Mansura Khatun

Ternary arsenides A2Zn2As3 (A = Sr, Eu) and their stuffed derivatives A2Ag2ZnAs3.

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.

Quaternary Arsenides AM1.5Tt0.5As2 (A = Na, K, Rb; M = Zn, Cd; Tt = Si, Ge, Sn): Size Effects in CaAl2Si2- and ThCr2Si2-Type Structures

Ten quaternary arsenides AM(1.5)Tt(0.5)As2 (A = Na, K, Rb; M = Zn, Cd; Tt = Si, Ge, Sn) have been prepared by stoichiometric reactions of the elements at 600-650 °C. Seven of them (NaZn(1.5)Si(0.5)As2, NaZn(1.5)Ge(0.5)As2, NaZn(1.5)Sn(0.5)As2, NaCd(1.5)Sn(0.5)As2, KZn(1.5)Sn(0.5)As2, KCd(1.5)Sn(0.5)As2, RbCd(1.5)Sn(0.5)As2) adopt the trigonal CaAl2Si2-type structure (Pearson symbol hP5, space group P3m1, Z = 1, a = 4.0662(3)-4.4263(7) Å, c = 7.4120(5)-8.4586(14) Å), whereas the remaining three (KZn(1.5)Si(0.5)As2, KZn(1.5)Ge(0.5)As2, RbZn(1.5)Ge(0.5)As2) adopt the tetragonal ThCr2Si2-type structure (Pearson symbol tI10, space group I4/mmm, Z = 2, a = 4.0613(10)-4.1157(5) Å, c = 14.258(3)-14.662(2) Å). Both structure types contain anionic [M(1.5)Tt(0.5)As2] slabs that are built from edge-sharing tetrahedra and that stack alternately with nets of A cations. A structure map delineates the formation of these structure types for AM(1.5)Tt(0.5)As2 as a function of simple radius ratios. Although these arsenides have charge-balanced formulations, band structure calculations on NaZn(1.5)Tt(0.5)As2 (Tt = Si, Ge, Sn) indicate that semimetallic behavior is predicted as a result of overlap of the valence and conduction bands.

Ternary arsenides A2Zn5As4 (A = K, Rb): zintl phases built from stellae quadrangulae.

Stoichiometric reaction of the elements at high temperature yields the ternary arsenides K(2)Zn(5)As(4) (650 °C) and Rb(2)Zn(5)As(4) (600 °C). They adopt a new structure type (Pearson symbol oC44, space group Cmcm, Z = 4; a = 11.5758(5) Å, b = 7.0476(3) Å, c = 11.6352(5) Å for K(2)Zn(5)As(4); a = 11.6649(5) Å, b = 7.0953(3) Å, c = 11.7585(5) Å for Rb(2)Zn(5)As(4)) with a complex three-dimensional framework of linked ZnAs(4) tetrahedra generating large channels that are occupied by the alkali-metal cations. An alternative and useful way of describing the structure is through the use of stellae quadrangulae each consisting of four ZnAs(4) tetrahedra capping an empty central tetrahedron. These compounds are Zintl phases; band structure calculations on K(2)Zn(5)As(4) and Rb(2)Zn(5)As(4) indicate semiconducting behavior with a direct band gap of 0.4 eV.

Quaternary arsenides ACdGeAs2 (A = K, Rb) built of ethane-like Ge2As6 units.

Reactions of the elements at high temperature resulted in the quaternary arsenides KCdGeAs2 (650 °C) and RbCdGeAs2 (600 °C). Single-crystal X-ray diffraction analysis reveals that they adopt a new triclinic structure type (space group P1̅, Pearson symbol aP20, Z = 4; a = 8.0040(18) Å, b = 8.4023(19) Å, c = 8.703(2) Å, α = 71.019(3)°, β = 75.257(3)°, γ = 73.746(3)° for KCdGeAs2; a = 8.2692(13) Å, b = 8.4519(13) Å, c = 8.7349(13) Å, α = 71.163(2)°, β = 75.601(2)°, γ = 73.673(2)° for RbCdGeAs2). Two-dimensional anionic layers [CdGeAs2](-) are separated by A(+) cations and are built from ethane-like Ge2As6 units forming infinite chains connected via three- and four-coordinated Cd atoms. Being Zintl phases, these compounds satisfy charge balance and are expected to be semiconducting, as confirmed by band structure calculations on KCdGeAs2, which reveal a band gap of 0.8 eV. KCdGeAs2 is diamagnetic.

Electron-deficient ternary and quaternary pnictides Rb4Zn7As7, Rb4Mn3.5Zn3.5Sb7, Rb7Mn12Sb12, and Rb7Mn4Cd8Sb12 with corrugated anionic layers.

The ternary pnictides Rb4Zn7As7 and Rb7Mn12Sb12 and their quaternary derivatives Rb4Mn3.5Zn3.5Sb7 and Rb7Mn4Cd8Sb12 have been prepared by reactions of the elements at 600 °C. They crystallize in two new structure types: orthorhombic Rb4Zn7As7-type (space group Cmcm, Z = 4; a = 4.1883(4) Å, b = 24.844(2) Å, c = 17.6056(17) Å for Rb4Zn7As7; a = 4.3911(8) Å, b = 26.546(5) Å, c = 18.743(4) Å for Rb4Mn3.5Zn3.5Sb7) and monoclinic Rb7Mn12Sb12-type (space group C2/m, Z = 2; a = 26.544(12) Å, b = 4.448(2) Å, c = 16.676(8) Å, β = 103.183(8)° for Rb7Mn12Sb12; a = 27.009(4) Å, b = 4.5752(7) Å, c = 16.727(3) Å, β = 103.221(2)° for Rb7Mn4Cd8Sb12). These related structures contain corrugated anionic layers built up by connecting ribbons of edge-sharing tetrahedra in a zigzag-like manner with chains of Mn-centered square pyramids located at the hinges. Homoatomic pnicogen-pnicogen bonding occurs in the form of Pn2 pairs. The compounds are formally deficient by one electron per formula unit, as confirmed by band structure calculations which reveal the location of the Fermi level just below a small gap in Rb4Zn7As7 or a pseudogap in Rb7Mn12Sb12.

NaGe6As6: Insertion of sodium into the layered semiconductor germanium arsenide GeAs

Abstract NaGe6As6 is a ternary arsenide prepared by reaction of the elements at 650 °C. It crystallizes in a new monoclinic structure type [space group C2/m, Z = 2, a = 22.063(2), b = 3.8032(4), c = 7.2020(8) Å, β = 92.7437(15)°] that can be considered to be derived by inserting guest Na atoms between [Ge6As6] layers identical to those found in the layered binary arsenide GeAs. An unusual feature in both structures is the presence of ethane-like Ge2As6 units in staggered conformation, with Ge–Ge dumbbells oriented either parallel or perpendicular to the layers. Electronic band structure calculations have shown that the electron excess in NaGe6As6 is accommodated by raising the Fermi level across a 0.6 eV band gap in semiconducting GeAs so that it cuts the bottom of the conduction band, resulting in an n-doped semiconductor.