No abstract

The synthesis and crystal structures of new ternary manganese tellurides, AMnTe2 (A = K, Rb, Cs), Na3Mn4Te6, and NaMn1.56Te2, are reported. These compounds are synthesized by solid-state reaction and cation exchange techniques in sealed Nb tubes. The single-crystal structures of AMnTe2 (A = K, Rb), Na3Mn4Te6, and NaMn1.56Te2 have been determined; KMnTe2:  a = 4.5110(4) Å, c = 14.909(2) Å, I4̄m2 (No. 119, Z = 2); RbMnTe2:  a = 4.539(1) Å, c = 15.055(2) Å, I4̄m2 (No. 119, Z = 2); Na3Mn4Te6:  a = 8.274(4) Å, b = 14.083(6) Å, c = 7.608(6) Å, β = 91.97(4)°, C2/m (No. 12, Z = 2); NaMn1.56Te2:  a = 4.4973(8) Å, c = 7.638(2) Å, P3̄m1 (No. 164, Z = 1). The fundamental building blocks of the title compounds are MnTe4 tetrahedra. AMnTe2 (A = K, Rb, Cs) are isostructural with TlFeS2, consisting of layers built up by four corner-shared MnTe4 tetrahedra. Manganese telluride layers in Na3Mn4Te6 consist of two hexagonal nets of Te atoms between which two-thirds of the tetrahedral interstices are filled with Mn atoms to form two 63 honeycomb nets. NaMn1.56Te2 adopts a defective CaAl2Si2 structure type, in which Mn atoms partially and randomly occupy 78% of tetrahedral sites. Temperature-dependent magnetic susceptibilities measurements show that AMnTe2 (A = K, Rb, Cs) exhibit Curie−Weiss paramagnetism, whereas Na3Mn4Te6 and NaMn1.56Te2 show paramagnetism with a weak dependence on temperature.

The layered structures of many transition metal chalcogenides are of interest because they are known to easily accommodate various metal ions or molecules in their van der Waals gaps. 1 Most early investigations of the first-row ternary transition metal chalcogenides concentrated on sulfides and selenides, which display a rich structural variety. [2][3][4][5][6] Only three types of ternary manganese telluride compounds, A 2 MnTe 2 (A ) K, Rb, Cs), A 6 -MnTe 4 (A ) Na, K), and A 2 Mn 3 Te 4 (A ) Rb, Cs), have been reported. 3,7,8 The first has a one-dimensional structure with edgesharing tetrahedra, the second possesses isolated [MnTe 4 ] 6tetrahedra, and the third compound has a layered structure. We have synthesized new layered compounds, LiMnTe 2 and NaMnTe 2 , by direct solid state and cation exchange reactions. These new compounds have unusual polar layers.

The synthesis of the group IV ternary chalcogenides Zr(6)MTe(2) (M = Mn, Fe, Co, Ni, Ru, Pt) and Zr(6)Fe(1)(-)(x)()Q(2+)(x)() (Q = S, Se) is reported, as are the single-crystal structures of Zr(6)FeTe(2), Zr(6)Fe(0.6)Se(2.4), and Zr(6)Fe(0.57)S(2.43). The structure of Zr(6)FeTe(2) was refined in the hexagonal space group P&sixmacr;2m (No. 189, Z = 1) with lattice parameters a = 7.7515(5) Å and c = 3.6262(6) Å, and the structures of Zr(6)Fe(0.6)Se(2.4) and Zr(6)Fe(0.57)S(2.43) were refined in the orthorhombic space group Pnnm (No. 58, Z = 4) with lattice parameters a = 12.737(2) Å, b = 15.780(2) Å, and c = 3.5809(6) Å and a = 12.519(4) Å, b = 15.436(2) Å, and c = 3.4966(6) Å, respectively. The cell parameters of Mn-, Co-, Ni-, Ru-, and Pt-containing tellurides were also determined. The Zr(6)ZTe(2) compounds are isostructural with Zr(6)CoAl(2), while Zr(6)Fe(1)(-)(x)()Q(2+)(x)() (Q = S, Se) were found to adopt a variant of the Ta(2)P-type structure. Chains of condensed M-centered, tetrakaidecahedra of zirconium constitute the basic structural unit in all these compounds. The modes of cross-linking that give rise to the Zr(6)FeTe(2) and Zr(6)Fe(1)(-)(x)()Q(2+)(x)() structures, differences among the title compounds, and the influence of chalcogen size differences are discussed. The stoichiometric nature of Zr(6)FeTe(2) and its contrast with sulfur and selenium congeners apparently result from a Te-Fe size mismatch. The importance of stabilization of both Zr(6)FeSe(2) and Zr(6)FeTe(2) compounds by polar intermetallic Zr-Fe bonding is underscored by a bonding analysis derived from electronic band structure calculations.

The synthesis and crystal structure determinations of Zr(0.30)ZrTe(2) and M(x)Zr(2)Te(2)As (M = Zr, Na) compounds are reported. The structure of Zr(0.30)ZrTe(2) was refined in the hexagonal space group P6(3)mc (No. 186, Z = 2) with lattice parameters a = 3.9840(3) Å and c = 13.366(3) Å; Zr(0.29)Zr(2)Te(2)As was refined in the rhombohedral space group R&thremacr;m (No. 166, Z = 3) with lattice parameters a = 3.9329(4) Å and c = 29.564(5) Å. Zr(0.30)ZrTe(2) and Zr(0.29)Zr(2)Te(2)As have close structural similarities to Zr(2)Se(3) and Ta(2)S(2)C, respectively, and are built up by stacking hexagonal layers with [Zr(0.30)-Te-Zr-Te] and [Zr(0.29)-Te-Zr-As-Zr-Te] sequences. Four-probe resistivity measurements (77-300 K) show both Zr(0.30)ZrTe(2) and Zr(0.29)Zr(2)Te(2)As to be metallic (Zr(0.29)Zr(2)Te(2)As: 8.9 x 10(-)(5) Omega cm at 273 K). Both compounds exhibit structures wherein Zr atoms are included between layers (ZrTe(2) and Zr(2)Te(2)As) by partially filling trigonal antiprismatic holes. The replacement of the included Zr ions in Zr(0.29)Zr(2)Te(2)As by Na ions has been demonstrated. Powder diffraction data showed that NaZr(2)Te(2)As is isostructural with Zr(0.29)Zr(2)Te(2)As. By use of Rietveld refinements, sodium ions were found to reside in the trigonal antiprismatic sites between the layers. Extended Hückel band calculations on the [Zr(2)Te(2)As](1.16)(-) layer indicate that it should be a metallic conductor and that the [Zr(2)Te(2)As] layer can bear a greater negative charge than has so far been observed. We suggest that the [Zr(2)Te(2)As] layered compounds may offer new opportunities as electron-donating hosts.