Synthesis,Crystal Structure,and Vibrational Spectroscopy of the Alkali Diselenates A2Se2O7 (A = Li – Cs)
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Authors: | Michael Daub Harald Hillebrecht |
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Institution: | 1. Institut für Anorganische und Analytische Chemie, Albert‐Ludwigs‐Universit?t, Albertstra?e 21, 79104 Freiburg, Germany;2. Freiburger Materialforschungszentrum FMF, Albert‐Ludwigs‐Universit?t, Stefan‐Meier‐Stra?e 25, 79104 Freiburg, Germany |
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Abstract: | The reaction of alkali carbonates and selenium acid yielded the “pyroanions” Se2O7]2– containing alkali diselenates. By varying the alkali carbonates we were able to synthesize and determinate the crystal structures of the whole row from Li to Cs. Li2Se2O7 crystallizes isotypic to Li2S2O7 Pnma, Z = 4, a = 13.815(3), b = 8.452(2) c = 5.0585(10) Å]. The structure of Na2Se2O7 P$\bar{1}$ , Z = 2, a = 6.9896(14), b = 6.9938(14), c = 7.0829(14) Å, α = 83.32(3), β = 64.56(3), γ = 83.18(3)°] is isotypic to Ag2S2O7. A2Se2O7 (A = K, Rb) A = K: C2/c, Z = 4, a = 12.851(3), b = 7.5677(15), c = 7.5677(15) Å, β = 93.35(3)°; A = Rb: C2/c, Z = 4, a = 13.118(3), b = 7.7963(16), c = 7.7811(16) Å, β = 94.03(3)°] are isotypic to K2S2O7. The crystal structure of Cs2Se2O7 P$\bar{1}$ , Z = 10, a = 7.7271(3), b = 16.2408(8), c = 18.4427(8) Å, α = 89.685(2), β = 89.193(2), γ = 76.251(2)°] seems to be isotypic to the averaged room‐temperature modification of Cs2S2O7. With exception of the caesium compound all diselenate anions show an ecliptic arrangement and can be therefore classified as dichromate‐like structures. In Cs2Se2O7 most of the Se2O7]2– units have a staggered alignment. The transition between both orientations can be explained by the increase of the cations size. Additionally the vibrational spectra of A2Se2O7 with A = Li – Cs are discussed as well as the resulting bond forces. |
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Keywords: | Crystal structure Vibrational spectroscopy Diselenates Bond valence sum Force constants |
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