Crystal structures of di-tin-hexa(seleno)hypodiphosphate, Sn2P2Se6, in the ferroelectric and paraelectric phase

The crystal and molecular structure of Sn2P2Se6 in the low temperature ferroelectric phase is determined by single crystal X-ray diffraction techniques. These results are compared with the crystal structure of the same compound in the paraelectric phase, which was redetermined at room temperature. In both phases, the structure crystallizes in the monoclinic system. The ferroelectric phase with a = 6.8145(3) Angstrom, b = 7.7170(3) Angstrom, c = 11.694(1) Angstrom, beta = 124.549(4)degrees, space group Pc, V = 506.52(6) Angstrom(3), Z = 2 was determined at T = 173 K and the paraelectric phase with a = 6.808(2) Angstrom, b = 7.682(3) Angstrom, c = 11.667(7) Angstrom, beta = 124.75(6)degrees, space group P2(1)/c, V = 501.4(5) A(3), Z = 2. was determined at T = 293 K. Both structures were solved by automated Patterson methods and refined by least squares methods. For the ferroelectric phase final refinement resulted in R = 0.038 for 3763 reflections (with I > 2 sigma(I)). Refinement of the paraelectric modification revealed disorder of the Sn2+ cations (the two Sn sites being separated by approximately 0.34 Angstrom) and resulted in R = 0.062 for 2211 reflections (with I > 2 sigma(I)). Comparison of the two structures showed that the tin ions shift to positions of about 0.13 Angstrom from an individual disorder-site in the high temperature phase (paraelectric) to the corresponding tin position in the low temperature phase (ferroelectric). The shift from the average Sn-position in the paraelectric phase to the Sn-positions in the ferroelectric phase is about 0.30 Angstrom (on average 10 degrees off the vector a + c), and is clearly related to the spontaneous polarization. Moreover, the average direction of these displacements is perpendicular to the modulation wave vector direction in the incommensurate phase, showing the prime importance of such movements to the incommensurate phase formation.