Novel Misfit Layer Systems: Synthesis and Characterization

Abstract

Stabilizing mechanisms and design considerations for generating misfit layer compounds with a variety of different structural motifs were explored using designed precursors consisting of elemental layers. Layer order in the precursor film and the behavior of binary reaction couples was used to avoid undesirable reaction intermediates.

Electron diffraction patterns of CuCr2 Se4 were inconsistent with prior reports that this compound has the spinel structure and were more consistent with a hexagonal R 3 ̄ structure. STEM imaging also suggests CuCr2 Se4 prepared using the compositionally modulated kinetic trapping approach is a new polymorph of the spinel structure. Electrical and magnetic properties were consistent with prior literature reports. Magnetic susceptibility measurements show pronounced hard and easy axes of magnetization not previously documented, which are consistent with a hexagonal crystal symmetry.

The [{(PbSe)m }0.99 (WSe2 )n ] r and [{(PbSe)m }1.00 (MoSe2 )n ] r systems were investigated by STEM, XRD and density functional theory (DFT) modeling. No crystallographic registration between MSe and TSe 2 layers was observed and the diffraction observed in the hk 0 and hkl directions, where h = k = 0, can be described by diffraction from discrete layers of finite thickness. A distortion of the MX structure for m > 4 was documented. The distortion in MSe layers was largest for m = 2 and independent of TSe2 thickness.

A novel family of compounds, [{(FeSe)m }1+y (NbSe 2 )n ]r , were synthesized inspired by a geological precedent. Single FeSe and NbSe2 layer thicknesses ((0.571 ± 0.005) nm and (0.653 ± 0.002) nm respectively) are consistent with literature values for the binary compounds. STEM-HAADF images of the [{(FeSe) 5 }1+y (NbSe2 )5 ]r revealed a multilayer structure with two distinct structural subunits. STEM-EELS analysis of the film showed no intermixing between the Nb and Fe regions within the limit of the measurement.

Another family of misfit layer compounds, [{(NbSe2 )m }1+y (CuCr2Se4)n]r, designed to test requirements for a stable misfit layer compound, were successfully synthesized. STEM analysis of the [{(NbSe2 )5 }1+y (CuCr2 Se4 ) 1 ]r compound showed a well segregated film with two distinct subunit structures. Thicknesses for individual layers of NbSe2 or CuCr2 Se4 ((0.648 ± 0.004) nm and (1.76 ± 0.01) nm respectively) are consistent with prior literature reports of the individual binary compounds.

This dissertation includes previously published and unpublished co-authored material.