  We consider an ensemble of three-level particles in lambda-configuration
interacting with two bosonic modes. The Hamiltonian has the form of a
generalized Dicke-model. We show that in the thermodynamic limit this model
supports a superradiant quantum phase transition. Remarkably, this can be both
a first and a second order phase transition. A connection of the phase diagram
to the symmetries of the Hamiltonian is also given. In addition, we show that
this model can describe atoms interacting with an electromagnetic field in
which the microscopic Hamiltonian includes a diamagnetic contribution. Even
though the parameters of the atomic system respect the Thomas--Reiche--Kuhn sum
rule, the system still shows a superradiant phase transition.
