Perovskite-like K3TiOF5 Exhibits (3 + 1)-Dimensional Commensurate Structure Induced by Octahedrally Coordinated Potassium Ions

Autor: P. Shiv Halasyamani, Christos D. Malliakas, Roberto dos Reis, Nenian Charles, Weiguo Zhang, Fenghua Ding, Matthew L. Nisbet, Vinayak P. Dravid, Jaye K. Harada, Kent J. Griffith, Chi Zhang, James M. Rondinelli, Kenneth R. Poeppelmeier
Rok vydání: 2021
Předmět:
Zdroj: Journal of the American Chemical Society. 143:18907-18916
ISSN: 1520-5126
0002-7863
DOI: 10.1021/jacs.1c05704
Popis: Elpasolite- and cryolite-type oxyfluorides can be regarded as superstructures of perovskite and exhibit structural diversity. While maintaining a similar structural topology with the prototype structures, changes in the size, electronegativity, and charge of cation and/or anion inevitably lead to structural evolution. Therefore, the nominal one-to-one relation suggested by a doubled formula of perovskite does not guarantee a simple 2-fold superstructure for many cases. Herein, the commensurately modulated perovskite-like K3TiOF5 was refined at 100 K from single-crystal X-ray diffraction data by using a pseudotetragonal subcell with lattice parameters of a = b = 6.066(2) A and c = 8.628(2) A. The length of the modulation vector was refined to 0.3a* + 0.1b* + 0.25c*. In the commensurate supercell of K3TiOF5, the B-site Ti4+ and K+ cations in [TiOF5]3- and [KOF5]6- octahedral units were found to be significantly displaced from the average atomic positions refined in the subcell. The displacements of the K+ cations are ±0.76 A, and those for the Ti4+ cations are approximately ±0.13 A. One- and two-dimensional solid-state 19F NMR measurements revealed two tightly clustered groups of resonances in a ratio of ca. 4:1, assigned to equatorial and axial fluorine, respectively, consistent with local [TiOF5]3- units. S/TEM results confirmed the average structure. Electronic structure calculations of the idealized I4mm subcell indicate the instability to a modulated structure arises from soft optical modes that is controlled by the octahedrally coordinated B-site potassium ions in the cryolite-type structure.
Databáze: OpenAIRE
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