# Pressure-Temperature Phase Diagram of Ta-H System up to 9 GPa and 600 \({}^{\circ}\)C
## 2 Materials and Methods  
We employed reagent-grade Ta powder (the Nilaco Corporation, 1-20-6 Ginza, chuo-ku, Tokyo 104-0061, Japan, purity 3N) as a starting material. The Ta powder was mixed with boron nitride (BN, the Nilaco Corporation, 1-20-6 Ginza, chuo-ku, Tokyo 104-0061, Japan, purity 2N) powder with a volumetric ratio of 2:3 to prevent solid-state grain growth of Ta under high temperature, which prevents the detection of X-ray diffraction peaks in the energy-dispersive mode. The powder mixture was compacted into a disk shape with a diameter of 0.4 mm and a height of 0.4 mm. The compacted Ta disk was inserted in a BN sample capsule, which was then placed in a high-pressure cell described below.  
High pressure was generated using a cubic-type multi-anvil apparatus. A cubic pressure medium with a 6 mm edge length containing the sample was squeezed by tungsten carbide pistons with a truncation edge length of 4 mm. Using the apparatus, a combination of the pressure medium and pistons can generate up to 10 GPa pressure. High-pressure hydrogenation was achieved employing the method initially developed by [PERSON] [12]. The method uses an internal hydrogen source placed in a cubic pressure medium and emits hydrogen when heated while the sample is under high pressure. The emitted hydrogen is confined in a hydrogen sealing capsule made of NaCl. A 0.1 mm thin BN disk separates the internal hydrogen source from the sample, preventing unfavorable reaction between the sample and the internal hydrogen source residue after the hydrogen evolution. Hydrogen permeation through the BN disk is quite fast; compared to the Ta-H system's hydrogenation/dehydrogenation reaction time, hydrogen permeation time through BN is negligible.  
In situ synchrotron radiation powder X-ray diffraction measurement was performed at BL14B1 and SPring-8. The sample was irradiated by white X-rays generated at a bending magnet source. In the energy-dispersive mode, a germanium solid-state detector detected the diffracted white X-rays. Throughout the measurements in this study, the diffraction angle was fixed at 2\(\theta\) = 6 \({}^{\circ}\). The sample pressure was estimated based on the unit cell volume of the NaCl placed in the cubic pressure medium along with the sample and the state thermal equation of NaCl reported by [PERSON] [13]. An alumel-chromel thermocouple wire was used to measure the temperature of the sample. Pressure and temperature uncertainties were less than 0.2 GPa and 10 \({}^{\circ}\)C, respectively.  
The sample was pressurized to the target pressures (9 GPa and 7 GPa) and then heated until the in situ measurements revealed the formation of distorted bcc TaH. After the formation of distorted bcc TaH, the sample was cooled to establish the phase boundary between distorted bcc TaH and hcp TaH\({}_{2}\). The pressure-temperature phase diagram of the Ta-H system under high pressure was then determined by repeating the depressurization, heating, and cooling steps. The typical heating or cooling rate was 20 K/min. Heating or cooling was carried out by applying a constant load to the high-pressure apparatus. Under such constant load conditions, a certain degree of pressure change occurs. Pressure values were determined by measuring the unit cell volume of NaCl before and after heating or cooling. Pressure values during heating or cooling were estimated by interpolation.