  The pressure response of the scheelite phase of CaWO4 (YLiF4) and the
occurrence of the pressure induced scheelite-to-wolframite (M-fergusonite)
transition are reviewed and discussed. It is shown that the change of the axial
parameters under compression is related with the different pressure dependence
of the W-O (Li-F) and Ca-O (Y-F) interatomic bonds. Phase transition mechanisms
for both compounds are proposed. Furthermore, a systematic study of the phase
transition in 16 different scheelite ABX4 compounds indicates that the
transition pressure increases as the packing ratio of the anionic BX4 units
around the A cations increases.
