Quasi-thermodynamic model on hydride formation in palladium–hydrogen thin films: Impact of elastic and microstructural constraints

The impact of elastic and microstructural constraints on structural phase transitions is investigated by using (10–300) nm Pd–H films of different microstructures. Hydrogen-induced stress mainly arises from the film's adhesion to a substrate. Stress changes the hydrogens' chemical potential μH, modifying the hydride phase stability. Microstructural constraints channel stress release in films. A thermodynamic model is proposed to deduce the H–H interaction energy EHH and an effective critical temperature Tceff of hydride formation in films. It allows for occasionally observed sloped plateaus of μH below Tceff. EHH (between 15 and 30 kJ/molH) and Tceff (340 K to 490 K) are reduced by up to 50% compared to bulk (EHH=36.8kJ/molH, Tc = 563 K), for all films. Concentration-dependent contributions of substrate-induced stress (of about (2–5) kJ/molH) and microstructure (of about (5–8) kJ/molH) are separated. For all films phase separation is still found at 300 K. •Hydride formation in adhered Pd–H films is investigated.•Stress and microstructure systematically destabilize hydride phase compared to bulk.•Chemical potential-concentration isotherms may reveal sloped plateaus below Tc.•A model is proposed accommodating for modified thermodynamic conditions.•Attractive H–H interaction strength and effective critical temperature Tc of films are deduced.