Harmonic Potentials for Quaternionic Symmetric Σ-Models

We construct N = 2 superspace lagrangians for quaternionic symmetric sigma-models G/H x Sp(1), or equivalently, quaternionic potentials for these symmetric spaces. They are homogeneous H invariant polynomials of order 4 which are similar to the quadratic Casimir operator of H. The construction is based on an identity for the structure constants specific for quaternionic symmetric spaces.