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 m onoclinic system. The ferroelectric phase with a = 6.8145(3) Angstrom, b = 7.7170(3) Angstrom, c = 11.694(1) Angstrom, beta = 124.549(4)degr ees, space group Pc, V = 506.52(6) Angstrom(3), Z = 2 was determined a t 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, s pace group P2(1)/c, V = 501.4(5) A(3), Z = 2. was determined at T = 29 3 K. Both structures were solved by automated Patterson methods and re fined by least squares methods. For the ferroelectric phase final refi nement resulted in R = 0.038 for 3763 reflections (with I > 2 sigma(I) ). Refinement of the paraelectric modification revealed disorder of th e 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 s hift to positions of about 0.13 Angstrom from an individual disorder-s ite in the high temperature phase (paraelectric) to the corresponding tin position in the low temperature phase (ferroelectric). The shift f rom the average Sn-position in the paraelectric phase to the Sn-positi ons in the ferroelectric phase is about 0.30 Angstrom (on average 10 d egrees off the vector a + c), and is clearly related to the spontaneou s polarization. Moreover, the average direction of these displacements is perpendicular to the modulation wave vector direction in the incom mensurate phase, showing the prime importance of such movements to the incommensurate phase formation.