This document summarizes a research paper that studied the incommensurate modulation and luminescence properties of CaGd2(1-x)Eu2x(MoO4)4(1-y)(WO4)4y phosphors. The researchers found that these materials exhibit incommensurate modulation of the cation ordering due to vacancies in the scheelite structure, which requires description in superspace. Replacing Mo6+ with W6+ switched the modulation from 3+2D to 3+1D, despite their similar sizes. Variations in Eu content changed luminescence intensity but not the modulation periodicity. The results contradict prior reports of simple ordered structures.

1. Incommensurate Modulation and
Luminescence in the
CaGd2(1-x)Eu2x(MoO4)4(1-y)(WO4)4y(0≤x≤1, 0≤y≤1)
Red Phosphors
A short summary of the paper
Chemistry of Materials, 25, 21 (2013) 4387-4395

2. This is the scheelite structure CaWO4
Ca
W
Many scheelites based structures are luminescent
materials.

3. Many scheelite based
compounds have
cation-vacancy order.

4. In the frame of our FWO project G039211N
we need to figure out what is the relation
between the cation order and the
luminescence and use it to improve the
luminescent properties.

5. So, we performed systematic cation
substitutions to control the order and to
find out the relation between the cation
order and the luminescence.
First system to investigate:
CaGd2(1-x)Eu2x(MoO4)4(1-y)(WO4)4y(0≤x≤1,
0≤y≤1)
=a model system where the incommensurate modulation can be monitored as a
function of cation size while the amount of the cation vacancies and the average
cation charge remain constant upon the isovalent cation replacement

6. We found that these materials are
incommensurately modulated.
This means that the ratio
of the periodicity of the
ordering in these
compounds to the
periodicity of the
underlying scheelite is
not a rational number.

7. We found that these materials are
incommensurately modulated.
...as a consequence these
materials need to be
described in more than 3D,
using superspace.

8. Also CaEu2(MoO4)4, CaGd2(MoO4)4
and CaEu2-xGdx(MoO4)4
are incommensurately modulated! Although in
literature* they were reported as normal 3D.
* Guo, C.; Yang, H. K.; Jeong, J.-H. J. Lumin. 2010, 130, 1390

9. All molybdates are 3+2D.
Superspace group
I41/a(a,b,0)00(-b,a,0)00
All tungstates are 3+1D.
Superspace group
I2/b(αβ0)00

10. Since a solution from XRD was
unsuccessful, we determined and refined
the 3+1D structure from precession
electron diffraction data.
PXRD patterns of
CaGd0.5Eu1.5(MoO4)4(1-y)(WO4)4y
solid solutions: y = 0 (1), 0.25 (2),
0.5 (3), 0.75 (4), 1 (5)

11. The model shows vacancies ordered in 3+1D.
Blue = cation
Orange=2 neighbouring vacancies
Yellow = 3 neighbouring vacancies

12. The model is supported by the
HRSTEM images we made.
Image
calculated using
the model.
Experimental
image.

13. Unfortunately, on changing the cation ratio
in CaGd2(1-x)Eu2x(MoO4)4(1-y)(WO4)4y(0≤x≤1,
0≤y≤1), the periodicity of the ordering does not
change, only the switch from 3+1D to 3+2D
occurs.

14. Also no change in the
luminescent properties occurs,
except for an increase in the
luminescence with the Eu
concentration.
Excitation (λem = 611 nm) (a) and
emission spectrum (λexc = 300 nm) (b) of
CaGd0.5Eu1.5(MoO4)4(1-y)(WO4)4y for
y= 0;0.25;0.5;0.75;1
Dependence of the
5D0 – 7F2 intensity
and the 5D1/5D0
emission ratio on the
Eu concentration in
CaGd2(1-x)Eu2x(BO4)4
after excitation at
300 nm. The circles
refer to B = Mo, the
triangles to B = W.

15. We found:
In contrast to the structures reported in literature
for CaEu2-xGdx(MoO4)4,these compounds as well
as their W-based analogues are not disordered
scheelites, but incommensurately modulated
structures due to ordering of the A cations and
vacancies.

16. We found:
Replacement of the smaller
Gd3+ (r = 1.053Å, CN = 8)
by the larger
Eu3+ (r = 1.066Å, CN = 8)
at the A sublattice does not affect the nature of
the modulation.

17. We found:
Replacement of Mo6+ by W6+ switches the modulation from
(3+2)D to (3+1)D regime.
Astonishing, if one takes into account that the Mo6+ and W6+
cations have almost identical ionic radii (r(Mo6+) = 0.41Å,
r(W6+) = 0.42Å). Thus, the charge and/or size difference
cannot be a driving force for this switching.

18. Therefore, a follow-up investigation is
ongoing with synchrotron X-ray diffraction.
Also, similar investigations will be
performed on a system with a varying
amount of vacancies.
Coming out soon.

19. You can find the details in the publication:
"Incommensurate Modulation and Luminescence in
the CaGd2(1-x)Eu2x(MoO4)(4(1-y))(WO4)(4y) (0 <= x <= 1, 0
<= y <= 1) Red Phosphors"
in Chemistry of Materials, 25, 21 (2013) 4387-4395
by Vladimir Morozov, Anne Bertha, Katrien Meert, Senne Van Rompaey, Dmitry Batuk,
Gerardo T. Martinez, Sandra Van Aert, Philippe F. Smet, Maria Raskina, Dirk Poelman, Artem
Abakumov, Joke Hadermann
This research was supported by FWO (projects
G039211N, G006410), Flanders Research Foundation and
the Russian Foundation for Basic Research (Grants 08-03-
00593, 11-03-01164, and 12- 03-00124).