The role of second coordination-sphere interactions in incommensurately modulated structures, using beta-K5Yb(MoO4)4 as an example.

The incommensurate palmierite-like structure of beta-K5Yb(MoO4)4, potassium ytterbium tetramolydate, has been refined in the (3 + 1)-dimensional monoclinic superspace group X2/m(0rho0)00, with X = [0 0 0 0; (1/2) (1/2) 0 0; 0 0 (1/2) (1/2); (1/2) (1/2) (1/2) (1/2)] and the unit-cell parameters a = 10.4054 (16), b = 6.1157 (12), c = 19.7751 (18) A, beta = 136.625 (10) degrees ; q = 0.6354 (30)b*. The occupations of the K and Yb atomic positions are described by crenel functions. The structure model reveals a balanced interaction between the atoms of the first and second coordination spheres. It is shown that the third coordination sphere should not be neglected in studies of modulated structures. The ordering of the K and Yb atoms appears to be the driving force for the modulation of all the other atoms.