Ternary Arsenides A₂Zn₂As₃ (A = Sr, Eu) and Their Stuffed Derivatives A₂Ag₂ZnAs₃

Abstract

The ternary arsenides A(2)Zn(2)As(3) and the quaternary derivatives A(2)Ag(2)ZnAs(3) (A = Sr, Eu) have been prepared by stoichiometric reaction of the elements at 800 °C. Compounds A(2)Zn(2)As(3) crystallize with the monoclinic Ba(2)Cd(2)Sb(3)-type structure (Pearson symbol mC28, space group C2/m, Z = 4; a = 16.212(5) Å, b = 4.275(1) Å, c = 11.955(3) Å, β = 126.271(3)° for Sr(2)Zn(2)As(3); a = 16.032(4) Å, b = 4.255(1) Å, c = 11.871(3) Å, β = 126.525(3)° for Eu(2)Zn(2)As(3)) in which CaAl(2)Si(2)-type fragments, built up of edge-sharing Zn-centered tetrahedra, are interconnected by homoatomic As-As bonds to form anionic slabs [Zn(2)As(3)](4-) separated by A(2+) cations. Compounds A(2)Ag(2)ZnAs(3) crystallize with the monoclinic Yb(2)Zn(3)Ge(3)-type structure (Pearson symbol mC32, space group C2/m; a = 16.759(2) Å, b = 4.4689(5) Å, c = 12.202(1) Å, β = 127.058(1)° for Sr(2)Ag(2)ZnAs(3); a = 16.427(1) Å, b = 4.4721(3) Å, c = 11.9613(7) Å, β = 126.205(1)° for Eu(2)Ag(2)ZnAs(3)), which can be regarded as a stuffed derivative of the Ba(2)Cd(2)Sb(3)-type structure with additional transition-metal atoms in tetrahedral coordination inserted to link the anionic slabs together. The Ag and Zn atoms undergo disorder but with preferential occupancy over four sites centered in either tetrahedral or trigonal planar geometry. The site distribution of these metal atoms depends on a complex interplay of size and electronic factors. All compounds are Zintl phases. Band structure calculations predict that Sr(2)Zn(2)As(3) is a narrow band gap semiconductor and Sr(2)Ag(2)ZnAs(3) is a semimetal. Electrical resistivity measurements revealed band gaps of 0.04 eV for Sr(2)Zn(2)As(3) and 0.02 eV for Eu(2)Zn(2)As(3), the latter undergoing an apparent metal-to-semiconductor transition at 25 K.

Crystal structure, electrical resistivity, and X-ray photoelectron spectroscopy of BaAg2As2

“…BaAg 2 As 2 belongs to the category of ThCr 2 Si 2 -type structures in which a contracted c/a ratio (2.361) is associated with the formation of homoatomic As-As bonds (2.517(1)Å) between the [Ag 2 As 2 ] layers. In fact, this As-As distance is the shortest among the few BaM 2 As 2 compounds possessing such pairs (cf., BaCu 2 As 2 , 2.583(2)Å [7]; BaPd 2 As 2 , 2.649(4)Å [13]), and comparable to single bond lengths ( $ 2.5Å) found in many polyarsenide Zintl phases (e.g., Sr 2 Zn 2 As 3 ) [26].…”

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

“…They are isostructural with the previously reported Ba 2 Cd 2 As 3 and Ba 2 Cd 2 Sb 3 [21], which are closely related to the CaAl 2 Si 2 -type [35]. Very recently, this series has been extended to include several other arsenides such as A 2 Zn 2 As 3 (A ¼Sr, Eu) [22] and the first phosphide Eu 2 Zn 2 P 3 also presented with this paper.…”

“…As part of these studies, we synthesized and structurally characterized the new compounds Eu 2 ZnP 2 , Eu 2 Zn 2 P 3 and Eu 2 Cd 2 As 3 , which are at the focus of this paper. 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.…”

Ternary CaCu4P2-type pnictides AAg4Pn2 (A=Sr, Eu; Pn=As, Sb)

“…Although these compounds are electronically charge-balanced (''A 2 þ (Ag þ ) 4 (Pn 3-) 2 '') and can be considered to be Zintl phases, this does not necessarily imply formation of a band gap, as has now been demonstrated in related systems [10,13]. The band structure of SrAg 4 As 2 does show the typical separation of mostly empty Sr states above the Fermi level and mostly filled Ag and As states below, consistent with the expected direction of electron transfer, but there is only a pseudogap near the Fermi level and semimetallic behavior is predicted (Fig.…”

“…For example, representatives containing trivalent rare-earth metals as the electropositive component can be prepared through aliovalent substitution in the quaternary phosphides RECuZnP 2 [10] or through defect formation in the ternary arsenides RE 0.67 Zn 2 As 2 [11]. Insertion of ''APn'' slabs between CaAl 2 Si 2 -type fragments results in ternary arsenides A 2 Zn 2 As 3 (A¼Sr, Eu) with the Ba 2 Cd 2 Sb 3 -type structure [12], which in turn may be stuffed with interstitial atoms to form isoelectronic quaternary arsenides A 2 Ag 2 ZnAs 3 (A¼Sr, Eu) [13].…”