X-ray powder diffraction characterization of the large-volume unit cell of the M8 murataite polytype

Abstract

We have used conventional X-ray powder diffraction to study one of the largest volume inorganic mixed oxide unit supercell structures done so far. This necessitated some SAXS-like observations at low angles from < 2° 2θ to concord with electron diffraction, which had indicated an 8×8×8 huge volume supercell of a fluorite-type basic sub-cell.Emphasis is on the detection of, possibly very weak, fingerprint, low-angle/long lines/peaks which will indicate the (often unsuspected) presence of complex polytypic arrangements of simple very strong basic sub-cells and so facilitate synthetic studies.

Synchrotron X-ray powder diffraction pattern of the M8 murataite polytype

“…Every strong peak is from 8 × ( h k l ) of the FCC sub-cell. The indexing of the new data firmly establishes a very large FCC unit cell, a = 39.277 Å (Maki et al , 2016).

Figure 1. Complete SPXRD pattern of M8 murataite.

…”

“…The specimen was prepared and characterized exactly as described in the earlier paper (Maki et al , 2016).…”

“…Previously (Maki et al , 2016), the authors illustrated how, for a powder X-ray diffraction (PXRD) pattern of a complex, very large face-centered cubic (FCC) unit cell ( a = 39.277(3) Å) oxide, M8, it becomes mandatory to investigate weak, low-angle 2 θ lines (≲2% of the maximum intensity line) to determine the correct space group(s) (SG). Originally, the authors did this with ordinary, but profligate, long scans on a typical laboratory X-ray unit.…”

A further investigation of the complex M3 murataite structure using Hf substitution and STEM-EELS techniques

“…This careful and exhaustive single crystal approach, undoubtably quite time-consuming, will need to be supported in future investigations for the whole complex polysomatic series, including M5, M7 and M8; for the latter, we have already reported the space group to be different (Maki et al, 2016;. We would like to emphasize that the newer methods (and future new methods) must endorse, support and confirm the purely computational crystallographic findings that may be performed on very complex, large unit-cells and/or many-atoms cases in the future.…”

“…There are no visible lines belonging to other members of murataite family, e.g. M5, M7 or M8 (Laverov et al, 1999;Stefanovsky et al, 1999;Stefanovsky et al, 2008;Krivovichev et al, 2012;Pakhomova et al, 2016;Maki et al, 2016), or apparently, the sometimes commonplace, coherent intergrowths. The chemical compositions of the cations of each M3 phase and expected/preferred coordination are shown in Table 1.…”