Gap closure in semiconducting APtSn cubic compounds (A = U, Th or Zr)

UPtSn is known as a semimetal with a bandgap in the electronic density of states. We report the results of measurements of the cubic lattice parameter a(T), coefficient of thermal expansion α(T), magnetic susceptibility χ(T), magnetization σ(T,B), specific heat C(T) and electrical resistivity ρ(T, B). We give arguments that this compound is antiferromagnetically ordered with T _N = 35 K, although this is not confirmed by neutron diffraction. For carefully annealed samples of UPtSn we provide a phenomenological description of electron transport properties in terms of the fact that the temperature-dependent bandgap is closed at T = 0 K. Results of electrical measurements on the isostructural compounds ThPtSn and ZrPtSn provide evidence for bandgap formation in these two compounds as well.