Structural Conditionality of the Ionic Conductivity of MTiORO4 (M = K, Rb; R = P, As) Single Crystals

The ionic conductivity sigma(parallel to c) along the crystallographic axis c and the structural imperfection of the lattices of KTiOPO4, RbTiOPO4, and RbTiOAsO4 single crystals with low defect concentration, grown by the high temperature solution growth technique, have been investigated by impedance spectroscopy and X-ray diffraction analysis. Isomorphic substitutions of Rb+ ions for conduction K+ cations in MTiOPO4 crystals decreases the sigma(parallel to c) value, whereas the substitution of As5+ ions for framework P5+ cations in RbTiORO4 crystals increases the sigma(parallel to c) value. The sigma(parallel to c) values at 573 K are found to be 1.0 x 10(-5), 5.7 x 10(-6), 2.0 x 10(-6), and 3.3 x 10(-5) S/cm for the KTiOPO4, RbTiOPO4 {100}, RbTiOPO4 {201}, and RbTiOAsO4 crystals, respectively (the growth zone of the crystalline boule from which the samples were cut is indicated in braces).