Carbon is one of the possible light elements in the cores of the terrestrial planets. The P wave velocity (VP) and density (ρ) are important factors for estimating the chemical composition and physical properties of the core. We simultaneously measured the VP and ρ of Fe‐3.5 wt % C up to 3.4 GPa and 1850 K by using ultrasonic pulse‐echo method and X‐ray absorption methods. The VP of liquid Fe‐3.5 wt % C decreased linearly with increasing temperature at constant pressure. The addition of carbon decreased the VP of liquid Fe by about 2% at 3 GPa and 1700 K and decreased the Fe density by about 2% at 2 GPa and 1700 K. The bulk modulus of liquid Fe‐C and its pressure (P) and temperature (T) effects were precisely determined from directly measured ρ and VP data to be K0,1700 K = 83.9 GPa, dKT/dP = 5.9(2), and dKT/dT = −0.063 GPa/K. The addition of carbon did not affect the isothermal bulk modulus (KT) of liquid Fe, but it decreased the dK/dT of liquid Fe. In the ρ‐VP relationship, VP increases linearly with ρ and can be approximated as VP (m/s) = −6786(506) + 1537(71) × ρ (g/cm3), suggesting that Birch's law is valid for liquid Fe‐C at the present P‐T conditions. Our results imply that at the conditions of the lunar core, the elastic properties of an Fe‐C core are more affected by temperature than those of Fe‐S core.

It is demonstrated that the multiple internal reflection infrared (IR) spectroscopy using a germanium prism is a very powerful nondestructive diagnostic technique for the study of silicon wafer surfaces in a wide range of IR irradiation region. The technique limits neither the shape of samples nor the IR range due to the absorption by silicon itself. With this technique, it is demonstrated that; (i) dangling bonds of a silicon surface treated with HF solution and de-ionized (DI) water are terminated mostly with H atoms, (ii) native oxide growth is enhanced by DI water rinsing, and the interstitial oxygen concentration in the silicon surface region increases during native oxide growth process, and (iii) DI water rinsing after HF etching replaces Si—F bonds with Si—H and Si—OH bonds on a silicon surface.

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