The existence of a monoclinic phase of hydroxyapatite, Ca(2)(PO(4))(4)OH, has been confirmed, by single-crystal structure analysis (weighted "reliability" factor = 3.9 percent on |F|(2)). The structure has space group P21/b, a = 9.4214(8) angstroms, b = 2a, c = 6.8814(7) angstroms, and gamma = 120 degrees , and is analogous to that of chlorapatite. The distortions from the hexagonal structure with which the monoclinic structure is pseudosymmetric are similar to those in chlorapatite, including enlargement of that triangular array of oxygen atoms in which the chlorine ion or, in hydroxyapatite, the hydroxyl hydrogen ion is approximately centered. The hydroxyapatite specimen was prepared by the conversion of a single crystal of chlorapatite in steam at 1200 degrees C, was mimetically twinned, and was approximately 37 percent monoclinic.

No abstract

No abstract

The whole-pattern least-squares fitting method (herein called the pattern-fitting structure-refinement, PFSR, method) developed by Rietveld and used by many with neutron powder data has been extended for use with X-ray powder diffractometer data. Examples of PFSR for specimens in five different space groups are given. Structural parameters generally agree, within 2a, with those from single-crystal results where available. Atom positions and site occupancies were determined for LaPO,,, CadPO4)3F, and quartz. R(pattern) values generally ranged from 9 to 20°'o while R(Bragg) values were a few percent lower. Reflection profiles were represented by either Gaussian or Cauchy functions: better representations are shown to be needed with an expected result of improved sensitivity accompanying lower R values. Possibilities for improved pattern-stripping for minor phase study via PFSR of the major phase are exhibited. Remarkably precise lattice parameters, e.g. 1 part in 35 000 from data all below 20 < 77 ~ in the fluorapatite case, and parametric characterization of the diffraction broadening of the Bragg profiles are obtained as additional benefits.

No abstract