  We use statistical mechanics -- variants of the six-vertex model in the plane
studied by means of the Yang-Baxter equation -- to give new deformations of
Weyl's character formula for classical groups of Cartan type B, C, and D, and a
character formula of Proctor for type BC. In each case, the corresponding
Boltzmann weights are associated to the free fermion point of the six-vertex
model. These deformations add to the earlier known examples in types A and C by
Tokuyama and Hamel-King, respectively. A special case for classical types
recovers deformations of the Weyl denominator formula due to Okada.
