Neutron diffraction investigation of the temperature dependence of crystal structure and thermal motions of red HgI₂

Abstract

The structure of, and anisotropic thermal motions in, the red semiconductor tetrahedral layer structure of HgI(2) have been studied with neutron powder diffraction as a function of temperature from 10 to 293 K. Average thermal displacement parameters U(eq) of the two atoms are comparable in size at 10 K, but U(eq)(Hg) increases considerably faster with temperature than U(eq)(I), the Hg-I bond being highly non-rigid. The anisotropic displacement tensor U(I) is strongly anisotropic with one term about twice as large as the others, while U(Hg) is nearly isotropic. All displacement tensor elements, except U(22)(I), increase faster with temperature than harmonic quantum oscillator curves indicating a softening of the isolated-atom potentials at large amplitudes. A lattice dynamical model provides arguments that the anisotropic thermal motions of I are dominated by a soft mode with a wavevector at the [(1/2) (1/2) 0] boundary of the Brillouin zone consisting essentially of coupled librations of the HgI(4) tetrahedra, and by translations of the entire layer. The large vibration amplitudes of Hg suggest weak Hg-I force constants compared with the I-I force constants, allowing Hg to move quite freely inside the tetrahedra. The libration mode induces dynamic deformations of the Hg-I bond with twice its frequency. This provides a mechanism for the anharmonicity and may explain the lightening of the color from red to orange upon cooling at ca 80 K.

Condensation of a tetrahedra rigid-body libration mode in HoBaCo4O7: the origin of phase transition at 355 K

“…In the anion-close-packed systems, the structure becomes highly susceptible to the libration-like motions when the proportion of "bridgelike" two-coordinated anions is large. For example, in the red mercury HgI 2 , where all the anions are bridgelike, the coupled librations of HgI 4 tetrahedra are unconstrainted and the thermal displacements of anions are strongly anisotropic [29]. This is not the case for the built of tetrahedra and octahedra structure of spinel M 3 O 4 , in which all the oxygens are four-coordinated.…”

Determination of the experimental equilibrium structure of solid nitromethane using path-integral molecular dynamics simulations

“…The volume term is equivalent to the thermal expansion coefficient, T . This approach is re- 42 and the harmonic frequency was treated as a fitting parameter. The dashed line in Fig.…”

ChemInform Abstract: The HgF2 Ionic Switch: A Triumph of Electrostatics Against Relativistic Odds.

“…[11,[16][17][18][19][20][21] The mercury dihalides HgCl 2 ,H gBr 2 ,a nd HgI 2 ,h owever,f orm weakly bound dimers andt rimers (Figure 1b,d ,e ) [13,14,[22][23][24][25][26] and they condenset of orm molecular extended solids, [27][28][29] even if alternative two-a nd three-dimensional polymorphs of HgI 2 are also known. [30,31] The more strongly bound dimer and trimer structures( ca nd fi n Figure 1) are preferred by the Zn and Cd dihalides, but not in any of the HgX 2 cases. HgF 2 is as plendid exception, however.F or some time now, it has been apparent that, for HgF 2 ,arare and spectacular thing happens on the way to the extended solid.…”