Hydrogen in niobium-tantalum superlattices

Abstract

Hydrogen has been dissolved in Nb/Ta superlattices for which the structural and thermodynamic properties of this new H-metal system were studied, in situ, by x-ray diffraction. It is found that H induces a strain modulation exhibiting a Curie-Weiss type temperature dependence, thus, providing a first example of a strained layer superlattice which can be strained after the growth of the structure. In addition, a lattice-gas lattice-liquid phase transition was observed where critical fluctuations exist only for wavelengths longer than the superlattice periodicity. The absence of short wavelength fluctuations prohibit phase separation and constitutes a· novel manifestation of a coherent phase transition. A mean field lattice gas model is presented, which, combined with x-ray data from the superlattice satellites, provides insight to the fundamental properties of a lattice gas in the presence of modulated interactions. Motivated by these experiments, a generalized theoretical framework from which to view coherent phase transitions is discussed. Due to the epitaxial constraints imposed by the substrate on the film, the lattice expansion due to H occurs only normal to the plane of the film, up to a maximum

lattice expansion of Delta d / d ~ .016, and beyond which, precipitation of oriented phases occurs. Also observed is an enhanced solubility for H where it takes two orders of magnitude less gas pressure to obtain the same H concentration as compared to bulk Nb. It is demonstrated that this enhancement is directly related to the one dimensional lattice expansion.