Ternary Arsenides A<sub>2</sub>Zn<sub>2</sub>As<sub>3</sub> (A = Sr, Eu) and Their Stuffed
Derivatives A<sub>2</sub>Ag<sub>2</sub>ZnAs<sub>3</sub>
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Abstract
The ternary arsenides A<sub>2</sub>Zn<sub>2</sub>As<sub>3</sub> and the quaternary derivatives A<sub>2</sub>Ag<sub>2</sub>ZnAs<sub>3</sub> (A = Sr, Eu) have been prepared by stoichiometric
reaction
of the elements at 800 °C. Compounds A<sub>2</sub>Zn<sub>2</sub>As<sub>3</sub> crystallize with the monoclinic Ba<sub>2</sub>Cd<sub>2</sub>Sb<sub>3</sub>-type structure (Pearson symbol <i>mC</i>28, space group <i>C</i>2/<i>m</i>, <i>Z</i> = 4; <i>a</i> = 16.212(5) Å, <i>b</i> =
4.275(1) Å, <i>c</i> = 11.955(3) Å, β =
126.271(3)° for Sr<sub>2</sub>Zn<sub>2</sub>As<sub>3</sub>; <i>a</i> = 16.032(4) Å, <i>b</i> = 4.255(1) Å, <i>c</i> = 11.871(3) Å, β = 126.525(3)° for Eu<sub>2</sub>Zn<sub>2</sub>As<sub>3</sub>) in which CaAl<sub>2</sub>Si<sub>2</sub>-type fragments, built up of edge-sharing Zn-centered tetrahedra,
are interconnected by homoatomic As–As bonds to form anionic
slabs [Zn<sub>2</sub>As<sub>3</sub>]<sup>4–</sup> separated
by A<sup>2+</sup> cations. Compounds A<sub>2</sub>Ag<sub>2</sub>ZnAs<sub>3</sub> crystallize with the monoclinic Yb<sub>2</sub>Zn<sub>3</sub>Ge<sub>3</sub>-type structure (Pearson symbol <i>mC</i>32, space group <i>C</i>2/<i>m</i>; <i>a</i> = 16.759(2) Å, <i>b</i> = 4.4689(5) Å, <i>c</i> = 12.202(1) Å, β = 127.058(1)° for Sr<sub>2</sub>Ag<sub>2</sub>ZnAs<sub>3</sub>; <i>a</i> = 16.427(1)
Å, <i>b</i> = 4.4721(3) Å, <i>c</i> =
11.9613(7) Å, β = 126.205(1)° for Eu<sub>2</sub>Ag<sub>2</sub>ZnAs<sub>3</sub>), which can be regarded as a stuffed derivative
of the Ba<sub>2</sub>Cd<sub>2</sub>Sb<sub>3</sub>-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<sub>2</sub>Zn<sub>2</sub>As<sub>3</sub> is a narrow
band gap semiconductor and Sr<sub>2</sub>Ag<sub>2</sub>ZnAs<sub>3</sub> is a semimetal. Electrical resistivity measurements revealed band
gaps of 0.04 eV for Sr<sub>2</sub>Zn<sub>2</sub>As<sub>3</sub> and
0.02 eV for Eu<sub>2</sub>Zn<sub>2</sub>As<sub>3</sub>, the latter
undergoing an apparent metal-to-semiconductor transition at 25 K