Structure and Conductivity of the NASICON Analog Na sub 3 SC sub 2 (PO Sub 4 ) sub 3

摘要

Neutron and x-ray diffraction data of Na sub 3 SC sub 2 (PO sub 4 ) sub 3 are presented that show the transition from an ordinary-conducting, high temperature phase to a still higher temperature, superionic conducting phase. There are at least four polymorphic modifications. At room temperature, the refined neutron diffraction data confirm the Cc space group reported by Efremov and Kalinin but with a = 16.0449(24), b = 8.9225(15), c = 9.0656(13)A, beta = 126.918(21), R/sub weighted pattern/ = 17.5%, and R/sub expected/ = 7%. Between 25 exp 0 C and 64 exp 0 C, a second polymorph appears. The structure is monoclinic but has not yet been refined. At 64 exp 0 C, a third polymorph C appears. It is rhombohedral R3c. Using hexagonal axes, a = 8.9273(2), c = 22.3668A, R/sub wp/ = 6.28% and R/sub e/ = 3.83% at 100 exp 0 C. At 166 exp 0 C, the high temperature polymorph D appears. It is the superconducting phase of NASICON(Sc). It, too, is rhombohedral R3c with a = 8.9274(1), c = 22.5493(6)A, R/sub wp/ = 5.81% and R/sub e/ = 3.87% at 225 exp 0 C. The diffraction data are correlated with ionic conductivity measurements as a function of temperature. (ERA citation 08:024058)