We report the synthesis, structure, and magnetic properties of a new Zintl phase and structure type, Eu11Zn4Sn2As12. The structure and composition of this phase have been established by single-crystal X-ray diffraction and electron microprobe analysis. Eu11Zn4Sn2As12 crystallizes in monoclinic space group C2/c (No. 15) with the following lattice parameters: a = 7.5679(4) Å, b = 13.0883(6) Å, c = 31.305(2) Å, and β = 94.8444(7)° [R 1 = 0.0398; wR 2 = 0.0633 (all data)]. The anisotropic structural features staggered ethane-like [Sn2As6]12– units and infinite ∞ 2[Zn2As3]5– sheets extended in the a–b plane. Eu cations fill the space between these anionic motifs. Temperature-dependent magnetic properties and magnetoresistance of this Zintl phase have been studied, and the electronic structure and chemical bonding were elucidated using first-principles quantum chemical calculations (TB-LMTO-ASA). Quantum chemical calculations show that the ethane-like units can be considered as consisting of covalent single bonds; however, the ∞ 2[Zn2As3]5– sheets are best described with delocalized bonding and there is evidence of Eu–As interactions. Temperature-dependent magnetization and transport properties between 2 and 300 K show a ferromagnetic transition at 15 K, a band gap of 0.04 eV, and negative colossal magnetoresistance.

Yb14MnSb11 is a p-type high-temperature thermoelectric material with operational temperatures as high as 1273 K. Rare-earth (RE) substitution into this phase has been shown to increase the melting point further while also decreasing the sublimation rate. Solid solutions of 3+ RE elements with Yb2+ in Yb14MnSb11 have shown to have increased stability against oxidation. Ce is an abundant RE element, and the substitution of Ce3+ on the Yb2+ sites should increase the thermoelectric efficiency of the material due to a decrease in carrier concentration. Samples of Yb14–x Ce x MnSb11 (x ∼ 0.4) were synthesized using ball milling, followed by annealing and consolidation via spark plasma sintering. The systematic addition of a small increase of excess Mn and the resulting compositions were investigated. Small amounts of impurities in the samples, such as Yb2O3 and Mn, are correlated with negative attributes in the resistivity data. Hall effect measurements revealed a reduced carrier concentration of ∼44% at 600 K over Yb14MnSb11, and adjusting the stoichiometry toward Yb13.6Ce0.4MnSb11 leads to increases in resistivity and the Seebeck coefficient with a reduction in thermal conductivity. Yb13.6Ce0.4MnSb11 shows an improved average zT avg = 0.80 when compared to Yb14MnSb11 (0.71) and no degradation when exposed to ambient air for 77 days at room temperature. Thermogravimetric analysis of air oxidation shows that Yb13.6Ce0.4MnSb11 and Yb14MnSb11 do not oxidize until 700 K.

Polymorphism and Second harmonic generation in a novel diamond-like semiconductor: Li 2 MnSnS 4 , Journal of Solid State Chemistry, http://dx. Abstract:High-temperature, solid-state synthesis in the Li2MnSnS4 system led to the discovery of two new polymorphic compounds that were analyzed using single crystal X-ray diffraction. The α-polymorph crystallizes in Pna21 with the lithium cobalt (II) silicate, Li2CoSiO4, structure type, where Z=4, R1=0.0349 and wR2=0.0514 for all data. The β-polymorph possesses the wurtz-kesterite structure type, crystallizing in Pn with Z=2, R1=0.0423, and wR2=0.0901 for all data. Rietveld refinement of synchrotron X-ray powder diffraction was utilized to quantify the phase fractions of the polymorphs in the reaction products. The α/β-Li2MnSnS4 mixture exhibits an absorption edge of ~2.6-3.0 eV, a wide region of optical transparency in the mid-to far-IR, and moderate SHG activity over the fundamental range of 1.1-2.1 μm. Calculations using density functional theory indicate that the ground state energies and electronic structures for α-and β-Li2MnSnS4, as well as the hypothetical polymorph, γ-Li2MnSnS4 with the wurtz-stannite structure type, are highly similar.

Polycrystalline samples of CoAsSb were prepared by annealing a stoichiometric mixture of the elements at 1073 K for 2 weeks. Synchrotron powder X-ray diffraction refinement indicated that CoAsSb adopts arsenopyrite-type structure with space group P2 1 /c. Sb vacancies were observed by both elemental and structural analysis, which indicate CoAsSb 0.883 composition. CoAsSb was thermally stable up to 1073 K without structure change but decomposed at 1168 K. Thermoelectric properties were measured from 300 to 1000 K on a dense pellet. Electrical resistivity measurements revealed that CoAsSb is a narrow-band-gap semiconductor. The negative Seebeck coefficient indicated that CoAsSb is an n-type semiconductor, with the maximum value of −132 μV/K at 450 K. The overall thermal conductivity is between 2.9 and 6.0 W/(m K) in the temperature range 300−1000 K, and the maximum value of figure of merit, zT, reaches 0.13 at 750 K. First-principles calculations of the electrical resistivity and Seebeck coefficient confirmed n-type semiconductivity, with a calculated maximum Seebeck coefficient of −87 μV/K between 900 and 1000 K. The difference between experimental and calculated Seebeck coefficient was attributed to the Sb vacancies in the structure. The calculated electronic thermal conductivity is close to the experimental total thermal conductivity, and the estimated theoretical zT based solely on electronic thermal conductivity agrees with experimental values in the high temperature range, above 800 K. The effects of Sb vacancies on the electronic and transport properties are discussed.

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Solid solutions of Yb 2−x A x CdSb 2 (A = Ca, Sr, Eu; x ≤ 1) are of interest for their promising thermoelectric (TE) properties. Of these solid solutions, Yb 2−x Ca x CdSb 2 has end members with different crystal structures. Yb 2 CdSb 2 crystallizes in the polar space group Cmc2 1 , whereas Ca 2 CdSb 2 crystallizes in the centrosymmetric space group Pnma. Other solid solutions, Yb 2−x A x CdSb 2 (A = Sr, Eu), crystallize in the polar space group for x ≤ 1, and compositions with x ≥ 1 have not been reported. Both structure types are composed of corner-sharing CdSb 4 tetrahedra condensed into sheets that differ by the stacking of the layers. Single crystals of the solid solution Yb 2−x Ca x CdSb 2 (x = 0−1) were studied to elucidate the structural transition between the Yb 2 CdSb 2 and Ca 2 CdSb 2 structure types. For x ≤ 1, the structures remain in the polar space group Cmc2 1 . As the Ca content is increased, a positional disorder arises in the intralayer cation sites (Yb2/Ca2) and the Cd site, resulting in inversion of the CdSb 4 tetrahedral chain. This phenomenon could be indicative of an intergrowth of the opposing space group. The TE properties of polycrystalline samples of Yb 2−x Ca x CdSb 2 (x ≤ 1) were measured from 300 to 525 K. The lattice thermal conductivity is extremely low (0.3−0.4 W/m•K) and the Seebeck coefficients are high (100−180 μV/K) across the temperature range. First-principles calculations show a minimum in the thermal conductivity for the x = 0.3 composition, in good agreement with experimental data. The low thermal conductivity stems from the acoustic branches being confined to low frequencies and a large number of phonon scattering channels provided by the localized optical branches. The TE quality factor of the Yb 1.7 A 0.3 CdSb 2 (A = Ca, Sr, Eu) series has been calculated and predicts that the A = Ca and Sr solid solutions may not improve with carrier concentration optimization but that the Eu series is worthy of additional modifications. Overall, the x = 0.3 compositions provide the highest zT because they provide the best electronic properties with the lowest thermal conductivity.

The structure, magnetic properties, and 151 Eu and 119 Sn Mossbauer spectra of the solid-solution Eu 11−x Sr x Zn 4 Sn 2 As 12 are presented. A new commensurately modulated structure is described for Eu 11 Zn 4 Sn 2 As 12 (R3m space group, average structure) that closely resembles the original structural description in the monoclinic C2/c space group with layers of Eu, puckered hexagonal Zn 2 As 3 sheets, and Zn 2 As 6 ethane-like isolated pillars. The solid-solution Eu 11−x Sr x Zn 4 Sn 2 As 12 (0 < x < 10) is found to crystallize in the commensurately modulated R3 space group, related to the parent phase but lacking the mirror symmetry. Eu 11 Zn 4 Sn 2 As 12 orders with a saturation plateau at 1 T for 7 of the 11 Eu 2+ cations ferromagnetically coupled (5 K) and shows colossal magnetoresistance at 15 K. The magnetic properties of Eu 11 Zn 4 Sn 2 As 12 are investigated at higher fields, and the ferromagnetic saturation of all 11 Eu 2+ cations occurs at ∼8 T. The temperature-dependent magnetic properties of the solid solution were investigated, and a nontrivial structure−magnetization correlation is revealed. The temperature-dependent 151 Eu and 119 Sn Mossbauer spectra confirm that the europium atoms in the structure are all Eu 2+ and that the tin is consistent with an oxidation state of less than four in the intermetallic region. The spectral areas of both Eu(II) and Sn increase at the magnetic transition, indicating a magnetoelastic effect upon magnetic ordering.

The band structure of Yb14MgSb11 is tuned by substituting the more earth-abundant cations, Ca and Ba, on the four crystallographically distinct Yb sites (Yb13–x Ca x BaMgSb11 (x = 1, 2, 3, 4, 5, 6)). Single crystals of composition Yb9.7(2)Ca3.85(5)Ba0.29(4)Mg1.13(3)Sb11.0(1) were grown from Sn flux revealing the cation site preferences. Magnetic measurements on this crystal show paramagnetic behavior consistent with the presence of ∼0.85 Yb3+. High-purity samples (>98%) with compositions close to nominal of Yb13–x Ca x BaMgSb11 (x = 1–6) were prepared by ball milling and spark plasma sintering. The carrier concentration can be rationalized with the presence of Yb3+ for all samples and decreases as a function of x in a systematic fashion at room temperature and increases above ∼600 K for x = 3–6. The temperature dependence of the carrier concentration can be understood considering the electronic structure with a light and heavy band valence band contributing to the properties and suggests the involvement of a localized flat band or impurity state that is active with increasing amounts of Ca. The effect of temperature leads to sustained high Seebeck coefficients with low electrical resistivity arising from the transitioning of the light to heavy band with localization of carriers in the flat band or impurity state for Ca-rich compositions. Speed of sound measurements show that the lattice stiffens with increasing x. Despite the stiffening lattice, the thermal conductivity decreases until x = 3, 4 at which point it increases slightly. The x = 4 sample reaches a peak figure of merit (zT) of 1.32 at 1273 K while being 16% lighter by the molar mass compared to Yb14MnSb11 thereby providing a more power dense material.