<p>We report preferential orientation control
in photochromic gadolinium oxyhydride (GdHO) thin films deposited by a two-step
process. Gadolinium hydride (GdH<sub>2-x</sub>) films were grown by reactive magnetron
sputtering, followed by oxidation in air. The preferential orientation, grain
size, anion concentrations, and photochromic response of the films are strongly
dependent on the deposition pressure. GdHO films show preferential orientation
along the [100] direction and exhibit photochromism when synthesized at
deposition pressures up to 5.8 Pa and. The photochromic contrast is larger than
20 % when the films are deposited below 2.8 Pa with 0.22 H<sub>2</sub>/Ar flow
ratio. We argue that the degree of preferential orientation defines the oxygen
concentration which is known to be a key parameter for photochromism in rare-earth
oxyhydride thin films. The experimental observations described above are
explained by the oxidation-induced decrease of the grain size as a result of
the increase of the deposition pressure of the sputtering gas. </p>
