Impulsive stimulated light scattering experiments were carried out on an ionic glass former Ca0.4K0.6(NO3)(1.4) to determine the temperature dependence of the Debye-Waller factor f(q)(T) in the q-->0 limit and to investigate the structural relaxation dynamics. A square-root cusp anomaly was observed in f(q-->0)(T) at a crossover temperature T-c of 378 K. This and the relaxation dynamics observed were consistent with predictions of the mode-coupling theory of the liquid-glass transition.