  Millimeter sized single crystals of Mo3Sb7 are grown using the self-flux
technique and a thorough characterization of their structural, magnetic,
thermal and transport properties is reported. The structure parameters for the
high-temperature cubic phase and the low-temperature tetragonal phase were, for
the first time, determined with neutron single crystal diffraction. Both X-ray
powder diffraction and neutron single crystal diffraction at room temperature
confirmed that Mo3Sb7 crystallizes in Ir3Ge7-type cubic structure with space
group Im-3m. The cubic-tetragonal structure transition at 53K is verified by
the peak splitting of (4 0 0) reflection observed by X-ray single crystal
diffraction and the dramatic intensity change of (12 0 0) peak observed by
neutron single crystal diffraction. The structural transition is accompanied by
a sharp drop in magnetic susceptibility, electrical resistivity, and
thermopower while cooling. A weak lambda anomaly was also observed around 53K
in the temperature dependence of specific heat and the entropy change across
the transition is estimated to be 1.80J/molMoK. The temperature dependence of
magnetic susceptibility was measured up to 750K and it follows a Curie-Weiss
behavior above room temperature. Analysis of the low-temperature magnetic
susceptibility suggests a spin gap of 110K around 53K. A typical phonon thermal
conductivity was observed in the low temperature tetragonal phase. A glassy
phonon thermal conductivity above 53K suggests a structural instability in a
wide temperature range. Superconductivity was observed at 2.35K in the as-grown
crystals and the dimensionless specific heat jump was determined to be 1.49,
which is slightly larger than the BCS value of 1.43 for the weak-coupling
limit.
