Si- and Ge-mullite single crystals doped with foreign cations
Final Report Abstract
This work covered the synthesis of (Al,Ge)-mullites doped with foreign cations which has a controlling influence on its optical properties. 5 mullites doped with Pb2+, Fe3+, Cr3+, Nd3+, and Sm3+ were synthesized by flux techniques and characterized by scanning electron microscopy, electron microprobe analyses and single-crystal X-ray diffraction. In addition, the refractive indices of the (Pb,Nd)-doped mullite were determined. Similarly, 2 crystals of a Tb-doped and undoped (Al,Si)- mullite were produced by Czochralski crystal growth and characterized, also using X-ray microtomography to visualize density variations in the crystals. Finally, we investigated a series of 9 single crystals of (Al,Ge)-mullites with varying Cr content, and a series of 11 (Al,Ge)-mullites synthesized by sol-gel methods with varying content of Ga replacing Al. Our studies have shown that Pb can be incorporated into the (Al,Ge)-mullite structure when a codopant element (here Fe, Cr, Nd, Sm) is present. It is not incorporated when the synthesis is done in a PbO-MoO3 flux without such a codopant element. The careful inspection of the singlecrystal X-ray diffraction data revealed that the weak superstructure reflections violating the reflection conditions can be attributed to λ/2 contributions in the primary X-ray beam. Most likely, this could be an interpretation for the effects observed in earlier work as well. Thus, there was no need to lower the symmetry to the noncentrosymmetric space group P21212 and all refinements could be done in the supergroup Pbam which is the standard space group for all (Al,Si)- and (Al,Ge)-mullites. The following phases were obtained: undoped mullite Al4.50Ge1.50O9.75; (Pb,Fe)-doped mullite Pb0.02Fe0.68Al3.95Ge1.37O9.70; (Pb,Cr)-doped mullite Pb0.01Cr0.63Al3.90Ge1.47O9.75; (Pb,Nd)-doped mullite Pb0.06Nd0.02Al4.82Ge1.18O9.69 and (Pb,Sm)-doped mullite Pb0.06Sm0.02Al4.55Ge1.45O9.79. Pb resides together with Nd or Sm in the interstitial oxygen vacancies created by the formation of triclusters of AlO4 and GeO4 tetrahedra. The occupancies follow a substitution scheme according to Pby(Nd,Sm)w(Cr,Fe)zAl4+2x+2y+3w-zGe2-2x- 2+ 2y-3wO10-x. I.e., charge compensation upon incorporation of Pb and (Nd,Sm)3+ is achieved by substituting Ge4+ by Al3+ according to the oxidation state of divalent Pb2+ and trivalent rare-earth elements. Refractive indices of (Pb,Nd)-mullite were determined by immersion methods as nx=1.697(3), ny=1.708(3), and nz=1.710(3) and 2Vz=122(4)°. In (Pb,Cr)-doped mullite as well as in the series of Cr-doped mullites, Cr was found in the octahedral site only, not being located in the TO4 or T*O4 sites. In Tb-doped Czochralski grown crystals (several cm size) Terbium concentrations were determined through laser ablation with ion coupled plasma mass spectrometry (LA-ICP-MS). X-ray microtomography showed a network of oriented cracks and pores, and LA-ICP-MS revealed that Tb was not homogeneously dispersed, with an average concentration of <20 ppm and single ablation spots containing >2000 ppm of terbium. In a small crystal cut from the whole crystal the site occupancy of Tb on various sites in 2:1 mullite could be investigated by single-crystal structure refinements. The series of 11 Ga-substituted mullites showed that there is a continuous solidsolution between the two endmembers Al4.56Ge1.44O9.72 and Ga4.58Ge1.42O9.71.
