PHASE TRANSITION IN La2NiO4.147 SYSTEM STUDIED BY LOW-FREQUENCY INTERNAL FRICTION TECHNIQUE

La2NiO4.147 sample with Fmmm symmetry is prepared by electrochemical oxidation process. It is found that there exists a low-frequency internal friction peak around 400 K with phase transition character, which just located in the position of one phase transition, reported by literature, from the low temperature orthorhombic Fmmm phase to high temperature tetragonal I4/mmm phase. The thermal hysteresis of the modulus and internal friction in heating and cooling processes as well as the fact that height of the internal friction peak is proportional to the rate of the heating and reciprocal to the work frequency mean that this phase transition belongs to a first order phase transition with martensitic type. No such phase transition observed in low δ sample of La2NiO4+δ suggests the above phase transition may have relationship with the three-dimensional ordering of excess oxygen in the low temperature orthorhombic Fmmm phase.