Imperfections in the structure of cobalt. I. Experimental work and proposed structure

X-ray powder photographs of hexagonal cobalt have a remarkable mixture of sharp and diffuse lines. The sharp lines are those with 1/3 (h - k) integral and those with l zero; all others are more or less diffuse. An explanation of this is produced, based on the possibility of faults occurring in the structure because there are two different ways in which one close-packed layer of atoms can be put on to another. It is shown theoretically that if such faults are repeated at regular intervals the reflexions with 1/3 (h - k) integral are not affected, but the positions and intensities of the other lines are modified although they remain sharp. These results suggest that a structure with random faults would give broadened reflexions, but the broadening is such that it would not show for lines with l zero on powder photographs. Evidence from oscillation photographs and from measurement of a powder photograph is adduced in support of this theory.