Phase transformation in La(Co_xNi_5-x)-H systems (x = 2, 3, 5) studied by in situ X-ray diffraction

The phase transformation in the La(Co_xNi_5-x)-H systems (x = 2, 3, 5) were investigated using in situ X-ray diffraction (XRD). La(Co_xNi_5-x) (x = 2, 3) formed α, β, γ and δ hydride phases with increasing hydrogen content while LaCo ₅ formed only α, β and γ hydride phases. The metal lattice of each phase is hexagonal in P6/mmm symmetry (α), orthorhombic in Cmmm symmetry (β) and orthorhombic in Im2m symmetry (γ), commonly observed for x = 2, 3, 5. The δ phase observed for x = 2, 3 has a hexagonal structure with P6/mmm symmetry, which was characterized for the first time in this study. The lattice expansion accompanied by the phase transformation was anisotropic for Co-rich compositions (x ≥ 2) compared with Ni-rich compositions (x = 0, 1). The lattice parameters changed with increasing hydrogen solution in the γ and δ single-phase regions for x = 2 and 3. Anisotropic lattice strain related with dense dislocations was not introduced through formation of any hydride phases. The isotropic strain increased upon formation of the δ phase but disappeared in subsequent phase transformation.