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Schlagwörter:
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Zusammenfassung:
A combined theoretical and experimental study of the natural Cu2+
-mineral callaghanite is presented. Its crystal structure features well
separated Cu-2(OH)(6) structural dimers with weakly bonded carbonate
groups and water molecules in between. Susceptibility, field-dependent
magnetization and specific-heat measurements reveal a compound with a
small spin gap of about 7 K. The observed magnetic properties are well
described by a model of isolated antiferromagnetic spin dimers. Possible
ferromagnetic interactions between the dimers amount to -1.5 K, at most.
Different flavors of electronic structure calculations have been
employed to locate the magnetic dimers in the crystal structure, i.e.,
to determine whether they coincide with the structural dimers or not.
Calculations of the coupling between the structural dimers clearly show
that magnetic and structural dimers are the same. For the intradimer
coupling, however, the computational results confirmed a coupling
strength close to zero but the sign of the coupling could not be
determined unambiguously. Based on this finding, we then discuss how the
reliability of the numerical methods depends on the characteristics of
exchange pathways and on structural features of the compound in general.
Eventually, we try to provide a minimum coupling strength that is needed
for a reliable computational description.
