and Breaking Bonds in Superconducting SrAl_4-xSi_x (0 <= x <= 2)

We explored the role of valence electron concentration in bond formation and superconductivity of mixed silicon-aluminum networks by using high-pressure synthesis to obtain the BaAl4-type structural pattern in solid solution samples SrAl4-xSix where 0 <= x <= 2. Local ordering of aluminum and silicon in SrAl4-xSix was evidenced by nuclear magnetic resonance experiments. Subsequent bonding analysis by quantum chemical techniques in real space demonstrated that the strong deviation of the lattice parameters in SrAl4-xSix from Vegard's law can be attributed to the strengthening of interatomic Al-Al and Al-Si bonds within the layers (perpendicular to [001]) for 0 <= x <= 1.5, followed by the breaking of the interlayer bonds (parallel to [001]) for 1.5 < x <= 2 and leading to the structural transition from the BaAl4 structure type with three-dimensional anionic framework at lower x values to the two-dimensional anion of the BaZn2P2 structure type with increasing x values. Low-temperature measurements of the resistivity and heat capacity reveal that SrAl2.5Si1.5 and SrAl2Si2 prepared at high pressures exhibit superconductivity with critical temperatures of 2.1 and 2.6 K, respectively

Enhanced thermoelectric properties of the Zintl phase BaGa₂Sb₂ via doping with Na or K

Na- or K-doped samples of Ba1-x(Na, K)(x)Ga2Sb2 were prepared by ball-milling followed by hot-pressing. The topological analysis of the electron density of BaGa2Sb2 implies a polar covalent nature of the Sb-Ga bonds in which the Sb atoms receive the electrons transferred from Ba rather than the Ga atoms. Successful doping of BaGa2Sb2 with Na or K was confirmed with combined microprobe and X-ray diffraction analysis. Alkali metal doping of BaGa2Sb2 increased the p-type charge carrier concentration to almost the predicted optimum values (similar to 10(20) h(+) cm(-3)) needed to achieve high thermoelectric performance. With increasing alkali metal concentration, electronic transport was shifted from non-degenerate semiconducting behaviour observed for BaGa2Sb2 to degenerate one for Na- or K-doped compounds. Overall, the thermoelectric figure of merit, zT, values reached up to similar to 0.65 at 750 K, considerably higher than the undoped sample (zT similar to 0.1 at 600 K), and a slight improvement relative to previously reported Zn-doped samples (similar to 0.6 at 800 K)

High temperature transport properties of BaZn_2Sn_2

BaZn_2Sn_2 (space group P4/nmm, a = 4.7459(5) Å, c = 11.330(2) Å, Z = 2) crystallizes in the CaBe_2Ge_2 structure type with a polyanionic framework comprising alternately stacked PbO-like {ZnSn_(4/4)} and anti-PbO-like {SnZn_(4/4)} layers along the c-axis. BaZn_2Sn_2 samples were obtained by either direct solid state reaction of the elements or from a Sn-flux method in very high yield with very small amount of β-Sn as the secondary phase. The samples were characterized by powder X-ray diffraction (PXRD) and scanning electron microscopy (SEM). The chemical compositions were determined to be off-stoichiometric with Zn/Sn ratio lower than 1.0 and Sn_2 atoms in the crystal structure were found to be either loosely bonded or not bonded which might lead to an incomplete charge balance. Electrical and thermal transport measurements have been performed in the temperature range 300–773 K. BaZn_2Sn_2 displays the electrical resistivity of a metal (or semimetal) along with very low Seebeck coefficients and relatively high thermal conductivity

Thermoelectric properties of the Zintl phases Yb₅M₂Sb₆ (M = Al, Ga, In)

Zintl compounds with chemical formula Yb5M2Sb6 (M = Al, Ga, and In) form one of two known A(5)M(2)Pn(6) structure types characterized by double chains of corner-linked MPn(4) tetrahedra bridged by Pn(2) dumb-bells. High temperature electronic and thermal transport measurements were used to characterize the thermoelectric properties of Yb5M2Sb6 compounds. All samples were found to exhibit similar high p-type carrier concentrations, low resistivity and low Seebeck coefficients in agreement with the band structure calculations. These results, combined with previous studies, suggest that Yb5M2Sb6 compounds are semi-metals (i.e., they lack an energy gap between the valence and conduction bands), in contrast to the semi-conducting alkaline earth (Ca, Sr, Ba) and Eu based A(5)M(2)Sb(6) compounds. Yb5M2Sb6 compounds have very low lattice thermal conductivity, comparable to other closely related A(5)M(2)Sb(6) and A(3)MSb(3) phases. However, due to the semimetallic behaviour, the figure of merit of investigated samples remains low (zT < 0.15)

Denken wie ein Chemiker: Thermoelektrika intuitiv

Abstract Effiziente thermoelektrische Materialien werden durch ein Zusammenspiel von stark voneinander abhängenden Transporteigenschaften erreicht. Um diese wissentlich zu beeinflussen, wird ein tieferes Verständnis der Chemie und Physik in Festkörpern benötigt. Auf der Grundlage von Molekülorbitaldiagrammen, der “Tight‐Binding”‐Methode und dem klassischem Verständnis von Bindungsstärken betrachten wir das gezielte Design thermoelektrischer Materialien. Hierbei werden Parameter wie Elektronegativität, Bandbreite, Orbitalüberlappung sowie Bindungsenergie und ‐länge herangezogen, um Trends der elektronischen Eigenschaften wie Größe und Temperaturabhängigkeit von Bandlücken, effektive Masse der Ladungsträger sowie Bandentartung und Bandkonvergenz zu erklären. Gitterwärmeleitfähigkeiten werden in Bezug auf die Kristallstruktur und Bindungsstärke behandelt, um den Einfluss von Bindungslängen zu verdeutlichen. Wir zeigen, wie Symmetrie und Stärke von Bindungen den Transport von Elektronen und Phononen beeinflussen und wie gezielte Strategien zu Veränderungen und zur Verbesserung thermoelektrischer Effizienz führen können