Luminescence of Fe3+ doped NaAlSiO4 prepared by gel to crystallite conversion

The polymorphs of NaAlSiO4, namely low-carnegieite, nepheline and high-carn egieite, are prepared through gel to crystallite conversion. As-prepared co mpound has the composition, NaAlSiO4. 1 . 5H(2)O (nepheline hydrate), and a cts as a precursor for low-carnegieite. The latter is obtained by decomposi ng the nepheline hydrate at 800 degrees C, which has an orthorhombic struct ure and the X-ray diffraction pattern is indexed based on the space group P maa. Low-carnegieite is metastable and can be rehydrated to form nepheline hydrate. At 950 degrees C, low-carnegieite transforms to nepheline which, i n turn, transforms to high-carnegieite at 1300 degrees C. Low-carnegieite a nd nepheline do not show any luminescence when doped with Fe3+, whereas the high-carnegieite exhibits the red emission. Non-emissivity of Fe3+ is corr elated to the higher site symmetry in low-carnegieite and nepheline having D-2 and C-3v, symmetry, respectively, according to infrared spectral analys is. Whereas, the high-carnegieite has the C-2v site symmetry, due to the rh ombic distortion of the tetrahedral sites during the phase transformation t hat accompanies the polyhedral rotation. Ferric ion doped high-carnegieite shows strong emission band maximising around 684 nm, due to the T-4(1) ((4) G) --> (6)A(1) (S-6) transition of Fe3+ (3d(5)). A weak emission around 714 nm arises from the cooperative vibronic transition and another band around 674 nm is attributed to the Fe3+ ion associated with the hole centre to fo rm a complex centre of the type Fe3+-O--Si4+ as shown by the EPR spectra. T he presence of the defect centre in the vicinity of Fe3+ changes the crysta l field. (C) 1998 Elsevier Science S.A. All rights reserved.