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Materialien/Werkstoffe Poster: Do., 13:00–15:30 D-P395
Disorder-induced diffuse scattering in mixed-cation scheelite-type compounds
Carsten Paulmann 1 , Thomas Malcherek 1 , Rigo Peters 2 , Klaus Petermann 2 ,
Ulrich Bismayer 1
1 Mineralogisch-Petrographisches Institut, Universität Hamburg, Grindelallee 48, D-
20146 Hamburg, Germany – 2 Institut für Laserphysik, Universität Hamburg, Luruper
Chaussee 149, D-22761 Hamburg, Germany
During the last decade, mixed-cation double tungstate and molybdate compounds
gained special interest due to their optical properties. Especially scheelite-type compounds
are favored as laser disc hosts due their uniaxial character and tunability. The
average structure of these compounds is isotypical to scheelite (CaWO4) with space
group I41/a (Z=4) and lattice constants of a = 0.5282 and c = 1.1466 nm. Mixedcation
occupation may also occur on the tetrahedrally coordinated W-position (4a) but
the most interesting compounds are synthesised with mixed occupation at the strongly
distorted dodecahedral Ca-positon (4b).
With prominent absorption bands in the range of 940 - 980 nm, Yb 3+ -doped NaGd(WO4)2
offers optimum spectroscopic properties for diode pumping and currrent research concentrates
on Yb 3+ -activated tungstate crystals [1]. A statistical distribution of Na +
and Gd 3+ was assumed earlier but recent X-ray powder investigations claimed evidence
for weak superstructure peaks (doubling of a- and c-axis) pointing to local order at
Wyckoff position 4b [2].
Single-crystal studies were performed at beamline F1 (Hasylab/DESY) using a Marresearch
CCD-detector and a wavelength of 0.050 nm to reduce absorption. Bragg
data collections were done with small distances (65 mm) and scan widths of 1 ◦ per
frame whereas diffuse scattering studies were performed with thin-slicing (0.2 ◦ ) and a
distance of 150 mm. The latter data were rescaled for the primary beam decay and
corrected for background and sample-external scattering (beamstop). Very weak intensities
for 00l-reflections with l=2n and hk0 with h,k=2n+1 give evidence for an at
least local symmetry I-4 instead of I41/a favored by incorporation of relatively small
Yb 3+ -ions which may lead to higher local ordering. Furthermore, reciprocal space
reconstructions show weak diffuse intensities pointing out an incommensurate modulation
with a vector parallel * and a modulation wavelength of ca. 0.95a. No
evidence was found for a modulation along c, in contrast to [2].
[1] P. Klopp et al., J. Appl. Phys. B 74 (2002) 185
[2] E. Zharikov et al., Inorg. Mater. 39 (2003) 151