Equation of state of TiH₂ up to 90 GPa: A synchrotron x-ray diffraction study and ab initio calculations

We performed high-pressure studies and ab initio calculations of titanium hydride (TiH₂), an important compound in hydrogen storage research. In situ, synchrotron x-ray diffraction studies were carried out in two separate compression runs: the first up to 19 GPa in quasihydrostatic conditions and the second up to 90 GPa in nonhydrostatic conditions, and followed by the subsequent decompression to ambient conditions. The pressure evolution of the diffraction patterns revealed a cubic [face-centered-cubic (fcc), Fm-3m] to tetragonal (body-centered-tetragonal (bct), I_4/mmm) phase transition in TiH ₂ occurring at or below 0.6 GPa. The high-pressure tetragonal phase persisted up to 90 GPa. Upon decompression to ambient conditions the observed phase transition appeared irreversible. A third order Birch-Murnaghan fit of the unit cell volume as a function of pressure for all experimental points, yielded a zero pressure bulk modulus K₀ =142 (7) GPa, and its pressure derivative K₀^′ =3.3 (0.2) for the high-pressure tetragonal phase of TiH₂ and with K0′ held at four, K ₀ =130 (5) GPa. The experimental value of bulk modulus confirmed our ab initio calculations where K₀ =139.9 GPa, and K₀ ^′ =3.7 for the high-pressure tetragonal phase of TiH ₂.