Syntheses and Structures of K₃MQ₄(M = Nb, Ta; Q = S, Se)

The four tetrachalcogenometalates K₃NbS₄, K₃NbSe₄, K₃TaS₄, and K₃TaSe₄have been obtained by direct reaction among the elements at 850 °C. The structures of K₃NbS₄and K₃NbSe₄have been determined by single-crystal X-ray diffraction methods. K₃TaS₄and K₃TaSe₄are isostructural with K₃NbS₄and K₃NbSe₄, as determined from X-ray Guinier photographs. All of them crystallize with four formula units in space group D¹⁶_2h-Pnma of the orthorhombic system in cells of dimensions a = 9.214 (8), b = 10.484 (6), and c = 9.319 (7) Å for K₃NbS₄, a = 9.599 (2), b = 11.042 (3), and c = 9.660 (2) Å for K₃NbSe₄, a = 9.283 (2), b = 10.806 (3), and c = 9.387 (2) Å for K₃TaS₄,and a = 9.682 (3), b = 11.276 (4), and c = 9.712 (3) Å for K₃TaSe₄. The final refinements of 43 variables lead to R and R_wvalues of 0.106 and 0.115 for K₃NbS₄and 0.069 and 0.083 for K₃NbSe₄. These structures are of the K₃VS₄type and contain discrete K₊and tetrahedral MQ₄^3-ions. In K₃NbS₄the Nb-S distances range from 2.241 (8) to 2.258 (8) Å and the S-Nb-S angles range from 108.3 (2) to 111.6 (3)°. In K₃NbSe₄the Nb-Se distances range from 2.387 (1) to 2.403 (1) Å and Se-Nb-Se angles range from 108.49 (3) to 111.68 (5)°.