Syntheses and Crystal Structures of a New Niobium Polysulfide K6Nb4S22 and two New Dimorphic Tantalum Polysulfides K6Ta4S22 |
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Authors: | Petra Stoll,Christian Nä ther,Wolfgang Bensch |
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Abstract: | The new compounds K6Nb4S22 and K6Ta4S22 ( I ) have been synthesised by the reaction of NbS2 or Ta metal in a K2S3 flux. Using TaS2 as educt a second modification of K6Ta4S22 ( II ) is obtained. K6Nb4S22 and K6Ta4S22 (form I ) crystallise in the monoclinic space group C2/c with a = 35.634 (2)Å, b = 7.8448 (4)Å, c = 12.1505 (5)Å, β = 100.853 (5)°, V = 3335.8 (3)Å3, and Z = 4 for K6Nb4S22 and a = 35.563 (7) Å, b = 7.836 (2)Å, c = 12.139 (2)Å, β = 100.56 (3)°, V = 3325.5 (2)Å3, and Z = 4 for K6Ta4S22 ( I ). The second modification K6Ta4S22 (form II ) crystallises in the monoclinic space group P21/c with a = 7.5835 (6)Å, b = 8.7115 (5)Å, c = 24.421 (2)Å, β = 98.733 (9)°, V = 1594.6 (2)Å3, and Z = 2. The structures consist of [M4S22]6— anions composed of two M2S11 sub‐units which are linked into M4S22 units via terminal sulfur ligands. The anions are well separated by the K+ cations. Differences between the structures of the title compounds and those with the heavier alkali cations Rb+ and Cs+ are caused by the different arrangement of the [M4S22]6— anions around the cations and the different S2—/S22— binding modes. The thermal behaviour of both modifications was investigated using differential scanning calorimetry (DSC). From these investigations there is no hint for a polymorphic transition between the two forms. After heating crystals of form II above the melting point and cooling the melt to room temperature a crystalline powder of form I can be isolated. |
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Keywords: | Niobium Tantalum Polysulfides Crystal structure Polymorphism |
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