I'm not sure if that's a right word. I'm not a linguist, and could not find much, but Polish wikipedia does not use Polish ia, ie io as example of "dyftong". Maybe because "i" is audible, but sounds like Polish j, and a is like any other a.
I think ia, io, ie are always either "softening previous consonant", "or "sound like j", but I can't think of an example with two syllables. I guess we would change spelling to ija, or -yja.
That is how we spell Maria.
Maria [Mar-ja] is a girls name
Maryja [Ma-ry-ja] is the Mother of Jesus.