### Abstract:

The object of research is the crystal structure of polymorphic modifications of the Ва6Ta2O11 compound. This
compound has low dielectric losses in the microwave (microwave) range and can be used in microwave technology.
The compound crystallizes in the structural type of cryolite with cubic lattice parameters a = 8.69 А°. At the same
time, Ва6Ta2O11 has inherent polymorphism. The paper proposes models of crystal structures for the a- and
b-phases of the compound for the spectra under the numbers 00-049-0899, and 00-049-0903 in the database of
powder diffraction patterns PDF-2 for 2004. The compound has a lattice that does not belong to the tetragonal
system, as suggested earlier, but to the orthorhombic one. For the-phase, a structural model is proposed: orthorhombic
system with lattice periods a = 6.218 A°; b = 8.509 A°; c = 6.227 A°. The space group P212121 is possible (19).
Odds factor R = 8.54707 %. For the b-phase: the orthorhombic system. The space group of symmetry Fmmm (69)
with lattice period is possible a = 8.668 (7) A°; b = 8.677 (8) A°; c = 8.685 (7) A°. Odds factor R = 7.03646 %. Let’s
assume that the phase transitions are associated not only with a change in the lattice symmetry (the appearance
of the second crystal symmetry elements), but also with structural disordering. The regular systems of points of
the a-and b-phases of the compound are not completely filled, which introduces defects into the crystal lattice.
Thus, the structure of the a-phase of the compound is completely occupied by the positions of the B2, B5, B6,
B7, Ta2, Ta3, O3, O10 atoms (it has the correct system of points 4a). The structure of the b-phase has regular
systems of points 4a, 4b, 8f, 32p, completely filled with atoms. This leads to distortion of the crystal structure of
the test compound. The structure of the b-phase additionally has elements of symmetry of the mirror reflection
plane, perpendicular to the x, y, z axes. They have elements of symmetry: the axis of symmetry of the 2nd order,
parallel to the axes x, y, z. Also, helical symmetry axes of the 2nd order (inherent in both structures), parallel to the
x, y, z axes, and grazing reflection planes perpendicular to the x, y, z axes, with n-slip along the diagonal. The
conducted studies of the crystal structure of the compound allo