The crystal chemistry of large cation dichromates, pyrophosphates, and related compounds with stoichiometry X2Y2O7

Crystals of composition X2Y2O7 in which the ionic radius of X is greater than 0.97 Å and that of Y is smaller than 0.60 Å crystallize in one of the dozen or so structures whose interrelationships are examined in this paper. A scheme is proposed for classifying the different structures and predicting the possible existence of other structures. It also predicts that the phase transition at 270°C in K2Cr2O7 is a displacive transition. The effect of crystal environment on the geometry of the Y2O7 group is examined.