In:
Zeitschrift für Kristallographie - Crystalline Materials, Walter de Gruyter GmbH, Vol. 238, No. 5-6 ( 2023-05-25), p. 151-166
Abstract:
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-MoO 3 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 (Al 4.50 Ge 1.50 O 9.75 ; a = 7.6559(4) Å, b = 7.7763(4) Å, c = 2.9233(2) Å, V = 174.04(2) Å 3 ); (Pb,Fe)-doped mullite (Pb 0.02 Fe 0.68 Al 3.95 Ge 1.37 O 9.70 ; a = 7.7125(7) Å, b = 7.8527(7) Å, c = 2.9528(2) Å, V = 178.83(3) Å 3 ); (Pb,Cr)-doped mullite (Pb 0.01 Cr 0.63 Al 3.90 Ge 1.47 O 9.75 ; a = 7.6917(6) Å, b = 7.8168(6) Å, c = 2.9522(2) Å, V = 177.50(2) Å 3 ); (Pb,Nd)-doped mullite (Pb 0.06 Nd 0.02 Al 4.82 Ge 1.18 O 9.69 ; a = 7.6585(7) Å, b = 7.7666(7) Å, c = 2.9164(3) Å, V = 173.47(3) Å 3 ); (Pb,Sm)-doped mullite (Pb 0.06 Sm 0.02 Al 4.55 Ge 1.45 O 9.79 ; a = 7.6563(3) Å, b = 7.7873(3) Å, c = 2.9236(1) Å, V = 174.31(1) Å 3 ); 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 AlO 4 and GeO 4 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 Al 4+2 v − z Ge 2−2 v O 10− v + q +3/2 r . 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 Al 4+2 v − z Ge 2−2 v O 10− v and then “stuffed” with q Pb 2+ + r (Nd 3+ ,Sm 3+ ) formula units where concurrently the number of available O3-vacancies is reduced by q + 3/2 r units of extra oxygen. Thus, charge compensation upon incorporation of Pb 2+ and (Nd,Sm) 3+ is achieved by adding the amount of oxygen corresponding to the oxidation state of divalent Pb 2+ 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 Fe 3+ and Cr 3+ are directly substituting Al 3+ . 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 2 V 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.
Type of Medium:
Online Resource
ISSN:
2194-4946
,
2196-7105
DOI:
10.1515/zkri-2022-0026
Language:
English
Publisher:
Walter de Gruyter GmbH
Publication Date:
2023
SSG:
13
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