If it is a typical spinel, it crystalises in the space group Fd-3m (#227), so it is cubic and the general reflection conditions for this space group are:

hkl: h+k=2n, h+l,k+l=2n 0kl: k+l=4n, k,l=2n hhl: h+l=2n h00: h=4n

Start with diffraction peak of lowest angle and lowest hkl,

e.g. 111 and try to index all the diffraction line upon increasing theta.

Of course there is software for indexing available.

Be careful (your question title): an XRD pattern is not a spectrum!

Hi,

Just to add the suggestion given by Mr. Lengauer above, there are some softwares suit for doing indexing/phase identification work, including High Score Plus, etc which is usually integrated with X-ray diffractometer plant. Some are free, e.g. Match! Try this link www.crystalimpact.com/match/

Hope this help.

Frida is completely right. If you don't find a software, here are the first (hkl) triples according to the rule given in my previous answer:

(111), (220), (311), (222), (400), (331), (422), (333), (511), (440), ...

Spinel phases have a large lattice parameter "a" on the order of e.g. 8.32 Angstrom. With this "a" you'll find the first peak (111) already at 18.66° (2-theta) if you use Cu-K-alpha radiation.