Alkali-Metal Thiogermanates: Sodium Channels and Variations on the La3CuSiS7 Structure Type

Five new isotypic quaternary chalcogenides
containing rare-earth metal atoms crystallizing in the hexagonal noncentrosymmetric
space group P63 (No. 173) with the La3CuSiS7 structure type have been synthesized by
reacting the appropriate anhydrous rare-earth trichloride with sodium
thiogermanate, Na2GeS3. The reaction between
LnCl3 and Na2GeS3 in an evacuated
fused-silica ampule produced high yields of good-quality crystals
of NaLn3GeS7 [Ln = Ce (I), Nd (II), Sm (III), Gd (IV), and Yb (V)], while a similar reaction between EuCl3 and
Na2GeS3 yielded a quinary chloride thiogermanate,
Na1.2Eu3.4Cl2Ge3S9 (VI), incorporating a cyclic trimeric Ge3S9 building unit and adopting a structure related
to La3CuSiS7. The crystal structure of the compounds
comprises a complex network of bicapped trigonal-prismatic LnS8 and GeS4 tetrahedra, which creates channels along
the [001] direction. The Na+ cations reside in these channels
within trigonally distorted octahedral coordination environments,
surrounded by six S atoms. For compounds III–V, the temperature dependence of the magnetic susceptibility
indicates that these compounds are paramagnetic with μeff. = 1.86, 8.01, and 3.87 μB, for III–V, respectively. The experimental μeff for IV is close to the theoretical value of
7.94 for free Gd3+ ions, while μeff values
for III and V deviate from their theoretical
values of 0.86 and 4.54 μB for Sm3+ and
Yb3+ ions, respectively. These compounds are semiconductors
with optical band gaps of around 1.3 eV for III and V. Extended Hückel calculations suggest that the valence
band comprises primarily S 3p and the bottom of the conduction band
is dominated by empty rare-earth 5d orbitals. Compound VI exhibits a sharp optical absorption of around 2.18 eV, which is
attributed to the f → d transition of EuII. The
effective magnetic moment of 7.94 μB/Eu is in excellent
agreement with the theoretical value of 7.94 μB for
the free Eu2+ ion.

五种新型的同位素四元硫族化合物，其中包含稀土金属原子，并以六方非中心对称空间群P63（编号173）结晶，其结构类型为La3CuSiS7，已通过将相应的无水稀土三氯化物与硫代锗酸钠（Na2GeS3）反应而合成。在真空熔融石英管中，LnCl3与Na2GeS3之间的反应生成了高质量的NaLn3GeS7晶体（其中Ln = Ce（I）、Nd（II）、Sm（III）、Gd（IV）和Yb（V）），产率较高。而EuCl3与Na2GeS3的类似反应则产生了一种五价氯化硫代锗酸盐Na1.2Eu3.4Cl2Ge3S9（VI），其中包含一个环状三聚体Ge3S9构建单元，并采用了与La3CuSiS7相关的结构。这些化合物的晶体结构由复杂的双帽三角棱柱形LnS8和GeS4四面体网络构成，沿[001]方向形成通道。Na+阳离子位于这些通道中，在三角畸变的八面体配位环境中，周围环绕着六个S原子。对于化合物III–V，磁化率的温度依赖性表明这些化合物是顺磁性的，其有效磁矩μeff分别为1.86、8.01和3.87 μB。对于化合物IV，其实验μeff值接近自由Gd3+离子的理论值7.94 μB，而III和V的μeff值分别偏离Sm3+和Yb3+离子的理论值0.86和4.54 μB。这些化合物是半导体，其光学带隙约为1.3 eV。扩展Hückel计算表明，价带主要由S 3p轨道组成，而导带底部则主要由空的稀土5d轨道主导。化合物VI表现出约2.18 eV的尖锐光学吸收，这归因于EuII的f → d跃迁。其有效磁矩7.94 μB/Eu与自由Eu2+离子的理论值7.94 μB吻合得非常好。