International Journal of Hydrogen Energy 31 (2006) 217 – 222 www.elsevier.com/locate/ijhydene Hydrogen-induced nonmonotonic hopping structure evolution in Pd-based alloys V.M.Avdyukhina a , ∗ , A.A. Anishchenko a , A.A. Katsnelson a , I.A. Lubashevsky a , A.I. Olemskoi b , G.P. Revkevich a a Physics Faculty, Moscow State University, Moscow 119992, Russia b Sumy State University, Sumy 244000, Ukraine Available online 1 July 2005 Abstract We show that the nonmonotonic discrete (“hopping”) structure evolution takes place in nonequilibrium Pd-based alloys after hydrogen charging. It can be explained by assuming the thermodynamic potential to be of multivalley form. The states of the system corresponding to these valley minima are long-lived states. The system hops between the long-lived states during the structural evolution due to the cooperative migration of hydrogen and vacancies between matrix and defect regions. The experimental results show that such jumps can cause the lattice period to increase or decrease.  2005 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved. Keywords: Structure evolution; Nonequilibrium Pd-based alloys; Effect of hydrogen Structure transformations in Pd-based alloys after being charged with hydrogen can exhibit nonmonotonic dynamics during relaxation (see Refs. [1,2] and references therein). Moreover, multiphase decomposition was observed in me- chanically strained Pd–Ta–H and Pd–Mo–H alloys just af- ter hydrogenation and the following long time relaxation is characterized by stochastic variations in the relative volume of the corresponding phases. To explain these phenomena a synergetic model was developed [3]. It is based on the assumption that the alloy thermodynamic potential is of a multivalley form, i.e. in the reciprocal space this thermody- namic potential possesses many local minima separated by potential barriers (Fig. 1) [4,5]. Under this assumption the structure evolution has to proceed via a stepwise dynamics, the system will jump between long-lived states matching these minima. For the first time such a discrete structure ∗ Corresponding author. E-mail address: valentina@solst.phys.msu.ru (V.M. Avdyukhina). 0360-3199/$30.00  2005 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2005.04.043 evolution observed experimentally was described in [2,6]. The purpose of the present paper is to study its characteristic features. To be specific, we consider the structure evolution in Pd-based alloys just after hydrogenation of mechanically strained specimens as well as after repeated charge with hydrogen. In this way, we clarify the factors governing the discrete structure evolution, which have to be taken into account in developing the appropriate mathematical models. Monochromatic Cu–K 1 irradiation was used to study specimens of Pd-based alloys with Mo or Ta solutes by the precision X-ray diffraction techniques. The details of preparing the specimens, measuring, and the experimental data analyses can be found in Refs. [1,2]. The X-ray diffrac- tion patterns obtained for Pd–Mo and Pd–Ta specimens, just after hydrogen charging as well as during the further re- laxation, turn out to be more complex than that of these specimens at the initial state. The latter feature means that the hydrogen charging induces multiphase decomposition, which gives rise to a complex phase structure of these alloys whose further relaxation is nonmonotonic in time.