SYNTHESIS OF RARE-EARTH POLYCHALCOGENIDES BY MODERATE TEMPERATURE SOLID-STATE METATHESIS

Alkali metal polychalcogenide salts of the form A(2)Q(x) (where A = Na , K; Q = S, Se, Te; x = 1, 2, 3, 4) have been used in moderate tempera ture solid-state metathesis reactions with lanthanoid halides, LnX(3) (Ln = La, Pr; X = Cl, Pr), to prepare four phases of lanthanoid chalco genides at temperatures below 400 degrees C. For lanthanum, alpha-LaS2 , beta-LaS2, LaSe2, and LaTe3 were prepared and an irreversible phase transformation at 450 degrees C between the metastable alpha- and ther modynamically stable beta-LaS2 was observed. For praseodymium, alpha-P rS2, PrSe2, and PrTe3 were prepared and no phase transition from alpha - to beta-PrS2 was observed. (C) 1995 Academic Press, Inc.