Structure determination from small crystals of two aluminophosphates, CrAPO-14 and SAPO-43

Small single crystals, ca 105 μm3, of two aluminophosphates have been used to determine their structures. Both compounds were synthesized with the aim of incorporating chromium into the framework, and for CrAPO-14 the X-ray diffraction results show that this has been achieved. For CrAPO-14, Al3.96Cr0.04(PO4)4(OH).C3H10N.H2O, data recorded on a Rigaku AFC-5R diffractometer (Cu Kα) and, independently, on an Enraf-Nonius FAST diffractometer (Mo Kα), showed the structure to be like that of GaPO-14; aluminium atoms occupy four-, five- and six-coordinated sites, and 4-5% of the aluminium in the six-coordinate site is replaced by chromium. For SAPO-43, Al2(PO4)2(C3H10N), data were recorded independently on the AFC-5R (Cu Kα) and on a FAST diffractometer with synchrotron radiation of wavelength 0.895 A; the structure is like that of gismondin, but replacement of Al by Cr to a significant extent could not be established. Framework Al, P and O atoms are located with e.s.d.'s of 0.001-0.005 A, template atoms with e.s.d.'s of ca 0.01 A in CrAPO-14, and larger e.s.d.'s in SAPO-43 where there is disorder in the template-molecule position. In all these respects the two independent determinations for each structure are in agreement. The effectiveness of the different methods of intensity-data collection is compared. (orig.)