Magnetic susceptibility temperature-dependent terms

We should mention that another possible explanation of the Pauli-like behavior has recently been proposed in terms of contributions of the triplet-excited bipolaronic states to the spin susceptibility [94]. As shown by Bussac and Zuppiroli, the polaron-bipolaron energy difference U is essentially determined by the interdopant distance Ld. The authors also consider the bipolaron triplet state, which reduces to two separate polarons for large Ld. Due to disorder, the Ld are distributed, giving rise to a distribution for the energy of the magnetic states. Summing over the distribution yields a Curie-like contribution, plus a smoothy temperature-dependent term, which resembles a Pauli contribution. 

Spin-orbit coupling in second order (Section 6.4.4.6) raises the degeneracy of orbitally nondegenerate ground terms by amounts of up to a few cm-1, occasionally more. For temperatures of the order of D and E, the magnetic susceptibility is a complicated function of temperature and varies strongly with direction, as the various levels for equation (65) become occupied to different extents.169 At temperatures much smaller than D and E, the susceptibility usually becomes independent of temperature but highly dependent on direction. 

The magnetic susceptibility is thus a temperature-dependent property of a bulk substance. It is meaningless to specify the susceptibility of a single molecule or complex ion. For convenience, inorganic chemists prefer to answer the question How paramagnetic is this substance in terms of the magnetic moment peff defined by the relation ... 

The Mossbauer spectra (between 4.2 K and 298 K) show two overlapping doublets with temperature dependent relative intensities. In conjunction with the data from magnetic susceptibility measurements (between 83 and 313 K) the Mossbauer spectra were interpreted in terms of a singlet triplet spin transition of the central iron(II) ion. No internal magnetic field was observed in a Mossbauer experiment under applied field at 4.2 K. The authors consider this complex the first authentic example of a singlet < triplet spin transition in the 3 d6 configuration of iron(II). Support has been received from UV-vis spectra and IR vibrational spectra201). 

Cu(tfd)2 units by 0.015 A in an orthogonal direction to fill in the space vacated by the TTF molecules. This existence of a progressive dimerization in a regular chain of molecules of spin 1/2 along with the observed activated temperature dependence of the magnetic susceptibility below Tc has been interpreted in terms of a spin-Peierls instability77-78) in a one-dimensional antiferromagnetically coupled chain. 

Stout and Gutowsky (384) have examined this point and found that the method of Horrocks agrees with an exact calculation of the susceptibility anisotropy for a J = state to within 01 % at temperatures down to 200 K, whereas Bleaney s equation gives a deviation of 1-49 %. However, inclusion of a term linear in T brings the agreement to within 0-27 % of the exact result. This deviation from dependence of the magnetic susceptibility has been further studied by Horrocks (385) for two axially symmetric Yb(iii) complexes and a non-axial Yb(iii) shift reagent. The appreciable deviations from T dependence can be accounted for by the Horrocks approach but not by the simpler Bleaney approach. 

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