Thermodynamic Aspects of the Interaction of Hydrogen with R2Fe17 (R = Gd, Dy, Lu)

The interaction of magnetic materials R2Fe17 (R = Gd, Dy, Lu) with hydrogen at 473 and 523 K and hydrogen pressure up to 5 MPa was studied by the Calvet calorimetry method. The dependences P = f(C) and ∆H = f(C) were obtained (P is the equilibrium pressure of hydrogen, ∆H is the enthalpy of absorption/desorption, C is the concentration of hydrogen in an intermetallic compound (IMC), C = H/IMC). It has been established that the enthalpy values of the reaction of hydrogen with IMC change with changes in the process temperature, the concentration of hydrogen in the metal matrix, as well as during the transition from one compound to another. The graphs of the dependence ∆H = f(C) show two sections where the enthalpy values are constant. It is assumed that the observed changes in the thermodynamic parameters of the process are related to the filling order of octahedral interstices 9e and tetrahedral interstices 18g, and the filling process of octahedral interstices 9e occurs in two stages with different energy characteristics.