Lattice Heat Capacity of Solid Hydrogen

The lattice heat capacity of solid hydrogen was measured at zero pressure, and at three constant volumes. The measurements extend from 2°K to the triple point for the zero-pressure data, from 2°K to the melting temperature at 22.56 cc/mole, and from 4° to 20°K at 19.83 and 18.73 cc/mole. The Debye thetas at 0°K are 128°, 169°, and 189°K at 22.56, 19.83, and 18.73 cc/mole, respectively. The temperature dependences of the Debye thetas are similar to those for other simple solids. Some information about other thermodynamic properties was obtained from the heat capacities. The isothermal compressibility at 16.35°K at the melting pressure is essentially the same as at 4.2°K at the same pressure. The thermal expansion coefficient at 82 atm appears to have a maximum at about 12°K. At zero pressure the thermal expansion coefficient is approximately proportional to the temperature. The thermodynamic expression for the Grüneisen relation is not obeyed in solid hydrogen.

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