Spin-glass behavior in the S = 1/2 fcc ordered perovskite SR₂CaReO₆

The ordered perovskite Sr₂CaReO₆ of monoclinic symmetry [space group P2₁ In,a = 5.7556(3) A,b = 5.8534(3) Å,c = 8.1317(4) Å,β = 90.276(5)° at T=4 K] has been synthesized using standard solid-state chemistry techniques. The difference in the size and charge of the cations induces an ordering of the B site Ca²⁺ and Re⁶⁺ ions which leads to a distorted fcc lattice of spin-1/2 Re⁶⁺ (5d¹) moments dc magnetic susceptibility measurements indicate a maximum at TG∼14 K and an irreversibility in the field-cooled and zero-field-cooled data at ∼22 K that is believed to be caused by the geometric frustration inherent in the fcc structure. Neutron-scattering measurements confirm the absence of magnetic long-range order, and muon spin relaxation experiments indicate the presence of an abrupt spin freezing at T_G. Specific heat measurements reveal a broad anomaly typical of spin glasses and no sharp feature. 65% of the spin entropy is released at low temperatures. The low-temperature data do not show the expected linear temperature dependence, but rather a T³ relationship, as is observed, typically, for antiferromagnetic spin waves. The material is characterized as an unconventional, essentially disorder-free, spin glass.