The anisotropy of pyrolytic graphite

Two proposed expressions, I₁(φ) = cos^m φ and I₂(φ) = (1 + b² sin² )⁻¹, for the orientation function, I(φ), of pyrolytic graphites have been shown analytically to give quite different values for the Bacon anisotropy factor (BAF) for oriented materials. BAF values derived from the angle of half-maximum intensity using I₁(φ) are within 20% of numerically calculated values for the BAF range 10–90, whereas values obtained using I₂(φ) are smaller by as much as a factor of twenty. The effect of sample preparation on such measurements has been found to be negligible. Diamagnetic susceptibility measurements on the graphites investigated validated the calculated BAF values in that the derived single-crystal susceptibilities were in reasonable agreement with known values. BAF values calculated via I₂(φ) produced unacceptable paramagnetic values parallel to the basal plane. It is suggested that I₁(φ) be used for rapid BAF determinations; although numerical calculation is preferred, the difficulty of measuring I(φ) at large φ, for highly oriented materials, may make the full numerical procedure impracticable and this approximate procedure the more desirable.