DETERMINATION OF THE ATOMIC GEOMETRIES OF THE (110) SURFACES OF CUCL AND CUBR BY DYNAMICAL LOW-ENERGY-ELECTRON DIFFRACTION INTENSITY ANALYSIS

The atomic geometries of the nonpolar (110) surfaces of CuCl and CuBr are determined by dynamical analysis of the intensities of 13 diffract ed beams for CuCl and 16 beams for CuBr associated with normally incid ent electrons at T=125 and 100 K, respectively. The structural model i s specified by six independent variables consisting of the tilt angles (omega(i)) of the top two layers, the three independent bond lengths (c(i)-a(j)) associated with atoms in the top two layers, and the inner potential. The focus of our analysis is the accuracy and precision wi th which these six structural parameters can be extracted from the mea sured intensities. A six-dimensional statistical error analysis was pe rformed for two electron-solid scattering models: one model in which a ll nonstructural parameters assumed the values used in a family of pre vious structural analyses for the (110) surface of CuCl and other bina ry zinc-blende structure materials, and one in which these parameters were treated as adjustable parameters to fit the measured intensities. The structural parameters emanating from these two analyses differ by Delta omega less than or equal to 4 degrees and Delta(c(i)-a(j))less than or equal to 0.3 Angstrom. Statistical analysis of uncertainties i n the structural parameters resulting only from uncertainties in the m easured intensities yields Delta omega less than or equal to 1.3 degre es, Delta(c(i)-a(j))less than or equal to 0.02 Angstrom. Thus, the unc ertainties in the structural parameters associated with the selection of the nonstructural parameters, especially the model of the electron exchange interaction, dominate those associated with the uncertainties in the experimental intensity data. Within these uncertainties we fin d the top-layer bond lengths are contracted by 0.3 Angstrom for CuCl(1 10) and 0.1 Angstrom or less for CuBr(110). Changes in the backbonds f rom the anion and cation cannot be established for CuCl using the exis ting intensity data in our analysis. For CuBr(110) the backbond from t he top-layer cation is contracted by 0.35+/-0.2 Angstrom. The tilt ang les are omega(1)=(53+/-2)degrees for CuCl and (35+/-2)degrees for CuBr with omega(2)=(-5+/-1)degrees for both. These results are compatible with previously identified structural systematics that the most ionic zinc-blende structures exhibit top-layer bond-length contractions whic h increase their tilt angles relative to the value of (29+/-3)degrees characteristic of bond-length-conserving rotational relaxations of cov alent III-V and II-VI (110) surfaces.