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.

Zn3B7O13Cl: A New Deep-Ultraviolet Transparency Nonlinear Optical Crystal with Boracite Structure.

Nonlinear optical crystals play important roles in modern laser science and technology. However, the design and growth of new nonlinear optical (NLO) materials is still a challenging issue for researchers. Due to the excellent performance of Mg3B7O13Cl crystal, we paid attention to the optimization of its structure, in order to find new NLO materials with favorable properties. Here, Zn3B7O13Cl crystals were obtained by a high-temperature solution method. Its structure was determined to be the trigonal symmetry with a polar space group of R3c, which is more highly symmetric than that of Mg3B7O13Cl (Pca21). The experimental and theoretical investigations demonstrated that the title compound exhibits a short absorption cutoff (band gap ∼6.53 eV), moderate SHG responses (2.2 times that of KDP at 1064 nm), and the improved birefringence, which results from the large distortion and anisotropy of borate groups and zinc polyhedrons. Therefore, the structural modification of Mg3B7O13Cl by zinc cations achieves a balance between the deep-ultraviolet transparency, the nonlinear optical effect, and the moderate birefringence, which is very significant for the design of practical NLO materials.

Ca2B5O9Cl and Sr2B5O9Cl: Nonlinear Optical Crystals with Deep-Ultraviolet Transparency Windows.

M2B5O9X is a prominent family with excellent nonlinear optical (NLO) responses, just as the Pb2B5O9I crystal with a large second harmonic generation (SHG) of 13.5 times that of KH2PO4. However, most of these compounds are limited to ultraviolet and visible regions because of their long absorption edge (small band gap). Here, we report two members of this family, which change the situation. Using a high-temperature solution method, we obtain Ca2B5O9Cl and Sr2B5O9Cl crystals, which exhibit a deep-ultraviolet (DUV) absorption edge of 170 nm (band gap ≈ 7.29 eV). It is an important breakthrough in the DUV transparency of the M2B5O9X family. Furthermore, Ca2B5O9Cl crystals display a phase-matching SHG response under a 1064 nm laser, which is further confirmed by the balance between the suitable birefringence and the small dispersion of refractive indexes in the wavelength range of 1064-532 nm. Therefore, they are promising DUV transparency windows and NLO candidates.

Abrupt Structural Transformation in Asymmetric ABPO4F (A = K, Rb, Cs).

An asymmetric structure is the necessary requirement for second-order nonlinear-optical (NLO) materials, which have important applications in modern science and technology. Here we report two isostructural asymmetric compounds, RbBPO4F and CsBPO4F. Both compounds crystallize in cubic space group P213 (No. 198) with three-dimensional (3D) gismondine-like structures. Remarkably, in spite of the same basic structural units BO3F and PO4, both structures are distinct from the previously reported derivative KBPO4F, which crystallizes in a monoclinic space group Cc (No. 9) with a two-dimensional (2D)-layered structure. Careful structural analysis reveals that this structural transformation (from a monoclinic 2D structure to cubic 3D structures) should be aroused by the different alkaline ionic radii. To the best of our knowledge, such an abrupt structural transformation by alkaline elements is reported in all-inorganic asymmetric compounds for the first time. The structural transformation from 2D to 3D structures is favorable to eliminate the layered growth habit. This study will shed deep insight in the structural modulation of asymmetric compounds.

Mixing Halogens To Assemble an All-Inorganic Layered Perovskite with Warm White-Light Emission.

Most of single-component white-light-emitting materials focus on organic-inorganic hybrid perovskites, metal-organic frameworks, as well as all-inorganic semiconductors. In this work, we successfully assembled an all-inorganic layered perovskite by mixing two halogens of distinct ionic radii, namely, Rb2 CdCl2 I2 , which emits "warm" white light with a high color rendering index of 88. To date, Rb2 CdCl2 I2 is the first single-component white-light-emitting material with an all-inorganic layered perovskite structure. Furthermore, Rb2 CdCl2 I2 is thermally highly stable up to 575 K. A series of luminescence measurements show that the white-light emission arises from the lattice deformation, which are closely related to the [CdCl4 I2 ]2- octahedra with high distortion from the distinct ionic radii of Cl and I. The first-principles calculations reveal that both the Cl and I components make significant contributions to the electronic band structures of Rb2 CdCl2 I2 . These findings indicate that mixing halogens is an effective route to design and synthesize new single-component white-light-emitting materials.

Non-Centrosymmetric RbNaMgP2O7 with Unprecedented Thermo-Induced Enhancement of Second Harmonic Generation.

It is of great difficulty to obtain deep-UV transparent materials with enhanced second harmonic generation (SHG), mainly limited by the theoretically poor transparency of these materials in the deep-UV spectral region. Here we report a new noncentrosymmetric, deep-UV transparent phosphate RbNaMgP2O7, which undergoes a thermo-induced reversible phase transition (at a high temperature of 723 K) and correspondingly an evident SHG enhancement up to ∼1.5 times. The phase transition is aroused by the twist of [P2O7]4- dimers with deviation from the P-O-P equilibrium positions. Theoretical analyses reveal that the enhanced SHG can be ascribed to the thermo-induced collective alignment of SHG-active [P2O7]4- dimers along the polar axis of high-temperature phase. This work provides an unprecedented physical routine (to SHG-enhanced materials) that is distinguished from the traditional one by chemical design and synthesis.

Crystal growth and optical characteristics of beryllium-free polyphosphate, KLa(PO3)4, a possible deep-ultraviolet nonlinear optical crystal.

Deep-ultraviolet nonlinear optical crystals are of great importance as key materials in generating coherent light with wavelength below 200 nm through cascaded frequency conversion of solid-state lasers. However, the solely usable crystal in practice, KBe2BO3F2 (KBBF), is still commercially unavailable because of the high toxicity of beryllium-containing and the extreme difficulty of crystal growth. Here, we report the crystal growth and characteristics of an beryllium-free polyphosphate, KLa(PO3)4. Centimeter-sized single crystals have been easily obtained by the flux method and slow-cooling technique. The second-harmonic generation efficiency of KLa(PO3)4 powder is 0.7 times that of KH2PO4; moreover, the KLa(PO3)4 crystal is phase-matchable. Remarkably, the KLa(PO3)4 crystal exhibits an absorption edge of 162 nm, which is the shortest among phase-matchable phosphates so far. These attributes make KLa(PO3)4 a possible deep-ultraviolet nonlinear optical crystal. An analysis of the dipole moments of the polyhedra and theoretical calculations by density functional theory were made to elucidate the structure-properties relationships of KLa(PO3)4.

Growth, structure, thermal properties and spectroscopic characteristics of Nd3+-doped KGdP4O12 crystal.

A single crystal of Nd3+-doped KGdP4O12 was successfully grown with the top-seeded solution growth and slow cooling (TSSG-SC) technique. It crystallizes in space group C2/c with cell parameters a = 7.812(2) Å, b = 12.307(3) Å, c = 10.474(2) Å, ß = 110.84(3)° and Z = 4. The IR and Raman spectra also indicated that the phosphoric polyhedra of Nd:KGdP4O12 has a cyclic symmetry. The chemical composition of the crystal was analyzed and the distribution coefficient of Nd3+ was calculated. The crystal morphology of KGdP4O12 was identified using X-ray diffraction. The compound has good thermal stability to 920°C. Its specific heat and thermal conductivity were determined for potential applications. The spectral properties of Nd:KGdP4O12 indicates that it exhibits broad absorption and emission bands, which are attributed to low symmetry of the crystal. The broad absorption band around 798 nm has a full-width at half-maximum (FWHM) of 14.8 nm and is suitable for AlGaAs laser diode pumping. Moreover, 5 at% Nd3+-doped KGdP4O12 crystal has a long luminescence lifetime of 300 µs and a high quantum efficiency of 96%.