Rare-Earth Germanate Visible, Near-Infrared, and Up-Conversion Emitters

Article abstract of DOI:10.1002/ejic.201800153

The hydrothermal synthesis and structural characterization of new trivalent rare-earth germanates, Na₄[(Y_1–aLn_a)₂Ge₄O₁₃] (Ln = Eu, Tb, or Yb and Er; a = 0.05, 0.1) and Na₄[(Yb_0.9Er_0.1)₂Ge₄O₁₃] are reported. The structure of these solids was solved by single-crystal X-ray diffraction (150 K). The materials crystallize in the orthorhombic non-centrosymmetric Pna2₁ space group, and exhibit a framework comprising unprecedented anionic tetrameric [Ge₄O₁₃]^10– units, tetrameric units of {GeO₄} tetrahedra, and isolated dimeric units encompassing two distinct distorted {REO₆} octahedra. These rare-earth germanates feature interesting photoluminescence properties in the visible and near-infrared spectral regions. Eu³⁺ photoluminescence confirms the presence of two distinct rare-earth sites. Energy transfer between the latter sites is shown for the Yb³⁺/Er³⁺ pair, both displaying significant near-infrared emission. Isomorphous Na₄[(Yb_0.9Er_0.1)₂Ge₄O₁₃] features up-conversion emission, an unusual property among rare-earth crystalline germanates.

Full text of DOI:10.1002/ejic.201800153

A Journal of
Accepted Article
Title: Rare-Earth Germanate Visible, Near-Infrared and Up-Conversion
Emitters
Authors: Duarte Ananias, Filipe Almeida Paz, Luis D Carlos, and Joao
Rocha
This manuscript has been accepted after peer review and appears as an
Accepted Article online prior to editing, proofing, and formal publication
of the final Version of Record (VoR). This work is currently citable by
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To be cited as: Eur. J. Inorg. Chem. 10.1002/ejic.201800153
Link to VoR: http://dx.doi.org/10.1002/ejic.201800153

European Journal of Inorganic Chemistry
10.1002/ejic.201800153
FULL PAPER
Rare-Earth Germanate Visible, Near-Infrared and Up-Conversion
Emitters
[
a]
[a]
[b]
[a]
Duarte Ananias,* Filipe A. Almeida Paz, Luís D. Carlos and João Rocha*
[
4]
Abstract:
characterization of new trivalent rare-earth germanates, Na
Ln Ge 13] (Ln = Eu, Tb or Yb and Er; a = 0.05, 0.1), and
Na Ge 13], are reported. The structure of these solids
was solved by single-crystal X-ray diffraction (150 K). The materials
crystallize in the orthorhombic non-centrosymmetric Pna2 space
group, and exhibit a framework comprising unprecedented anionic
The
hydrothermal
synthesis
and
structural
in the same material.
In contrast with RE silicates, luminescence accounts on RE
4
[(Y1-
3
+
a
a
)
2
4
O
germanates are scarce. In 2008 Chen et al., studied the Eu
room-temperature luminescence of KEuGe
cyclogermanate exhibiting zigzag chains of edge-sharing {EuO
4
[(Yb0.9Er0.1
)
2
4
O
2
O
6
,
a
6
}
[
5]
1
octahedra. Xu and collaborators, in 2012, studied the effect of
[
6]
pressure on the luminescence properties of both, NaEuGeO
a material bearing {EuO } octahedra linear chains and isolated
{GeO } tetrahedra, and of NaEu
membered ring channels of {EuO
NaLnGeO (Ln = Sm, Eu, Gd, Tb) has been further investigated
by Yeon et al.
[Tb (OH)
Tb energy-transfer process in K
4
1
0-
tetrameric [Ge
and isolated dimeric units encompassing two distinct distorted
REO } octahedra. These rare-earth germanates feature interesting
photoluminescence properties in the visible and near-infrared
4
O
13
]
units, tetrameric units of {GeO
4
} tetrahedra,
6
[
7]
4
3
(GeO
4
)
2
(OH)
2
,
featuring six-
{
6
6
} octahedra. Luminescence of
4
3
+
[8]
spectral regions. Eu photoluminescence confirms the presence of
The tunable emission capability of
[
9]
3+
two distinct rare-earth sites. Energy transfer between the latter sites
K
3
x
Eu1−xGe
3
O
8
2
] (x = 1, 0.88, 0.67, 0) and the Gd -to-
Ge (OH) ] (x = 0,
a family of layered compounds structurally
3
+
3+
3+
is shown for Yb /Er pair, both displaying a significant near-infrared
emission. Isomorphous Na Ge features up-
unusual property among rare-earth
3
[Gd1−xTb
x
3
O
8
2
[
10]
4
[(Yb0.9Er0.1
)
2
4
O
13
0.3, 0.1, 1),
analogous to K
have also been discussed.
Here, we wish to report the new RE germanate system, Na
Ln Ge 13] (Ln = Eu, Tb or Yb and Er; a = 0.05, 0.1), whose
crystal structure consists of {GeO tetrahedra forming
units, isolated dimeric units
{REO octahedral, and
3
+
3+
3+
3+ [2a]
conversion emission,
crystalline germanates.
a
3 3 8 2
LnSi O (OH) (Ln = Y , Eu , Tb , Er ),
4
[(Y1-
a
a
)
2
4
O
4
}
Introduction
10-
unprecedented tetrameric [Ge
4 13
O ]
comprising two distinct distorted
6
}
In the past two decades we have been particularly interested on
the synthesis and characterization of the luminescence
properties of microporous, layered, and dense rare-earth (RE)
+
charge-balancing Na cations. These materials were prepared
under mild hydrothermal conditions and present significant
luminescence properties in the visible and near infrared spectral
regions. Up-conversion emission was also demonstrated with
3
+
[1-
silicates bearing optically-active trivalent lanthanide ions (Ln ).
3
]
Some of these materials display unique and unusual
the isomorphic compound Na
4 2 4
[(Yb0.9Er0.1) Ge O13].
photoluminescence. A case in point is the first example of a
3
+
ratiometric cryogenic thermometer based on a dense Ln
silicate (Ln=Gd, Tb, Eu) whose emission properties depend on
Crystal Structure Description
[
3c]
structural disorder and a phase transition. Another example is
Na
suitable for X-ray diffraction (Experimental Section), crystallizing
in the orthorhombic non-centrosymmetric Pna2 space group
Table 1) with two crystallographically distinct RE centers in the
4 2 4 13
[(Y0.95Eu0.05) Ge O ] (1) was isolated as single-crystals
microporous K
7
+
[Eu
3
2
Si12O32]•xH O, an extended solid hosting
3
3+
framework Ln -O-Ln dimers that behave like a molecule, as
1
[
1a]
far as PL is concerned. Yet another example is microporous
(
3
+
chiral Na
3
[(Y,Ln)Si
3
O
9
]•3H
2
O
for which Eu
luminescence
asymmetric unit (Figure 1). An unrestrained refinement of the
spectroscopy, with excitation by unpolarized light in the absence
of an external magnetic field, is able to identify enantiomeric
3+
crystal structure for the rates of occupancy of the mixed Y and
3+
Eu sites yielded ratios of 95% and 5%, similar to the synthesis
[
1b]
domains at 12 K.
Such enantiomeric detection was also
gel and, thus, they were kept in the final structural model (and
4
+
4+
achieved with partial substitution of Si ions (up to 2/3) by Ge
empirical formula).
A search in the Inorganic Chemistry
Database shows 1 is isotypical with a scandium-based material
[
11]
reported in the 1970s’ by Gorbunov et al.
[a]
D. Ananias, J. Rocha, F. A. A. Paz
3+
The two RE centers coordinate to six {GeO
4
} tetrahedra in a
}, with the two
Department of Chemistry
typical distorted octahedral geometry, {REO
6
CICECO – Aveiro Institute of Materials, University of Aveiro, 3810-
1
93 Aveiro, Portugal
adjacent sites sharing an octahedral edge (Figure 1), forming a
dimeric unit with a 3.5716(5) Å intermetallic RE···RE distance.
Consider Table S1 in the supporting Information (SI). The two
E-mail: dananias@ua.pt
rocha@ua.pt
Web page: www.ciceco.ua.pt
{
6
REO } coordination polyhedra have very similar RE–Oaverage
distances and Δmax(RE–O) maximum values: while for RE1 the
RE–O distances range from 2.174(4) to 2.339(4) Å (Δ=0.165 Å),
for RE2 they are in the range 2.175(4)-2.329(4) Å (Δ=0.154 Å).
[b]
L. D. Carlos
Department of Physics
CICECO – Aveiro Institute of Materials, University of Aveiro, 3810-
193 Aveiro, Portugal
Supporting information for this article is given via a link at the end of the
document.
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European Journal of Inorganic Chemistry
10.1002/ejic.201800153
FULL PAPER
equivalent atoms: (i) –x+1/2, y-1/2, z+1/2; (ii) x, y-1, z; (iii) x-1/2, -y+1.5, z;
This similarity is also witnessed by the similar cis and trans O–
(
iv) x-1/2, -y+1/2, z; (v) –x+1/2, y-1/2, z-1/2.
RE–O bond angles (Table S2 in the SI).
4 2 4
Table 1. Crystal and structure refinement data of Na [(Y0.95Eu0.05) Ge O13] (1).
Formula
Formula weight
Crystal system
Space group
a/Å
4 4 13
Na Y1.90Eu0.10Ge O
3097.78
Orthorhombic
Pna2
1
14.7595(7)
5.5203(3)
15.4331(7)
1257.44(11)
1
b/Å
c/Å
Volume/Å
3
Z
-
3
D /g cm
µ(Mo-Kα)/mm
c
4.091
-
1
18.805
Crystal size/mm
Crystal type
θ range
0.06 Í 0.05Í 0.05
Colourless block
3.82 to 29.11
Index ranges
-16 ≤ h ≤ 20
-
-
7 ≤ k ≤ 7
21 ≤ l ≤ 10
5
960
Reflections collected
Independent reflections
Completeness to θ = 25.24º
2767 [Rint = 0.0234]
99.7%
1
0-
Figure 2. Tetrameric [Ge
4
O
13
]
4 2 4
unit in Na [(Y0.95Eu0.05) Ge O13] (1) viewed
down the a-axis of the unit cell, depicting the individual Ge···Ge distances.
Atoms are represented as thermal ellipsoids drawn at the 95% probability level.
Selected bond lengths and angles are given in Tables 2 and 4, respectively.
R1 = 0.0193
wR2 = 0.0446
R1 = 0.0205
wR2 = 0.0450
m = 0.0287
[
a,b]
Final R indices [I>2σ(I)]
[
a,b]
Final R indices (all data)
[
c]
Weighting scheme
n = 1.7109
-
3
The anionic unit comprises four crystallographically independent
0
.578 and -0.737 eÅ
4
-
edge-shared {GeO
4
}
units with inter T-site distances ranging
Largest diff. peak and hole
from 3.076(1) to 3.156(1) Å (Figure 2). Geometrical features of
4
-
a] ꢀ1 = ∑||ꢀ! | − |ꢀ! || ⁄ ꢀ! . [b] ꢀꢁ2 = ∑[ꢀ ꢀ! − ꢀ!! !]
ꢀ
!
[ꢀ ꢀ !! !]. [c]
⁄
the central {GeO4} units are slightly different from the periphery
ones. First, for Ge1 and Ge3 the Ge–O distances are generally
slightly longer and more regular than the distances observed for
Ge2 and Ge4. This is witnessed by the calculated Ge–Oaverage
distances and Δmax(Ge–O) values given in Table S2 in the SI.
Second, it is interesting to note that the less regular Ge2 and
Ge4 tetrahedra also exhibit the smaller dispersion in internal O–
Ge–O tetrahedral angles: while for Ge2 and Ge4 the tetrahedral
angles are in the ranges 102.4(2)-114.3(2)º [Δ=11.9º] and
[
!
!
!
!
= 1 ⁄ ꢀ ꢀ! + ꢀꢁ + ꢀꢁ where ꢀ = (ꢀ! + 2ꢀ! )/3
1
03.3(2)-114.47(19)º [Δ=11.2º], respectively, for Ge1 and Ge3
they spun 101.0(2)-120.39(19)º [Δ=19.4º] and 101.5(2)-122.1(2)º
Δ=20.6º] (Table S4 in the SI). These structural variations are
attributed to the steric pressure imposed on the two central
[
4
-
{
GeO
4
} moieties by the tetrahedral corner sharing to form the
1
0-
tetrameric [Ge
4
O
13
]
unit. Because of this, the two peripheral
moieties are slightly more liberated to accommodate a more
regular tetrahedral environment.
1
0-
The anionic tetrameric [Ge
4
O
13
]
unit interacts strongly with
3
+
+
neighbouring RE and Na cations in the dense crystal structure
4
-
4
of 1. As shown in Figure S1 in the SI, each {GeO } moiety is
3
+
+
connected to three RE centers and, in general, to six Na
+
cations (except for Ge2, which only interacts with five Na ). The
closest Ge···RE distance is ca. 3.54 Å for Ge2, while the longest
distance (ca. 3.74 Å) is observed for Ge3. Concerning the
Figure 1. Schematic representation of the two distorted {REO
coordination environments in Na Ge 13] (1). Atoms forming the
RE coordination sphere are represented as thermal ellipsoids drawn at the
5% probability level. Selected bond lengths and angles are given in Tables
S1 and S2, respectively. Symmetry transformations used to generate
6
} octahedral
+
4
[(Y0.95Eu0.05
)
2
4
O
charge-balancing Na cations, Ge4 exhibits the smallest Ge···Na
3
+
dispersion (ca. 3.22-3.80 Å), with the opposite being observed
for Ge2 (ca. 2.93-3.84 Å) (Figure S1 in the SI).
9
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European Journal of Inorganic Chemistry
10.1002/ejic.201800153
FULL PAPER
[
14]
[15]
The crystal packing of 1 (Figure 3) may be rationalized by
as revealed by Mercury
and PLATON.
The available
+
considering the close packing and mutual interactions between
crystallographic voids are instead occupied by Na cations
(Figure 3c).
1
0-
building units. The anionic tetrameric [Ge
typical herringbone fashion in the ac plane of the unit cell (Figure
a), in what is one of the most efficient ways to head-to-tail close
pack discrete linear units. {Ge 10} binuclear units, in turn, are
also distributed in a herringbone fashion, and establish physical
4
O
13
]
units occur in a
4
n-
To reduce the anionic [RE
2
Ge
4
O
13
]
n
network to nodes and
3
connecting rods based on the structural features previously
1
0-
2
O
described, both the tetrameric [Ge
4
O
13
]
unit and the RE dimer
[
16]
are considered nodes. Using the software package TOPOS, it
is possible to infer that both nodes are six-connected and
equivalent topologically, with the uninodal network exhibiting the
pcu α-Po primitive cubic topological type. The framework
topological description is depicted in Figures 3d and 3e.
1
0-
connections between adjacent tetrameric [Ge
4
O
13
]
units (each
dimer connecting to six symmetry-related anionic units), forming
4
n-
an anionic [RE
2
Ge
4
O
13
]
n
network. This charged network has
no solvent accessible volume (for a typical 1.2 Å probe radius),
1
0-
4n-
Figure 3. (a) Herringbone distribution of anionic tetrameric [Ge
connection between adjacent tetramers via {Ge
Na Ge 13] (1) viewed along the [010] direction of the unit cell. Topological analysis of the inorganic framework: (d) identification of the anionic
tetrameric [Ge
4
O
13
]
units in the ac plane of the unit cell. (b) Anionic [RE
2
Ge
4
O
13
]
n
network formed by
2
10
O } dimers. (c) Mixed polyhedral and ball-and-stick representation of the crystal packing of
4
[(Y0.95Eu0.05
)
2
4
O
10-
4
O
13
]
units and {Ge
2
O
10} dimers as 6-connected nodes; (e) uninodal 6-connected network with the pcu α-Po primitive cubic topological type.
This article is protected by copyright. All rights reserved.

European Journal of Inorganic Chemistry
10.1002/ejic.201800153
FULL PAPER
3
+
5
7
Photoluminescence Properties
ꢀ
300 K) and 12 K monitoring the strongest Eu
D
0
→ F
emission transitions (Figure 5). A broad UV band
dominates the spectra, ranging from 230 to 275 nm and from
2
, and the
3
+
5
7
Tb
4 5
D → F
Trivalent lanthanide ions present exceptional luminescence
properties, ranging from the UV-Visible to the near infrared
3
+
3+
235 to 300 nm for Eu and Tb excitations, respectively. These
bands are attributed to the inter-configurational spin-forbiden
[
17]
regions,
and find major technological applications in lasers,
8
7
1
3+
6
5
1
telecommunications, displays, lighting, pressure sensors,
security inks and marking, barcoding, luminescent thermometry,
4f →4f 5d transition of Tb , and to spin-allowed 4f →4f 5d
3
+
transition of Eu because their energies are very similar to the
[
18]
3+
3+
[2a]
bioimaging and immunoassays.
materials were obtained by replacing 10% of optically silent Y ,
Here, photoluminescent
energy reported for layered Eu and Tb silicates.
The
3
+
additional sharp lines in the spectra of 2 and 3 are ascribed to
3
+
3+
6
7
5
5
5
5
5
used as a diluting element, by Eu and Tb for visible emission,
the intra-4f
F
0,1→ D1-4
,
L
7
6
,
G
2-6
5
,
H
5
3-7 and
F
5
1-5 transitions of
3
+
3+
3+
8
and Yb and Er for near infrared emission. Additionally, the
up-conversion emission is achieved with a sample containing
Eu , and to the intra-4f
F
6
J 7
→ D2-4, G and H transitions of
3
+
Tb .
3
+
3+
only Yb and Er . We have prepared Na
4
[(Y0.9Eu0.1
)
)
2
Ge
Ge
4
O
O
13
]
]
(
(
2), Na
4
[(Y0.9Tb0.1
)
2
Ge
4
O
13
]
(3), Na
4
[(Y0.90Yb0.09Er0.01
2
4
13
8
6
1
4) and Na
4
[(Yb0.9Er0.1
)
2
Ge
4
O
13] (5) (Experimental Section). The
4
f
→
ꢀ4f 5d
experimental powder X-ray diffraction patterns of these samples
are identical and in accord with the powder pattern simulated
form the single-crystal X-ray structure (Figure 4). As shown for 2
Na [(Y 0.9Tb ) G e O13]
(
Figure S2 in SI), these materials are thermally stable up to 1273
K. At 1373 K the crystal structure begins decomposing, resulting
in the crystallization of some Na3.70Ca1.15Ge (ICDD no. 04-
09-4326), where Ca is replaced by RE ions with a decrease
4
0.1
2
4
7
5
→
5
F 6
G ,ꢀ D
J
3
7
5
→
3
O
9
F 6
D 4
2
+
3+
0
6
5
1
+
4f →ꢀ4f 5d
of
the
Na
charge
. At 1473 K, the as-synthesized material is
totally transformed into a mixture of the latter phase and
NaY(GeO ) (ICDD no. 04-008-8315; Figure S2).
balancing
cations,
affording
3 9
Na2.55(Y,Eu)1.15Ge O
Na [(Y 0.9E u0.1) G e O13]
4
2
4
7
5
4
F → L
0
6
7
5
F → D
0
2
7
5
7
5
→
F 0,1→ D 1
F 0
D 3
2
50
300
350
400
450
500
550
W avelengthꢀ(nm)
Na [(Y b0.9E r0.1)2G e O 13]
4
4
Figure 5. Excitation spectra of Na
4 2 4
[(Y0.9Eu0.1) Ge O13] (λEm.= 610 nm) and
Na Ge 13] (λEm.= 551 nm) recorded at room temperature (black
4
[(Y0.9Tb0.1
)
2
4
O
lines) and 12 K (red lines).
Na [(Y 0.90Y b0.09E r0.01)2G e O 13]
4
4
The emission spectra of 2 and 3 recorded at room temperature
and 12 K, excited at 392.5 and 377 nm, respectively, are shown
Na [(Y 0.9T b0.1)2G e O 13]
3+
3+
4
4
in Figure 6. The sharp Eu and Tb emission lines are
5
7
5
7
assigned to the D
The emission spectra of 2 show a dominance of the
0 4
→ F0-4 and D → F6-0 transitions, respectively.
5
7
0 2
D → F
5
7
3+
Na [(Y 0.9E u ) G e O 13]
4
0.1
2
4
over the D
emission spectrum exhibits for the
transitions minimum of five and six Stark components,
respectively, in accord with the presence of at least two low-
0 1
→ F transition. Even at room temperature, the Eu
5
7
5
7
D
0
→ F
1
and
0 2
D → F
a
S imulated
3
+
symmetry Eu environments in the crystal structure. In addition,
the 12 K emission spectra excited at 392.5 and 250 nm (inset in
Figure 6), bears evidence to two distinct sets of three Stark
1
0
20
30
40
50
5
7
components in the
D
0
→ F
transitions. These features are in perfect
agreement with crystal structure, which calls for the presence of
1
region, and the presence of two
2
ꢀT hetaꢀ(degrees )
5
7
very close
0 0
D → F
Figure 4. Experimental and simulated powder X-ray diffraction patterns of
Na Ge 13], Na Ge 13], Na Ge
and Na Ge 13].
3
+
two distorted octahedral RE centers in a dimeric unity. The
4
[(Y0.9Eu0.1
)
2
4
O
4
[(Y0.9Tb0.1
)
2
4
O
4
[(Y0.90Yb0.09 Er0.01
)
2
4 13
O ]
3
+
4
[(Yb0.9Er0.1
)
2
4
O
assignment of the Eu emission features of each site may be
accomplished using the relationship between the covalency of
3
+
the local Eu environments, the nephelauxetic effect (affecting
5
7
[19]
The excitation spectra of Na
4
[(Y1.8Eu0.2)Ge
4
O
13
]
and
the energy of the
D
0
→ F
0
7
transition),
and the energy
7
[20]
Na 13] were recorded at room temperature (ca.
4
[(Y1.8Tb0.2)Ge
4
O
difference between the outer F
1
Stark components, ΔE( F ).
1
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European Journal of Inorganic Chemistry
10.1002/ejic.201800153
FULL PAPER
In accordance, site II is more covalent than site I because it
The excitation spectra of Na
4
[(Y1.8Yb0.18Er0.02)Ge
4
O
13] detecting
5
7
7
3+
3+
presents lower
D
0
→ F
0
transition energy, and lower ΔE( F
1
)
the Er
and the Yb
near infrared emissions at room
3
+
(
inset of Figure 6). However, the similarity of the two RE
temperature and 12 K are shown in Figure 7. The spectra
present a broad charge transfer UV band from 250 to 350 nm.
According to Van Pieterson et al., the associated charge-transfer
process comprises the transfer of an electron from the ligand to
crystallographic sites does not allow an unambiguous
assignment. Although with a residual contribution, at 12 K the
5
7
3+
1
D → F0-4 emission transitions of Eu are observed (Figure S3
5
5
3+
3+
in the SI). We have measured the D
0
and D
4
lifetimes of Eu
the Yb , with reorganization of the charge density distribution on
3
+
5
[21]
and Tb materials using intra-4f excitation at 392.5 ( L
6
level of
the metal ion.
Under excitation in the charge-transfer
3
+
5
3+
3+
Eu ) and 376 nm ( D
3
level of Tb ), respectively. At 12 K, it was
absorption band the 4f levels of Yb are populated via non-
5
3+
3+
possible to measure the
D
0
lifetime of the two individual Eu
radiative relaxation. The Er excitation spectra also show
1
1
3+
sites by selective detection at 589.2 nm (site I: 3.61±0.01 ms)
and 590.6 nm (site II: 3.06±0.01 ms) (Figure S4a in the SI).
However, at room temperature selective detection was not
possible as the decay curves monitored at 589.2 and 590.6 nm
prominent sharp lines, ascribed to intra-4f Er transitions
4
4
4
2
4
between the I15/2 and the F9/2,7/2,5/2,3/2, S3/2, H11/2,9/2, G11/2, and
2
3+
K
15/2 levels. Both at room temperature and 12 K, Er excitation
3
+
3+
is more efficient via the Yb -to-Er energy transfer process
3
+
2
2
are identical (and essentially mono-exponentials). An average
since the Yb
dominates the spectra. On the other hand, at room temperature,
F7/2→ F5/2 transition (from 875 to 1000 nm)
5
0
D lifetime of 3.01±0.01 ms was determined by measuring the
3
+
3+
3+
decay curve at 610 nm (emission maximum). For Tb , selective
detection of the two sites was not possible at both room
temperature and 12 K. Monitoring at 551 nm (emission
maximum), lifetimes of 3.38±0.02 ms (room temperature) and
Er -to-Yb energy transfer also occurs, as witnessed by the
3
+
2
4
2
3+
Er
spectrum. Thus, both Yb -to-Er
transfer processes occur (inset in Figure 7). However, in
K
15/2
,
G
11/2 and
H
3
11/2 levels in the Yb
excitation
+
3+
3+
3+
and Er -to-Yb
energy
3
+
3+
3
.70±0.02 ms (12 K) were obtained (Figure S4b in the SI).
contrast with the Yb -to-Er energy transfer, which arises
2
3+
4
These data, point out the similarity of the lifetimes of the two
between two resonant excited levels ( F5/2 of Yb and
I9/2 of
3
+
3+
3+
4
RE coordination sites.
Er ), the energy gap between the Er donor level ( F9/2) and the
3
+
2
-1
Yb emitting level ( F5/2), ca. 2800 cm , is too large for an
efficient energy transfer process to take place.
5
6
7
D → F
4
4
5
4
F 9/2
4
Na [(Y 0.9Tb ) G e O13]
4
0.1
2
4
I15/2
→
G 11/2
14
12
10
8
2
4
4
5
7
F 5/2
I
9/2
D → F
4
I1
1/2
2
5
7
5
7
2
5
7
H 11/2
D → F D → F
K 15/2
D → F
4
4
4
4
3
2
4
2,1,0
I13/2
6
H 9/2
4
ꢀ
4
4
F 7/2
5
7
5
7
4
D → F 1
7
2
2
4
0
D → F 0
5
F 3/2,ꢀ5/2
S 3/2
0
F 7/2
I15/2
D → F
4
0
2
0
F 9/2
3+
3+
S iteꢀII
Y b
E r
S iteꢀI
5
7
Na [(Y 0.9E u0.1) G e O13]
4 2 4
D → F
0
1
2
2
F 7/2→ F 5/2
halfꢀorderꢀof
theꢀdetec tion
5
7
5
7
D → F
D → F
0
4
0
3
5
80
585
590
595
600
C T
5
00
550
600
650
700
W avelengthꢀ(nm)
3
00
400
500
600
700
800
900
1000
Figure 6. Emission spectra of Na
Na Ge
lines) and 12 K (red lines). The inset shows the Eu
4 2 4
[(Y0.9Eu0.1) Ge O13] (λExc.= 392.5 nm) and
4
[(Y0.9Tb0.1
)
2
4
O
13] (λExc.= 376 nm) recorded at room temperature (black
4 2 4
Figure 7. Excitation spectra of Na [(Y0.90Yb0.09Er0.01) Ge O13] with the
3+ 3+
3
+
5
7
D
1
→ F0,1 transitions
detection fixed at 1010 nm (Yb emission; bottom) and at 1540.5 nm (Er
emission; top) recorded at room temperature (black lines) and 12 K (red lines).
region recorded at 12 K using excitation wavelengths of 376 (red line) and 250
nm (cyan line).
3
+
3+
The inset shows a simplified energy diagram of Yb and Er with the possible
mechanism for the room-temperature energy transfer processes.
Room temperature absolute emission quantum yields were
measured for the red (2) and green (3) emitters. For 2, the
values measured for 254 nm and 393 nm excitation were
Figure 8 shows the near infrared emission spectra of 4 (excited
2
3+
at 903 nm; F5/2 level of Yb ) recorded at room temperature and
2
2
0
.42±0.04 and 0.26±0.03, respectively, while for 3 the values
12 K. The emission lines are assigned to the
F
5/2→ F7/2
3
+
4
4
measured at 254 nm and 377 nm excitation were 0.020±0.002
and 0.22±0.02, respectively.
emission transition of Yb and to the I13/2→ I15/2 transition of
3
+
4
4
Er . The 12 K emission spectrum displays I13/2→ I15/2 local-field
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European Journal of Inorganic Chemistry
10.1002/ejic.201800153
FULL PAPER
structure with, at least, 10 Stark components, again supporting
2
3
+
4H11/2
(
a)
the presence of two distinct optically active Er sites, in accord
with the crystal structure. Upon raising the temperature from 12
K to room temperature, both emissions decrease significantly:
S 3/2
1
6
4
4
4
F 9/2
F 9/2→ I15/2
3
+
whereas the integrated intensity of the Yb transition decreases
12
8
4
2
I9/2
3
+
F 5/2
four fold, the Er transition decreases only 1.5 times. This
distinct temperature dependence of the two transitions is due to
4
I11/2
3
+
3+
the progressive thermal intensification of the Yb -to-Er energy
transfer process.
4
I13/2
4
2
4
0
F 7/2
I15/2
3
+
3+
Y b
E r
4
4
I13/2
→
I15/2
4
4
3
+
S 3/2→ I15/2
ꢀ
ꢀꢀ(E r )
2
2
F 5/2
→
F 7/2
3
+
ꢀ
ꢀꢀꢀ(Y b
)
520
560
600
640
680
W avelenꢀgthꢀ(nm)
(
b)
4
4
F 9/2→ I15/2
0
2
0
.
4
±
.
2
=
ꢀ
1
eꢀ
op
s l
4
4
S 3/2→ I
15/2
1
±0.
3
0
9
50
1000
1050
1100
1450
1500
1550
1600
4
4
.
5
W avelengthꢀ(nm)
ꢀ=
ꢀ
1
S 3/2→ I15/2
ope
0.03
s l
±
6
7
.
1
ꢀ
=
ꢀ
e
p
o
Figure 8. Emission spectra of Na
4
[(Y0.90Yb0.09Er0.01
)
2
Ge
4
O
13] (λExc.= 903 nm)
s l
recorded at room-temperature (black line) and 12 K (red line).
0
.0
0.4
0.8
1.2
-
2
L ogꢀ(densityꢀpowerꢀ/ꢀW cm
)
A preliminary luminescence study of 5 demonstrates that the
Na
property seldom reported for rare-earth crystalline germanates.
Upon 980 nm excitation, Na Ge 13] emits red light
9/2→ I15/2 emission transition (Figure 9a). The
4 2 4
[RE Ge O13] system features up-conversion emission, a
Figure
Na
Wcm (black line) and 6.67 Wcm (grey line) from a 980 nm continuous wave
9. (a) Normalized up-conversion emission spectra of
4
[(Yb0.9Er0.1
)
2
Ge 13] recorded using a power density excitation of 1.33
4
O
−
2
−2
4
[(Yb0.9Er0.1
)
2
4
O
3
+
3+
3
+
4
4
laser diode. The inset depicts Yb /Er partial energy level diagrams showing
attributed to Er
green emissions, H11/2→ I15/2 and S3/2→ I15/2, are also observed
F
3
+
3+
2
4
4
4
the most likely Yb -to-Er energy transfer pathway. (b) Corresponding
3+
double-log plot of the Er red and green up-conversion emission bands
3
+
with much lower intensity. These three visible Er emissions
present a distinct dependence on the excitation power, as
shown by double-log plots of the up-conversion emission versus
power density and determined slopes (Figure 9b).
versus power density.
Conclusions
4 a 2 4
A new rare-earth germanate system, Na [(Y1-aLn ) Ge O13] (Ln =
Eu, Tb or Yb and Er; a = 0.05, 0.1), exhibiting visible and near
infrared emission was synthesized. These materials crystallize in
an orthorhombic non-centrosymmetric Pna2
1
structure, thermally
stable up to 1273 K. The very dense framework is built up of
1
0-
unprecedented anionic tetrameric [Ge
units of {GeO tetrahedra, and isolated dimeric units
encompassing two distinct distorted {REO } octahedral. The
Eu photoluminescence, in particular at 12 K, confirmed the
4
O
13
]
units, tetrameric
4
}
6
3
+
3
+
presence of two very similar RE local sites. However, the
crystallographic similarities of these two sites do not allow an
unequivocal assignment of the individual transition lines and
3
+
lifetimes to each of the observed Eu sites. Incorporation of
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European Journal of Inorganic Chemistry
10.1002/ejic.201800153
FULL PAPER
3
+
3+
Yb and Er shows that this rare-earth germanate system is
also suitable for up-conversion emission.
anisotropic displacement parameters. All structural refinements were
[
29]
performed using the graphical interface ShelXle.
The structure was
found to be disordered concerning the occupancy of the heavy sites: the
3
+
two crystallographically independent RE sites may be occupied by both
3
+
3+
Y
and Eu ; the site occupancy was found from unrestrained refinement
Experimental Section
3+
3+
with complementary rates of occupancy for each Y /Eu pair; upon full
least-squares convergence the respective site occupancy was fixed to
give the molecular formulae quoted for each material. During the
refinement stage, crystals of 1 yielded crystallographic evidence of being
twinned by inversion. During the last stages of the least-squares
refinement, the twin law (-1 0 0, 0 -1 0, 0 0 -1) (inversion operator) was
considered along with one BASF parameter (equivalent to the Flack
Synthesis. Rare-earth germanates were synthesized under static
hydrothermal conditions using Teflon-lined autoclaves, in ovens
-
3
preheated at 503 K. Rare-earth nitrate aqueous solutions (0.4 mol•dm )
were prepared by dissolving the corresponding oxides (99.99% purity in
rare-earth oxide, Jinan Henghua Sci. & Tec. Co. Ltd.) with stoichiometric
amounts of diluted nitric acid under heating.
[
30]
parameter ), which refined to the value in Table 1. The last difference
-
3
Fourier map synthesis showed the highest peak (0.578 eÅ ) and
-
3
deepest hole (-0.737 eÅ ) at 0.99 Å and 0.74 Å from Ge3 and the mixed
site Y1/Eu1, respectively. Structural drawings were created with the
Typical synthesis of Na
crystals for X-ray analysis). An alkaline solution was made by dissolving
.80 g GeO (99.99%, Jinan Henghua Sci. & Tec. Co. Ltd.) with 9.00 g
O and 3.13 g NaOH (99%, AnalaR Normapur®). A mixture of 0.50 ml
4 2 4
[(Y0.95Eu0.05) Ge O13] single-crystal sample (best
[
31]
software package Crystal Impact Diamond.
Information concerning
1
2
crystallographic data collection and structure refinement details is
summarized in Table 1. Detailed tabulated data on bond lengths and
H
2
-
3
-3
Eu(NO
solutions was added to this solution and stirred thoroughly. The gel, with
composition 2.27 Na O: 1.00 GeO : 0.052 Y : 0.0058 Eu : 29.06
O, was reacted in Teflon lined autoclaves (volume 21 cm ) for 6 days
3 3 3 3
) 0.4 mol•dm and 4.50 ml Y(NO ) 0.4 mol•dm aqueous
3
+
4+
angles for the octahedral (RE ) and tetrahedral (Ge ) environments are
given in Tables 2 to 4. Crystallographic information (excluding structure
factors) may be obtained free of charge from Fachinformationszentrum
Karlsruhe, 76344, Eggenstein-Leopoldshafen, Germany (e-mail:
crysdata@fiz-karlsruhe.de), on quoting the depository numbers CSD-
434102.
2
2
2
O
3
2 3
O
3
H
2
at 503 K. After synthesis, the autoclaves were removed from the oven
and quenched in cold water. The final product consisted of single-crystals
and very small particles of an unknown impurity, which were discarded.
The single-crystals were separated by precipitation in five cycles using
distilled water. The final sample was dried at 353
K
overnight.
Ge
13] samples were
General Instrumentation. The crystal phase and purity of the samples
Na
Na
4
[(Y0.9Eu0.1
)
2
Ge
4
O
13],
Na
4
[(Y0.9Tb0.1
)
2
4
O
13],
were established by powder X-ray diffraction data collected at ambient
4
[(Y0.90Yb0.09Er0.01
)
2
Ge
4
O
13] and Na
4
[(Yb0.9Er0.1
)
2
Ge
4
O
temperature on
a
Empyrean PANalytical diffractometer (CuKα1,2 X-
=1.544426 Å) equipped with a fast PIXcel 1D
3
+
synthesized in a similar way, by adjusting the Ln amounts (from 0.4
radiation, λ =1.540598 Å, λ
1
2
-
3
mol•dm nitrate solutions) to the desired concentration in the initial gel,
but ca. 5 mg of pure sample (obtained, as mentioned above, by five-
cycles precipitation) was added to the initial gel as seeds to ensure the
achievement of the pure phase samples. These samples were collected
as pure phases only by simple filtration. The syntheses give close to
detector and a flat-plate sample holder in a Bragg-Brentano para-
focusing optics configuration (45 kV, 40 mA). Intensity data were
collected by the step-counting method (step 0.04º), in continuous mode,
in the 5 ≤ 2θ ≤ 50º range. SEM (Scanning Electron Microscopy) images
were collected either with either a Hitachi S4100 or a high-resolution
Hitachi SU-70, both working at 15 kV. EDS (Energy Dispersive X-Ray
Spectroscopy) data was recorded using the latter microscopes coupled
with a Bruker Quantax 400 or a Sprit 1.9 EDS microanalysis system.
3
+
1
00% yields in terms of the Ln inserted in the parent gel, thus the
relative concentration of lanthanides. The rare-earth compositions of the
samples were ascertained by EDS, which did not show significant
deviations from the nominal synthesis composition, and also showing a
3
+
3+
homogeneous distribution of Ln and Y ions over the crystals. The
morphology of all samples was ascertained by SEM (Figure S5 in the SI).
Photoluminescence. The emission and excitation spectra were
®
recorded on
a
Fluorolog-2
Horiba Scientific (Model FL3-2T)
spectroscope, with a modular double grating excitation spectrometer
(fitted with a 1200 grooves/mm grating blazed at 330 nm) and a TRIAX
320 single emission monochromator (fitted with a 1200 grooves/mm
Single-Crystal X-ray Diffraction Studies. Small single crystals of
compound Na
4 2 4
[(Y0.95Eu0.05) Ge O13] (1) were directly harvested from the
-
1
reaction vessel and immersed in highly viscous FOMBLIN
Y
grating blazed at 500 nm, reciprocal linear density of 2.6 nm·mm ),
coupled to a R928 (visible detection) or a H9170 (near infrared detection)
Hamamatsu photomultiplier, using the front face acquisition mode. The
excitation source was a 450 W Xe arc lamp. The emission spectra were
corrected for detection and optical spectral response of the
spectrofluorometer and the excitation spectra were corrected for the
spectral distribution of the lamp intensity using a photodiode reference
detector. Time-resolved measurements were carried out on a 1934D3
phosphorimeter coupled to the Fluorolog®-3, and a Xe-Hg flash lamp (6
µs/pulse half width and 20-30 µs tail) was used as the excitation source.
The low-temperature measurements were performed using a helium-
perfluoropolyether vacuum oil (LVAC 140/13) purchased from Sigma-
Aldrich. After careful inspection and selection, a suitable single-crystal of
[
22]
the material was mounted on Hampton Research CryoLoops with the
help of a Stemi 2000 stereomicroscope equipped with Carl Zeiss lenses.
Data were collected at 150(2) K on a Bruker X8 Kappa APEX II charge-
coupled device (CCD) area-detector diffractometer (Mo K
α
graphite-
monochromated radiation, λ = 0.71073 Å) controlled by the APEX2
[
23]
software package , and equipped with an Oxford Cryosystems Series
7
00 cryostream monitored remotely using the software interface
[
24]
Cryopad.
Images were processed using the software package
and data was corrected for absorption by the multi-scan
[
25]
-6
SAINT+,
closed cycle cryostat with vacuum system measuring ca. 10 mbar and a
[
26]
semi-empirical method implemented in SADABS.
The crystal structure
Lakeshore 330 auto-tuning temperature controller with a resistance
heater. For the up-conversion measurements, a 980 nm continuous wave
laser diode (CrystaLaser, DL980-3W0-TO) was used as the excitation
was solved using the Patterson synthesis algorithm implemented in
[
27]
SHELXS-97
, which allowed the immediate location of most of the
3
+
2
atoms in the asymmetric unit, in particular the rare-earth (RE ) centers
source, with the laser beam area of 0.15 cm , according to the
4
+
and the tetrahedral Ge sites. All remaining non-hydrogen atoms were
located from difference Fourier maps calculated from successive full-
manufacturer. The laser power was tunable up to a maximum output
power of 3 W.
2
matrix least squares refinement cycles on F using 2017 SHELXL
[
27a, 28]
release.
All non-hydrogen atoms were successfully refined using
This article is protected by copyright. All rights reserved.

European Journal of Inorganic Chemistry
10.1002/ejic.201800153
FULL PAPER
The absolute emission quantum yields were measured at room
temperature using a quantum yield measurement system C9920-02 from
Hamamatsu with a 150 W Xenon lamp coupled to a monochromator for
wavelength discrimination, an integrating sphere as sample chamber and
a multi-channel analyser for signal detection. Three measurements were
made for each sample so that the average value is reported. The method
is accurate to within 10 %.
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[
We would like to thank Fundação para a Ciência e a Tecnologia
(
FCT, Portugal) for funding. This work was developed in the
scope of the project CICECO – Aveiro Institute of Materials
POCI-01-0145-FEDER-007679 (Ref. FCT
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European Journal of Inorganic Chemistry
10.1002/ejic.201800153
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The framework of the new family of
Rare-Earth Germanates*
rare-earth germanates Na
Ln Ge 13] (Ln = Eu, Tb or Yb and
Er; a = 0.05, 0.1) comprises
4
[(Y1-
a
a
)
2
4
O
Duarte Ananias,* Filipe A. Almeida Paz,
Luís D. Carlos and João Rocha*
1
0-
unprecedented [Ge
4
O
13
]
tetramers,
Page No. – Page No.
among other units. These materials
feature interesting photoluminescence
properties in the visible and near-
infrared spectral regions, while
Title
isomorphous Na
4 2 4 13
[(Yb0.9Er0.1) Ge O
features up-conversion emission.
*
one or two words that highlight the emphasis of the paper or the field of the study
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