Polarized Raman, IR, and time-domain THz spectroscopy of orthorhombic lead zirconate single crystals have yielded a comprehensive picture of temperature-dependent quasiharmonic frequencies of its low-frequency phonon modes. It is argued that these modes primarily involve vibrations of Pb ions and librations of oxygen octahedra. Their relation to phonon modes of the parent cubic phase is proposed. Counts of the observed IR and Raman active modes belonging to distinct irreducible representations agree quite well with group-theory predictions. Analysis of the results yields insight into the phase transition mechanism, involving a soft ferroelectric branch coupled by a trilinear term to another two oxygen octahedra tilt modes.

In this review we describe and discuss recent results on the linear dielectric response of high-permittivity dielectric crystals and ceramics in the wide frequency range 10²–10¹⁴Hz. Our attention is paid to materials which exhibit some interesting low-frequency polar-phonon anomalies in combination with some additional dielectric dispersion below the polar phonon region. The following compounds are discussed: microwave ceramics Bi_1.5Zn_1.0Nb_1.5O₇, antiferroelectric PbZrO₃ceramics and AgNbO₃crystals, ferroelectric LiNaGe₄O₉, LaBGeO₅, Cd₂Nb₂O₇and SrBi₂Ta₂O₉crystals, incommensurate ferroelectric Sr₂Nb₂O₇, Ba₂NaNb₅O₁₅and BCCD crystals, dipolar-glass crystal of Rb_1/2(ND₄)_1/2D₂PO₄, relaxor ferroelectric Pb(Sc_1/2Ta_1/2)O₃and PLZT ceramics and PMN crystals, antiferroelectric PLZT 2/95/5 ceramics and relaxor-based PMN-PT, PZN-PT and BiScO₃-PT crystals and ceramics. For these materials the polar phonon spectra are discussed together with their flow-frequency dielectric response and dispersion regions in between, in the 10–300 K or higher temperature range, depending on the existing phase transitions of interest. Some universal features of the disorder and anharmonicities are pointed out: central-mode phenomena near displacive phase transitions and polar nano-clusters which appear at rather high temperatures and remain present down to low temperatures if the material remains structurally disordered. The manifestation of the latter in the dielectric spectra consists in extreme broadening of the relaxation region on cooling which at low temperatures results in constant loss spectra (1/fnoise).

High-frequency dielectric properties of ${\mathrm{A}}_{\mathrm{5}}{\mathrm{B}}_{\mathrm{4}}{\mathrm{O}}_{15}$ (A=Ba, Sr, Mg, Zn, Ca; B=Nb, Ta) dielectric ceramics are studied by means of the microwave cavity technique, a combination of far-infrared reflection and transmission spectroscopy and time-resolved terahertz transmission spectroscopy. Microwave permittivity ε^′ and $\mathrm{Q\times f}$ factor vary, depending on the chemical composition, between 11 and 51, and 2.4 and 88 THz, respectively. The temperature coefficient ${\tau }_f$ varies between −73 and 232 ppm/°C, and in two samples ${\mathrm{|\tau }}_f|$ is less than 15 ppm/°C. It is shown that the microwave permittivity ε^′ of the ceramics studied is determined by the polar phonon contributions and that linear extrapolation of the submillimeter dielectric loss ε^″ down to the microwave region is in agreement with the microwave data of single phase samples. The relationship among phonon spectra, the crystal structure, and the unit cell volume is discussed.

A systematic study of the Raman spectra of $\mathrm{Pb}(\mathrm{Z}{\mathrm{r}}_{1-x}\mathrm{T}{\mathrm{i}}_x){\mathrm{O}}_3$ (PZT) ceramics has been performed in a broad temperature interval (10–600 K) and a broad Ti/Zr concentration range around the morphotropic phase boundary $(x=0.25\text{–}0.70)$. The number of the spectral components was estimated by a standard fitting procedure with damped harmonic oscillators as well as by counting the number of peaks and shoulders with the help of a purposely designed mathematical analysis based on frequency derivatives of the Raman spectra. This last method proves to be very useful to study Raman spectra of disordered materials. For the case of PZT, the comparison with the Raman modes of $\mathrm{PbTi}{\mathrm{O}}_3$ allows us to assign the phonon bands on both sides of the morphotropic phase boundary, and the crossover from the tetragonal to rhombohedral phase spectra is clearly visible. However, there are no indications of a systematic splitting of the $E$-symmetry modes into monoclinic $A^{\prime }-A^{\prime \prime }$ doublets in the morphotropic samples. Detailed adjusting of the response function to the spectrum requires to assume additional Raman-active modes, but this holds for a much broader concentration range than that of the anticipated monoclinic phase. In particular, the lowest frequency transverse optic mode of $E$-symmetry (soft mode of the ferroelectric phase transition) is split into two components, a THz frequency anharmonic (central modelike) component and a resonant component (at $\omega \sim 80\mathrm{c}{\mathrm{m}}^{-1})$, both related to the same normal coordinate. The additional Raman band appearing in this frequency range $(\omega \sim 65\mathrm{c}{\mathrm{m}}^{-1})$ at low temperatures is rather associated with the antiphase tilt vibrations of the oxygen octahedra.

Infrared (IR) reflectivity spectra of nominally pure Pb(Zr${}_{1–x}$Ti${}_x$)O${}_3$ ceramics with different Ti/Zr concentration ($x$ $=$ 0.42–0.58) were measured and evaluated, along with the time-domain terahertz transmittance spectra in the temperature range 10 K–900 K. The temperature dependence of the low-frequency vibrations, related to Pb atoms, was analyzed in terms of two overdamped modes—a soft mode and an anharmonic hopping central mode—in the cubic and high-temperature ferroelectric phase and three main vibrations in the low-temperature ferroelectric phase with the doubled unit cell: two E-symmetry modes (the soft mode and a mode corresponding to antiphase vibrations of neighboring Pb atoms in the terahertz range) and the antiferrodistortive mode producing the antiphase tilts of the oxygen octahedra. The last bare mode is not IR active, but it becomes activated by coupling with the soft mode. As predicted by theory, the intrinsic permittivity of Pb(Zr${}_{1–x}$Ti${}_x$)O${}_3$ has a maximum at the morphotropic phase boundary, although this represents just a small percentage of the total permittivity at lower frequencies. Its maximum is linked to the softening of the anharmonic vibrations of Pb ions, perpendicular to the polarization, and shifts from $x$ $=$ 0.48 at room temperature to $x$ $=$ 0.52 at 10 K.

Far-infrared reflectivity spectra of $[\mathrm{Pb}({\mathrm{Zn}}_{\text{1/3}}{\mathrm{Nb}}_{\text{2/3}}){\mathrm{O}}_3{]}_{\text{0.92}}\mathrm{–[}{\mathrm{PbTiO}}_3{]}_{\text{0.08}}$ and $[\mathrm{Pb}({\mathrm{Mg}}_{\text{1/3}}{\mathrm{Nb}}_{\text{2/3}}){\mathrm{O}}_3{]}_{\text{0.71}}\mathrm{–[}{\mathrm{PbTiO}}_3{]}_{\text{0.29}}$ single crystals were investigated between 10 and 530 K, micro-Raman spectra were recorded between 300 and 800 K. No phonon softening was observed near either of the ferroelectric phase transitions. The low-frequency dielectric anomaly in the paraelectric phase is caused by contribution of dynamic polar nanoclusters with the main dispersion in the microwave range. Infrared and Raman spectra confirm the locally doubled unit cell ${\mathrm{(Z}}_{\mathrm{prim}}\text{=2)}$ in the paraelectric and ferroelectric phases due to the ordering in the perovskite B sites and occurrence of polar nanoclusters in the paraelectric phase. The lowest-frequency transverse optical $({\mathrm{TO}}_1)$ phonon mode active in the infrared spectra is underdamped in contrast to the recent result of inelastic neutron scattering, where no ${\mathrm{TO}}_1$ mode could be observed for the wave vectors ${\text{q⩽0.2\hspace{0.17em}Å}}^{\text{−1}}.$ This discrepancy was explained by different q vectors probed in infrared and neutron experiments. The infrared probe couples with very long-wavelength phonons ${\text{(q≈10}}^{\text{−5}}{\mathrm{\hspace{0.17em}\AA }}^{\text{−1}})$ which see the homogeneous medium averaged over the nanoclusters, whereas the neutron probe couples with phonons whose wavelength is comparable to the nanocluster size ${\text{(q⩾10}}^{\text{−2}}{\mathrm{\hspace{0.17em}\AA }}^{\text{−1}}\mathrm{).}$