Incommensurate Jahn-Teller Phase Transitions in R2PbCu(NO2)6 (R: K, Rb, Tl). I. Huang Scattering

X-ray and neutron scatterings at the incommensurate Jahn-Teller phase transitions in R 2 PbCu(NO 2 ) 6 (R: K, Rb, Cs, Tl) are studied in order to investigate microscopic origins to stabilize these incommensurate phases. We discuss the characteristics of diffuse scattering which do not allow the conventional interpretation as critical scattering associated with correlations of an order parameter. It has been shown that major component of the diffuse scattering is well explained as `Huang scattering' due to tetragonal `defects.' In the disordered phase, each Cu(NO 2 ) 6 -group is already locally distorted, with the axis of tetragonal elongation taking one of the cubic principal axes at random (either spatially or temporally) and this local distortion is `dressed' by a strain field. Such a view point establishes a possible origin of long range interaction to stabilize the incommensurate phases.