Restricting the palladium cap layer thickness to =<9nm on top of 170nm Pr films during in situ hydrogen loading has been shown to result in nanocrystallite size PrH 3 - δ films even though the deposited Pr films are of large crystallite size. The effect is attributed to hydrogen-induced stresses in the PrH 3 - δ films, which trigger structural rearrangement. These nanocrystalline films show a blue shift of the transmittance edge with respect to PrH 3 - δ films of large crystallite size. The approximate size of the nanocrystallites calculated from the blue shift using an effective mass approximation (EMA) theory is supported by XRD, TEM and AFM measurements.