# Pressure-Temperature Phase Diagram of Ta-H System up to 9 GPa and 600 \({}^{\circ}\)C
## 3 Results  
Figure 1 shows a series of in situ synchrotron radiation powder X-ray diffraction patterns of Ta and BN powder mixture at 8.17 GPa. Discontinuous peak shifts to the lower energy sides were observed. The peak shifts correspond to lattice expansion. The hydrogenation reaction of bcc Ta resulted in distorted bcc TaH. The value of the lattice expansion was calculated as \(\Delta V\) = 2.5 A\({}^{3}\)/f. u. Using the value of the hydrogen-induced volume expansion \(\Delta V_{\rm H}\) = 2.4 A\({}^{3}\)/f. u. reported by [PERSON] et al., the hydrogen content was calculated to be TaH\({}_{1.0}\). The observed hydrogenation temperature of 642 \({}^{\circ}\)C at 8.17 GPa corresponds to the decomposition pressure-temperature conditions of AlH\({}_{3}\)[14] that were used as the internal hydrogen source in this study.  
The sample was then cooled to room temperature under high pressure. Figure 2 shows a series of in situ synchrotron radiation powder X-ray diffraction patterns at 7.97 GPa. New Bragg peaks emerged at 320 \({}^{\circ}\)C. The peaks can be indexed to a hexagonal unit cell, indicating a phase transition from distorted bcc TaH to hcp TaH\({}_{-2}\). The temperature at which the distorted bcc TaH Bragg peaks vanished was approximately 100 \({}^{\circ}\)C lower than the temperature at which hcp TaH\({}_{-2}\) formed; distorted bcc TaH and hcp TaH\({}_{-2}\) coexisted in the temperature region.  
Figure 1: Series of in situ synchrotron radiation powder X-ray diffraction patterns of Ta and BN powder mixture at 8.17 GPa during heating from 599 \({}^{\circ}\)C to 659 \({}^{\circ}\)C. Discontinuous lattice expansion was observed at 642 \({}^{\circ}\)C and was caused by the hydrogenation reaction of bcc Ta to form distorted bcc TaH. The peaks on the dashed lines correspond to fluorescence from Ta.  
Figure 3 shows the pressure-temperature phase diagram of the Ta-H system. Cooling and heating runs were used to establish the pressure-temperature conditions at which hcp TaH\({}_{2}\) and distorted bcc TaH formed. As can be seen in Figure 3, transition temperatures obtained during heating and cooling runs were different, indicating that the phase transition involves hysteresis.  
As illustrated by the solid line in Figure 3, the phase boundary was defined as the midpoints of dehydrogenation and hydrogenation _p-T_ conditions. The phase transition temperature monotonously increased with increasing pressure, reaching 600 \({}^{\circ}\)C at 10 GPa. At 2.92 GPa, the phase boundary crosses the room temperature line at T = 27 \({}^{\circ}\)C. This is consistent with the previously reported transition pressure (2.2 GPa) from TaH\({}_{2}\) to TaH at room temperature (open circle in Figure 3) [10].  
The lattice parameters of distorted bcc TaH and hcp TaH\({}_{-2}\) were calculated to be \(a\) = 3.4073 \(\pm\) 0.0006 A at 3.55 GPa and \(a\) = 3.186 \(\pm\) 0.005 and \(c\) = 5.069 \(\pm\) 0.009 A at 6.14 GPa, respectively. These values are comparable to those previously reported ones, e.g., \(a\) \(\sim\) 3.41 A at 3.5 GPa and \(a\) \(\sim\) 3.19 and \(c\) \(\sim\) 5.10 A (c/_a_ \(\sim\) 1.6) at 6.1 GPa which were estimated from FIG. 3 in ref. [10].  
Figure 2: Series of in situ synchrotron radiation powder X-ray diffraction patterns of Ta and BN powder mixture at 7.97 GPa cooling from 400 \({}^{\circ}\)C to 193.4 \({}^{\circ}\)C. The peaks on the dashed lines correspond to fluorescence from Ta.