FeSe-derived superconductors show some unique behaviors relative to iron-pnictide superconductors, which are very helpful to understand the mechanism of superconductivity in high-T c iron-based superconductors. The low-energy electronic structure of the heavily electron-doped A x Fe 2 Se 2 (A=K, Rb, Cs) demonstrates that interband scattering or Fermi surface nesting is not a necessary ingredient for the unconventional superconductivity in iron-based superconductors 1,2 . The superconducting transition temperature (T c ) in the one-unit-cell FeSe on SrTiO 3 substrate can reach as high as ~65 K 3-6 , largely transcending the bulk T c of all known iron-based superconductors. However, in the case of A x Fe 2 Se 2 , the inter-grown antiferromagnetic insulating phase makes it difficult to study the underlying physics. Superconductors of alkali metal ions and NH 3 molecules or organic-molecules intercalated FeSe 7-9 and single layer or thin film FeSe on SrTiO 3 substrate 3-6 are extremely air-sensitive, which prevents the further investigation of their physical properties. Therefore, it is urgent to find a stable and accessible FeSe-derived superconductor for physical property measurements so as to study the underlying mechanism of superconductivity. Here, we report the air-stable superconductor (Li 0.8 Fe 0.2 )OHFeSe with high temperature superconductivity at ~40 K synthesized by a novel hydrothermal method. The crystal structure is unambiguously determined by the combination of X-ray and neutron powder diffraction and nuclear magnetic resonance. It is also found that an antiferromagnetic order coexists with superconductivity in such new FeSe-derived superconductor. This novel synthetic route opens a new avenue for exploring other superconductors in the related systems. The combination of different structure characterization techniques helps to complementarily determine and understand the details of the complicated structures

A new series of alkaline beryllium borates NaBeB(3)O(6), alpha-KBe(2)B(3)O(7), beta-KBe(2)B(3)O(7), gamma-KBe(2)B(3)O(7), and RbBe(2)B(3)O(7) were synthesized by spontaneous crystallization with molten fluxes based on A(2)O-B(2)O(3) (A = Na, K, Rb) solvent. For KBe(2)B(3)O(7), three polymorphous phases were found and are referred to as alpha-, beta-, and gamma-KBe(2)B(3)O(7) relative to their crystallization temperature from high to low. All of the materials are noncentrosymmetric except alpha-KBe(2)B(3)O(7). NaBeB(3)O(6) and alpha-KBe(2)B(3)O(7) consist of a new anionic group [Be(2)B(3)O(11)](9-), which is similar to a naphthalene molecule that is rarely found in inorganic matter. beta-KBe(2)B(3)O(7), gamma-KBe(2)B(3)O(7), and RbBe(2)B(3)O(7) consist of 2D alveolate beryllium borate layers [Be(2)BO(5)](infinity), which are connected by strong covalent bonds. The UV-vis diffuse reflectance spectroscopy on powder samples indicated that the short-wavelength absorption edges of noncentrosymmetric materials are all below 200 nm. Second-harmonic generation (SHG) on powder samples was measured using the Kurtz and Perry technique, which indicated that NaBeB(3)O(6), beta-KBe(2)B(3)O(7), gamma-KBe(2)B(3)O(7), and RbBe(2)B(3)O(7) are all phase matchable materials, and their measured SHG coefficients were approximately 1.60, 0.75, 0.68, and 0.79 times as large as that of d(36) (KDP), respectively.