A non-typical sequence of phase transitions in (NH4)(3)GeF7: optical and structural characterization

作者：Mel'nikova S V; Molokeev M S^*; Laptash N M; Misyul S V

来源：Dalton Transactions, 2016, 45(12): 5321-5327.

DOI：10.1039/c5dt04907e

Single crystals of germanium double salt (NH4)(3)GeF7 = (NH4) 2GeF6 center dot NH4F = (NH4)(3)[GeF6]F were grown and studied by the methods of polarization optics and X-ray diffraction. The birefringence Delta n = (n(o) - n(e)), the rotation angle of the optical indicatrix phi(T) and unit cell parameters were measured in the temperature range 100-400 K. Three structural phase transitions were found at the temperatures: T-1 down arrow = 279.2 K (T-1 up arrow = 279.4 K), T-2 up arrow = 270 K (T-2 down arrow = 268.9 K), T-3 down arrow = 218 K (T-3 up arrow = 227 K). An unusual sequence of symmetry transformations with temperature change was established: P4/mbm (Z = 2) (G1) <-> Pbam (Z = 4) (G2) <-> P2(1)/c (Z = 4) (G3) <-> Pa (3) over bar (Z = 8) (G(4)). The crystal structures of different phases were determined. The experimental data were additionally interpreted by a group-theoretical analysis of the complete condensate of order parameters taking into account the critical and noncritical atomic displacements. Strengthening of the N-H center dot center dot center dot F hydrogen bonds can be a driving force of the observed phase transitions.

• 出版日期2016