Spectroscopical Investigation of Order-disorder in Some Compounds with Scheelite Structure

Abstract

Infrared and Raman spectra have been measured of scheelites with formula ALnW8O8, ALnMo2O8 (A = Li, Na, K; Ln = La, Y), Y2MWO8 M = Ge, Si), Y2GeMoO8 and the zircon Y2SiMoO8 in order to get information about the crystallographic order. The A+ and Ln3+ ions in the scheelites ALnW2O8 and ALnMo2O8 are probably ordered in layers Ln-Ln-A-A perpendicular to the c-axis. The order is short-range only. The degree of order decreases in the sequence K, Na, Li. The compounds Y2MWO8 and Y2GeMoO8 are not ordered, whereas the Si4+ and Mo6+ ions in Y2SiMoO8 are ordered in alternate layers perpendicular to the c-axis. The use of vibrational spectra for the determination of crystallographic order is discussed.