k_f}$, for given face are chosen to satisfy condition of constructive interference $({\bf 
k}_f-{\bf k}_i)_{\parallel} =l{\bf g}_1+m{\bf g}_2 $, where $l, 
m=0, \pm 1, \ldots$ and the symbol $\parallel$ denotes vector component parallel to the face.

We calculated structure factors for the wave vector of the incident electron, $ k_i =2\pi/a( 0, 0, 1.43)$, normal to the bcc(111) surface. Thus energy of incident electrons is equal to 31 eV for $a=3.14\mbox{\AA}$, similarly as in LEED experiment for Pd/Mo(111) [@song95]. Figure \[figure8\] shows LEED patterns obtained in computer simulation for the model parameter $K=-1.25$. In the ordered phase {211} we see besides 3 clovers (groups of 3 spots) observed experimentally, the forth clover in the center of the image, and six single spots more distant from the center. The pattern of disordered phase consists of three spots which positions correspond to the centers of external clovers in the diffracted pattern of the faceted phase. Apparent lack of spots from bcc(111) surface confirms that disordered phase has not the planar structure.

![ Temperature dependence of the structure factor for the wave vector corresponding to a spot of the disordered phase on warming (triangles) and on cooling (squares). The hysteresis loop for large 