(NH4)3CrO8: A new S = 1/2 system exhibiting no magnetic transition down to 0.3 K

Recent studies related to quantum computation suggest a need for developing a well-characterized low-spin, e.g., S = 1/2, metal complex with limited or no significant hyperfine interactions. To this end, we here report the synthesis, crystal structure and magnetic and thermodynamic characterization of ammonium peroxychromate, (NH4)3CrO8, a new compound of Cr(5+), with a 3dx2 y2 , S = 1/2 ground state, with about 92% molecules devoid of any metal hyperfine interaction. It is the first Cr(5+) compound exhibiting a structural phase transition (at 137.5 K) tentatively assigned to the order–disorder motion of the NH4 þ ions, but it remains paramagnetic down to 0.3 K. The compound is stable for years if stored in a refrigerator, but it could explode if crushed or subjected to high pressure, or heated to well above room temperature. 2005 Published by Elsevier Ltd.