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Browsing by Author "Kumar, Amit"

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    Elastic distortion determining conduction in BiFeO3 phase boundaries
    (Royal Society of Chemistry, 2020) Holsgrove, Kristina M.; Duchamp, Martial; Moreno, M. Sergio; Bernier, Nicolas; Naden, Aaron B.; Guy, Joseph G. M.; Browne, Niall; Gupta, Arunava; Gregg, J. Marty; Kumar, Amit; Arredondo, Miryam; Queens University Belfast; Nanyang Technological University & National Institute of Education (NIE) Singapore; Nanyang Technological University; CEA; Communaute Universite Grenoble Alpes; UDICE-French Research Universities; Universite Grenoble Alpes (UGA); University of St Andrews; University of Alabama Tuscaloosa
    It is now well-established that boundaries separating tetragonal-like (T) and rhombohedral-like (R) phases in BiFeO3 thin films can show enhanced electrical conductivity. However, the origin of this conductivity remains elusive. Here, we study mixed-phase BiFeO3 thin films, where local populations of T and R can be readily altered using stress and electric fields. We observe that phase boundary electrical conductivity in regions which have undergone stress-writing is significantly greater than in the virgin microstructure. We use high-end electron microscopy techniques to identify key differences between the R-T boundaries present in stress-written and as-grown microstructures, to gain a better understanding of the mechanism responsible for electrical conduction. We find that point defects (and associated mixed valence states) are present in both electrically conducting and non-conducting regions; crucially, in both cases, the spatial distribution of defects is relatively homogeneous: there is no evidence of phase boundary defect aggregation. Atomic resolution imaging reveals that the only significant difference between non-conducting and conducting boundaries is the elastic distortion evident - detailed analysis of localised crystallography shows that the strain accommodation across the R-T boundaries is much more extensive in stress-written than in as-grown microstructures; this has a substantial effect on the straightening of local bonds within regions seen to electrically conduct. This work therefore offers distinct evidence that the elastic distortion is more important than point defect accumulation in determining the phase boundary conduction properties in mixed-phase BiFeO3.
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    The Synthesis, Characterization and Dehydrogenation of Sigma-Complexes of BN-Cyclohexanes
    (Wiley-VCH, 2016) Kumar, Amit; Ishibashi, Jacob S. A.; Hooper, Thomas N.; Mikulas, Tanya C.; Dixon, David A.; Liu, Shih-Yuan; Weller, Andrew S.; University of Oxford; Boston College; University of Alabama Tuscaloosa
    The coordination chemistry of the 1,2-BN-cyclohexanes 2,2-R-2-1,2-B,N-C4H10 (R-2=HH, MeH, Me-2) with Ir and Rh metal fragments has been studied. This led to the solution (NMR spectroscopy) and solid-state (X-ray diffraction) characterization of [Ir(PCy3)(2)(H)(2)(eta(2)eta(2)-H2BNR2C4H8)][BAr4F] (NR2=NH2, NMeH) and [Rh(iPr(2)PCH(2)CH(2)CH(2)PiPr(2))(eta(2)eta(2)-H2BNR2C4H8)][ BAr4F] (NR2=NH2, NMeH, NMe2). For NR2=NH2 subsequent metal-promoted, dehydrocoupling shows the eventual formation of the cyclic tricyclic borazine [BNC4H8](3), via amino-borane and, tentatively characterized using DFT/GIAO chemical shift calculations, cycloborazane intermediates. For NR2=NMeH the final product is the cyclic amino-borane HBNMeC4H8. The mechanism of dehydrogenation of 2,2-H,Me-1,2-B,N-C4H10 using the {Rh(iPr(2)PCH(2)CH(2)CH(2)PiPr(2))}+ catalyst has been probed. Catalytic experiments indicate the rapid formation of a dimeric species, [Rh-2(iPr(2)PCH(2)CH(2)CH(2)PiPr(2))(2)H-5][BAr4F]. Using the initial rate method starting from this dimer, a first-order relationship to [amine-borane], but half-order to [Rh] is established, which is suggested to be due to a rapid dimer-monomer equilibrium operating.