Interplay between Size and Electronic Effects in Determining the Homogeneity Range of the A₉Zn_4+xPn₉ and A₉Cd_4+xPn₉ Phases (0 ≤ x ≤ 0.5), A = Ca, Sr, Yb, Eu; Pn = Sb, Bi

Abstract

Seven cadmium- and zinc-containing Zintl phases, A9Zn(4+x)Pn9 and A9Cd(4+x)Pn9 (0 < or = x < or = 0.5), A = Ca, Sr, Yb, Eu; Pn = Sb, Bi, have been synthesized, and their structures have been determined by single-crystal X-ray diffraction. All compounds are isostructural and crystallize in the centrosymmetric orthorhombic space group Pbam (no. 55, Z = 2), and their structures feature tetrahedra of the pnicogens, centered by the transition metal. The tetrahedra are not isolated but are connected through corner sharing to form ribbons, which are separated by the divalent cations. The occurrence of a small phase width and its variation across this family of compounds has been systematically studied by variable temperature crystallography, resistivity, and magnetic susceptibility measurements, and these results have been reconciled with electronic structure calculations performed using the tight-binding linear muffin-tin orbital (TB-LMTO-ASA) method. These analyses of the crystal and electronic structure indicate that the polyanionic subnetwork requires 19 additional electrons, whereas only 18 electrons are provided by the cations. Such apparent "electron deficiency" necessitates the presence of an interstitial atom in order for an optimal bonding to be achieved; however, an interplay between the sizes of the cations and anions and the total valence electron concentration (governed by the stoichiometry breadth) is suggested as a possible mechanism for achieving structure stability. The structural relationship between these and some known structures with two-dimensional layers are discussed as well.

Flux growth and structure of two compounds with the EuIn2P2structure type,AIn2P2(A= Ca and Sr), and a new structure type, BaIn2P2

“…This was attributed to the difference in cation coordination: octahedral for the cation site in CaAl 2 Si 2 versus cubic in the ThCr 2 Si 2 structure type. Electronic considerations have been shown to play an important role in dictating structure and properties for other isoelectronic series (Alemany et al, 2008;Xia & Bobev, 2007, 2008Kim et al, 2008). For AIn 2 P 2 (A = Ca and Sr), the EuIn 2 P 2 structure type, the six-membered ring structure of the nonmetal layer cannot accommodate the size of the Ba cation and a new structure type is adopted.…”

Gallium Pnictides of the Alkaline Earth Metals, Synthesized by Means of the Flux Method: Crystal Structures and Properties of CaGa2Pn2, SrGa2As2, Ba2Ga5As5, and Ba4Ga5Pn8 (Pn = P or As)

“…[10] Motivated by the discoveries of high ZT among such compounds, [6][7][8][9][10] our research group has been very active in this field. Past efforts have been focused on the search for new thermoelectric materials in the ternary systems A-MPn (A = Ca, Sr, Ba, Eu, Yb; M = Zn, Cd; Pn = P, As, Sb, Bi), and we have already reported a number of new compounds, such as Ca 2 CdSb 2 and Yb 2 CdSb 2 , [11] Na 2 ACdSb 2 and K 2 ACdSb 2 , [12] Eu 11 Zn 6 Pn 12 and Eu 11 Cd 6 Sb 12 , [13] Ba 11 Cd 8 Bi 14 , [14] A 9 Zn 4+x Pn 9 and A 9 Cd 4+x Pn 9 , [15] Ba 3 Cd 2 Sb 4 , [16] and Ba 2 Cd 2 Pn 3 . [17] The synthesis of most of these materials has been facilitated by the metal-flux method, [18] which, unlike the traditional solidstate route, allows the reactants to dissolve in a "solvent" and thereby achieve higher diffusion rates at lower temperatures.…”

Optoelectronic and transport properties of Zintl phase KBa2Cd2Sb3 compound

“…The sharp peaks produced in L(x) around 10.5, 10.0 and 11.0 eV are due to the plasma oscillation [47]. The above cases show considerable anisotropy among the three tensor components in the energy range (10-12.5) eV and (15)(16)(17)(18)(19)(20)(21)(22)(23) eV while rest of range shows isotropic behavior among the three components.…”

“…The Saprov et al [10] synthesized and structurally characterized many novel ternary compounds, where Sr 9 Cd 4+x Sb 9 and Ca 9 Zn 4+x Sb 9 [22,23], Eu 11 Cd 6 Sb 12 and Eu 11 Zn 6 Sb 12 [24], Ba 21 Cd 4 Bi 18 and Ba 21 Cd 4 Sb 18 [25], Ba 3 Cd 2 Sb 4 [26], Yb 2 CdSb 2 [27], are few examples with the divalent alkaline-earth and rare-earth metals. Saprov et al [10] also synthesized five new zintl phases KA 2 Cd 2 Sb 3 (A = Ca, Sr, Ba, Eu, Yb) compounds.…”