Compression of [121]Tetramantane to over 50 GPa - Phase transformations and elastic properties

The hydrocarbon [121]tetramantane (C₂₂H₂₈) is a promising precursor material for the high-pressure high-temperature synthesis of nanodiamonds due its diamond-like carbon bonding network. We present a comprehensive study of its structural behaviour under pressure using single-crystal and synchrotron X-ray diffraction, Raman spectroscopy, and density functional theory calculations. At ambient conditions, [121]tetramantane crystallises in monoclinic space group P2₁/n with lattice parameters of a = 7.7192(1) Å, b = 8.0112(1) Å, c = 12.8281(2) Å, β = 104.635(2)°, Z=2, and a density of 1.265(3) g/cm³. Under non-hydrostatic diamond anvil cell compression, we observe two phase transitions: first to a phase with space group P2/n at 1 GPa, followed by a transition to a high-pressure phase above 8 GPa indexed to a unit cell with space group P2/m and lattice parameters a = 14.62(1) Å, b = 7.78(1) Å, c = 6.39(1) Å, β = 94.4(2)°, and Z=3, and a density of 2.010(3) g/cm³ at 31 GPa. The isothermal bulk modulus B₀ increases significantly from the value of the ambient phase of (19.4 ± 0.9) GPa to that of the high-pressure phase of (147.5 ± 1.7) GPa. Upon recovery, [121]tetramantane transforms back to a P2/n phase with similar lattice parameters compared to the initial P2₁/n structure, but possesses significantly different relative peak intensities. This study provides valuable insights on the structure-property relationships of [121]tetramantane and suggests an optimal pressure of 8 GPa for the use of [121]tetramantane as a precursor for nanodiamond synthesis.