C.Veitsch et al. / Materials Research Bulletin 43 (2008) 168–175
175
added either as BaO to the parent compound BaMgF4 prepared from the melt or by heating melt grown pure BaMgF4
under 50 Torr of saturated water vapour pressure to 850 8C for 3 days. The resulting samples show two broad
luminescence bands centered at 22000 cmÀ1 (454 nm) and 17300 cmÀ1 (580 nm) with a steadily increasing excitation
band with a maximum at 49000 cmÀ1 (205 nm). Similar emission bands can be seen in the SrMgF4 samples annealed
at 400 8C for 30 min at around 490 nm and 580 nm.
4. Conclusions
It is possible to prepare strontium magnesium fluoride via a solution chemical route. Since the precipitated powder
is very fine and/or amorphous it has to be annealed at a minimum temperature of 350 8C to achieve X-Ray powder
patterns with discrete reflections. Annealing at 450 8C yields single phase crystalline SrMgF4.
The superstructure-reflexes reported by Ishizawa [5] are visible as well. Fig. 12 shows, that the measured data
corresponds to a powder pattern derived from the single crystal data published in [5] (the superstructure-reflexes are
especially magnified). The annealing time and temperature can be used to adjust the mean particle size in this system.
TEM-micrographs confirm the mean particle size calculated with X-ray data refinement. One can also observe a
growth of the mean particle size along with higher annealing temperatures. This corresponds quite well with the
crystallite growth calculated from X-ray data. Further HR-TEM analysis is needed to determine the exact constitution
of the grains and to gain information about the size of the crystalline regions which are responsible for X-ray
diffraction.
When heating the precipitated powder to higher temperatures it starts to decompose at 500 8C. Compared to
previous results [4] this temperature is rather low. Therefore we conclude, that the heating speed and the grainsize
distribution have an impact on the compounds melting and decomposition properties. An intensive broadband
luminescence with afterglow, was observed after annealing the samples. This luminescence is probably caused by
defects, since the pure parent compound BaMgF4 is fully transparent from 180 nm (7 eV) to 8000 nm [10], and
probably even further in the VUV region (pure SrF2 and MgF2 are transparent even above 8 eV). It does not appear
likely that the small grain size can shift the absorption edge by as much as at least 2 eV to generate an absorption close
to 250 nm (ca. 5 eV) associated with the observed emission. The emissions maxima shift with different annealing
temperatures and times. Therefore it is concluded, that these factors must have some kind of impact on the defects
which are causing the fluorescence and phosphorescence. The most intense luminescence can be observed, if the
samples are treated for 30 min at 350 8C.
Acknowledgements
This work was supported by the Swiss National Science Foundation; TEM photographs were taken at USTEM—
University of Technology Vienna.
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