Access Type

Open Access Dissertation

Date of Award

January 2013

Degree Type

Dissertation

Degree Name

Ph.D.

Department

Chemistry

First Advisor

Stephanie L. Brock

Abstract

This dissertation study focuses on (1) probing the magneto-structural phase transformation in nanoscale MnAs; (2) evaluation of the size-dependent phase stability of type-B MnAs (prepared by rapid injection); and (3) developing a general synthetic method for transition metal arsenide nanoparticles.

Discrete MnAs nanoparticles that adopt different structures at room temperature (type-A, α-structure and type-B, β-structure) have been prepared by the solution-phase arrested precipitation method. Atomic pair distribution and Rietveld refinement were employed on synchrotron data to explore the structural transitions of the bulk and nanoparticle samples, and these results were compared to AC magnetic susceptibility measurements of the samples. The results confirm that the structural transition and the magnetic transition are highly correlated in bulk MnAs and suggest that for type-A MnAs nanoparticles, there is a similar magneto-structural phase transition occurring in the same temperature region as that for bulk MnAs. However, for type-B MnAs nanoparticles, there is no magneto-structural phase transition, consistent with that type-B MnAs nanoparticles being kinetically trapped in the β-structure.

Type-B MnAs nanoparticles adopting the β-strucuture undergo a transformation from β to α upon cooling. Temperature dependent XRD studies and magnetic measurements suggest that the TP for α → β conversion is suppressed to lower temperatures relative to bulk and type-A MnAs nanoparticles and that the transformation is reversible but has an enhanced hysteresis, which results in a large coexistence temperature range for the α and β structure. The transformation temperature correlated with the compression of the lattice parameters of the type-B MnAs nanoparticles due to the decrease in the particle size or the presence of chemical doping, or both.

A new general synthetic method for transition metal arsenide (Ni11As8, FeAs and CoAs) nanocrystals synthesis was developed by directly injecting the metal precursor into pre-reacted arsenic precursors. This method enables more control of the nanoparticle growth and monodispersity than is achieved by the direct conversion of metal nanoparticles.

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