A Model for the Formation of BPIII and Experimental Evidence

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Abstract
Calculations of the effects of fluctuations on the cubic blue phases (BPI and BPII) have shown that these lattices are always unstable in the limit of sufficiently high chirality. The point of instability, as estimated by the Lindemann melting criterion, is the same as the temperature at which BPIII appears. To test this theory measurements have been made of the Debye-Waller factors as a function of temperature for large single crystals of BPII in various orientations as the transition to BPIII is approached from below. We observe large temperature effects consistent with the “melting” mechanism outlined above. There are other evidences of strong temperature dependent dynamical processes, possibly related to thermal diffuse scattering, which are also discussed. Three possible models for the structure of BPIII which could result from this scenario are considered. One is that of an optically isotropic fluid whose orientational order parameter is octupolar rather than quadrupolar, as is usual. The other two models involve two different ways of realizing the state of orientational disorder.

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