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](http://www.chemtube3d.com/solidstate/_fluorite%28final%29.htm)). This is pretty typical behaviour for a transition metal hydride. 


I don't think radius ratio rules are going to be useful in predicting the behaviour of these systems; there is a lot of covalency going on (hydride-hydride interactions and Nb-Nb interactions), and the metal lattices will end up being distorted by the correlated interstital occupancies of the hydrides. Hydride has a radius of around 130 pm while Niobium has a radius of around 150 pm so you wouldn't predict the observed behaviour based on radius ratios. 


More information on this particular system can be found in [Hauck (1976), Ordering of Hydrogen in Niobium Hydride phases](http://journals.iucr.org/a/issues/1977/01/00/a14075/a14075.pdf).


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