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The atomic arrangement for AB₂O₄ type ternary compounds is known to depend on the relative sizes of A and B type cations; the radius ratio determines the stability of the relevant structure type. Although simple considerations based upon the cation sizes lead to the prediction that chromous orthosilicate should adapt the olivine structure, Cr₂SiO₄ was found to crystallize in the thenardite type (see Annual Report 1993), which is otherwise only known for larger B cations relative to the Si cation on the A site.

The chromium atoms in Cr₂SiO₄ are strongly displaced from the central position of the octahedral site relative to the positions in isotypic thenardite-type compounds. This leads to an unusual reduction of the Cr-Cr distance between adjacent polyhedra. The Cr-Cr distance (2.75Å) falls between the distances known for quadruple Cr-Cr bonds (1.90-2.55 Å) in binuclear [Cr₂]²⁺ complexes and non-interactive metal-metal distances (>3.10 Å). This Cr-Cr distance suggests that there is significant electronic interaction or perhaps metal-metal bonding between the Cr atoms. Experimental work in the subsolidus equilibrium stability field of Cr₂SiO₄ has now enabled the growth of larger single crystals suitable for X-ray diffraction, spectroscopic and magnetic measurements to investigate this bonding further.

From high-pressure studies of the crystal structures of Cr₂SiO₄ and isotypic Cd₂SiO₄ we found distinct differences in the compressional behaviour what can be attributed to the different stereochemical requirements of bonding for the octahedrally coordinated M (=Cr, Cd) cations: (1) the bulk modulus of Cr₂SiO₄ (94.7 GPa) is smaller than that of Cd₂SiO₄ (119.2 GPa) although its molar volume (45.93 cm³ mol^-1) is smaller than that of Cd₂SiO₄ (52.38 cm³ mol^-1) this is in contrast to the general observation that the bulk modulus decreases with increasing molar volume in a series of isostructural compounds; (2) the compressional anisotropy of the unit cell is different reflecting the differences in compression of the MO₆ octahedra (Fig. 3.3-2); (3) the pressure-induced displacements of the oxygen atoms leads to an increased planarity of the CrO₄ configuration with a higher compressibility of the short Cr-Cr contact relative to those of the non-interactive Cr-Cr contacts.

Fig. 3.3-2: Compression curves for the orthorhomic unit cells of Cd2SiO4 and Cr2SiO4. As a result of the differences in the coordination geometries of the M2+ site, the c axis in Cr2SiO4 is much more compressible than in Cd2SiO4 whereas the anisotropy of compression of the a and b axes is greater in Cd2SiO4.

Polarized optical absorption spectra with E//c (the c axis is parallel to the vector of metal-metal interaction) reveal one strong absorption band with an unusually large half width (~5000 cm^-1), which is typical of metal-metal electronic transfer. Moreover, the absorption for E//c was found to increase relative to the total absorption with increasing pressure suggesting an enhanced metal-metal electronic transfer. Clear evidence for interaction between the Cr atoms is given from the diamagnetic response measurements. Cr₂SiO₄ has an unmeasureable (<10^-8 Am²) magnetic momentum compared to the typical paramagnetic response for an isolated square-planar high-spin d⁴ Cr²⁺ complex (e.g. 1.4 10^-6 Am² in gillespite-type CaCrSi₄O₁₀) indicating pairing of electrons. Furthermore, molecular orbital calculations reveal a sigificant orbital delocalization for the d_Z² orbitals indicative of direct Cr-Cr interaction. This explains the strong antiferromagnetic behaviour of the binuclear Cr-Cr system and finally provides clear evidence for a metal-metal bonding interaction and the presence of a binuclear [Cr₂]⁴⁺ complex in Cr₂SiO₄, the first known in any silicate.