Yes, EXPANSIVITY is a Scrabble word!

Letter values in Scrabble US: E 1, X 8, P 3, A 1, N 1, S 1, I 1, V 4, I 1, T 1, Y 4.

Is it a valid word?

EXPANSIVITY is a valid word in the official Scrabble dictionary (US), the UK Scrabble Dictionary and Words with Friends (ENABLE).

the fractional change in length or area or volume per unit change in temperature at a given constant pressure
a quality characterized by magnificence of scale or the tendency to expand
a friendly open trait of a talkative person

Also: coefficient of expansion.