Thermally Stimulated Luminescence and First-Principle Study of

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Thermally Stimulated Luminescence and First-Principle Study of Defect Configurations in the Perovskite-Type Hydrides LiMH:Eu (M = Sr, Ba) and the Corresponding Deuterides 3

2+

Nathalie Kunkel, Atul D. Sontakke, Stephan Kohaut, Bruno Viana, and Pieter Dorenbos J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b10088 • Publication Date (Web): 02 Dec 2016 Downloaded from http://pubs.acs.org on December 6, 2016

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The Journal of Physical Chemistry

Thermally Stimulated Luminescence and First-Principle Study of Defect Configurations in the Perovskite-Type Hydrides LiMH3:Eu2+ (M = Sr, Ba) and the Corresponding Deuterides. Nathalie Kunkel,∗,†,‡ Atul D. Sontakke,† Stephan Kohaut,¶ Bruno Viana,† and Pieter Dorenbos§ PSL Research University, Chimie ParisTech, CNRS, Institut de Recherche de Chimie Paris, 11 rue Pierre et Marie Curie, 75005 Paris, France, Chair of Inorganic Chemistry with Focus on Novel Materials, Department of Chemistry, Technical University of Munich, Lichtenbergst. 4, 85747 Garching, Germany, Physical and Theoretical Chemistry, Saarland University, Campus B 2.2, 66123 Saarbr¨ ucken, Germany, and Delft University of Technology, Faculty of Applied Sciences, Department of Radiation Science and Technology (FAME-LMR), Mekelweg 15, 2629 JB Delft, Netherlands E-mail: [email protected] Phone: +49 89 289 13109. Fax: +49 89 289 13186

∗

To whom correspondence should be addressed PSL Research University, Chimie ParisTech, CNRS ‡ Technical University of Munich ¶ Saarland University, Germany § Delft University of Technology †

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Abstract Temperature-dependent photoluminescence (PL) as well as thermoluminescence (TL) were studied in the Eu2+ -doped hydrides LiMH3 (M = Sr, Ba) and the corresponding deuterides. Here, thermally stimulated luminescence was observed for the first time in a Eu2+ -doped hydride. The onset temperature of quenching (T95 %) and the quenching temperature (T50 %) were determined from photoluminescence intensities and the energy barrier for thermal quenching was estimated. Then, a scheme with the localization of divalent and trivalent lanthanide 4f and 5d levels for the example LiSrH3 was proposed. Thermal quenching far below room temperature was observed. In deuterides, the quenching temperatures are slightly higher than in hydrides, which can be related to the influence of the different phonon frequencies. In the TL measurements we observed very shallow and intense TL glow peaks in all samples. We also used density functional methods in order to show qualitative trends for the stability of possible defects. The model calculation suggest that energetically favorable defects are the introduction of Eu2+ on a Sr2+ or Ba2+ site, the substitution of barium by strontium and vice versa, lithium and hydrogen vacancies and combinations and possibly clusters thereof. The observed persistent luminescence for several minutes despite of the lack of intense traps at room temperature suggest that the material could in principle be promising for the design of long-lasting phosphors, if traps could be stabilized, which can be released at higher temperature.

Introduction When an insulator or semiconductor had been previously exposed to ionizing radiation for a sufficient time, thermoluminescence may be observed by thermally stimulating the material. 1 The excitation energy is stored in traps originating from lattice defects and information on the depth of the charge carrier trap can be obtained from the temperature of the thermoluminescence peak maximum. 2 Depending on the application of the material, different trap depths are desirable. In case of dosimetry, X-ray and neutron storage phosphors 3–6 deep traps are 2 ACS Paragon Plus Environment

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needed, whereas in case of persistent phosphor applications the traps should be rather shallow. 2 Persistent luminescence (afterglow) may last from seconds to even days and is used for a number of applications, such as traffic or emergency exit signs, displays or bio imaging. 7–12 A common type of persistent phosphors are Eu2+ -doped materials, which are often co-doped with other rare earth ions. 13 A number of Eu2+ -doped halides, 14,15 oxides, 16–19 silicates, 20–22 phosphates, 23 nitrides, and oxynitrides, 24 which show long-lasting luminescence are known. However, thermoluminescence or persistent luminescence in Eu2+ -doped hydrides or mixed hydrides have never been studied, probably because such compounds have only recently been used as host lattices for Eu2+ luminescence. 25,26 In the present work, we studied persistent luminescence and thermoluminescence in Eu2+ -doped LiMH3 (M = Sr, Ba) and the corresponding deuterides. The compounds crystallize in the inverse cubic perovskite structure type 27,28 with octahedrally coordinated lithium ions and cuboctahedrally coordinated strontium or barium ions and recent electron paramagnetic resonance studies confirmed the cubic coordination of the Eu2+ -dopant. 29 Photoluminescence in LiMH3 :Eu2+ (M = Sr, Ba) was recently studied 28 and it was found that LiBaH3 :Eu2+ emits green and LiSrH3 :Eu2+ yellow luminescence. In order to gain more insights into the possible traps we also carried out density functional calculation considering a number of different defect configurations. Even though spectroscopic properties cannot be extracted from these calculations, because DFT does not take excited states into account, not all possible configurations can be considered and the calculated defect density is probably higher than in reality due to computational limitations, density functional calculations can still be useful in showing qualitative trends. For instance, recent density functional calculations on antisite defect configurations in the persistent phosphor ZnGa2 O4 :Cr 30 were in good agreement with experimental results, e.g. from electron paramagnetic resonance spectroscopy. 31



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Methods Experimental details The metal hydrides were prepared via melting of the corresponding metals and subsequent hydrogenation of the alloys according to the procedure described in Refs. 28,32 (Eu ingots, Alfa Aesar, 99.9 %, strontium pieces, Alfa Aesar, 99.8 %, barium rod, Chempur, 99.3 % (rest strontium), lithium wire, Alfa Aesar, 99.8 %). Europium concentrations were about 0.5 % with regard to the strontium or barium amount and values determined by ICP-MS can be found in Ref. 32 Structural characterization was carried out using X-ray and neutron powder diffraction (D20 at the Institute Laue-Langevin) and crystal structures were refined using TOPAS 4.2 (Bruker AXS, Karlsruhe, Germany) 33 and the fundamental parameter approach; 34 details are listed in Refs. 28,32 Obtained lattice parameters at RT are as follows: LiSrH3 :Eu2+ 383.498(3) pm, LiSrD3 :Eu2+ 382.535(3) pm, LiBaH3 :Eu2+ 402.284(3) pm, LiBaD3 :Eu2+ 401.459(3) pm. Temperature-dependent photo- and thermoluminescence measurements were recorded using a high pressure 365 nm mercury lamp as excitation source and an Acton Spectra Pro 2150 Dual Grating Monochromator and a Princeton Instruments PI 100 CCD camera for detection. Except for persistent decay measurements at RT, samples were placed inside a Sumitomo Cryogenics HC-4E closed cycle cryostat connected to a Lakeshore 340 Temperature controller with integrated heating. Samples were cooled down to 10 K and then heated with a rate of 10K/min up to 373 K. For thermoluminescence measurements, samples were irradiated for 10 min at 10 K and spectra were recorded after a wait time of 2 min. For persistent decay measurements, samples were irradiated for 5 min. Spectra were corrected for photomultiplier sensitivity at the respective measurement temperatures. Furthermore, thermoluminescence spectra were corrected for photoluminescence intensity. For the optical measurements, samples were enclosed in sealed silica tubes of 0.5 cm diameter (transparent for visible and UV light down to 180 nm) and attached to the cold finger using high purity


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silver paint. Due to alignment differences an accurate quantitative luminescence intensity comparison for the ampoules is hampered.

Computational details Density-functional calculations of different defect configurations were performed using the Vienna ab initio simulation package (VASP 5.3) 35,36 and projector augmented waves (PAWs). 37 For all metal potentials, semicore states were taken into account (sv), meaning that 10 valence electrons were considered for barium and strontium and 3 for lithium. For Eu2+ a frozen core (Kr4d) potential was used. The exchange-correlation effects were treated within the generalized gradient approximation of Perdew, Burke and Ernzerhof revised for solids, (PBEsol) 38 since several tests with PBE and PBesol showed that the cell volumes and distances obtained with PBEsol were closer to the experimental data. A cut-off of 600 eV for the plane wave expansion of the electronic orbitals was used. Similar to Ref., 39 we tested a plane wave expansion cut-off of 700 eV and hard potentials for hydride (h) and did not find significantly different results. In order to model different defect configurations for the perovskites LiMH3 (M = Sr, Ba) we constructed 3 x 3 x 3 super cells similar to Ref. 39 in the inverse cubic perovskite structure type. Originally, these super cells were built up starting from the unit cell in space group Pm ¯3m via an isomorphic transition with tripling of the axes and the symmetry elements were then removed by introducing different types of defects. This super cell contained 27 Li, 27 strontium or barium and 81 hydrogen atoms. Therefore, replacing for example a lithium atom by another species or a vacancy leads to a doping percentage of 1/27, which is slightly higher than expected experimentally. Due to the high computational cost a lower percentage of dopant or vacancies is hard to realize. In some cases we used the Bilbao Crystallographic Server in order to confirm the correctness of the cells we had set up. 40–42 The electronic properties were evaluated using the tetrahedron method with Blöchl corrections 43 on a Γ-centered grid (6 x 6 x 6 for the perovskite super cells, 15 x 15 x 15 for LiH and 10 x 16 x 8 for SrH2 , EuH2 and BaH2 crystallizing in the PbCl2 structure type). 5 ACS Paragon Plus Environment


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Lattice parameters and positional parameters were allowed to relax and a electronic energy convergence of 0.01 meV and a convergence of the forces to 0.001 meV/pm were obtained.

Results and discussion Spectroscopic measurements In order to study the thermal quenching behavior of Eu2+ in LiMH3 and LiMD3 (M = Sr, Ba) and possible traps, we carried out temperature-dependent photoluminescence (PL) and thermoluminescence (TL) measurements as well as persistent decay measurements at room temperature. The temperature-dependent integrated and normalized PL, the integrated TL glow curves and the corrected TL glow-curves are shown in Figs. 1a-1d and 2. Since the TL glow curves are influenced by the thermal quenching behavior, it is common to correct the TL intensity for the PL intensity. 44,45 From the integrated photoluminescence intensities, the onset temperature of quenching (T95% ) and the quenching temperature (T50% ) were estimated. At these temperatures the photoluminescence intensity becomes 95% and 50%, respectively, of the photoluminescence intensity found at low temperatures. All presently investigated compounds, and especially the strontium compounds show a very well resolved vibrational fine structure at low temperatures, which is a typical sign for intermediate coupling strength of electronic transitions with vibrations. A careful analysis of the vibronic structure 28 yielded Huang-Rhys coupling parameters 46 S between 0.7 and 0.8 for the different compounds and vibrations. As a consequence, thermal quenching according to the configuration coordinate model can be excluded and thermally activated photoionization is the probable thermal quenching mechanism. This is because in the configuration coordinate model the quenching temperature is supposed to be strongly correlated with the Huang-Rhys parameter S, meaning that a low quenching temperature, as it is observed 6 ACS Paragon Plus Environment

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4

1 ,0

4

0 ,4 2 x 1 0

4

1 x 1 0

4

0 ,2

0 5 0

7 5

1 0 0

1 2 5

6 ,0 x 1 0

3

T L c o rre c te d T L

1 5 0

1 7 5

2 0 0

2 2 5

2 5 0

2 7 5

3 0 0

3 2 5

3 5 0

0 ,8

P L n o rm 0 ,6

3

4 ,0 x 1 0

0 ,4

3

2 ,0 x 1 0

0 ,2

0 ,0

0 ,0 2 5

3

0 ,8

0 ,6 3 x 1 0

8 ,0 x 1 0

0 ,0

3 7 5

5 0

1 0 0

1 5 0

T (K )

T L c o rre c te d T L P L n o rm

2 ,0 x 1 0

3

1 ,5 x 1 0

3

1 ,0 x 1 0

3

0 ,4

5 ,0 x 1 0

2

0 ,2

0 ,0

In te g r . T L in te n s ity ( a .u .)

In te g r . P L in te n s ity ( n o r m .)

0 ,8

0 ,6

1 0 0

1 2 5

1 5 0

1 7 5

2 0 0

2 2 5

2 5 0

2 7 5

3 0 0

3 2 5

3 5 0

1 ,0


8 ,0 x 1 0

4

6 ,0 x 1 0

4

4 ,0 x 1 0

4

0 ,4

2 ,0 x 1 0

4

0 ,2

0 ,8

0 ,6

0 ,0

0 ,0 7 5

3 5 0

5

1 ,0 x 1 0

1 ,0

5 0

3 0 0

(b) LiBaD3 :Eu2+ .

3

2 5

2 5 0

T (K )

(a) LiBaH3 :Eu2+ 2 ,5 x 1 0

2 0 0

3 7 5


4 x 1 0

4


4


5 x 1 0

1 ,0



6 x 1 0


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0 ,0 2 5

5 0

7 5

T (K )

1 0 0

1 2 5

1 5 0

1 7 5

2 0 0

2 2 5

2 5 0

2 7 5

3 0 0

3 2 5

3 5 0

3 7 5

T (K )

(c) LiSrH3 :Eu2+ .

(d) LiSrD3 :Eu2+ .

Figure 1: Thermoluminescence (TL), photoluminescence intensity (PL) and thermoluminescence curves corrected by photoluminescence intensity of LiMH3 :Eu2+ and LiMD3 :Eu2+ (M = Sr, Ba). Excitation source: 365 nm mercury lamp. For TL measurements the samples were irradiated for 10 min and data recording was started 2 min after irradiation.



1 ,0

L iB a H In te g r . n o r m . T L in te n s ity

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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L iB a D

0 ,8

3

L iS r H L iS r D

0 ,6

3 3

2 +

:E u 3

2 +

:E u :E u

2 +

:E u

2 +

0 ,4

0 ,2

0 ,0 5 0

1 0 0

1 5 0

2 0 0

2 5 0

3 0 0

3 5 0

T (K )

Figure 2: Comparison of the normalized integrated thermoluminescence (TL) curves of LiMH3 :Eu2+ and LiMD3 :Eu2+ (M = Sr, Ba). Excitation source: 365 nm mercury lamp. for LiSrH3 :Eu2+ , is related to a strong electron-phonon coupling (S 1). However, for LiSrH3 :Eu2+ a small S, a small Stokes shift and a relatively narrow emission band are found and thus, thermally activated cross-over as quenching mechanism is ruled out. In case of thermally activated photoionization the emitting Eu2+ 4f6 5d state is located just below the conduction band and the emission is quenched by thermally activated ionization from the Eu2+ excited state to the conduction band. Furthermore, the phonon energies were determined for hydrides and deuterides from the fine structures and it was shown that the phonon energies are slightly higher for hydrides than deuterides. 28 For instances, the lines ν1−0−0 and ν2−0−0 appear at 99 and 204 cm−1 in Eu2+ -doped LiSrH3 , but at 96 and 198 cm−1 in the corresponding deuterides. This relatively higher value of the phonon energies in hydrides in comparison to deuterides can lead to higher attempt rates for thermal ionization, which is believed to be responsible for the observed slightly faster quenching in hydrides.

The energy barrier ∆E for thermal quenching, which is the difference between the relaxed lowest d-states and the conduction band, can be estimated using the following equation: 47

∆E

T50% [eV ] 680

= 8

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(1)

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The estimated values for ∆ E as well as T95% and T50% for LiMH3 :Eu2+ and LiMD3 :Eu2+ (M = Sr, Ba) are listed in Table 1. For comparison, also the quenching temperatures (T50% ) and the energy barriers from excited state lifetimes (LT) are given. 28 Table 1: Onset temperatures of quenching, quenching temperatures and estimated energy barriers ∆E for thermal quenching via photoionization from PL intensities and , for comparison, quenching temperatures and energy barriers from excited state lifetimes (LT) for LiMH3 :Eu2+ and LiMD3 :Eu2+ (M = Sr, Ba). Compound LiSrH3 :Eu2+ LiSrD3 :Eu2+ LiBaH3 :Eu2+ LiBaD3 :Eu2+

T95% [K] T50% [K] ∆E [eV] (PL) T50% [K] (LT) ∆E [eV] (LT) 120 225 0.33 165 0.24 140 325 0.48 175 0.26 175 345 0.51 210 0.31 185 375 0.55 230 0.34

The quenching temperatures estimated from photoluminescence intensities in the present work are higher than the quenching temperatures determined from lifetime measurements reported in a previous work. 28 A possible explanation can be that the thermally ionized electron may be re-trapped by the luminescence center leading to delayed emission. Such contribution is not seen in the decay curve but does add to the luminescence intensity. Effectively then the luminescence quenching is somewhat delayed in temperature leading to an seemingly higher energy barrier when analyzed with the simple Arrhenius equation. Additionally, heat transfer may be hampered due to the necessity to measure the samples in closed ampoules due to their air-sensitivity, which may also result in temperatures differences between different cryostat set-ups.

The results show that thermal quenching due to photoionization is occurring at slightly higher temperatures in the barium compounds. Consequently, the energy barrier for thermal quenching is higher in the barium compounds than the strontium compounds. Furthermore, the quenching temperatures for deuterides is slightly higher than for hydrides. This is because the phonon frequencies in hydrides are slightly higher than in deuterides, which was 9 ACS Paragon Plus Environment


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determined from the vibronic structure at low temperatures. 28 In fact, the phonon energies are related to the photoionization behavior, which can be understood from the Arrhenius relation in equ. 2 below, describing the thermal quenching behavior in systems, which follow an Arrhenius behavior. Here, the temperature-dependence of the luminescence intensity I(T ) is given by equ. 2: 47

I(T )

=

I(0) 1 +

Γ0 exp( k−∆E ) Γν B T

(2)

with the Boltzmann’s constant kB , the energy barrier for thermal quenching ∆ E, the radiative decay rate of the Eu2+ 5d states Γν and the attempt rate for thermal quenching Γ0 . The attempt rate has a similar magnitude as the maximum phonon frequency 44 and can therefore be expected to be higher for hydrides than deuterides, thereby causing a slightly faster thermal quenching in hydrides than in deuterides. Our data shows a qualitative agreement with this relation, even though unfortunately the quenching behavior does not exactly follow an Arrhenius behavior. This is believed to be due to the complex electron recapturing mechanism of thermally ionized detrapped electrons, which contribute to the PL intensity with temperature.

In Fig. 3 an exemplary estimation of the vacuum referred binding energy (VRBE) of an electron in the 4f and 5d levels of lanthanide ions in LiSrH3 is shown. Here, we assume that the VRBE in the Eu2+ ground state is about -3.7 eV and in the Eu3+ ground state at 6.3 eV lower energy. These are similar to that typical for iodide and sulfide compounds. We observed 2.38 eV for the 4f to 5d energy, which leads to a VRBE of -1.32 eV for the electron in the lowest 5d level. We derived a thermal quenching barrier of 0.24 eV (0.33 eV from PL analysis). 28 From these values and assuming a host exciton creation energy of 5.4 eV with exciton binding energy of 0.2 eV, the VRBE at the bottom


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L iS r H 3

0 -1 -2 -3

V R B E (e V )

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-4 -5 -6 -7 -8 -9 -1 0

L a

C e

P r

N d

P m

S m

E u

G d

T b

D y

H o

E r

T m

0

1

2

3

4

5

6

7

8

9

1 0

1 1

1 2

n u m b e r o f e le c tr o n s n in th e 4 f s h e ll o f L n

Y b

1 3

1 4

3 +

Figure 3: VRBE scheme of divalent and trivalent lanthanide 4f and 5d levels in LiSrH3 . Vacuum referred binding energies are shown. of the conduction band (EC ) of -1.08 eV, the VRBE of the electron in the exciton (EX ) of -1.28 eV and at the top of the valence band (EV ) of -6.68 eV is obtained. The 5.4 eV exciton creation energy is based on the known value of 6.1 eV for exciton creation MgH2 and the known fundamental absorption edge at 5 eV in SrH2 48 From this scheme it can be seen that Eu2+ can act as a hole trap. Furthermore, the VRBE scheme shows that it is possible to stabilize, for instance, divalent ytterbium in this host.

The thermoluminescence glow curves were recorded after excitation into the Eu2+ 5d state using a 365 nm high pressure mercury lamp. Figs. 1a-1d show the TL glow curves after irradiation for 10 min. The glow peak maxima are listed in Table 2 and a comparison of the different thermoluminescence curves is also shown in Fig. 2. Table 2: Glow peak maxima (K) for LiMH3 :Eu2+ and LiMD3 :Eu2+ (M = Sr, Ba). Compound LiSrH3 :Eu2+ LiSrD3 :Eu2+ LiBaH3 :Eu2+ LiBaD3 :Eu2+

T [K] 105, 155 108, 158 104, 169 121, 181

Most of the observed traps show intense emission, but appear at rather low temperatures



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and are therefore shallow. At room temperature, no significant thermoluminescence glow peaks are observed. It is striking that for the deuterides all intense glow peaks appear at slightly higher temperatures than for the hydrides. Similar to the observation of slightly higher quenching temperatures for deuterides, the slightly deeper traps in deuterides seem to be related to the differences in the phonon energies. As shown in equ. 2, the attempt rate Γ0 for thermal quenching relates to the maximum phonon frequency. Since the phonon frequencies in hydrides are slightly higher, the probability for photoionization or transfer of a trapped electron to the conduction band is also higher. In Fig. 4 the persistent decay curves of LiBaH3 :Eu2+ at room temperature and 150 K and in Figs. 5a and 5b some representative persistent luminescence spectra are depicted. Even though the samples do not show significantly intense TL glow peaks at room temperature, they still show persistent luminescence for about 10 min. At lower temperature, the persistent luminescence last at least twice as long. This suggests that much longer and intense persistent luminescence could be reached if deeper traps can be implemented in the materials. Considering the observation of persistent luminescence at room temperature despite of the lack of significant traps and the observation of traps with intense TL emission at lower temperatures, it seems worthwhile trying to develop similar materials where the traps are shifted to higher temperatures. The VRBE diagram shown in Fig. 3 suggest that co-doping with Dy3+ might lead to slightly deeper traps. An electron ionized from Eu2+ to the conduction band could then be transferred and stabilized near the surroundings of Dy3+ . The recombination would then yield a glow peak at higher temperatures then observed at present. Furthermore, it might also be thinkable to slightly influence the energy gap of the host, the crystal field strength or the nephelauxetic effect by partial substitution of the host cations or anions. For example, it was already shown that barium can be partially substituted by strontium and hydride by fluoride. 39,49


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1 0

6

1 0

5

1 0

4

1 0

3

R T 1 5 0 K

1

1 0

1 0 0

1 0 0 0

t (s )

Figure 4: Persistent luminescence decay curves (from integrated intensities) of LiBaH3 :Eu2+ after 5 min irradiation using a 365 nm mercury lamp. 2 ,5 x 1 0

4

2 ,0 x 1 0

4

1 ,5 x 1 0

4

1 ,0 x 1 0

4

5 ,0 x 1 0

4

1 ,5 x 1 0

1 s 5 s 1 0 s 2 0 s 3 0 s 6 0 s

L iB a D 3

:E u

2 +

, R T

p e r s is te n t lu m in e s c e n c e a fte r 5 m in ir r a d ia tio n w ith 3 6 5 H g la m p

4

1 ,0 x 1 0

In te n s ity ( a .u .)

In te n s ity ( a .u .)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


In te g r a te d in te n s ity ( a .u .)

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5 ,0 x 1 0

2 +

1 s

L iB a H

5 s 1 0 2 0 3 0 6 0

p e r s is te n t lu m in e s c e n c e a f te r 5 m in ir r a d ia tio n w ith 3 6 5 H g la m p

s s

3

:E u

, R T

s s

3

3

0 ,0

0 ,0 4 0 0

5 0 0

6 0 0

7 0 0

4 0 0

8 0 0

5 0 0

6 0 0

7 0 0

8 0 0

W a v e le n g th ( n m )


(a) LiBaD3 :Eu2+ .

(b) LiBaH3 :Eu2+ .

Figure 5: Persistent luminescence spectra of LiBaD3 :Eu2+ and LiBaH3 :Eu2+ after 5 min irradiation using a 365 nm mercury lamp at RT.

Theoretical model In order to gain inside into possible defect configurations with favorable energies, we constructed a simplified theoretical model using a super cell, where we introduced different defects. We considered both defects in the non-doped host lattices as well as in the Eu2+ doped compounds. Here, we studied single Schottky defects (combined vacancies in the anion and cation sub lattice), combined Schottky defects, substitutional atoms and combinations



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of those. For instance, we considered lithium and hydrogen vacancies,

× 0 • Li× Li + HH ↔ VLi + VH + LiH

(3)

barium and hydrogen vacancies,

× • 00 Ba× Ba + 2 HH ↔ VBa + 2 VH + BaH2

(4)

combined barium, lithium and hydrogen vacancies,

× × 00 0 Ba× Ba + LiLi + 3 HH ↔ VBa + VLi

+

(5)

3 VH• + BaH2 + LiH

antisite defects on the barium and lithium sites,

× 0 • Ba× Ba + LiLi ↔ LiBa + BaLi

(6)

substitution of lithium or barium by cations as europium or strontium, because europium is used as a dopant and the starting material barium is likely to contain small amounts of strontium and vice versa, as for example

× Ba× Ba + EuH2 ↔ EuBa + BaH2

(7)

• 0 2 Li× Li + EuH2 ↔ EuLi + VLi + 2 LiH

(8)

or


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or the combination of Schottky defects and europium doping

× × × 0 • Ba× Ba + LiLi + HH + EuH2 ↔ EuBa + VLi + VH + BaH2

(9)

F-centers (electron on an anion vacancy) were not considered and no evidence for F-centers was found by EPR. We then studied the structural and energetic effects using VASP. We calculated both the energy of formation (∆ Eform. ) as well as the defect energy (∆ Edefect ). For calculating the energies of formation the stability with regard to the binary hydrides was considered. For example, for a simple Schottky pair as a vacancy on a lithium and a hydrogen site in LiBaH3 , the energy of formation in our model is calculated as ∆ Eform. [LiBaH3 : VLi0 , VH• ] =

(10)

E[LiBaH3 : VLi0 , VH• ] − (E[LiH] + E[BaH2 ])

in our case of the 3 x 3 x 3 super cell this corresponds to

∆ Eform. [Li26 Ba27 H80 ] =

(11)

E[Li26 Ba27 H80 ] − (26 E[LiH] + 27 E[BaH2 ])

in formula units. The defect energy (∆ Edefect ) is calculated as follows: ∆ Edefect [LiBaH3 : VLi0 , VH• ] =

(12)

(E[LiBaH3 : VLi0 , VH• ] + E[LiH]) − (E[LiBaH3 ])

corresponding to

∆ Edefect [Li26 Ba27 H80 ] = (E[Li26 Ba27 H80 ] + E[LiH]) − (E[Li27 Ba27 H81 ])


(13)


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for our example. For each type of defect, we listed only the lowest energy configurations, which correspond to configurations where the differently charged vacancies are close to each other (see Tables 3 and 4). A number of configurations where the vacancies are more far away from each other were also calculated and found to be higher in energy. This suggests that, in general, configurations with differently charged vacancies close to each other are most favorable. Since the calculations were carried out for 0 K, the obtained defect energy is usually positive. The formation of defects becomes important for higher temperatures and we suppose that defects with low positive defect energies obtained by our calculation are likely to be energetically favorable at higher temperatures. Table 3: Energies of formation and defect energies for different defect configurations in LiBaH3 and Eu2+ -doped LiBaH3 . Energies refer to the 3 x 3 x 3 supercell.

LiBaH3 LiBaH3 : SrBa LiBaH3 : EuBa LiBaH3 : V0Li , V•H LiBaH3 : EuBa , V0Li , V•H LiBaH3 : 2 V0Li , 2 V•H LiBaH3 : V00Ba , 2 V•H LiBaH3 : Eu•Li , V0Li LiBaH3 : Li0Ba , Ba•Li LiBaH3 : V0Li , V00Ba , 3 V•H

∆ Eform. [eV] ∆ Edefect [eV] -9.082 -8.812 0.269 -8.694 0.387 -7.718 1.363 -7.366 1.716 -6.370 2.712 -6.341 2.741 -5.548 3.533 -5.420 3.662 -4.841 4.241

The theoretical results suggest that the Eu2+ ion substitutes only the divalent cation on the cuboctahedral site and not the Li+ ion on the octahedral site, which would require a charge compensation. Furthermore, only a substitutional atom with the same charge, such as Eu2+ or Sr2+ on a Ba2+ site, simple lithium and hydrogen vacancy pairs and the combination of both are energetically favorable. Such defects show defect energies comparable to those found in Ref. 30 Other defects, such as barium or strontium vacancies or barium on a lithium site are much less energetically favorable and therefore unlikely to occur. For instance, 16 ACS Paragon Plus Environment

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Table 4: Energies of formation and defect energies for different defect configurations in LiSrH3 and Eu2+ -doped LiSrH3 . Energies refer to the 3 x 3 x 3 super cell.

LiSrH3 LiSrH3 : BaSr LiSrH3 : EuSr LiSrH3 : EuSr , V0Li , V•H LiSrH3 : V0Li , V•H LiSrH3 : V00Sr , 2 V•H LiSrH3 : 2 V0Li , 2 V•H LiSrH3 : Li0Sr , Sr•Li LiSrH3 : V0Li , V00Sr , 3 V•H LiSrH3 : Eu•Li , V0Li

∆ Eform. [eV] -4.608 -4.641 -4.641 -3.227 -3.210 -2.273 -1.815 -0.994 -0.513 0.197

∆ Edefect [eV] -0.032 -0.032 1.381 1.398 2.336 2.792 3.614 4.095 4.801

strontium on a barium site in LiBaH3 can easily be present in small concentrations, because the starting materials already contained small amounts of strontium (e.g. 99.3 % barium, rest strontium). Vacancies on lithium and hydrogen sites may be introduced during heating and hydrogenations of the alloys. Another result of the calculations is that configurations with differently charged vacancies close to each other are more favorable than those where the vacancies are more far away. When different defects are combined, it also seems that configurations with the defects close to each other are most favorable. For example, isolated EuBa and V0Li , V•H in LiBaH3 is slightly less stable than the combination of both in close proximity. Figs. 6a and 6b show a detail from a LiBaH3 super cell without and with simple V0Li and V•H vacancies close to each other. With ∆ Edefect of 1.363 eV, the configuration with V0Li and V•H as close to each other as possible in the model (197.9 pm in LiBaH3 ) is the most stable one, whereas all other configurations with larger distances (> 400 pm) between V0Li and V•H show defect energies of ∼ 1.70 - 1.75 eV. Replacing Ba2+ by Eu2+ leads to a slight change in the inter atomic distances. In non-doped LiBaH3 the calculated inter atomic distance for all Ba-H distances is 279.89 pm. In Eu2+ 17 ACS Paragon Plus Environment


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(b) LiBaH3 : V0Li , V•H .

(a) LiBaH3 .

Figure 6: Detail from the 3 x 3 x 3 super cell of LiBaH3 without (a) and with (b) a V0Li and V•H vacancy. Structural data from VASP calculations. doped LiBaH3 the closest Eu-H calculated inter atomic distance is only 269.16 pm, whereas the Ba-H atomic distance of H next to Eu is slightly larger than in the non-doped compound (288.45 pm). In spite of this change in the inter atomic distances, the surrounding of the europium dopant still remains cuboctahedral. The Ba-H inter atomic distance in the next further coordination sphere is then 278. 81 pm and all Ba-H inter atomic distances of atoms, which are further away from the europium dopant then become similar to the Ba-H inter atomic distance in non-doped LiBaH3 . In Fig. 7 the first cationic coordination shell is shown exemplary.

Figure 7: Illustration of inter atomic distances in the non-doped LiBaH3 host and Eu2+ doped LiBaH3 . For clarity only one cationic shell is shown, however, note that the impurity ion also influences the second cationic shell. In LiSrH3 the calculated inter atomic Sr-H distances are 265.57 pm in the host and in 18 ACS Paragon Plus Environment

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the Eu2+ -doped compound, the Eu-H inter atomic distance is slightly shorter (261.18 pm). The next Sr-H inter atomic distance is then 264.17 pm and in the next outer shell 266.46 pm. Consequently, the model calculation suggests that energetically favorable defects are the introduction of Eu2+ on a Sr2+ or Ba2+ site, the substitution of barium by strontium and vice versa, lithium and hydrogen vacancies and combinations and possibly clusters thereof. Other defects, such as strontium or barium vacancies or barium on a lithium site and vice versa are energetically not favorable. Since the thermoluminescence curves show rather broad glow peaks and at least two maxima, the co-existence of different defects seems likely. Excitation at 365 nm excites an electron from the Eu2+ 4f7 ground state to the 4f6 5d1 excited state. Due to the proximity of the 5d state to the conduction band, the electron can then easily escape and be trapped at the available traps. It is evident that the traps are in the close surroundings of Eu2+ ions, since the persistence luminescence spectrum and the TL glow spectrum are slightly different than the PL spectrum (see Fig. 8). This suggests that the Eu2+ ions contributing to the persistence luminescence by recombination of detrapped electron is in a more distorted site, which can be due to the presence of defects within a few coordination spheres. In order to further clarify the nature of the traps, which are responsible for the thermoluminescence, it may be worthwhile to try controlling the different defect concentrations and study their influence on the TL curves. It might also be worthwhile testing different ratios of lithium and strontium or barium during the synthesis in order to further investigate possible lithium defects. Emission due to defects without the involvement of europium are unlikely, since the non-doped hosts do not emit yellow or green emission.



1 ,2

N o r m a liz e d In te n s ity ( a r b . u n it)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

λP

u m p

P L s P e rs P e rs T L g

: 3 6 5 n m

1 ,0

p e c L u L u lo w

tru m m . s p m . s p s p e c

(3 0 0 e c tru e c tru tru m

K ) m (1 5 0 K ) m (3 0 0 K ) (1 6 0 K )

0 ,8

0 ,6

0 ,4

0 ,2

0 ,0 4 5 0

5 0 0

5 5 0

6 0 0

6 5 0

7 0 0

7 5 0


Figure 8: Comparison of the PL spectrum at 300 K, the persistent luminescence spectra at 150 and 300 K and the TL glow spectrum at 160 K for LiBaH3 :Eu2+ .

Conclusion In the present work, we studied for the first time thermoluminescence in Eu2+ -doped hydrides and also carried out temperature-dependent photoluminescence measurements. We estimated the energy barriers for thermally activated photoionization and propose a scheme with the localization of divalent and trivalent lanthanide 4f and 5d levels for the example LiSrH3 and suggest that it should be possible to stabilize Yb2+ in the host. The determination of the quenching temperatures also shows that deuterides quench at slightly higher temperatures than hydrides. Since the photoionization rate is not only determined by the energy gap between Eu2+ 5 d levels and the conduction band but also by the phonon frequencies, we relate this observation to the higher phonon energies in hydrides. In the TL measurements we observed very shallow and intense TL glow peaks in all samples. In order to clarify the nature of these glow peaks we also carried out theoretical calculations and modeled different defect configurations. The results suggest that the thermoluminescence might be caused by an electron trapped by defects. Even though the samples do not show significantly intense TL glow peaks at room temperature, the samples showed persistent luminescence, which lasted approximately 10 min. This suggests that these materials are in principle promising materials for persistent luminescence, if traps at higher temperatures can


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be stabilized. Thus, it seems worthwhile to develop similar materials with slightly deeper traps by, for example, co-doping with Dy3+ or Tm3+ .

Acknowledgement Financial support by the Deutsche Forschungsgemeinschaft (DFG, project no. KU 3427/11) is gratefully acknowledged. N. K. would also like to thank H. Kohlmann for providing synthesis facilities, M. Springborg and N. Louis and C3 M Saar for providing computational resources and the Fonds der Chemischen Industrie for a Liebig fellowship. Furthermore the authors would like to thank Patrick Aschehoug and Jean-Fran¸cois Engrand for technical support.

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