Phase diagram and equation of state of TiH2 at high pressures and high temperatures

We determined the phase diagram and the equation of state (EoS) of TiH2 at high pressures up to 8.7 GPa and high temperatures up to 600 °C by in situ synchrotron radiation X-ray diffraction measurements. Compression induced strain inhibited the phase transition from the low-temperature bct phase to the high-temperature fcc phase, making the phase diagram difficult to determine. However, heating around 600 °C relieved the strain, and the phase boundary between the bct and fcc phases was elucidated. The phase transition temperature at ambient pressure increased from around room temperature to 200 °C at 8.7 GPa. The high temperature Birch–Murnaghan EoS was determined for the fcc phase. With the pressure derivative of the bulk modulus K′0 = 4.0, the following parameters were obtained: ambient bulk modulus K0 = 97.7 ± 0.2 GPa, ambient unit cell of the fcc phase V0 = 88.57 ± 0.02 Å3, temperature derivative of the bulk modulus at constant pressure (∂K/∂T)P = −0.01 ± 0.02, and volumetric thermal expansivity α = a + bT with a = 2.62 ± 1.4 × 10−5 and b = 5.5 ± 4.5 × 10−8. K0 of fcc TiH2 was close to those for pure Ti and bct TiH2 reported in previous studies.

► We determined the phase diagram of TiH2 at high pressures and high temperatures. ► Compression induced stain inhibited the phase transition from the bct to fcc phase. ► The phase boundary was appropriately determined using a sample with heat treatment. ► The high temperature Birch–Murnaghan equation of state of fcc TiH2 was firstly determined.