Structure and conductivity of the NASICON analog Na/sub 3/SC/sub 2/(PO/sub 4/)/sub 3/

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/sup 0/C and 64/sup 0/C, a second polymorph appears. The structure is monoclinic but has not yet been refined. At 64/sup 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/sup 0/C. At 166/sup 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/sup 0/C. The diffraction data are correlated with ionic conductivity measurements as a function of temperature