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32.
Zintl Ions Fassler, Thomas F
2011, 20110512, 2014-07-30, Volume:
140
eBook
This book presents critical reviews of the present state and future trends in research concerned with chemical structure and bonding, concentrating on Zintl Ions and related materials.
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Zintl phase compounds with large unit cells and complex anionic structures such as Yb11Sb10 hold potential for being good thermoelectric materials. Single crystals of Ge‐doped Yb11Sb10 were ...synthesized using a molten Sn‐flux technique. Single crystal X‐ray diffraction data were obtained and resulted in a composition of Yb11Sb9.3Ge0.5 which was verified by microprobe. Yb11Sb9.3Ge0.5 is isostructural to Ho11Ge10, crystallizing in a body‐centered, tetragonal unit cell, space group I4/mmm, with Z = 4. The unit cell parameters of Yb11Sb9.3Ge0.5 are a = 11.8813(4), c = 17.1276(13) Å with a volume of 2417.8(2) Å3. These parameters correlate well with the structural refinement of previously published Yb11Sb10. The structure consists of 16 isolated Sb3− anions, 8 $\rm Sb^{4-}_{2}$ dumbbells, 2 $\rm Sb^{4-}_{4}$ square planar rings and 44 Yb2+ cations. The Ge, doped in at 28 % occupancy, was found to be site specific, residing on the 2 $\rm Sb^{4-}_{4}$ square planar rings. Single crystal X‐ray diffraction is most consistent with the site that makes up the square ring being less than fully occupied. The doped compound is additionally characterized by X‐ray powder diffraction, differential scanning calorimetry and thermogravimetry. High temperature (300–1200 K) thermoelectric properties show that the doped compound has extremely low thermal conductivity (10–30 mW/cmK), lower than that of Yb11Sb10. Temperature dependent resistivity is consistent with a heavily doped semiconductor. Yb11Sb9.3Ge0.5 shows p‐type behavior increasing from ∼22 μV/K at room temperature to ∼31 μV/K at 1140 K. The low value and the temperature dependence of the Seebeck coefficient suggest that bipolar conduction produces a compensated Seebeck coefficient and consequently a low zT.
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Large samples (6–8
g) of Yb
11Sb
10 and Ca
11Sb
10 have been synthesized using a high-temperature (1275–1375
K) flux method. These compounds are isostructural to Ho
11Ge
10, crystallizing in the ...body-centered, tetragonal unit cell, space group
I4/
mmm, with
Z=4. The structure consists of antimony dumbbells and squares, reminiscent of Zn
4Sb
3 and filled Skutterudite (e.g., LaFe
4Sb
12) structures. In addition, these structures can be considered Zintl compounds; valence precise semiconductors with ionic contributions to the bonding. Differential scanning calorimetry (DSC), thermogravimetry (TG), resistivity (
ρ), Seebeck coefficient (
α), thermal conductivity (
κ), and thermoelectric figure of merit (
zT) from room temperature to at minimum 975
K are presented for
A
11Sb
10 (
A=Yb, Ca). DSC/TG were measured to 1400
K and reveal the stability of these compounds to ∼1200
K. Both
A
11Sb
10 (
A=Yb, Ca) materials exhibit remarkably low lattice thermal conductivity (∼10
mW/cm
K for both Yb
11Sb
10 and Ca
11Sb
10) that can be attributed to the complex crystal structure. Yb
11Sb
10 is a poor metal with relatively low resistivity (1.4
mΩ
cm at 300
K), while Ca
11Sb
10 is a semiconductor suggesting that a gradual metal–insulator transition may be possible from a Ca
11−
x
Yb
x
Sb
10 solid solution. The low values and the temperature dependence of the Seebeck coefficients for both compounds suggest that bipolar conduction produces a compensated Seebeck coefficient and consequently a low
zT.
Large samples (6–8
g) of Yb
11Sb
10 and Ca
11Sb
10 have been synthesized from a Sn-flux method. Thermoelectric measurements from room temperature to 1075
K have been obtained. Both
A
11Sb
10 (
A=Yb, Ca) materials exhibit remarkably low lattice thermal conductivity (∼10
mW/cm
K for both Yb
11Sb
10 and Ca
11Sb
10) that can be attributed to the complex crystal structure.
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We found the minimum-energy structures of Ag
n
Rb
n
(n = 2-10) clusters by a combination of density functional theory (DFT) and taboo search global optimization. The global minimum geometry is mixed ...for n ≤ 4 and segregated, with a core-shell arrangement, for n > 4. There is a change in the nature of the bonding, from ionic to metallic, between n = 4 and n = 5. Although metallic bonding dominates at n > 4, large atomic charges (in the order of ±0.5) persist. These atomic charges (negative on the interior Ag atoms, positive on the surface Rb atoms) make Ag
n
Rb
n
clusters analogous to Zintl compounds and could prevent them from coalescing. This makes them intriguing potential building blocks for cluster-assembled materials. Ag
4
Rb
4
is relatively stable compared with other Ag
n
Rb
n
clusters; it has a nearly cubic shape, a large HOMO-LUMO gap (2 eV), and a highly ionic character with atomic charges equal to roughly ±1 au.
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In contrast to the reactions of E(NMe
2)
3 (E=As, Sb, Bi) with RNHM (M=Li–Cs) which generate stable Group 15 imido anions (such as the dianions E
2(NR)
4
2− and trianions E(NR)
3
3−), the phosphide ...analogues formed in the analogous reactions using RPHM decompose readily to give Zintl compounds. This process is accompanied by the formation of RP
n
rings, indicating that the alloy-forming reaction is driven thermodynamically by the bond energy of P–P bonds. The apparent intermediates in this reaction are heterocyclic anions of the type (RP)
n
E
– (
n=3 or 4). The latter are interesting new ligands for a range of metals, with the potential ability to behave as sources of metal atoms (E). The anions exhibit unusual reactivity with transition metal metallocenes, adding to the cyclopentadienyl rings in preference to the attack of metal–halogen bonds. In addition, the reactions of the cyclic anions with electrophiles (R′X) furnish a simple selective route to neutral oligophosphine ligands of the type (RP)
n
R′
2.
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This text explores 3D modeling and analysis of inorganic structures, including relationships between intermetallic clathrates, porous tectosilicates and clathrates hydrates, crystal structures of ...inorganic oxoacid salts perceived as cation arrays and more.
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