Possible charge-density-wave/spin-density-wave in the layered pnictide-oxides Na₂Ti₂Pn₂O (Pn = As, Sb)

The compounds Na₂Ti₂Pn₂O (Pn = As, Sb) crystallize in the anti-K₂NiF₄ structure type in the space group, I4/mmm, with Z = 2 and the lattice parameters a = 4.0810(9) Å and c = 15.311(3) Å for the As analogue at 310 K and a = 4.160(2) Å and c = 16.558(7) Å for the Sb analogue at 150 K. The structure consists of edge-shared _∞ ² [Ti_4/2Pn₂O_4/4]^2- layers separated by double layers of Na⁺. These compounds exhibit an anomalous transition in the temperature-dependent magnetic susceptibility at T_c ^onset = 330 K for the As analogue and T_c ^onset = 120 K for the Sb analogue. Temperature-dependent powder neutron diffraction has been performed to investigate the magnetic spin ordering and structure symmetry breakdown of the compounds; however, no scattering due to magnetic spin ordering or symmetry change has been detected. The temperature-dependent electrical resistivity of these compounds also exhibits an anomaly reminiscent of CDW (charge-density-wave)/SDW (spin-density-wave) materials. The As analogue shows an insulator-to-insulator transition around 135 K whereas the Sb analogue shows a metal-to-metal transition around 120 K that corresponds well to the onset of the anomaly in the magnetic susceptibility. The similarity and difference in the physical properties between the As and Sb analogues as well as related compounds will be discussed.