Order-disorder transformations in graphite nitrates

Abstract

Using synthetic near-ideal graphite obtained by hot-pressing and annealing pyrolytic material, we have prepared graphite nitrates from the first to the fourth sequence, under controlled conditions. For the first sequence compound, present determinations on form I (room temperature) agree with previous publications and confirm the stacking sequence A | A | A | A . Form I sequences 2, 3 and 4 show well defined stacking of the graphite networks, characterized by X-ray methods as follows: sequence network stacking* system unit cell (Å) space group 2 A | AB | BC | CA | A rhombohedral a H = 2⋅46 R 3 ¯ m c H = 33⋅45 3 A | ABA | ACA | A orthorhombic a = 2⋅46 Cmc 2 1 b = 4⋅26 c = 28⋅96 4 A | ABAB | BCBC | CACA | A rhombohedral a H = 2⋅46 R 3 ¯ m c H = 53⋅5 * The vertical lines imply layers of intercalate. In all these compounds, the stacking sequences found can be generated from the stacking in normal graphite, ABAB , if the change AB → A | A on entry of successive intercalate layers involves the movement of a boundary dislocation through the structure. Since it is observed that the end-product (sequence N ) under any given conditions is formed by way of a systematic numerical succession of higher sequence compounds ( N +2, N +1) as intermediates, this fundamental stacking change must reverse so that AB ⇌ A | A . When any of these sequences of graphite nitrate are cooled through a λ transformation around —20°C, increased ordering appears in the low temperature structures (form II). Each intercalate layer now exhibits two-dimensional long range order. In addition, successive intercalated layers within any crystallite are stacked in a non-random way, which in the case of the first sequence compound implies a structure that is periodic in three dimensions. Within any intercalated layer in form I, the molecular packing resembles that in liquids. In a given crystal compound the numerical sequence of such layers is regular, and successive intercalate layers occur after N carbon hexagon planes. In form II, molecular packing in the intercalated layers is crystalline. Some correlation also appears between the occupied sites in different filled layers. Implications of this order-disorder transformation are briefly discussed.