In recent years, compounds doped with transition metal ions are more and more focused ... as host materials for the Cr-substitution because of the identical electrical charges of both ions. ... covalent sensor â eight types of aluminate host.
T O M Tailored Optical Materials
The Luminescence Properties of Cr‐doped Aluminates 1,2Beata Malysa, 2Andries Meijerink, and 1Thomas Jüstel 1Münster University of Applied Sciences, Stegerwaldstrasse 2Universiteit Utrecht, Princetonlaan
39, D‐48565 Steinfurt, Germany 1, Utrecht, The Netherlands
Background In recent years, compounds doped with transition metal ions are more and more focused because of a wide range of application both in solid state lasers and non‐linear optic devices. Particularly, Cr3+ is widely used as a dopant in solid state crystals. This cheap activator yields deep red to NIR radiation, bright luminescence and broad optical absorption bands. Materials doped with Cr3+ show a broad band luminescence at room temperature and their spectra are easy to interpret on the basis of the Tanabe‐Sugano diagram. Aluminate materials are selected as host materials for the Cr‐substitution because of the identical electrical charges of both ions. Hence, no charge compensation is required. Moreover, these ions have a similar ionic radius, viz. 67,5 pm for Al3+ and 75,5 pm for Cr3+.
Optical Properties Table 1. The Racah parameters (B, C), nephelauxetic ratio (β), crystal field splitting energy (10Dq) of Cr‐doped of Cr doped aluminates. aluminates.
GdAl3(BO3)4:Cr(1,0%)
1,0
ß = B/B0 0,71 0,70 0,67 0,65 0,64 0,63 0 61 0,61 0,60 (b)
100
ex = 420 nm
= 720 nm
80
Intensity [a.u.]
4
4
T2g-> A2g
0,6
60
0,4
40
0,2
20
0,0 300
400
500
600
700
800
Reflecttance [%]
0,8 em
C/C0 0,768 0,778 0,786 0,803 0,820 0,823 0 834 0,834 0,844 1,0
BaMgAl10O17:Cr(1,0%) 4
4
T2g 80
0,6
60
0,4
40
ex = 420 nm
0,0 300
400
-1
60
0,4
40
20
14600
E( Eg) = 16281 ' - 2392,4 R = 0,98
0,0 500
600
700
Wavelength [nm]
800
0 900
YAB GAB
Sr4Al7
14500
SrAl BAM
14450
LMA
B0 = 995 cm
CaAl
14400
400
0 900
800
SrAl6
14550
2
0,2
2
2
80
Reflectance [%]
4
Eg-> A2g
Eg state energy [cm ]
Intensity [a.u.]
2
0,6
300
700
2
14650
em = 690 nm
600
Dependence of Eg energy on ' parameter
(d)
100
ex = 560 nm 0,8 ,
500
Wavelength [nm]
SrAl2O4:Cr(1,0%)
1,0
20
em = 720 nm
Wavelength [nm]
(c)
100
4
0,8
0,2
0 900
In order to investigate a possibility to use Cr3+ as a covalent sensor – eight types of aluminate host materials were synthesized by conventional solid state reaction (Table 1). Crystal structures were examined by X‐ray diffraction patterns. Then, samples were characterized by measuring of the excitation, emission and reflection spectra. The spectroscopic parameters were derived on the basis off experimental i l data d and d by b using i some equations i [1].
10 Dq [cm‐1] 16667 16750 17730 17699 17065 17825 18050 17857
β’ 1,045 1,046 1,034 1,031 1,039 1,034 1 036 1,036 1,038
A2g-> T1g
Intensity [a.u.]
(a)
B [cm‐1] C [cm‐1] 705 2792 696 2828 668 2858 643 2922 636 2979 623 2995 611 3034 601 3070
1,028
-1
C0 = 3637 cm 1,032
1,036
1,040 2
1,044
-1
1,048
2 1/2
' = [(B/B0) + (C/C0) ]
Fig.1. Reflection (blue line), excitation (red line) and emission spectra (black line) of selected Cr‐doped aluminates: a) GdAl3(BO3)4 (b) BaMgAl10O17 (c) SrAl2O4. Additionally, plot (d) reveals the dependence of 2Eg energy on the β’ parameter.
Reflecttance [%]
Cr‐phosphor GdAl3(BO3)4 YAl3(BO3)4 BaMgAl10O17 LaMgAl11O19 SrAl12O19 Sr4Al14O25 C Al2O4 CaAl SrAl2O4
Among synthesized phosphors both a strong (SrAl2O4:Cr) and intermediate (BaMgAl10O17:Cr, GdAl3(BO3)4) crystal field materials are presented (Fig.1). As can be observed in Table 1, the derived magnitudes of Crystal Field Strength, B Racah parameters and β‐covalency varies with the nature of the host materials. The incorporation of Cr3+ into aluminates causes a decreasing of these parameters as a consequence of the nephelauxetic effect. The role p played y byy covalence effects in the considered systems can be analyzed from another point of view, by using the non‐dimensional quantity β’ investigated by M. Brik [2]. It turns out that the dependence of the energy of 2Eg state on the new β’ parameter exhibits linear correlation. In the herein discussed Cr‐phosphors a monotonic increase of 2Eg transition energies with increasing β’ can be observed. observed This is obviously related to the calculated Racah parameters.
Conclusions Determination of Crystal Field Energy and Racah parameters allows to deduce the kind of emission band and to derive the covalent character of bonds of the activator ion in the respective crystalline environment. It turns out that B and C Racah parameters determine the position of the 2Eg level. level A high covalency causes a strong nephelauxetic effect and 2Eg 4A2g emission is red shifted. shifted On the other hand a high ionicity causes a strong nephelauxetic effect and 2Eg 4A2g emission is blue shifted. Therefore, Cr3+ ion can serve as a good and stable probe, which helps to distinguish between covalent and ionic compounds.
References [1] M.G. Brik, A.M. Srivastava, Electronic Energy Levels of the Mn4+ Ion in the Perovskite, CaZrO3, ECS J. Solid State Sci. Technol., 2(7), 2013, R148‐R152. [2] G. Wang et al., Novel laser gain media based on Cr3+ doped mixed borates RX3(BO3)4, Appl. Phys. Lett. 67(26), 1995, 3906‐3908.
Beata Malysa: beata.malysa@fh‐muenster.de, Thomas Jüstel: tj@fh‐muenster.de
