Incommensurate composite crystal

From Online Dictionary of Crystallography

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Incommensurate Composite Crystal

Cristal composite (Fr.)


An incommensurate composite crystal is a compound with two or more (N) subsystems that are themselves modulated structures, with basis structures that are mutually incommensurate. Each subsystem (numbered by ν) has a reciprocal lattice for its basic structure with three basis vectors ai. There is a basis of the module of diffraction spots that has at most 3N basis vectors Aj* such that

[math]{\bf a}_i^{*\nu}~=~\sum_{j=1}^n Z_{ij}^{\nu} {\bf A}_j^* ~~~(i=1,2,3),[/math]

where Zijν are integer coefficients. If n is larger than the dimension of space (three), the composite crystal is an aperiodic crystal.


Examples are intergrowth crystals and adsorbed monolayers. To the former belongs Hg3-δAsF6 with two systems of Hg chains inside the host lattice formed by AsF6 octahedra. Another example is nonadecane in the channels of a urea host crystal.