A first-principles study of the structural, electronic and elastic properties of the FeO-FeOHe system under high pressure

The origin of the wave velocity anomalies at the core-mantle boundary (CMB) has been controversial. The primordial helium reservoir in the deep lower mantle remains elusive even with geochemical evidence for its existence. Here, we calculated the density and wave velocity of the FeO 2 -FeO 2 He system under the CMB conditions using first principles. The FeO 2 and FeO 2 He of pyrite-type can exist stably under the CMB conditions without melting, and the incorporated helium increases the stability of the system. The electrical properties of FeO 2 and FeO 2 He are not related to pressure. Doped helium reduces the density of the system but increases the elastic modulus. Our results suggest that FeO 2 can be used as a viable material composition of ultra-low velocity zones (ULVZs), and FeO 2 He can explain the D′′ seismic discontinuity instead of ULVZs. The primordial helium reservoir possibly formed by the accumulation of FeO 2 He, the only stable solid helium-bearing compound under the CMB conditions, may coincide with the location of the D′′ layer. The effect of pressure on the electrical properties of FeO 2 -FeO 2 He is studied, and its structural and elastic properties reveal a deep primordial helium reservoir and candidate for seismic velocity anomalies at the core-mantle boundary.