Speaker
Dr
Mashikoane Mogodi
(European Synchrotron Radiation Facility (ESRF))
Description
Keywords: borophosphates, high-temperature, dehydration, thermal-induced amorphisation, in-
situ synchrotron XRPD
Theme: Crystal Engineering and Structural Chemistry
Four CZP (chiral zincophosphate) zeolite topology compounds [1] with the general formula
MIMII (H2O)2[BP2O8].yH2O (MI = Na, NH4 and MII = Mn, Co, y = 0.5, 1) have been prepared
under mild hydrothermal conditions (at 180 °C). Such microporous compounds with
aesthetically interesting crystal structures can have interests in fields such as catalysis, storage,
separation and ion-exchange. One compound of this family,
(NH4)0.5Co1.25(H2O)2[BP2O8].(H2O)0.5, has been studied by variable temperature high resolution
powder X-ray diffraction
experiments carried out from 298 to
1073 K. Complete Rietveld
refinements were achieved by
combining stereochemical restraints
with the powder diffraction data. At
room temperature, this compound
crystallizes in the P65 (No. 170)
space group with Z = 4 belonging to
the hexagonal system. The unit cell
parameters obtained were: a = 9.4330(2) Å, c = 15.5203(2) Å, V = 1196.01(5) Å
3
. This
reciprocal space analysis also revealed the presence of positional disorder via large refined
MSDs. The crystal structure consists of a helical anionic framework, ∞[BP2O8]
3-
, composed of
corner sharing BO4 and PO4 tetrahedra. Water and ammonia molecules are found within the
helical channels running along the [001] direction. This compound undergoes a series of
dehydration, de-ammoniation (analysis augmented by thermogravimetric experiments and
Fourier analysis) and finally long range structural decomposition into an amorphous phase.
Total scattering analysis [2] was applied for the first time coupled to the above conventional
structural refinement approach to map the identified positional disorder into real space, and to
further unravel the gaseous dissociation and subsequent decomposition pathway of the rigid
host structure.
Primary author
Dr
Mashikoane Mogodi
(European Synchrotron Radiation Facility (ESRF))
