Single crystals of (1-x)PbMg_1/3Ta_2/3O₃ - xPbTiO₃ solid solution of the concentration 0≤x≤0,4 were examined by two methods: X-ray powder (using a Siemens D-5000 diffractometer with Cu Kα radiation) and single crystal diffraction (using a four-circle KM4 diffractometer with graphite-monochromatized Mo Kα radiation).
The samples were prepared by the flux method [1]. The crystals show that with the increase of Ti content, at room temperature, symmetry changes from cubic to tetragonal.
For the PbMg_1/3Ta_2/3O₃, on the diffracted patterns obtained for single crystal as well as for powder apart the main lines related to cubic, perovskite structure, the additional, weak lines are observed. The calculations for the cubic model give the value of R_B about 0,10. When the model of structure with the tilts of oxygen octahedral, and with the double lattice cell was assumed the discrepancy factor is about 0,08. For these reason the model of the modulated structure has been proposed and calculated for the single crystal data.
The picture present the experimantal powder spectrum (1-x)PbMg_1/3Ta_2/3O₃ - xPbTiO₃ for x=0, with permit observe the additional lines. It assume that these lines are the superstructure ones.
The next studies were performed for the mixed crystals (1-x)PbMg_1/3Ta_2/3O₃ - xPbTiO₃. The diffracted pattern shows the splitting of the main diffracted lines, indicating on tetragonal distortion. The structure has been refinement using the Glazer model [2,3]. The data from the single crystal experiment confirmed the model proposed from powder.

[1] A. Kania, A. Leonarska, Z. Ujma - J.Crystal Growth 310 (2008) 594 - 598
[2] A.M. Glazer - Acta Cryst. (1972). B28, 3384
[3] A.M. Glazer - Acta Cryst. (1975). A31, 756
                       Experimental powder spectrum (1 - x)PMT - xPT for x=0

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