Cation−Anion Interactions as Structure Directing Factors:  Structure and Bonding of Ca₂CdSb₂ and Yb₂CdSb₂

Abstract

Two new transition-metal-containing Zintl phases, Ca2CdSb2 and Yb2CdSb2, have been synthesized by flux reactions, and their structures have been determined by single-crystal X-ray diffraction. Yb2CdSb2 crystallizes in the noncentrosymmetric orthorhombic space group Cmc21 (No. 36, Z = 4). Ca2CdSb2 crystallizes in the centrosymmetric orthorhombic space group Pnma (No. 62, Z = 4). Despite the similarity in their chemical formulas and unit cell parameters, the structures of Yb2CdSb2 and Ca2CdSb2 are subtly different: Ca2CdSb2 has a layered structure built up of infinite layers of CdSb4 tetrahedra connected through corner-sharing. These layers are stacked in an alternating AA-1AA-1 sequence along the direction of the longest crystallographic axis (A denotes a layer; A-1 stands for its inversion symmetry equivalent), with Ca2+ cations filling the space between them. The structure of Yb2CdSb2 features the very same [CdSb2]4- layers of CdSb4 tetrahedra, which because of the lack of inversion symmetry are stacked in an AAAA-type fashion and are separated by Yb2+ cations. Electronic band structure calculations performed using the TB-LMTO-ASA method show a small band gap at the Fermi level for Ca2CdSb2, whereas the gap closes for Yb2CdSb2. These results suggest narrow gap semiconducting and poorly metallic behavior, respectively, and are confirmed by resistivity and magnetic susceptibility measurements. The structural relationship between these new layered structure types and some well-known structures with three-dimensional four-connected nets are discussed as well.

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.…”

New ternary phosphides and arsenides. Syntheses, crystal structures, physical properties of Eu2ZnP2, Eu2Zn2P3 and Eu2Cd2As3

“…36, Pearson symbol oC20). This structure belongs to the Yb 2 CdSb 2 structure type, which has been discussed in an earlier publication [3]. The asymmetric unit of the structure contains two Eu, one Zn, and two pnictogen atoms, located at the special position 4a (Table 2).…”

“…Two of the newly synthesized compounds-Eu 2 Zn 2 P 3 and Eu 2 Cd 2 As 3 -belong to the monoclinic Ba 2 Cd 2 Sb 3 or Ba 2 Cd 2 As 3 structure type [21], first identified by us and then enriched by a series of members such as Sr 2 Zn 2 As 3 , Eu 2 Zn 2 As 3 [22], Sr 2 Ag 2 ZnAs 3 and Eu 2 Ag 2 ZnAs 3 [22]. Eu 2 ZnP 2 on the other hand is isostructural with the orthorhombic Yb 2 CdSb 2 [3], also discovered by us not long ago. Very recently, we extended this system to include several more arsenides and phosphides [15], but Eu 2 ZnP 2 is the only zinccontaining compound (so far) with this structure.…”

“…Over the past decades, the studies of pnictogen-based compounds have proven to be a fertile field for exploration, very much fueled by their diverse structural chemistry and unusual physical properties [1][2][3]. Specifically, we can mention the recently demonstrated superconductivity in a series of phosphides or arsenides represented by BaNi 2 P 2 [4], BaIr 2 P 2 [5], LaRu 2 P 2 [6], AFe 2 As 2 (A¼ K, Rb, Cs, Sr, Ba) [7] and AFeAs (A¼Li, Na) [8]; such ThCr 2 Si 2 -type or PbFCl-type compounds have truly taken the condensed-matter world by a storm.…”

Optoelectronic and transport properties of Zintl phase KBa2Cd2Sb3 compound

“…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.…”