Microphases deriving from Ba2ZrF8 structure type: II: Crystal structure of Sr17Zr9F70 and Sr17.145Zr9F70.29.

In the domain Sr 1−x Zr x F 2+2 × studied at 670 °C between the limits 0.333 ≤ x ≤ 0.353, Sr 2 ZrF 8 (x = 0.333) cannot be obtained under our synthesis conditions but a new phase Sr 17 Zr 9 F 70 (x = 0.346) is characterized which clearly shows a great analogy with Ba 2 ZrF 8 and Pb 2 ZrF 8 already described. It presents the same Pnma space group and derives from this basic type by the relation: a = 28.812(10) Å = 3 × a (Ba 2 ZrF 8 ); b = 16.566(6) Å = 3 × b (Ba 2 ZrF 8 ); c = 11.433(4)Å = c (Ba 2 ZrF 8 ). The superlattice along a results from a change in the step of zigzag parallel rows of respectively Sr cations and ZrF 8 polyhedra. The zigzag step is not composition-dependent but more likely depends on the size of the M 2+ cation. The superlattice along b results from an ordered creation of Sr vacancies of multiplicity m = 3 (1 vacancy for 3 Sr) in some Sr raws which changes the Sr/Zr ratio from 2/1 to 1.889/1 (x = 0.346). Then the superstructure along b accommodates the change in stoichiometry in Sr within this phase from Sr 18 Zr 9 F 72 (9 × ”Sr 2 ZrF 8 ”) to Sr 17 Zr 9 F 70 . A second structure of close composition (x = 0.344) shows that the vacant Sr site can be partially occupied in a disordered way by a small proportion of Sr, giving a composition slightly closer to “Sr 2 ZrF 8 ”. Compared to Sr 17 Zr 9 F 70 , the over-stoichiometry mechanism is a solid solution model. For Zr contents comprised between x = 0.346 and 0.353, a series of modulated phases corresponding to various more or less ordered sequences of Sr cations and Sr vacancies along b are characterized by electron diffraction and TEM lattice imaging. They can be interpreted as intermediate between limit phases Sr 17 Zr 9 F 70 (x = 0.346) and Sr 11 Zr 6 F 46 (x = 0.353), of respective multiplicities m = 3 and m = 2 along the same b direction and of modulation vectors q = 1/3b* and q = 1/2b*. Several modulated phases with q modulation vector comprised between 1/3 b* and 1/2 b* can be characterized. [ABSTRACT FROM AUTHOR]