Resonant Ultrasound Studies of Mo$_{3}$Sb$_{7}$*

The elastic behavior of a series of Mo$_{3-x}$M$_{x}$Sb$_{7-y}$X$_{y}$ (M= Cr, Ru, X= Te) single crystals is examined with resonant ultrasound spectroscopy (RUS) as a function of temperature (300 K - 5 K). The elastic response of the parent compound Mo$_{3}$Sb$_{7}$ reveals a transition at around 53K, evidenced by a dramatic softening in the shear modulus c=(c$_{11}$-c$_{12}$)/2. This softening is associated with a cubic-to-tetragonal structural transition as well as a spin gap formation. The transition temperature is lowered by a few degrees upon doping with Cr. Doping with Ru and Te is known to suppress the structural transition, but the softening in the shear modulus suggests that a structural instability remains present in these compounds.