Apatite metaprism twist angle (phi) as a tool for crystallochemical diagnosis

作者：Lim S C^*; Baikie Tom; Pramana Stevin S; Smith Ron; White T J

来源：Journal of Solid State Chemistry, 2011, 184(11): 2978-2986.

DOI：10.1016/j.jssc.2011.08.031

• Metaprism twist angle
• Apatite crystal chemistry
• Neutron diffraction

[A(I)](4)[A(II)](6)(BO4)(6)X-2 apatites can flexibly accommodate numerous cationic, metalloid and anionic substitutions. Using a combination of new refinements and published structures, this paper reviews correlations between substituent type and framework adaptation through adjustment of the A(I)O(6) metaprism twist angle, phi. These systematics are illustrated through powder neutron diffraction refinement of the crystal chemistry of A(10)(PO4)(6)F-2 (A = Ca, Sr) fluorapatites. Variations in phi reflect changes in the relative size of the A(4)(I)(BO4)(6) framework and A(6)(II)X(2) tunnel content and can be used to quantitatively assess the reliability of A(I)/A(II) cation partitioning coefficients determined by Rietveld analysis. In the simplest cases of bi-ionic substitution, the metaprism twist systematics conform to three principle trends
(i) For A-type divalent substitution, the larger A(2+) species preferentially enters the channel before partitioning to the framework. This leads to parabolic modification in phi across the compositional series;
(ii) For B-type pentavalent compounds, the phi variation will be linear in accord with the relative B5+ ionic size; and
(iii) For X-type substitution of halide anions, phi will be reduced as the average size increases.
Departures from these trends may indicate polymorphism, compositional anomalies, A(I)/A(II) order disequilibrium, or poor structure refinement, and may be extended to chemically complex apatites with simultaneous substitutions over the A, B and X sites.

• 出版日期2011-11