As AD&D's Phoenix never dies, can you ever get experience points for defeating it?
The Monster Manual II (1982) phoenix can, in fact, die normally from, for instance, attacks by +3 or better magic weapons or, instead, can suicide in "a double-strength (40th level) combination of fire storm (2" high × 5" wide × 8" deep) and incendiary cloud.... This destroys the phoenix but leaves a gem-like egg behind from which a new phoenix will arise in 20 days" (101). Participants gain normal experience points for slaying that phoenix.
However, the Deities and Demigods (1980) phoenix has the entry "Level/X. P. Value: IX/not applicable" (47-8), so, according to that text, slaying that phoenix garners no experience points. Whether this was the topic of great debate at the TSR offices or someone's idea of a joke, we'll probably never know.
Defeating doesn't have to mean killing (even though it often ends up that way).
If you can defeat a monster in a non-lethal way, then you still get XP for it.