Project
Synthesis and properties in the system Li_xWO₃ with macroscopic phase interfaces and of nano-crystalline material

PD Dr. Claus H. Rüscher
Institute of Mineralogy,
University of Hannover, Germany

The perovskite tungsten bronzes (PTB) Li_xWO₃ have attracted much attention for example because of the electrochromic properties of thin film related material [1]. On the other hand single crystal data concerning structural details and optical, magnetic and electrical properties have been missing. According to this there may have been large discrepancies in the interpretation of their electronic density of states [2]. Therefore, the optimal synthesis conditions of crystalline powders of submicron up to mm sized crystals for the Li_xWO₃ system has been studied. A detailled knowledge concerning the effects of high Li mobility (e.g. surface reactivity under atmospheric conditions) and the complex x dependent phase stability fields was obtained [3,4,5]. Slices of typical crystals (Fig 1) obtained using chemical vapour transport method of Li_xWO3 compositions within the two phase field between the PTB_c (cubic modification) and PTB_t (tetragonal modification) show a size tailored effect depending on bulk composition x [5]. It is observed by LA ICP OES microspot technique that here the darker parts and brighter lamellars are of composition x ≈ 0.38 and x ≈ 0.17, respectively [6,7]. Therefore, these values indicate the room temperature lower and upper stability limits for PTB_c and PTB_t , respectively. The study of their optical properties by using polarized micro reflectivity technique reveals a Drude free carrier type dominated near infrared response for the cubic phase. The excitation spectrum of the tetragonal phase, as well as the lower x orthorhombic and monoclinic modifications indicate the formation of self localized electrons (polarons).

Based on these results it is our aim to explore within this project the tailored properties of the basic tungsten bronze systems. One important part concerns the x dependent effects on nanocrystalline sized forms. For the preparation of such materials the use of milling techniques as well as soft chemical methods will be developed. Possible applications for low temperature oxygen sensoring advanced materials could be straight forward. Another aim concerns the design of macroscopic interfaces, in particular of metallic domains within a matrix of localized electrons and vice versa. Variations as a function of temperature and applied stress will be measured. This problem concerns the effect of superconducting layers as observed along twin boundaries doped with Na in WO₃ [8], too. Our preliminary studies have shown uncommon superconductivity for certain x compositions of Li_xWO3 [9]. Reconfirmation requires, however, a further systematic approach. Finally the optimized conditions arrived for the preparation of Li_xWO₃ crystal will be transferred to the Na_xWO₃ system in order to explore here the x dependent crossover to the lower symmetric phases in more detail.

Fig. 1: Polished crystal individua of Li0.35WO3 (left) and Li0.3WO3 (right) bulk compositions in the optical microscope (reflexion mode)

Literature:
1. Granqvist C. G., 2000, Solar Energy Materials & Solar Cells 60, 201–262: Electrochromic tungsten oxide films: Review of progress 1993-1998.
2. Likalter A. A., 2002, Physica B 315, 252-260:
3. Dey K. R., Thesis, Faculty of Geosciences and Geography at the University of Hannover, Gemany: Synthesis and Characterisation of Alkali Metal Tungsten Bronzes, Li_xWO₃ and M_xM_y’W_1-yO₃ (M = Li, Na, Cs and M’ = Nb, Mo) systems.
4. Dey K. R., Hussain A, Rüscher C. H., 2002, Z. Anorg. Allg. Chem. 628, 2141: Synthesis and Characterisation of non-stoichiometric Li_xNb_yW_1-yO3 (PTB).
5. Dey K. R., Hussain A, Rüscher C. H., 2004, ASCA 2004, book of abstracts: Crystal growth by chemical transport of perovskite tungsten bronzes, Li_xWO₃, and their optical properties.
6. Rüscher C. H., Dey K. R., Horn I., 2005a, Z. Krist suppl. No. 22, in press: The effect of Li-exsolution in Li_xWO₃ crystals in the two phase region of PTB_cubic and PTB_tetragonal.
7. Rüscher C. H., Dey K. R., Horn I., 2005b, in preparation: Crystal growth in the perovskite tungsten bronze (PTB) system Li_xWO₃ by chemical vapour transport: Single crystal optical properties of the PTB_cubic, PTB_tetragonal, PTB_orthorhombic phases.
8. Aird A., Salje E. K. H., 1998, J. Phys.: Condens. Matter 10, L377-L380: Sheet superconductivity in twin walls: experimental evidence of WO_3-x
9. Dey K. R., Gesing Th. M., Hatcher S., Hussain A., Rüscher C. H., Urland W., 2003, Proc. Silver Jubilee Conference Bangladesh Chemical Society, 82-84: Phase relation, optical absorption and superconductivity of lithium tungsten bronzes Li_xWO₃.