The crystal structures of La 2 O 2 CO 3 II and Nd 2 O 2 CO 3 II have been shown by means of high-resolution powder neutron (PND) and synchrotron X-ray di4raction (SXRD) combined with selected area electron di4raction (SAED) studies to be far more complex than earlier anticipated, owing to ordering of carbonate groups between (Ln 2 O 2؉
2 ) n layers. In contrast to earlier descriptions, the carbonate groups appear to be rather regular. Relative to an average model, the SAED patterns show additional scattering in the form of closely distributed, but essentially discrete, spots along 1 1 3 , 1 3 , l2*. Most of the observed scattering, H, can be described as H ؍ G<mq 1 ؉nq 2 , where G is the Bragg re6ections of the underlying average
3 ]*, and m and n are integers. The additional scattering re6ects ordering of the carbonate groups into trigonal layers between the (Ln 2 O 2؉
2 ) n layers, but it remains open whether q 1 and q 2 represent two separate structures with di4erent stacking sequences of such layers or whether they correspond to an even more complex stacking sequence. In any case, some disorder and rotational domain twinning are present. Two structure models, one for each modulation wave vector, were constructed. Rietveld-type re5nements of PND data of La 2 O 2 CO 3 II were performed, approximating the complex, and at least partly disordered, stacking sequence as a two-phase mixture of the two modulated phases. Satisfactory convergence was achieved with R p ؍ 6.4%, R wp ؍ 8.3%, and 2 ؍ 3.32. The isothermal expansivities, p , for La 2 O 2 CO 3 II and Nd 2 O 2 CO 3 II between 298 and 893 K were determined as 2.92؋10 ؊5 and 2.70؋10 ؊5 K ؊1 , respectively.