Rapid communicationExperimental and computational study of crystal structure and thermal expansion of barium hollandites BaM2Ti6O16 (M = Al, Cr, Ga)
Introduction
The unit-cell formula of hollandites is AxB8O16 (x ≤ 2) where A represents ions in the tunnel cavities of the structure (e.g. Ba, Cs) and B are smaller cations such as Al3+, Ga3+, Cr3+and Ti4+occupying octahedral sites.
Hollandites can be tetragonal (space group I4/m) or monoclinic (C2/m).
Barium hollandites are used as matrix forms for radioactive Cs immobilization. Bax[Al2xTi8-2x]O16 is currently one of the major components of SYNROC ceramics for storing radioactive waste. Bax[Ga2xTi8-2x]O16 and Bax[Cr2xTi8-2x]O16 have captured great attention recently as its promising substitutes.
The purpose of this study was to establish structural evolution in hollandite across Ba[Al2Ti6]O16–Ba[Cr2Ti6]O16–Ba[Ga2Ti6]O16 series. Similar experimental studies have already been carried out earlier [1], however, Al-, Ga- and Fe-doped hollandites were studied there, in which aluminum and gallium hollandites are tetragonal, and Fe is monoclinic. This fact slightly complicates interpretation of the results. In addition, the method of neutron diffraction used in the work does not allow one to establish the coordinates of B (and, therefore, the lengths of the BO bonds) as the negative scattering length of the Ti cancels the positive scattering length of the other ions on this site giving a resultant scattering length close to zero.
Rietveld refinement provides only average interatomic distances and gives no information about short range cation ordering. To gain insight into local structure of barium hollandites we performed density functional theory (DFT) calculations.
Al-, Ga-, and Zn-doped hollandites were studied computationally in Ref. [2]. However to maintain charge balance, the trivalent Al and Ga are incorporated with a higher concentration than the divalent Zn which leads to incomparability of the corresponding structures.
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Sample preparation and identification
Barium hollandites were prepared by mixing equimolar quantities of the starting materials Ba(NO3)2, TiO2 and the appropriate sesquioxide (Al2O3, Cr2O3 or Ga2O3). The specimens were calcined at 773 K for 5 h for decomposing barium nitrate. Solid state synthesis had been carrying out in a muffle furnace at 1553 K for 12 h. Before each stage and every 1–2 h reaction mixtures were homogenized by grinding in an agate mortar.
All synthesized compounds were identified by X-ray diffraction. X-ray
Details of ab initio calculations
Quantum-mechanical DFT calculations were performed on Lobachevsky cluster at the University of Nizhny Novgorod using the generalized gradient approximation (GGA) with the Perdew-Burke-Ernzerhof functional [8] for the electron correlation and exchange energy as implemented in the Vienna Ab-initio Simulation Package (VASP). PAW pseudopotentials were used to describe the behavior of the core electrons and their interactions with the valence electrons, while Ba 5s25p66s2, Ga 3d104s24p1, Cr 3p63d54s1
Conclusion
Thus, hollandites BaM2Ti6O16 with M = Al, Cr, Ga have been studied both computationally and experimentally. Their crystal structures have been refined by the Rietveld method. Local structures of barium hollandites were calculated using density functional theory. Thermal expansion coefficients of all studied compounds were determined by the low- and high-temperature X-ray diffraction in the range 173–1173 K.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
The authors are grateful to the Ministry of Science and Higher Education of the Russian Federation for financial support through grant (Number Mnemonic 0729-2020-0039).
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Cited by (1)
Salt-flux synthesis, crystal structure and theoretical characterization of Rb<inf>0.74</inf>Ga<inf>6.62</inf>Ti<inf>0·38</inf>O<inf>11</inf>
2020, Solid State SciencesCitation Excerpt :This interest has led a number of groups, including ours, to explore structures related to the hollandite structure by changing the cations occupying the octahedral sites. Among the hollandites, there are only a limited number of gallium-based compositions known that contain K or Ba as the A-site cation, making it of interest to determine if other gallium-based systems that incorporate larger alkali cations and that crystallize in the hollandite or related structure types can be synthesized [14,15]. Utilizing exploratory flux crystal growth, Rb0.74Ga6.62Ti0·38O11 (RGTO) was isolated in a reaction intended to crystallize a Rb-Ga-Ti-O hollandite [16].