(PDF-4+ 2016, PDF-4/Minerals 2016)
A multi-year effort among world leading scientists has led to the development and growth of the modulated structure subfile. A structural analysis program was developed to produce up to 6-dimensional indexing, superspace group identification, and classification of modulation vectors. This project also entailed re-extracting the world’s historic literature and analyzing it with our editorial program, resulting in a standardized library of reference materials. This is now an ongoing effort with annual extraction of newly discovered modulated structures.
New Capability: More entries, new molecular graphics for visualizing various modulations.
Through strategic partnerships, the International Centre for Diffraction Data (ICDD) has developed leading edge technology for the analysis and standardization of modulated structures. In Release 2015, the modulated structure subfile received a substantial entry increase and a new graphics package addition allows modulations to be visualized.
An example of a large Fe modulation in strontium ferrite (PDF 05-001-0301).
"A new light on the iron coordination in Sr4Fe6O13±d: Super space formalism and structural mechanism". O. Pérez, B. Mellenne, R. Retoux, B. Raveau, M. Hervieu Solid State Sci. 8, 431 (2006).
Modulated and composite crystals are fascinating and complex structures, which violate the traditional idea that a perfect crystal structure must possess a 3-dimensional periodicity. Generally considered aperiodic, these structures actually do obey rules of periodicity, but their description requires 4 or more dimensions for this periodicity to become obvious. These modulations are a result of fluctuations in the position or occupation of the atoms that compose the crystal. The relevance of these structures goes beyond that of pure mathematical curiosity. Many exhibit interesting ferroelectric, magnetic, and superconducting properties, as well as other properties, which cannot be fully understood by looking solely at the average structure but can be understood in a modulated presentation.