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Phase-matching of light waves is a critical condition for maximizing the efficiency of nonlinear frequency conversion processes in nonlinear optical crystals; however, phase-matching, commonly achieved by tuning birefringence, is often difficult to achieve over a wide wavelength range. Here, full-wavelength phase-matching crystals that can avoid phase-mismatching across the entire optical transparency range are proposed. The anisotropic strength of bonding in the dimension of energy is confirmed theoretically to be the key to the full-wavelength phase-matching ability. We demonstrate that a crystal of guanidinium tetrafluoroborate (C(NH2)3BF4) can be phase-matched throughout its entire optical transparency range and is able to generate harmonic light as short as ~193.2 nm, which is close to its deep-ultraviolet cut-off edge. Importantly, this crystal is stable, cheap and efficient compared with commercially available nonlinear optical crystals for generation of 266 nm light. This work lays the foundation for finding a new class of crystals in which the phase-matching wavelength fully covers its optical transparency range, and also provides a high-performance crystal for generating light at 266 nm—the fourth-harmonic of a commercial 1,064 nm laser.A new nonlinear optical crystal offers efficient harmonic generation in the ultraviolet and deep-ultraviolet regions.

Symmetry is an essential concept in physics, chemistry and materials science. Comprehensive, authoritative and accessible symmetry theory can provide a strong impetus for the development of related materials science. Through the sustained efforts of physicists and crystallographers, researchers have mastered the relationship between structural symmetry and ferroelectricity, which demands crystallization in the 10 polar point groups. However, the symmetry requirement for antiferroelectricity is still ambiguous, and polar crystals possessing antiferroelectricity seem contradictory. This work systematically and comprehensively studies the transformation of dipole moments under symmetry operations, using accessible geometric methods and group theory. The results indicate crystals that crystallize in polar point groups 2 ( C 2 ), m ( C 1h ), mm 2 ( C 2v ), 4 ( C 4 ), 4 mm ( C 4v ), 3 m ( C 3v ), 6 ( C 6 ) and 6 mm ( C 6v ) also possess anti-polar structure and are capable of Kittel-type antiferroelectricity. The anti-polar direction of each point group is also highlighted, which could provide a straightforward guide for antiferroelectric property measurement. Like ferroelectric crystals, antiferroelectric crystals belonging to polar point groups have great potential to become a family of important multifunctional electroactive and optical materials. This contribution refines antiferroelectric theory, will help facilitate and stimulate the discovery and rational design of novel antiferroelectric crystals, and enrich the potential functional applications of antiferroelectric materials.

PbZr1−xTixO3 (PZT) and Pb(Mg1/3Nb2/3)1−xTixO3 (PMN-xPT) are complex lead-oxide perovskites that display exceptional piezoelectric properties for pseudorhombohedral compositions near a tetragonal phase boundary. In PZT these compositions are ferroelectrics, but in PMN-xPT they are relaxors because the dielectric permittivity is frequency dependent and exhibits non-Arrhenius behavior. We show that the nanoscale structure unique to PMN-xPT and other lead-oxide perovskite relaxors is absent in PZT and correlates with a greater than 100% enhancement of the longitudinal piezoelectric coefficient in PMN-xPT relative to that in PZT. By comparing dielectric, structural, lattice dynamical, and piezoelectric measurements on PZT and PMN-xPT, two nearly identical compounds that represent weak and strong random electric field limits, we show that quenched (static) random fields establish the relaxor phase and identify the order parameter.

Lanthanide doping is widely employed to tune structural change temperature and electrical properties in ABO3-type perovskite ferroelectric materials. However, the reason that A-site lanthanide doping leads to the decrease of the Curie temperature is still not clear. Based on the reported Curie temperature of lanthanides (Ln) doped in two classic ferroelectrics PbTiO3 and BaTiO3 with A2+B4+O3-type perovskite structure, we discussed the relationship between the decrease rate of Curie temperature (ΔTC) and the bond strength variance of A-site cation (σ). For Nd ion doped Pb(Mg1/3Nb2/3)O3-PbTiO3 (Nd-PMNT) ferroelectric crystal as an example, the internal factors of the dramatic decline of the Curie temperature induced by A-site Nd doping were investigated under a systematic study. The strong covalent bonds of Ln-O play an important role in A-site Ln composition-induced structural change from ferroelectric to paraelectric phase, and it is responsible for the significant decrease in the Curie temperature. It is proposed that the cells become cubic around the Ln ions due to the strong covalent energy of Ln-O bonding in A-site Ln doped A2+B4+O3 perovskite ferroelectrics.

As a fundamental parameter of the optical crystals, birefringence plays a vital role in many optical applications, such as phase modulation, light splitting, and polarization, especially the phase matching process of the nonlinear optical crystals. The big birefringence not only benefits to the miniaturization of related devices, but also broadens the phase-matching wavelength range of nonlinear optical crystals. The design and synthesis of crystals with large birefringence becomes a hot research topic due to its more and more important applications in the optical modulation and laser technology fields. Herein, crystals with birefringence greater than 0.05 in the borate system are reviewed and classified according to different birefringent active groups, and the relationship between structure and properties is thoroughly explored. It is hoped that this review will provide a clear understanding of what kinds of building units and arrangements would have more opportunity to get adequate birefringence in borate systems and provide the statistical references to encourage the emergence of better crystal materials with large birefringence.

The phase diagram of the Pb(Lu0.5Nb0.5)O3-PbTiO3 (PLN-PT) binary system was previously reported based on XRD and dielectric measurements results. Unusually, the Curie temperature of PLN-PT with low PT obtained from the phase diagram is much lower than that of PLN and PT end members, which is different from others, such as PZT. Therefore, the complex structure of PLN-PT with low PT is desired to be studied. In this work, PLN-PT single crystals with low PT were grown for the study of their super-lattice structure and phase evolution. The super-lattice reflections were identified by X-ray diffraction. Domains and their evolution by heating from room temperature to 150 °C were observed under a polarized light microscope. The phase transition from the ferroelectric phase to the paraelectric phase was determined by dielectric spectra and polarized light microscopy. A precursor/intermediate phase exhibiting pinched hysteresis loops was displayed above the Curie temperature, which originates from some polar region embedded in the non-polar matrix. The coexistence of the ferroelectric and antiferroelectric domains leads to peculiarities of the phase transitions, such as a lower Curie temperature compared with PLN and PT. The studies of the phase evolution of PLN-PT with low PT single crystal is a supplementary amendment of the PLN-PT phase diagram as previously reported.

Lead-free (K0.5Na0.5)NbO3-LiNbO3 (KNN-LN) and (K0.5Na0.5)NbO3-LiTaO3 (KNN-LT) ferroelectric single crystals, with the dimensions of 11 ´ 11 ´ 5 mm3 and 5 ´ 5 ´ 3 mm3, were grown successfully using the top-seeded solution growth (TSSG) method, respectively. The crystal structures were analyzed by means of X-ray diffraction, showing orthorhombic symmetry for KNN-LN single crystals and coexistence of orthorhombic and tetragonal symmetry for KNN-LT single crystals at room temperature. The orthorhombic-tetragonal (TO-T) and tetragonal-cubic (TC) phase transition temperatures are 195 °C and 420 °C for the KNN-LN single crystals, and 130 °C and 280 °C for KNN-LT single crystals, respectively. The remnant polarization (Pr) is 27.8 μC/cm2 with a coercive field (Ec) of 17 kV/cm for KNN-LT single crystals. The two single crystals showed 90° domains with layers in (parallel) straight lines, while KNN-LT single crystals have a larger domain region. The actual stoichiometry deviates easily from the original composition in the process of crystal growth, thus, an appropriate nominal composition and optimized crystal growth method is desired to get high-quality crystals in the future.

As structural variants of famous hexagonal tungsten bronzes, hexagonal tungsten oxides (HTO) represent an important family with fascinating functional properties, such as piezoelectric, ferroelectric, pyroelectric, and nonlinear optical (NLO) properties. However, none of them are transparent in the deep-UV spectral region, which limits their applications. Herein, we report the first HTO-type monofluorophosphate K 3 Sc 3 (PO 4 )(PO 3 F) 2 F 5 ( I ) with deep-UV transparency. Such a monofluorophosphate is NLO-active with a phase-matchable powder second harmonic generation efficiency of 0.9 times that of KH 2 PO 4 at 1064 nm. Importantly, the UV-Vis reflectance spectrum indicates that it is deep-UV transparent down to 200 nm. This work pushes the transparent window of NLO materials with HTO-type structures down to the deep-UV spectral region for the first time and opens up a new door for HTO materials.

BiFeO 3 -Pb(Mg 1/3 Nb 2/3 )O 3 -PbTiO 3 (BF-PMN-PT) ternary ceramics with pure perovskite phase were prepared through a two-step solid reaction method. Based on structural analysis, the ternary phase diagram of BF-PMN-PT solid solution at room temperature has been established. The Curie temperature T C , remnant polarization P r and piezoelectric constant d 33 vary in the range of 138 to 225 °C, 15.12 to 23.65 μC/cm 2 and 129 to 276 pC/N, respectively. The coercive field Ec increases gradually from 5.77 to 29.56 kV/cm upon PT content increasing. The magnetic study suggests that the magnetism turns from diamagnetism for PMN-PT to paramagnetism for BF-PMN-PT by adding BiFeO 3 into PMN-PT and adding more content of BF does not change the paramagnetism further.

BiFeO sub(3)-Pb(Mg sub(1/3)Nb sub(2/3))O sub(3)-PbTiO sub(3) (BF-PMN-PT) ternary ceramics with pure perovskite phase were prepared through a two-step solid reaction method. Based on structural analysis, the ternary phase diagram of BF-PMN-PT solid solution at room temperature has been established. The Curie temperature T sub(C), remnant polarization P sub(r) and piezoelectric constant d sub(33) vary in the range of 138 to 225 degree C, 15.12 to 23.65 mu C/cm super(2) and 129 to 276 pC/N, respectively. The coercive field Ec increases gradually from 5.77 to 29.56 kV/cm upon PT content increasing. The magnetic study suggests that the magnetism turns from diamagnetism for PMN-PT to paramagnetism for BF-PMN-PT by adding BiFeO sub(3) into PMN-PT and adding more content of BF does not change the paramagnetism further.