Experimental study of the critical-point behaviour of the hydrogen in palladium system. I. Lattice gas aspects

Abstract

For the hydrogen in palladium (PdH) system, the critical point has been located at T_c=566+or-1K, rho _c=0.29+or-0.01 atomic H/Pd ratio, P_c=19.89+or-0.05 atm (hydrogen pressure surrounding the palladium). Critical-point exponents delta , gamma and Delta have been determined for the critical isotherm, the isothermal compressibility of the hydrogen lattice gas (this being proportional to the relaxation strength of the elastic after-effect for the interstitial hydrogen in the palladium) and the relaxation time of the elastic after-effect, respectively. The experimental values of these exponents ( delta =3.2+or-0.3, gamma =1.01+or-0.01, Delta =1.01+or-0.1) and information from the specific heat measurements indicate that the critical-point behaviour of the PdH system is described by mean-field theory at least for epsilon =(T-T_c)/T_c approximately=10^-3. This behaviour is compatible with the interaction between the interstitial hydrogens being a long-range elastic type between lattice defects in the palladium.