To synthesize a novel titanium-containing hydride, a Co3Ti alloy with a Cu3Au-type structure was hydrogenated at 9 GPa and 900 °C. Structural changes under high pressure were monitored via in situ synchrotron radiation x-ray diffraction at BL14B1, SPring-8. The in-situ measurement reveals that novel hydride Co3TiH~4 is formed at 9 GPa and 900 °C. When the sample was depressurized at room temperature, the formed hydride decomposed to (the previously reported) Co3TiH~1 at a pressure of ~1 GPa. Co3TiH~1 was recovered at ambient conditions and decomposed gradually into the Co3Ti alloy. Co3TiH~4 is thermodynamically stable only at pressures above 1 GPa.
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