X-ray diffraction study of a structural phase transition in (NbSe4)3I

The second-order structural phase transition was observed by X-ray diffraction technique in (NbSe₄)₃I. The transition temperature (T_c) was 274.2 K. No evidence of the formation of the charge-density wave was found out. Temperature dependence of the intensity of (0,5,12) reflection obeys the Landau theory of the second kind of phase transition near T_c. Temperature dependence of the integral breadth of (0,0,12) reflection indicates the effect of the soft-phonon mode below T_c. The existence of the structural phase transition, probably not associated with charge-density-wave formation, suggests the unique nature of (NbSe₄)Z₃I in the new series of compounds (MSe₄)_xI (M=Nb, Ta).