Mecanisme d'hydratation des niobates alcalins lamellaires de formule A4Nb4O17 (A = K,Rb, Cs)

Two double oxides, K₄Nb₆O₁₇ and K₂Rb₂Nb₆O₁₇ · 5H₂O, have been synthesized as single crystals. Unit cell dimensions are a = 7.83, b = 33.21, c = 6.46 Å for the first and a = 7.84, b = 41.75, c = 6.50 Å for the second. The common space group is P2₁nb (Z = 4). The K₄Nb₆O₁₇ structure was solved by X-ray diffraction using three-dimensional Patterson and Fourier techniques with 1401 reflections collected on films. Comparison between the structure types of K₄Nb₆O₁₇ and Rb₄Nb₆O₁₇ · 3H₂O leads to elaboration of a pattern for the pentahydrate which was confirmed by the crystal structure. These results show that the hydration mechanism leads only to compounds with three or five molecules of water and they explain the differences of stability found between several compounds of the same type.