Application of diamond anvil cell (DAC) to the investigations of metal - hydrogen systems has extended the limit of hydrogen pressure by almost two orders of magnitude[1]. Due to the strong deviation from ideality, activity of hydrogen expressed as fugacity, could be increased by many orders of magnitude [2,3]. As a result chemical potential of hydrogen could be increased to the extent so far impossible to obtain, giving the possibility of synthesis of new hydrides and determination of thermodynamic properties of this compounds as hydrogen activity is well defined in DAC. By combining DAC with X-ray diffraction technique one can follow crystal structure and volume changes of the sample within DAC as function of hydrogen pressure. Since the incorporation of the hydrogen atom into metal lattice causes increase of the host lattice volume of about 2.9 A3 [4] calculated per metal atom, measurements of lattice volume allow for the calculation of hydrogen concentration in the metal.
Due to hysteresis phenomenon, only the pressure of decomposition of hydride phase, provides us with the information about true equilibrium[5]. Thus determination of the pressure of decomposition is essential feature for the calculation of thermodynamic properties of these compounds.
The purpose of this paper is presentation of the possibilities of DAC technique in determination of the thermodynamic properties of metal-hydrogen system. Semi-theoretical models predicting pressures of formation of the hydrides so far unknown will be discussed

References
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4. H. Peisl, Hydrogen in Metals I, in Topics in Applied Physics vol. 28, G. Alefeld and V(lkl (eds) 1978. p.53
5. B. Baranowski, K. Bochenska, Z.Phys. Chem. (N.F.) 45 140 (1965)