Neutron Rietveld analysis of structural changes in NASICON solid solutions Na 1+xZr 2Si xP 3−xO 12 at elevated temperatures: x=1.6 and 2.0 at 320°C

Abstract

Neutron Rietveld analyses of the structures of NASICON solid solutions as a function of composition have been extended to 320°C for the high-conductivity compositions x=1.6 and 2.0. The transformation from the room temperature monoclinic C2/c structure to the hexagonal R3c high temperature phase involves small atomic displacements, ranging from 0.385Å for Na(2) down to shifts of only a few hundredths of an Ångstrom for several framework ions. The Na(1) interstice remains fully occupied to the temperature presently examined. No evidence for partial occupancy of the Zr octahedron is found, a non-stoichiometry which is possible but not obligatory for NASICON. The distortions of the framework are largest at x=2.0 as at room temperature. The radius of the windows between Na sites at 320°C remain largest at the composition with x=2.0 for both a Na(1)-Na(2) jump and a Na(2)-Na(2) jump. The radii are significantly larger than the maximum value available among the three symmetry-independent paths in the room-temperature monoclinic structures for both types of diffusion paths.