X-RAY STUDIES OF MAGNETICALLY INTERESTING COMPOUNDS IN THE TEMPERATURE RANGE 10-300K. A. E. Goeta, R. C. B Copley, C. W. Lehmann, J. A. K. Howard, University of Durham, Durham Chemical Crystallography Group, UK

We present here an application of low temperature crystallography to the study of structure/property relationships. Two systems previously reported to show magnetic phase transitions at approximately 36K and 23K have been investigated using the Fddd high intensity cryogenic diffractometer at Durham, UK.

(i) [p-CNSSN^.C₆F₄CN]^_ The ß-phase of this compound is the first purely organic material to exhibit spontaneous magnetisation above liquid helium temperatures. The compound crystallises in the orthorhombic space group Fdd2 and keeps the same symmetry from room temperature down to 10K. The observed magnetic behaviour has been explained in terms of a phase transition, at the unprecedentedly high temperature of 36K, from a one-dimensional antiferromagnetic state to a weak ferromagnetic state. The fact that the structure remains in a non-centrosymmetric space group is a very positive finding since it is in accordance with the weak ferromagnetic signal postulated to result from the Dzyaloshinsky-Moriya interaction.

(ii) [(T^5-Cp^*)Cr(T⁵-P₅)Cr(T^5*)](SbF₆) A previous SQUID investigation on this triple-decker compound showed it to undergo a spin crossover at approximately 23K. The diffraction study carried out clearly shows a sharp and non-destructive structural phase change at 21K and initial analysis of the full and extensive data suggests that the Cr-Cr separation has shortened from 3.1Å at room temperature to 2.7Å at 12K. This is a first and is seen as clear and exciting experimental evidence for the spin pairing of the chromium atoms.