# How to predict if hydrogen atoms could be placed in the interstials of a lattice

Palladium hydride is an example of a hydride which contains H atoms in palladium's crystal lattice (which is fcc).

I would like to know if it's possible to predict if another hydride, $\ce{NbH_{x}}$ could exhibit the same behavior.

I know that in an ionic solid, the fraction $$\frac{r_{+}}{r_{-}}$$ Can be used to predict the geometry around a cation. Can I use this expression for niobium hydride? In the case of niobium, the crystal structure is bcc, so it contains tetrahedral and octahedral interstial sites.

Your intuition is correct; $\ce{NbH_{x}}$ retains the bcc Nb lattice, with the hydrides in the tetrahedral interstital sites, up to x = 2, when the compound switches to a fcc Nb lattice (the Fluorite structure). This is pretty typical behaviour for a transition metal hydride.