Low-temperature specific heat of BaCuO2 and BaCuO2.14 in magnetic fields to 7 T

The specific heats of samples of BaCuO2 and BaCuO2.14, on which magnetization and/or neutron-diffraction measurements had been made earlier, were measured for the temperature range 0.35 less than or equal to T less than or equal to 30 K in magnetic fields to 7 T. BaCuO2+x has a complex structure with 90 formula units in a bcc unit cell; 6 Cu are "lone spins,'' 48 are in 8 Cu6O12 "ring clusters," and 36 are in 2 Cu18O24 "sphere clusters." The ring and sphere clusters have ferromagnetically ordered ground states with spins S=3 and 9, respectively. Antiferromagnetic ordering of the ring clusters occurs with a Neel temperature T-N(0)similar to 15 K. The specific heat of BaCuO2 shows a cooperative ordering anomaly associated with the antiferromagnetic ordering of the ring clusters. Schottky-like anomalies, having maxima at similar to 5 and similar to 0.7 K, are identified with the ordering of the sphere clusters and the lone spins, respectively. Only Schottky-like anomalies are observed for the specific heat of BaCuO2.14. It is suggested that the increase in the Cu oxidation state, due to the addition of 0.14 mol of O, increases Cu-O covalent bonding (spin compensation) and/or produces nonmagnetic Cu3+, which in addition to the known increase in the Cu-O bond lengths, disrupts the superexchange paths that lead to the antiferromagnetic ordering of the ring clusters in BaCuO2. For BaCuO2 the magnetic entropy was 90% of that predicted for the ordering of the three Cu structures. On the other hand, the magnetic entropy for BaCuO2.14 was only 65% of that predicted, which suggests a relative large suppression of some magnetic entities due to the addition of 0.14 mol of O. Although BaCuO2+x is an insulator, the specific heat has a T-proportional component that is magnetic field dependent and is presumably associated with the magnetic degrees of freedom.