Structure of graphite by neutron diffraction

Abstract

THE structure of graphite¹ consists of layers of linked hexagons of carbon atoms. The layers are stacked in a ⋯ ABAB ⋯ sequence so that half the atoms in a layer are directly above and below carbon atoms in the adjoining layers, and half are directly above and below centres of the hexagons. The hexagonal unit cell contains atoms at the (0, 0, 1/4), (1/3, 2/3, 1/4), (0, 0,−1/4) and (2/3, 1/3,−1/4) positions. Modifications to this structure have been proposed by several workers. Lukesh² reported diffraction patterns indicating a lower symmetry, and Pauling³ has proposed an orthorhombic structure in which the interconnected six-membered rings are distorted so that two-thirds of the bonds are longer. Pauling based his model on the high compressibility of the basal planes, and considerations of stacking bonds in the layered structure. Ergun⁴ suggested that data on pyrolitic graphite indicated an apparent quinoid structure in the graphite layers, but errors in his analysis have been reported⁵. X-ray and neutron diffraction studies of graphite were undertaken in our laboratory to study the details of the electron charge density and atomic motion in the graphite structure. The results of the neutron diffraction work show that all in plane carbon–carbon distances have the same value 1.422±0.001 Å.