Abstract. Inversion of the magnetic field in a large-scale model of αΩ-dynamo with nonlocal α-effect is under the investigation. The model allows us to reproduce the main features of the geomagnetic field reversals. It was established that the polarity intervals in the model are distributed according to the power law. Model magnetic polarity time scale is fractal. Its dimension is consistent with the dimension of the real geomagnetic polarity time scale.

Abstract.NH4Pb2C15, monoclinic, P2~/c, a = 9-018 (3), b = 7.981 (6), c = 12.502 (4)A, /~ = 90.09(2) ° , Z=4, Dc=4.50 g cm -3. NH4Pb2CI5 adopts the same structure as PbU2Se 5. This structure could be assigned to a number of other halides in the series AB2X 5 with A = K, Rb, TI or NH4; B = Pb or Sr and X = CI or Br.Introduetlon. Jansen (1968) reported the unit-cell dimensions of a series of compounds AB2X 5, with A = K, Rb or NH4; B = Pb or Sr and X = CI or Br. In this series a tetragonal modification with I4/mcm symmetry occurs (Poweli & Tasker, 1937) which is an ordered version of the Cr5B 3 (anti-) type structure (Bertaut & Blum, 1953). The other modification gave powder X-ray diagrams which could be indexed on the basis of an orthorhombic unit cell (Jansen, 1968). The systematic extinctions hk0: h = 2n and 00l: l=2n, however, do not fit an orthorhombic space group. The lattice constants of this modification are comparable with those of U3Se 5 (Moseley, Brown & Whittaker, 1972), if the difference between the ionic radii of Se 2-, and C1-and Br-is taken into account. Cation substitutions in the U3Se 5 structure lead to a structure with Pnma symmetry (Jeitschko & Donohue, 1975) and to one with P2~/c symmetry (Potel, Brochu & Padiou, 1975). In order to distinguish between the two possibilities it is sufficient to look for the occurrence of reflexions Okl: k + l = 2n + 1, which are forbidden in Pnma. In this way P2~/c symmetry could be assigned to this modification. statistics. Nearly half the reflexions had I < 2o(I) and were considered non-significant. Absorption corrections were applied with a computer program written by de Graaff (1973). The transmission factors ranged between 0.37 and 0.46 (p = 380 cm-l). Together with the absorption corrections new standard deviations were computed, taking into account the inaccuracy of the absorption correction and the attenuation factors, leaving 183 5 independent observed reflexions.The intensities were reduced to F values and a Wilson (1942) plot was calculated, yielding approximate values for the scale factor and the initial overall isotropic thermal parameter B.All crystallographic calculations were carried out on the Leiden University IBM 370/158 computer, with a set of computer programs written or modified by Rutten-Keulemans and de Graaff. Atomic scattering * NH 4 has an isotropic temperature factor of 0.7 (2) A 2.

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