Physical properties of the layered pnictide oxides Na2Ti2P2O (P=As, Sb)

The compounds Na2Ti2P2O (P=As,Sb) with anti-K2NiF4 structure were characterized by magnetic susceptibility, magnetoresistance, Hall coefficient (R-H), heat capacity, and thermoelectric power (TEP). These compounds exhibit an anomalous transition in resistivity and susceptibility at T-S similar to 320 K for Na2Ti2As2O and T-S similar to 115 K for Na2Ti2Sb2O. The Hall coefficient of Na2Ti2Sb2O shows a pronounced rise below T-S, indicating a decrease in carrier density. The sign of TEP changes from positive to negative and magnetoresistance pronounced increase at T-S for Na2Ti2Sb2O. Heat capacity shows an anomalous peak at T-S similar to 110 K, a typical behavior for spin-density-wave (SDW)/charge-density-wave instability or a structure distortion. These behaviors of Na2Ti2Sb2O are quite similar to that of FeAs-based high-T-C parent compound RFeAsO and AFe(2)As(2) with a SDW instability. Na2Ti2Sb2O could be considered as a possible parent compound for superconductivity.