ECM20

August 25-31, 2001

[Continued from Volume 10 number1]

The emphasis in the microsymposium 'Polymorphism and Solid State Chemistry' was on inorganic systems.

The microsymposium was organized jointly by the 'Mineralogical Crystallography' (SIG5) and the 'Molecular Interaction and Recognition' (SIG7) SIG’s of the ECA.

M. Nespolo (NIMS-AML, Japan) reviewed the IUCr definition of polytypism and pointed out its shortcomings, a lack of geometricity and overemphasis on the chemical aspects, rather than structural features. Polytypes are defined in terms of building layers with a partly idealized geometry (layer archetypes) and symmetry operations relating pairs of layers. Chemical composition differentiates the various families of polytypes.

E.L. Belokoneva (Moscow U., Russia), discussed the inter-transformations among the polymorphs in the LnBGeO₅ (Ln = La - Er) family of borogermanates, showing that the structure is analogous to datolite for Ln = Nd-Er and to stillwellites for Ln = La-Pr. The phase transformation from the polar P3₁ space-group type to the non-polar P3₁2₁ space-group type could be described as a complex movement of the B-O tetrahedra along the 3₁ polar axis having both displacive and order-disorder character.

J. Hybler (Czech Academy of Sciences, Praha) discussed 'Cronstedtite polymorphism - an X-ray and HRTEM study' showing the first experimental occurrence of intergrowth of polytypes belonging to different OD subfamilies in this group of 1:1 layer silicates. In some of the pyramidal crystals investigated, the proportion of different subfamilies varies with the location inside the sample, with intergrowth in the central part, and separation of the subfamilies in the external parts.

M. Ellner (Max Planck Inst. for Metal Research, Germany) discussed the mechanism of transformation from the interstitial solid solution Pd(B), which forms above 723K, to Pd5B, which occurs at 658K. The atomic radius ratio for B and Pd is 0.71 and, according to Hagg’s rule, it is too high for the formation of an interstitial solid solution. The hard-sphere model cannot explain its occurrence. Pd5B is the Pd-richest boride phase of the Pd-B system and is based on a sub cell of the Pd(B) phase.

M. Hostettler (U. de Lausanne, Switzerland) discussed three polymorphs of 'Mercuric Iodide' that crystallize concomitantly. In a lovely series of carefully controlled experiments he also observed that the three phases could be obtained in stages, from least stable to most stable. Of particular interest is the intermediate metastable phase, whose complex structure comprises two MDO (Maximum Degree of Order) OD polytypes. The stable phase differs from the intermediate phase only in the distribution of Hg, but the nearest-neighbor coordination is preserved.

Joel Bernstein and Massimo Nespolo