The correlated block model

Sabine, T. M.

doi:10.1107/97809553602060000603

International
Tables for
Crystallography
Volume C
Mathematical, physical and chemical tables
Edited by E. Prince

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International Tables for Crystallography (2006). Vol. C. ch. 6.4, p. 611

Section 6.4.10.1. The correlated block model

T. M. Sabine^a

^a ANSTO, Private Mail Bag 1, Menai, NSW 2234, Australia

6.4.10.1. The correlated block model

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For this model of the real crystal, the variable x is given by equation (6.4.8.6), with $[\ell]$ and g the refinable variables. Extinction factors are then calculated from equations (6.4.5.3), (6.4.5.4), and (6.4.5.5). For a reflection at a scattering angle of $[2\theta]$ from a reasonably equiaxial crystal, the appropriate extinction factor is given by (6.4.7.1) as E(2θ) = $[E_{{L}}\cos\!{^2}\,2\theta +E_{{B}}\sin\!{^2}\,2\theta ]$ .

It is a meaningful procedure to refine both primary and secondary extinction in this model. The reason for the high correlation between $[\ell]$ and g that is found when other theories are applied, for example that of Becker & Coppens (1974), lies in the structure of the quantity x. In the theory presented here, x is proportional to F² for pure primary extinction and to $[Q_\theta ^2]$ for pure secondary extinction.

References

Becker, P. J. & Coppens, P. (1974). Extinction within the limit of validity of the Darwin transfer equations. I. General formalisms for primary and secondary extinction and their application to spherical crystals. Acta Cryst. A30, 129–147.Google Scholar

International Tables for Crystallography (2006). Vol. C. ch. 6.4, p. 611