Thermal expansion in Prussian Blue Analogs: A combined neutron and X-ray scattering study.

摘要

Prussian Blue Analogs are important materials because of their characteristics as molecular magnets and their technologically important responses to external stimuli, e.g., photo-excitation, hydrostatic pressure and reduced pressure. They are also the potential candidates for hydrogen storage and carbon dioxide storage/sequestration. While some of them are known to show anomalous negative thermal expansion behavior, few have been studied in detail. Because it is possible to systematically vary ion size and charge in these materials, they represent an interesting playground to study negative thermal expansion and to establish possible correlations with electronic and crystal structures. Unlike silicates displaying NTE and having tetrahedral units linked with apical oxygens, the octahedra units in Prussian Blue compounds are linked with a linear cyanide ligand. This introduces more degrees of freedom in the (mostly) cubic structures.;For my thesis, synthesis of a series of Prussian Blue Analogs via chemical precipitation, characterization using standard techniques, and investigation of thermal expansion behavior of the materials using a combination of X-ray (laboratory and synchrotron) and neutron scattering techniques are presented. First I investigate the thermal expansion behavior of three families of Prussian Blue Analogs, MII3[CoIII(CN) 6]2.nH2O, MII3[Fe III(CN)6]2.nH2O, and MII2[FeII(CN)6].nH 2O (M = Mn, Fe, Co, Ni, Cu or Zn; n = number of water molecules per formula unit). I found that nine compounds exhibit negative thermal expansion magnitude of which can be as large as 39.7 x 10 -6 K-1 for Co3[Co(CN)6 2.12H2O. All of the MII 3[CoIII(CN)6]2.nH2O compounds show negative thermal expansion behavior, which correlates with the Irving-Williams series for metal complex stability. In addition, two compounds (M = Cu, Zn) of the MII 3[CoIII(CN)6]2.nH2O family are found to exhibit negative thermal expansion behavior while positive thermal expansion behavior is observed for other compounds. The oxidation states of the transition metals in the Prussian Blue Analogs and local coordination environment around them (qualitatively) were characterized using X-ray absorption near-edge spectroscopy (XANES). All the transition metals were found to be either in a single oxidation state or in a mixed oxidation state. The effect of the hydration and deuteration on the thermal expansion behavior of a representative Prussian Blue Analog, Fe3[Co(CN) 6]2.xH2O was investigated via X-ray diffraction. Finally we explore the inelastic incoherent neutron scattering spectrum of another Prussian Blue Analog, Zn3[Fe(CN)6] 2.14H2O, which opens up the possibility of correlating negative thermal expansion with the temperature dependence of the mode Grüneisen parameter.