Quasi-Isolated Pairs in Trirutile Compounds

The MCr206 (M = Te, W) oxides show an ordered trirutile structure, first reported by Bayer [61] with space group P42/mnm. This structrure can be described as (Cr2O10)14- species built up from two octahedra sharing an edge  

Using the Heisenberg model, the relationship giving the temperature dependence of the molar susceptibility, per (Cr2O10)14- unit, is given by  

Clearly, for 0=0, the obtained J values (Table 5) for both compounds disagree with the expected trend from the Cr(III)-Cr(III) distance. The agreement is greatly improved when considering intercluster couplings, as displayed in Fig. 23 for the best lit parameters listed in Table 5. Note that the calculated j parameters show a sign reversal which is fully consistent with neutron diffraction results. 

It is of interest to compare the J values with that obtained in a similar entity isolated in Cr2Te4Ou [63]. This example is the only one to our knowledge which shows binuclear units (Cr2O10)14. The variation of the coupling with respect to the Cr(III)-Cr(III) distance (d) is plotted in Fig. 24 as log J against log d. In the explored bond range, the data follow quite well a law of the type  

Finally, at low temperatures a discrepancy appears between experimental and theoretical curves. The influence of intercluster interaction being significant, one can expect a crossover from Od to 3d behaviors. A treatment with a mean field approximation then becomes questionable for giving a good picture of the system. 

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