Optical Anisotropy and Phase Transitions in Lead Halide Perovskites

Manipulation and applications of ultrafast all-optical switching based on transient absorption and dispersion in KTN crystals

Potassium tantalate niobate (KTN) represents a noteworthy category of optical crystals known for their superior nonlinear optical properties. In this study, we conducted measurements of femtosecond time-resolved transient absorption (TA) spectra in KTa0.57Nb0.43O3 crystals. Notably, a rapid and pronounced "plateau" phase, ∼1.5 ps in duration, was detected at the onset of the TA kinetics and succeeded by two distinct decay components, exhibiting lifetimes of ∼140 ps and over 10 ns, respectively. We attribute these observations to a decay process involving two-photon absorption, dispersion characteristics, and excited state absorption. Based on this unique TA characteristic of KTN crystals, an all-optical switching strategy was proposed and utilized to measure the ultrafast lasing dynamics of single-crystal CH3NH3PbBr3 nanowires. This polarization-independent TA gate approach offers an adjustable gate width combining ps and ns time scales and introduces a versatile tool for advanced optical applications.

Pioneering Two-Dimensional Perovskites Featuring Four-Layer Organic Spacers for Polarization-Sensitive Photodetection

Hybrid perovskites have great potential in photovoltaics and photodetection. Specially, two-dimensional (2D) hybrid perovskites have been discovered to show distinctive applications in polarization-sensitive photodetection due to their intrinsic anisotropy. Herein, we designed a new type of 2D perovskite by introducing bifunctional alkylammonium as an organic spacer, (β-Ala)4PbBr4 (1, where β-Ala+ is 3-aminopropanoic), which has four organic spacers in adjacent inorganic layers and adjacent organic layers are linked by hydrogen bonding. The pioneering structure with four organic spacers enables an intrinsic high strong anisotropy, facilitating polarization-sensitive detection. The analysis of the crystal structure and optical properties further elucidates the natural anisotropic properties of 1. Strikingly, 1 has a strong optical dichroism (αc/αb ≈ 7.4 in 405 nm), and the polarization-sensitive detector on single crystals of 1 exhibits a large polarization ratio (Imax/Imin ≈ 2.0). This result highlights that the employment of bifunctional cations is efficient to explore new type 2D perovskites for potentially high-performance polarization-sensitive detection.

The synthesis of novel lanthanum hydroxyborate at extreme conditions

The novel structure of lanthanum hydroxyborate La2B2O5(OH)2 was synthesized by the reaction of partially hydrolyzed lanthanum and boron oxide in a diamond anvil cell under high-pressure/high-temperature (HPHT) conditions of 30 GPa and ∼2,400 K. The single-crystal X-ray structure determination of the lanthanum hydroxyborate revealed: P 3 ¯ c 1 , a = 6.555(2) Å, c = 17.485(8) Å, Z = 6, R1 = 0.056. The three-dimensional structure consists of discrete planar BO3 groups and three crystallographically different La ions: one is surrounded by 9, one by 10, and one by 12 oxygen anions. The band gap was estimated using ab initio calculations to be 4.64 eV at ambient pressure and 5.26 eV at 30 GPa. The current work describes the novel HPHT lanthanum hydroxyborate with potential application as a deep-ultraviolet birefringent material.

Nonlinear terahertz control of the lead halide perovskite lattice

Lead halide perovskites (LHPs) have emerged as an excellent class of semiconductors for next-generation solar cells and optoelectronic devices. Tailoring physical properties by fine-tuning the lattice structures has been explored in these materials by chemical composition or morphology. Nevertheless, its dynamic counterpart, phonon-driven ultrafast material control, as contemporarily harnessed for oxide perovskites, has not yet been established. Here, we use intense THz electric fields to obtain direct lattice control via nonlinear excitation of coherent octahedral twist modes in hybrid CH3NH3PbBr3 and all-inorganic CsPbBr3 perovskites. These Raman-active phonons at 0.9 to 1.3 THz are found to govern the ultrafast THz-induced Kerr effect in the low-temperature orthorhombic phase and thus dominate the phonon-modulated polarizability with potential implications for dynamic charge carrier screening beyond the Fröhlich polaron. Our work opens the door to selective control of LHP's vibrational degrees of freedom governing phase transitions and dynamic disorder.

Two Carboxylate-Cyanurates with Strong Optical Anisotropy and Large Band Gaps

The first metal carboxylate-cyanurates, namely, K(H₃C₃N₃O₃)(HCO₂) (I) and Ba₂(H₂C₃N₃O₃)(CH₃CO₂)₃(H₂O) (II), which contain π-conjugated carboxylate and cyanurate groups, have been synthesized by hydrothermal methods. They crystallize in centrosymmetric space groups of P1̅ and P2₁/n, respectively. Compound I exhibits a novel three-dimensional (3D) structure based on a [K(H₃C₃N₃O₃)]⁺ cationic framework with 12-membered ring (12-MR) channels, and the (HCO₂)^- anions are located within the 12-MR channels. The [K(H₃C₃N₃O₃)]⁺ cationic framework is composed of K⁺ ions interconnected by H₃C₃N₃O₃ ligands. Compound II features a 3D network formed by [Ba₂(CH₃CO₂)₃]⁺ cationic double chains bridged by (H₂C₃N₃O₃)^- anions. The [Ba₂(CH₃CO₂)₃]⁺ cationic double chain is composed of (CH₃CO₂)^- anions and Ba²⁺ ions. Optical property measurements show that both compounds exhibit short ultraviolet cutoff edges (I, 208 nm; II, 218 nm) and wide band gaps (I, 5.43 eV; II, 5.20 eV). Importantly, K(H₃C₃N₃O₃)(HCO₂) (I) features a large birefringence of 0.285@532 nm due to the parallel alignment of π-conjugated H₃C₃N₃O₃ and (HCO₂)^- groups, indicating that K(H₃C₃N₃O₃)(HCO₂) (I) is a promising short-wave ultraviolet birefringent material. Detailed theoretical calculations elucidate that their excellent optical properties originate from the synergetic effect of both types of π-conjugated groups.

Finding a Deep-UV Borate BaZnB4 O8 with Edge-sharing [BO4 ] Tetrahedra and Strong Optical Anisotropy

The polarization modulation of deep-UV light is an important process that incorporates functionality to selectively respond to light-mater interaction. Typically, optical anisotropy is foremost to the use efficiency of deep-UV birefringent crystals. Herein, a new congruently melting polyborate with extremely large birefringence (Δn₍₀₀₁₎ =0.14@589.3 nm) and band gap (6.89 eV) is discovered as a high performance birefringent crystal, which breaks the current deadlock of deep-UV polyborates that usually show small birefringence. The rigid tetrahedra, including [ZnO₄ ] and edge-sharing [BO₄ ] tetrahedra, make all the planar [BO₃ ] triangles in the lattice adopt preferential arrangement and thereby lead to an extraordinary large birefringence that is larger than all the deep-UV borates with experimentally measured values. Structural analyses with the additional theoretical calculations were used to study the origin of strong optical anisotropy in BaZnB₄ O₈ .

CsB3O4(OH)2: a new deep-ultraviolet birefringent crystal with [B3O4(OH)2] anionic group

A new cesium hydroxyborate CsB₃O₄(OH)₂, was designed and synthesized by a hydrothermal method. Remarkably, CsB₃O₄(OH)₂ presents novel [B₃O₄(OH)₂]_∞ chains formed by [B₃O₄(OH)₂] fundamental building blocks (FBBs). The report of less common [B₃O₄(OH)₂] FBBs and [B₃O₄(OH)₂]_∞ chains in CsB₃O₄(OH)₂ enriches the structural diversity of hydroxyborates. In addition, CsB₃O₄(OH)₂ has a wide transparent window in the DUV spectral range and a large birefringence.

Double-Modification Oriented Design of a Deep-UV Birefringent Crystal Functionalized by [B12 O16 F4 (OH)4 ] Clusters

Polarization modulation of deep-UV light is of significance to current technologies, and to this end, the birefringent crystal has emerged as an invaluable material as it allows for effective light modulation. Herein, a double-modification strategy driven by F and OH anions that makes double effects towards the critical property enhancement of deep-UV birefringent crystals is proposed. This leads to a new hydroxyborate (NH₄ )₄ [B₁₂ O₁₆ F₄ (OH)₄ ] with giant cluster as a deep-UV birefringent crystal with large birefringence (Δn_exp. =0.12@546.1 nm). This birefringence is a record among inorganic hydroxyborates with experimentally measured birefringence. Structural analysis shows that the near-plane arrangement of [B₁₂ O₁₆ F₄ (OH)₄ ] cluster is responsible for the large optical anisotropy. Theoretical calculations indicate that its π-conjugated [BO₃ ] and [BO₂ OH] units are the main source of this large optical anisotropy.

Tunable thermotropic phase transition triggering large dielectric response and superionic conduction in lead halide perovskites

Lead halide perovskites show tunable structural phase transition, accompanied by large dielectric response and superionic conduction.

Two metal-free cyanurate crystals with a large optical birefringence resulting from the combination of π-conjugated units

Benefiting from the planar π-conjugated (H_xC₃N₃O₃)^x-3 (x = 0-3) groups, cyanurate crystals have recently become a research hotspot in birefringent materials. Herein, by combining the (H_xC₃N₃O₃)^x-3 (x = 0-3) group with the (CN₃H₆)⁺ cationic group, two metal-free cyanurates, GU(H₂C₃N₃O₃) (I) and GU₃(H₂C₃N₃O₃)₃(H₃C₃N₃O₃) (II), were obtained by the hydrothermal method. These compounds have wide band gaps (∼5 eV) and a large birefringence (∼0.40@400 nm), demonstrating their potential to be ultraviolet birefringent crystals. Moreover, first-principles calculations indicate that their large birefringence values originated from the synergistic effect of the (CN₃H₆)⁺ cations and (H_xC₃N₃O₃)^x-3 (x = 0-3) groups. These findings provide a new design strategy for exploring low-cost UV birefringent crystals with a large birefringence.

Hydroxyfluorooxoborate Na[B3 O3 F2 (OH)2 ]⋅[B(OH)3 ]: Optimizing the Optical Anisotropy with Heteroanionic Units for Deep Ultraviolet Birefringent Crystals

Maximizing the optical anisotropy in birefringent materials has emerged as an efficient route for modulating the polarization-dependent light propagation. Currently, the generation of deep-ultraviolet (deep-UV) polarized light below 200 nm is essential but challenging due to the interdisciplinary significance and insufficiency of high-performing birefringent crystals. Herein, by introducing multiple heteroanionic units, the first sodium difluorodihydroxytriborate-boric acid Na[B₃ O₃ F₂ (OH)₂ ]⋅[B(OH)₃ ] has been characterized as a novel deep-UV birefringent crystal. Two rare heteroanionic units, [B₃ O₃ F₂ (OH)₂ ] and [B(OH)₃ ], optimally align to induce large optical anisotropy and also the dangling bonds are eliminated with hydrogens, which results in an extremely large birefringence and band gap. The well-ordered OH/F anions in [B₃ O₃ F₂ (OH)₂ ] and [B(OH)₃ ] were identified and confirmed by various approaches, and also the origin of large birefringence was theoretically discussed. These results confirm the feasibility of utilizing hydrogen involved heteroanionic units to design crystals with large birefringence, and also expand the alternative system of deep-UV birefringent crystals with new hydroxyfluorooxoborates.