MS16-P17 Pure gyrotropic phase transitions in the arcanite related materials PbMGeO4 (M = Ba, Sr)Gyrotropic phase transitions are characterized by the appearance of a spontaneous optical activity . The appearance of such activity is very common in ferroelectric materials. In such materials, the optical activity is a secondary order parameter and is coupled to the primary order parameter which is the electrical polarization. However, only very rare examples are known of a pure gyrotropic phase transition. Among those, one can cite BiFeO3 under strain , (C5H11NH3)2ZnCl4 as function of temperature . In both cases, the transition exhibits a change from Pnma to P212121 symmetry.
In this contribution, we have investigated 2 materials belonging to the BaNdGaO4 structural type, namely PbBaGeO4 and PbSrGeO4 using powder X-ray diffraction as function of temperature. While PbBaGeO4 exhibits a first order phase transition from P212121 to Pnma symmetry. This behaviour is similar to the one reported for (C5H11NH3)2ZnCl4. However, PbBaGeO4 exhibits phase coexistence and competition over a wide temperature range of about 200°C. On the contrary, PbSrGeO4 shows a 2nd order type phase transition towards a Pnma structure with nearly metrically hexagonal cell.
This work demonstrates that both materials are purely gyrotropic and identify BaNdGaO4 structural type as a new source for such materials. Furthermore, those phase transitions seem unusual in light of the complex behavior reported here for these 2 compounds.
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