  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.
