Synthesis, X-ray crystal structure and dielectric measurements of a tetragonal tungsten bronze: Pb0.75K1.80Li1.70Nb5O15

Typical crystals Pb0.75K1.80Li1.70Nb5O15, belonging to tetragonal tungsten bronze materials, were grown by the slow cooling technique. The crystals were characterized at room temperature by X-ray diffraction (XRD) and high resolution transmission electron microscopy (HREM). The structure, solved combining information obtained by HRTEM images and Direct Methods application to single crystal XRD data, was refined by full-matrix least-squares technique in the P4bm space group to a final R = 0.031. The lattice constants are a = 12.540(2), c = 4.037(1) Angstrom, V = 634.8(2) Angstrom(3), at 293 K. There are two formulas per unit cell, for a calculated density of 4.928 Mg/m(3). Dielectric measurements, carried out along the [001] direction in the frequency 10(3) Hz, showed the existence of the paraelectric-ferroelectric transition at 628 K and of a further maximum of epsilon(r)(') (with a very weak intensity) at T-1 = 150 K.