La3S4 is known to crystallize in a defect cubic Th3P4 structure. The effect of alkali metal ion doping in the lacunar La3-xS4 structure has been studied. Compounds with molecular formula La(3-x)A(x)S(4) (A = Li, Na and K) with varying A/La ratios (0.1, 0.15, 0.2, 0.25 and 0.3) have been synthesized by gas-solid reaction method. The band gap increases with increase in dopant concentration. The optical properties show an increase in yellowness of the doped compositions when compared to the parent phase. (c) 2006 Elsevier Ltd. All rights reserved.