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

20 nm thick Pd coatings deposited on Si substrates with 800 nm SiO 2 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 Pd 3 H 4 . 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.