High-resolution temperature-dependent Brillouin scattering studies of ferroelectric K3Nb3O6(BO3)2

High-resolution micro-Brillouin scattering studies of K3Nb3O6(BO3)2, exhibiting superionic and ferroelectric properties, were performed in the temperature range of 300–870 K. Strong increase in damping of longitudinal and transverse acoustic phonons corresponding to the c33 and c44 elastic constants was observed above 650 K. This increase in damping was correlated with anomalous decrease in the c33 and c44 elastic constants and strong increase in central peak intensities. The observed behavior has been attributed to melting of the K+ sublattice as a result of the order-disorder phase transitions. The obtained results also revealed a critical slowing down of the E[double-prime]-symmetry relaxation mode near the phase transitions at 388 and 690 K as well as the broad A1[prime]-symmetry relaxation mode near the phase transition at 750 K. This behavior indicates that the relaxation modes play an important role in the mechanism of these phase transitions. In particular, this behavior suggests that the phase transition near 690 K is induced by instability of the E[double-prime]-symmetry relaxation mode

Heat capacity, Raman, and Brillouin scattering studies of M2O–MgO–WO3–P2O5 glasses (M=K,Rb)

The authors report the results of temperature-dependent Brillouin scattering from both transverse and longitudinal acoustic waves, heat capacity studies as well as room temperature Raman scattering studies on M2O–MgO–WO3–P2O5 glasses (M=K,Rb). These results were used to obtain information about structure and various properties of the studied glasses such as fragility, elastic moduli, ratio of photoelastic constants, and elastic anharmonicity. They have found that both glasses have similar properties but replacement of K+ ions by Rb+ ions in the glass network leads to decrease of elastic parameters and P44 photoelastic constant due to increase of fragility. Based on Brillouin spectroscopy they show that a linear correlation between longitudinal and shear elastic moduli holds over a large temperature range. This result supports the literature data that the Cauchy-type relation represents a general rule for amorphous solids. An analysis of the Boson peak revealed that the form of the frequency distribution of the excess density of states is in agreement with the Euclidean random matrix theory. The reason of the observed shift of the maximum frequency of the Boson peak when K+ ions are substituted for Rb+ ions is also briefly discussed

Temperature-dependent studies of the geometrically frustrated pyrochlores Ho2Ti2O7 and Dy2Ti2O7

A temperature-dependent Raman study of Ho2Ti2O7 and Dy2Ti2O7 single crystals was performed in the 5–873 K temperature range. Polarized spectra allowed us to establish the symmetries of the observed bands and revise the mode assignment made in previous works. Our studies revealed also two additional bands near 287 and 300 cm−1 for Dy2Ti2O7, which can be assigned to crystal field transitions in Dy3+ ions. Temperature dependent Raman studies showed large increase of linewidths. These changes have been analyzed in terms of strong third-order phonon-phonon anharmonic interactions. The Raman spectra also showed anomalous softening of the majority of phonon modes upon cooling in the whole temperature range studied. In contrast to this behavior, the F2g 310 cm−1 and Eg 330 cm−1 phonon modes showed hardening upon cooling down to about 100–120 K and then anomalous softening below this temperature. This anomalous behavior of phonon wave numbers has been attributed to the increase in octahedral distortion upon cooling

Optical Spectra of Neodymium and Europium Tungstates and Molybdates

Absorption and excitation spectra of lanthanides molybdates and tungstates were measured and analyzed. Probabilities of electronic transitions were analysed on the basis of the Judd-Ofelt theory. The significant differences of intensities were observed. Those differences could result from the symmetry changes, cooperative effects and electron-phonon interaction. The elucidation of intensities given by separate mechanisms needs further investigation

Comparative Studies of Vibronic Transitions in One- and Two-Photon Spectra of Gd3+\text{}^{3+} in Cs2\text{}_{2}NaGdCl6\text{}_{6}

Vibronic spectra of Gd$\text{}^{3+}$ in Cs$\text{}_{2}$NaGdCl$\text{}_{6}$ were measured under one- and two-photon excitations. Following the vibrational analysis of the Cs$\text{}_{2}$NaGdCl$\text{}_{6}$ crystal an assignment of vibronic transitions was proposed. It was found that the vibronic transitions in one- and two-photon spectra are combined with the odd parity modes