K2NiF4型Sr2CrO4的磁电性质研究

以SrO和CrO₂为原料, 在高温高压的条件下直接反应生成纯相的K₂NiF₄结构的Sr₂CrO₄多晶样品. 结构用粉末X射线衍射及GSAS精修表征. 磁化率测试显示样品存在一个弱的反铁磁相变, 奈尔温度为T_N=95 K. 在奈尔温度以上, 磁化率随温度的变化遵循居里-外斯定律. 对样品进行了电阻测试, 结果显示了样品的绝缘特性.

Magnetic and electrical properties of K2NiF4-type Sr2CrO4

Sr₂CrO₄ with a K₂NiF₄ structure can be synthesized by different methods under ambient pressure and high pressure, while the properties reported are quite different. In this paper, pure phase Sr₂CrO₄ with K₂NiF₄ structure is obtained by one-step solid state reaction under high pressure and high temperature. Powders of SrO and CrO₂ are used as the starting materials. The Sr₂CrO₄ structure at room temperature is determined by powder XRD measurement and GSAS Rietveld refinement. Sr₂CrO₄ crystal is of tetragonal symmetry with space group I4/mmm and its lattice parameters a = 3.8191 Å and c=12.5046 Å. There are two oxygen sites, apical O₁ and equatorial O₂. The CrO₆ octahedron is slightly elongated along the c-axis, forming a longer bond Cr–O₁=1.9180 Å and a shorter bond Cr–O₂=1.9096 Å. Temperature dependence of the magnetic susceptibility is measured in the temperature range of 2-300 K under the magnetic field 1 T. A weak antiferromagnetic transition can be seen at T_N=95 K. Above T_N, the susceptibility obeys Curie-Weiss law. The Curie-Weiss fitting gives the Weiss constant θ =-364 K and the effective magnetic moment μ_eff=2.9 μ_B, in good agreement with the theoretical value of localized Cr⁴⁺, indicating the localized electronic state. Field dependence of susceptibility has been measured at different temperatures. The magnetic properties here are different from those in the previous reports, and this discrepancy is attributed to the quite different sample synthesis methods. Temperature dependence of electrical resistivity of Sr₂CrO₄ shows insulating behavior. Activation energy Δ is estimated by the relation ρ ∝ exp(Δ/k_BT) at temperature range 150-300 K. In the temperature range 150-200 K and 200-300 K the activation energies are Δ_L=0.134 eV and Δ_H=0.168 eV, respectively. The insulating behavior is consistent with the previous experiment reports and the theoretical calculation, which is possibly attributed to the suppression of orbital degree of freedom, resulting from the elongation of CrO₆ octahedron and the narrow band width induced by the two-dimensional crystal structure.