A Study of Structure, Electric and Magnetic Properties of the New Layered Cobalt Oxides Sr₂Y_1-x Ca _x Co₂O_7-z

Abstract

The crystal structure of Sr ₂ Y _0.8 Ca _0.2 Co ₂ O ₆ is orthorhombic below 270 K (Immm, a = 3.84029(7) Å, b = 3.80691(6) Å and c = 19.4980(3) Å at 20 K), transformed from tetragonal (I4/mmm, a = 3.82765(6) Å and c = 19.5795(3) Å) at 295 K. The crystal structure distortion is accompanied by an anomalous temperature dependence of the degree of buckling of the CoO ₂ plane and is correlated with the development of anisotropic antiferromagnetic ordering of magnetic moments (2.93(2) μ _B per Co at 20 K). A magnetically glassy state below about 35 K and variable range hopping conduction below about 64 K for $\begin{gathered} Sr_2 Y_{1 - x} Ca_x Co^{3.4 + } _2 O_{7 - z} (0.2 \leqslant x \leqslant 0.5,0.21 \leqslant z \leqslant 0.38) \hfill \\ \hfill \\ \end{gathered}$ were found by magnetic susceptibility and electrical resistivity measurements.