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Ordered Hydrides of Rare-Earths and Their Alloys

#G-713


Hydrides of Rare-Earth Metals and Their Alloys. Influence of Hydrogen Ordering on Physical Properties

Tech Area / Field

  • MAT-ALL/High Performance Metals and Alloys/Materials
  • PHY-SSP/Solid State Physics/Physics

Status
3 Approved without Funding

Registration date
23.04.2001

Leading Institute
Tbilisi State University / Institute of Physics (Ge), Georgia, Tbilisi

Collaborators

  • CNRS / Ecole Polytechnique / Laboratoire des Solides Irradies, France, Palaiseau\nUS Department of Commerce / National Institute of Standards and Technology, USA, MD, Gaithersburg\nFlorida State University / National High Magnetic Field Laboratory, USA, FL, Tallahassee

Project summary

In a number of rare-earth superstoichiometric dihydrides RH2+c (R – metal atom, H – hydrogen atom, 0<c< 1), which are technically significant and scientifically very interesting compounds, the subsystem of hydrogen atoms undergoes a disorder-order transition accompanied by the changes of mechanical, electric and magnetic properties of the compound. As it was established, in different RH2+c hydrides (R = La, Ce, Tb) the hydrogen superstructures are described by the identical distribution functions containing two long-range-order (LRO) parameters.

Theoretical consideration of the two-parameter ordering processes has shown that they are sufficiently complicated and reveal the following specific features:

– a stepped character of the ordering process;


– existence of two different equilibrium low-temperature ordered configurations;
– changes of the type of disorder-order and order-order transitions;
– different sequences of equilibrium ordered states (different shapes of trajectories of ordering processes in the plane of order parameters) depending upon the values of two main parameters of the system – hydrogen concentration c and the energy parameter p (defined as the ratio of Fourier-components of the H-H interaction potential).

Due to the latter point, in different RH2+c compounds the ordering processes follow different scenario (in spite of the identical distribution functions).

In our knowledge of physical properties of rare-earth dihydrides RH2+c there exists a certain gap associated with the two-parameter character of hydrogen ordering. Within the frames of the Project we intend to fill this gap using several methods of experimental and theoretical physics. Particularly, to obtain the actual information the calorimetric, internal friction, gravimetric, nuclear magnetic resonance and X-ray measurements will be performed, while to describe the sequences of equilibrium configurations of different subsystems in RH2+c compounds the method of numerical solutions of equilibrium equations defining the free-energy minimum will be applied.

The given investigation can be attributed to the category of basic research.

The key problems considered in the project can be formulated as follows:

1. An attempt to observe for the first-time the specific characteristics of the ordering systems with two LRO-parameters in the case of superstoichiometric dihydrides of rare-earth metals.

2. Determination of the hydrogen ordering influence on the properties of the host lattice and other subsystems of the compound.

3. Examination of analogous phenomena in the hydrides of rare-earth alloys.

Consideration of the mentioned problems is separated into five tasks, among which four are experimental tasks: "Preliminary (control) measurements", "Investigation of the step-character of hydrogen ordering in a number of hydrides RH2+c (R = La, Ce, Lay Ce1-y)", "Determination of phase transition type change points in the case of hydrogen ordering in compounds RH2+c (R = La, Ce, Lay Ce1-y)", "Metal-insulator transitions in the dihydrides of rare-earth metals and their alloys", and one is a theoretical task – "Thermodynamic description of the equilibrium configurations in dihydrides of rare-earth metals and their alloys".

Thus, the given investigation proposes to explain the role of the hydrogen ordering in the behavior of RH2+c compounds and to predict their main associated properties both for hydrides of different R-metals and of their alloys, that will be very significant for technical applications of the mentioned compounds.

The main results of the Project successful completion, coming from the Instructions for Proposal Preparation concerning a basic research, can be classified in the following categories.

New knowledge expected from the Project.

– correction of calculated (c, T) phase diagrams for compounds LaH2+c and CeH2+c, tentative(c,T) phase diagrams for a number of compounds Lay Ce1-y H2+c, features of a three-dimensional (c, y, T) phase diagram for compounds Lay Ce1-y H2+c;


– the response of different subsystems of RH2+c compounds (R = La, Ce, Lay Ce1-y) on the hydrogen ordering processes.
– properties of RH2+c compounds in the critical points of phase diagrams, particularly, in the phase transition type change points and in the vicinity of metal-insulator transition;
– dependence of the main energy parameter p in Lay Ce1-y H2+c compounds on the hydrogen concentration c and on the composition of the host lattice y. (Estimation of the function p(c, y)).

Influence of the Project on the future development in the field.

In the Project is established a set of system's parameters characterizing the ordering process in RH2+c compounds (the main energy parameter p, concentration c, a system of temperature parameters) and there are introduced the criteria to estimate the hydrogen ordering influence on other subsystems of the compound, that is very important for the subsequent practical applications. The elaborated scheme of description of RH2+c compounds can be applied to other M-H systems.

Successful project completion will stimulate systematic investigations of M-H compounds based on the alloys of different rare-earth metals.

Realization of the Project tasks is mainly based on the efforts of scientists of Metal Physics Laboratory (former Department of Irradiated Materials Low-Temperature Investigations) of E.Andronikashvili Institute of Physics, specialists in the fields of Solid State Physics, Theoretical Physics, Radiation Effects, Helium Temperatures, Nuclear Magnetic Resonance and X-Ray measurements, etc. Metal-Hydrogen compounds were one of the most investigated systems in the Metal Physics Laboratory. The Project team mainly consists of weapon-specialists and the financial support within the frames of the Project will stimulate their incorporation in the world-wide peaceful activity. The Project gives possibilities to revive the experimental investigations in the Laboratory and to drive them into the direction of basic research of M-H compounds which are recognized as advanced energy storage materials and have as well a number of other very interesting applications (e.g. the "switchable mirrors").

We hope that the experience of the Project team members and an intensive collaboration with our foreign colleagues, the leader specialists in the field of metal-hydrogen physics, will provide the highest degree of the planed investigations and will give fruitful results.


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