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non-electron interaction o> TO is expressed as 4 C^TO -^TO -(27r)3 J w \ 2 M.vW £ C + 2E ' W where w is the bandwidth. This equation indicates that the CD TO 2 can most easily become negative near E G = 0. This means that T co peaks at around x =0.47 and coincides exactly with Tb.In summary, an anomalous increase in Raman intensity due to unscreened LO phonons was observed at both temperatures T c and T b which correspond to the displacive phase transition and the band inversion, respectively. It is shown that the electron-soft-TO-phonon interaction plays an important role in the displacive phase transition in PbSnTe.We wish to acknowledge fruitful discussions with Professor H. Kawamura of Osaka University and also wish to thank Y. Kato for stimulating discussions and sample preparation, and J. Umeda for his encouragement during this work. *N. Kristoffel and P. Konsin, Phys. Status Solidi 21,We show here that a solid Kr layer adsorbed on the basal plane of graphite undergoes an apparently second-(or higher-) order phase transition from an in-registry, (V3~xV3~)30° structure to an out-of-registry, compressed structure as the Kr pressure above the substrate is increased. This transition was first inferred from vapor-pressure isotherms, 1 but was not observed in an earlier low-energy-electron diffraction (LEED) investigation. 2 We observe this transition with highresolution LEED 3 and report the first detailed measurements of mean overlayer lattice constant as a function of the equilibrium vapor pressure P. Our observations are inconsistent with the first-K39 (1967).order, in-registry to out-of-registry transition recently proposed for Kr on graphite. 4 However, our results show remarkable agreement with a one-dimensional dislocation model of monolayer epitaxy. 5 The data provide a unique opportunity to test theories of monolayer epitaxy 5 " 8 in a case where the forces involved are fairly well known.Kr monolayers were condensed on a cleaved, natural graphite crystal mounted in an ultrahighvacuum system. 3 The Kr pressure was increased in small increments while monitoring the LEED pattern. Parameters for the normal-incidence electron beam were 144 eV, 3 nA, and 0.2 mm diam. Electrons backscattered within 30° of nor-Lattice-constant measurements of epitaxial solid krypton monolayers adsorbed on the basal plane of graphite reveal an apparently second-(or higher-) order structural phase transition from an in-registry (\f3x V3)30° structure to an out-of-registry compressed structure. Measurements as a function of equilibrium vapor pressure were made with a high-re solution, low-energy-electron diffraction apparatus for three temperatures near 57 K. Indirect evidence for spatial modulation of the compressed structure by the substrate potential is presented.146

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