Perovskite ferroelectrics with prominent nonlinear optical absorption have attracted great attention in the field of photonics. However, they are traditionally dominated by inorganic oxides and exhibit relatively small nonlinear optical absorption coefficients, which hinder their further applications. Herein, we report a new organic-inorganic hybrid bilayered perovskite ferroelectric, (CHNH)(NHCHNH)PbBr (1), showing an above-room-temperature Curie temperature (∼322 K) and notable spontaneous polarization (∼3.8 μC cm). Significantly, the unique quantum-well structure of 1 results in intriguing two-photon absorption properties with a giant nonlinear optical absorption coefficient as high as 5.76 × 10 cm GW, which is almost two-orders of magnitude larger than those of mostly traditional all-inorganic perovskite ferroelectrics. To our best knowledge, 1 is the first example of hybrid ferroelectrics with giant two-photon absorption coefficient. The mechanisms for ferroelectric and two-photon absorption are revealed. This work will shed light on the design of new ferroelectrics with two-photon absorption and promote their potentials in the photonic application.

Halide double perovskites have recently bloomed as the green candidates for optoelectronic applications, such as X‐ray detection. Despite great efforts, the exploration of promising organic–inorganic hybrid double perovskites toward X‐ray detection remains unsuccessful. Now, single crystals of the lead‐free hybrid double perovskite, (BA)2CsAgBiBr7 (BA+ is n‐butylammonium), featuring the unique 2D multilayered quantum‐confined motif, enable quite large μτ (mobility‐lifetime) product up to 1.21×10−3 cm2 V−1. This figure‐of‐merit realized in 2D hybrid double perovskites is unprecedented and comparable with that of CH3NH3PbI3 wafers. (BA)2CsAgBiBr7 crystals also exhibit other intriguing attributes for X‐ray detection, including high bulk resistivity, low density of defects and traps, and large X‐ray attenuation coefficient. Consequently, a vertical‐structure crystal device under X‐ray source yields a superior sensitivity of 4.2 μC Gyair−1 cm−2.

It is challenging to explore deep-ultraviolet (deep-UV) nonlinear optical (NLO) materials that can achieve a subtle balance between deep-UV transparency and high NLO activity. Known deep-UV NLO materials are almost exclusively limited to borates, except few newly discovered phosphates despite their small NLO activities. Here we report two asymmetric phosphates, RbBa2(PO3)5 (I) and Rb2Ba3(P2O7)2 (II), which feature [PO3]∞ chains and [P2O7](4-) dimers formed by condensation of [PO4](3-) units, respectively. Remarkably, I achieves the desired balance, with the shortest deep-UV absorption edge at 163 nm and the largest NLO activity of 1.4 × KDP (KH2PO4) in deep-UV NLO phosphates. According to first-principles calculations, the enhanced macroscopic SHG response of I can be attributed to the [PO3]∞ chains which exhibit significantly larger microscopic SHG coefficients as compared with the [P2O7](4-) dimers.

Large-size crystals of organic-inorganic hybrid perovskites (e.g., CH 3 NH 3 PbX 3 , X = Cl, Br, I) have gained wide attention since their spectacular progress on optoelectronic technologies. Although presenting brilliant semiconducting properties, a serious concern of the toxicity in these lead-based hybrids has become a stumbling block that limits their wide-scale applications. Exploring lead-free hybrid perovskite is thus highly urgent for high-performance optoelectronic devices. Here, a new lead-free perovskite hybrid (TMHD)BiBr 5 (TMHD = N,N,N,N-tetramethyl-1,6-hexanediammonium) is prepared from facile solution process. Emphatically, inch-size high-quality single crystals are successfully grown, the dimensions of which reach up to 32 × 24 × 12 mm 3 . Furthermore, the planar arrays of photodetectors based on bulk lead-free (TMHD)BiBr 5 single crystals are first fabricated, which shows sizeable on/off current ratios (≈10 3 ) and rapid response speed (τ rise = 8.9 ms and τ decay = 10.2 ms). The prominent device performance of (TMHD)BiBr 5 strongly underscores the lead-free hybrid perovskite single crystals as promising material candidates for optoelectronic applications.

X-ray detectors with high sensitivity are of great significance in both civil and military fields. Over the past decades, great efforts have been made to improve the sensitivity in conventional inorganic materials, but mainly at the cost of increasing the energy consumption with a quite high operating voltage. Developing photosensitive ferroelectrics directly as detector materials may be a conceptually new strategy in view of the strong ferroelectric spontaneous polarization (P s ) that assists photoinduced carriers separation and transport. = 5 µC cm −2 ) is fabricated and exhibits an ultrahigh X-ray sensitivity up to 6.8 × 10 3 µC Gy air −1 cm −2 even at a relatively low operating voltage, which is over 300-fold larger than that of state-of-the-art α-Se X-ray detectors. Such a brilliant figure-of-merit is largely attributed to the superior mobility-lifetime products associated with the strong ferroelectric polarization of BA 2 EA 2 Pb 3 Br 10 . As pioneering work, these findings inform the exploration of hybrid halide perovskite ferroelectrics toward highperformance photoelectronic devices.

Multiaxial molecular ferroelectrics, in which multiple-directional switching of spontaneous polarization creates diverse properties, have shown many intriguing advantages, making them indispensable complements to conventional inorganic oxides. Despite recent blooming advances, multiaxial molecular ferroelectric with bulk photovoltaic effects still remains a huge blank. Herein, we report a biaxial lead halide ferroelectric, EA 4 Pb 3 Br 10 (1, EA = ethylammonium), which adopts the unique trilayered perovskite motif with a high Curie temperature of ∼384 K. Particularly, for 1, the distinct symmetry breaking with 4/mmmFmm2 species leads to the emergence of four equivalent polarization directions in the ferroelectric phase. Based on its biaxial nature, the bulk photovoltaic effect of 1 can be facilely tuned between such multiple directions through electric poling. As far as we know, this is the first report on biaxial hybrid perovskite ferroelectric showing directionally tunable photovoltaic activity. This work provides an avenue to control the bulk physical properties of multiaxial molecular ferroelectrics, and highlights their potential for further applications in the field of smart devices.

Cesium-lead halide perovskites (e.g. CsPbBr ) have gained attention because of their rich physical properties, but their bulk ferroelectricity remains unexplored. Herein, by alloying flexible organic cations into the cubic CsPbBr , we design the first cesium-based two-dimensional (2D) perovskite ferroelectric material with both inorganic alkali metal and organic cations, (C H NH ) CsPb Br (1). Strikingly, 1 shows a high Curie temperature (T =412 K) above that of BaTiO (ca. 393 K) and notable spontaneous polarization (ca. 4.2 μC cm ), triggered by not only the ordering of organic cations but also atomic displacement of inorganic Cs ions. To our knowledge, such a 2D bilayered Cs -based metal-halide perovskite ferroelectric material with inorganic and organic cations is unprecedented. 1 also shows photoelectric semiconducting behavior with large "on/off" ratios of photoconductivity (>10 ).

The changeable molecular dynamics of flexible polar cations in the variable confined space between inorganic chains brings about a new type of two-step nonlinear optical (NLO) switch with genuine "off-on-off" second harmonic generation (SHG) conversion between one NLO-active state and two NLO-inactive states.