Tetracoordinate planar carbon (that is, carbon atoms coordinated by four other atoms in a square-planar arrangement), first proposed by Hoffmann et al. [1] over thirty years ago, was recently observed in the vibrationally averaged D 4h [Al 4 C] À , [2] C 2v Al 3 XC, [Al 3 XC] À (X = Si,Ge), [3,4] and [CAl 4 ] 2À .[5] Tetracoordinate planar (TP) Si and Ge centers were also discovered in MAl 4 and [MAl 4 ] À .[6] The TP bonding character of Group 14 elements, contrary to the conventional concept of tetrahedral C, Si, and Ge, is supported by excellent agreements between the measured photoelectron spectra and ab initio vertical detachment energies (VDE) of these molecules in the TP structures.Here we extend the range of TP centers to include B, N, and O and the ligands from p-block elements to d-block transition metals (Cu and Ni). On the basis of ab initio optimization results, we present the first theoretical evidence that, in the form of the X-centered hydrometals M 4 H 4 X, these first-row nonmetals X (X = B, C, N, O) are tetracoordinated by four transition-metal ligands M (M = Cu, Ni) in perfect squares. Hydrocopper Cu 4 H 4 and hydronickel Ni 4 H 4 are found to be suitable for hosting these tetracoordinate planar nonmetals (TPNs) with the high symmetry of D 4h . To the best of our knowledge, neither experimental nor theoretical data on these unusual molecules are available to date. Our work was stimulated by the proposal of aromatic hydrocopper Cu 4 H 4 at the DFT level. [7] Various initial structures obtained at the B3LYP/Lanl2dz level were optimized at the B3LYP/6-311 + G(3df,p) level, and imaginary frequencies were checked at the same level. The DFT binding energies of 23.1 eV for D 4h Ni 4 H 4 C and 18.7 eV for C 4v Cu 4 H 4 C demonstrate the stability of these complexes with respect to dissociation. The B3LYP geometries were further refined with the second-order Møller-Plesset (MP2) procedure. We calculated the NMR shielding tensors using the gauge-independent atomic orbital (GIAO) procedure [8,9] at the B3LYP/6-311 + G(3df,p) level, and VDEs, ionization potentials (IPs), and electron affinities (EAs) utilizing the restricted outer valence Green function method (ROVGF) [10,11] with a smaller basis of 6-31 + G(d,p). The optimized MP2 bond parameters, normal vibrational frequencies, and electronic properties of these systems are provided in the Supporting Information. All the calculations were performed with the Gaussian 03 program. À and Cu 4 H 4 C, on the other hand, were found to be transition states with imaginary frequencies of 266i and 135i cm À1 , respectively, and lie 0.962 and 0.124 eV higher in energy than the corresponding tetracoordinate pyramidal C 4v structures. These imaginary frequencies correspond to the A 2u vibrational mode, in which the central atom X moves up and down along the fourfold axis. Distortion of the D 4h structure in the A 2u mode leads to the C 4v global minimum, in which the X atom lies about 0.

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Organic-inorganic hybrid halide perovskites, represented by CH 3 NH 3 PbX 3 (X ¼ Cl, Br, I), have demonstrated excellent optoelectronic and radiation detection properties. [1-3] All-inorganic CsPbX 3 perovskites (X ¼ Cl, Br, I) with better long-term stability are also considered as promising materials for optoelectronic devices and semiconductor γ-ray detectors. [4-6] Recently, there has been a surge of interest in low-dimensional perovskites due to their high photoluminescence quantum yield (PLQY). For example, the crystal structure of Cs 4 PbBr 6 that consist of spatially isolated [PbBr 6 ] 4À octahedra surrounding with Cs þ ions can be regarded as a 0D structure at molecular level, which leads to an intense quantum confinement effect. Excitons are strongly confined at each [PbBr 6 ] 4À octahedron, enabling a high exciton binding energy of 353 meV and a high PLQY of between 42% and 45%. [7,8] Quite a few low-dimensional allinorganic perovskites with remarkable luminescent properties were also reported as sensitive and efficient scintillators. Bulk crystals of 0D Cs 4 CaI 6 :Eu, Cs 4 SrI 6 :Eu, and Cs 4 EuBr 6 perovskites, isostructural to K 4 CdCl 6 trigonal system, have excellent light yields from 51 800 to 78 000 photons MeV À1 and energy resolutions from 3.3% to 4.3% at 662 keV. [9,10] However, due to a small Stokes shift of Eu 2þ ions, these materials suffer from strong self-absorption effect when scaling-up crystal size. Nanocrystals of 0D CsPbBr 3 /Cs 4 PbBr 6 perovskites show a high light yield of 64 000 photons MeV À1 and a fast decay time of <10 ns. [11] Onedimensional materials were also reported as sensitive X-ray scintillators, such as Rb 2 CuBr 3 and Rb 2 CuCl 3. [12,13] In particular, the Rb 2 CuBr 3 , that is self-absorption free and nonhygroscopic, was reported to achieve an ultrahigh scintillation yield of 90 000 photons MeV À1. [12] All-inorganic 0D perovskite Cs 3 Cu 2 I 5 was recently reported as a highly efficient blue-emitting material with a PLQY of 91.2%, and regarded as promising for application in photodetectors, light-emitting diodes, and memristors afterward. [14-17] In 2020, Cs 3 Cu 2 I 5 nanocrystals were developed for X-ray imaging with a light yield of 80 000 photons MeV À1. [18] To the best of our knowledge, the X-ray and γ-ray detection capability of bulk Cs 3 Cu 2 I 5 single crystal has not been reported. Thus, the aim of this work is to study the physical and optical properties and the scintillation performance under X-ray and γ-ray radiation of high-quality Cs 3 Cu 2 I 5 perovskite single crystal grown by the Bridgman method. The 7 mm diameter single crystal of Cs 3 Cu 2 I 5 was grown by the vertical Bridgman method. High-purity powders of CsI (99.99%, Grirem Advanced Materials) and CuI (99.999%, Sigma-Aldrich) were used as raw materials. These starting materials were mixed consistent with stoichiometric ratio and loaded into a quartz ampoule in a glovebox with <0.1 ppm moisture and oxygen. The loaded ampoule was sealed under a vacuum of 10 À6 torr after drying at 180 C for ...

Metallocenes, including ferrocene, [(h 5 -C 5 H 5 ) 2 Fe], and its 18-electron analogues [(h 5 -E 5 ) 2 M] (E = P, N, As, Sb, and Bi; M = transition-metal atom), have found important applications in both fundamental research and materials science.[ (n = 4, 6). [9][10][11] In these sandwich-type structures, the transition-metal center, M, is coordinated between two aromatic h n -E n monocycles (n = 4-6), each of which has 6-p electrons. The p-d interactions between the delocalized p molecular orbitals (MOs) of the ligands and the partially occupied d orbitals of the transitionmetal center play a crucial role in stabilizing the systems. Designing new forms of metallocenes and their sandwich-type analogues requires the right match between the monocyclic ligands and transition-metal centers, both geometrically and electronically. Inspired by the proposed 6-p electron aromatic D 6h B 6 C 2À ion featuring a carbon atom in a planar hexacoordinate environment (denoted phC) at the center of a perfect B 6 hexagon, [12] we present herein an investigation by density functional theory (DFT) of a new class of sandwich-type complexes, D 6d [(h 6 -B 6 X) 2 M] (X = C, N; M = Mn, Fe, Co, Ni). These complexes are unique in that they contain two parallel h 6 -B 6 X hexagons centered with two nearly planar hexacoordinate carbon or nitrogen atoms (phN) located along the sixfold molecular axis. The results obtained in this work provide an important extension to the traditional concept of sandwich-type complexes by incorporating hexacoordinate carbon or nitrogen atoms in the systems and present a viable possibility to stabilize and characterize phC or phN atoms in future experiments. To the best of our knowledge, there have been no investigations reported to date on B 6 C 2À ligands in metallocene-like complexes.DFT structural optimizations at the B3LYP/6-311 + G-(3df) level [13][14][15] were performed on the sandwich complexes under investigation and imaginary frequencies and DFT wavefunction instabilities checked at the same theoretical level. Natural bond orbital (NBO) [16] analyses were implemented to gain insight into the bonding pattern of the complexes. Figure 1 and Figure 2 depict the optimized structures of Fe-containing complexes, and Figure 3 shows the MO pictures of the top 13 occupied energy levels of D 6d [(h 6 -B 6 C) 2 Fe] 2À (with one orbital shown for two degenerate MOs). These optimum structures are well-maintained when symmetry constraints are removed. Co-, Ni-, and Mncentered complexes have the same geometries and similar MOs but have different energy levels. Table 1 tabulates the calculated bond lengths, natural atomic charges, lowest vibrational frequencies (ñ min ), highest occupied molecular orbital (HOMO) energies, and total Wiberg bond indices (WBIs) [12,17] of the constituent atoms for the complexes under investigation. Detailed geometrical and electronic properties of 28 neutral and charged complex ions that contain B 6 X 2À