Formation of Nets of Corner-Shared Bicapped Gold Squares in SrAu3Ge: How a BaAl4-Type Derivative Reconciles Fewer Valence Electrons and the Origin of Its Uniaxial Negative Thermal Expansion

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) Å, c = 5.5082(9) Å, Z = 2 at room temperature. The structure of SrAu3Ge, a reapportioned √2 × √2 × 1 superstructure of CeMg2Si2 (P4/mmm), exhibits checkerboard nets of corner-shared bicapped Au squares (or corner-shared Au­(Au4/2)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 5d10 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 × 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.SrAu3Ge通过混合元素高温直接熔融后经退火处理合成，其室温结构通过单晶X射线衍射法测定，空间群为（Pearson符号：tP10）P4/nmm，晶胞参数a=6.264(1) Å，c=5.5082(9) Å，Z=2。SrAu3Ge的结构是CeMg2Si2（空间群P4/mmm）经重排得到的√2×√2×1超结构，呈现出由共角顶双帽金原子正方形（或共角Au(Au4/2)Ge八面体）构成的棋盘格网状结构，相邻网状结构中的顶点Au-Ge原子对沿c轴方向发生强相互键合。该结构基元与常见的BaAl4（空间群I4/mmm）原型结构存在显著差异：后者对应层中的Al四方环会交替被来自上方或下方的Al原子单帽封端。BaAl4型结构的典型价电子数（valence electron count，简称vec）介于12至16之间，CeMg2Si2的价电子数与之相近，为15。SrAu3Ge的价电子数为9，其特殊稳定性源于金的5d10电子态对Au-Ge及Au-Au键合的显著相对论效应贡献。这一效应也体现在SrAu3Ge中Au与Ge的位点占有率相较于CeMg2Si2发生了显著的重新分配。SrAu3Ge沿c轴方向表现出显著的单轴负热膨胀，其热膨胀系数为-1.57×10^-5 K^-1，而a、b轴方向的热膨胀系数为2.16×10^-5 K^-1。升温过程中，网状Au5Ge八面体层在ab平面内发生热膨胀，而层间较强的Au-Ge键则保持固定。