Χαίρετε!
Dr. Mastronarde’s Introduction to Attic Greek mentions a semivocalic iota here and there. For example, on page 387:
“α. When y is added to dental stems, the result is a stem in ζ, such as
κομίζω from κομιδ- and ἐλπίζω from ἐλπιδ-. Note, however, that
-ίζω and -άζω became productive suffixes in their own right for
denominative verbs, as in νομίζω from νόμος or δικάζω from δίκη.
β. A ζ results similarly for a few other plosive stems, such as ἁρπάζω
from a root that appears in other forms as ἁρπαγ-.
γ. When y is added to velar plosive stems, the combination produces
Attic ττ and non-Attic σσ: e.g., φυλάττω (φυλακ-), πράττω (πραγ-),
τάττω (ταγ-).
δ. When y is added to stems in a liquid or nasal, the combination
produces either a doubling of λ or a compensatory lengthening and
alteration of the vowel preceding ν or ρ: e.g., ἀγγέλλω (ἀγγελ-),
βάλλω (βαλ-), φαίνω (φαν-), κτείνω (κτεν-), φθείρω (φθερ-),
κρῑν- (κρῐν-).”
Was the semivocalic iota an actual letter that was used, or was it more an added sound that certain consonants made when pronounced?