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 Å for a free-electron description or 5.215 Å for Overhauser's charge-density-wave (CDW) model for the ground state of potassium. Barrett's (1956) x-ray measurement gave $a_0=5.225\mathrm{}$ Å, a value intermediate between the two predictions. We obtain an average value $a_0=5.2295\mathrm{}\pm 0.0007$ Å at 5.2°K, which is the result of back-reflection measurements ($2\theta \approx 168.5°$) 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 Å) 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.