Dynamic Jahn-Teller effect in a manganese(III) complex. Synthesis and structure of hexakis(urea)manganese(III) perchlorate

Finally, the spontaneous moments seem to be associated with the cubic phase. These moments are actually induced moments that disappear when the external magnetic field is removed. The small moments observed in samples of C S ~ , ~ ~ ~ V F ~ and Cs0,,,VF3 are probably a measure of crystal defects in those phases. Magnetic Behavior in Rb,VF3. The distortion of the orthorhombic phase from cubic symmetry is much larger in Rb,VF3 than in Cs,VF,, although its origin is the samenamely, from ionic ordering. This distortion in Rb,VF3 is not simple as in Cs,VF3 but involves moving V and probably F to positions of lower symmetry. This change in the packing structure is reflected in the magnetic properties of Rb,VF3. As in Cs,VF,, the values of CM indicate V3' orbital moments are partially quenched in the cubic phase and totally quenched in the orthorhombic phase of Rb,VF3. The 6 values in RbxVF3 change significantly with composition. In Rb0.45VF3 a considerable amount of the cubic phase is present. As in Cs,VF,, delocalization in cubic Rb,VF3 makes a contribution to the magnetic interactions that all but cancels the antiferromagnetic contribution made by correlation effects. In the orthorhombic phase of Rb,VF3, however, distortion of the V and F ions significantly reduces the extent of delocalization; hence, 6 values become considerably more negative. The TN values of Rb,VF3 are quite explicitly defined by plots of u vs. T shown in Figure 3. Interestingly, TN varies with x. This is again related to the changes in magnetic interactions, which involve both ferromagnetic and antiferromagnetic components. Apparently, the decrease in the ferromagnetic component reduces the total ordering energy, resulting in TN values 5 K. The magnetically ordered states of both Rb,VF3 and Cs,VF3 are antiferromagnetic overall. An analysis of the ordering would require a knowledge of the ordered structure, which could only be obtained by neutron diffraction. It is certain that the ordered structure is complex as there exist two constraints to magnetic ordering per V atom. The ordered structure, therefore, is a frustrated one in which magnetic moments are forced antiparallel to directions preferred by some of their nearest-neighbor interactions. Finally, there appears to be small spontaneous moments associated with the cubic phase in Rb,VF3, as in Cs,VF3. The moments in Rb,VF3 are smaller than those in Cs,VF3 and decrease to about 0.02 pB at x = 0.50, as they did in Cs,VF,. At x = 0.55, the moment increases again, which is probably a measure of structural defects. Conclusions. In both RbxVF3 and Cs,VF3 systems, the magnetic data support X-ray and optical results and the conclusions regarding V2+-V3+ ionic ordering. In the paramagnetic region (above -30 K), CM values indicate the orbital moment is partially quenched in the cubic phase but totally quenched in the orthorhombic. The 6 values support the conclusion that delocalization (ferromagnetic contribution) as well as correlation (antiferromagnetic contribution) is involved in the magnetic exchange mechanism. Delocalization is optimized in the cubic phase. In Cs,VF,, orthorhombic distortion is small and delocalization is decreased only slightly; but in Rb,VF,, the orthorhombic distortion considerably reduces delocalization as seen by the significant change in 6. The TN values in Cs,VF3 are invariant because atomic positions are not significantly altered by ionic ordering, so correlation and delocalization remain in balance. The Rb,VF, system, however, reflects an imbalance of these effects in its orthorhombic phase as delocalization becomes reduced. At temperatures below TN small induced magnetic moments are associated with the cubic phase, which further supports the conclusion that V2+ and V3' are random in this phase. Electronic ordering has been found in the pseudohexagonal A,VF3 phases? in the tetragonal K,VF3 phase,2b and now in the modified pyrochlore A,VF3 phases. It appears that electronic ordering in mixed V2+-V3+ fluoride systems is the rule rather than the exception.