Lattice distortion of hcp solid helium under pressure

Abstract

The lattice distortion of hcp solid He under pressure is calculated using semiempirical and first-principle approaches. While three-body forces tend to flatten the lattice at all compressions, the effect of pair forces changes from the flattening at small compression to elongation at large one. At large compressions, the lattice distortion due to the triple forces is more than twice as large as those due to pair forces and the lattice is slightly flattened. First-principles results show that over approximately fivefold compressions higher-order, many-body forces become important.

Sound velocities in solid hydrogen under pressure

“…Systematic high-pressure studies of the EOS of solid hydrogens were started in the 1970s [1][2][3][4][5][6], and at present they reach the pressures up to ~2 Mbar [7][8][9][10][11][12][13]. The highest volume compression reached in the EOS experiments is 10.4 for solid hydrogen [13] (7.6 for solid D 2 [12]), higher than for solid helium (8.4) [14]. This is an enormous change of molecular volume.…”

“…As known, this problem arises because of neglecting of the three body and higher-order terms in the intermolecular potential [14,[18][19][20][21][22][23][24][25]. Our many-body potential includes not only pair forces, but triple forces as well.…”

Lattice distortion in hcp rare gas solids

“…It has been shown in our previous paper [6] that the hcp structure of solid He is stabilized by strong zero-point vibrations.…”

Molecular rotation in p-H2 and o-D2 in phase I under pressure

“…The lattice distortion parameter δ = -0.03 was obtained for 94 GPa at 83 K, ∼25% smaller than the room temperature single-crystal value by Loubeyre et al [13]. When these results for the lattice distortion are compared with that for rare-gas solids ( at 100 GPa [1,2]) it is apparent that the difference in more than the order of magnitude is due to the rotational degrees of freedom.…”

“…Typical values are of the order of 10 -2 . For solid helium δ is an order of magnitude smaller [1,2]. In the case of hydrogens the situation under discussion is rather controversial.…”