Independent q→ and 2q→ Distortions in the Incommensurately Modulated Low-Temperature Structure of NiTa2Se7

✍ Scribed by Jens Lüdecke; Martin Schneider; Sander van Smaalen

• Publisher: Elsevier Science
• Year: 2000
• Tongue: English
• Weight: 137 KB
• Volume: 153
• Category: Article
• ISSN: 0022-4596
• DOI: 10.1006/jssc.2000.8766

✦ Synopsis

The modulated low-temperature structure of NiTa 1.98 Nb 0.02 Se 7 has been determined by X-ray di4raction using synchrotron radiation. The superspace group of the modulated structure at 15 K is C2/m(0, q b , 0) s0 with q b ‫؍‬ 0.483(2). The lattice parameters were determined to a ‫؍‬ 13.807(6) A s , b ‫؍‬ 3.483(1) A s , c ‫؍‬ 18.564(11) A s , and ‫؍‬ 108.95(4)3 with Z ‫؍‬ 4. Satellite re6ections up to second order were measured. Re5nement with 5rstand second-order harmonic components of the modulation functions converged to R ‫؍‬ 0.059. The 5rst-order harmonic was found predominantly on the double chain of Ni and Se2 atoms. Shifts were found parallel to the al and cl axes, in good agreement with earlier investigations. The major part of the second harmonic was found on the Ta2 atoms, with amplitudes of secondary importance on the surrounding Se atoms. Shifts of the Ta2 atoms were found parallel to bo , thus forming a longitudinal wave. It is argued that the modulations according to the 5rst-order and the second-order harmonics should be considered as independent CDW:s.