Haüyne: phase transition and high-temperature structures obtained from synchrotron radiation and Rietveld refinements

The structural behaviour of a haüyne with a chemical composition of Na_4.35Ca_2.28K_0.95[Al₆Si₆O₂₄]-(SO₄)_2.03, at room pressure and from 33 to 1035°C on heating, was determined by using in situ synchrotron X-ray powder diffraction data (λ = 0.92249(5) Å). The satellite reflections in haüyne are lost at ~400°C and a true substructure results because of this phase transition. There is a discontinuity in the a unit-cell parameter at ~585°C. The a parameter increases rapidly and non-linearly to 585°C, but above 585°C, the expansion rate decreases. The percent volume change between 33 and 576°C is 2.0(3)%, and 0.6(3)% between 593 and 1035°C. Between 33 and 1035°C, the Al–O, Si–O and S–O distances are constant. Between 33 and 576°C, the angle of rotation of the AlO₄ tetrahedron, φ_Al, changes from 11.5 to 5.8°, while the angle of rotation of the SiO₄ tetrahedron, φ_Si, changes from 12.4 to 6.3°. The Al–O–Si bridging angle changes from 150.05(2) to 153.08(1)° from 33 to 576°C. Beyond 585°C, φ_Al and φ_Si angles remain nearly constant even though the maximum rotation of the tetrahedral is not achieved. Moreover, the Al–O–Si angle continues to increase at a slower rate from 585 to 1035°C by 1.05(2)°. From 33 to ~585°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°C, all the atomic positions of the interstitial cations (Na⁺, K⁺, Ca²⁺) remain nearly constant and the expansion of the structure is retarded.