Electron paramagnetic resonance of triplet diphenylmethylene in single crystal benzophenone: Evidence for a low temperature phase transition

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Abstract
The angular variation of the field for resonance for diphenylmethylene in benzophenone at T?2 °K has been measured over the entire unit sphere. Both the values of the fine structure tensors T and g and the spatial relationships between the four substitutional sites in the unit cell change as the temperature is lowered from 77 °K to ∼2 °K. The geometrical changes are consistent with a benzophenone phase transition from P212121 to P21 symmetry.