Abstract
The hcp lattice distortion parameter δ, the deviation of the ratio c∕a from the ideal hcp value 8∕3, is calculated for solid He under pressure, taking into account two- and three-body interatomic forces. The resulting lattice distortion parameter is small and negative, i.e., the lattice is slightly flattened compared with the ideal hcp lattice. It increases monotonically in absolute value with pressure and reaches 10−3 for a molar volume of ∼2.5cm3∕mol. Such small distortions are most likely beyond the capabilities of x-ray or neutron experiments but can be detected by optical methods based on measurements of the birefringence. The data on δ can be used as a probe of the many-body forces.
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