Powder samples of the anhydrous compounds M2+(ReO4)2 (M = Mn, Fe, Co, Ni, and Cu) were prepared. All have trigonal symmetry, except M = Cu which is Jahn-Teller distorted to monoclinic symmetry, I2/m. Single crystals of the Fe and Cu compounds were grown by vapor transport and their layered structures determined. Ferromagnetism, deduced from ac susceptibility and dc magnetization studies, is exhibited by the Fe (TCurie = 8.5 K), Co (TCurie = 4.7 K), and Ni (TCurie = 12.5 K) phases. They show 2-D ferromagnetic (intralayer) interactions as well as a 3-D ferromagnetic (interlayer) ground state with both of these effects obvious in the heat capacity of Ni(ReO4)2. Low-temperature powder neutron diffraction confirms the ferromagnetism of Ni(ReO4)2 below 12 K, and also reveals a structural phase transition occurring between room temperature and 77 K. The origin of the 3-D ferromagnetic ground states of the Fe, Co, and Ni perrhenates is the result of dipolar interlay er interactions, as opposed to superexchange between the layers. Susceptibility results of the Cu perrhenate indicate intralayer ferromagnetism and any long-range 3-D order occurring at T ≤ 1.5 K. The Mn compound is antiferromagnetic.
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