@murrayjames ，

呲cī only has one sound. zī牙 is a different character 龇 - They are interchangeable.

呲： to reprimand; to rebuke.常用儿化音 － 呲儿 i.e. 我呲儿了她两句她就哭了。

龇： to show; to bare. 龇牙咧嘴 to show one's teeth; to look ferocious.

So from the meanings of these two characters, we can see龇zī牙is officially correct;), but 呲cī牙 is also commonly used.

Best,

Grace