One Ion, Many Facets: Efficient, Structurally and Thermally Sensitive Luminescence of Eu2+ in Binary and Ternary Strontium Borohydride Chlorides

Article abstract of DOI:10.1021/acs.chemmater.9b03048

The Eu²⁺-doped mixed alkaline metal strontium borohydride chlorides ASr(BH₄)_3-xCl_x (A = K, Rb, Cs) and Eu²⁺-doped strontium borohydride chloride Sr(BH₄)_2-xCl_x have been prepared by mechanochemical synthesis. Intense blue photoluminescence for Sr(BH₄)_2-xCl_x (λ_em= 457 nm) and cyan photoluminescence for the perovskite-type mixed alkaline metal strontium borohydride chlorides ASr(BH₄)_3-xCl_x (A = K, Rb, Cs) (λ_em = 490 nm) is already observable after short milling times. Temperature dependent luminescence measurements reveal an appreciable blue shift with increasing temperature for all ASr(BH₄)_3-xCl_x (A = K, Rb, Cs) until 500 K. This extremely large shift, caused by structural relaxation, as well as the vibrationally induced emission band broadening can serve as a sensitive response signal for temperature sensing, and this unique behavior has, to the best of our knowledge, not been reported in any Eu²⁺ doped phosphor so far. Additionally, bright luminescence, high quantum efficiencies, and very low thermal quenching of these Eu²⁺-doped borohydrides show that such host materials could serve in solid state lighting applications.

Full text of DOI:10.1021/acs.chemmater.9b03048

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Article
One ion, many facets: Efficient, structurally and thermally sensitive
luminescence of Eu2+ in binary and ternary strontium borohydride chlorides
Thomas Wylezich, Atul D. Sontakke, Victor Castaing, Markus
Suta, Bruno Viana, Andries Meijerink, and Nathalie Kunkel
Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.9b03048 • Publication Date (Web): 07 Oct 2019
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One ion, many facets: Efficient, structurally and thermally sensitive
luminescence of Eu2+ in binary and ternary strontium borohydride
chlorides
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Thomas Wylezich,a Atul D. Sontakke,b Victor Castaing,c Markus Suta,b Bruno Viana,c Andries
Meijerink,b and Nathalie Kunkel *,a, d
a. Chair for Inorganic Chemistry with Focus on Novel Materials, Department of Chemistry, Technical University of
Munich, Lichtenbergstr. 4, 85747 Garching, Germany.
b. Condensed Matter and Interfaces, Debye Institute for Nanomaterials Science, Department of Chemistry, Utrecht
University, Princetonplein 1, 3584 CC Utrecht, Netherlands.
c. Chimie ParisTech, PSL University, CNRS, Institut de Recherche de Chimie Paris, 11 rue Pierre et Marie Curie, 75005
Paris, France.
d. Institut für Anorganische Chemie, Georg-August-Universität Göttingen, Tammannstrasse 4, 37077 Göttingen,
Germany.
ABSTRACT: The Eu2+ - doped mixed alkaline metal strontium borohydride chlorides ASr(BH4)3-xClx (A = K, Rb, Cs) as well as
Eu2+ - doped strontium borohydride chloride Sr(BH4)2-xClx have been prepared by mechanochemical synthesis. Intense blue
photoluminescence for Sr(BH4)2-xClx (λem= 457 nm) and cyan photoluminescence for the perovskite-type mixed alkaline metal
strontium borohydride chlorides ASr(BH4)3-xClx (A = K, Rb, Cs) (λem = 490 nm) is already observable after short milling times.
Temperature dependent luminescence measurements reveal an appreciable blue shift with increasing temperature for all
ASr(BH4)3-xClx (A = K, Rb, Cs) until 500 K. This extremely large shift, caused by structural relaxation, as well as the vibrationally
induced emission band broadening can serve as a sensitive response signal for temperature sensing and this unique
behaviour has, to the best of our knowledge, not been reported in any Eu2+ doped phosphor so far. Additionally, bright
luminescence, high quantum efficiencies and very low thermal quenching of these Eu2+ - doped borohydrides show that such
host materials could serve for solid state lighting applications.
Introduction
vibrational fine structures of the Eu2+-based emission bands
is observable in these compounds and in the binary
hydrides MH2 (M = Ca, Sr, Ba) it was shown that the high
polarizability of the hydride anion leads to the hitherto
largest redshifts of the energies of the lowest excited 4f65d1
electronic levels among all known inorganic host
compounds.23 Besides ionic hydrides, also Eu2+-doped
borohydrides and mixed-anionic hydrides have been
studied lately.
Applications of rare earth-doped inorganic materials are
manifold, ranging from solid state lighting to scintillators,
lasers, biomedical applications, multiphoton processes,
temperature sensing, quantum information technology and
many more.1-11 Especially the 4fn-15d-4fn transitions of Eu2+
(n = 7) and Ce3+ (n = 1) are of great importance for
phosphors used in phosphor-converted light-emitting
diodes (pc-LEDs).1, 4, 12, 13 They usually show high emission
intensities and also a strong dependence of the emission
energy on the host material. While Eu2+ and Ce3+ emission
in a number of host materials ranging from halides to
nitrides has been studied for some time,14-16 hydride,
borohydride and mixed anionic hydrides have only been
considered as host lattices recently.17
A general finding is that in borohydrides the emission
energies of the Eu2+-based 4f65d1-4f7 emission so far are
rather comparable with those observed typically in halides.
For instance, broad blue luminescence and a quantum yield
24
of 75% was reported for [Eu(BH4)2(THF)2] and different
shades of blue were also reported for an Eu2+-related
emission in the perovskite-analogue borohydrides
For instance, the hydride perovskites LiMH3 and LiMD3
(M = Sr, Ba) doped with Eu2+ have been thoroughly studied
using luminescence, thermoluminescence, electron
paramagnetic resonance and electron nuclear double
resonance spectroscopy.18-22 In particular, well-resolved
26
RbEu(BH4)3, CsEu(BH4)3 or CsCa(BH4)3:Eu2+.25,
On the
other hand, recently, particular interest has grown on
mixed anionic hydrides. In those compounds, the Eu2+-
based emission energies strongly depend on the type and
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number of anions other than hydride. Examples for various
per “temperature step” with an imaging plate distance of
200 mm resulting in a diffraction range of 2θ = 4° - 35°.
classes of compounds are hydride fluorides,27-29 hydride
chlorides,30, 31 hydride oxide chlorides,32 and even silicate
hydrides.33 In oxide hydrides, only luminescence of trivalent
terbium and europium ions has been investigated so far.34
Luminescence spectroscopy. Due to moisture and air
sensitivity, all samples were enclosed in silica ampoules (5
mm diameter).
In this work, we present the luminescence properties of
the Eu2+-doped strontium borohydride chloride
Sr(BH4)2-xClx:Eu2+ and the alkaline metal strontium
borohydride chlorides ASr(BH4)3-xClx:Eu2+ (A = K, Rb, Cs) for
the first time. All discussed ternary compounds show
efficient cyan emission at room temperature as well as a
strongly temperature-dependent shift of the emission
energies. This unusually strong response of the
luminescence properties of Eu2+ with regard to temperature
changes may be interesting for potential applications in the
field of thermometry.
Photoluminescence excitation spectra at room
temperature were recorded using a Xenon plasma lamp
(Energetic EQ99X) with
a
Jobin-Yvon HR 250
9
monochromator (1200 grooves/mm) for excitation and an
Acton Spectra Pro 2150 Dual Grating Monochromator
together with a Pixis 100 CCD camera for detection.
Photoluminescence excitation and emission spectra at 4.2 K
were acquired with a Xenon arc lamp (450 W), an
Edinburgh FLS920 spectrofluorometer with a single 0.25 m
monochromator combined with appropriate band pass and
long pass filters. Emission was detected with a Hamamatsu
R928 photomultiplier tube (PMT). The samples were cooled
with an Oxford Instruments liquid He flow cryostat with
external temperature control unit. All emission spectra
were corrected for the wavelength-dependent PMT
sensitivity and monochromator light output while
excitation spectra were corrected for the reference signal of
the Xe lamp.
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Experimental section
Synthesis. Due to moisture and air sensitivity all
compounds were handled in an argon-filled glove box. The
mixed metal borohydrides ASr(BH4)3-xClx:Eu2+ (A = K, Rb,
Cs) were prepared via mechanochemical reaction of LiBH4
(Alfa Aesar, 95%), SrCl2 (Alfa Aesar, 99.995%), the
corresponding alkaline borohydrides (KBH4, Alfa Aesar
98%; RbBH4 and CsBH4 prepared from Rb or Cs metal and
NaBH4 in ethanol according to the procedure described in
Ref 35) as well as EuH2 (2 mol% doping; synthesized via
hydrogenation of europium metal in an autoclave made
from hydrogen-resistant Böhler L718 alloy) in a Fritsch
Pulverisette Premium Line 7 in a ZrO2 bowl with an
overpressure valve. Sr(BH4)2-xClx:Eu2+ was prepared via
mechanochemical reaction of stoichiometric amounts of
SrCl2, LiBH4 and 2 mol% EuH2. 10 ZrO2 balls with a diameter
of 10 mm are used together with approximately 0.3 g of
reactants. Caution! Overpressure may build up during
milling via decomposition of the borohydrides. Reactants
were milled during 19 cycles with 2 minutes of milling at
600 rpm followed by 3 minutes pauses to prevent heating
of the samples. To increase crystallinity, the samples were
annealed at 200 °C in fused silica ampoules for 2 days.
Temperature-dependent photoluminescence emission
measurements were carried out from 14 K to 490 K in a
Sumitomo Cryogenics HC-4E closed-cycle cryostat with a
Lakeshore 340 temperature controller. A xenon lamp
(Energetic EQ99X) was used as excitation source and
spectra were recorded using an Acton Spectra Pro 2150
Dual Grating Monochromator (300 grooves/mm, 550 nm
center wavelength) and a Princeton Instruments PI 100
CCD camera. The samples were attached to a cold finger
with aid of silver paint and copper tape.
Quantum yields were measured on
a Hamamatsu
Quantaurus-QY Absolute PL quantum yield spectrometer
equipped with a 150 W xenon light source, a Czerny-Turner
type spectroscope and a spectralon coated Ulbricht sphere.
Ampoules containing borohydrides were opened prior to
the measurements and powders were transferred
immediately to the integrating sphere for measurements.
Powder x-ray diffraction (XRD). For powder XRD
analysis, a STOE STADI P diffractometer (Cu-Kα1 radiation,
λ = 1.54051 Å (determined from a silicon standard)), Ge-
(111)-monochromator) including a DECTRIS Mythen 1 K
detector was used. Samples were enclosed in 0.3 mm glass
capillaries. Data was collected in the range of 2θ= 10° - 100°
within 2 hours. Structural analysis was performed with the
Thermal analysis (DSC). The differential scanning
calorimetry (DSC) measurements were carried out using a
Netzsch DSC 200 F3 Maja calorimeter in the temperature
range from 295 K to 673 K and nitrogen atmosphere with
heating and cooling rates of 10 K/min. Approximately 5 mg
of the samples were loaded into aluminium crucibles and
sealed with a press (Netzsch).
36
program package Topas V5
and the fundamental
parameter approach.37 Cell parameters, atomic positions of
the metal atoms, hydrogen and chlorine content, thermal
displacement parameters, zero shift, background, size and
strain parameters were refined. For more details of the
refinement process the reader is referred to table S2 in the
Supporting Information.
Results and discussion
Synthesis and crystal structure. The alkaline metal
strontium borohydride chlorides ASr(BH4)3-xClx (A = K, Rb,
Cs) were obtained from the mechanochemical metathesis
reaction of lithium borohydride, the alkaline metal
borohydride and strontium chloride according to the
following equation:
Temperature dependent XRD patterns from 140 K to 290
K with intermediate steps of 25 K were recorded on a STOE
IPDS 2T single crystal diffractometer in powder acquisition
mode with Mo-radiation (λ = 0.71073 Å). Samples were
prepared in 0.3 mm capillaries and measured over 3 min
2 LiBH4 + ABH4 + SrCl2 → ASr(BH4)3-xClx + (2-x) LiCl + x
LiBH4
(1)
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Table 1: Crystallographic information for the mixed
metal borohydrides ASr(BH4)3-xClx (A = K, Rb, Cs) and
Sr(BH4)2-xClx.
with A = K, Rb, or Cs. For Eu2+-doped samples, EuH2 was
added (2 mol%). Mechanochemical metathesis reactions of
this kind are a well-known tool for the synthesis of mixed-
anion and mixed-cation borohydrides.26, 38-41
KSr(BH4)3- RbSr(BH4)3- CsSr(BH4)3- Sr(BH4)2-
Compound
xClx
xClx
xClx
xClx
Space
group
P 21 c n (33) P b n 21 (33) P 2 21 21 (18) P b c n (60)
ASr(BH4)3-xClx (A = K, Rb, Cs) crystallize in perovskite-
related orthorhombic structures as reported before for the
pure borohydrides ASr(BH4)3 (A = K, Rb, Cs) by Møller et
al.42 In ASr(BH4)3 (A = K, Rb, Cs) the Sr2+ ions are surrounded
by 6 [BH4]- - tetrahedra forming a distorted octahedron.
These octahedra are connected via corners to build up a
three-dimensional network. The structure of KSr(BH4)3 is
shown in Fig. 1 (for A = Rb, Cs see Fig. S1 in the Supporting
Information). All three compounds share the same
structural motif. Due to the similar ionic radii of Sr2+ and
Eu2+,43 it can be expected that Eu2+ occupies the strontium
site and is therefore coordinated by a distorted octahedron
of [BH4]- units.
Cell
parameters
[Å]
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a
b
c
11.3749
8.3074
7.5106
8.2717
8.0830
11.5538
6.0840
8.0941
8.0867
6.9160
8.3096
7.5129
d(Sr – B) [Å] 2.71 – 3.54
2.38 – 3.65
3.00 ± 0.17
6
2.84 – 3.24
3.01 ± 0.06
6
2.96 – 3.13
3.02 ± 0.08
6
davg(Sr – B)
3.06 ± 0.12
[Å]
CN (Sr)
6
x (via
Rietveld)
0.20
0.14
0.26
0.26
Figure 1: Representative crystal structure of KSr(BH4)3
according to Ref. 42. Sr (blue) is coordinated by six [BH4]-
tetrahedra (grey) building up a three-dimensional network.
Hydrogen atoms of the [BH4]- units are omitted for clarity.
Table 1 summarizes selected structural information for
ASr(BH4)3-xClx (A = K, Rb, Cs) and Sr(BH4)2-xClx obtained
from Rietveld refinement. A detailed list of the atomic
positions is given in the Supporting Information, Table S1.
Figure 2: Rietveld refinement of KSr(BH4)3-xClx. Structural
refinement yields a substitution of [BH4]- - units with Cl- of x =
0.20. After ball milling approx. 32 wt% of LiCl is obtained as a
secondary phase, blue triangles mark a non-identifiable phase.
Bragg markers from top to bottom: KSr(BH4)2.80Cl0.20, LiCl.
For the chloride content x, 0.20, 0.14, and 0.26 were
obtained for A = K, Rb, and Cs, respectively. The partial
chloride substitution in mixed metal borohydrides
Sr(BH4)1.74Cl0.26:Eu2+
was
obtained
from
the
prepared by mechanochemical milling is in good agreement
mechanochemical reaction of SrCl2 with two equivalents of
LiBH4 and 2 mol% EuH2 for Eu2+-doping as reported by
Ravnsbæk et al.40 Sr(BH4)2 crystallizes orthorhombically in
space group Pbcn and Sr is coordinated by six [BH4]- - units,
which form a slightly distorted octahedron.
40
with previously reported works.26,
Exemplarily, the
Rietveld refinement of the structure of KSr(BH4)2.80Cl0.20 is
shown in Fig. 2. Additional Rietveld refinements of the
structures of Sr(BH4)1.74Cl0.26
,
RbSr(BH4)2.86Cl0.14 and
CsSr(BH4)2.74Cl0.26 are compiled in Figs. S2 – S4 of the
Supporting Information.
For KSr(BH4)2.80Cl0.20, a DSC measurement showed no
phase transition in the range of -160°C to 250°C (see Fig.
S14 in the Supporting Information). At 252°C, an
endothermic signal indicates the decomposition at much
lower temperatures compared to the pure borohydride
analogue KSr(BH4)3 reported by Møller et al.,42 which
decomposes at approximately 400°C. Møller et al. suggested
that at 258°C, a structural transformation to a HT-
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polymorph of KSr(BH4)3 occurs. The authors came to that
conclusion based on additional reflections observed in high
temperature synchrotron diffraction patterns. Yet, in their
DSC measurements they did not observe any heat flow
changes in the corresponding temperature range, but only
a strong endothermic signal at 400°C. The borohydride
chloride KSr(BH4)2.80Cl0.20 presented in this work, in turn,
does not show any strong signal at 400°C, but at
significantly lower temperature (252°C), which suggests
decomposition as also confirmed by ex-situ XRD. For
samples that were heated to 250°C and 300°C, respectively,
XRD analysis showed two phases of KSr(BH4)3-xClx with
different x values at 250°C (see Fig. S5 in the supporting
info). At 300°C, the sample clearly darkened and only one
KSr(BH4)3-xClx phase is present. After treatment at 400°C,
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Figure 3: Photographs of ASr(BH4)3-xClx (A = K, Rb, Cs; x = 0.20,
0.14, 0.26) and Sr(BH4)2-xClx (x = 0.26) from left to right. Top:
Samples under daylight, bottom: Samples under 365 nm
excitation in an argon filled glovebox.
the powder appeared black, which suggests
a
decomposition of the remaining phase at higher
temperatures.
Temperature-dependent crystal structure data from X-
ray powder diffraction patterns collected on a single crystal
diffractometer from 140 K to room temperature showed a
temperature dependence of the lattice parameters for the
mixed metal borohydride chlorides ASr(BH4)3-xClx (A = K,
Rb, Cs) (see Fig. S6 to S13 in the supporting information). In
the range between room temperature and 600 K, DSC and
ex-situ X-ray diffraction was used (see Figs. S14 and S5,
respectively, in the Supporting Information). For all ions A,
an increase of temperature results in a linear increase of the
cell parameters. Accordingly, positive volumetric thermal
expansion coefficients can be deduced, namely 14.5 x 10-
5 K-1 (A = K), 9.2 x 10-5 K-1 (A = Rb) and 6.9 x 10-5 K-1 (A = Cs)
according to equation (2),
1
∂푉
∂푇
훼푉 =
(2)
(
)
푉
Figure 4: (a) Photoluminescence excitation (dashed) and
emission spectra (solid) of Eu2+ in ASr(BH4)3-xClx (A = K, Rb, Cs;
x = 0.20, 0.14, 0.26) at 4.2 K (b) same measurements at room
temperature. For λex and λem of the respective spectra, see
Table 2. The arrows mark the estimated positions of the
emissive 4f6(7F0)5d1 states
푝
where the subscript indicates constant (ambient)
pressure conditions. The magnitude of the thermal
expansion coefficient is particularly related to the
anisotropy in different bonds within a given crystal or,
alternatively, the increasing degree of anharmonicity of the
respective collective vibrations. According to the crystal
structure data of the pure ternary borohydrides, the
decreasing magnitude from A = K to A = Cs also becomes
readily evident upon already regarding the coordination
sphere of the Sr2+ ions as a representative example: the
coordination spheres of the [Sr(BH4)6]4- octahedra become
less distorted upon variation of A = K to A = Cs. This critical
point will be reviewed in terms of its effects on the
photoluminescence properties of Eu2+ below in detail.
Luminescence spectroscopy. After short milling times
of only 90 min at 600 rpm the Eu2+-doped samples already
exhibit intense cyan luminescence in mixed cationic
borohydride-chlorides and blue in Sr-borohydride-chloride
under UV excitation (Fig. 3)
Figure 4 depicts the photoluminescence excitation and
emission spectra of ASr(BH4)3-xClx (A = K, Rb, Cs; x = 0.20,
0.14, 0.26) at 4.2 K and room temperature after having
annealed the samples at 200°C for 2 days. All three Eu2+-
activated ternary borohydride compounds show a broad
emission located in the cyan range at around 490 – 500 nm.
The broad banded appearance of the emission is typical for
the parity-allowed 4f65d1 – 4f7 transition of Eu2+. Similar
emission spectra have been previously recorded by
26
Schouwink et al.25,
in the mixed borohydrides
CsCa(BH4)3:Eu2+ and CsEu(BH4)3 that were also prepared by
mechanochemical synthesis. Another example is the
molecular compound [Eu(BH4)2(THF)2] reported by Marks
et al.24 In contrast, there is only the corresponding chloride
analogue CsSrCl3:Eu2+ reported to show the respective
4f65d1 → 4f7 emission transition at around 431 nm.44,45
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Table 2: Characteristic optical parameters deduced from the photoluminescence spectra of ASr(BH4)3-xClx (A = K, Rb,
Cs; x = 0.20, 0.14, 0.26). Values refer to 4.2 K if not noted otherwise.
2
3
4
Excitation maximum
Emission maximum
Zero phonon line (ZPL)
FWHM
[cm-1]a
1970
Stokes shift
[cm-1]a
5600
Decay timeb
Compound
[nm]
[cm-1]a
29690
29130
27270
[nm]
504
499
497
[cm-1]a
19830
19990
20000
[nm]
436
438
444
[cm-1]a
22630
22565
22400
[ns]
5
6
7
KSr(BH4)2.80Cl0.20
330
608
RbSr(BH4)2.86Cl0.14 335
2085
5140
598
8
CsSr(BH4)2.74Cl0.26
339
1940
4800
558
9
a
Obtained by proper conversion of the photon counts per constant wavelength interval into wavenumber domain by
푑휈
푑휆
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=
inclusion of the factor
.
|
|
휆2
b Values refer to room temperature.
Since Rietveld analyses result in low chloride
substitution for the here reported mixed anionic alkaline
metal
photoluminescence properties origin mainly from the
borohydride ligands and, as will also argued below, the Cl-
ligands only have a negligible effect thereon.
The gradual redshift of the excitation maximum from the
K- to Cs-based compound may be asserted by a repulsive
effect of the second coordination sphere on the Eu2+ ions
and thus, counteracting destabilization of the excited 4f65d1
configuration. Since K+ ions are smaller than Rb+ and Cs+
ions, the former may approach the Eu2+ ions more closely.43
The alike positive charge then leads to a repulsive
contribution, which in turn destabilizes the 4f65d1
configuration and should in total induce a higher energy of
that configuration in the K-based compound compared to
the energy in the Cs-based compound. This interpretation is
additionally suggested by various longer and more
anisotropic Sr-B bond lengths in the ground state structure
of the K representative compared to the Cs representative
(see also Table 1 and Figure 5).
strontium
borohydride
chlorides
the
The maxima in the photoluminescence excitation spectra
of these compounds are systematically redshifted from K
(330 nm) over Rb (335 nm) to Cs (339 nm) and exhibit a
broadened fine structure both at 4.2
K and room
temperature (see Figure 4.(a), (b), dashed lines). The fine
structure is typically argued to be related to the underlying
spin-orbit splitting of the 4f6 core into the different FJ (J =
7
0…6) levels in the excited 4f65d1 configuration. It should be
noted, however, that this is only a valid interpretation
scheme if the Coulomb and isotropic exchange interactions
between the 4f6 core and the 5d electron are negligibly
small compared to the crystal field and spin-orbit coupling
interaction. Yanase and Kasuya46 or Weakliem47 have shown
that a fine structure can, however, also be explained taking
this interaction explicitly into account. Both came to the
conclusion that the dominant intensities stem from
transitions of the 8S7/2 ground level to the still almost spin-
pure octet-based Kramers’ doublets. Strongly ionic
compounds such as fluorides, borates or halides48,49 often
exhibit a yet well-resolved fine structure that would not be
readily expected based on the stronger interaction between
the 4f and 5d electrons. Typically, this is related to the high
symmetry and the related largely retained degeneracy of
many of the crystal field states that supports a good
resolution. In contrast, more covalent host compounds
often tend to show less resolved fine structure in the
excitation spectra although an almost negligible 4f-5d
Coulomb-exchange interaction is expected for these cases
based on the expanded 5d orbitals by means of the
nephelauxetic effect. This limited resolution in the
presented borohydride chlorides even at 4.2 K is a result of
several consequences. Strong electron-phonon coupling
Figure 5: Bond lengths of the three ternary strontium
borohydride chlorides in the respective [Sr(BH4)6] octahedra.
The structural anisotropy of the octahedra decreases from K
over Rb to Cs. Hydrogen atoms are omitted for clarity.
In line with the consequences of such a repulsive impact,
excitation from
a
very localized and shielded 4f7
configuration of Eu2+ to a more open or covalent 4f65d1
configuration should induce a higher degree of excited state
relaxation in the K-based compound to allow the ligand field
to minimize its energy with respect to this higher degree of
Coulomb repulsion. Thus, it is expected that the bond length
in the excited 4f65d1 configuration changes more strongly
for A = K than for A = Cs. This is in line with the observed
trend of Stokes shifts (see Table 2). Due to the higher
number of different excited states underlying the broad
excitation bands of Eu2+, an accurate determination of the
Stokes shift for this divalent lanthanide ion is far from trivial
and typically only approximately possible. Based on the
-
induced by the more polarizable BH4 ions in addition to
symmetry distortions from octahedral symmetry have a
counteracting effect on the resolution of spectra and lead to
severe homogeneous broadening effects. In addition, the
presence of Cl- impurities in the presented compound may
additionally affect local symmetries and may lead to
inhomogeneous broadening such that the overall resolution
of the spectra is low.
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positions of the zero-phonon lines (ZPLs) determined by
process.50 It can be estimated from the emission spectra by
the crossing point between emission and excitation spectra
at 4.2 K (see Figure 4.(a)), the Stokes shifts may be
approximated by twice the energy difference between the
ZPL and the emission maximum (see also discussion on the
Huang-Rhys-Pekar factors below). This approximation
affords the values of ∆S = 5600 cm-1, 5140 cm-1 and 4800
cm-1 for A = K, Rb and Cs, respectively. Besides the expected
decreasing trend from A = K to Cs and the connected
successive symmetrisation of the coordination sphere of
Eu2+, these values reasonably assess the position of the
emissive 4f6(7F0)5d1 state assuming a valid interpretation of
decoupled interaction scheme between 4f and 5d electrons
(see arrows in Figure 4.(a)) as the lowest energetic
shoulders of the broad excitation bands.
comparison of the integrated intensity I0 of the zero-phonon
transition (being estimated by the crossing point between
emission and excitation spectra) relative to that of the
vibrational sidebands, Ip, 50
퐼0 = 퐼푝e ―푆
(3)
eff
9
It should be noted that due to the different nature of
chemical bond in the 4f7 (8S7/2) and 4f65d1 electronic states,
respectively, also the Huang-Rhys-Pekar factors for the
excitation transition and emission transition then differ,
respectively. Seff is then the geometric mean of these two
values and a reliable effective measure, especially if it is
small. Based on eq. (3), the values for Seff for the different
chloride borohydrides may be estimated (errors ~ ± 0.3) as
Seff = 6.3 (A = K), Seff = 5.6 (A = Rb) and Seff = 5.0 (A = Cs).
Although the estimated values of the different Huang-Rhys-
Pekar factors should not be considered as very accurate
based on the weakly resolved vibronic fine structure only,
the difference between the Huang-Rhys-Pekar factors is
significant. This is noteworthy given the fact that the alkali
ions only reside in the second coordination sphere of Eu2+
and thus, illustrate the yet sensitively detectable local
response of the Eu2+-based luminescence thereon.
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Another measure for the non-radiative relaxation in the
excited 4f65d1 configuration is the emission bandwidth of
the corresponding 4f65d1 → 4f7 transition. In contrast to the
Stokes shift, it can be accurately determined over the whole
temperature range between 4.2 K to around 500 K and is
often in the same order of magnitude as the Stokes shift. For
the considered Eu2+-activated borohydride chlorides, the
corresponding full widths at half maximum (FWHMs) read
1970 cm-1 for A = K, 2085 cm-1 for A = Rb and 1940 cm-1 for
A = Cs (see also Table 2). Interestingly, the Eu2+-related
emission band FWHM in the Rb representative is larger
than in the other two compounds, which is a consequence
of inhomogeneous broadening induced by the largest
anisotropy of the Eu-B bond lengths in the first coordination
sphere among all presented borohydride chlorides (see
Table 1 and Figure 5). Thus, the emission band FWHMs
reflect the local structural anisotropies around the Eu2+ ions
more accurately than the Stokes shifts do.
Additionally,
a
lower effective Huang-Rhys-Pekar
parameter should give better resolved vibrational fine
structure, which is in agreement with the observations on
CsSr(BH4)2.74Cl0.26 (cf. Figure 4.(a)).
From the Huang-Rhys-Pekar factors Seff and the
determined Stokes shifts ∆S (see Table 2), an average value
for the vibrational mode that is coupled to an electronic
transition is estimated by the high FWHM limit51
For
a
discussion of the structure-luminescence
relationship of Eu2+, the emission spectra of Eu2+ at 4.2 K
have to be regarded (see Figure 4.(a), solid lines). The
emission maxima shift from 504 nm for A = K over 499 nm
for A = Rb to 497 nm for A = Cs to systematically higher
energies. This behaviour is also in agreement with the
previous discussion on the variation of the excitation
energies and Stokes shifts. At room temperature, the
emission bands of all three chloride borohydrides display a
thermally induced blueshift and thereby strongly overlap
(see Figure 4.(b)).
∆푆 = 2푆effℏ휔eff
(4)
ℏ휔eff = (460 ± 10)
and gives an average value of
cm-1
among the three Eu2+-activated ternary chloride
borohydrides. This value agrees very well with the
observed highest energetic librational mode in the pure
25, 52
borohydride perovskites ACa(BH4)3
and also fits to the
expectation that the electronic states of Eu2+ should couple
to a vibrational mode that also influences the Eu-B bonds
instead of internal vibrational modes of the [BH4]- units
themselves. In particular, the relevance of this librational
mode for the anharmonic structural phase transitions in the
pure borohydrides was emphasized.52 Furthermore, the
photoluminescence decay curves at room temperature have
been recorded for ASr(BH4)3-xClx:Eu2+ (Fig. S16 – S19
Supporting Information and Table 2). In all compounds, the
Eu2+-based emission intensity shows a single exponential
Only the emission spectrum of Eu2+ in CsSr(BH4)2.74Cl0.26
at 4.2 K displays a weakly resolved vibrational fine
structure at the higher energy side. The lacking resolution
may be partially attributed to the presence of the Cl- ions
that induce inhomogeneous broadening effects, but also the
covalent Eu-BH4 bonds. Apart from that impact, however,
the regularity in trends from both the excitation and
emission spectra indicate that the Cl- ligands do not have a
significant effect on the appearance of the luminescence
spectra of the presented compounds and most of the Eu2+
ions may be coordinated by solely [BH4]- anions.
Nonetheless, the decreasing Stokes shift from A = K to Cs
relates to a correspondingly decreasing Huang-Rhys-Pekar
parameter Seff that serves as a measure for the strength of
the electron-vibrational coupling and denotes the average
number of vibrations emitted during the relaxation
decay and lifetimes of about 608 ns for KSr(BH4)2.80Cl0.20
,
598 ns for RbSr(BH4)2.86Cl0.14 and 558 ns for
CsSr(BH4)2.74Cl0.26 were obtained. This is in reasonable
agreement with decay times reported for the Eu2+ emission
in metal hydrides17, fluorides53 or other complex oxides54 at
room temperature. Lifetimes found in hydrides and
borohydrides are usually somewhat shorter than in other
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classes of materials, like halides or oxides, at comparable
Since all reported Eu2+-doped compounds show very
intense photoluminescence even up to 500 K, internal
quantum yield measurements at room temperature were
carried out to determine the efficiency of these phosphors.
Table 3 compares the quantum yield for Eu2+ emission on
ASr(BH4)3-xClx (A = K, Rb, Cs) with previously reported
quantum yields in similar compounds.
energies. A possible explanation might be the different
refractive index, which has a strong influence on the
radiative decay rate.55 Due to the higher polarizability of
hydrides, one might expect the refractive index to be higher
than, for instance, in some oxides and this will lead to a
faster radiative decay. Comparingly, the here reported
lifetimes are shorter than the values reported for the pure
halides CsMBr3 and CsMI3 (M = Mg, Ca, Sr) at room
temperature, which are in the range of around 800 – 900 ns
at room temperature instead.48,49 This variation is
attributed to an impact of non-radiative decay, which
correlates with the vibrational energies of the local modes
or phonons that the electronic states of the Eu2+ ions couple
to. The vibrational coupling energy in the presented
borohydride chlorides at around 460 cm-1 is right in
between the maximum phonon energies found in fluorides
or several complex oxides between 350 cm-1 to 700 cm-1,56
thus explaining the similarity in the respective decay times
of the 4f65d1 configuration at room temperature. In
contrast, the phonon energies of bromides and iodides are
in the range of 160 cm-1 to 120 cm-1,57 and as a result, the
corresponding 4f65d1-related decay times become slightly
longer. An additional influence stems from photonic effects
on the radiative decay time such as the refractive index
inducing a local field correction in a dielectric, but also the
휆 ―3
Among the mixed metal borohydride chlorides ASr(BH4)3-
xClx (A = K, Rb, Cs), the highest efficiency is found for A = K
with 93% internal quantum yield followed by Cs (76%) and
Rb (53%). For comparison, Marks et al. reported an
absolute quantum yield of 75% for [Eu(BH4)2(THF)2] and
were able to increase the quantum yield to 93% by
deuteration of the compound.24
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emission wavelength (
dependence of the Einstein
em
coefficient of spontaneous emission).58 These effects
become, however, only clearly distinguishable at very low
temperatures when non-radiative decay is negligible.
Temperature dependent luminescence emission spectra
of Eu2+ have been recorded for all ternary chloride
borohydrides and are shown in Fig. 6. While all compounds
are characterized by a vibrationally induced broadening of
the emission bands (see also Fig. S23 for the temperature
evolution of the full widths at half maximum), the integrated
emission intensity remains very stable even up to 500 K
(decrease by less than 10%, see Figures S20-S22) and
indicate that despite the potentially high energetic
stretching vibrations of the [BH4]- ligands at around 2400
cm-1,52 quenching of the Eu2+ luminescence in the chloride
borohydrides is inefficient. Decomposition of the
borohydrides hinder further recording of luminescence
spectra above 500 K. However, the present data clearly
suggest that the quenching temperature T50% should be well
above 500 K for A = K, Rb and Cs (see Figs. S20 – S22 of the
Supporting Information).
Table 3: Absolute quantum yield (QY) of different Eu-
containing borohydride compounds at 293 K.
λexc
λems
Quantum yield
Compound
[nm]
[nm]
[%]
KSr(BH4)2.80Cl0.20:Eu2+
RbSr(BH4)2.86Cl0.14:Eu2+
CsSr(BH4)2.74Cl0.26:Eu2+
[Eu(BH4)2(THF)2] [23]
330
330
350
360
360
489
492
486
490
458
93
53
76
75
5
Figure 6: Temperature dependent photoluminescence
spectra of the 4f65d1
→
4f7 transition of Eu2+ in
CsSr(BH4)2.74Cl0.26 (top, λex = 350 nm), RbSr(BH4)2.86Cl0.14
(middle, λex = 330 nm) and KSr(BH4)2.80Cl0.20 (bottom, λex = 330
nm) excited with a xenon lamp.
[Eu(BH4)(THF)5][BPh4] [23]
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In addition to the previously characterized features of the
thermal population of higher vibrational states does not
temperature-dependent luminescence of Eu2+, an
extraordinary blueshift of the 4f65d1 → 4f7-related emission
maxima with increasing temperatures is observed in all
three ternary mixed borohydrides ASr(BH4)3-xClx (see
Figure 6) and this observation remains unchanged after
repeated heating and cooling steps. As depicted in Figure 7,
the Eu2+-based luminescence in all three compounds first
display a slight redshift within the range of 15 K to 150 K,
while a blueshift emerges at higher temperatures. The net
blueshift of the temperature-dependent emission maxima
systematically decrease from ∆Eem = 1500 cm-1 for A = K
over ∆Eem = 990 cm-1 for A = Rb to ∆Eem = 700 cm-1 for A =
Cs between 14 K and 495 K. This feature is perfectly
correlated to the trend of thermal expansion coefficients as
affect the energetic position of the emission maximum.51
In addition, also the thermal excitation into the
7
energetically closely lying spin-orbit FJ levels (J = 1, 2) of
the 4f6 core of the excited configuration may induce a
systematic blue shift and has been interpreted to occur in
other Eu2+-activated halides before.48 However, given the
7
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energy difference of only 400 cm-1 between the F0 and F1
level in the 4f6 core,60 this mechanism alone cannot be
responsible for the significantly strong thermochromic
blueshift of the Eu2+-based emission observed in these
mixed anionic borohydrides (see Figure 6) in the range
between 1000 -1500 cm-1 in all three compounds. Upon
comparison to conventionally observed thermochromic
blueshifts of the 4f65d1 → 4f7 configuration of Eu2+ in other
host compounds48, 49 in the order of only ~ 100 cm-1, the
presented borohydride chlorides are unique in their
thermal Eu2+-related photoluminescence response. For
instance, such compounds could, in principle, be used as
internal temperature sensors in in situ neutron diffraction
studies where transparent reaction chambers are used.61
Certainly, this would probably require additional pressure
calibration due to the dependence of the emission on the
interatomic distances and the related vibrational modes.
Further options could be the encapsulation into a polymer
matrix to overcome air and moisture sensitivity in the
future and using the particles in reaction monitoring under
different atmospheres as is currently investigated for
various processes for other phosphors with temperature-
dependent peak positions.62 Due to decomposition of the
KSr(BH4)2.80Cl0.20:Eu2+ at higher temperatures, no
luminescence emission could be studied above 500 K. The
steady and sensitive blueshift as a response to external
temperature changes in the region between 200 and 475 K
combined with the fact that this mixed anionic borohydride
does not decompose in this temperature region under
atmospheric inert gas pressure or under hydrogen
atmosphere make especially this compound a promising
local and sensitive temperature sensor, for example during
hydrogenation reactions, as will also quantitatively
assessed below.
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derived
from
the
temperature-dependent
XRD
measurements and indicates that the Eu2+-based
luminescence sensitively reacts on the successively
anisotropically increasing bond lengths from 140 K on (see
Fig. S9, S11 and S13).
The temperature-dependent peak maxima for A = K, Rb
and Cs are shown as a function of temperature in Fig. 7.
Figure 7: Temperature dependent peak maximum of the Eu2+
photoluminescence emission in ASr(BH4)3-xClx:Eu2+ (A = K, Rb,
Cs; x = 0.20, 0.14, 0.26).
As the 4f65d1 configuration reacts sensitively to local
structural changes in the coordination environment of the
Eu2+ ions, we propose the following mechanistic
explanation for the large temperature induced blueshift. At
low temperatures, the bond lengths between Eu2+ (being as
large as Sr2+) and the surrounding [BH4]- units are highly
anisotropic, which give rise to a deviation from perfect
octahedral symmetry of the coordination sphere of the Eu2+
ions. Correspondingly, the lowest energetic emissive
4f65d1(t2g) crystal field states of Eu2+ will lose their
degeneracy and split up into at most three states. Since the
smaller K+ ions in the second coordination sphere may
approach closer to the Eu2+ ions than the bulkier Rb+ and Cs+
ions, the Eu-B bonds in the first coordination sphere
increase from the Cs to the K compound in average (see also
Table 1 and Figure 5). In turn, the low symmetry splitting of
the 5d orbitals of Eu2+ should decrease from
Predictions of the direction and magnitude of
temperature-dependent shifts of the emission maxima in,
e.g. Eu2+- and Ce3+ activated phosphors are not always
straightforward. Yan59 proposed that one reason for the
observation of a redshift can be the broadening of the
emission band with increasing temperature, which may
lead to an increased self-absorption due to a larger overlap
of the photoluminescence excitation and emission spectra.
A blueshift, on the other hand, can have several origins.
One important cause is lattice expansion, which increases
the activator-ligand distance and thus, results in a smaller
crystal field splitting and decreasing covalency of the
lanthanide-ligand bond. Furthermore, thermally induced
population of higher vibrational states may lead to a higher
emission energy with increasing temperature due to the
presence of hot vibrational side bands. For approximately
harmonic vibronic states, however, it was shown that
CsSr(BH4)2.74Cl0.26
over
RbSr(BH4)2.86Cl0.14
to
KSr(BH4)2.80Cl0.20 due to the overall increasing bond lengths
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in that order. Correspondingly, any excitation from the
illustratively show the very large temperature-dependent
2
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shielded 4f7 to the chemically more covalent 4f65d1
configuration will affect the bond length most in the A = K-
related representative, which is additionally accompanied
by large Stokes shifts. This explains the energetic order of
the 4f65d1 → 4f7-based emission bands at low temperatures
(see Figure 4.(a)).
shifts of the 4f65d1 → 4f7 emission energies of Eu2+ by
tailoring the local environment, which makes them
potentially good candidates to investigate temperature-
dependent phenomena and also for a new class of
temperature sensors.
The emission energies and europium – ligand distances
found in the present work are subsequently compared to
similar compounds with octahedrally coordinated
europium/strontium sites and different anions reported
earlier and can be seen in Table 4.
8
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If the temperature is raised, the collective vibrational
motion of the host crystal (including the local coordination
sphere of the Eu2+ ions) systematically makes the Eu-B
bonds more isotropic in thermal average. Correspondingly,
at some temperature, the octahedral symmetry is
approximately retained and the 4f65d1(t2g) levels recover
their degeneracy. In agreement with that picture, the
emission should be systematically blueshifted and can
cover larger energy difference dependent on the symmetry-
induced splitting of the t2g states, in agreement to
observations. Alternatively, the thermally induced lattice
expansion should generally increase the Eu-B bonds in
average and thus, lead to a generally observable blueshift of
the Eu2+-related emission with increasing temperature in all
three compounds. A strong indication for the relevance of
vibrations for the photoluminescence properties of these
compounds is given by the different magnitudes of blue
shifts among the three ternary borohydride chlorides. Their
values are in perfect line with the decreasing size of the
thermal expansion coefficients αV from A = K to A = Cs (see
above and Figures S9, S11, S13). As the thermal expansion
coefficient is an indirect measure for the response of a
crystal to thermally excited (in particular, anharmonic)
vibrations, it is also expected that the A = K representative
should show the largest blueshift of the Eu2+-related
emission. In turn, that also implies that at a certain
temperature, the three compounds should emit at around
the same wavelength, which is in fact the case at around
room temperature (see Figure 4.(b) or 7). At higher
temperatures, it is then expected that the emission energies
reverse their order and that the A = K should show the most
blueshifted emission, also in agreement with observations.
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Table 4: Various europium doped strontium
compounds with the coordination number (CN = 6),
their emission wavelength and atomic distances d(Sr –
X). The given atomic distances in compounds with
simple anions correspond to the europium – anion
distance, whereas in compounds with complex anions,
the distances are given as distances between europium
and the central atom of the complex anion.
d(Sr – X) davg(Sr – X) Emission Referen
Compound
[Å]
[Å]
[nm]
ce
SrSO4
3.40 – 3.95 3.57
3.33 – 3.90 3.68
378
[63]
[64]
[65]
[66]
SrLiB9O15
Sr3(PO4)2
Sr3P4O13
CsSrBr3
CsSrI3
385 b)
413 c)
415 d)
3.44
3.44
3.25 – 3.82 3.48
3.05 – 3.08 3.06
428-430 [48, 67]
446-454 [49, 68]
3.36
3.36
NaSrPO4
3.20 – 4.15 3.67
451
[69]
[Eu(BH4)(THF)5]
[BPh4]
a)
a)
a)
a)
458
[24]
Eu(BH4)2
465
457
485
485
486
488
490
490
490 c)
492
564
588 c)
620
625
[25]
Sr(BH4)1.74Cl0.26
CsEu(BH4)3
CsCa(BH4)3
2.96 – 3.14 3.02
this work
[25]
a)
a)
a)
a)
[25]
The observed redshifts of the Eu2+-based 4f65d1 → 4f7
emission in all compounds ASr(BH4)3-xClx (see Figure 7) at
temperatures below 150 K may be easily explained in the
framework of this model if αV is negative at those
temperatures. Future studies are already ongoing to verify
both this aspect and also the applicability of this model in
other related compounds. Another impact can be the
balance between thermally induced vibrational broadening
of the emission and corresponding symmetrisation of the
coordination sphere around Eu2+ at higher temperatures. In
fact, the full widths at half maximum (FWHMs) of
particularly the Rb- and Cs-based compound increase
almost linearly with temperature even below 150 K, while
in the K-based compound, the Eu2+-based emission FWHM
remains constant at those temperatures (see Fig. S23 in the
Supporting Information). A hint on the validity of this
balancing interpretation is the vanishing of the blueshift of
the 4f65d1 → 4f7 emission of Eu2+ in KSr(BH4)2.80Cl0.20 in
exactly the temperature range of constant FWHMs.
CsSr(BH4)2.74Cl0.26 2.84 – 3.24 3.01
KSr(BH4)2.80Cl0.20 2.71 – 3.54 3.06
this work
this work
[24]
a)
a)
[Eu(BH4)2(THF)2]
[Eu(BD4)2(THF)2]
Sr2SiO4
a)
a)
[24]
3.15 – 4.22 3.79
[70]
RbSr(BH4)2.86Cl0.14 2.38 – 3.65 3.00
this work
[71]
SrSe
3.12
3.26
3.01
2.61
3.12
3.26
3.01
2.61
LiSr4[BO3]3
SrS
[72]
[73]
SrO
[74]
a)
b)
c)
not reported [BO4]5- units second polyhedron with other CN
than 6 d) second and third polyhedron with other CN than 6
Combined with the high quantum yields and thermally
stable luminescence, the Eu2+-activated chloride
borohydrides may be considered as potential luminescent
thermometers. The vast majority among the currently still
developed luminescent thermometers work with
Compared to well-established classes of host materials
for phosphors such as oxides, borohydride hosts
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luminescence intensity ratios as those signals are stable
wavelength shift) itself being much lower than the
towards absolute uncertainties of the intensity, but merely
depend on relative intensity uncertainties only. In contrast,
the most prominent thermal response of the presented
three chloride borohydrides ASr(BH4)3-xClx:Eu2+ (A = K, Rb,
Cs) is the emission band shift. A figure of merit for the
performance of a luminescent thermometer is the relative
sensitivity, which relates its thermal response to the
reference signal (energy or wavelength). It is, however,
similarly possible to quantify the blueshift by means of an
analogous ratiometric area analysis. By integration of the
low- and high-energy side of the temperature-dependent
emission spectra (see Figs. S28 and S29 in the Supporting
Information), which mimics the usage of corresponding
bandpass filters, it is possible to achieve an alternative
corresponding signal at
a
given temperature.75,76
quantification for the blueshift. A similar operating
9
Specifically, it reads for any band shift
principle is used by ratio pyrometers that detect the emitted
black body radiation of any object at two different
wavelengths. If R denotes the ratio between the integrated
intensity of the high-energy portion I2 and low-energy
portion I1, the corresponding relative sensitivity may be
given by
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∂퐸em
∂휆em
1
1
푆r(푇) =
=
(5)
|
|
|
|
퐸em ∂푇
휆em ∂푇
with Eem and λem as the emission energy or wavelength at
temperature T, respectively. It should be explicitly
mentioned that the relative sensitivity is invariant with
respect to the chosen quantity for the emission position (Eem
and λem). The relative sensitivity readily allows to compare
different thermometers irrespective of their underlying
operating principle. With eq. (5), maximum relative
sensitivities of Sr = 2.85 x 10-2 % · K-1 for A = K (at 245 K),
Sr = 2.12 x 10-2 % · K-1 for A = Rb (at 324 K) and Sr = 1.51 x
10-2 % · K-1 for A = Cs (at 178 K) were derived. Given the
commonly observed blue shifts of an Eu2+-related emission
of around 100 cm-1 between 10 K and 300 K,48,49,77,78 the
relative sensitivities of the 4f65d1 → 4f7 emission in the
chloride borohydrides are about one order of magnitude
larger and thus, make these compounds unique among Eu2+-
activated phosphors in the context of thermal emission
band position response. It is noteworthy that they are even
competitive to the reported relative thermal sensitivities of
CdSe quantum dots (Sr = 1.59 x 10-2 % · K-1) using the
1∂푅
푆r(푇) =
(6)
|
|
푅∂푇
The thus obtained relative thermal sensitivities are much
higher and read Sr = 0.91 % · K-1 for A = K (at 211 K), Sr =
0.66 % · K-1 for A = Rb (at 278 K) and Sr = 0.83 % · K-1 for A
= Cs (at 229 K). In fact, they are even competitive with
conventional ratiometric thermometers (Sr ≥ 1 % · K-1)
using two emission transitions from thermally coupled
excited states.92-97 In that sense, the presented compounds
are also promising single-band luminescent thermometers.
Properties of Sr(BH4) 1.74Cl0.26:Eu2+
In addition to the ternary strontium borohydride
chlorides, the binary compound can be synthesized in the
same manner. After ball milling of 2 equivalents LiBH4 with
SrCl2 (2 mol% EuH2) the blue emitting phosphor
Sr(BH4)1.74Cl0.26:Eu2+ is obtained and its photoluminescence
emission spectrum reveals a broad emission band with a
maximum at 457 nm at room temperature (see Fig. S15 in
the Supporting Information). Rietveld refinement,
temperature-dependent emission position shift as
response signal,79,80 while detection of the peak shift of a
Nd3+-based 4f3(4F3/2 4f3(4I11/2
R2) Z1) emission
a
,
→
,
transition in the crystal field of ALa(PO3)4 (A = Li - Rb) was
shown to act an order of magnitude more sensitively (Sr =
0.48 % · K-1).81
crystallographic
information,
photoluminescence
excitation and decay measurements can be viewed in the
Supporting Information. Similar to the ternary compounds
a high quantum yield of 82% is achieved.
In contrast, however, the absolute temperature-
dependent luminescence intensity or fluorescence decay
time of CdSe82-84 or, more recently, Ag2S quantum dots85,86
have been shown to react by orders of magnitude more
sensitively (Sr > 2 % · K-1) to small temperature changes.
Also, the usage of a luminescence intensity ratio of two
emission bands using lanthanides as emitters has already
been shown to response more sensitively to small
temperature changes (even Sr > 10 % · K-1) in various
temperature regimes.76,87-91 The reader is referred to
several excellent reviews on that topic for a broad overview
of this quickly emerging field.92-96 Thus, although the
presented Eu2+-activated chloride borohydrides ASr(BH4)3-
xClx (A = K, Rb, Cs) are very sensitive in terms of their
temperature-dependent emission band position, detection
of the emission maximum as itself does not provide the
thermal relative sensitivities already achievable in practical
luminescent thermometers. This circumstance is related to
the type of detected temperature response (energy or
Expectedly, the bond lengths in the binary compound
Sr(BH4)1.74Cl0.26 are more isotropic and generally shorter
due to the lack of a repulsive alkali cation and thus, the
coordination sphere of Eu2+ is closer to a perfect octahedral
symmetry. Accordingly, a more blueshifted excitation band
compared to the presented ternary representatives should
be expected due to the recovery of degenerate 4f65d1(t2g)
crystal field states, as is also observed (cf. Figure S15 in the
supporting information). A similar model was already
presented to explain the difference between the
photoluminescence of Eu2+ in the iodides CsCaI3 compared
to CsMgI3 and CsSrI3, respectively.49
Conclusion
The mixed metal borohydride chlorides ASr(BH4)3-xClx (A
= K, Rb, Cs) and Sr(BH4)1.74Cl0.26 have been synthesized by
ACS Paragon Plus Environment

Page 11 of 15
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Chemistry of Materials
This research was supported by a Liebig fellowship including
mechanochemical reaction. These borohydride host show
2
doctoral fellowship of the Fonds der Chemischen Industrie
(FCI), grant nr. Li 197/02, the German Research Foundation
(DFG, KU 3427/4-1) and the Bavarian-French Academy Center
for a Mobility aid (Az. FK03_2017). M. S., B. V. and A. M.
acknowledge funding from the EU-FET-Open project
nanoTBtech (grant agreement no.: 801305).
very efficient luminescence at room temperature upon
doping with 2 mol% europium and very low thermal
quenching. All mixed metal borohydride chlorides show
thermally induced significant shifts of the emission energies
and also a unique sensitivity towards local changes even in
the second coordination sphere. In addition, the 4f65d1 →
4f7 emission of the Eu2+-activated ternary chloride
borohydrides shows thermally induced blueshifts in the
order of 1000 to 1500 cm-1 so far never observed for Eu2+ to
the best of our knowledge, which makes them promising
candidates for temperature sensing applications in a
relevant temperature regime (200 K – 500 K), e.g. for the in
situ studies of hydrogenation reactions. Despite the fact that
the relative thermal sensitivity of this blueshift is below the
much better optimized ratiometric or luminescence decay
time thermometers, it is competitive to even quantum dot-
based thermometers with the same operating principle of a
thermally induced change in emission position.
3
4
5
6
7
8
9
ACKNOWLEDGMENT
N.K. and T. W. would like to thank Prof. Fässler for hosting our
group, P. Walke for help with DSC measurements, Dr. C. Jandl
for help with quantum yield measurements, Dr. W. Klein and
Dr. V. Hlukhyy for help with the temperature dependent XRD
measurements and C. Kriebisch as well as K. Weber for help
with the syntheses.
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AUTHOR INFORMATION
Corresponding Author
* nathalie.kunkel@uni-goettingen.de
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Funding Sources
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