Single crystals of five (Al,Ge)-mullites incorporating Pb, and four of which also incorporating foreign cations (Fe,Cr,Nd,Sm) were grown by flux techniques in a PbO-MoO3 flux. They were characterized by scanning electron microscopy, electron microprobe analyses, single-crystal X-ray diffraction. In addition, the refractive indices of mullite containing Nd were determined by spindle-stage optical investigations. Careful inspection of the single-crystal X-ray diffraction data revealed that weak superstructure reflections observed in all doped crystals violating the reflection conditions can be attributed to λ/2 contributions in the primary X-ray beam. Consequently, all crystal structures were refined in space group Pbam, thus avoiding a symmetry lowering to a noncentrosymmetric subgroup as done in earlier work on a (Al,Ge)-mullite doped with Pb and Nd (Saalfeld & Klaska, Z. Kristallogr. 1985, 172, 129–133). The following phases with chemical compositions used in the refinements were obtained: undoped mullite (Al4.50Ge1.50O9.75; a = 7.6559(4) Å, b = 7.7763(4) Å, c = 2.9233(2) Å, V = 174.04(2) Å3); (Pb,Fe)-doped mullite (Pb0.02Fe0.68Al3.95Ge1.37O9.70; a = 7.7125(7) Å, b = 7.8527(7) Å, c = 2.9528(2) Å, V = 178.83(3) Å3); (Pb,Cr)-doped mullite (Pb0.01Cr0.63Al3.90Ge1.47O9.75; a = 7.6917(6) Å, b = 7.8168(6) Å, c = 2.9522(2) Å, V = 177.50(2) Å3); (Pb,Nd)-doped mullite (Pb0.06Nd0.02Al4.82Ge1.18O9.69; a = 7.6585(7) Å, b = 7.7666(7) Å, c = 2.9164(3) Å, V = 173.47(3) Å3); (Pb,Sm)-doped mullite (Pb0.06Sm0.02Al4.55Ge1.45O9.79; a = 7.6563(3) Å, b = 7.7873(3) Å, c = 2.9236(1) Å, V = 174.31(1) Å3); Pb is only incorporated into the crystal structure when a co-dopant element is present. Then it resides together with Nd or Sm in the oxygen-vacancy sites created by the formation of triclusters of AlO4 and GeO4 tetrahedra. In the case of (Pb,Fe)-doped mullite, Fe shares the same position as Al and Ge. In contrast to the (Al,Si)-mullites, Ge is located in both tetrahedral sites T and T*. The occupancies follow a substitution scheme according to Pb q (Nd,Sm) r (Cr,Fe) z Al4+2v−z Ge2−2v O10−v+q+3/2r . With v = number of vacancies, such a mullite can be understood as a “stuffed mullite” derived from a related “open mullite” (no vacancies filled with large cations) of composition (Cr,Fe) z Al4+2v−z Ge2−2v O10−v and then “stuffed” with qPb2+ + r(Nd3+,Sm3+) formula units where concurrently the number of available O3-vacancies is reduced by q + 3/2r units of extra oxygen. Thus, charge compensation upon incorporation of Pb2+ and (Nd,Sm)3+ is achieved by adding the amount of oxygen corresponding to the oxidation state of divalent Pb2+ and trivalent rare-earth elements. Based on this description, the maximum number of large cations which can be stuffed into the mullite structure can directly be calculated from the v-value of the related “open mullite”. In contrast, the smaller cations Fe3+ and Cr3+ are directly substituting Al3+. In the stuffed mullites, Pb and (Nd,Sm) could not be distinguished and were refined with a mixed occupancy on the same site. In addition to structure analysis, refractive indices of (Pb,Nd)-mullite were determined by immersion methods using a micro-refractometer spindle stage yielding n x = 1.697(3), n y = 1.708(3), and n z = 1.710(3) and 2V z = 122(4)°. The mean refractive index corresponds closely to the corresponding parameter calculated from the chemical composition whereas it would be significantly off if the extra cations were ignored, thus representing an independent evidence for the incorporation of Pb and Nd into the crystal structure.

中文翻译：

---

在 Fe、Cr、Nd 和 Sm 存在下，Pb 在 (Al,Ge)-mullites 中的掺入

五种 (Al,Ge)-mulites 的单晶包含 Pb, 其中四种还包含外来阳离子 (Fe,Cr,Nd,Sm) 通过助熔剂技术在 PbO-MoO 中生长3个通量。它们通过扫描电子显微镜、电子微探针分析、单晶 X 射线衍射进行表征。此外，含 Nd 的莫来石的折射率是通过主轴台光学研究确定的。仔细检查单晶 X 射线衍射数据表明，在所有违反反射条件的掺杂晶体中观察到的微弱超结构反射可归因于初级 X 射线束中的 λ/2 贡献。因此，所有晶体结构都在空间群中得到细化Pbam，从而避免对称性降低到非中心对称子群，如早期研究掺杂 Pb 和 Nd 的 (Al,Ge)-莫来石 (Saalfeld & Klaska, Z. Kristallogr. 1985, 172, 129–133)。获得了以下具有用于细化的化学成分的相：未掺杂的莫来石（Al4.50葛1.50欧9.75;A= 7.6559(4) Å,b= 7.7763(4) Å,C= 2.9233(2) Å,V= 174.04(2) Å3个); (Pb,Fe)-掺杂莫来石 (Pb0.02铁0.68阿尔3.95葛1.37欧9.70;A= 7.7125(7) Å,b= 7.8527(7) Å,C= 2.9528(2) Å,V= 178.83(3) Å3个); (Pb,Cr)-掺杂莫来石 (Pb0.01铬0.63阿尔3.90葛1.47欧9.75;A= 7.6917(6) Å,b= 7.8168(6) Å,C= 2.9522(2) Å,V= 177.50(2) Å3个); (Pb,Nd)-掺杂莫来石 (Pb0.06钕0.02阿尔4.82葛1.18欧9.69;A= 7.6585(7) Å,b= 7.7666(7) Å,C= 2.9164(3) Å,V= 173.47(3) Å3个); (Pb,Sm)-掺杂莫来石 (Pb0.06史密斯0.02阿尔4.55葛1.45欧9.79;A= 7.6563(3) Å,b= 7.7873(3) Å,C= 2.9236(1) Å,V= 174.31(1) Å3个); 当存在共掺杂元素时，Pb 仅结合到晶体结构中。然后它与 Nd 或 Sm 一起存在于由 Al2O3 的三簇形成而产生的氧空位中4个和氧化锗4个四面体。在 (Pb,Fe) 掺杂莫来石的情况下，Fe 与 Al 和 Ge 共享相同的位置。与 (Al,Si)-莫来石相反，Ge 位于四面体位点 T 和 T*。占用率遵循 Pb 替代方案 q (钕、锑) r (铬、铁) z 阿尔4+2v−z 葛2−2v 欧10−v+q+3/2r . 和v= 空位数，这样的莫来石可以理解为“填充莫来石”，源自成分为 (Cr,Fe) 的相关“开放式莫来石”（无空位填充大阳离子） z 阿尔4+2v−z 葛2−2v 欧10−v 然后“塞满”q铅2++r（钕3+,史密斯3+) 公式单元，同时可用 O3 空缺的数量减少了q+ 3/2r额外氧气的单位。因此，加入 Pb 后的电荷补偿2+和 (Nd,Sm)3+通过添加与二价 Pb 的氧化态相对应的氧气量来实现2+和三价稀土元素。基于此描述，可以直接计算出可以填充到莫来石结构中的大阳离子的最大数量v-相关“开放莫来石”的价值。相反，较小的阳离子 Fe3+和铬3+直接替代Al3+. 在填充莫来石中，Pb 和(Nd,Sm) 无法区分，并且在同一位置混合占据而被细化。除了结构分析外，(Pb,Nd)-mulite 的折射率通过浸入法使用微折射仪主轴台屈服来确定n X = 1.697(3),n 是 = 1.708(3)，和n z = 1.710(3) 和 2V z = 122(4)°。平均折射率与根据化学成分计算的相应参数密切相关，而如果忽略额外的阳离子，它将显着偏离，因此代表了 Pb 和 Nd 掺入晶体结构的独立证据。