Order–Disorder Transformations in Graphite Nitrates

Abstract

GRAPHITE nitrates, in which the layers are intercalated in first, second and third sequences, all show an anomaly in thermal expansion around − 20° C, attributed to a lambda transformation from form I to form II on cooling these solids1. Structural investigations by X-ray methods have now confirmed that order–disorder transformations occur in each of these crystal compounds. Moreover, some of the findings indicate that this thermal transformation exhibits several unusual features. It has been shown that: (1) In form II, the layers of intercalated molecules (HNO3 and NO3) assume a two-dimensional crystalline arrangement. On warming to give form I, these crystalline layers ‘melt’, that is, they pass into a much more disordered two-dimensional assembly, akin to a liquid or glass (cf. Figs, 1a and 1b). Transformations between forms I and II occur quite readily on cycling the temperature around the transformation point. Little is yet known regarding the symmetry and cell dimensions of form II, except that the c-axis repeat distance doubles in all cases. (2) Although the four compounds show lambda transformations at effectively the same temperature, suggesting that each intercalate layer undergoes transformation independently, in fact the hkl superlattice reflexions indicate that in the first sequence compound (Fig. 1b) the sites occupied by the intercalate molecules are ordered in three principal directions. Furthermore, an interesting finding of the present research is that some (although not all) of the hkl superlattice reflexions are also sharp in compounds of sequences 2, 3 and 4 (Fig. 1c). The simultaneous occurrence of sharp and ‘streaked’ reflexions (diffuse in one dimension) suggests that the ideal three-dimensional order is perturbed by stacking faults. Even so, considerable three-dimensional order persists between intercalate layers in form II of any of these three sequences. This contrasts markedly with the behaviour reported for the bromine layers in C8Br (ref. 2). Conceivably, positional correlation between different layers may appear at a lower temperature than the two-dimensional crystallization referred to in (I), but, if so, the temperature interval cannot be greater than a few degrees. (3) The distances d(Å) between layers which show such correlations of position in the ordered structures of form II are as follows: These distances are large, and it seems unlikely that conventional interaction forces between molecules in different layers can account for the lambda transformation at about −20° C.

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UBBELOHDE, A., PARRY, G. & NIXON, D. Order–Disorder Transformations in Graphite Nitrates. Nature 206, 1352–1354 (1965). https://doi.org/10.1038/2061352b0

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