Neutron-Diffraction Measurement of the Lattice Parameter of Potassium Metal at 5.2°K

This paper reports the measurement of the lattice parameter of potassium metal at liquid-helium temperature using neutron diffraction. This experiment was motivated by the recent absolute de Haas-van Alphen (dHvA) measurement of Thomas and Turner, who concluded that the lattice parameter should be 5.236 {} for a free-electron description or 5.215 {} for Overhauser's charge-density-wave (CDW) model for the ground state of potassium. Barrett's (1956) x-ray measurement gave a₀=5.225 {}, a value intermediate between the two predictions. We obtain an average value a₀=5.2295±0.0007 {} at 5.2^∘{}K, which is the result of back-reflection measurements (2θ≈168.5^∘) on two single-crystal samples. Although this lattice parameter is closer to the free-electron prediction, we definitely cannot infer that this model is correct, since the difference ({~}0.0065 {}) is considerably larger than the experimental error. Consequently, it appears that there must be additional (unknown) effects on the shape (or size) of the Fermi surface, or on its dHvA measurement.