Fragile structural transition in Mo3Sb7

Mo${}_3$Sb${}_7$ single crystals lightly doped with Cr, Ru, or Te are studied in order to explore the interplay between superconductivity, magnetism, and the cubic-tetragonal structural transition. The structural transition at 53,K is extremely sensitive to Ru or Te substitution which introduce additional electrons, but robust against Cr substitution. No sign of a structure transition was observed in superconducting Mo${}_{2.91}$Ru${}_{0.09}$Sb${}_7$ and Mo${}_3$Sb${}_{6.975}$Te${}_{0.025}$. In contrast, 3at% Cr doping only slightly suppresses the structural transition to 48,K while leaving no trace of superconductivity above 1.8,K. Analysis of magnetic properties suggests that the interdimer interaction in Mo${}_3$Sb${}_7$ is near a critical value and essential for the structural transition. All dopants suppress the superconductivity of Mo${}_3$Sb${}_7$. The tetragonal structure is not necessary for superconductivity.