Haueyne; phase transition and high-temperature structures obtained from synchrotron radiation and Rietveld refinements

The structural behaviour of a hauyne with a chemical composition of Na (sub 4.35) Ca (sub 2.28) K (sub 0.95) [Al (sub 6) Si (sub 6) O (sub 24) ]-(SO (sub 4) ) (sub 2.03) , at room pressure and from 33 to 1035 degrees C on heating, was determined by using in situ synchrotron X-ray powder diffraction data (lambda = 0.92249(5) Aa). The satellite reflections in hauyne are lost at approximately 400 degrees C and a true substructure results because of this phase transition. There is a discontinuity in the a unit-cell parameter at approximately 585 degrees C. The a parameter increases rapidly and non-linearly to 585 degrees C, but above 585 degrees C, the expansion rate decreases. The percent volume change between 33 and 576 degrees C is 2.0(3)%, and 0.6(3)% between 593 and 1035 degrees C. Between 33 and 1035 degrees C, the Al-O, Si-O and S-O distances are constant. Between 33 and 576 degrees C, the angle of rotation of the AlO (sub 4) tetrahedron, phi (sub Al) , changes from 11.5 to 5.8 degrees ?, while the angle of rotation of the SiO (sub 4) tetrahedron, phi (sub Si) , changes from 12.4 to 6.3 degrees . The Al-O-Si bridging angle changes from 150.05(2) to 153.08(1) degrees from 33 to 576 degrees C. Beyond 585 degrees C, phi (sub Al) and phi (sub Si) angles remain nearly constant even though the maximum rotation of the tetrahedra is not achieved. Moreover, the Al-O-Si angle continues to increase at a slower rate from 585 to 1035 degrees C by 1.05(2) degrees . From 33 to approximately 585 degrees C, the K atom position migrates at a slower rate than the Na and Ca atoms, and the structure expands at a high rate. Beyond 585 degrees C, all the atomic positions of the interstitial cations (Na (super +) , K (super +) , Ca (super 2+) ) remain nearly constant and the expansion of the structure is retarded.