Structural changes during phase transitions, critical and noncritical order parameters in the elpasolite $\mathrm{Cs_2RbDyF}_6$

According to $X$-ray experiment on the powder sample determined the structure of the initial cubic and the low-temperature monoclinic phases of the crystal $\mathrm{Cs_2RbDyF}_6$. The refinement of the profile and structural parameters has been carried out by the technique implemented in the DDM program, which minimizes the differences between the derivatives of the calculated and measured $X$-rays intensities over the entire profile of the $X$-ray diffraction pattern. The results obtained have been discussed using the group-theoretical analysis of the complete order parameters condensate, which takes into account the critical and noncritical atomic displacements and permits the interpretation of the experimental data obtained previously. It has been reliable established that the sequence of changes in the symmetry during phase transitions in $\mathrm{Cs_2RbDyF}_6$ can be represented as $Fm\bar 3m\xrightarrow[(\varphi,0,0)]{11-9(\Gamma^+_4)}I4/m\xrightarrow[(\varphi,\varphi,0)]{11-9(\Gamma^+_4)}I2/m\xrightarrow[(\varphi,\varphi,\psi)]{11-9(\Gamma^+_4)\oplus10-3(\mathrm X^+_3)}P2_1/n$.