Neutron scattering study of the phase transition in s-triazine at high pressure

Pressure and Laser-Induced Reactivity in Crystalline s-Triazine

“…I−II Phase Transition. At ambient conditions, the crystal structure of s-triazine, indicated as phase I, is rhombohedral R3̅ c (D 3d 6 ; Z = 2), but on cooling at ambient pressure a transition, suggested to be second order, 16,23 to a monoclinic C2/c structure (C 2h 6 ; Z = 2) is observed at 198 K. 24−26 This monoclinic phase, indicated as phase II, has also been observed by 14 N nuclear quadrupole resonance, 16 neutron diffraction 17 and Raman scattering 18 once the phase I crystal was compressed at temperatures above 200 K. At ambient temperature, the I−II phase transition is found at 0.5 GPa.…”

“…S-triazine is also an appealing system to give rise to layered graphitic (C,N) materials that have been suggested as potential precursors of superhard carbon nitrides . Two derivatives of s-triazine, melamine (2,4,6-triamino-1,3,5-triazine) and cyanuric chloride (2,4,6-trichloro-1,3,5-triazine), both widespread employed in industrial applications, have been extensively studied to this purpose. , Extended oligomeric planar units form at ambient pressure through ring condensation of these compounds at temperatures in excess of 500 °C. − Planar carbon nitrides sheets have also been synthesized at high pressure and high temperature through the polycondensation of melamine in the presence of hydrazine at 3 GPa and through a solid-state reaction between melamine and cyanuric chloride just above 1 GPa. , Contrary to the interest for these two derivatives, s-triazine has been poorly investigated at high pressure, being the only exception the characterization of the low pressure (0.4–0.5 GPa at ambient temperature) rhombohedral to monoclinic phase transition. − The highest pressure reached in these studies was 4 GPa in Raman experiments, , but no information was provided about the chemical stability of this molecule at higher pressure.…”

Structural and Electronic Competing Mechanisms in the Formation of Amorphous Carbon Nitride by Compressing s-Triazine

“…s -Triazine presents two different crystal structures. Phase I, trigonal R 3̅ c ( D 3 d 6 , Z = 2), exists at ambient pressure down to 198 K, where it transforms to phase II, monoclinic C 2/ c (C 2 h 6 , Z = 2). ,, The I–II phase boundary has been partly characterized with increasing pressure up to ambient temperature by different techniques. ,− …”

Tuning the Aromaticity of s-Triazine in the Crystal Phase by Pressure

“…In studying the electronic properties of crystalline s -triazine it is useful to recall that the crystal structure at ambient conditions is rhombohedral R 3̅ c ( D 3 d 6 ; Z = 2), indicated as phase I, transforming below 198 K and ambient pressure to a monoclinic C 2/ c structure ( C 2 h 6 ; Z = 2). , This transition has also been observed by compressing the phase I crystal at temperatures above 200 K. − At ambient temperature the I–II phase transition occurs at 0.5 GPa and, according to group theory, all the aforementioned electronic transitions are predicted to be both one- and two-photon allowed in the high pressure monoclinic phase.…”