Orthorhombic superstructures of the polar potassium tungsten bronze knbw₂o₉

Abstract

Selected area electron diffraction and dark-field electron microscopy revealed three superstructures coexisting within the nonstoichiometric phase K_1-y,(Nb_1-y, W_2+y)O₉. The pseudohexagonal unit cell parameters are Type I: a = a_h, c = 2a_h , Type II: a = 2a_h, c = 2c; and Type III: a = 3a, c = 2c. In fact the superstructures have orthorhombic symmetry. Possible orthorhombic space groups and unit cell parameters are derived for the three phases. The specimen stoichiometry was measured to be K_0.92(Nb_0.87, W_2.12)O₉; thus the M₃O₉ octahedral framework of the hexagonal tungsten bronze subcell is retained apparently and the three different superstructures represent distinct ordering mechanisms for the potassium vacancies and/or the (Nb, W) displacements which occur within the octahedral framework.