Abstract
LaF3/SrF2 multilayer heterostructures with thicknesses of individual layers in the range 5–100 nm have been grown on MgO(100) substrates using molecular beam epitaxy. The longitudinal conductivity of the films has been measured using impedance spectroscopy in the frequency range 10−1–106 Hz and a temperature range 300–570 K. The ionic DC conductivities have been determined from Nyquist impedance diagrams and activation energies from the Arrhenius–Frenkel equation. An increase of the DC conductivity has been observed to accompany decreased layer thickness for various thicknesses as small as 25 nm. The greatest conductivity has been shown for a multilayer heterostructure having thicknesses of 25 nm per layer. The structure has a conductivity two orders of magnitude greater than pure LaF3 bulk material. The increasing conductivity can be understood as a redistribution of charge carriers through the interface due to differing chemical potentials of the materials, by strong lattice-constant mismatch, and/or by formation of a solid La1-xSrxF3-x solution at the interface during the growth process.
Highlights
Combinations of MF2 and RF3 fluorides (M- alkalineearth and R- rare-earth elements) are perspective materials that demonstrate high ionic conductivity.[1]
Films were grown on epi-ready MgO(100) substrates by the MTI company (Richmond, USA) using the molecular beam epitaxy (MBE) method in an ultra-high vacuum chamber equipped with reflection high-energy electron diffraction (RHEED)
X-ray diffraction (XRD) pattern obtained by angular integration of the maps recorded in the (HKL) reciprocal space presented in Figure 1, left panel
Summary
Combinations of MF2 and RF3 fluorides (M- alkalineearth and R- rare-earth elements) are perspective materials that demonstrate high ionic conductivity.[1] Growing of superionic materials by molecular beam epitaxy (MBE) allows creation of composite materials with defined thicknesses and physical properties; this is useful for decreasing power consumption of devices,[2] and for cardinally varying physical properties of materials Based on this growth technique, fluoride sensors,[3] oxygen sensors,[4] batteries,[5] and transistors [6] have been proposed. We expect that the production of heterostructures based on fluoride materials with different LaF3/SrF2 crystal structure could be interesting for applications and as a subject for fundamental studies Such structures may demonstrate a greater increase of conductivity as a function of layer thickness than BaF2/CaF2 heterostructures with consideration of the layer thickness
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