Strong anisotropy in the electromagnetic properties of Na ₂Ti₂X₂O (X = As, Sb) crystals

Na₂Ti₂X₂O (X = As, Sb) crystals have been grown from the flux method. X-ray diffraction characterization revealed an anti-K₂NiF₄-type layered structure (tetragonal, space group I4/mmm) for both compounds. Magnetic susceptibility (χ(T)) and electrical resistivity (ρ(T)) measurements revealed major kinks at ∼115 K (T_s1) and ∼320 K (T_s2) for Na₂Ti ₂Sb₂O and Na₂Ti₂As₂O, respectively, signifying possibly the opening of density wave gaps. Both Na ₂Ti₂Sb₂O and Na₂Ti ₂As₂O showed remarkably strong anisotropy in their electromagnetic transport properties, and values of γ_ρ (ρ_c/ρ_ab) even reached ∼140 and ∼430, respectively, being much larger than that of iron pnictide BaFe ₂As₂ (γ_ρ ∼ 2-5). The γ_ρ of Na₂Ti₂Sb₂O changed slightly with cooling, though a small drop at T_s1 occurred. In contrast, the γ_ρ of Na₂Ti₂As ₂O changed strikingly by exhibiting not only a small change at T _s2 but also a sudden decrease of ∼50 K, reduced nearly 1/3. Specific heat measurement indicated that Na₂Ti₂Sb ₂O was only partially gapped with γ₁ = 4.1 mJ mol^-1 K^-2, though a long-range order was established at Ts₁, while Na₂Ti₂As₂O was fully gapped. The remarkably strong electromagnetic anisotropy revealed in Na ₂Ti₂X₂O suggests the crucial role of the TiO₂X₄ layer for the transport properties of layered titanium oxypnictides.