Crystal structure features in a new compound C 4 B 25 Mg 1.42
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TAL CHEMISTRY
Crystal Structure Features in a New Compound C4B25Mg1.42 S. V. Konovalikhin and V. I. Ponomarev Institute of Structural Macrokinetics and Materials Science, Russian Academy of Sciences, Chernogolovka, Moscow, 142432 Russia e-mail: [email protected] Received December 16, 2014
Abstract—The composition of C4B25Mg1.42 crystal obtained by self-propagating high-temperature synthesis was determined using X-ray diffraction. This is the first crystalline structure where all boron atoms in the В12 icosahedron occupy crystallographically independent positions; this circumstance allowed us to analyze the effect of substituents on bond lengths in the icosahedron. The crystal structure features, including the channels filled with disordered Mg atoms and the spread of В–В endo- and exo-bond lengths in the icosahedra, are described. A crystallochemical analysis of pair bonds has been performed for the first time. DOI: 10.1134/S1063774515050053
INTRODUCTION Self-propagating high-temperature synthesis (SHS) of a new compound C4B25Mg1.42 (1) was reported in [1, 2]. It was found that structure 1 is a combination elements of α-boron [3] and boron carbide crystals [4–6]. Since all the boron atoms in two В12 icosahedra occupy crystallographically independent positions, it has become possible to estimate for the first time the effect of exo-bonds on the icosahedron structure. In crystal 1, В12 boron atoms form exobonds of four types. We hope this will help solve the longstanding problem of boron carbide crystals concerning the place and character (ordered/disordered) of incorporation of С atoms into the boron carbide structure [6]. In this study, we describe in detail the crystal structure of compound 1. EXPERIMENTAL Synthesis of crystals 1 was described in [1, 2]. The chemical composition of the compound was established by comparing the chemical [1] and X-ray diffraction (XRD) data. For XRD investigations, we chose a dark red single crystal 1 in the form of a parallelepiped 0.25 × 0.25 × 0.3 mm in size. We measured 3326 reflections with I ≥ 2σ(I) on an automatic four-circle diffractometer in МоKα radiation (λ = 0.7107 Å). The range of θ angles was 2.97° < θ < 35.84°. The structure was determined directly using the SHELXL-86 program with subsequent construction of a Fourier synthesis series. Atomic positions were refined by the full-matrix leastsquares method in the anisotropic approximation of thermal displacements of all atoms using the SHELXL-97 program. The final value was R = 0.032.
The main crystallographic data are а = 9.626(1) Å, b = 11.329(1) Å, c = 8.966(1) Å, β = 105.80(3)°, V = 940.8(2) ų, sp. gr. P21/c, dcalc = 2.505 g/cm³, and μ = 0.20 mm–1. Atoms in the independent part of unit cell and the bond lengths discussed below are shown in Fig. 1. Atomic coordinates and other crystallographic parameters were deposited to the Cambridge Structural Database (ССDС no. 974255). RESULTS AND DISCUSSION The crystal structure of compound 1 simultaneously comprises elements of the α-boron (2) [3] and В13С2 (3) and В12С3 (4) bo
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