Experimental studies of structurally incommensurate crystal phases

Dielectric and light scattering spectroscopy are the most sensitive methods for the direct observation of low-frequency excitations in incommensurate phases. In several cases they have been successfully used for the observation of amplitudon and phason modqs. Whereas the amplitudon below the normal-incommensurate transition temperature behaves in a similar way to soft modes at common structural transitions, the phason represents a qualitatively new feature of incommensurate phases. All the corresponding experimental data published up to the middle of 1980 are reviewed and consistently interpreted taking into account the specific selection rules recenily derived. The necessary theoretical background is discussed qualitatively.

Dielectric and light scattering spectroscopy of incommensurate phases in crystals

Crystal chemical aspects of ferroelectrics AnMnO3n+2 with layered perovskite-like structure (the idealized and real structures, nature of phase transitions, the Curie temperatures, peculiar properties of Sr2Ta2O7, stability of the layered structure, formation of solid solutions) are discussed.

Crystal chemical aspects of the layered perovskite-like oxide ferroelectrics of the AnMnO3n 2 type

Polarized far-infrared reflectivity and transmission measurements of an Sr2Nb2O7 single crystal were performed in the temperature interval 10-550 K for the E||a and E||c polarizations. Modes at the centre of the Brillouin zone were analysed in the subsequent paraelectric-ferroelectric-incommensurate-monoclinic phases and the factor group analysis together with the correlation of the modes is presented. The ferroelectric A1 soft mode revealed in Raman spectra below Tc = 1613 K was found in the E||c polarized infrared spectra. An amplitudon which softens to Ti = 488 K was observed in the incommensurate phase. On cooling the amplitudon anti-crosses a hard excitation near 33 cm-1 located in discommensurations and then, at about 100 K, also the ferroelectric soft mode. No evidence was found for the earlier suggested multiplication of the unit cell below the phase transition at T0 = 117 K. Neglecting the small volume of discommensurations, we suggest the Pb11-C2S space group (Z = 4) for the monoclinic phase below T0.

https://iopscience.iop.org/article/10.1088/0953-8984/13/12/307/pdf

Polar phonons and far-infrared amplitudon in Sr2Nb2O7

We evaluate theoretically the thermoelectric efficiency of the layered perovskite ${\mathrm{Sr}}_{2}{\mathrm{Nb}}_{2}{\mathrm{O}}_{7}$ via calculations of the electronic structure and transport coefficients within density-functional theory and Bloch-Boltzmann relaxation-time transport theory. The predicted figure-of-merit tensor $ZT$, computed with energy-, chemical potential--, and temperature-dependent relaxation times, has one component increasing monotonically from around 0.4 at room temperature to 2.4 at 1250 K at an optimal carrier density of around $2\ifmmode\times\else\texttimes\fi{}{10}^{20}\phantom{\rule{0.28em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}3}$, while the other components are small. The Seebeck coefficient is about 250 to 300 $\ensuremath{\mu}\mathrm{V}$/K at optimal doping and reaches 800 $\ensuremath{\mu}\mathrm{V}$/K at lower doping. We provide a python code implementing various approximations to the energy-dependent relaxation-time transport, which can be used to address different systems with an appropriate choice of material parameters.

H. Kakinuma

Papers

Optical mode softening in the incommensurate phase of Sr2Nb2O7