A reassessment of the Burns temperature and its relationship to the diffuse scattering, lattice dynamics, and thermal expansion in the relaxor PMN

Abstract

We have used neutron scattering techniques to characterize the diffuse scattering and lattice dynamics in single crystals of the relaxor PMN from 10K to 900K. We observed two distinct types of diffuse scattering. The first is weak, relatively temperature independent, persists to at least 900 K, and forms bow-tie-shaped patterns in reciprocal space centered on (h00) Bragg peaks. We associate this primarily with chemical short-range order. The second is strong, temperature dependent, and forms butterfly-shaped patterns centered on (h00) Bragg peaks. This diffuse scattering has been attributed to the PNR because it responds to an electric field and vanishes near Td ~ 620K when measured with thermal neutrons. Surprisingly, it vanishes at 420K when measured with cold neutrons, which provide ~4 times superior energy resolution. That this onset temperature depends on the instrumental energy resolution demands a reassessment of the Burns temperature Td. Neutron backscattering measurements made with 300 times better energy resolution confirm the onset temperature of 420+/-20K. The energy width of the diffuse scattering is resolution limited, indicating that the PNR are static on timescales of at least 2 nsec. Transverse acoustic (TA) phonon lifetimes are temperature independent up to 900K for q < 0.2 1/A. This motivates a physical picture in which sufficiently long-wavelength TA phonons average over the PNR; only those TA phonons having wavelengths comparable to the size of the PNR are affected. The PMN lattice constant changes by less than 0.001 Angstroms below 300K, but expands rapidly at a rate of 2.5x10^-5 1/K at high temperature. The regimes of low and high thermal expansion bracket the revised value of Td, which suggests the anomalous thermal expansion results from the condensation of static PNR.

arXiv comment16 pages, 12 figures, accepted for publication in Physical Review B

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• Origin of Diffuse Scattering in Relaxor Ferroelectrics
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• Dielectric and electro-mechanic nonlinearities in perovskite oxide ferroelectrics, relaxors and relaxor ferroelectrics
• Random Electric Field Instabilities of Relaxor Ferroelectrics
• Paraelectric and Ferroelectric States in a Model for Relaxor Ferroelectrics
• Evidence for anisotropic polar nanoregions in relaxor PMN: A neutron study of the elastic constants and anomalous TA phonon damping
• A two-component model of the neutron diffuse scattering in the relaxor ferroelectric PZN-4.5%PT
• Temperature independent diffuse scattering and elastic lattice deformations in relaxor PbMg1/3Nb2/3O3
• Spontaneous decay of a soft optical phonon in the relaxor ferroelectric PbMg${}_{1/3}$Nb${}_{2/3}$O${}_3$
• Coupled magnetic and ferroelectric excitations in PbFe_{1/2}Nb_{1/2}O_{3}
• Structure Factor of a Relaxor Ferroelectric
• Freezing of the local dynamics in the relaxor ferroelectric PZN-4.5PT
• Macroscopic polarization in the nominally ergodic relaxor state of lead magnesium niobate
• Anelastic relaxor behavior of Pb(Mg1/3Nb2/3)O3
• Direct observation of monoclinic polar nanoregions in the relaxor ferroelectric Pb(Yb${}_{1/2}$Nb${}_{1/2}$)O${}_3$-PbTiO${}_3$
• Broadband critical dynamics in disordered lead-based perovskites
• Tetragonal tungsten bronze compounds: relaxor vs mixed ferroelectric - dipole glass behavior
• Phonon coupling to dynamic short-range polar order in a relaxor ferroelectric near the morphotropic phase boundary
• Fast broadband cluster spin-glass dynamics in PbFe${}_{1/2}$Nb${}_{1/2}$O${}_3$
• Soft Phonon Modes and Diffuse Scattering in Pb(In1/2Nb1/2)O3-Pb(Mg1/3Nb2/3)O3-PbTiO3 Relaxor Ferroelectrics
• Topological protection breakdown: a route to frustrated ferroelectricity
• Intrinsic structure of relaxor ferroelectrics from first principles