Atomic disorder of Li_0.5Ni_0.5O thin films caused by Li doping: estimation from X-ray Debye-Waller factors

Cubic type room-temperature (RT) epitaxial Li_0.5Ni_0.5O and NiO thin films with [111] orientation grown on ultra-smooth sapphire (0001) substrates were examined using synchrotron-based thin-film X-ray diffraction. The 1[Formula: see text]1 and 2[Formula: see text]2 rocking curves including six respective equivalent reflections of the Li_0.5Ni_0.5O and NiO thin films were recorded. The RT B₁ factor, which appears in the Debye-Waller factor, of a cubic Li_0.5Ni_0.5O thin film was estimated to be 1.8 (4) Ų from its 1[Formula: see text]1 and 2[Formula: see text]2 reflections, even though the Debye model was originally derived on the basis of one cubic element. The corresponding Debye temperature is 281 (39) K. Furthermore, the B₂ factor in the pseudo-Debye-Waller factor is proposed. This parameter, which is evaluated using one reflection, was also determined for the Li_0.5Ni_0.5O thin film by treating Li_0.5Ni_0.5O and NiO as ideal NaCl crystal structures. A structural parameter for the atomic disorder is introduced and evaluated. This parameter includes the combined effects of thermal vibration, interstitial atoms and defects caused by Li doping using the two Debye-Waller factors.