  There is a long-standing confusion concerning the physical origin of the
anomalous resistivity peak in transition metal pentatelluride HfTe5. Several
mechanisms, like the formation of charge density wave or polaron, have been
proposed, but so far no conclusive evidence has been presented. In this work,
we investigate the unusual temperature dependence of magneto-transport
properties in HfTe5. We find that a three dimensional topological Dirac
semimetal state emerges only at around Tp (at which the resistivity shows a
pronounced peak), as manifested by a large negative magnetoresistance. This
accidental Dirac semimetal state mediates the topological quantum phase
transition between the two distinct weak and strong topological insulator
phases in HfTe5. Our work not only provides the first evidence of a
temperature-induced critical topological phase transition in HfTe5, but also
gives a reasonable explanation on the long-lasting question.
