Determination of large unit cells of ordered surface structures by tensor LEED

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Surface structure sequence 2x1*3x3*4x4 formed on Cu (001) by Li deposition at room temperature has been determined by automated tensor LEED analysis. All three structures include considerable displacement of substrate surface Cu atoms. The 2x1 structure is formed by missing of every second row of the Cu (001) plane, and Li atoms are located in the missing-row sites. This is substitutional adsorption of Li atoms. At the completion of the 2x1 structure (Li coverage 0.4), no substitutional site remains. Then surface Cu and Li atoms move around and form space for Li adatoms. This is the 3x3 structure, in which a single Li adatom is located on a small island of four Cu atoms separated and joined by pairs of substitutional Li atoms. Then, Li coverage increases to 0.556, while the average adsorption energy per Li atom decreases because of appearance of Li adatoms having smaller binding energy than the substitutional Li atoms. With increasing Li coverage further, similar 4x4 structure is formed. Again the average adsorption energy decreases because of increase of Li adatoms in the 4x4. Thus the surface accommodates additional Li atoms at the cost of the average adsorption energy.