OK sorry. I had forgotten about nitrides because although they exist and are useful they do not seem to come into ordinary chemistry much.
I had had in mind to say you can manage these days to put the most unexpected combinations of atoms together violating traditional chemistry intuition but then I thought such chemistry would be irrelevant for you as it is for most of us.
It seems I was not far wrong for sodium nitride. It seems to be a laboratory rarity - extreme rarity. I now read that it is synthesised using atomic beams and deposited on a sapphire surface, which mean that the merest smidgens of it have ever been made and this nitride is said to be very unstable. I don't know that the reaction you mention has ever been observed, sodium azide can be made to decompose just to nitrogen and sodium which of course will soon react with something else. (In airbags).
I found this: "Na3N—An Original Synthetic Route for a Long Sought After Binary Nitride (pages 1701–1702)
The game of hide and seek is over: The existence of a binary sodium nitride Na3N (see structure) has been the subject of speculation for a long time. Recently, through a novel experimental method, the synthesis and structural characterization of this compound has been successful. The compound crystallizes in the unexpected anti-Re3O structure type.
http://onlinelibrary.wiley.com/doi/10.1002/1521-3773%2820020517%2941:10%3C%3E1.0.CO;2-F/issuetoc"[/I] [Broken] So it looks to be a solid which I would expect and the crystal structure has even been determined.
Quite a recent discovery, 2002. Chemistry teachers usually discourage you from following up oddballs and rarities as you have to know the things you are likely to meet which is hard enough. You are not very likely to end up doing any chemistry that requires sapphire for instance. But knowing about such splendid obscurities has a horrible fascination and is motivating if you are really attracted by chemistry. I've learned something now from this.