Because this is elemental nickel, we already know it’s density. For compounds such as ‘SiO2’ we would have to specify an additional density=2.634 parameter.

Common materials defined in materialdb:

air, water, silicon, sapphire, …

Specific elements, molecules or mixtures can be added using the classes in refl1d.material:

SLD unknown material with fittable SLD Material known chemical formula and fittable density Mixture known alloy or mixture with fittable fractions