## 3 Results
#### 3.1.2 Crystal Structure  
The alloy's crystal structure was investigated by X-ray powder diffraction. Diffraction patterns of co-melt Ti\({}_{1.56}\)V\({}_{0.18}\)(FeV)\({}_{0.18}\)Cr\({}_{1.08}\) + 4%Zr\({}_{7}\)Ni\({}_{10}\) taken in as cast, hydrogenated and dehydrogenated states are shown in Figure 3. The as-cast pattern only shows the BCC structure and there is no evidence of a secondary phase. As seen in Figures 1 and 2, the proportion of secondary phase is quite small, thus making it harder to identify by X-ray diffraction. Also, it is clear from the atomic compositions of both phases presented in Table 1 that both BCC alloys have an almost similar bulk composition. Therefore, even if the crystal structure of the secondary phase remains unknown, there is a strong probability that it exhibits the same crystal structure as the matrix phase. According to our calculations that are based on the diameter of every element provided by the Shannon's Table, the average metallic radii in both the matrix and the secondary phase were estimated to be, respectively, 0.1392 nm and 0.1401 nm [23]. This means that, if the matrix and secondary phase have both a BCC structure and if one of them has a much lower abundance than the other, then it may be effectively impossible to distinguish them using the powder X-ray diffraction technique. Assuming the same crystal structure and the average metallic radius, we calculated a shift in the Bragg's peaks of only 0.01\({}^{\circ}\) at 42\({}^{\circ}\) and 0.1\({}^{\circ}\) at 104\({}^{\circ}\). As the secondary phase abundance is much lower than the main phase, its peak will be effectively 'buried' under the main phase peaks.  
Figure 2: Linear chemical analysis of the as-cast co-melt Ti\({}_{1.56}\)V\({}_{0.18}\)(FeV)\({}_{0.18}\)Cr\({}_{1.08}\) + 4% Zr\({}_{7}\)Ni\({}_{10}\)alloy; lines positions are indicated on the backscattered electrons (BSE) SEM micrograph.  
Rietveld refinements were performed on the three patterns and results are shown in Table 2. The values that are reported in this table are essentially identical to the one reported in our previous investigation on the alloy Ti\({}_{1.56}\)V\({}_{0.36}\)Cr\({}_{1.08}\) + 4%Zr\({}_{7}\)Ni\({}_{10}\)[21]. The main discrepancies are the slightly less important reduction of crystallite size upon dehydrogenation and a larger increase of micro-strain after hydrogenation. Nevertheless, it seems that the partial replacement of vanadium has no important effect on the crystal structure of the BCC alloy.  
Rietveld refinements of the patterns that are displayed in Figure 3 also enable us to estimate the stoichiometry of the hydride sample. From the lattice parameters, the formula unit volume was calculated to be 14.89 A\({}^{3}\) for the BCC structure of the as-cast sample. Regarding its Face Centred Cubic (FCC) hydride counterpart, a volume of 20.24 A\({}^{3}\) was estimated, corresponding to an expansion of 5.35 A\({}^{3}\) per formula unit. Using the [PERSON]'s criteria claiming a volume change comprised between 2 and 3 A\({}^{3}\) for each hydrogen atom absorbed, the cell structure expansion is consistent with a dihydride FCC phase (Ti\({}_{1.56}\)V\({}_{0.36}\)Cr\({}_{1.08}\))H\({}_{2}\).