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

Microphases deriving from Ba2ZrF8 structure type: II: Crystal structure of Sr17Zr9F70 and... In the domain Sr1−xZrxF2+2 × studied at 670°C between the limits 0.333 ≤ x ≤ 0.353, Sr2ZrF8 (x = 0.333) cannot be obtained under our synthesis conditions but a new phase Sr17Zr9F70 (x = 0.346) is characterized which clearly shows a great analogy with Ba2ZrF8 and Pb2ZrF8 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(Ba2ZrF8); b = 16.566(6) Å = 3 × b(Ba2ZrF8); c = 11.433(4)Å = c(Ba2ZrF8).The superlattice along a results from a change in the step of zigzag parallel rows of respectively Sr cations and ZrF8 polyhedra. The zigzag step is not composition-dependent but more likely depends on the size of the M2+ cation.The superlattice along b results from an ordered creation of Sr vacancies of multiplicity m = 3 (1 vacancy for 3Sr) 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 Sr18Zr9F72 (9 × ”Sr2ZrF8”) to Sr17Zr9F70.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 “Sr2ZrF8”. Compared to Sr17Zr9F70, 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 Sr17Zr9F70 (x = 0.346) and Sr11Zr6F46 (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. http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Journal of Solid State Chemistry Elsevier

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

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Publisher
Elsevier
Copyright
Copyright © 2018 Elsevier Ltd
ISSN
0022-4596
eISSN
1095-726X
D.O.I.
10.1016/j.jssc.2018.01.005
Publisher site
See Article on Publisher Site

Abstract

In the domain Sr1−xZrxF2+2 × studied at 670°C between the limits 0.333 ≤ x ≤ 0.353, Sr2ZrF8 (x = 0.333) cannot be obtained under our synthesis conditions but a new phase Sr17Zr9F70 (x = 0.346) is characterized which clearly shows a great analogy with Ba2ZrF8 and Pb2ZrF8 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(Ba2ZrF8); b = 16.566(6) Å = 3 × b(Ba2ZrF8); c = 11.433(4)Å = c(Ba2ZrF8).The superlattice along a results from a change in the step of zigzag parallel rows of respectively Sr cations and ZrF8 polyhedra. The zigzag step is not composition-dependent but more likely depends on the size of the M2+ cation.The superlattice along b results from an ordered creation of Sr vacancies of multiplicity m = 3 (1 vacancy for 3Sr) 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 Sr18Zr9F72 (9 × ”Sr2ZrF8”) to Sr17Zr9F70.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 “Sr2ZrF8”. Compared to Sr17Zr9F70, 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 Sr17Zr9F70 (x = 0.346) and Sr11Zr6F46 (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.

Journal

Journal of Solid State ChemistryElsevier

Published: Apr 1, 2018

References

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