## 4 Discussion  
After heating the TiZrv film at 120 \({}^{\circ}\)C for 0.5 h, the metal states of Ti 2p, V 2p and Zr 3d appeared, the intensity of the O 1s peak significantly reduced and metal carbide states of C 1s emerged. Therefore, it is reliable to state that the activation temperature of the TiZrv film is 120 \({}^{\circ}\)C. This is the lowest activation temperature reported for TiZrv. In other words, the TiZrv film can be fully activated by heating at 120 \({}^{\circ}\)C for a longer time. In a similar manner, the activation temperature for Zr and ZrvHf films were found to be 300 \({}^{\circ}\)C and 200 \({}^{\circ}\)C. Comparing to reference [19] (TiZrv film activated at 120 \({}^{\circ}\)C for 0.5 h), and reference [14] (TiZrv film activated at 150 \({}^{\circ}\)C for 1 h), the reduction degree of the TiZrv film found in this work after heating at 120 \({}^{\circ}\)C for 0.5 h is to a higher extent. In addition, there are three phenomena worth noting. The first phenomenon is that as the heating temperature increases, the transformation of metal-C bond follows the order of V-C, Ti-C, Zr-C, Hf-C. The first phenomenon is found for films containing the same element. The temperature required to break the Zr-O bond on Zr, ZrvHf and TiZrv films is about 300 \({}^{\circ}\)C, 200 \({}^{\circ}\)C and 120 \({}^{\circ}\)C, respectively, suggesting that the difficulty of the reduction of Zr oxide in different NEG films follows the order of Zr film > ZrvHf film > TiZrv film. By comparing the content ratios of the V(0), V(sub) and V(V) states on TiZrv and ZrvHf films, the activation of V in TiZrv film is easier than that in ZrvHf film. The third phenomenon is found in the activation of different elements in the same film. The elemental carbon absorbed on the surface of the Zr, ZrvHf and TiZrv films started to transform to metal carbide phase after heating at 300 \({}^{\circ}\)C, 200 \({}^{\circ}\)C and 120 \({}^{\circ}\)C, respectively. On TiZrv film, the metal carbide phase was fully removed after heating at 250 \({}^{\circ}\)C or above. This confirmed the reported order of stabilities of the metallic oxides as follows: ZrO\({}_{2}\) > TiO\({}_{2}\) > V\({}_{2}\)O\({}_{5}\)[34]. Combining the comparison of content ratios of Hf (sub)/Hf (IV) to Zr (sub)/Zr (IV) on ZrvHf film after heating 200 \({}^{\circ}\)C, the complete difficulty of the reduction of oxides follows the order of HfO\({}_{2}\) > ZrO\({}_{2}\) > TiO\({}_{2}\) > V\({}_{2}\)O\({}_{5}\). The above phenomena can be explained by the thermodynamics of materials. The ZrvHf and TiZrv alloy films are substitutional solid solutions. The atomic radii of Ti, Zr, V and Hf elements are 176 pm, 206 pm, 171 pm and 208 pm, respectively. The electronegativities of Ti, Zr, V and Hf elements are 1.38, 1.32, 1.53, 1.16, respectively. Since they possess similar crystal structures, their atomic radius difference is within 15% and their electronegativities are similar, the solid solutions have high solubility. For NEG elements, at a typical temperature of activation, the free energy of formation of the oxide is larger than that of the solid solution per oxygen/carbon atom, so the solid solution phase is favored, that is, during activation, O and C diffuse into the bulk of the films. As the electronegativities of H, C, O are much higher and their atomic radii are much smaller, they diffuse in NEG crystals by interstitial diffusion. The overall diffusion coefficient of H, C, O in single crystals follows the order of V > Ti > Zr > Hf [8]. The difficulty of the reduction can also be explained by analysis of Gibbs free energy of the formation of oxides: \(\Delta G_{T}^{\Theta}=\Delta H_{298}^{\Theta}-T\Delta\Phi_{T}^{\prime}\) in which \(\Delta H_{298}^{\Theta}\) and \(\Delta\Phi_{T}^{\prime}\) is the enthalpy difference and Gibbs free energy of reaction at 298 K as a reference [35]. The order of the stability of these metal oxides is HfO\({}_{2}\) > ZrO\({}_{2}\) > TiO\({}_{2}\) > V\({}_{2}\)O\({}_{5}\). The alloy films prepared by sputtering are nanocrystalline or even amorphous.