Synthesis of layered sodium lanthanum selenide through ion exchange reactions

Layered hexagonal KLaSe2 (α-NaFeO2-type) was synthesized using the reactive flux method and analyzed by powder XRD to determine its lattice constants (space group R-3m, a = 4.40508(5) A, c = 22.7838(5) A). NaLaSe2, which normally crystallizes as a disordered rock salt structure with mixed Na+/La + 3 sites, was synthesized through a solid state ion exchange reaction at 585 deg. C from a 1:3 molar ratio mixture of KLaSe2:NaI. The product of this reaction was hexagonally layered NaLaSe2 (space group R-3m, a = 4.3497(3) A, c = 20.808(2) A) isostructural to KLaSe2. This product was analyzed by comparison with members of the set of solid solutions Na(1-x)K(x)LaSe2 to confirm that the extent ion exchange in this reaction was complete. Cubic (disordered) NaLaSe2 was also reacted with KI to yield the poorly crystalline hexagonally layered product with the approximate formula Na0.79K0.21LaSe2