Defect-induced room-temperature modulation in NbSe2

The quasi-two-dimensional compound NbSe2 was studied by scanning tunneling microscopy. Small domains of a root 13a(0) x root 13a(0) superstructure, fo rming an angle of about +/-14 degrees with the average structure, were obse rved at the edges of some surface defects.. The superstructure is unstable and disappears gradually during scanning under the influence of the strong local field between the tip and the sample. It is attributed to a modulatio n analogous to those known in other d(1) transition-metal dichalcogenides M X2 with octahedrally coordinated X-M-X layers. It is assumed to be triggere d by a local nonstoichiometry, which transforms the trigonal prismatic 2H s tacking into the octahedral 1T one. [S0163-1829(99)13331-9].