Hexagonal to cubic phase transition in YH3 under high pressure

X-ray diffraction studies on bulk yttrium trihydride, in a diamond anvil cell, have been carried out up to 25 GPa. Pressure induced hexagonal-to-cubic phase transformation in YH3 has been found at pressure of about 8 GPa. The lattice parameter of the new cubic phase was determined as equal to 5.28 Å. This finding confirms the theoretical predictions based on first principle calculations of such a transformation. Equations of state have been determined for both the hexagonal hcp and cubic fcc YH3 phases. As compared to the pure yttrium metal, bulk modulus for YH3 is about four times bigger. The similarity of this transition to that observed in the other 4-f trivalent hydrides has been discussed.