W. Paraguassu

Raman scattering study of NaAl(MoO4)2 crystal under high pressures

The high-pressure Raman spectra of ferroelastic NaAl(MoO4)2 have been measured at room temperature. The studies indicated that this crystal exhibits two pressure-induced phase transitions at about 1.1 and 3.3 GPa. The first transition is connected with slight rotation of the MoO4−2 tetrahedra with loss of the inversion centre. The second transition is connected with significant distortion of the MoO42− tetrahedra and Al3+ coordination sphere. By performing the lattice dynamics calculations in the starting phase (monoclinic C2h6) we have been able to make an assignment of the Raman modes of this material. The deep knowledge of the modes helped us to get fundamental insights into the mechanism driving the structural changes occurring in NaAl(MoO4)2. The two phase transitions observed in the 0.0–4.2 GPa pressure range are completely reversible.

Two new low-temperature phase transitions in the Li(NH4)1 − x Na x SO4 system

Dielectric measurements, X-ray diffraction and Raman scattering techniques have been used to investigate Li(NH4)0.99Na0.01SO4 (LASS1%) and Li(NH4)0.80Na0.20SO4 (LASS20%) crystals in the temperature range from 200 to 10 K. These are interesting systems because of the presence of hydrogen bonds and structural defects and because the mother crystal, LiNH4SO4, is ferroelectric at room temperature. From the analysis of the results we can conclude that the substitution of Na+ ions in the structure of LiNH4SO4, even at quantities as low as 1%, is sufficient to induce additional phase transitions to the LiNH4SO4 system. The new phase transitions are observed at 181 and 115 K for LASS1% and at 208 and 133 K for LASS20% and both are reversible.