Marta Herraiz

Behavior of ammonium ion in β-LiRb1−x(NH4)xSO4 mixed crystals (0.5 ≤ x ≤ 1.0)

The influence of replacement of an ammonium ion by rubidium on the x-T phase diagram in the concentration range 0.50 ≤ x ≤ 1.0 has been studied by X-ray and neutron powder diffraction over a wide temperature range. It is shown that a decrease in the ammonium concentration is accompanied by an increase in the temperature of the II ⇔ I phase transition and stabilization of phase II up to low temperatures (20 K). The changes occurring in the dynamics of mixed crystals are studied by inelastic incoherent neutron scattering. The spectra of the generalized vibrational density of states obtained allowed one to establish the difference in the phonon modes corresponding to phase III of β-LiNH4SO4 and phase II of β-LiRb1−x(NH4)xSO4 mixed crystal with x = 0.91 and x = 0.77 at 20 K. It is shown that a mixed crystal with x = 0.77 at 20 K is in the orientational glass state.

Neutron studies of the structure and dynamics of Rb1 − x(NH4)xI mixed crystals

The structural and dynamical properties of the x-T phase diagram of the system of Rb1 − x(NH4)xI mixed crystals is of great interest, because such solid solutions are almost free of internal stresses due to almost equal ionic radii of ammonium and rubidium. The x-T phase diagram of Rb1 − x(NH4)xI is studied on samples with ammonium concentration ranging from 0.01 to 0.73 over the temperature range from 15 to 300 K by the methods of powder neutron diffraction and inelastic incoherent neutron scattering. The results obtained by powder neutron diffraction show that the α-β phase transition at low temperature is rather extended and occurs at the ammonium concentrations x = 0.50 and 0.66. The region of orientational state glass is determined by inelastic incoherent neutron scattering at the concentrations x = 0.29 and 0.40 at the temperature T = 20 K.

Synthesis and Characterization of a New Aurivillius Phase

Several new Aurivillius phases with the general formula Bi2.5+xLi0.5−3xNb2O9 (0.04 x 0.08) were synthesized. The crystal structure of the compound with x = 0.04 was studied by X-ray powder diffraction at room temperature, and by neutron powder diffraction in temperature intervals from 10 to 290 K. A single-phase compound was obtained. The crystal structure can be described by a space group, Cmc21, with