periodical vs half-yearly

periodical is a noun but half-yearly is not a noun.

Word	Noun	Adjective	Verb	Adverb
periodical	Yes	Yes	No	No
half-yearly	No	Yes	No	Yes

periodical (noun)	half-yearly (noun)
a publication that appears at fixed intervals

periodical (adjective)	half-yearly (adjective)
happening or recurring at regular intervals	occurring or payable twice each year

periodical (adverb)	half-yearly (adverb)
	every half year, every six months