  Raman spectroscopy experiments on single crystals of CuSb2O6 and CoSb2O6
quasi-one-dimensional antiferromagnets with trirutile crystal structure were
performed, with a focus on the first material. The observed Raman-active phonon
modes and previously reported infrared-active modes were identified with the
aid of ab-initio lattice dynamics calculations. The structural transition
between monoclinic beta-CuSb2O6 and tetragonal alpha-CuSb2O6 phases at Ts=400 K
is manifested in our spectra by a "repulsion" of two accidentally
quasi-degenerate symmetric modes below Ts, caused by a phonon mixing effect
that is only operative in the monoclinic beta-CuSb2O6 phase due to symmetry
restrictions. Also, two specific phonons, associated with CuO6 octahedra
rotation and with a Jahn-Teller elongation mode, soften and broaden appreciably
as T -> Ts. A crossover from a displacive to an order-disorder transition at Ts
is inferred.
