taru-adjective, adverb taking the 'to' particle
sweetly smiling; gracious
Literary or formal term describing a beautiful, charming smile, often used in classical or poetic contexts. Functions as a taru-adjective or adverb with と.
彼女は艶然と微笑んだ。
She smiled sweetly.
その絵の女性は嫣然とした表情を浮かべている。
The woman in the painting has a gracious expression.
にこやか is a common, everyday word for a cheerful, smiling demeanor, while 艶然 is literary and specifically evokes a beautiful, alluring smile.
The exact derivation is uncertain; the kanji 艶 (charming, glossy) and 嫣 (beautiful, captivating) both contribute to the sense of a lovely smile.