Spect,roscopic and specific heat measurements are reported on two singlet magnets, TbAsO, and TbVO,. The ratios between the antiferromagnetic ordering temperature T N and the splitting A between the two singlets are TN/A = 1.58 and TN/A = 0.45, respectively. The experimentsshow considerable deviationsfroma molecular field theory but remarkably better agree with a self-consistent RPA calculation in which the experimentally determined exchange and dipole fields are used.Es wird iiber spektrdskopische Messungen und uber Messungen der spezifischen Warme an den beiden Singulett-Magnetika TbAsO, und TbVO, berichtet. Das Verhaltnis der antiferromagnetischen Ordnungstemperatur TN zum Abstand A zwischen den beiden Singuletts betragt T,/A = 1,58 fur TbAsO, und T,/A = 0,45 fur TbVO,. Die Experimente zeigen betrachtliche Abweichungen von der Molekularfeldtheorie; die Ubereinstimmung mit einer selbstkonsistenten RPA-Rechnung, in welcher die experimentell bestimmten Austauschund Dipolfelder eingesetzt werden, ist wesentlich besser.
The structure of orthorhombic hexachloroethane has been refined at 294 and 140 K. At room temperature 983 symmetry-independent reflections were measured on a four-circle neutron diffractometer. The final R(F) was 0..066 including an isotropic extinction parameter. At 140 K 793 symmetry-independent reflections refined to R(F) = 0-056. The temperature parameters have been analyzed in terms of the tensors T, L and S describing the rigid-body movement of the C2C16 molecule. The temperature variation of the T and L tensors can be understood with the assumption of strong anharmonic contributions. There is, however, no indication of rotational disorder in the orthorhombic phase.
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