Crystal Structure, EXAFS Study and Low Temperature Magnetic Properties of Ammonium Hexabromorhenate

Crystal and molecular structures of ammonium hexabromorhenate (NH4)2ReBr6 have been determined by the heavy atom method. The title compound crystallizes red-violet crystals in the space group Fm3m with Z = 4. The Re atom is of six-coordinate octahedral configuration, being bonded to six bromide anions. The magnetic behaviour of (NH4)2ReBr6 has been investigated over the temperature range 1.72-300 K. These measurements revealed that this compound is an antiferromagnet with Neél temperature TN = 15.5 K. The magnetic data are interpreted in terms of zero-field splitting of the ground 4A2g term (D = 22.8 cm-1), large because of the high spin-orbit coupling constant for the Re+4 ion, with antiferromagnetic exchange coupling (J = -17.7 cm-1).