SrAu3Ge was synthesized by direct fusion of the mixed elements at high temperature followed by annealing treatments, and its structure was determined by single crystal X-ray diffraction means in space group (Pearson symbol: tP10) P4/nmm, a = 6.264(1) angstrom, c = 5.5082(9) angstrom, Z = 2 at room temperature. The structure of SrAu3Ge, a reapportioned root 2 x root 2 x 1 superstructure of CeMg2Si2 (P4/mmm), exhibits checkerboard nets of corner-shared bicapped Au squares (or corner-shared Au(Au-4/2)Ge octahedra), in which the apical Au Ge pairs in adjoining nets are strongly interbonded in the c direction. This motif contrasts with that of the common BaAl4 (I4/mmm) prototype in which Al squares in comparable layers are alternately monocapped by Al from the top or the bottom. Typical examples show valence electron counts (vec) between 12 and 16 for the BaAl4 type and that for CeMg2Si2 is similar, 15. The special stability of SrAu3Ge, with vec = 9, derives from significant relativistic contribution of the Au 5d(10) states to the Au-Ge and Au-Au bonding. These factors are also recognized in the marked redistribution of Au and Ge site occupancies from those in CeMg2Si2. SrAu3Ge exhibits a pronounced uniaxial negative thermal expansion along c, with a coefficient of -1.57 versus 2.16 x 10(-5) K-1 in a and b. The reticulated Au5Ge octahedral layers expand in the ab plane on heating, whereas the strong, interlayer Au-Ge bonds remain fixed.