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

Abstract The polymorphs of NaAlSiO4, namely low-carnegieite, nepheline and high-carnegieite, are prepared through gel to crystallite conversion. As-prepared compound has the composition, NaAlSiO4·1·5H2O (nepheline hydrate), and acts as a precursor for low-carnegieite. The latter is obtained by decomposing the nepheline hydrate at 800°C, which has an orthorhombic structure and the X-ray diffraction pattern is indexed based on the space group Pmaa. Low-carnegieite is metastable and can be rehydrated to form nepheline hydrate. At 950°C, low-carnegieite transforms to nepheline which, in turn, transforms to high-carnegieite at 1300°C. Low-carnegieite and nepheline do not show any luminescence when doped with Fe3+, whereas the high-carnegieite exhibits the red emission. Non-emissivity of Fe3+ is correlated to the higher site symmetry in low-carnegieite and nepheline having D2 and C3v, symmetry, respectively, according to infrared spectral analysis. Whereas, the high-carnegieite has the C2v site symmetry, due to the rhombic distortion of the tetrahedral sites during the phase transformation that accompanies the polyhedral rotation. Ferric ion doped high-carnegieite shows strong emission band maximising around 684 nm, due to the 4 T 1 ( 4 G )→ 6 A 1 ( 6 S ) transition of Fe3+ (3d5). 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 form a complex centre of the type Fe3+–O−–Si4+ as shown by the EPR spectra. The presence of the defect centre in the vicinity of Fe3+ changes the crystal field.