High pressure phase transitions in scheelite structured fluoride: ErLiF₄

Our synchrotron based angle dispersive x-ray diffraction studies on scheelite structured ErLiF₄ show that it undergoes two phase transitions, at ~11.5 and ~15.5 GPa to lower symmetry monoclinic phases, before becoming (irreversibly) amorphous at ~28 GPa. The first high pressure phase transformation to the fergusonite structure (space group I2/a) is found to be of thermodynamically second order. The second high pressure phase could be fitted to the P2/c space group, but detailed analysis rules out the wolframite structure (P2/c space group), common to many scheelite compounds under high pressures. We also suggest that despite the ionic character of the LiF₄ tetrahedra, the compressibility of LnLiF₄ (Ln=Eu–Lu) kind of scheelites is more affected by the LnF₈ dodecahedra than the LiF₄ tetrahedra. - Graphical abstract: Volume per formula unit of the scheelite and high pressure phases of ErLiF₄ as a function of pressure. - Highlights: • ErLiF₄ transforms to fergusonite and P2/c phase at high pressure. • Polyhedra of LnF₈ affects compressibility of LnLiF₄ (Ln=Eu–Lu) more than LiF₄. • Amorphization pressure varies inversely in LnLiF₄ with ionic size of Ln cation. • In ErLiF₄a/c ratio reduces with pressure in contrast to reported increase in YLiF₄