Microphases deriving from Ba2ZrF8 structure type: III: Structural approach of dis-Sr17Zr9F70, Sr23Zr12F94, Pb29Hf15F118. General model of formation of M′2p-1MpF8p-2 microphases

The domain Sr1-xZrxF2+2x studied at 670°C between the limits 0.340≤x≤0.353 show the presence of a new phase Sr17Zr9F70 and of modulated phases, all deriving from Ba2ZrF8 structure type. In the same domain studied below 600°C, Sr17Zr9F70 tends to lose its long range ordering between Sr cations and Sr vacancies, tripling the b lattice parameter; moreover a new monoclinic superstructure Sr23Zr12F94 [a=3sinβ×a(Ba2ZrF8); b=b(Ba2ZrF8); c=c(Ba2ZrF8), β=105.31°] is also characterized. The homologous Hf phases are isostructural. With Pb, between 555 and 530°C, another superstructure Pb29Zr15F118 [a=5×a(Ba2ZrF8); b=b(Ba2ZrF8); c=c(Ba2ZrF8] and its isotypic Pb29Hf15F118 are evidenced and their structure is solved in great part with a similar cationic disorder between Pb cations and Pb vacancies. Between 530 and 510°C, a second much more complex superstructure exists but has not been characterized. Moreover, at 630°C, single crystals of Pb2HfF8 are obtained which allows to confirm that, contrary to Ba2ZrF8, Pb2HfF8 is non-centrosymmetric (Pn21a space group). but the atomic positions deviate only slightly from centrosymmetry A general structural model for all these M'2p-1MpF8p-2 (p = 6, 9, 12, 15) microphases is developed.