Extending Unique 1D Double-Chains to 2D Layers with Birefringent Gain by Introducing a Hydroxyl Group.

It remains a significant hurdle for discovering birefringent materials in the deep ultraviolet (DUV, λ < 200 nm). It is well-known that the OH anions are recognized for their capability to eliminate the dangling bonds from terminal oxygen atoms, promoting the ultraviolet (UV) cutoff edge blueshift and regulating the crystal structure. Here, two new barium hydroxyborates, Ba3B11O18(OH)3(H2O) (BaBOH) and Na2BaB10O16(OH)2(H2O)2 (NaBaBOH), were designed and synthesized while displaying different dimensions. Remarkably, BaBOH presents novel one-dimensional (1D) [B22O37(OH)6]∞ double-chains formed by a new fundamental building block (FBB)[B11O21(OH)3]. NaBaBOH possesses a 2D [B10O16(OH)2]∞ layer with a less common FBB [B10O19(OH)2]. They enrich the structural diversity of hydroxyborates. Moreover, NaBaBOH exhibits a broad transparent window within the DUV spectral range (<190 nm) and possesses a favorable birefringence of 0.064. Furthermore, detailed summaries and structural comparisons have been implemented for all hydroxyborates containing alkali and alkaline-earth metals. This reveals that the OH group modulation strategy can be appropriately employed for the structural design.

A Sharp Improvement of Sulfate's Birefringence Induced by the Synergistic Effect of Heteroleptic and Dimeric Strategies.

As for tetrahedron-based ultraviolet nonlinear optical crystals, it is so difficult to achieve a sufficient birefringence to satisfy the phase-matching condition. Thereover, it is necessary to greatly increase the polarizability anisotropy of tetrahedra. Meanwhile, the tetrahedra should be arranged as uniformly as possible. Based on these ideas, a new strategy that dimerizes the heteroleptic tetrahedra with heteroleptic atoms as bridges is proposed and implemented, resulting in a new compound, Na2 S3 O6 , which crystallizes in a centrosymmetric monoclinic phase (phase-I) and a non-centrosymmetric orthorhombic one (phase-II). In the phase-I, a large birefringence of 0.103 @ 546 nm comparable to those of many triangle-based crystals is confirmed experimentally, indicating the ability of [S3 O6 ] group in improving the birefringence. Besides, the phase-II exhibits an improved birefringence of 0.056 @ 546 nm and a moderate second harmonic generation response of 1.4 × KH2 PO4 . In particular, the phase-II exhibits a typical phase-matching behavior under the irradiation of 532 nm laser. Moreover, the excellent optical properties of [S3 O6 ] group are further verified by theoretical calculations. These results completely illustrate the validity of the design strategy. Therefore, this work lights a new route for exploring novel ultraviolet nonlinear optical crystals.

Mg assists in modulating the dimensionalities of the anionic frameworks of borates.

Three Mg-containing borates were obtained by high-temperature spontaneous crystallization. In the (A2O)- or (A2O-MO)-MgO-B2O3 system (A is alkali metal and M is alkaline-earth metal) reported in the ICSD, Li4Mg3SrB12O24 is the first compound that contains one-dimensional infinite anionic chains, and the two examples of the isostructural A2Mg3B16O28 (A = Rb, Cs) exhibit a two-dimensional infinite bilayer structure for the first time, which contributes to the enrichment of the structural chemistry of Mg-containing borates. Besides, the results of comparison and analysis in this system clearly show that Mg not only affects the anionic frameworks of borates to produce low-dimensional structures but, together with the ratio of Ncation/NB, is responsible for the dimensionalities of the anionic frameworks in borates. The optical properties of the three compounds also show that they all have short cutoff edges, and Cs2Mg3B16O28, in particular, could reach the deep-ultraviolet region (<200 nm).

Rare-Earth Scandium Borate Fluoride with a Deep-Ultraviolet Cutoff Edge.

Through reasonable selections of raw materials and experimental methods, a new rare-earth borate fluoride K11Sc5(B5O10)4F6 is synthesized successfully by the high-temperature solution method in a closed system, which is the first noncentrosymmetric scandium borate fluoride. It crystallizes in the Fdd2 space group of the orthorhombic crystal system and features an extremely complicated structure constructed by the fundamental building blocks [B5O10] units, Sc-based, and K-based polyhedra. To our knowledge, K11Sc5(B5O10)4F6 is the only rare-earth borate that contains two kinds of [B5O10] groups and crystallizes in the Fdd2 space group, enriching the structural chemistry of rare-earth borates and rare-earth borate fluorides. Additionally, it is discussed in detail how F can significantly improve performance by modifying the modules in a comparison of structures. Discussion on rational synthetic conditions is instructive for obtaining rare-earth borate fluorides. Furthermore, a short cutoff edge (<190 nm) is experimentally confirmed, indicating the potential application of K11Sc5(B5O10)4F6 in ultraviolet/deep-ultraviolet regions.

Introduction of the [B-O/F] Units Enhances the Band Gap and Birefringence from Na6Mg3B10O18F6 to K3NaB10O16F2.

The borate family is the main source of deep-ultraviolet (DUV) birefringent crystals, and it has attracted a lot of attention due to versatile [B-O] basic units. Herein, two new borate-based compounds Na6Mg3B10O18F6 and K3NaB10O16F2 were discovered. Their fundamental building blocks are [B5O11] and [B5O10F] units, respectively. The calculated results showed that the band gap and birefringence of K3NaB10O16F2 (Eg = 6.93 eV, Δn = 0.047 at 1064 nm) are greater than those of Na6Mg3B10O18F6 (Eg = 5.40 eV, Δn = 0.039 at 1064 nm). Furthermore, the effects of [B-O/F] units on band gap and birefringence were analyzed by the charge-transfer model and response electron distribution anisotropy method. The results show that introducing the [B-O/F] units can improve the band gap and birefringence. These findings will boost the exploration of DUV birefringent opticals.

Rational Design of the First Ammonium Magnesium Borate with Deep-Ultraviolet Cutoff Edge and Moderate Birefringence and Further Investigation into the Nature of Ammonium in the Borate System.

Through the rational design of the experimental method, the first combination of ammonium and magnesium in the borate system was successfully achieved. In this paper, a case of ammonium magnesium borate, (NH4)2{Mg(H2O)2[B6O7(OH)6]2}·2H2O, was successfully synthesized by a mild hydrothermal method at a relatively low temperature. A brief review was performed to show the participation of NH4+ in the recent development of optical materials. By discussing the optimum synthesis method of ammonium-containing borates and the main factors affecting the dimensionality of B-O anionic groups in their structures, the design strategy for synthesizing ammonium-containing borate and adjusting its structure has been put forward. Relevant experimental measurement results and the first-principles calculation results show that the title compound has a deep-UV cutoff edge (<200 nm) and moderate birefringence (Δncal. = 0.064 @546 nm), which indicates its potential application in the deep-UV region.

A2B6O9F2 (A = NH4, K): new members of A2B6O9F2 family with deep-UV cutoff edges and moderate birefringence.

The fluorooxoborate A2B6O9F2 family (A = alkali metal) has attracted much attention because of its diverse structures and properties. Herein, two new members of this family, (NH4)2B6O9F2 and K2B6O9F2, have been synthesized which exhibit different two-dimensional [B6O9F2]∞ layered structures. Millimeter-sized crystals of (NH4)2B6O9F2 have been grown with NH4F as the flux. The transmittance spectrum indicates that it has a wide UV transparency window with a cutoff edge lower than 180 nm. First-principles calculations show that both (NH4)2B6O9F2 and K2B6O9F2 have deep-UV cutoff edges and moderate birefringence. The contributions of the constituent structural groups to the optical properties are discussed.

Symmetry of antiferroelectric crystals crystallized in polar point groups.

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), mm2 (C 2v), 4 (C 4), 4mm (C 4v), 3m (C 3v), 6 (C 6) and 6mm (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.

Enhanced mechanical quality factor of BiScO3-PbTiO3 piezoelectric ceramics using glass composition.

Compared with pure Pb-based perovskite ferroelectric materials, Bi(Me)O3-PbTiO3 (Me = Sc3+, In3+, Yb3+) have attracted attention due to their remarkable advantage in their Curie temperature. Among them, BiScO3-PbTiO3 piezoelectric ceramic is a potential piezoelectric material in high-temperature applications for its high Curie temperature and excellent piezoelectric coefficient. However, its shortcomings are high dielectric loss and low mechanical quality factor. Herein, we report the improvement of the mechanical quality factor of 0.36BS-0.64PT ceramics through the addition of glass composition (GeO2). There is a small change in the Curie temperature after GeO2 addition. The piezoelectric coefficient d 33 and planar electromechanical coupling factor k p increase first and then decrease, and the mechanical quality factor Q m monotone increases with an increase in GeO2. The 0.36BS-0.64PT + 0.5 mol%GeO2 ceramics have optimal electrical properties with T C of 455 °C, d 33 of 385 pC N-1, k p of 58%, and Q m of 90. In addition, the thermal stability of 0.36BS-0.64PT + xGeO2 and 0.36BS-0.64PT ceramics is almost the same. It was concluded that the mechanical quality factor of BS-PT ceramics can be enhanced by the addition of GeO2 with other properties remaining unchanged.

Phosphogermanate Crystal: A New Ultraviolet-Infrared Nonlinear Optical Crystal with Excellent Optical Performances.

The phase matching ability is a key factor for nonlinear optical crystals to realize coherent output. Herein, a new design strategy combining ultraviolet and infrared functional groups into a ferroelectric was put forward. Thus, a phosphogermanate crystal, KGeOPO4, was designed and studied. It exhibits a wide transparency window (0.22-9.70 µm), a strong second harmonic generation response (5× KH2PO4), a high laser-induced damage threshold (1.61 GW/cm2), and the typical ferroelectricity (coercive field ∼ 9.8 kV/cm and remnant polarization ∼7.6 µC/cm2). In the infrared region, it could realize coherent output by the birefringence phase matching method, while it could generate ultraviolet coherent lights by the quasi-phase matching technique. Therefore, this work designs a promising ultraviolet-infrared nonlinear optical crystal and provides a new perspective for exploring nonlinear optical crystals.

Complete Sets of Material Constants of [001]-Poled 0.72Pb(Mg1/3Nb2/3)O3- 0.28PbTiO3 Single Crystals Using Alternating Current Poling.

Alternating current poling (ACP) is an effective method to improve the piezoelectric performance of relaxor-PbTiO3 (PT) ferroelectric single crystal. 0.72Pb(Mg1/3Nb2/3)O3-0.28PbTiO3 (PMN-PT) single crystals have been used to fabricate piezoelectric transducers for medical imaging. Up to date, there are no reports about the full matrix material constants of PMN-0.28PT single crystals poled by ACP. Here, we report the complete sets of elastic, dielectric, and piezoelectric properties of [001]-poled PMN-0.28PT single crystals by direct current poling (DCP) and ACP through the resonance method. The results show that [001]-poled rhombohedral PMN-0.28PT single crystals exhibit the enhancement of longitudinal and transverse piezoelectric properties ( d33  âˆ¼ 2000 pC/N, d31  âˆ¼ -1010 pC/N) after ACP. Compared with DCP samples ( d33  âˆ¼ 1660 pC/N, d31  âˆ¼ -780 pC/N), the values of d33 and d31 increase 20% and 29%, respectively. While the d15 value decrease from 110 pC/N for DCP sample to 90 pC/N for ACP sample, showing the decrease in transverse shear piezoelectric properties. In addition, the elastic stiffness coefficients c11 , c12 , c13 , the elastic compliance coefficients s11 , s12 , and the dielectric constants ε11 , ε33 have great change compared with DCP and ACP samples. This variation of the property matrices provides a reference for high-performance piezoelectric device design.

Non-π-Conjugated Deep-Ultraviolet Nonlinear Optical Crystal K2Zn3(SO4)(HSO4)2F4.

Deep-ultraviolet (deep-UV) (wavelengths of <200 nm) nonlinear optical (NLO) materials are playing an increasingly important role because of their significant technological applications in advanced scientific instruments. In recent years, the non-π-conjugated systems have received extensive attention as new emerging sources of deep-UV NLO materials. Here, a new non-π-conjugated deep-UV NLO material, K2Zn3(SO4)(HSO4)2F4, has been successfully obtained by the hydrothermal method. It has a layered structure formed by [Zn3(SO4)(HSO4)2F3]∞ layers bridged via K-O and K-F bonds. Powder second-harmonic generation shows that K2Zn3(SO4)(HSO4)2F4 can achieve phase matching, and the response is ∼0.3 times that of KH2PO4. Remarkably, the single-crystal transmittance spectrum confirms that the absorption edge of K2Zn3(SO4)(HSO4)2F4 is below 200 nm, and the experimentally measured birefringence is 0.0126 at λ = 546 nm. In-depth first-principles calculations illustrate well the microscopic origin of the optical properties. This work enriches the structrual diversity of non-π-conjugated deep-UV NLO materials.

Giant Second-Harmonic Generation Response and Large Band Gap in the Partially Fluorinated Mid-Infrared Oxide RbTeMo2O8F.

Strong second-harmonic generation (SHG) and a wide band gap are two crucial but often conflicting parameters that must be optimized for practical nonlinear optical (NLO) materials. We report herein the first d0-transition-metal (TM) tellurite with half of the d0-TM-octahedra partially fluorinated, namely, quinary RbTeMo2O8F, which exhibits giant SHG responses (27 times that of KH2PO4 (KDP) and 2.2 times that of KTiOPO4 (KTP) with 1064 and 2100 nm laser radiation, respectively), the largest SHG values among all reported metal tellurites. RbTeMo2O8F also possesses a large band gap (3.63 eV), a wide optical transparency window (0.34-5.40 µm), and a significant birefringence (Δn = 0.263 at 546 nm). Theoretical calculations and crystal structure analysis demonstrate that the outstanding SHG responses can be definitively attributed to the uniform alignment of the polarized [MoO5F]/[MoO6] octahedra and the seesaw-like [TeO4], and the consequent favorable summative polarization of the three distinct SHG-active polyhedra, both induced by partial fluorine substitution on the [MoO6] octahedra.

Optimizing the Piezoelectric and Dielectric Properties of Pb(In1/2Nb1/2)O3-PbTiO3 Ferroelectric Crystals via Alternating Current Poling Waveform.

The alternating current poling (ACP) method has been attracting significant attention because of the enhanced piezoelectric and dielectric properties of relaxor-based ferroelectric (FE) crystals with advantages of high-efficiency, time-savings, and ease of operation. The most commonly used poling waveform is the bipolar triangle. Other waveforms have been seldom applied in ACP. However, the waveform plays a significant influence on the fatigue performance of FE materials. Therefore, optimizing the ACP waveform to gain high piezoelectric performance and decrease dielectric loss factors is technologically essential. Here, the effects of ACP waveforms on the piezoelectric and dielectric properties of [001]c-oriented 0.66 PIN-0.34 PT FE crystals were studied. The highest piezoelectric coefficient d33 (1450 pC/N) and dielectric permittivity εT33/ε0 (3180) were obtained using the sesquipolar triangle waveform while the lowest dielectric loss factors tan δ (0.99%) was obtained using the unipolar triangle waveform. FE hysteresis loop measurements revealed that the lower coercive field and current density are beneficial for higher piezoelectric performance. Domain analysis showed that ACP samples exhibit a regular domain structure and high density 109° domain walls, which are the critical reasons for enhanced piezoelectric performance. This work provides guidance in designing an ACP waveform to achieve higher performance.

π-Conjugated Trigonal Planar [C(NH2)3]+ Cationic Group: A Superior Functional Unit for Ultraviolet Nonlinear Optical Materials.

Employing π-conjugated anionic groups in molecular construction has been proven to be an effective strategy to find superior ultraviolet (UV) nonlinear optical (NLO) crystals over the decades. Herein, unlike the traditional π-conjugated anionic groups, we identify that a π-conjugated cationic group, viz., [C(NH2)3]+, is also an excellent UV NLO-active functional group in theory. Furthermore, we identify a [C(NH2)3]+-containing compound, C(NH2)3ClO4, as a promising UV NLO candidate due to its short UV cutoff edge (200 nm), remarkable second-harmonic generation effect (∼3 × KDP), and moderate birefringence of 0.076@1064 nm. Additionally, C(NH2)3ClO4 has excellent ferroelectric properties and reversal of domains, which also enables it to produce ultraviolet coherent light as short as 200 nm by a quasi-phase matching technique with a periodically poling method. Our study may provide not only a promising UV NLO crystal but also a new π-conjugated functional unit, [C(NH2)3]+, which will open a path to finding new classes of high-performance UV NLO crystals.

UV Solar-Blind-Region Phase-Matchable Optical Nonlinearity and Anisotropy in a π-Conjugated Cation-Containing Phosphate.

Wide ultraviolet (UV) transparency, strong second-harmonic generation (SHG) response, and sufficient optical birefringence for phase-matching (PM) at short SHG wavelengths are vital for practical UV nonlinear optical (NLO) materials. However, simultaneously optimizing these properties is a major challenge, particularly for metal phosphates. Herein, we report a non-traditional π-conjugated cation-based UV NLO phosphate [C(NH2 )3 ]6 (PO4 )2 ⋅3 H2 O (GPO) with a short UV cutoff edge. GPO is SHG active at 1064 nm (3.8 × KH2 PO4 @ 1064 nm) and 532 nm (0.3 × ß-BaB2 O4 @ 532 nm) and also possesses a significant birefringence (0.078 @ 546 nm) with a band gap >6.0 eV. The PM SHG capability of GPO can extend to 250 nm, indicating GPO is a promising UV solar-blind NLO material. Calculations and crystal structure analysis show that the rare coexistence of wide UV transparency, large SHG response, and optical anisotropy is due to the introduction of π-conjugated cations [C(NH2 )3 ]+ and their favorable arrangement with [PO4 ]3- anions.

Large Second-Harmonic Response and Giant Birefringence of CeF2(SO4) Induced by Highly Polarizable Polyhedra.

Second-harmonic generation (SHG) response and birefringence are two critically important properties of nonlinear optical (NLO) materials. However, the simultaneous optimization of these two key properties remains a major challenge because of their contrasting microstructure requirements. Herein, we report the first tetravalent rare-earth metal fluorinated sulfate, CeF2(SO4). Its structure features novel net-like layers constructed by highly distorted [CeO4F4] polyhedra, which are further interconnected by [SO4] tetrahedra to form a three-dimensional structure. CeF2(SO4) exhibits the strongest SHG effect (8 times that of KH2PO4) and the largest birefringence for sulfate-based NLO materials, the latter exceeding the birefringent limit for oxides. Theoretical calculations and crystal structure analysis reveal that the unusually large SHG response and giant birefringence can be attributed to the introduction of the highly polarizable fluorinated [CeO4F4] polyhedra as well as the favorable alignment of [CeO4F4] polyhedra and [SO4] tetrahedra. This research affords a new paradigm for the designed synthesis of high-performance NLO materials.

Broad bandwidth emission and in situ electric field modulation of photoluminescence in Nd-doped ferroelectric crystals.

The electric field modulation of photoluminescence in ferroelectric-optical materials as a novel in situ, non-damaging and real-time controllable method has drawn much research focus. The broad bandwidth emission of 33 nm and a tuneable luminescence contrast of 28% were achieved in Nd-doped Pb(Mg1/3Nb2/3)O3-PbTiO3 (Nd:PMNT) tetragonal ferroelectric-optical crystals arising from spontaneous polarization. The study of Nd:PMNT ferroelectric crystals under a cyclic, triangular alternating current voltage wave showed that the change in the photoluminescence intensity subjected to an electric field is mainly related to the 180° domain distribution, rather than the 90° domain. The physical process of electric field tuning luminescence is to control the 180° domain distribution using the external electric field, which thereby effectively tunes the luminescence. This finding restricts the limitation of a specific phase change region, which greatly increases the range of materials used and has guiding significance for research in the electric field modulation of luminescent technology.

Effects of alternating current poling on the dielectric and piezoelectric properties of Pb(In0.5Nb0.5)O3-PbTiO3 crystals with a high Curie temperature.

The alternating current poling (ACP) method has been become more and more popular recently because of its advantages of being low cost, time saving and highly efficient. Few ACP studies have focused on relaxor-PT crystals with a high coercive field and high Curie temperature or the effects of ACP on intrinsic and extrinsic contributions. The effects of the electric field, frequency, and number of cycles of ACP on the piezoelectric and dielectric properties of 〈001〉-oriented Pb(In0.5Nb0.5)O3-PbTiO3 ferroelectric crystals were studied. The dielectric permittivity ε 33 T /ε 0 and piezoelectric coefficient d 33 of an ACP sample are 3070 and 1400 pC N-1, respectively, which are 14% and 18% larger than those of a DCP sample. Rayleigh analysis reveals that both intrinsic and extrinsic contributions are enhanced after ACP. The poling electric field, frequency and cycle number can influence the intrinsic and extrinsic contributions. The intrinsic contribution is significantly affected by the poling electric field, and cycle number, but it is not very sensitive to frequency, while the poling electric field, frequency and cycle number are very important for the extrinsic contribution. This work demonstrates that the uniform domain patterns are a critical factor for the enhancement of the piezoelectric properties.

Giant Optical Anisotropy in the UV-Transparent 2D Nonlinear Optical Material Sc(IO3 )2 (NO3 ).

Birefringence is a fundamental optical property for linear and nonlinear optical (NLO) materials. Thus far, it has proved to be very difficult to engineer large birefringence in optical crystals functioning in the UV region. Herein, we report the first 2D rare-earth iodate-nitrate crystal Sc(IO3 )2 (NO3 ) (SINO), which is shown to exhibit giant optical anisotropy. Air-stable SINO possesses a short UV absorption edge (298 nm), a strong NLO response (4.0 times that of benchmark KH2 PO4 ) for the nitrate family, and the largest birefringence (Δn=0.348 at 546 nm) of inorganic oxide optical crystals. The unusually large birefringence and NLO response can be attributed to an optimized 2D layered structure, combined with highly polarizable and anisotropic building units [IO3 ]- and [NO3 ]- . These findings will facilitate the development of UV linear and NLO materials with giant optical anisotropy and promote their potential application in optoelectronic devices.