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) A and c = 15.311(3) A for the As analogue at 310 K and a = 4.160(2) A and c = 16.558(7) A for the Sb analogue at 150 K. The structure consists of edge-shared [Ti4/2Pn2O4/4]2- layers separated by double layers of Na+. These compounds exhibit an anomalous transition in the temperature-dependent magnetic susceptibility at = 330 K for the As analogue and = 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 tran...