Crystal structure and resistivity characteristics of new tantalum bronze K6Ta6.5O15+xF6+y

The structure of K6Ta6.5O15+xF6+y (I) with partly reduced tantalum has been determined by the methods of X-lay diffraction analysis (SYNTEX P1 diffrac tometer, lambda MoKalpha radiation, 808 independent reflections): sp. gr. P 6/m, a = 13.118(4), c = 3.862(1) Angstrom, R-F = 0.0292. The resistivity o f metal-like crystals I is almost independent of temperature in the tempera ture range T = 4-300 K. The physical propel ties studied allow one to relat e the crystals to the group of tantalum bronzes. The structure of compound I is compared with that of the well-known crystals of the transparent diele ctric K6Ta6.5O14.5F9.5 (Ia) containing only pentavalent tantalum not belong ing to the group of bronzes. The I and Ia structures have the same basic fr ameworks but different distributions of additional Ta atoms located along t he channel axes and different degrees of delocalization as well as the numb ers and degrees of delocalization of the surrounding anions. The relation b etween these structural features acid the resistivity characteristics of th e two compounds is also discussed. (C) 2001 MAIK "Nauka/Interperiodica".