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

This is the scheelite structure CaWO4

Ca

W

Many scheelites based structures are luminescent materials.

Many scheelite based compounds have

cation-vacancy order.

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.

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

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.

We found that these materials are incommensurately modulated.

...as a consequence these materials need to be described in more than 3D, using superspace.

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

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

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)

The model shows vacancies ordered in 3+1D.

Blue = cation Orange=2 neighbouring vacancies Yellow = 3 neighbouring vacancies

The model is supported by the HRSTEM images we made.

Image calculated using the model.

Experimental image.

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.

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.

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.

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.

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.

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.

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).