Crystal Structure, Electronic Structure, and Optical Properties of the Novel Li4cdge2s7, a Wide-Bandgap Quaternary Sulfide with a Polar Structure Derived from Lonsdaleite

The novel quaternary thiogermanate Li4CdGe2S7 (tetralithium cadmium digermanium heptasulfide) was discovered from a solid-state reaction at 750 °C. Single-crystal X-ray diffraction data were collected and used to solve and refine the structure. Li4CdGe2S7 is a member of the small, but growing, class of I4-II-IV2-VI7 diamond-like materials. The compound adopts the Cu5Si2S7 structure type, which is a derivative of lonsdaleite. Crystallizing in the polar space group Cc, Li4CdGe2S7 contains 14 crystallographically unique ions, all residing on general positions. Like all diamond-like structures, the compound is built of corner-sharing tetrahedral units that create a relatively dense three-dimensional assembly. The title compound is the major phase of the reaction product, as evidenced by powder X-ray diffraction and optical diffuse reflectance spectroscopy. While the compound exhibits a second-harmonic generation (SHG) response comparable to that of the AgGaS2 (AGS) reference material in the IR region, its laser-induced damage threshold (LIDT) is over an order of magnitude greater than AGS for λ = 1.064 μm and τ = 30 ps. Bond valence sums, global instability index, minimum bounding ellipsoid (MBE) analysis, and electronic structure calculations using density functional theory (DFT) were used to further evaluate the crystal structure and electronic structure of the compound and provide a comparison with the analogous I2-II-IV-VI4 diamond-like compound Li2CdGeS4. Li4CdGe2S7 appears to be a better IR nonlinear optical (NLO) candidate than Li2CdGeS4 and one of the most promising contenders to date. The exceptional LIDT is likely due, at least in part, to the wider optical bandgap of ∼3.6 eV

Ternary Rare-Earth Arsenides REZn₃As₃ (RE = La–Nd, Sm) and RECd₃As₃ (RE = La–Pr)

Ternary rare-earth zinc arsenides REZn₃As₃ (RE = La–Nd, Sm) with polymorphic modifications different from the previously known defect CaAl₂Si₂-type forms, and the corresponding rare-earth cadmium arsenides RECd₃As₃ (RE = La–Pr), have been prepared by reaction of the elements at 800 °C. LaZn₃As₃ adopts a new orthorhombic structure type (Pearson symbol <i>oP</i>28, space group <i>Pnma</i>, <i>Z</i> = 4, <i>a</i> = 12.5935(8) Å, <i>b</i> = 4.1054(3) Å, <i>c</i> = 11.5968(7) Å) in which ZnAs₄ tetrahedra share edges to form ribbons that are fragments of other layered arsenide structures; these ribbons are then interconnected in a three-dimensional framework with large channels aligned parallel to the <i>b</i> direction that are occupied by La³⁺ cations. All remaining compounds adopt the hexagonal ScAl₃C₃-type structure (Pearson symbol <i>hP</i>14, space group <i>P</i>6₃/<i>mmc</i>, <i>Z</i> = 2; <i>a</i> = 4.1772(7)–4.1501(2) Å, <i>c</i> = 20.477(3)–20.357(1) Å for REZn₃As₃ (RE = Ce, Pr, Nd, Sm); <i>a</i> = 4.4190(3)–4.3923(2) Å, <i>c</i> = 21.4407(13)–21.3004(8) Å for RECd₃As₃ (RE = La–Pr)) in which [M₃As₃]^3– layers (M = Zn, Cd), formed by a triple stacking of nets of close-packed As atoms with M atoms occupying tetrahedral and trigonal planar sites, are separated by La³⁺ cations. Electrical resistivity measurements and band structure calculations revealed that orthorhombic LaZn₃As₃ is a narrow band gap semiconductor

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 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 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

Syntheses and crystal structures of the quaternary thiogermanates Cu4FeGe2S7 and Cu4CoGe2S7

© 2020. The quaternary thiogermanates Cu4FeGe2S7 (tetracopper iron digermanium heptasulfide) and Cu4CoGe2S7 (tetracopper cobalt digermanium heptasulfide) were prepared in evacuated fused-silica ampoules via high-temperature, solid-state synthesis using stoichiometric amounts of the elements at 1273K. These isostructural compounds crystallize in the Cu4NiSi2S7 structure type, which can be considered as a superstructure of cubic diamond or sphalerite. The monovalent (Cu+), divalent (Fe2+ or Co2+) and tetravalent (Ge4+) cations adopt tetrahedral geometries, each being surrounded by four S2- anions. The divalent cation and one of the sulfide ions lie on crystallographic twofold axes. These tetrahedra share corners to create a three-dimensional framework structure. All of the tetrahedra align along the same crystallographic direction, rendering the structure non-centrosymmetric and polar (space group C2). Analysis of X-ray powder diffraction data revealed that the structures are the major phase of the reaction products. Thermal analysis indicated relatively high melting temperatures, near 1273K

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