Electron-Deficient Ternary and Quaternary Pnictides Rb₄Zn₇As₇, Rb₄Mn_3.5Zn_3.5Sb₇, Rb₇Mn₁₂Sb₁₂, and Rb₇Mn₄Cd₈Sb₁₂ with Corrugated Anionic Layers

The ternary pnictides Rb₄Zn₇As₇ and Rb₇Mn₁₂Sb₁₂ and their quaternary derivatives Rb₄Mn_3.5Zn_3.5Sb₇ 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 <i>Cmcm</i>, <i>Z</i> = 4; <i>a</i> = 4.1883(4) Å, <i>b</i> = 24.844(2) Å, <i>c</i> = 17.6056(17) Å for Rb₄Zn₇As₇; <i>a</i> = 4.3911(8) Å, <i>b</i> = 26.546(5) Å, <i>c</i> = 18.743(4) Å for Rb₄Mn_3.5Zn_3.5Sb₇) and monoclinic Rb₇Mn₁₂Sb₁₂-type (space group <i>C</i>2/<i>m</i>, <i>Z</i> = 2; <i>a</i> = 26.544(12) Å, <i>b</i> = 4.448(2) Å, <i>c</i> = 16.676(8) Å, β = 103.183(8)° for Rb₇Mn₁₂Sb₁₂; <i>a</i> = 27.009(4) Å, <i>b</i> = 4.5752(7) Å, <i>c</i> = 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

Ternary Arsenides A₂Zn₅As₄ (A = K, Rb): Zintl Phases Built from <i>Stellae Quadrangulae</i>

Stoichiometric reaction of the elements at high temperature yields the ternary arsenides K₂Zn₅As₄ (650 °C) and Rb₂Zn₅As₄ (600 °C). They adopt a new structure type (Pearson symbol <i>oC</i>44, space group <i>Cmcm</i>, <i>Z</i> = 4; <i>a</i> = 11.5758(5) Å, <i>b</i> = 7.0476(3) Å, <i>c</i> = 11.6352(5) Å for K₂Zn₅As₄; <i>a</i> = 11.6649(5) Å, <i>b</i> = 7.0953(3) Å, <i>c</i> = 11.7585(5) Å for Rb₂Zn₅As₄) with a complex three-dimensional framework of linked ZnAs₄ 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 <i>stellae quadrangulae</i> each consisting of four ZnAs₄ tetrahedra capping an empty central tetrahedron. These compounds are Zintl phases; band structure calculations on K₂Zn₅As₄ and Rb₂Zn₅As₄ indicate semiconducting behavior with a direct band gap of 0.4 eV

Ternary Arsenides A₂Zn₅As₄ (A = K, Rb): Zintl Phases Built from <i>Stellae Quadrangulae</i>

Stoichiometric reaction of the elements at high temperature yields the ternary arsenides K₂Zn₅As₄ (650 °C) and Rb₂Zn₅As₄ (600 °C). They adopt a new structure type (Pearson symbol <i>oC</i>44, space group <i>Cmcm</i>, <i>Z</i> = 4; <i>a</i> = 11.5758(5) Å, <i>b</i> = 7.0476(3) Å, <i>c</i> = 11.6352(5) Å for K₂Zn₅As₄; <i>a</i> = 11.6649(5) Å, <i>b</i> = 7.0953(3) Å, <i>c</i> = 11.7585(5) Å for Rb₂Zn₅As₄) with a complex three-dimensional framework of linked ZnAs₄ 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 <i>stellae quadrangulae</i> each consisting of four ZnAs₄ tetrahedra capping an empty central tetrahedron. These compounds are Zintl phases; band structure calculations on K₂Zn₅As₄ and Rb₂Zn₅As₄ indicate semiconducting behavior with a direct band gap of 0.4 eV

Quaternary Arsenides <i>A</i>CdGeAs₂ (<i>A</i> = 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 <i>P</i>1̅, Pearson symbol <i>aP</i>20, <i>Z</i> = 4; <i>a</i> = 8.0040(18) Å, <i>b</i> = 8.4023(19) Å, <i>c</i> = 8.703(2) Å, α = 71.019(3)°, β = 75.257(3)°, γ = 73.746(3)° for KCdGeAs₂; <i>a</i> = 8.2692(13) Å, <i>b</i> = 8.4519(13) Å, <i>c</i> = 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 <i>A</i>⁺ 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

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

The ternary arsenides A₂Zn₂As₃ and the quaternary derivatives A₂Ag₂ZnAs₃ (A = Sr, Eu) have been prepared by stoichiometric reaction of the elements at 800 °C. Compounds A₂Zn₂As₃ crystallize with the monoclinic Ba₂Cd₂Sb₃-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 Sr₂Zn₂As₃; <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 Eu₂Zn₂As₃) in which CaAl₂Si₂-type fragments, built up of edge-sharing Zn-centered tetrahedra, are interconnected by homoatomic As–As bonds to form anionic slabs [Zn₂As₃]^4– separated by A²⁺ cations. Compounds A₂Ag₂ZnAs₃ crystallize with the monoclinic Yb₂Zn₃Ge₃-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 Sr₂Ag₂ZnAs₃; <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 Eu₂Ag₂ZnAs₃), which can be regarded as a stuffed derivative of the Ba₂Cd₂Sb₃-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 Sr₂Zn₂As₃ is a narrow band gap semiconductor and Sr₂Ag₂ZnAs₃ is a semimetal. Electrical resistivity measurements revealed band gaps of 0.04 eV for Sr₂Zn₂As₃ and 0.02 eV for Eu₂Zn₂As₃, the latter undergoing an apparent metal-to-semiconductor transition at 25 K

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

The ternary arsenides A₂Zn₂As₃ and the quaternary derivatives A₂Ag₂ZnAs₃ (A = Sr, Eu) have been prepared by stoichiometric reaction of the elements at 800 °C. Compounds A₂Zn₂As₃ crystallize with the monoclinic Ba₂Cd₂Sb₃-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 Sr₂Zn₂As₃; <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 Eu₂Zn₂As₃) in which CaAl₂Si₂-type fragments, built up of edge-sharing Zn-centered tetrahedra, are interconnected by homoatomic As–As bonds to form anionic slabs [Zn₂As₃]^4– separated by A²⁺ cations. Compounds A₂Ag₂ZnAs₃ crystallize with the monoclinic Yb₂Zn₃Ge₃-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 Sr₂Ag₂ZnAs₃; <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 Eu₂Ag₂ZnAs₃), which can be regarded as a stuffed derivative of the Ba₂Cd₂Sb₃-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 Sr₂Zn₂As₃ is a narrow band gap semiconductor and Sr₂Ag₂ZnAs₃ is a semimetal. Electrical resistivity measurements revealed band gaps of 0.04 eV for Sr₂Zn₂As₃ and 0.02 eV for Eu₂Zn₂As₃, the latter undergoing an apparent metal-to-semiconductor transition at 25 K

Quaternary Arsenides <i>AM</i>_1.5<i>Tt</i>_0.5As₂ (<i>A</i> = Na, K, Rb; <i>M</i> = Zn, Cd; <i>Tt</i> = Si, Ge, Sn): Size Effects in CaAl₂Si₂- and ThCr₂Si₂‑Type Structures

Ten quaternary arsenides <i>AM</i>_1.5<i>Tt</i>_0.5As₂ (<i>A</i> = Na, K, Rb; <i>M</i> = Zn, Cd; <i>Tt</i> = Si, Ge, Sn) have been prepared by stoichiometric reactions of the elements at 600–650 °C. Seven of them (NaZn_1.5Si_0.5As₂, NaZn_1.5Ge_0.5As₂, NaZn_1.5Sn_0.5As₂, NaCd_1.5Sn_0.5As₂, KZn_1.5Sn_0.5As₂, KCd_1.5Sn_0.5As₂, RbCd_1.5Sn_0.5As₂) adopt the trigonal CaAl₂Si₂-type structure (Pearson symbol <i>hP</i>5, space group <i>P</i>3̅<i>m</i>1, <i>Z</i> = 1, <i>a</i> = 4.0662(3)–4.4263(7) Å, <i>c</i> = 7.4120(5)–8.4586(14) Å), whereas the remaining three (KZn_1.5Si_0.5As₂, KZn_1.5Ge_0.5As₂, RbZn_1.5Ge_0.5As₂) adopt the tetragonal ThCr₂Si₂-type structure (Pearson symbol <i>tI</i>10, space group <i>I</i>4/<i>mmm</i>, <i>Z</i> = 2, <i>a</i> = 4.0613(10)–4.1157(5) Å, <i>c</i> = 14.258(3)–14.662(2) Å). Both structure types contain anionic [<i>M</i>_1.5<i>Tt</i>_0.5As₂] slabs that are built from edge-sharing tetrahedra and that stack alternately with nets of <i>A</i> cations. A structure map delineates the formation of these structure types for <i>AM</i>_1.5<i>Tt</i>_0.5As₂ as a function of simple radius ratios. Although these arsenides have charge-balanced formulations, band structure calculations on NaZn_1.5<i>Tt</i>_0.5As₂ (<i>Tt</i> = Si, Ge, Sn) indicate that semimetallic behavior is predicted as a result of overlap of the valence and conduction bands