Magnetic and electronic nature of layered transition-metal oxysulfides

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Novel layered transition-metal oxysulfide Sr_2Cu_2CoO_2S_2 (I/4mmm), which crystallizes in an unusual intergrowth structure with alternating square-planar CoO_2 sheets and SrCu_2S_2 layers, have been studied to reveal the electronic and magnetic nature as follows.Sr_2Cu_2CoO_2S_2 is a p-type antiferromagnetic (AF) semiconductor with TN (=200K) determined by the magnetic susceptibility (χ) ana the neutron powder diffraction (NPD) measurements. Above T_t (=140K), the. χ shows a broad maximum indicative of two-dimensional (2D) AF nature as normally seen for K_2NiF_<4->type AF compounds. The derivative permeability (dμ/dT) curves have a kink near T_t at which the AF nature is changed from 2D to 3D ; T_t extremely rises with increasing pressure (dT./dP-13.5K/kbar), indicating that 3D-type AF state is more stable under pressure as compared to 2D-type AF state. Tlle T_t in the dFL /dT curves correspond to the bending point of the resistivity, and above T_t the nearest-neighbor hopping conduction occurs. The Cu ion of die Cu_2S_2 layer is in the mono-valent state (Cu_+ : d_<10>) and the CoO_2 square-planes have the strongly correlated electronic nature (with the O 2p-to-Co 3d charge transfer energy Δ=4.2eV and the on-site d-d Coulomb. repulsion energy U=5eV), according to experimental and theoretical x-ray photoemission spectroscopy (XPS) study. Therefore, the magnetic nature is not originated from the Cu_2S_2 layers but the strongly correlated CoO_2 square-planes, which is consistent with the NPD analysis. In Cu-doped Sr_2Cu_2CoO_2S_2 with Co_<1-x>Cu_xxO2 Planes, the samples with x<0.4 shows a semiconducting behavior, while the samples with x=0.4 and x=0.5 exhibits a metal-semiconductor transition at 200K and 175K, respectively. This feature of Cu-doping into the CoO_2 planes suggests that Sr_2Cu_2(Co, Cu)O_2S_2 situates near the strongly correlated metallic region in the so-called Zaanen-Sawatzky-Allen electronic diagram.