INELASTIC-NEUTRON-SCATTERING STUDY OF A SOFT ROTATIONAL MODE IN ND2CUO4

We have found evidence for an incipient structural phase transformation in Nd2CuO4 by inelastic-neutron-scattering experiments. In this compound, Cu is square-planar coordinated by four O(1) ions which can rotate as a rigid entity around the 001-axis. The frequency of this rotational mode was found to be rather low, i.e. v = 3.2 THz at room temperature. A further softening by 10% was observed on cooling. In contrast with all other phonons investigated in this compound, the rotational mode shows an anomalous lineshape, indicating a strongly anharmonic character of this type of vibration. Model calculations indicate that the rotational mode will be rather soft for all compounds crystallizing in the T'-phase or even exhibit a zero frequency.