Possible charge-densitgr-wave/spin-density-wave in the layered pnictide-oxides: Na(2)Ti(2)Pn(2)O (Pn = As, sb)

The compounds Na(2)Ti(2)Pn(2)O (Pn = As, Sb) crystallize in the anti-K2NiF4 structure type in the space group, I4/mmm, with Z = 2 and the lattice para meters a = 4.0810(9) Angstrom and c = 15.311(3) Angstrom far the As analogu e at 310 K and a = 4.160(2) Angstrom and c = 16.558(7) Angstrom for the Sb analogue at 150 K. The structure consists of edge-shared (2)(infinity)[Ti(4 /2)Pn(2)O(4/4)](2-) layers separated by double layers of Na+. These compoun ds exhibit an anomalous transition in the temperature-dependent magnetic su sceptibility at T-c(onset) = 330 K for the As analogue and T-c(onset) = 120 K far the Sb analogue. Temperature-dependent powder neutron diffraction ha s been performed to investigate the magnetic spin ordering and structure sy mmetry breakdown of the compounds; however, no scattering due to magnetic s pin ordering or symmetry change has been detected. The temperature-dependen t electrical resistivity of these compounds also exhibits an anomaly remini scent of CDW (charge-density-wave)/SDW (spin-density-wave) materials. The A s analogue shows an insulator-to-insulator transition around 135 K whereas the Sb analogue shows a metal-to-metal transition around 120 K that corresp onds well to the onset of the anomaly in the magnetic susceptibility. The s imilarity and difference in the physical properties between the As and Sb a nalogues as well as related compounds will be discussed.