Magnetic and electronic nature of layered transition-metal oxysulfides

研究了一种新型层状过渡金属硫化物Sr2Cu2CoO2S2(I/4mmm)，它以一种奇特的共生结构结晶，具有交替的方形平面CoO2片层和SrCu2S2层。由磁化率(χ)和中子粉末衍射(NPD)测量确定，Sr2Cu2CoO2S2是一种p型反铁磁(AF)半导体，Tn=200K。在T_t(=140K)以上，χ显示了一个广泛的最大值，表明了二维(2D)AF性质，就像通常看到的K_2NiF_1；4-型AF化合物一样。导数渗透率(dμ/dt)曲线在T_t附近有一个拐点，在T_t处，房颤性质由2D变为3D；T_t随压力的增加而极大地升高(dt./dp-13.5K/kbar)，表明3D型房颤态在压力下比2D型房颤态更稳定。DFL/DT曲线上的TlE T_t对应于电阻率的拐点，在T_t以上发生最近邻跳跃导电。Cu2S2层的铜离子处于单价态(Cu2+：D&lt；10&gt；)，CoO2方面具有强关联的电子性质(O2p-to-Co3d电荷转移能Δ=4.2 eV和在位d-d库仑)。根据实验和理论x射线光电子能谱(XPS)研究，排斥能U=5 eV。因此，磁性不是来源于Cu2S2层，而是强相关的CoO2方面，这与NPD分析是一致的。在具有Co&lt；1-x&Gt；CuxxO2晶面的掺铜Sr2Cu2CoO2中，x=0.4的样品表现出半导体行为，而x=0.4和x=0.5的样品分别在200K和175K表现出金属-半导体转变。在CoO_2晶面中掺入铜的这一特征表明，在所谓的Zaanen-Sawatzky-Allen电子图中，Sr_2Cu2(Co，Cu)O_2S_2位于强关联金属区附近。

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.