(Left) Crystal structure of the 1111 FeAs material (after ). Fluorine (green) replaces oxygen (gray), donating electrons to the FeAs layers. Other atoms shown are iron (yellow), arsenic (purple), and lanthanum (light blue). (Right) Crystal structure of the 122 FeAs material (after ). The parent compound of both materials has iron moments (red arrows) that form a striped antiferromagnetic pattern. Calcium atoms are shown in blue.