On the Crystal Structure of Ln2O2CO3 II (Ln=La and Nd)

The crystal structures of La2O2CO3 II and Nd2O2CO3 II have been shown by means of high-resolution powder neutron (PND) and synchrotron X-ray diffraction (SXRD) combined with selected area electron diffraction (SAED) studies to be far more complex than earlier anticipated, owing to ordering of carbonate groups between (Ln2O2+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 < , , l>*. Most of the observed scattering, H, can be described as H=G±mq1+nq2, where G is the Bragg reflections of the underlying average P63/mmc lattice, q1=[ , , ± ]*, q2=[ , , ± ]*, and m and n are integers. The additional scattering reflects ordering of the carbonate groups into trigonal layers between the (Ln2O2+2)n layers, but it remains open whether q1 and q2 represent two separate structures with different 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 refinements of PND data of La2O2CO3 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 Rp=6.4%, Rwp=8.3%, and 2=3.32. The isothermal expansivities, p, for La2O2CO3 II and Nd2O2CO3 II between 298 and 893 K were determined as 2.92×10-5 and 2.70×10-5 K-1, respectively.