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Ferroelectric lead-free ${\mathrm{K}}_{x}{\mathrm{Na}}_{1\ensuremath{-}x}{\mathrm{NbO}}_{3}$ (KNN) perovskite, as the representative of the preferred oxides with good dielectric and piezoelectric behaviors, has attracted broad attentions in recent years. Here we systematically investigate the complete process of temperature dependent structural evolution in manganese (Mn)-doped ${\mathrm{K}}_{0.5}{\mathrm{Na}}_{0.5}{\mathrm{NbO}}_{3}\text{\ensuremath{-}}{\mathrm{LiBiO}}_{3}$ single crystals, presenting the ultrahigh piezoelectric coefficient of about 1050 pC ${\mathrm{N}}^{\ensuremath{-}1}$ and the excellent dielectric and ferroelectric performances, by demonstrating phonon thermodynamics associated with all Raman active modes at a wide temperature range of 80--800 K. All-round unpolarized and polarized scattering characteristics reflecting various phonon and structure properties are specified in detail. Symmetry difference, symmetry breaking, and structure rearrangement among the molecular vibrations have been proven to be related to a multiphase coexistence and the discontinuity of first-order phase transition. In comparison with the lattice dynamics, a complete phase transition ordering and the shift of transition point have been observed in $\mathrm{KNN}\text{\ensuremath{-}}{\mathrm{LiBiO}}_{3}$ crystals with the different Mn contents. This work aims at deeply revealing the details of good ferroelectric/dielectric performance, structure, and phonon thermodynamics, as well as understanding the mechanisms of first-order phase transition under the doping manipulation in KNN systems.

We systematically investigate the detailed dynamics of the phonon and local structure of rare-earth orthoferrites ${\mathrm{GdFeO}}_{3}$ single crystal with temperature and pressure induced structural/magnetic phase transition by Raman spectroscopy. Phonon evolution related to the motion of octahedra reveals paramagnetic to antiferromagnetic ordering transition of ${\mathrm{Fe}}^{3+}$ ions at N\'eel temperature ${\phantom{\rule{4pt}{0ex}}T}_{N,Fe}$. By quantifying the polarized Raman spectra, especially the cross-polarized geometry, the lattice dynamics and distortion with local structure rearrangement during ferromagnetic transition has been also discovered. Particularly, we claim that the depolarization ratio could be quantified and used to precisely determine ferromagnetic phase transition of ${\mathrm{GdFeO}}_{3}$ and symmetry evolution simultaneously. Additionally, pressure dependence (up to 25.03 GPa) of collective phonon behavior indicates that antiphase tilt in ${\mathrm{FeO}}_{6}$ octahedra is more susceptible to the stress field than the in-phase one. The ${\mathrm{FeO}}_{6}$ octahedra presents better compressible than ${\mathrm{GdO}}_{12}$ dodecahedra in the ${\mathrm{GdFeO}}_{3}$ lattice with respect to pressure. This work has discovered the physical mechanism underlying variation of local structural symmetry, octahedra tilt, and phonon dynamics in ${\mathrm{GdFeO}}_{3}$, which can be regarded as the basic view for a series of ${\mathrm{GdFeO}}_{3}$-type perovskites and more R${\mathrm{FeO}}_{3}$ system.

We have developed the multiperspective methods for discussing phase transition and evolution of molecular vibration using unpolarized and polarized Raman scatterings. There are three scattering characteristics serving as the criteria to judge the vibrational modes, molecular symmetry, and first-order phase transition. Except for the frequency collection of one or more phonon modes, remarkably, we first demonstrate in theory that polarized Raman scattering exhibits high sensitivity on dissecting first-order phase transition, such as the symmetry breaking and discontinuity during phase transition. Experimental results confirm that temperature dependencies of peak intensity and the depolarization ratio extracted from polarized spectra present the abrupt changes associated with the discontinuity of first-order phase transition. Thermal discontinuous transformation of the ${(\mathrm{K},\mathrm{Na})\mathrm{NbO}}_{3}$ lattice brings into physical correspondence with the molecule polarizability and order parameter of the system, i.e., spontaneous polarization, which is the first derivative of Gibbs free energy. Hence, these methods based on the scattering criteria could be broadly adapted for more extensive studies on the phase transition in condensed matter.

Atomically thin materials hold mutative electronic transition properties by virtue of the inevitable strains adding to the lattice distribution. A quantitative approach and clear systematical verification have seldom been presented to effectively illustrate the substrate-induced strain along the lattice distribution, although the substrate perturbations for monolayers have been repeatedly mentioned. Here, a model of difference analysis is introduced to reveal the structural strain of monolayers from the substrate, which is derived from the mismatch effect based on the variation of the thermal expansion coefficient between the substrate and monolayers. A coupling coefficient is proposed to quantitatively manifest the substrate-induced strain and mismatch effect. Furthermore, combined with the coupling coefficient, the lattice disorder from the mismatch effect is demonstrated, experimentally using the ${\mathrm{WS}}_{2}$ monolayer as reference. This study could promote broad investigations of some fundamental issues from preparation to electrical structural performances in atomically thin materials and further practical applications.

Ferroelectrics have long been recognized as one of the candidate class of materials for applications in photovoltaic devices. Recently, ferroelectric perovskite $(\text{K},\text{Ba})(\text{Ni},\text{Nb}){\mathrm{O}}_{3\ensuremath{-}\ensuremath{\delta}}$ has been successfully synthesized and demonstrated to have a near-optimal band gap (1.39 eV), exhibiting good photovoltaic performance. However, the connection between the structural order-disorder transformation, electronic structure, bulk photovoltaic, and photocatalytic properties remains not well understood. Here, we investigate the phase transition evolutions of lead-free ${[{\mathrm{KNbO}}_{3}]}_{1\ensuremath{-}x}{[{\mathrm{BaNi}}_{1/2}{\mathrm{Nb}}_{1/2}{\mathrm{O}}_{3\ensuremath{-}\ensuremath{\delta}}]}_{x}$ ($\mathrm{KBNNO}x$, $x=0--0.5$) ceramics via x-ray diffraction (XRD), Raman scattering, and computational evidences. The lattice dynamics and the origin of the successive rhombohedral$\ensuremath{\rightarrow}$orthorhombic$\ensuremath{\rightarrow}$tetragonal$\ensuremath{\rightarrow}$cubic phase transitions have been systemically explored based on temperature-dependent XRD peak positions and phonon modes under different geometries. Moreover, the differences in the phase transition temperature and interior structure between the solid solution $x=0.2$ and the end member $x=0$ highlight local and nonlocal characteristics, which are helpful for understanding the photovoltaic mechanisms. Additionally, the robust photocatalytic decoloration effect on methylene blue can further confirm the photon-generated carrier behavior in the partly structural disordered orthorhombic phase. This identification of structural phases, combined with the ability to perform photocatalytic decoloration, give some insights on promising oxide applications as semiconducting ferroelectric absorbers and carrier-separating layers in photocatalytic or photovoltaic devices.