Getting more out of X2T2O7 compounds with thortveitite structure: The bond-valence model

C-M2Si2O7 (M=RE and In) and crystals of composition X2T2O7 (T=P, As, Ge) with ionic radius of X less than 0.97 Å (X=Ni, Cd, Mg, Zn, Cu, Ca) are isostructural with the natural-occurring mineral thortveitite. In those compounds, the T–O bridging distance values vary considerably and there is no explanation to this fact in the literature. This paper will bring their structural characteristics out by the bond-valence model. It has been concluded that T–O bridging distance and mean T–O distance are linearly correlated to the total atomic valence of the bridging oxygen and the T atom (T=Si, P, As, Ge), respectively, and they are a function of the principal quantum number (n) in the valence shell of the atom T. Finally, we have applied successfully the model for the prediction of Ge–O distances of (In,A)2Ge2O7 (A=Fe,Y,Gd) compounds.