Ramanspektren anionen‐ und kationensubstituierter ternärer Oxide des Molybdäns und Wolframs mit Pyrochlor‐ und hexagonaler Wolframbronzen‐Struktur Einkristallstrukturuntersuchungen an CsZnMoO₃F₃, Rb_0,3Nb_0,3W_0,7O₃ und Rb_0,3Ga_0,1W_0,9O₃

Abstract

Die Ramanspektren der gemischten Oxide Ay(BxW1−x)O3 (A = K, Rb, Cs; B = Mg, Al, Ga, Ti, V, Nb, Ta) sowie der Oxidfluoride Cs(BMO3F3) (M = Mo, W; B = Ni, Cu, Zn) und AM2O5F (A = Rb, Cs; M = Nb, Ta) mit Defektpyrochlor‐ bzw. hexagonaler Wolframbronzen(HTB)‐Struktur wurden gemessen. Alle Spektren enthalten Banden mit Wellenzahlen größer als 900 cm−1, d. h. im Bereich terminaler MO‐Valenzschwingungen (M = MoVI, WVI bzw. NbV, TaV). Sie werden den Valenzschwingungen von MOB‐Gruppen zugeordnet, die durch die partielle Substitution von M durch B entstehen. Hierin ist die MO‐Bindung stark kovalent und besitzt Mehrfachbindungscharakter, die BO‐Bindung ionisch und vergleichsweise schwach. Weitere Banden werden den MOM‐Bindungen zugeordnet. Diese sind in Verbindungen mit HTB‐Struktur zum Teil alternierend kurz und lang. Die Zuordnung der Spektren wird durch Strukturuntersuchungen an Einkristallen von CsZnMoO3F3 (Raumgruppe Fd3m, a = 1 039,0(2) pm, Defektpyrochlor‐Struktur), Rb0,3Nb0,3W0,7O3 (Raumgruppe Cmm2, a = 2 196(2), b = 1 266(1), c = 388,9(4) pm, Überstruktur einer HTB‐Struktur) und Rb0,3Ga0,1W0,9O3 (Raumgruppe P6mm, a = 726,9(5), c = 376,0(2) pm. HTB‐Struktur) bestätigt.

Vibrational spectra of KNbW2O9 hexagonal tungsten bronze

“…Our present explanation for the discussed bands is different from that presented on the basis of studies performed for Rb 0.75 Ta 0.75 W 2.25 O 9 and Rb 0.81 Ga 0.27 W 2.73 O 9 ,13 crystallizing with structures similar to KNbW 2 O 9 . The terminal character of some oxygen atoms was explained as a result of substitution of W atoms by Ta (or Ga) atoms.…”

“…This explanation is reasonable for the above-mentioned Ga compound. However, in the case of the Ta compound and KNbW 2 O 9 the explanation proposed by Mattes et al 13 does not seem appropriate. In these cases the valence of substituting atoms is high (C5) and therefore relatively small shortening is expected for some of the W-O bonds (the valence 7/6 D 1.166 is much smaller than the valence for a really terminal -W O bond, which is 2).…”

Vibrational properties of ferroelectric hexagonal tungsten bronzes ABxW3−xO9, where A = K, Rb, Cs and B = Nb, Ta, Zr, Cr

“…Only in a few cases have ambient-temperature x-ray diffraction and temperature-dependent dielectric studies of single crystals been performed. 1,3,7,8 Vibrational spectroscopy is a useful tool in studies of such materials since it may provide important information about lattice distortion, disorder, local symmetry of atoms, defects, valences of atoms, etc. For example, vibrational techniques proved to be very useful in understanding the properties of photochromic WO 3 oxide.…”

Crystal structure of rubidium niobium tungsten bronzes, RbxNbyW1-yO3(x~0.3;y=0.13,0.19)

“…The present structure investigations show that tungsten can be substituted by niobium according to Rb^Nb^Wi-yCb here with y=0.13 (jc=0.31) and y = 0.19 (jc = 0.30) which is in reasonable agreement with the preceding nominal composition. Mattes et al [3] reported for a HTB-type structure with a e-lattice parameter of 3.862 A. However, the present structure refinement leads to a double c-lattice parameter due to the shift of W and 0(2) as given in Table 2 and Table 4.…”