Abstract
Synthesis of apatites, Na 1− x K x Pb 4(PO 4) 3 0≤ x≤1, with anion vacancy was carried out using solid state reactions. The solid solution of apatite-type structure crystallizes in the hexagonal system, space group P6 3/m (No 176). Rietveld refinements showed that 75% of Pb 2+ cations are located in the (6h) sites; the ninefold coordination sites (4f) are equally occupied by the other 25% lead cations and the K + and Na + monovalent ions. The structure can be described as built up from [PO 4] 3− tetrahedra and Pb 2+ of sixfold coordination cavities (6h positions), which delimit void hexagonal tunnels running along [0 0 1]. These tunnels are connected by cations of mixed sites (4f) half occupied by Pb 2+ and half by Na +/K + mixed cations. The assignment of the observed frequencies in the Raman and infrared spectra is discussed on the basis of a unit cell group analysis and by comparison with other apatites. The Raman modes of all the compositions show some linear shifts of the frequencies as a function of the composition toward lower values due the substitutions of Na + by K + with a larger radius.
Published Version
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