Bararite [Trigonal (NH$_{4}$)$_{2}$SiF$_{6}$, $J1_{6}$] Structure: A6B2C_hP9_164_i_d_a-001
| Prototype | Fe$_{6}$(NH$_{4}$)$_{2}$Si |
| AFLOW prototype label | A6B2C_hP9_164_i_d_a-001 |
| Strukturbericht designation | $J1_{6}$ |
| Mineral name | bararite |
| ICSD | 18027 |
| Pearson symbol | hP9 |
| Space group number | 164 |
| Space group symbol | $P\overline{3}m1$ |
| AFLOW prototype command |
aflow --proto=A6B2C_hP9_164_i_d_a-001
--params=$a, \allowbreak c/a, \allowbreak z_{2}, \allowbreak x_{3}, \allowbreak z_{3}$ |
- Bararite is a trigonal form of (NH$_{4}$)$_{2}$SiF$_{6}$, metastable at room temperature. The room temperature stable form is cubic cryptohalite, which takes on the $J1_{1}$ structure. Except for the hydrogen atoms, this structure is very similar to $J1_{13}$, K$_{2}$GeF$_{6}$. (Schlemper, 1966) state that the hydrogen atoms are on (2d) and (6i) sites, but were not able to determine the coordinates because of large thermal fluctuations. They were to study the system at 77K, but we have not found any evidence that this work was ever published.
- Originally (Hicks, 2021) we had the wrong sign for $z_{3}$. This has been corrected.
\[ \begin{array}{ccc}
\mathbf{a_{1}}&=&\frac{1}{2}a \,\mathbf{\hat{x}}- \frac{\sqrt{3}}{2}a \,\mathbf{\hat{y}}\\\mathbf{a_{2}}&=&\frac{1}{2}a \,\mathbf{\hat{x}}+\frac{\sqrt{3}}{2}a \,\mathbf{\hat{y}}\\\mathbf{a_{3}}&=&c \,\mathbf{\hat{z}}
\end{array}\]
Basis vectors
| Lattice coordinates | Cartesian coordinates | Wyckoff position | Atom type | |||
|---|---|---|---|---|---|---|
| $\mathbf{B_{1}}$ | = | $0$ | = | $0$ | (1a) | Si I |
| $\mathbf{B_{2}}$ | = | $\frac{1}{3} \, \mathbf{a}_{1}+\frac{2}{3} \, \mathbf{a}_{2}+z_{2} \, \mathbf{a}_{3}$ | = | $\frac{1}{2}a \,\mathbf{\hat{x}}+\frac{\sqrt{3}}{6}a \,\mathbf{\hat{y}}+c z_{2} \,\mathbf{\hat{z}}$ | (2d) | N I |
| $\mathbf{B_{3}}$ | = | $\frac{2}{3} \, \mathbf{a}_{1}+\frac{1}{3} \, \mathbf{a}_{2}- z_{2} \, \mathbf{a}_{3}$ | = | $\frac{1}{2}a \,\mathbf{\hat{x}}- \frac{\sqrt{3}}{6}a \,\mathbf{\hat{y}}- c z_{2} \,\mathbf{\hat{z}}$ | (2d) | N I |
| $\mathbf{B_{4}}$ | = | $x_{3} \, \mathbf{a}_{1}- x_{3} \, \mathbf{a}_{2}+z_{3} \, \mathbf{a}_{3}$ | = | $- \sqrt{3}a x_{3} \,\mathbf{\hat{y}}+c z_{3} \,\mathbf{\hat{z}}$ | (6i) | F I |
| $\mathbf{B_{5}}$ | = | $x_{3} \, \mathbf{a}_{1}+2 x_{3} \, \mathbf{a}_{2}+z_{3} \, \mathbf{a}_{3}$ | = | $\frac{3}{2}a x_{3} \,\mathbf{\hat{x}}+\frac{\sqrt{3}}{2}a x_{3} \,\mathbf{\hat{y}}+c z_{3} \,\mathbf{\hat{z}}$ | (6i) | F I |
| $\mathbf{B_{6}}$ | = | $- 2 x_{3} \, \mathbf{a}_{1}- x_{3} \, \mathbf{a}_{2}+z_{3} \, \mathbf{a}_{3}$ | = | $- \frac{3}{2}a x_{3} \,\mathbf{\hat{x}}+\frac{\sqrt{3}}{2}a x_{3} \,\mathbf{\hat{y}}+c z_{3} \,\mathbf{\hat{z}}$ | (6i) | F I |
| $\mathbf{B_{7}}$ | = | $- x_{3} \, \mathbf{a}_{1}+x_{3} \, \mathbf{a}_{2}- z_{3} \, \mathbf{a}_{3}$ | = | $\sqrt{3}a x_{3} \,\mathbf{\hat{y}}- c z_{3} \,\mathbf{\hat{z}}$ | (6i) | F I |
| $\mathbf{B_{8}}$ | = | $2 x_{3} \, \mathbf{a}_{1}+x_{3} \, \mathbf{a}_{2}- z_{3} \, \mathbf{a}_{3}$ | = | $\frac{3}{2}a x_{3} \,\mathbf{\hat{x}}- \frac{\sqrt{3}}{2}a x_{3} \,\mathbf{\hat{y}}- c z_{3} \,\mathbf{\hat{z}}$ | (6i) | F I |
| $\mathbf{B_{9}}$ | = | $- x_{3} \, \mathbf{a}_{1}- 2 x_{3} \, \mathbf{a}_{2}- z_{3} \, \mathbf{a}_{3}$ | = | $- \frac{3}{2}a x_{3} \,\mathbf{\hat{x}}- \frac{\sqrt{3}}{2}a x_{3} \,\mathbf{\hat{y}}- c z_{3} \,\mathbf{\hat{z}}$ | (6i) | F I |
References
- E. O. Schlemper and W. C. Hamilton, On the Structure of Trigonal Ammonium Flourosilicate, J. Chem. Phys. 45, 408–409 (1966), doi:10.1063/1.2716548.
- D. Hicks, M. J.Mehl, M. Esters, C. Oses, O. Levy, G. L. W. Hart, C. Toher, and S. Curtarolo, The AFLOW Library of Crystallographic Prototypes: Part 3, Comput. Mater. Sci. 199, 110450 (2021), doi:10.1016/j.commatsci.2021.110450.
Found in
- J. Fábry, J. Chval, and V. Petrícek, A new modification of diammonium hexafluorosilicate, (NH$_{4}$)$_{2}$SiF$_{6}$, Acta Crystallogr. Sect. E 57, i90–i91 (2001), doi:10.1107/S160053680101501X.
Prototype Generator
aflow --proto=A6B2C_hP9_164_i_d_a --params=$a,c/a,z_{2},x_{3},z_{3}$