  Colloidal glasses form from hard spheres, nearly hard spheres, ellipsoids and
platelets or their attractive variants have been studied in detail.
Complementing and checking theoretical approaches and simulations, the many
different types of model systems have significantly advanced our understanding
of the glass transition in general. Despite their early prediction, however, no
experimental charged sphere glasses have been found at low density, where the
competing process of crystallization prevails. We here report the formation of
a transient amorphous solid formed from charged polymer spheres suspended in
thoroughly deionized water at volume fractions of 0.0002-0.01. From optical
experiments, we observe the presence of short-range order and an enhanced shear
rigidity as compared to the stable polycrystalline solid of body centred cubic
structure. On a density dependent time scale of hours to days, the amorphous
solid transforms into this stable structure. We further present preliminary
dynamic light scattering data showing the evolution of a second slow relaxation
process possibly pointing to a dynamic heterogeneity known from other colloidal
glasses and gels.We compare our findings to the predicted phase behaviour of
charged sphere suspensions and discuss possible mechanisms for the formation of
this peculiar type of colloidal glass.
