Cs3reIIIGe3O9 (RE = Pr, Nd, and Sm?Yb) and Cs8TbIII2TbIVGe9O27: A rare example of a mixed-valent Tb(III)/Tb(IV) oxide

Article abstract of DOI:10.1021/acs.inorgchem.9b01033

Single crystals of 12 new cesium rare earth germanates crystallizing in two new structure types were grown from a CsCl/CsF flux. Cs₃REGe₃O₉ (RE = Pr, Nd, and Sm?Yb), a new family of germanates that form for almost the entire series of rare earth elements, crystallizes in orthorhombic space group Pna2₁ with lattice parameters in the following ranges: a = 13.7033(4)?14.022(2) ?, b = 7.0545(2)?7.2405(12) ?, and c = 12.6672(4)?12.836(2) ?. Surprisingly, the Tb reaction yielded both Cs₃TbGe₃O₉ and Cs₈Tb₃Ge₉O₂₇, a rare example of a mixed-valent Tb(III)/ Tb(IV) compound. Cs₈Tb₃Ge₉O₂₇ crystallizes in space group P3 with the following lattice parameters: a = 11.2906(4) ?, and c = 7.9605(3) ?. The mixed-valent oxidation state of Tb was confirmed by structure solution, bond-valence sums, X-ray photoelectron spectroscopy data, and magnetic data. Optical and magnetic properties are reported for both sets of compounds.

Full text of DOI:10.1021/acs.inorgchem.9b01033

Article
Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
pubs.acs.org/IC
III
III
IV
Cs RE Ge O (RE = Pr, Nd, and Sm−Yb) and Cs Tb Tb Ge O : A
3
3 9
8
2
9 27
Rare Example of a Mixed-Valent Tb(III)/Tb(IV) Oxide
Gregory Morrison,†,§ Nicholas R. Spagnuolo, Stavros G. Karakalos,
†,§
‡
,
†
and Hans-Conrad zur Loye*
†
‡
Department of Chemistry and Biochemistry and Department of Chemical Engineering, University of South Carolina, Columbia,
South Carolina 29208, United States
*
S Supporting Information
ABSTRACT: Single crystals of 12 new cesium rare earth
germanates crystallizing in two new structure types were
grown from a CsCl/CsF flux. Cs REGe O (RE = Pr, Nd, and
3
3
9
Sm−Yb), a new family of germanates that form for almost the
entire series of rare earth elements, crystallizes in
orthorhombic space group Pna2 with lattice parameters in
1
the following ranges: a = 13.7033(4)−14.022(2) Å, b =
7
.0545(2)−7.2405(12) Å, and c = 12.6672(4)−12.836(2) Å.
Surprisingly, the Tb reaction yielded both Cs TbGe O and
3
3
9
Cs Tb Ge O , a rare example of a mixed-valent Tb(III)/
8
3
9
27
Tb(IV) compound. Cs Tb Ge O crystallizes in space group
8
3
9
27
P3 with the following lattice parameters: a = 11.2906(4) Å,
and c = 7.9605(3) Å. The mixed-valent oxidation state of Tb
was confirmed by structure solution, bond-valence sums, X-ray photoelectron spectroscopy data, and magnetic data. Optical and
magnetic properties are reported for both sets of compounds.
INTRODUCTION
conditions through the reaction of Tb O and GeO in the
presence of 20 M KOH as a mineralizer.
As uncommon as Tb(IV) oxides are mixed Tb(III)/Tb(IV)
compounds. A limited number of mixed-valent terbium
4
7
2
■
1
7
Rare earth silicates make up an extensively studied class of
materials that have attracted attention due to their potential
applications, primarily as optical and luminescent materials.
1
−8
fluorides have been achieved through the thermal decom-
The rare earth germanates, while offering the same potential
22−25
position of TbF4.
For the oxides, a series of binary phases
9
applications, are considerably less studied. In many cases, the
26−28
exists between Tb O and TbO ,
with “Tb O ”, a mixture
2
3
2
4 7
germanates adopt the same structures as the corresponding
of Tb O and Tb O , being the most commercially available
1
0−12
7
12
11 20
silicates.
However, in some instances, new structure types
terbium oxide. Few higher-order Tb(III)/Tb(IV)-containing
and even compositions exist for the germanates that are not
29
oxides are known.
1
3
known for the silicates. The 12 cesium rare earth germanates
reported in this work crystallize in two new structure types
with no analogous silicate compositions.
We have been exploring the rare earth germanate phase
12,13,30,31
space via flux growth.
In particular, we have found
mixed alkali halide fluxes to be particularly well suited for the
12,13
Typically, the rare earths adopt the +3 oxidation state in
oxides. The most common exceptions to this are cerium, which
synthesis of alkali rare earth germanates.
Recently, we
reported on the synthesis and structures of Ce(IV) germanates
1
3−15
32
readily oxidizes to a +4 state under oxidizing conditions,
and europium, which can be reduced to a +2 state in flux
growth reactions by metal reducing agents such as Zn. The
latter case, Eu(II), occurs due to the resulting half-filled 4f
subshell. Terbium(IV) also contains a half-filled 4f subshell but
is typically considered quite oxidizing in nature and is not
crystallizing in the Wadeite structure type and two Dy(III)
13
germanates that crystallize in related structures. In our
continued exploration of this phase space, we discovered
Cs REGe O (RE = Pr, Nd, and Sm−Yb), a family of
1
6
3
3
9
germanates that crystallize in a new structure type and have no
compositionally analogous silicates. Surprisingly, the Tb flux
1
7
usually obtained under common synthetic conditions.
growth reaction also produced Cs Tb Ge O27, a mixed-valent
8 3 9
1
8
Besides TbO2, few other examples of Tb(IV) oxides exist
Tb(III)/Tb(IV) oxide. Herein, we report the syntheses,
structures, optical properties, and magnetic properties of
1
9−21
and are mostly perovskites and related materials.
Typically, these are produced via solid-state reactions at high
temperatures (1300 °C) and/or under an O atmosphere
Cs REGe O and Cs Tb Ge O .
3
3
9
8
3
9
27
2
1
20
2
to achieve the Tb(IV) state. A non-perovskite-related Tb(IV)
oxide, K TbGe O , was obtained under hydrothermal
Received: April 9, 2019
2
2
7
©
XXXX American Chemical Society
A
DOI: 10.1021/acs.inorgchem.9b01033
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Figure 1. Single crystals of (a) Cs PrGe O , (b) Cs DyGe O , and (c) Cs Tb Ge O .
3
3
9
3
3
9
8
3
9
27
distinguished by color (RE = Pr, Tb, and Ho) or morphology (RE =
Eu and Gd). PXRD data for the ground samples indicated a small,
unidentified impurity in Cs Tb Ge O (see Figure S1). The
EXPERIMENTAL SECTION
■
Reagents. RE O (RE = Nd, Sm−Gd, and Dy−Yb) (99.9%, metal
2
3
8
3
9
27
basis, Alfa Aesar), CsCl (99.9%, VWR), CsF (99.9%, Alfa Aesar), and
remaining samples were phase pure, as indicated by PXRD (Figures
S2−S6).
GeO (99.9%, BeanTown Chemical) were used as received with no
2
further modifications. Tb O and Pr O (99.9%, metal basis, Alfa
4
7
6
11
A phase pure sample of Cs TbGe O was synthesized via a solid-
3
3
9
Aesar) were reduced to Tb O and Pr O under a flow of 5%
2
3
2
3
state reaction. Extra precautions had to be taken due to the
hygroscopic nature of Cs CO and the propensity of Tb(III) to
hydrogen in nitrogen gas in a tube furnace at 800 or 1000 °C,
respectively, for 12 h. The purities of the synthesized rare earth oxides
were confirmed via powder X-ray diffraction (PXRD) data (vide
infra).
2
3
partially oxidize to Tb(IV) when heated in air. Stoichiometric
amounts of GeO and Cs CO (plus 10% extra to account for its
2
2
3
evaporative loss at elevated temperatures) were ground in a glovebox
for 30 min. To decompose the carbonate, this mixture was placed in
an alumina crucible covered by an alumina lid and heated in a furnace
at 450 °C for 8 h and 750 °C for 8 h. The reaction mixture was then
cooled to 250 °C, at which point the crucible was quickly transferred
Synthesis. All compounds were synthesized by molten flux crystal
growth with a CsCl/CsF flux. Cs REGe O (RE = Pr, Nd, and Sm−
3
3
9
Tm) and Cs Tb Ge O were synthesized by layering a mixture of 11
8
3
9
27
mmol of CsCl and 9 mmol of CsF over a mixture of 2 mmol of GeO2
and 0.5 mmol of RE O . The mixtures were placed in either a 14 mL
2
3
back into the glovebox to prevent the reaction of any formed Cs O
2
high-form silver crucible or a cylindrical silver crucible that is 1.2 cm
in diameter and 5.6 cm in height. No differences in reaction products
were observed on the basis of the reaction vessel. Both crucibles were
loosely covered with a silver lid, heated to 900 °C at a rate of 600 °C/
h, held for 12 h, slowly cooled to 400 °C at a rate of 6 °C/h, and
rapidly cooled to room temperature by shutting the furnace off. After
cooling, the resulting crystals were separated from the solidified flux
by dissolving the flux with water aided by sonication for 45−60 min
followed by vacuum filtration. Cs YbGe O was synthesized utilizing
with water vapor to form CsOH. The stoichiometric amount of
Tb O was added, and the mixture was ground for an additional 30
2
3
min. The ground powder was loaded into a pellet dye, pressed into a
pellet (diameter of 12.5 mm) at a load of ∼4.5 t, wrapped in Ag foil,
and sealed under vacuum in a flame-dried fused silica tube. The sealed
tube was heated to 875 °C, held at this temperature for 48 h, and then
cooled back to room temperature by shutting off the furnace. The
fused silica tube was attacked by the Cs vapor, which led the tube to
crack, presumably during cooling (shorter dwell times or lower dwell
temperatures decreased the extent of the attack on the tube and
prevented it from cracking but did not result in a phase pure product).
The resulting pellet contained some yellow material on its surface,
likely an oxidized product that formed after the tube cracked as
previous reactions in which the tube survived did not produce any
yellow product. This material was removed by sanding the pellet with
sandpaper and then sonicating the pellet in acetone to remove the
residual SiC powder from the surface. This process produced a phase
3
3
9
the conditions described above except in a high-form Pt crucible.
Attempts to grow the La analogue of Cs REGe O were unsuccessful.
No attempt was made to grow the Ce analogue due to its propensity
to oxidize to Ce(IV).
3
3
9
All reactions yielded single crystals of Cs REGe O , representative
3
3
9
crystals of which are shown in Figure 1, along with other phases. If the
Tb reaction is excluded, the identified impurity phases are as follows:
Cs RE Ge O (RE = Pr and Nd), RE (GeO ) O or a related
6
2
11 29
9.33
4
6
2
33
apatite (RE = Pr, Nd, and Sm), and RE Ge O (RE = Nd and Sm−
2
2
7
34
pure sample of Cs TbGe O as indicated by PXRD data.
Yb). The reactions performed in the cylindrical crucibles also
produced a large amount of AgCl, which resulted from the flux
attacking the tube. This was removed before vacuum filtration by
several iterations of swirling the reaction mixture in water under
sonication and decanting off the suspended AgCl.
3
3
9
Structure. Structure determination for each compound was
carried out using single-crystal X-ray diffraction (SXRD) data
collected on a Bruker D8 Quest diffractometer equipped with a Mo
Kα microfocus source (λ = 0.71073 Å). The raw data were integrated
3
5
using SAINT+ and corrected for absorption effects using SADABS.
The Tb reaction yielded yellow crystals of Cs Tb Ge O as the
major product (Figure 1c) along with small, colorless crystals of
8
3
9
27
36
An initial structure solution was obtained with SHELXT and refined
using SHELXL in the OLEX2 interface. Both Cs REGe O and
37
38
Cs TbGe O and some unidentified phase(s) indicated by PXRD
3
3
9
3
3
9
Cs Tb Ge O crystallize in noncentrosymmetric space groups and
data. Yb reactions performed in Ag reaction vessels did not produce
any Cs YbGe O but instead produced a cesium silver ytterbium
8
3
9
27
were refined as inversion twins. Crystallographic data for each
compound are listed in Table S1.
3
3
9
germanate that appears to crystallize in space group P1 with lattice
parameters a ≈ 11.23 Å, b ≈ 11.23 Å, c ≈ 27.34 Å, α ≈ 90.42°, β ≈
Powder X-ray diffraction data were collected on a Bruker D2
Phaser equipped with a Cu Kα source (λ = 1.5418 Å). Data were
analyzed using the MDI Jade 9 software. Patterns were indexed using
reflection data from the ICDD PDF-4+ 2018 database and calculated
diffraction patterns based on the CIFs.
9
0.21°, and γ ≈ 119.89° and is comprised of Ge O chains connected
2
6
into a three-dimensional structure by YbO -isolated polyhedra and
6
Yb O face-sharing dimers. Due to the large, low-symmetry unit cell,
2
9
combined with poor crystal quality and the presence of twinning,
attempts to obtain a publishable crystal structure have so far been
unsuccessful. A possible second related compound with similar lattice
parameters except for c ≈ 32.81 Å has also been observed, although
the different c axis length may be the result of further twinning. These
compounds were also observed in the Tm reaction but were less
pervasive.
Elemental analysis via energy dispersive spectroscopy (EDS) was
performed on several analogues of Cs REGe O and Cs Tb Ge O
3 3 9 8 3 9 27
using data collected on a TESCAN Vega-3 SBU instrument equipped
with an EDS detector. Qualitatively, elemental analysis using this data
confirmed the presence of each expected element and did not indicate
the presence of any other elements in the analyzed crystals.
Samples of Cs REGe O (RE = Pr, Eu, Gd, Dy, and Ho) and
X-ray photoelectron spectroscopy (XPS) measurements on
Cs Tb Ge O were performed with a Kratos AXIS Ultra DLD XPS
system using a monochromatic Al Kα source that was operated at 15
3
3
9
Cs Tb Ge O for the measurement of properties were obtained by
8
3
9
27
8
3
9
27
manual separation, where the crystals of the desired phase could be
B
DOI: 10.1021/acs.inorgchem.9b01033
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Table 1. Lattice Parameters and Single-Crystal X-ray Diffraction Refinement Statistics for Cs REGe O (RE = Pr, Nd, and
3
3
9
Sm−Yb), in Space Group Pna2 , and Cs Tb Ge O , in Space Group P3
1
8
3
9
27
a
compound
Cs PrGe O
a (Å)
b (Å)
c (Å)
V (Å3)
R1(F)
Δρmax, Δρmin
14.022(2)
7.2405(12)
7.2281(2)
7.1921(6)
7.1723(2)
7.1594(2)
7.1375(2)
7.1204(3)
7.1054(2)
7.0903(2)
7.0731(3)
7.0545(2)
11.2906(4)
12.836(2)
1303.2(4)
0.0215
0.0254
0.0283
0.0189
0.0196
0.0274
0.0243
0.0290
0.0180
0.0171
0.0247
0.0364
2.591, −1.874
2.979, −2.105
2.884, −2.152
2.678, −1.822
2.611, −1.956
2.805, −2.545
2.774, −2.259
2.532, −2.494
2.169, −1.352
1.801, −1.280
2.162, −1.906
3.049, −3.064
3
3
9
Cs NdGe O
13.9959(5)
13.9327(12)
13.9054(4)
13.8827(3)
13.8466(4)
13.8165(5)
13.7914(3)
13.7625(4)
13.7380(6)
13.7033(4)
11.2906(4)
12.8284(4)
12.8026(11)
12.7751(3)
12.7676(3)
12.7469(4)
12.7299(5)
12.7172(3)
12.6982(3)
12.6837(5)
12.6672(4)
7.9605(3)
1297.77(7)
1282.89(19)
1274.11(6)
1268.99(5)
1259.78(6)
1252.35(9)
1246.20(5)
1239.09(6)
1232.48(9)
1224.54(6)
878.83(7)
3
3
9
Cs SmGe O
3
3
9
Cs EuGe O
9
3
3
Cs GdGe O
3
3
9
9
9
Cs TbGe O
3
3
Cs DyGe O
3
3
Cs HoGe O
9
3
3
Cs ErGe O
9
3
3
Cs TmGe O
9
3
3
Cs YbGe O
9
3
3
Cs Tb Ge O
27
8
3
9
a
For F 2 > 2σ(F ).
2
o
o
Figure 2. Crystal structure of Cs NdGe O showing (a) a silicate chain, (b) the rare earth germanate framework, and (c) the overall structure. Ge
3
3
9
polyhedra are colored gray, Nd polyhedra orange, Cs atoms pink, and oxygen atoms red.
keV and 150 W and a hemispherical energy analyzer. The X-rays were
incident at an angle of 45° with respect to the surface normal. Analysis
was performed at a pressure of 1 × 10− mbar. High-resolution core
level spectra were measured with a pass energy of 40 eV, and analysis
of the data was carried out using XPSPEAK 41 software.
RESULTS AND DISCUSSION
■
Synthesis. Alkali halide fluxes, especially mixed halide
fluxes, have been found to be excellent media for the growth of
9
12,13,41,42
rare earth silicates and germanates.
It has been our
experience that halide flux reaction mixtures containing a
Tb(III) starting material maintain this oxidation state and
produce no mixed oxidation state or Tb(IV) products.
Properties. Ultraviolet−visible (UV−vis) diffuse reflectance data
for Cs TbGe O and Cs Tb Ge O were collected using a
3
3
9
8
3
9
27
43,44
For
PerkinElmer Lambda 35 UV/vis scanning spectrophotometer
equipped with an integrating sphere. Data were collected for the
range of 200−900 nm and converted to absorbance using the
example, in the case of Cs TbSi O F , which was synthesized
3
4
10 2
at 900 °C from a mixture of TbCl and SiO in a CsCl/CsF
3
2
flux, no difference was observed between the resulting products
and the products of reaction mixtures containing a rare earth
with only a +3 oxidation state available. Despite the
similarities between the syntheses of Cs TbSi O F and
39
Kubelka−Munk equation. Fluorescence data for Cs REGe O (RE
3
3
9
=
Eu and Tb) were collected on a PerkinElmer LS55 luminescence
42
spectrometer. For Cs EuGe O , the excitation spectrum was recorded
3
3
9
3
4
10 2
with an emission wavelength of 546 nm and the emission spectrum
with an excitation wavelength of 383 nm. For Cs TbGe O , the
Cs Tb Ge O , the latter reaction readily promotes the
8
3
9
27
3
3
9
oxidation of Tb with Cs Tb Ge O being the dominant
8 3 9 27
emission and excitation wavelengths were 607 and 235 nm,
respectively.
product. This suggests that the germanium plays a role in the
formation of Tb(IV) in this reaction, either by catalyzing the
oxidation of terbium or by stabilizing the +4 oxidation state
once it forms. It is worth noting that the two Tb(IV)-
containing oxides synthesized under crystal growth conditions
that are not considered highly oxidizing, hydrothermally grown
K TbGe O and flux-grown Cs Tb Ge O , are both germa-
Magnetic measurements on Cs REGe O (RE = Gd, Tb, and Ho)
3
3
9
and Cs Tb Ge O were collected on a Quantum Design SQUID
8
3
9
27
magnetometer (QD MPMS3). Samples of ground single crystals with
masses between 15 and 30 mg were massed on a balance sensitive to
0.01 mg and loaded into VSM powder holders. Temperature-
2
2
7
8
3
9
27
17
dependent susceptibility data were collected from 2 to 300 K at an
applied field of 1000 Oe under zero-field-cooled (zfc) and field-cooled
nates.
The Cs Tb Ge O structure type contains Tb(III) and
8 3 9 27
(fc) conditions. No deviation was observed between the zfc and fc
Tb(IV). Among the rare earths, only Ce, Pr, and Tb
commonly adopt both the +3 and +4 oxidation states. While
the Tb reaction yielded both Cs TbGe O and Cs Tb Ge O ,
data. Field-dependent data were collected at 2 K under applied fields
of −5 to 5 T. All raw moments were corrected for sample shape and
3
3
9
8
3
9
27
40
radial offset effects.
the Pr reaction yielded only Cs PrGe O , indicating that either
3
3
9
C
DOI: 10.1021/acs.inorgchem.9b01033
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Figure 3. Crystal structure of Cs Tb Ge O showing (a) the view down the c-axis, (b) a view of a slab parallel to the (100) plane, and (c) the
8
3
9
27
arrangement of Cs atoms within the channels. Ge polyhedra are colored gray, Tb polyhedra purple, Cs atoms pink, and oxygen atoms red.
Polyhedra are lightened in panel c to make the Cs atoms more visible.
Cs Pr Ge O does not exist or the Pr was not oxidized to
2.161(10) Å (see Table 2). The bond-valence sums (BVS)
for the three sites, calculated using the literature parameters for
8
3
9
27
Pr(IV) under our growth conditions. In our experience, Ce
completely adopts the +4 oxidation state in a flux growth
reaction open to the air, meaning a Ce analogue of
Cs Tb Ge O cannot be grown using the reported conditions.
Table 2. Terbium Bond Distances and Bond-Valence Sums
for Cs TbGe O and Cs Tb Ge O
8
3
9
27
3
3
9
8
3
9
27
It is possible that the mixed rare earth compounds
Cs CeRE Ge O exist where Ce(IV) occupies the Tb(IV)
interaction
Cs TbGe O
interaction
Cs Tb Ge O
8 3 9 27
3
3
9
8
2
9
27
Tb−O(1)
Tb−O(2)
Tb−O(4)
Tb−O(5)
Tb−O(7)
Tb−O(8)
BVS
2.288(7)
2.314(6)
2.273(6)
2.276(6)
2.258(7)
2.306(6)
3.17
Tb(1)−O(3) × 3
Tb(1)−O(9) × 3
2.293(9)
2.261(10)
3.10
2.300(9)
2.304(9)
2.89
2.141(10)
2.161(10)
4.35
site and RE(III) occupies the Tb(III) sites (vide infra). A
reaction using a stoichiometric mixture of CeO and Dy O
3
2
2
a
BVS
appeared to produce Cs CeDy Ge O on the basis of powder
8
2
9
27
Tb(2)−O(6) × 3
Tb(2)−O(8) × 3
diffraction data and unit cell determination via SXRD.
However, all of the analyzed crystals were multiply twinned,
inhibiting structure solution.
a
BVS
Tb(3)−O(4) × 3
Tb(3)−O(5) × 3
Structure. Cs REGe O compounds crystallize in ortho-
3
3
9
rhombic space group Pna2 with lattice parameters in the
1
a
BVS
following ranges: a = 13.7033(4)−14.022(2) Å, b =
a
7
.0545(2)−7.2405(12) Å, and c = 12.6672(4)−12.836(2) Å.
Calculated using bond-valence parameters for Tb(III).
Lattice parameters for each analogue are listed in Table 1 and
follow the trend expected due to lanthanide contraction (see
Figure S7). The asymmetric unit contains three cesium sites, a
rare earth site, three germanium sites, and nine oxygen sites.
The RE atom is coordinated by six oxygen atoms to form an
octahedron with bond distances ranging from 2.320 to 2.369
Å, and the germanium atoms are tetrahedrally coordinated by
four oxygen atoms with bond distances ranging from 1.711 to
4
5
Tb(III), are 3.10, 2.89, and 4.35, respectively. This suggests
that Tb(1) and Tb(2) are trivalent and Tb(3) is tetravalent.
Such ordering provides the requisite ratio of oxidation states,
III
IV
Cs Tb Tb Ge O , required for charge balance. The large
8
2
9
27
difference between the assigned oxidation state and BVS for
Tb(3) is likely due to the use of Tb(III) bond-valence sum
parameters.
As bond-valence parameters have not been reported for
Tb(IV), the Tb valence was also analyzed using charge
1
.781 Å, both for the Nd analogue. The GeO tetrahedra
4
corner-share to form zigzag chains with a Ge O repeating
6
18
unit (shown in Figure 2a). These chains are connected
4
6
distribution analysis (CHARDI) calculated by CHAR-
together into a framework by REO octahedra, which corner-
6
47
DI2015. Whereas BVS rely on a set of empirical parameters
share with two GeO4 tetrahedra each from three chains
determined from known structures, CHARDI uses assigned
charges and differences in bond lengths within a single
structure to determine bond strengths and, in turn, formal
oxidation numbers of each atom. As shown in Table 3, good
(
shown in Figure 2b). The connectivity is such that the chains
are aligned with chains that are adjacent in the y-direction but
rotated relative to chains that are adjacent in the x-direction.
The Cs atoms occupy voids within the structure (shown in
Figure 2c). When RE = Pr, Nd, and Sm, disorder is observed in
all three unique Cs sites, with one dominant Cs position
Table 3. Assigned Oxidation States (asgnd ox.) and Formal
Oxidation Numbers (calc. ox.) Calculated Using CHARDI
for Each Tb
(
occupancy of >80%) and one minor Cs position. For the Eu−
Yb analogues, disorder is observed in only two of the Cs sites.
Cs Tb Ge O , shown in Figure 3, crystallizes in space
8
3
9
27
Tb(1)
asgnd
Tb(2)
asgnd
Tb(3)
asgnd
group P3 with the following lattice parameters: a = 11.2906(4)
Å, and c = 7.9605(3) Å. The asymmetric unit contains three
cesium sites, three terbium sites, three germanium sites, and
nine oxygen sites. The Tb atoms are coordinated by six oxygen
atoms to form octahedra. Tb(1) and Tb(2) have bond
distances ranging from 2.261(1) to 2.304(9) Å, whereas Tb(3)
has considerably shorter bond distances of 2.141(10)-
calc.
ox.
calc.
ox.
calc.
ox.
ox.
ox.
ox.
all Tb3+
+3
+3.33
+4
3.07
3.32
3.77
3.07
+3
+3.33
+3
3.07
3.31
3.77
3.07
+3
+3.33
+4
3.32
3.58
4.06
4.06
all Tb3.33+
all Tb4+
charge ordered
+3
+3
+4
D
DOI: 10.1021/acs.inorgchem.9b01033
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Figure 4. XPS data for Cs Tb Ge O showing the Tb 3d photoelectron peak (left) and the Tb 4d photoelectron peak (right). The deconvolution
8
3
9
27
into the Tb4+ and Tb components and the overall fit are shown as pink, teal, and black lines, respectively. The intense peak with a binding energy
3+
of ∼158 eV is the Cs 4p3/2 photoelectron peak.
agreement between the assigned and calculated oxidation
states of all three Tb sites is achieved only when Tb(1) and
Tb(2) are assigned to be +3 and Tb(3) is assigned to be +4.
This agrees with the charge distribution suggested by the BVS.
In Cs Tb Ge O , the germanium atoms are tetrahedrally
8
3
9
27
coordinated by oxygen atoms with bond distances ranging
from 1.719 to 1.806 Å. The germanium tetrahedra corner-
share to form spiral chains with a Ge O repeat unit. These
3
9
chains are connected by TbO octahedra into a framework
6
with the same connectivity observed in Cs REGe O .
3
3
9
However, in Cs Tb Ge O , all of the germanate chains are
8
3
9
27
aligned in the z-direction. This creates channels in the c-
direction in which two of the Cs atoms sit. One of these Cs
sites is disordered with a major position [occupancy of
Figure 5. UV−vis spectra of Cs TbGe O (blue) and Cs Tb Ge O
3
3
9
8
3
9
27
9
3.8(3)%] and a minor position. The two remaining Cs atoms
(green).
lie directly between two terbium octahedra.
To confirm the presence of Tb(IV) in Cs Tb Ge O , XPS
compounds with the dominant emission peaks at 587 and
8
3
9
27
5
7
5
7
data were collected. As shown in Figure 4, the Tb 3d and Tb
d photoelectron peaks are very wide, indicating the presence
607 nm corresponding to the D − F and D − F transitions,
0
1
0
2
4
respectively. The high intensity of the latter relative to the
prior is indicative of the Eu sitting on a site with no inversion
of more than one oxidation state. These peaks can be
deconvoluted into two components each with binding energies
of 148.5 and 151.1 eV (Tb 4d) and 1238.6 and 1241.4 eV (Tb
48
symmetry. The emission spectrum of Cs TbGe O contains
3
3
9
four bands with maxima at 482, 546, 574, and 614 nm,
5
7
5
7
5
7
3
d). These binding energies are in very good agreement with
corresponding to the D − F , D − F , D − F , and
4
6
4
5
4
4
3+
4+
5 7
48
those previously reported for Tb and Tb with the higher-
D − F transitions, respectively. No luminescence was
4
3
4+
29
energy component of each peak corresponding to Tb .
observed in Cs Tb Ge O .
8 3 9 27
Fitting of the deconvoluted peaks provides an oxidation-state
composition of 64% Tb(III) and 36% Tb(IV), which is in
excellent agreement with the expected values.
The magnetic properties of Cs REGe O (RE = Gd, Tb, and
3 3 9
Ho) and Cs Tb Ge O are shown in Figure 7 and
8
3
9
27
summarized in Table 4. All four compounds exhibit para-
magnetic behavior with no magnetic ordering down to 2 K.
Fitting the high-temperature susceptibility data (50−300 K)
with a Curie−Weiss law results in effective moments of
7.73(3) μ /Gd, 9.62(3) μ /Tb, and 10.55(3) μ /Ho for
Properties. Figure 5 shows the UV−vis spectra for the two
Tb analogues. The spectrum for Cs TbGe O is typical of
3
3
9
colorless oxides, where the dominant feature is the absorption
band at low wavelengths due to the band gap, calculated to be
B
B
B
∼
3.4 eV. The spectrum for Cs Tb Ge O exhibits a broad
Cs REGe O , which are in good agreement with the calculated
moments of 7.94(3) μ /Gd, 9.72(3) μ /Tb, and 10.61(3) μ /
B B B
8
3
9
27
3 3 9
absorption band centered at ∼420 nm that corresponds to the
absorption of blue and violet light and explains the yellow
color of the compound. This is in agreement with other Tb
compounds in which both Tb(III)- and Tb(IV)-containing
Ho for the respective trivalent rare earths. The Curie−Weiss fit
for Cs Tb Ge O results in an effective moment of 8.80(3)
8
3
9
27
μB/Tb that is somewhat lower than the calculated moment of
17
compounds are colorless, whereas mixed-valent Tb com-
pounds are light brown or light orange in color.
Figure 6 shows the luminescence properties of Cs REGe O
9
9.17 μ /Tb for a 2/1 Tb(III)/Tb(IV) mix calculated as the
B
25
square root of the squared moments times their respective
fractions. The low observed moment may be the result of the
small, unidentified impurity in the picked sample being
nonmagnetic in nature. The field-dependent magnetization
for each compound is typical of paramagnetic materials. The
3
3
(
RE = Eu and Tb) which are weakly fluorescent and exhibit
their characteristic red and green emissions, respectively. The
emission spectrum of Cs EuGe O is typical of Eu(III)
3
3
9
E
DOI: 10.1021/acs.inorgchem.9b01033
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Figure 6. Fluorescence data for Cs EuGe O (left) and Cs TbGe O (right). Optical images of the two analogues under exposure to 254 nm (Eu)
3
3
9
3
3
9
or 365 nm (Tb) light are shown in the center.
Figure 7. Magnetic properties of Cs REGe O (RE = Gd, Tb, and Ho) and Cs Tb Ge O showing the inverse susceptibility (left) and
3
3
9
8
3
9
27
magnetization as a function of field (right).
Table 4. Magnetic Properties of Cs REGe O (RE = Gd, Tb,
synthetic conditions. As similar reactions using SiO instead of
2
3
3
9
and Ho) and Cs Tb Ge O
GeO2 do not yield any Tb(IV)-containing products, this
suggests that the germanate plays an important role in
oxidizing the Tb O starting material or stabilizing the formed
Tb(IV). For this reason, further exploration of the terbium
germanate phase space is warranted.
8
3
9
27
μcalc (μ /RE)
θ (K)
μeff (μ /RE)
B
B
2
3
Cs GdGe O
7.94
9.72
1.0(3)
−8.7(3)
−5.7(3)
−2.0(3)
7.73(3)
9.62(3)
10.55(3)
8.80(3)
3
3
9
Cs TbGe O
3
3
9
Cs HoGe O
9
10.61
9.17
3
3
Cs Tb Ge O
8
3
9
27
ASSOCIATED CONTENT
■
*
S
Supporting Information
magnetization of Cs GdGe O reaches 6.61 μ /Gd at 5 T,
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.9b01033.
3
3
9
B
close to the calculated saturation magnetization of 7 μ /Gd.
The remaining moments of 3.81 μ /Tb (Cs TbGe O ), 5.36
B
B
3
3
9
μ /Ho (Cs HoGe O ), and 4.72 μ /Tb (Cs Tb Ge O ) are
considerably less than the calculated saturation magnetizations
B
3
3
9
B
8
3
9
27
Powder X-ray diffraction data, crystallographic data, and
plots of the lattice parameters and volume of
Cs REGe O as a function of rare earth (PDF)
of 9 μ /Tb (Cs TbGe O ), 10 μ /Ho (Cs HoGe O ), and
B
3
3
9
B
3
3
9
3
3
9
8
.33 μ /Tb (Cs Tb Ge O ), indicating that the magnetization
B 8 3 9 27
does not saturate up to an applied field of 5 T.
Accession Codes
CCDC 1909033−1909044 contain the supplementary crys-
tallographic data for this paper. These data can be obtained
free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by
emailing data_request@ccdc.cam.ac.uk, or by contacting The
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
CONCLUSIONS
■
Single crystals of Cs REGe O (RE = Pr, Nd, and Sm−Yb)
3
3
9
were grown from a CsCl/CsF flux. The Eu and Tb analogues
were each found to be weakly luminescent, exhibiting the
typical colors and emission spectra for their respective RE
ions. Magnetic measurements were performed on Cs REGe O
9
3
+
3
3
(
RE = Gd, Tb, and Ho), and all three analogues exhibited
AUTHOR INFORMATION
Corresponding Author
E-mail: zurLoye@mailbox.sc.edu.
■
Curie−Weiss behavior down to 2 K.
The Tb flux growth reaction yielded both Cs TbGe O and
3
3
9
*
Cs Tb Ge O , the latter of which is a rare example of a mixed-
valent Tb(III)/Tb(IV)-containing oxide. The mixed-valent
8
3
9
27
ORCID
Gregory Morrison: 0000-0001-9674-9224
Stavros G. Karakalos: 0000-0002-3428-5433
Hans-Conrad zur Loye: 0000-0001-7351-9098
nature of Cs Tb Ge O was confirmed by structure solution,
BVS, XPS data, and magnetic data. Tb(IV) is considered quite
oxidizing in nature and is not usually achieved under common
8
3
9
27
F
DOI: 10.1021/acs.inorgchem.9b01033
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Author Contributions
Article
First Cerium-Containing Cationic Frameworks. Inorg. Chem. 2012,
§
5
(
1, 10083−10085.
16) Abeysinghe, D.; Gerke, B.; Morrison, G.; Hsieh, C. H.; Smith,
M. D.; Pottgen, R.; Makris, T. M.; zur Loye, H.-C. Synthesis,
G.M. and N.R.S. contributed equally to this work.
Notes
̈
The authors declare no competing financial interest.
characterization, and properties of reduced europium molybdates and
tungstates. J. Solid State Chem. 2015, 229, 173−180.
(
17) Fulle, K.; Sanjeewa, L. D.; McMillen, C. D.; Wen, Y.;
ACKNOWLEDGMENTS
■
Rajamanthrilage, A. C.; Anker, J. N.; Chumanov, G.; Kolis, J. W. One-
pot hydrothermal synthesis of Tb
Research was conducted by the Center for Hierarchical
Wasteform Materials (CHWM), an Energy Frontier Research
Center (EFRC). The research was supported by the U.S.
Department of Energy, Office of Basic Energy Sciences,
Division of Materials Sciences and Engineering, under Award
DE SC0016574.
III13
(GeO ) O (OH) and
4 6 7
IV
K Tb Ge O : Preparation of a stable terbium(4+) complex. Inorg.
2
2
7
Chem. 2017, 56, 6044−6047.
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̈
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2 3 2 3
(
Allg. Chem. 1988, 556, 97−108.
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DOI: 10.1021/acs.inorgchem.9b01033
Inorg. Chem. XXXX, XXX, XXX−XXX