Orbital-fluctuation freezing and magnetic-nonmagnetic phase transition in alpha-TiBr3

We present a detailed study on the structural phase transition in alpha -TiBr3, which is deeply connected with the lattice and orbital degree of freedoms. A chemical vapor transport method is adopted to synthesize the alpha -TiBr3 single crystal samples, and the structural phase transition at about 180K is characterized by x-ray diffraction (XRD), magnetic susceptibility, and specific heat capacity. To further the understanding in the physical nature of this phase transition, a systematic Raman spectroscopic study is performed on alpha -TiBr3 crystals. With temperature decreasing, a large frequency blue shift and peak width narrowing are observed in the vibrational mode associated with Ti in-plane relative movement, which indicates the formation of Ti-Ti bonding and orbital-fluctuation freezing at low temperatures. These results are fully consistent with magnetic-nonmagnetic phase transition resolved by the measurement of magnetic susceptibility and lattice changes by XRD.