CRYSTAL-STRUCTURES AND MAGNETISM OF OXYGEN MONOLAYERS ON HEXAGONAL BORON-NITRIDE

We have measured X-ray diffraction (XRD) and magnetic susceptibility ( chi) of oxygen monolayers on hexagonal boron nitride (h-BN) as a funct ion of coverage and temperature, and determined the phase diagram, whi ch is discussed compared with that of a graphite substrate. The covera ge dependence reveals two distinct phases. A high-density monolayer ze ta-phase has a triangular lattice, which is characterized by a magneto elastic transition to the epsilon-phase at T-N = 11 K, similar to a gr aphite substrate. On the other hand, for the low-density monolayer lam bda-phase in which all the molecules are lying on the surface, there a ppears a structural transition of the first-order to kappa-phase, incl uding the change of lattice constants and a simultaneous rotation of m olecules. This transition is unique to h-BN and not observed for a gra phite substrate. The shrink of the nearest neighbor distance at low te mperatures is a result of a magnetoelastic transition and in kappa-pha se; the magnetic exchange energy (J) is expected to be much stronger f or the kappa-phase (J = 28.7 K) than the lambda-phase(J = 14.5 K) and form a quasi-one-dimensional structure (J(nnn)/J(nn) = 0.01). Moreover , chi shows an exponential temperature dependence below T-c, suggestin g the possibility of an energy gap, discussed as a Haldane gap. A rema rkably slow kinetic characteristic of log t is observed both in XRD an d chi measurements, which might be related to the domain growth in the quasi-one-dimensional system. (C) 1998 Elsevier Science B.V. All righ ts reserved.