Synthesis, Crystal Structure, and Properties of Two Modifications of MgB₁₂C₂

Abstract

Single crystals of two modifications of the new magnesium boride carbide MgB(12)C(2) were synthesized from the elements in a metallic melt by using tantalum ampoules. Crystals were characterized by single-crystal X-ray diffraction and electron microprobe analysis (energy-dispersive (EDX) and wavelength-dispersive (WDX) X-ray spectroscopy). Orthorhombic MgB(12)C(2) is formed in a Cu/Mg melt at 1873 K. The crystal structure of o-MgB(12)C(2) (Imma, Z=4, a=5.6133(10), b=9.828(2), c=7.9329(15) A, 574 reflections, 42 variables, R(1)(F)=0.0208, wR(2)(I)=0.0540) consists of a hexagonal primitive array of B(12) icosahedra with Mg atoms and C(2) units in trigonal-prismatic voids. Each icosahedron has six exohedral B--B and six B--C bonds. Carbon is tetrahedrally coordinated by three boron atoms and one carbon atom with a remarkably long C--C distance of 1.727 A. Monoclinic MgB(12)C(2) is formed in an Al/Mg melt at 1573 K. The structure of m-MgB(12)C(2) (C2/c, Z=4, a=7.2736(11), b=8.7768(13), c=7.2817(11) A, beta=105.33(3) degrees , 1585 reflections, 71 variables, R(1)(F)=0.0228, wR(2)(I)=0.0610) may be described as a distorted cubic close arrangement of B(12) icosahedra. Tetrahedral voids are filled by C atoms and octahedral voids are occupied by Mg atoms. The icosahedra are interconnected by four exohedral B--B bonds to linear chains and by eight interstitial C atoms to form a three-dimensional covalent network. Both compounds fulfill the electron-counting rules of Wade and Longuet-Higgins.

LiB12PC, the First Boron‐Rich Metal Boride with Phosphorus—Synthesis, Crystal Structure, Hardness, Spectroscopic Investigations

“…The rules of Wade and Longuet-Higgins are fulfilled with every B 12 icosahedron requiring two electrons, provided by the metal atoms. [15] This assignment of electrons is confirmed by the fact that single crystals are almost colourless is supported by band structure calculations in the cases of Li 2 B 12 Si 2 , [11] LiB 13 C 2 , [16] Li 2 B 12 C 2 [17] and B 6 P. [17] Reacting the elements in molten metals using boron nitride crucibles welded in Ta-ampoules, [18][19][20] we were able to synthesise LiB 12 PC, the first boron-rich metal boride with Abstract: We present synthesis, crystal structure, hardness, and IR/Raman and UV/Vis spectra of a new compound with the mean composition LiB 12 PC. Transparent single crystals were synthesised from Ga, Li, B, red phosphorus and C at 1500 8C in boron nitride crucibles welded in Ta ampoules.…”

“…We obtained transparent brown and red single crystals with different P and C content and colourless single crystals with the "ideal" composition LiB 12 PC. Crystal structures were solved and refined based on single crystal data and were found to be close to o-MgB 12 C 2 , [12] MgB 7 , [21,22] and ScB 13 C. [23] Results and Discussion LiB 12 PC: Single crystals of LiB 12 PC are colourless platelets or columns (Figure 1, right). In the following the characteristics of this structure are discussed with respect to our interpretation of the single crystal data and EDX results, that is, the appearance of disordered PÀC units.…”

“…Brown LiB 12 P 0.89 C 1.11 is very similar, but with a lower P content. Red single crystals of LiB 12 P 1.13 C 0.87 have a larger unit cell with a = 10.4097 (18) , b = 5.9029(7) , c = 8.2044 (12) . EDX measurements confirm that the red crystals contain more phosphorus than the other ones.…”

“…[5] No other boron-rich boride containing phosphorus has been synthesised until now. [8] In the course of our research on boron-rich lithium and magnesium borides, we found several new compounds including the first colourless and electron-precise boron-rich borides Li 2 B 12 C 2 , [9] LiB 13 C 2 , [9] MgB 12 Si 2 , [10] Li 2 B 12 Si 2 , [11] MgB 12 C 2 , [12] Mg 2 B 24 C, [13] and Mg 3 B 50 C 8 . [14] The crystal structures of these compounds are remarkably simple.…”

“…EDX measurements confirm that the red crystals contain more phosphorus than the other ones. The crystal structure is characterized by a covalent network of B 12 phosphorus. We obtained transparent brown and red single crystals with different P and C content and colourless single crystals with the "ideal" composition LiB 12 PC.…”