Abstract
A first-principles-based approach is developed to model dynamical coupling between different degrees of freedom in the terahertz frequency range at any temperature. Such an approach is applied to $\text{Pb(Zr,Ti)}{\text{O}}_{3}$ alloys, which exhibit both ferroelectric (FE) and antiferrodistortive (AFD) motions. It reproduces very well the existence of two modes (rather than the single $E(1\text{TO})$ soft mode) in the 50--75 cm${}^{\ensuremath{-}1}$ range for temperatures smaller than $\ensuremath{\simeq}200$ K. It also provides an insight into the existence of these two modes, which originate from the coupling between long-range-ordered FE and AFD motions. The proposed scheme is further used to reveal a field-induced anticrossing involving FE and AFD degrees of freedom.
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