Anomalous lattice expansion of metal-hydrogen thin films
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We report x-ray diffraction experiments investigating the lattice expansion due to hydrogen in Nb/Ta superlattices and thin Nb films. For low hydrogen concentrations (cH < 0.09 H/metal), the lattice expands only normal to the plane of the film due to the epitaxial constraints imposed by the substrate. At higher concentrations, corresponding to strains on the order of the critical strains expected for the formation of misfit dislocations, we find a structural instability which leads to the formation of two metal-H phases, one of which has a lattice expansion in the plane of the film. These results differ significantly from the expected phase diagram and properties of the bulk metal-H systems.
i. INTRODUCTION
Since studies of the physical properties of materials often employ thin films, it is essential to understand what role the substrate plays in determining film properties. In particular, we expect that physical properties which are sensitive to displacements of the lattice sites will be most affected by the substrate-film interaction. Such effects have been the subject of recent experimental and theoretical interest.1"3 Because hydrogen-metal systems exhibit a rich variety of critical phenomena4 associated with the lattice expansion due to the interstitial hydrogen, properties of thin metal-H films should echo the effects of the interaction of the film with the substrate. Studies of the thermodynamics of H in metallic superlattices have been reported previously.5"8 In this work, we report x-ray scattering studies on the lattice expansion of Nb/Ta superlattices and thin Nb films containing interstitially dissolved hydrogen. We find that the lattice expansion due to H is strongly influenced by the substrate and thus, the H-metal phase diagram is dramatically affected. II. EXPERIMENTAL
The Nb/Ta superlattices and thin films were grown by molecular beam epitaxial methods (MBE) along the [110] direction on [1120] sapphire substrates. More details about the growth procedure are provided in Ref. 9. Both Nb and Ta have body-centered cubic lattices with nearly identical lattice parameters (3.302 A and 3.306 A, respectively), providing a negligible intrinsic strain in the superlattice. The thermal expansion coefficients of both constituents are similar and are
III. EXPERIMENTAL RESULTS
a)
Current address: Bell Communications Research, Red Bank, New Jersey 07701. b) Current address: Fakultat fur Physik und Astronomie, Ruhr Universitat Bochum, 4630 Bochum 1, Germany. 964
also close to that of the substrate, such that thermal strain plays no role upon cooling the superlattices from their growth temperature. High quality crystalline films are obtained which exhibit x-ray mosaic widths of ^0.1 degrees, typical of metallic crystals, and structural coherence lengths equal to the film thickness perpendicular to the film, while being much longer in the plane of the film. The x-ray scattering results were obtained on a double-axis x-ray spectrometer using a Si(lll) monochromator with MoKal radiation. In-plane scans were perfo
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