Phase transformations in highly electrostrictive (1-x)Pb(Mg1/3Nb2/3O3)-xPbTiO3 and magnetostrictive (Fe-xGa) crystals

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Date/Time:Thursday, 03 Mar 2011 - Saturday, 05 Mar 2011
Location:PHYSICS Room 5
Channel:Condensed Matter Physics
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Hu (Tiger) Cao Indiana University Cyclotron Facility 2401 N Milo B. Sampson Lane, Bloomington, IN 47408

Phase transformations in highly electrostrictive (1-x)Pb(Mg1/3Nb2/3O3)-xPbTiO3 and magnetostrictive (Fe-xGa) crystals: structural inhomogeneity and history dependent phase stability

Ferroelectric and ferromagnetic materials have been extensively studied for the potential applications in medical imaging devices, sensors, actuators and transducers. Among them, highly electrostrictive (1-x)Pb(Mg1/3Nb2/3)-xPbTiO3 (PMN-xPT) (ferroelectric) and highly magnetostrictive Fe-xGa alloys (ferromagnetic) are such two novel "smart" materials which are highly responsive and have the inherent capability to sense and react according to the changes in the environment.
Our studies by x-ray, neutron and electron scattering have revealed the structural origin of ultrahigh performance observed in these two materials- structural inhomogeneity on a nanoscale. In PMN-xPT, various intermediate monoclinic (M) phases have been found, which structurally 'bridge' the rhombohedral (R) and tetragonal (T) ones across the morphtropic phase boundary (MPB). Systematic investigations of <001> and <110> electric (E) field-temperature phase diagrams of PMN-xPT crystals have demonstrated that the phase stability of PMN-xPT crystals is quite fragile: depending not only on modest changes in E (≤ 0.5kV/cm), but also on the direction along which E is applied. An alternative interpretation for the observed phase fragility is the "ferroelectric adaptive phase" model, which theorized that the monoclinic phases are miniaturized T or R nanotwins (~10nm) determined by elastic lattice accommodation under the misfit strain and electric field. In Fe-xGa alloys, the high resolution transmission electron microscopy showed a DO3 second phase nano-dispersion within an A2 matrix, whereas the diffuse neutron scattering surprisingly established a confined distortion of nano-dispersed DO3 precipitates with an extra-large strain. Investigations as a function of Ga content showed that the diffuse scattering intensity is strongest near Fe-0.19Ga where the magnetostriction is maximal: This confirms that the enhanced magnetostriction is directly related to the structural heterogeneity of DO3 nano-precipitates.