ionicity

The change in the coordination increases the ionicity of the Si-O bond.

You may wish to change the premise of your experiment just a little bit: i.e., "freezing point depression vs. degree of ionicity" or some such.

In the Coulson-Fischer description, orbitals used for bonding utilize a ligand orbital component to account for the ionicity of the bond.

Phillips invented the exact theory of the ionicity of chemical bonding in semiconductors, as well as new theories of compacted networks (including glasses, high temperature superconductors, and proteins).

PP culminated in his "exact" dielectric ionicity theory (1968), which still is the only theory to improve on the previously best ionicity theory of Linus Pauling.

Some compounds with intermediate ionic-covalent character like boron trifluoride, silicon tetrafluoride, boron trioxide and silicon dioxide do not have a full octet valence shell due to their high ionicity.

In such cases, they can still be drawn with full octets as the electron-deficiency on the central atom is not due to a lack of formal bonding interactions but due to the high ionicity of the bonds.

It was here especially that Pauling's electronegativity concept was particularly useful; the electronegativity difference between a pair of atoms will be the surest predictor of the degree of ionicity of the bond.