 Solution

For any indexed cubic reflection,Use ${\lambda }_{\mathrm{Mo}}\simeq 0.07107\mathrm{nm}$ for the unresolved molybdenum $K_{\alpha }$ doublet; the tabulated components are approximately $0.0709300$ and $0.0713590\mathrm{nm}$. These are external spectral constants, not an assertion about the unavailable historical Data Book.^[ref]

Reading broad ring centers in the supplied image more closely gives representative radii $5.96,6.99,10.63,13.23\mathrm{mm}$. Their corresponding estimates are about $0.3471,0.3473,0.3478,0.3480\mathrm{nm}$. Their small scatter understates possible common scale and camera-distance errors. In particular,for a first-order signed perturbation; independent uncertainties are combined in quadrature instead. A reading uncertainty of order $0.2\mathrm{mm}$, plus the printed-scale and setup uncertainty, justifies a deliberately modest precision:Nickel is a plausible identification, since it is FCC with room-temperature edge about $0.352\mathrm{nm}$.^[ref] The source photograph alone does not chemically identify the specimen; nearby elemental lattice constants and the scale uncertainty preclude a rigorous unique assignment. The estimate is not reported with artificial precision inherited from pixel fitting.