TY - JOUR
T1 - Geometrical parameterization of the crystal chemistry of P63/m apatites
T2 - Comparison with experimental data and ab initio results
AU - Mercier, Patrick H.J.
AU - Le Page, Yvon
AU - Whitfield, Pamela S.
AU - Mitchell, Lyndon D.
AU - Davidson, Isobel J.
AU - White, T. J.
PY - 2005/12
Y1 - 2005/12
N2 - Experimental structure refinements and ab initio simulation results for 18 published, fully ordered P63/m ()()(BO4)6X 2 apatite end-member compositions have been analyzed in terms of a geometric crystal-chemical model that allows the prediction of unit-cell parameters (a and c) and all atom coordinates. To an accuracy of ± 0.025 Å, the magnitude of c was reproduced from crystal-chemical parameters characterizing chains of ...-AII-O3-B-O3-AII-⋯ atoms, whereas that of a was determined from those describing (A IO6)-(BO4) polyhedral arrangements. The c/a ratio could be predicted to ±0.2% using multi-variable functions based on geometric crystal-chemical model predictions, but could not be ascribed to the adjustment of a single crystal-chemical parameter. The correlations observed between algebraically independent crystal-chemical parameters representing the main observed polyhedral distortions reveal them as the minimum-energy solution to accommodate misfit components within this flexible structure type. For materials with given composition, good agreement (within ± 0.5-2.0%) of ab initio crystal-chemical parameters was observed with only those from single-crystal refinements with R≤ 4.0%. Agreement with single-crystal work with R > 4.0% was not as good, while the scatter with those from Rietveld refinements was considerable. Accordingly, ab initio cell data, atomic coordinates and crystal-chemical parameters were reported here for the following compositions awaiting experimental work: (Zn,Hg) 10(PO4)6(Cl,F)2, (Ca,Cd) 10(VO4)6Cl2 and (Ca,Pb,Cd) 10(CrO4)6Cl2.
AB - Experimental structure refinements and ab initio simulation results for 18 published, fully ordered P63/m ()()(BO4)6X 2 apatite end-member compositions have been analyzed in terms of a geometric crystal-chemical model that allows the prediction of unit-cell parameters (a and c) and all atom coordinates. To an accuracy of ± 0.025 Å, the magnitude of c was reproduced from crystal-chemical parameters characterizing chains of ...-AII-O3-B-O3-AII-⋯ atoms, whereas that of a was determined from those describing (A IO6)-(BO4) polyhedral arrangements. The c/a ratio could be predicted to ±0.2% using multi-variable functions based on geometric crystal-chemical model predictions, but could not be ascribed to the adjustment of a single crystal-chemical parameter. The correlations observed between algebraically independent crystal-chemical parameters representing the main observed polyhedral distortions reveal them as the minimum-energy solution to accommodate misfit components within this flexible structure type. For materials with given composition, good agreement (within ± 0.5-2.0%) of ab initio crystal-chemical parameters was observed with only those from single-crystal refinements with R≤ 4.0%. Agreement with single-crystal work with R > 4.0% was not as good, while the scatter with those from Rietveld refinements was considerable. Accordingly, ab initio cell data, atomic coordinates and crystal-chemical parameters were reported here for the following compositions awaiting experimental work: (Zn,Hg) 10(PO4)6(Cl,F)2, (Ca,Cd) 10(VO4)6Cl2 and (Ca,Pb,Cd) 10(CrO4)6Cl2.
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U2 - 10.1107/S0108768105031125
DO - 10.1107/S0108768105031125
M3 - Article
C2 - 16306671
AN - SCOPUS:33744493048
SN - 0108-7681
VL - 61
SP - 635
EP - 655
JO - Acta Crystallographica Section B: Structural Science
JF - Acta Crystallographica Section B: Structural Science
IS - 6
ER -