Fermi resonance involving nonlinear dynamical couplings in Pb(Zr,Ti)O3 solid solutions.

We have used first-principles-based simulations and Raman scattering techniques to reveal a novel dynamical phenomenon in Pb(Zr,Ti)O(3) solid solutions: a Fermi resonance (FR) emerging from the nonlinear coupling between ferroelectric (FE) motions and tiltings of oxygen octahedra. This FR manifests itself as the doubling of a nominally single FE mode in a purely FE phase, when the resonant frequency of the FE mode is close to the first overtone of the tiltings. We show that the FR is the result of a nonlinear coupling that is proportional to the spontaneous polarization of the material and derive an analytical model that captures the essence of the effect.