Dielectric constant, polarisability, density, and thermal expansion of solid parahydrogen
Boris G Udovidchenko, Valentin B Esel’son, Anatolii N Aleksandrovskii
The effects of a local field in solid parahydrogen (p-H2), the features of thermal expansion of solid p-H2, and its density on the melting line have been investigated. The measurements of absolute values of the dielectric constant, e, of solid p-H2 have been made from temperatures T ~ 0.5Tm up to the melting temperature, Tm, under different constant pressures, p, up to 60 MPa with a measuring capacitive cell of volumetric geometry. The precise density data (dr ~= 0.1%) and the values of thermal volume expansion coefficients (db/b ~= 7%) have been obtained in the whole p – T range of our measurements by transforming e to r. On the basis of the results of the calculation of polarisability, a, of hydrogen molecules ab initio and the experimental data on shifts of vibron frequencies with pressure as well as crystallisation of H2 and D2, it has been shown that the influence of a crystalline field on atomic degrees of freedom of molecules is the main mechanism of a changes in solid p-H2 at p/MPa ~< 103. It has been shown that the universal dependence obtained on the basis of a melting adiabatic criterion and which describes the crystal density along a melting curve is fulfilled for p-H2 and 4He in a region Tm << Q, where Q is the Debye temperature.