A SUPERSPACE APPROACH TO THE MODULATED STRUCTURES OF MA(X)TE(2) (M=NB,TA A=SI,GE 1/3-LESS-THAN-OR-EQUAL-TO-X-LESS-THAN-OR-EQUAL-TO-1/2), EXEMPLIFIED BY NBGE3/7TE2

MA(x)Te(2) (M = Ta, Nb and A = Si, Ge) compounds show a great variety of commensurately and incommensurately modulated structures depending on the value for x. It is shown that the theory of superspace groups can be used to standardize the structures and to predict the 3D symmetry of the commensurately modulated structures. The commensurately modulated structure of NbGe3/7Te2 is determined with the aid of single crystal x-ray data. A comparison with some already published structures in the series shows that all structures with 1/3 less than or equal to x less than or equal to 1/2 can be regarded as being built from definite numbers of units of the MA(1/3)Te(2) and MA(1/2)Te(2) type.