Understanding the nature of "superhard graphite"

Numerous experiments showed that on cold compression graphite transforms into a new superhard and transparent allotrope. Several structures with different topologies have been proposed for this phase. While experimental data are consistent with these models, the only way to solve this puzzle is to find which structure is kinetically easiest to form. Using state-of-the-art molecular-dynamics transition path sampling simulations, we investigate kinetic pathways of the pressure-induced transformation of graphite to various superhard candidate structures. Unlike hitherto applied methods for elucidating nature of superhard graphite, transition path sampling realistically models nucleation events necessary for physically meaningful transformation kinetics. We demonstrate that nucleation mechanism and kinetics lead to $M$-carbon as the final product. $W$-carbon, initially competitor to $M$-carbon, is ruled out by phase growth. Bct-C$_4$ structure is not expected to be produced by cold compression due to less probable nucleation and higher barrier of formation

Large amplitude fluxional behaviour of elemental calcium under high pressure

Experimental evidences are presented showing unusually large and highly anisotropic vibrations in the “simple cubic” (SC) unit cell adopted by calcium over a broad pressure ranging from 30–90 GPa and at temperature as low as 40 K. X-ray diffraction patterns show a preferential broadening of the (110) Bragg reflection indicating that the atomic displacements are not isotropic but restricted to the [110] plane. The unusual observation can be rationalized invoking a simple chemical perspective. As the result of pressure-induced s → d transition, Ca atoms situated in the octahedral environment of the simple cubic structure are subjected to Jahn-Teller distortions. First-principles molecular dynamics calculations confirm this suggestion and show that the distortion is of dynamical nature as the cubic unit cell undergoes large amplitude tetragonal fluctuations. The present results show that, even under extreme compression, the atomic configuration is highly fluxional as it constantly changes

Na12Ge17: A compound with the zintl anions [Ge-4](4-) and [Ge-9](4-) - Synthesis, crystal structure, and Raman spectrum

Na12Ge17 is prepared from the elements at 1025 K in sealed niobium ampoules. The crystal structure reinvestigation reveals a doubling of the unit cell (space group:P2(1)/c; a = 22.117(3) Angstrom, b = 12.803(3) Angstrom, c = 41.557(6) Angstrom, beta = 91.31(2)degrees, Z = 16; Pearson code: mP464), furthermore, weak superstructure reflections indicate an even larger C-centred monoclinic cell. The characteristic structural units are the isolated cluster anions [Ge-9](4-) and [Ge-4](4-) in ratio 1:2, respectively The crystal structure represents a hierarchical cluster replacement structure of the hexagonal Laves phase MgZn2 in which the Mg and Zn atoms are replaced by the Ge-9 and Ge-4 units, respectively The Raman spectrum of Na12Ge17 exhibits the characteristic breathing modes of the constituent cluster anions at v = 274 cm(-1) ([Ge-9](4-)) and v = 222 cm(-1) ([Ge-4](4-)) which may be used for identification of these clusters in solid phases and in solutions. Raman spectra further prove that Na12Ge17 is partial soluble both in ethylenediamine and liquid ammonia. The solution and the solid extract contain solely [Ge-9](4-). The remaining insoluble residue is Na4Ge4. By heating the solvate Na4Ge9(NH3)(n) releases NH3 and decomposes irreversibly at 742 K, yielding Na12Ge17 and Ge