Synthesis, structure and optical properties of layered M4Nb6O17⋅nH2O (M = K, Rb, Cs) hexaniobates

Abstract

Potassium hexaniobate (K4Nb6O17) with 3D-layered and 2D-nanostructured morphological features has been primarily of academic interest due to its catalytic and photophysical properties. Although its microscopic properties are well described in the literature, there remains an absence of support information on the spectroscopic nature arising from the insertion of larger counterions in the crystalline lattice. In the present work, a new protocol for the preparation of layered M4Nb6O17⋅3H2O (M = K, Rb, Cs) hexaniobates is reported, starting from a citric acid aqueous solution comprising the alkali cations and a niobium citrate complex. X-ray powder diffraction, Raman and photoluminescence spectroscopies were employed to examine the implications of alkali metal incorporation. Crystal structures determination was accomplished by Rietveld profile refinement method. The observed occurrence of a pseudo Jahn-Teller effect (PJTE) implies a reduction of NbO6 octahedral symmetry due to the shortening of terminal Nb−OM bond lengths along with O–Nb–O bond angles tilted from 180°. The Raman spectra point to a local structural disorder by increasing the metal radius with characteristic frequencies assigned to variations of Nb–O bond order. Crystal lattice compositions Rb4Nb6O17⋅3H2O and Cs4Nb6O17⋅3H2O materially induce changes in the luminescent properties and strongly distorted octahedral groups yield a brighter PL blue emission at low temperature (77 K).