Structural and conductivity studies in LiFeP2O7, LiScP2O7, and NaScP2O7

Structural studies of LiScP2O7 by Rietveld refinement confirm that this material is isostructural with LiFeP2O7 studied previously. However, NaScP2O7 shows a structure different from the structural types of the basic group of (NaMP2O7)-M-I-P-IIIP known thus far. Systematic ranges for the six structural types of A(I)M(III)P(2)O(7) are presented in terms of ion radii sums and ratios. The framework of LiMP2O7 (M = Sc, Fe) has rather wide tunnels running along the crystallographic c-axis. This feature has determined our interest to check the ion conductivity in A(I)M(III)P(2)O(7) (A = Li, Na; M = Sc, Fe). The bulk conductivity, however, is low in these compounds, 10(-6)-10(-7) S/cm at 300 degrees C, as determined by impedance spectroscopy. In order to facilitate the conductivity via normal lithium sites, heterovalent substitution is used.