Journal of Conference Abstracts

Geometrical Origin and Theory of Negative Thermal Expansion in Framework Structures

Volker Heine¹ (vh200@phy.cam.ac.uk), Patrick R. L. Welche¹ (prlw1@cam.ac.uk) & Martin T. Dove²

¹ Theory of Condensed Matter, Cavendish Laboratory, Madingley Road, Cambridge CB3 0HE.

² Dept. of Earth Sciences, University of Cambridge, Downing Street, Cambridge CB2 3EQ.

By framework structures are meant materials consisting of relatively stiff units such as octahedra or tetrahedra joined by shared oxygen (or other) atoms at the corners. Examples are 127 (octahedra) and many aluminosilicates (tetrahedra). Rigid rotation of the units often gives a reduction of the volume or of some lattice constant as a purely geometrical effect. The theory of this effect is developed and shown to give a negative contribution to the thermal expansion coefficient. The effect varies through the phonon band structure, being strongest for low frequencies, though the sign of the effect may be reversed above a soft mode phase transition.

Leucite Structure Type Variability: NMR and XRD Studies

C. M. B. Henderson¹ (chenderson@fs2.ge.man.ac.uk) & S. C. Kohn² (simon.kohn@bris.ac.uk)

¹ Dept. of Earth Sciences, Manchester University, Manchester M13 9PL.

² Dept. of Geology, Bristol University, Bristol BS8 1RJ.

Leucite (KAlSi₂O₆) has a three-dimensional framework of silicate and aluminate tetrahedra; other minerals with the same framework topology are pollucite (CsAlSi₂O₆), analcime (NaAlSi₂O₆.H₂O) and wairakite (CaAl₂Si₄O₁₂.2H₂O). A wide range of atomic substitutions are possible involving both tetrahedral and cavity sites. Thus samples can be synthesized with Rb and Tl⁺ in cavity sites and with univalent (Li), divalent (e.g., Mg, Cd, Co, Cu), trivalent (e.g., B, Fe, Ga), quadrivalent (Ge), and pentavalent (P) tetrahedral cations. Leucite-type structures are also known with alkali feldspar stoichiometry. Structural studies for K-, Rb- and Cs-ion-exchanged analcime and synthetic samples of stoichiometry X₂YSi₅O₁₂ (X = K, Rb, Cs; Y = Mg, Zn, Cd, Ni, Cu) show interesting features of tetrahedral site cation ordering. Recent MAS NMR ²⁹Si and ²⁷ MAS NMR, and high-resolution synchrotron XRD and time-of-flight neutron diffraction studies, have allowed the structural relations and stabilities of some of these phases to be established. For example, K₂MgSi₅O₁₂, Rb₂ZnSi₅O₁₂, and K₂ZnSi₅O₁₂ are monoclinic P2₁/c with 12 tetrahedral (T) sites while Cs₂CdSi₅O₁₂ and Cs₂NiSi₅O₁₂ are orthorhombic Pbca with 6 T sites. The sample of natural analcime used for the ion-exchange experiments has about 80% of total Si as the Q⁴(2Al) species. The K-, Rb-, and Cs-analogues prepared from the analcime inherit this feature, but show changes in the relative intensities of the peaks in the ²⁹Si and ²⁷Al NMR spectra. The ion exchange experiments were carried out at temperatures in the cubic stability fields for the leucite analogues, and the peak intensity variation reflects the local collapse of the framework about individual T sites as the samples pass through the cubic-tetragonal phase transition.