Solution of hydrogen in non-stoichiometric Pd3P1–x compounds

Abstract

The solution of H₂(D₂) in a series of Pd₃P_1–x compounds has been investigated for x between 0 and 0.20. H₂ solubility is significant for compounds with a large degree of non-stoichiometry and negligible for the stoichiometric compound. A lattice parameter anomaly occurs at x= 0.08 for the H-free phosphides; the crystallographic axes expand differently with increase of x above and below the anomaly. H₂ solubility falls to very small values for x values corresponding to those above the anomaly, i.e., with x less than 0.08. The maximum solubility observed is H/Pd₃P_0.80= 0.15 (296 K, 93.3 kPa); there is no indication of hydride phase formation. The lattice expansions accompanying H solution have been measured and these are anisotropic, i.e., the b-axis expands less than half as much as the a and c-axes for all of the Pd₃P_1–x compounds. The volume change per H is smaller than usually observed for solid-H systems.

With the aid of information on H positions derived from a recent neutron diffraction study on deuterated Pd₃P_0.80, a model has been proposed which describes the pressure—H content data over a wide range of H contents. This model is based on H occupation of interstices in two of the quadrilateral faces of trigonal prisms not occupied by central P atoms and the assumption that occupation of one interstice in the prism by H blocks the other interstice in the same prism for H occupation. The relative partial enthalpy (standard) for H₂ solution in Pd₃P_1–x with x= 0.20 is –16.3 kJ(mol H)^–1 and the relative (to H₂ at 1 atm) partial entropy (standard) is –48.7 J(mol H)^–1 K^–1 compared to corresponding values of –9.8 kJ(mol H)^–1 and –56.5 (mol H)^–1 K^–1 for Pd-H.