The increase of ionicity results in the decrease of CrO bond length [27]. Thus, the decreasing
bond length of CrO (a) for the 0-Pa film sample leads to an increasing crystal field. On the other hand, the decreasing ligand charge (Z) due to the formation of oxygen vacancies decreases the crystal field [28]. Based on the above analyses, we conclude that these two effects tend to cancel each other for the crystal field and thus the crystal field strength parameter 10Dq does not change. In Fig. 6(a), the PL spectra of films show a broad band from 650 to 750 nm, with a red emission maximum at 694 nm, which is consistent with that reported for ZnGa2O4:Cr3+ thin film phosphors by a sol–gel process [7].