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Elastic and anelastic properties of poled and depoled single crystals of Pb (In 1/2 Nb 1/2) O 3-Pb (Mg 1/3 Nb 2/3) O 3-PbTiO 3 with compositions close to the morphotropic boundary have been investigated over the temperature range 5–700 K by resonant ultrasound spectroscopy (RUS) at frequencies of 0.1–1.2 MHz. Steep elastic softening occurs in a temperature interval of at least 250 K as the Vogel-Fulcher freezing interval and cubic→ tetragonal transition point, T c, are approached from above. This is understood in terms of coupling between acoustic modes and central peak mode (s) associated with dynamic polar nano regions (PNR's) below the Burns temperature. Acoustic losses occur in a temperature interval of∼ 50 K above T c, associated with slowing down of the PNR dynamics. The cubic↔ tetragonal and tetragonal↔ rhombohedral transitions are accompanied by steep minima in elastic properties …