Browsing by Author "Rumsey, MS"

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    Ultra-flexible framework breathing in response to dehydration in liskeardite, [(Al,Fe)16(AsO4)9(OH)21(H2O)11]·26H2O, a natural open-framework compound
    (Elsevier, 2015-05-02) Grey, IE; Brand, HEA; Rumsey, MS; Gozukara, Y
    Dehydration of the natural open-framework compound, liskeardite, [(Al,Fe)16(AsO4)9(OH)21(H2O)11]·26H2O, is accompanied by a change in the sign of the thermal expansion from positive to negative above room temperature, and at ~100 °C the structure undergoes a dramatic 2D contraction by co-operative rotation of heteropolyhedral columns that constitute the framework walls. Monoclinic liskeardite, I112 with a≈b≈24.7 Å, c ≈7.8 Å and β≈90° is transformed to a tetragonal phase, I-4 with a≈20.6 Å, c ≈7.7 Å. The associated 30% decrease in volume is unprecedented in inorganic microporous compounds. The flexibility of the contraction is related to the double-hinged nature of the column rotations about [001]. Octahedra in adjacent columns are interconnected by corner-sharing with the two pairs of anions forming opposing edges of AsO4 tetrahedra, so a double-hinged rotation mechanism operates. Thermal analysis and mass spectroscopic results for liskeardite show that the phase transition at ~100 °C is related to removal of the channel water. The tetragonal phase shows exceptionally large NTE behaviour. Over the temperature range 148–178 the NTE along a and b is close to linear with a magnitude of the order of −900×10−6 °C−1. The contraction along the channel direction is smaller but still appreciable at −200×10−6 °C−1. © 2015 Elsevier Inc.