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Abstract

A new method for the measurement of bound coherent neutron scattering lengths is reported. It is shown that a relative measurement of the neutron scattering length, (b) over bar, of an element can be made by analysis of the neutron correlation function of a suitable oxide crystal powder. For this analysis, it is essential to take into account the average density contribution to the correlation function, as well as the contributions arising from distances between atoms in the crystal. The method is demonstrated and verified by analysis of the neutron correlation function for the corundum form of Al2O3, yielding a value (b) over bar = 3.44 (1) fm for Al, in good agreement with the literature. The method is then applied to the isotopes of iridium, for which the values of the scattering lengths were unknown, and which are difficult to investigate by other methods owing to the large cross sections for the absorption of neutrons. The neutron correlation function of a sample of Sr2IrO4 enriched in Ir-193 is used to determine values (b) over bar = 9.71 (18) fm and (b) over bar = 12.1 (9) fm for Ir-193 and Ir-191, respectively, and these are consistent with the tabulated scattering length and cross sections of natural Ir. These values are of potential application for obtaining improved neutron diffraction results on iridates by the use of samples enriched in Ir-193, so that the severe absorption problems associated with Ir-191 are avoided. Rietveld refinement of the neutron diffraction pattern of isotopically enriched Sr2IrO4 is used to yield a similar result for Ir. However, in practice the Rietveld result is shown to be less reliable because of correlation between the parameters of the fit.