Direct evidence of soft mode behavior near the Burns temperature in thePbMg1/3Nb2/3

Abstract

Inelastic neutron scattering measurements of the relaxor ferroelectric PbMg 1/3 Nb 2/3 O3 (PMN) in the temperature range 490 K<T<880 K directly observe the soft mode (SM) associated with the Curie-Weiss behavior of the dielectric constant ε(T). The results are treated within the framework of the coupled SM and transverse optic (TO1) mode and the temperature dependence of the SM frequency at q=0.075 a* is determined. The parameters of the SM are consistent with the earlier estimates and the frequency exhibits a minimum near the Burns temperature (≈ 650K)

Diffuse neutron scattering in relaxor ferroelectric PbMg1/3Nb2/3O3

“…It was proposed by Burns that these PNR are formed at some temperature T d (now usually called Burns temperature) much higher than T g [5], for instance, in the case of PMN T d ~ 630K. Recently it was demonstrated that the formation of the PNR is related to the transverse optic (TO) mode softening [6][7][8][9]. The most striking difference of the PNR in relaxors from the precursor clusters in majority of compounds undergoing a structural phase transition is the essential difference of the eigenvectors of the soft mode and PNR.…”

The Lineshape of Inelastic Neutron Scattering in Relaxor Ferroelectrics

“…Thus, adjusting the a, b and p parameters allows us to modify the distribution of the points freely, and therefore to include the knowledge of an expert into the LH sampling method. Such knowledge in our case is, for example, the choice of the proper reciprocal lattice vector τ (in [3], it is (3,0,0)) to clearly observe the soft mode. In particular, the authors of [3] have shown that observation of the soft phonon mode in PMN is much easier (as compared to previous results performed in the Brillouin zone centered at (2,0,0)) in the [010] direction near the (3,0,0) reciprocal lattice vector.…”

“…Note that most often the neutron scattering measurements in relaxors are performed for τ = (2, 0, 0) and (3,0,0) (see, e. g. [3,8]). We will use this fact below for planning experiments by the LHS method.…”

“…Inelastic neutron scattering permits to obtain the detailed information about soft phonon mode (and actually any other phonon mode) dispersion throughout entire Brillouin zone. Such investigations for relaxor ferroelectric PbMg 1/3 Nb 2/3 O 3 have been performed, e. g., in [3]. It is also possible to study the thermal displacements of ions from their equilibrium positions in the elementary cell, also during ferroelectric phase transition.…”

“…The experimental investigations of relaxor ferroelectrics by inelastic neutron scattering method are dependent on many parameters (see, e. g. [3,6,7]) like temperature, concentrations of different kinds of defects and impurities (sometimes called degree of disorder), vectors of the reciprocal lattice etc. To draw the correct conclusions from such multifactor experiment, we should perfectly know, which measurements (i. e. for which parameter values) give the most significant information for the system considered.…”