resulting in breaking of the ground state orbital degeneracy and a soft acoustic mode in the $lim_{q\rightarrow 0}$.

The simultaneous broadening and softening of an acoustic phonon over a range in wavevector transfers is indicative of coupling to some other degree of freedom (see for example discussion of coupling in Ref. ). Coupling of acoustic phonons to crystal field [@Bruhl78] driven distortions have been experimentally studied with examples being the TA2 phonon anomalies reported in DyVO$_{4}$ and TbVO$_{4}$. [@Melcher72; @Sandercock72] Similar coupling effects have also been discussed in UO$_{2}$. [@Allen68] These examples involve the coupling of an acoustic phonon to a crystal field level with a similar energy scale to that of the phonon and this is also predicted to be required in the case of magnetic pyrochlores. [@Yamashita00] While the $dd$ transitions in $3d$ transition metal ions are large ($\sim$ 1-2 eV) [@Haverkort07; @Kant08; @Schooneveld12; @Kim11; @Cowley13] making them unlikely to be involved with this process, the lower energy spin-orbit split levels may have a more appropriate energy scale. As discussed by Tchernyshyov [@Tcherny04], the ground state of V$^{3+}$ derived by diagonalising the spin-orbit Hamiltonian is a $j_{eff}$=2 quintuplet which is separated by an energy scale of $\sim$10 meV 