METAL-INSULATOR-TRANSITION AND ANOMALOUS MAGNETIC ORDER IN THE TL(1212) TLBA2(CA1-XRX)CU2O7 (R=PR, Y) SYSTEM

Magnetic order and superconductivity of the TlBa2(Ca1-xRx)Cu2O7 system (R = Pr, Y) are reported. For the Ca-free semiconducting compound TlBa2RCu2O7-delta (x = 1.0, delta less-than-or-equal-to 0.06), Cu2+ moments ordered antiferromagnetically above room temperature. Low-temperature data indicate an anomalously high antiferromagnetic Pr ordering with Neel temperature T(N)(Pr) = 8-10 K and a magnetic entropy change of DELTAS congruent-to 0.5 R ln 2 between 2 and 10 K. Through structural consideration, these observations correlated to the almost identical but still puzzling features of PrBa2Cu3O7-y(123) with T(N)(Pr) = 10-17 K. Metal-insulator transition occurs around x congruent-to 0.5 for the Ca-substituted TlBa2(Ca1-xRx)Cu2O7 Compounds. No superconducting transition was observed down to 2 K for x greater-than-or-equal-to 0.5. In the metallic region, a maximum T(c) of 110 K was observed for TlBa2(Ca0.8Pr0.2)Cu2O7 and 105 K for TlBa2(Ca0.7Y0.3) Cu2O7, which separate the underdoped region from the overdoped region.