Structural investigations of the two polymorphs of synthetic Fe-cordierite and Raman spectroscopy of hexagonal Fe-cordierite

Abstract

The crystal structures of synthetic hexagonal and orthorhombic Fe-cordierite polymorphs with the space groups P6/mcc and Cccm were refined from single-crystal X-ray diffraction data to R1, hex = 3.14 % and R1, ortho = 4.48 %. The substitution of the larger Fe2+ for Mg leads to multiple structural changes and an increase of the unit cell volumes, with a, c (hex) = 9.8801(16) Å, 9.2852(5) Å and a, b, c (ortho) = 17.2306(2) Å, 9.8239(1) Å, 9.2892(1) Å in the end-members. Furthermore Fe incorporation results in an increase of the volumes of the octahedra, although the diameters of the octahedra in direction of the c-axis decrease in both polymorphs. X-ray powder diffraction analysis indicates a high degree of Al/Si ordering in the orthorhombic polymorph, the Miyashiro distortion index is ~0.24. Estimations of site occupancies based on the determined tetrahedral volumes result in the following values for hexagonal Fe-cordierite: ~73 % Al for T1 and ~28 % Al for T2. For the first time Raman spectroscopy was performed on the hexagonal Fe-cordierite polymorph. In the hexagonal Fe-cordierite polymorph most Raman peaks are shifted towards lower wavenumbers when compared with the Mg-end-member.Electronic supplementary materialThe online version of this article (doi:10.1007/s00710-013-0313-3) contains supplementary material, which is available to authorized users.

Towards new zero-thermal-expansion materials: Li-free quartz solid solutions stuffed with transition metal cations

“…Glass-ceramic powders containing Qss crystals stuffed with transition metal cations were synthesized for the first time within this work, relying on surface crystallization of glass powders and preferring short heat treatments to avoid the formation of more stable competing phases at these compositions, such as indialite/cordierite, spinel, non-stuffed quartz and cristobalite (see for instance Fig. S7 and previous literature [66][67][68][69][70][71][72]). Qss is commonly obtained as the first metastable devitrification product in a variety of aluminosilicate glasses containing Li, Mg and/or Zn [8,9], possibly due to close structural analogies with these amorphous precursors, in which Si 4+ and Al 3+ build up a network of interconnected tetrahedra with M + and M 2+ cations acting as charge compensators.…”