Abstract

The 'almost forbidden' Bragg reflection 061 and the very weak Bragg reflection 0,1,12 of a Zn single-crystal sphere have been carefully analysed to study the antisymmetric and symmetric features of vibrational anharmonicity. The intensity measurements were carried out at room temperature using synchrotron radiation with = 0.7100 (3) A taking into account multiple-beam effects. Data recorded on the Hamburger Synchrotronstrahlungslabor (HASYLAB) are discussed in terms of the anharmonic atomic vibrations using the effective one-particle-potential formalism. The outcome concerning third- and fourth-order anharmonicity is in accordance with previous results of the authors derived by least-squares fitting of measured Bragg intensities and disprove results given by Merisalo & Larsen Acta Cryst. (1979) A35, 325-327 and Merisalo, Jarvinen & Kurittu Phys. Scr. (1978), 17, 23-25. The measured very weak intensity of the almost forbidden 061 reflection can be well interpreted in terms of a small but significant antisymmetric anharmonic thermal motion of the Zn atoms characterized by the third-order anharmonic temperature parameter 33 = -0.16 (2) x 10-19 J A-3.