Structure and Physical Properties of the Layered Pnictide-Oxides: (SrF)₂Ti₂Pn₂O (Pn = As, Sb) and (SmO)₂Ti₂Sb₂O

Abstract

The novel family of pnictide-oxides compounds (SrF)2Ti2Pn2O (Pn = As and Sb) and (SmO) 2Ti2Sb2O were predicted from the stacking of the well-known fluorite type block [Sr2F2]2+ or [Sm2O2]2+ alternating regularly with the anti-CuO2-type Ti2O square planar layer and subsequently synthesized. The samples (SrF)2Ti2Pn2O and (SmO)2Ti2Sb2O with an anti-K2NiF4 type were characterized by X-ray diffraction, resistivity, susceptibility, Hall coefficient (RH), thermoelectric power (TEP), and heat capacity. These compounds exhibit an anomalous transition in resistivity and susceptibility at TS ∼ 380 K for the As analogue, at TS ∼ 200 K for the Sb analogue, and at TS ∼ 230 K for (SmO)2Ti2Sb2O, respectively, being similar to the anomaly observed in FeAs-based high-TC parent compounds REFeAsO/AEFeAsF and AEFe2As2 with a spin density wave (SDW) ordering. A drop in carrier concentration evidenced by a pronounced rise in RH is observed below TS for (SrF)2Ti2Sb2O. Additionally, TEP changes sign below TS for (SrF)2Ti2Pn2O. Heat capacity demonstrates an anomalous peak at TS ∼ 198 K for (SrF)2Ti2Sb2O and TS ∼ 230 K for (SmO)2Ti2Sb2O, respectively, consistent with that observed in resistivity and susceptibility. Such an anomaly could arise from the SDW/CDW instability or structure distortion. An anomaly associated with the Sm3+ ions antiferromagnetic order is observed at 3.2 K in the specific heat for the sample (SmO)2Ti2Sb2O.