Formation of palladium hydrides in low temperature Ar/H₂-plasma

20 nm thick Pd coatings deposited on Si substrates with 800 nm SiO₂ and 1 nm Cr buffer layers were treated in a 2.45 GHz microwave plasma source at 700 W plasma power and 40 Pa working pressure without substrate heating. For obtaining information on the effect of energy influx due to ion energy on the palladium films the substrate potential was varied from U_sub = 0 V to − 150 V at constant gas flow corresponding to mean ion energies E_i from 0.22 eV ∙ cm⁻² ∙ s⁻¹ to 1.28 eV ∙ cm⁻² ∙ s⁻¹. In contrast to high pressure reactions with metallic Pd, under plasma exposure we do not observe solid solutions over a wide range of hydrogen concentration. The hydrogen incorporation in Pd films takes place discontinuously. At 0 V substrate voltage palladium hydride is formed in two steps to PdH_0.14 and PdH_0.57. At − 50 V substrate voltage PdH_0.57 is formed directly. However, substrate voltages of − 100 V and − 150 V cause shrinking of the unit cell. We postulate the formation of two fcc vacancy palladium hydride clusters PdH_Vac(I) and PdH_Vac(II). Under longtime plasma exposure the fcc PdH_Vac(II) phase forms cubic PdH_1.33. The fcc PdH_0.57 phase decomposes at temperatures > 300 °C to form metallic fcc Pd. The hydrogen removal causes a decrease of lattice defects. In situ high temperature diffractometry measurements also confirm the existence of PdH_Vac(II) as a palladium hydride phase. Stoichiometric relationship between cubic PdH_1.33 and fcc PdH_Vac(II) becomes evident from XR measurements and structure considerations. We assume both phases have the chemical composition Pd₃H₄. Up to 700 °C we observe phase transformation between both the fcc PdH_Vac(II) and cubic PdH_1.33 phases. These phase transformations could be explained analog to a Bain distortion by displacive solid state structural changes. - Highlights: • Thin Pd films were treated under low pressure conditions by an Ar/H₂-plasma. • The formation of Pd hydrides depends on the substrate voltage and the plasma exposition time. • Substrate voltages up to − 50 V cause a lattice dilatation in the fcc Pd unit cell. • Higher biasing conditions cause a lattice shrinking of the fcc Pd structure. • Under longtime plasma exposure the fcc PdH_Vac phases convert into cubic PdH_1.33.