(First published in J. Amer. Math. Soc. in 13 (2000), 1-54 , published by the American Mathematical Society) © Copyright 1999 American Mathematical Society

The oxygen-deficient double perovskite YBaCo 2 O 5 , containing corner-linked CoO 5 square pyramids as principal building units, undergoes a paramagnetic to antiferromagnetic spin ordering at 330 K. This is accompanied ...