Scholarly Works, Materials Science and Engineering (MSE)

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Browsing Scholarly Works, Materials Science and Engineering (MSE) by Author "Ahart, Muhtar"

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    Lead-free and lead-based ABO(3) perovskite relaxors with mixed-valence A-site and B-site disorder: comparative neutron scattering structural study of (Na1/2Bi1/2) TiO3 and Pb(Mg1/3Nb2/3)O-3
    Ge, Wenwei; Devreugd, Christopher P.; Phelan, Daniel; Zhang, Qinhui; Ahart, Muhtar; Li, Jiefang; Luo, Haosu; Boatner, Lynn A.; Viehland, Dwight D.; Gehring, Peter M. (American Physical Society, 2013-11-01)
    We report the results of neutron elastic-scattering measurements made between -250 degrees C and 620 degrees C on the lead-free relaxor (Na1/2Bi1/2)TiO3 (NBT). Strong, anisotropic, elastic diffuse scattering intensity decorates the (100), (110), (111), (200), (210), and (220) Bragg peaks at room temperature. The wave-vector dependence of this diffuse scattering is compared to that in the lead-based relaxor Pb(Mg1/3Nb2/3)O-3 (PMN) to determine if any features might be common to relaxors. Prominent ridges in the elastic diffuse scattering intensity contours that extend along < 110 > are seen that exhibit the same zone dependence as those observed in PMN and other lead-based relaxors. These ridges disappear gradually on heating above the cubic-to-tetragonal phase transition temperature T-CT = 523 degrees C, which is also near the temperature at which the dielectric permittivity begins to deviate from Curie-Weiss behavior. We thus identify the < 110 >-oriented ridges as a relaxor-specific property. The diffuse scattering contours also display narrower ridges oriented along < 100 > that are consistent with the x-ray results of Kreisel et al. [Phys. Rev. B 68, 014113 (2003)]; these vanish near 320 degrees C, indicating that they have a different physical origin. The < 100 >-oriented ridges are not observed in PMN. We observe no equivalent relaxor-specific elastic diffuse scattering from the homovalent relaxor analogues K0.95Li0.05TiO3 (A-site disordered) and KTa0.95Nb0.05O3 (B-site disordered). This suggests that the < 110 >-oriented diffuse scattering ridges are correlated with the presence of strong random electric fields and invites a reassessment of what defines the relaxor phase. We find that doping NBT with 5.6% BaTiO3, a composition close to the morphotropic phase boundary with enhanced piezoelectric properties, increases the room-temperature correlation length along [1 (1) over bar0] from 40 to 60 angstrom while doubling the associated integrated diffuse scattering. Similar behavior was reported by Matsuura et al. [Phys. Rev. B 74, 144107 (2006)] for morphotropic compositions of PMN doped with PbTiO3. Finally, we comment on the recent observation of monoclinicity in NBT at room temperature by placing a strict bound on the strength of the (1/2 1/2 1/2) superlattice reflection associated with the Cc space group based on the atomic coordinates published in the x-ray study by Aksel et al. [Appl. Phys. Lett. 98, 152901 (2011)] for NBT. We show that a skin effect, analogous to that reported in the relaxors PZN-xPT and PMN-xPT, can reconcile our neutron single-crystal data with the x-ray powder data of Aksel et al. [Appl. Phys. Lett. 98, 152901 (2011)]. Our finding of a skin effect in a lead-free, A-site disordered, heterovalent relaxor supports the idea that it arises in the presence of strong random electric fields.
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    X-ray diffraction study of the pressure-induced bcc-to-hcp phase transition in the highly magnetostrictive Fe0.81Ga0.19 alloy
    Ahart, Muhtar; Devreugd, Christopher P.; Li, Jiefang; Viehland, Dwight D.; Gehring, Peter M.; Hemley, Russell J. (American Physical Society, 2013-11-07)
    High-pressure x-ray diffraction measurements were performed at room temperature on single crystals of the highly magnetostrictive alloy Fe0.81Ga0.19 (galfenol). This alloy has a bcc crystal structure at ambient pressure but undergoes a bcc-to-hcp phase transition at 24 GPa on compression. A large hysteresis loop is observed in which the reversed transition occurs at 13 GPa on decompression. The midpoint of this transition is 18.5 GPa. The measured bulk modulus of this material is 182 (+/- 17) GPa, which is comparable to that of pure iron. As with iron, the hcp structure of the alloy can be derived from a compression of the bcc lattice along [001] that is accompanied by shearing along [1 (1) over bar0]. Our results indicate that the addition of Ga shifts the bcc-to-hcp transition from 13 GPa in pure iron to 18.5 GPa, and we speculate that this is due to the larger atomic radius of Ga. A uniaxial loading of 3 GPa completely suppresses the diffuse scattering in Fe0.81Ga0.19. We ascertain that the magnetostrictive properties of the alloy are reduced under pressure.