  Comprehensive vibrational studies on the multiferroic-to-normal-ferromagnetic
crossover in isostructural Ni3-xCoxV2O8 (NCVO, x = 0 - 3.0) are performed using
Raman scattering spectroscopy. The systematically red-shifted phonon modes are
discussed in terms of the mode Gr\"uneisen parameters, and are interpreted as a
chemical pressure effect. In addition, we present evidence that the local
symmetry is broken in the multiferroic phase.
