First, the influence of pO2 on the chemical interactions during initial growth was investigated. Figure 1a shows XPS spectra of 6 nm YSZ films grown at different pO2, while the total pressure and laser repetition rate were kept constant at respectively 2*10−2 mbar Ar and 14 Hz. Zr silicide formation was clearly observed when a pO2 of 1*10−6 mbar was used, as concluded from the existence of Zr0 peaks together with a shoulder at the low binding energy side of the Si2p bulk peak25. The intensities of these features were lower at 1*10−5 mbar, and were completely absent when pO2 of 1*10−4 mbar or higher were used. A similar change was obtained by changing the flux of Zr and Y using different laser repetition rates. The XPS spectra in Fig. 1b show that silicide formation decreased when the laser repetition rate was decreased from 28 to 14 Hz at a constant pO2 of 1*10−5 mbar, whereas no silicide formation was detected anymore at 7 Hz. Thus, the formation of Zr silicides can be controlled by tuning the ratio between flux of oxygen from the background gas and Zr and Y from the plasma. Although similar trends can be expected for Y, the changes in binding energies are too small to clearly identify the different phases (see Supplementary Fig. S1).Figure 1(a) XPS Zr3d and Si2p spectra of films grown at total pressures of 2*10−2 mbar at (a) different pO2 or (b) with different laser repetition rates. A low flux of oxygen compared to Zr, caused by low pO2 or high laser repetition rate, led to silicide formation and an increased ratio of silicate/SiOx to SiO2 bonds.