Syntheses, Crystal Structures, NMR Spectroscopy, and Vibrational Spectroscopy of Sr(PO3F)·H2O and Sr(PO3F)

Article abstract of DOI:10.1002/ejic.201501143

Single crystals of Sr(PO₃F)·H₂O {P2₁/c, Z = 4, a = 7.4844(2) ?, b = 7.0793(2) ?, c = 8.4265(2) ?, β = 108.696(1)°, V = 422.91(2) ?³, 2391 F_o², 70 parameters, R₁[F² > 2σ(F²)] = 0.036; wR₂(F² all) = 0.049, S = 1.054} were grown from an aqueous solution by a metathesis reaction. The structure comprises [SrO₈] polyhedra and PO₃F tetrahedra that form a layered arrangement parallel to (100). The topotactic dehydration of this phase proceeds between 80 and 140 °C to afford Sr(PO₃F). The monazite-type crystal structure of Sr(PO₃F) was elucidated from the X-ray powder data by simulated annealing [P2₁/c, Z = 4, a = 6.71689(9) ?, b = 7.11774(11) ?, c = 8.66997(13) ?, β = 128.0063(7)°, V = 326.605(8) ?³, R_p = 0.010, R_wp = 0.015, R_F = 0.030]. During dehydration, the structure of Sr(PO₃F)·H₂O collapses along [100] from a layered arrangement into a framework structure, accompanied by a change of the coordination number of the Sr²⁺ ions from eight to nine. The magic-angle spinning (MAS) NMR and vibrational spectroscopy data of both phases are discussed.

Full text of DOI:10.1002/ejic.201501143

DOI: 10.1002/ejic.201501143
Full Paper
Phosphors
Syntheses, Crystal Structures, NMR Spectroscopy, and
Vibrational Spectroscopy of Sr(PO3F)·H2O and Sr(PO3F)
Stephan G. Jantz,[a] Leo van Wüllen,[b] Andreas Fischer,[b] Eugen Libowitzky,[c]
Enrique J. Baran,[d] Matthias Weil,*[e] and Henning A. Höppe*[a]
Abstract: Single crystals of Sr(PO3F)·H2O {P21/c, Z = 4, a = the X-ray powder data by simulated annealing [P21/c, Z = 4, a =
7.4844(2) Å, b = 7.0793(2) Å, c = 8.4265(2) Å, ꢀ = 108.696(1)°, 6.71689(9) Å, b = 7.11774(11) Å, c = 8.66997(13) Å, ꢀ =
2
V = 422.91(2) Å3, 2391 Fo , 70 parameters, R1[F2 > 2σ(F2)] = 128.0063(7)°, V = 326.605(8) Å3, Rp = 0.010, Rwp = 0.015, RF =
0.036; wR2(F2 all) = 0.049, S = 1.054} were grown from an aque- 0.030]. During dehydration, the structure of Sr(PO3F)·H2O col-
ous solution by a metathesis reaction. The structure comprises lapses along [100] from a layered arrangement into a frame-
[SrO8] polyhedra and PO3F tetrahedra that form a layered ar- work structure, accompanied by a change of the coordination
rangement parallel to (100). The topotactic dehydration of this number of the Sr2+ ions from eight to nine. The magic-angle
phase proceeds between 80 and 140 °C to afford Sr(PO3F). The spinning (MAS) NMR and vibrational spectroscopy data of both
monazite-type crystal structure of Sr(PO3F) was elucidated from phases are discussed.
Introduction
Therefore, by this approach, the emission of structure dopant
Eu2+ ions can be tuned accordingly.
Monofluorophosphates are isoelectronic with the corre-
sponding sulfates; they are interesting owing to their high tech-
nological relevance, for example, as toothpaste additives, active
agents during the biomineralisation of fluoroapatite, solubility
inhibitors for lead in potable water sources, wood preservatives
or corrosion inhibitors.[2] In systematic studies devoted to struc-
ture determinations and solid-state NMR investigations of
monofluorophosphates with closed-shell d10 metal cations, we
have so far elucidated the properties of Ag2(PO3F),[10]
Hg2(PO3F)[11] and Ba(PO3F).[2] In addition to that of Ba(PO3F),
the crystal structure of another alkaline-earth representative,
namely, Ca(PO3F)·2H2O, has already been determined.[12] For
Sr(PO3F)·H2O, various preparation methods towards polycrystal-
line samples as well as their thermal behaviour have been re-
ported;[13–17] with respect to structural data, only a set of lattice
parameters was given.[17]
In this contribution, we report the syntheses and crystal
structures of Sr(PO3F)·H2O and its intermediate thermal decom-
position product Sr(PO3F). To unequivocally show the presence
of P–F bonds, to exclude the presence of P–OH bonds and to
prove a possible O/F ordering in the tetrahedral (PO3F)2– anions,
which is difficult to derive solely from X-ray diffraction experi-
ments, we also included careful solid-state NMR and vibrational
spectroscopy studies. Electrostatic calculations based on the
Madelung part of lattice energy (MAPLE) concept confirmed the
spectroscopic and diffraction data.
In the course of our investigations of silicate-analogous materi-
als[1–8] focussing on alkaline-earth compounds as potential host
structures for doping with divalent europium or manganese
ions for application as phosphors, we started a systematic study
of the crystal structures of substituted phosphates (PO3X)m–
(X = S, m = 3; X = F, m = 2). For instance, we clarified the
chemical and crystallographic properties of the monothiophos-
phates NaM(PO3S)·9H2O (M = Mg, Ca, Ba),[1,7] which are subject
to decomposition at relatively low temperatures owing to the
evaporation of H2S. Although thio derivatives with the (PO3S)3–
anion would provide a softer coordination environment for lu-
minescent inner or outer transition-metal ions, fluorido deriva-
tives with the (PO3F)2– anion should act as harder ligands in the
sense of the hard and soft acids and bases (HSAB) concept.[9]
[a] Lehrstuhl für Festkörperchemie, Institut für Physik, Universität Augsburg,
Universitätsstr. 1, 86159 Augsburg, Germany
E-mail: henning.hoeppe@physik.uni-augsburg.de
http://www.ak-hoeppe.de
[b] Lehrstuhl für Chemische Physik und Materialwissenschaften, Institut für
Physik, Universität Augsburg,
Universitätsstr. 1, 86159 Augsburg, Germany
[c] Institute for Mineralogy and Crystallography, Faculty of Geosciences,
Geography and Astronomy, University of Vienna,
Althanstr. 14, 1090 Vienna, Austria
[d] Centro de Quımica Inorganica (CEQUINOR/CONICET, UNLP), Facultad de
Ciencias Exactas, Universidad Nacional de La Plata,
C. Correo 962, 1900 La Plata, Argentina
[e] Institute for Chemical Technologies and Analytics, Division Structural
Chemistry, Vienna University of Technology,
Results and Discussion
Getreidemarkt 9/164-SC, 1060 Vienna, Austria
E-mail: matthias.weil@tuwien.ac.at
Crystal Structure of Sr(PO3F)·H2O
http://www.cta.tuwien.ac.at/sc/experimental_solid_state_chemistry/
Supporting information and ORCID(s) from the author(s) for this article are
available on the WWW under http://dx.doi.org/10.1002/ejic.201501143.
Strontium monofluorophosphate hydrate, Sr(PO3F)·H2O (1),
crystallises in a new structure type in the monoclinic space
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group P21/c. All of the atoms in the asymmetric unit occupy
positions on Wyckoff site 4e. The Sr2+ cations are arranged in
zigzag chains running parallel to [001] with Sr–Sr distances of
4.47 Å. The fluorophosphate anions are aligned in rows along
the same direction with alternating orientations (up or down)
of the PO3F tetrahedra with P–P distances of 4.44 Å (Figure 1).
Figure 1. The crystal structure of Sr(PO3F)·H2O viewed along [001]; strontium
grey, oxygen red, fluorine green, hydrogen white; the (PO3F)2– ions are drawn
as closed tetrahedra.
Figure 2. Coordination environment of the Sr2+ ions in Sr(PO3F)·H2O (the col-
our scheme is the same as that used in Figure 1); all non-H atoms are drawn
with their displacement ellipsoids at a 75 % probability level.
The Sr2+ ions are eightfold-coordinated by oxygen atoms
only, two of which stem from water molecules (OW), and the
remaining six originate from five PO3F tetrahedra. The Sr–O
bond lengths range from 2.48 to 2.82 Å, and the Sr–OW bond
lengths lie between 2.58 and 3.00 Å. The average Sr–O bond
length of 2.67 Å is in excellent agreement with the sum of the
effective ionic radii of 2.64 Å.[18] The resulting [SrO8] polyhedron
might be described as a snub disphenoid (Johnson polyhedron
J50), as shown in Figures 2 and S6 (Supporting Information).
The oxygen and fluorine atoms in the (PO3F)2– anion are well-
ordered. The P–F bond is significantly longer than the P–O
bonds, and the O–P–O angles are slightly larger than the O–P–
F angles. Selected interatomic distances and angles of 1 are
listed in Table 1. These are typical values for PO3F tetrahedra
and match those determined for related monofluorophos-
phates.[2,10,12,19] A compilation of the lengths of the longest P–F
and the longest P–O bonds in numerous other fluorophosphate
structures with metal or organic countercations was given re-
cently.[20] The deviation of the PO3F moiety in Sr(PO3F)·H2O
from an ideal tetrahedron is very small (–0.4 %) and was calcu-
lated by applying the method suggested by Balic–Žunic and
Makovicky on the basis that the positions of all experimentally
determined ligands are enclosed by a sphere leading to an
idealised tetrahedron's volume, which is then compared with
the experimentally determined volume.[21,22] A short descrip-
tion of the method has been given previously.[8]
Table 1. Relevant ranges of interatomic distances [Å] and angles [°] in
Sr(PO3F)·H2O and Sr(PO3F) [the estimated standard deviations (esds) are in
parentheses].
Sr(PO3F)·H2O
Sr(PO3F)
Sr–O
Sr–OW
Sr–F
P–O
P–F
2.4828(11)–2.8202(11)
2.5752(11)–3.0039(13)
–
1.4945(11)–1.5186(11)
1.5801(9)
2.515(4)–2.944(4)
–
2.683(3)–2.869(4)
1.528(5)–1.539(4)
1.576(3)
O–P–O
O–P–F
110.21(6)–116.13(7)
103.90(6)–105.53(6)
110.2(4)–117.5(5)
101.3(3)–110.2(3)
ter molecules and the oxygen and fluorine atoms of the PO3F
unit. The distances between the donors (D) and the oxygen and
fluorine atoms as acceptor (A) atoms range between 2.7 and
3.0 Å (Table 2) and, hence, point to the possible formation of
medium–strong H···F or H···O hydrogen bonds; however, the
corresponding O–H···F angles are below 110°, which makes a
significant interaction between water and fluorine relatively un-
likely.[23,24] Consequently, the adhesion of the layers seems to
be entirely dominated by O–H···O hydrogen bonds. This obser-
vation follows a general trend that hydrogen bonds of the type
O–H···F do not play a significant role in water- or OH-containing
Table 2. Possible hydrogen bonds in Sr(PO3F)·H2O; D = donor, A = acceptor.
A distinct feature of the crystal structure of Sr(PO3F)·H2O is
its layered character. The [SrO8] polyhedra share edges and cor-
ners to form sheets parallel to (100), and the (PO3F)2– anions
are attached below and above these sheets. The fluorine atoms
of the PO3F tetrahedra point towards the interlayer space. Adja-
cent sheets are held together by hydrogen bonds between wa-
D–H
A
d(D–H) [Å]
d(H–A) [Å]
∠DHA [°]
d(D–A) [Å]
OW–H1
OW–H2
OW–H2
OW–H2
O3
O2
F
0.95
0.97
0.97
0.97
1.92
1.91
2.31
2.48
141
166
108
110
2.717
2.863
2.771
2.953
F
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fluorophosphates.[25–29] Overall, one has to consider that the tine) crystallises isotypically with baryte in the space group
positions of the hydrogen atoms determined by the X-ray dif- Pbnm.
fraction analysis are somewhat uncertain; therefore, only a
highly probable scenario is suggested in this discussion.
Synthesis of Sr(PO3F)
Unlike the previously mentioned monothiophosphates
NaM(PO3S)·9H2O (M = Mg, Ca, Sr,[7,30] Ba), which decompose
by the loss of H2S upon mild heating, Sr(PO3F)·H2O dehydrates
without the evaporation of HF to yield phase-pure Sr(PO3F)
above 80 °C. In a previous investigation on the thermal behav-
iour of Sr(PO3F)·H2O, it was shown by simultaneous thermogra-
vimetry and mass-spectroscopic measurements that the de-
composition of the hydrous phase follows a multistage mecha-
nism until the end-products α-Sr2P2O7 and the apatite-type
phase Sr5(PO4)3F form above 750 °C.[16] For the intrinsic dehy-
dration, the authors determined a decomposition range of 80–
140 °C and a mass loss corresponding to ca. 0.8 mol of water
Figure 3. Observed (circles) and calculated (line) X-ray powder diffraction pat-
tern (Cu-Kα radiation) as well as the difference profile of the Rietveld refine-
under the chosen conditions (heating range 10 °C/min, flowing
Ar atmosphere). Both the previously reported decomposition
range and the partial dehydration were confirmed by our obser-
vations. However, the dehydration of Sr(PO3F)·H2O itself ap-
pears not to follow a simple one-step mechanism. Differential
scanning calorimetry (DSC) measurements revealed at least four
well-resolved endothermic effects that accompany the loss of
water in the temperature range 80–140 °C (Figure S1). Whether
this multistage mechanism points to a stepwise (partial) dehy-
dration or accompanying structural rearrangements or both
could not be resolved with our experimental methods. The de-
hydration seems to be controlled kinetically and can be moni-
tored at different heating rates and heating times of the sam-
ples. A partial dehydration occurs if the material is heated
quickly or only for a short time above the onset of the dehydra-
tion temperature (as in the previous thermogravimetric
study[16] or for the preparation of samples for solid-state NMR
studies, see below), whereas slow heating rates and the reten-
tion of the chosen temperature for several hours or even days
lead to complete dehydration. The differential weighing of a
hydrous sample treated at 115 °C for two weeks under atmos-
pheric conditions revealed a dehydration degree of more than
99 %. This sample did not contain any remaining crystal water
as evidenced by infrared spectroscopy. The as-prepared
Sr(PO3F) material is stable for at least five months when stored
under ambient conditions, as shown by repeated powder X-ray
diffraction measurements.
ment of Sr(PO3F); the row of vertical lines indicates the possible peak posi-
tions of Sr(PO3F).
The close similarities between the crystal structures of the
hydrous and anhydrous strontium fluorophosphates point to-
wards a topotactic dehydration reaction, and this situation re-
sembles the dehydration of layered SrTeO3·H2O to yield the
framework structure of ε-SrTeO3.[33]
In Sr(PO3F), the formerly layered arrangement of
Sr(PO3F)·H2O transforms into a framework structure owing to
the removal of water molecules. The similarities between both
become apparent through the observation of the similar lattice
parameters b and c in the two structures, the same type of
zigzag chains of strontium cations (Sr–Sr distance 4.62 Å) and
the same alignment of the PO3F tetrahedra (P–P distance
4.59 Å) in an up-and-down orientation parallel to [001] in 2
(Figure 4).
Figure 4. Representation of the crystal structures of Sr(PO3F)·H2O (left) and
Sr(PO3F) (right) viewed along [001] (the colour scheme is the same as that
used in Figure 1); with the loss of H2O during dehydration, a shrinks signifi-
cantly, whereas b and c change only slightly; the shown coordinate system
applies for both unit cells.
Crystal Structure of Sr(PO3F)
The crystal structure of the anhydrous phase was solved from
The collapse of the crystal structure of 1 along [100] is re-
the X-ray powder diffraction data by simulated annealing; the flected in drastic changes to a (change of 0.77 Å) and the
resulting structure model was refined by the Rietveld method monoclinic angle (change of 19.3°). During the dehydration, the
(Figure 3). Sr(PO3F) (2) crystallises in the monoclinic space PO3F tetrahedra are tilted with respect to the strontium cation,
group P21/c with all atoms occupying general Wyckoff sites 4e the coordination number of which changes from eight to nine
and adopts a monazite-type crystal structure.[31] This is in con- in the anhydrous phase. The resultant coordination polyhedron
trast to the assumption that fluorophosphates are isotypic to is a distorted triaugmented triangular prism composed of seven
the corresponding isoelectronic sulfates,[13,32] as SrSO4 (celes- oxygen atoms and two fluorine atoms (Johnson polyhedron
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J51). Five PO3F tetrahedra act as monodentate ligands through over, calculations with different assignments of the fluorine and
oxygen or fluorine atoms, and two act as bidentate ligands oxygen atoms in the PO3F unit yielded significantly larger devi-
through oxygen and fluorine atoms (Figure 5 and Figure S6 in ations and, thus, confirmed the O/F ordering of our structure
the Supporting Information).
model.
Table 3. Result of the MAPLE calculations for Sr(PO3F) compared with those
for SrO (ICSD no. 109461),[34] P2O5 (ICSD no. 40865),[35] and SrF2 (ICSD no.
40414).[36]
MAPLE (SrPO3F)
MAPLE (SrO)
MAPLE (P2O5)
MAPLE (SrF2)
1/2 MAPLE (SrO + P2O5 + SrF2)
Δ
24703 kJ/mol
3762 kJ/mol
42887 kJ/mol
2790 kJ/mol
24719.5 kJ/mol
0.1 %
Solid-State NMR Spectroscopy
To verify the presence of P–F bonds as well as the O/F ordering
experimentally, 19F and 31P solid-state magic-angle spinning
(MAS) NMR spectra were recorded. For 19F–31P bonds, the sig-
nals in the 19F and 31P spectra should be split into doublets as
1
a consequence of the JP, F coupling. The 31P MAS NMR spectra
for both title compounds are shown in Figure 6. For
Sr(PO3F)·H2O (Figure 6, upper spectrum), a doublet centred at
δ ≈ –3.4 ppm with a 1JP, F coupling of (890 10) Hz is observed,
whereas the spectrum of SrPO3F (Figure 6, lower spectrum)
comprises a doublet [1JP, F = (840 10) Hz] centred at δ ≈
–2.1 ppm. The spectra were recorded with the cross-polarisa-
tion (CP) of the 31P nuclei in a 31P{19F} CPMAS NMR experiment
without 19F decoupling during acquisition. We note that the
single-pulse acquisition spectra exhibit the presence of some
minor phosphorus-containing impurities (phosphates); how-
ever, the cross-polarisation of 31P from 19F produces spectra in
which only those 31P species with direct proximity to 19F nuclei
contribute. Thus, the spectra confirm the presence of only one
phosphorus position in each compound. The observed values
Figure 5. The coordination environment of the strontium ions in the crystal
structure of Sr(PO3F); the colour scheme is the same as that used in Figure 1.
The Sr–O bond lengths range from 2.52–2.94 Å, and the Sr–
F bond lengths range from 2.68 to 2.87 Å. The average coordi-
nation distances of 2.78 Å for the Sr–F bonds and 2.62 Å for
the Sr–O bonds are in reasonable agreement with the sum of
the effective ionic radii[18] of 2.64 Å for Sr–O and 2.62 Å for Sr–
F bonds.
1
for JP, F are typical for monofluorophosphates[2,11,40,41] and
show the presence of only one phosphorus position in each
The PO3F tetrahedron in the anhydrous phase shows basi-
cally the same features as that in the hydrous phase. The P–F
bond length is longer than the average P–O bond length (1.58
vs. 1.53 Å), and the O–P–O angles are larger than the O–P–F
angles. The deviation from an ideal tetrahedron in 2 is some-
what larger than that in 1 and amounts to –1.3 %. However,
this larger deviation is presumably caused by the less accurate
structure determination from powder diffraction data rather
than by structural reasons. Selected interatomic distances and
angles of Sr(PO3F) are listed in Table 1.
Electrostatic Calculations
We checked our structure model of Sr(PO3F) for electrostatic
reasonability by using calculations based on the MAPLE con-
cept.[37–39] A structure model is considered as electrostatically
consistent if the sum of the MAPLE values of chemically similar
compounds deviates from the MAPLE value of the compound
of interest by less than 1 %. According to our calculations, the
structure model shows electrostatic consistency (Table 3). More-
Figure 6. 31P{19F} CPMAS NMR spectra (no 19F decoupling) for Sr(PO3F)·H2O
(top) and SrPO3F (bottom).
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compound. The 19F MAS NMR spectra (not shown) also confirm
the presence of PO3F anions in both compounds. Here, we find
doublets centred at δ = –70.0 pm [1JF,P = (880 20) Hz] for 1
and a signal at δ = –64.5 ppm [1JF,P = (840 20) Hz] for 2.
The downfield shift by 5.5 ppm for 2 indicates a lower elec-
tron density around the F atom. This is in accordance with the
coordination to the Sr2+ ion in 2 instead of the H+ ion in 1. In
both spectra, an additional broad signal at δ ≈ –87 ppm is
observed. The observed chemical shift and the considerable
line width indicate the presence of small amounts of SrF2 (δiso
=
–84.5 ppm, strong 19F–19F dipolar interactions)[42] not detecta-
ble by powder XRD.
Vibrational Spectroscopy
The attenuated total reflectance (ATR) IR spectra of 1 and 2
are well-resolved (Figure 7) and show similar patterns, although
some of the bands in the spectrum of the anhydrous material
present additional splittings, which suggest the presence of cor-
Figure 8. Raman spectrum of Sr(PO3F)·H2O.
exclusion principle becomes operative, as the IR and Raman
active modes belong to phonons of different parity.
relation field effects. The broad bands in the region ν = 3600–
˜
Table 4. Factor-group analysis of the (PO3F)2– vibrations of the two investi-
gated strontium monofluorophosphates.[a]
3100 cm–1 and ν ≈ 1660 cm–1 can be assigned to the OH va-
˜
lence and deformation vibrations in 1, and these bands are
absent in the spectrum of 2.
Vibrational
mode
Free anion
Site symmetry Factor group
(C3v
)
(C1)
(C2h)
ν1
ν2
ν3
ν4
ν5
ν6
ν(P–F)
A1
A1
A1
E
E
E
A
A
A
2A
2A
2A
Ag+Bg+Au+Bu
Ag+Bg+Au+Bu
Ag+Bg+Au+Bu
2Ag+2Bg+2Au+2Bu
2Ag+2Bg+2Au+2Bu
2Ag+2Bg+2Au+2Bu
νs(PO3)
δ(FPO3)
νas(PO3)
δ(PO3)
ρ(PO3)
[a] Activity: free anion: A1 and E: IR and Raman; site symmetry: A: IR and
Raman; factor group: Ag, Bg: Raman; Au, Bu: IR.
The simplicity of the vibrational spectra of 1 and 2 suggests
that no significant correlation field effects operate in these
cases; therefore, the spectroscopic analysis can be performed
on the basis of the site-symmetry approximation, and the pro-
posed assignment is shown in Table 5.
Table 5. Assignment of the vibrational spectra of Sr(PO3F)·H2O and Sr(PO3F);
band positions given in cm–1 units.
Infrared
Raman
Assignment
Sr(PO3F)·H2O
Figure 7. ATR infrared spectra of Sr(PO3F)·H2O (black) and Sr(PO3F) (red; dried
in air at 115 °C for two weeks).
3380 (vs), 3270 (sh)
1660 (w)
3381 (w)
1647 (vw)
ν(OH)
δ(H2O)
The Raman spectrum of Sr(PO3F)·H2O (Figure 8) is also well-
resolved, whereas that of the anhydrous fluorophosphate is
poorly resolved (Figure S2) owing to very high background fluo-
rescence and was, therefore, not useful for this analysis.
Taking into account the mentioned characteristics of the
Sr(PO3F) spectra, it appears useful to perform an initial factor-
group analysis of the investigated lattices[43–45] on the basis of
the known structural data and to correlate the symmetry of the
“free” (PO3F)2– anion (C3v) with its site symmetry (C1) and factor
group (C2h). The results of this correlation are shown in Table 4,
and it becomes evident that the three doubly degenerate E
modes are split under site-symmetry conditions, and all vibra-
tions present IR and Raman activities. Furthermore, under fac-
1168 (m), 1106 (vs)
1013 (s)
801 (s)
657 (w)
512 (vs)
1181(vw), 1160 (w), 1130 (w) ν4, νas(PO3)
1022 (vs), 997 (w)
816 (m)
668 (vw)
ν2, νs(PO3)
ν1, ν(P–F)
see text
567 (w), 545 (w)
399 (w), 380 (w)
ν5, δ(PO3)
ν6, ρ(PO3)
Sr(PO3F)
1192 (m), 1125 (vs), 1080 (vs)
1011 (vs), 995 (sh)
844 (w), 749 (m)
564 (sh), 546 (sh), 514 (s)
434 (w)
ν4, νas(PO3)
ν2, νs(PO3)
ν1, ν(P–F)
ν5, δ(PO3)
ν3, δ(FPO3)
Regarding the vibrations of the water molecules, the O–H
tor-group symmetry, additional splittings are predicted, and the stretching modes are seen as a unique relatively strong band
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with a weak shoulder on the lower-energy side. The position of are always observed between the corresponding IR and Raman
these bands is characteristic for the presence of hydrogen bands. Such differences are considered as a valuable criterion
bridges of medium strength[46] and correlates[47] very well with for the evaluation of the strength of the coupling effects in
the lengths determined for the hydrogen bonds in this struc- crystal structures[48,49] and confirm that they are relatively weak
ture (Table 2).
in the present case. For 2, the correlation field effects are more
evident from the spectroscopic behaviour; although the factor
predictions are not totally fulfilled, significant splitting is gener-
ally observed for all infrared bands.
In agreement with the predictions of the site-symmetry anal-
ysis, the antisymmetric ν(PO3) vibration presents two clear IR
components. In the Raman spectrum, three relatively weak sig-
nals are observed in this region; this suggests the presence of
weak correlation field effects in this case. The corresponding
symmetric stretching vibration is the strongest Raman band
and is accompanied by a very weak satellite band; this band is
also relatively strong in the IR spectrum.
Conclusions
The crystal structures of Sr(PO3F)·H2O and the monazite-type
Sr(PO3F) were determined by single-crystal and powder X-ray
diffraction, respectively. The structural changes during the top-
otactic dehydration of the hydrous phase were followed by
complementary spectroscopic methods (MAS NMR and vibra-
tional spectroscopy), and DSC measurements showed very
good agreement with the structural data. The very intense fluo-
rescence of anhydrous Sr(PO3F), possibly caused by defects
generated during the dehydration, and the concomitant very
low resolution of the Raman spectra recorded at the red and
blue laser lines did not allow a reasonable assignment of the
corresponding Raman bands. As such a strong fluorescence was
not observed in the Raman spectrum of Sr(PO3F)·H2O, the
change of the coordination environment of the Sr2+ ion from
coordination number eight and an all-oxygen environment to
coordination number nine with two fluorine ligands seems to
have a crucial influence and makes Sr(PO3F) a promising host
for phosphor materials. Systematic studies to incorporate differ-
ent dopants and dopant concentrations in the Sr(PO3F) struc-
ture are underway.
The ν(P–F) vibration can be clearly identified in both spectra
at a slightly higher energy than that observed in the respective
solution Raman spectrum (ν = 795 cm–1).[46] The different posi-
˜
tion of the ν(P–F) bands at somewhat lower wavenumbers for
2 can be explained by the coordination of a F– ion to a Sr2+
ion, which only occurs in 2 (Sr–F 3.32–3.52 Å in 1, Sr–F 2.68–
2.87 Å in 2). Hence, the P–F bond strength is reduced in 2, and
the ν(P–F) vibration can be excited at lower energies.
The very weak IR band at ν = 657 cm–1 with a very weak
˜
Raman counterpart at ν = 668 cm–1 is probably a combination
˜
band. For the deformational modes, only δ(PO3) could be iden-
tified as a very intense but unique IR band; in the Raman spec-
trum, this band is found as a clearly split vibration at somewhat
higher wavenumbers, as predicted (cf. Table 4). No signals for
the δ(FPO3) mode could be found. In the Raman spectrum of a
(PO3F)2– solution, both vibrations are reported at the same en-
ergy (ν = 520 cm–1),[46] although both vibrations were identified
˜
at slightly different wavenumbers for crystalline Hg2(PO3F) with
ν5 > ν3.[41] The corresponding ν6 PO3 rocking mode was identi-
fied as a very weak doublet in the Raman spectrum only.
Experimental Section
Synthesis of Polycrystalline Sr(PO3F)·H2O: Polycrystalline
Sr(PO3F)·H2O was synthesised in aqueous solution by metathesis.
The proposed assignment for the IR spectrum of Sr(PO3F) is
presented in Table 5. The two ν(PO3) vibrations are clearly split;
for the antisymmetric mode, three of the four factor-group-pre-
dicted vibrations (2Au+2Bu) are observed, whereas the two ex- Na2(PO3F) (200 mg, 1.39 mmol, Aldrich, purity 95 %) was dissolved
in water (ca. 7 mL), and a solution of Sr(NO3)2 (294 mg, 1.39 mmol,
Fluka, ≥99 %) or SrCl2·6H2O (371 mg, 1.39 mmol, Aldrich, 99 %) in
water (ca. 7 mL) was added dropwise. The colourless product pre-
cipitated upon the addition of acetone (ca. 100 mL, VWR, technical
grade) and was collected by filtration (yield ca. 89 %). Alternatively,
(NH4)2(PO3F)·H2O (prepared according to ref.[50]) can be used as the
fluorophosphate source. The addition of concentrated aqueous
SrCl2 or Sr(NO3)2 solutions to a concentrated (NH4)2(PO3F) solution
also affords the direct precipitation of Sr(PO3F)·H2O. All reactions
were performed in polytetrafluoroethylene (PTFE) beakers. The pu-
rity of the freshly prepared product was checked by powder X-ray
diffraction. The observed intensities were in very good agreement
with the diffraction pattern calculated from the single-crystal data
(Figure S3).
pected components (Au+Bu) are seen for the symmetric mode.
In the poorly defined Raman spectra (Figure S2), only a weak
doublet with components at ν = 1015 and 996 cm–1 can be
˜
observed. These bands clearly represent the two expected com-
ponents (Ag+Bg) of the νs(PO3) mode. As expected, the ν(P–F)
vibration is also seen as a well-defined doublet, and the mean
value of the observed components practically coincides with
the solution value measured by Raman spectroscopy (ν =
˜
795 cm–1).[46]
In this case, we have again assigned ν5 > ν3, and this last
vibration has been tentatively assigned to the weak feature at
ν = 434 cm–1. The vibrational mode ν is found as a single
˜
5
strong band with two weak shoulders on the higher-energy
side, that is, three of the four predicted components (2Au+2Bu)
are observed.
Growth of Sr(PO3F)·H2O Single Crystals: To obtain single crystals,
two crystal-growth procedures were employed. Firstly, the reaction
was performed as described above; however, instead of fast precipi-
tation through the addition of acetone, the solution was allowed to
evaporate overnight in a desiccator with silica gel as a drying agent,
and a few block-shaped colourless crystals were obtained. Secondly,
Ag2(PO3F)[10] was initially dissolved in water. To this solution, the
In summary, the observed vibrational spectroscopic behav-
iour of 1 is in good agreement with the site-symmetry predic-
tions, although evidence for the presence of correlation field
effects is observed in some spectral regions. These effects also
become evident through the fact that small energy differences stoichiometric amount of an aqueous SrCl2 solution was added. The
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1126 © 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Paper
precipitated AgCl was removed by filtration, and the resulting solu- through the different bond lengths within the PO3F tetrahedron, in
tion was left to evaporate to dryness under ambient conditions
over several days; again, block-shaped colourless crystals could be
isolated.
analogy with previously determined fluorophosphates structures.
All hydrogen atoms were located by difference Fourier syntheses
and refined isotropically with fixed Uiso = 0.03 Å2 and restrained O–
H distances of 0.99(1) Å. The details of the X-ray data collection are
summarised in Table 6. The positional and displacement parameters
for all atoms are listed in the Supporting Information (Tables S1
and S2).
Synthesis of Sr(PO3F): Polycrystalline Sr(PO3F) was obtained by de-
hydration of Sr(PO3F)·H2O in air at 80–115 °C in a compartment
dryer. This method yielded Sr(PO3F) of poor crystallinity. Through
very slow heating during a temperature-dependent X-ray powder
diffraction measurement, we obtained a sample of much higher
crystallinity (Figure S4) that was suitable for structure determination
and subsequent Rietveld analysis (Figure 3). A Hilgenberg glass cap-
illary was filled with finely ground Sr(PO3F)·H2O and mounted on a
Bruker D8 Advance powder diffractometer equipped with a furnace.
The sample was heated at 0.1 °C/s to the desired temperature and
held there for 3 h, during which a diffraction pattern was acquired;
this procedure was repeated every 5 °C between 47 and 117 °C
(Figure S5).
Powder X-ray Diffraction: The powder X-ray diffraction data were
collected with a Bruker D8 Advance diffractometer equipped with
a furnace and a capillary mounting system by using Cu-Kα radiation
(LynxEye 1-D detector, steps of 0.2°, acquisition time 12–17 s/step,
Soller slits 4°, fixed divergence slit 1 mm, transmission geometry).
The generator was driven at 40 kV and 40 mA. The samples were
finely ground and prepared in Hilgenberg glass capillaries (outer
diameter 0.3 mm). The X-ray powder diffraction pattern of Sr(PO3F)
was recorded in the 2θ range 5.0–140.0° with steps of 0.2° and an
acquisition time of 17 s/step. Profile fitting and structure solution
through simulated annealing were performed with EXPO2013.[52,53]
The obtained structure model was refined with the program suite
FullProf and the graphical user interface WINPlotR.[54] The results
are summarised in Table 6 and shown in Figure 3. The atomic coor-
dinates and their respective isotropic displacement parameters are
listed in the Table S3.
Crystal-Structure Determination: The single-crystal X-ray diffrac-
tion data were collected at room temperature with a Bruker D8
Advance diffractometer equipped with a SMART APEXII 4k CCD de-
tector by using Ag-Kα radiation; the data were corrected for absorp-
tion by applying a multiscan approach. The crystal structure of
Sr(PO3F)·H2O was solved by direct methods with the SHELXTL pro-
gram package[51] and refined with anisotropic displacement param-
eters for all non-H atoms; the F and O atoms could be distinguished
Further details on the crystal structure investigations may be ob-
tained from the Fachinformationszentrum Karlsruhe, 76344 Eggen-
stein-Leopoldshafen, Germany (fax: +49-7247-808-666; e-mail:
crysdata@fiz-karlsruhe.de), on quoting the depository numbers
CSD-429339 [Sr(PO3F)·H2O] and -429338 [Sr(PO3F)].
Table 6. Crystal data and details of structure refinements.
Sr(PO3F)·H2O
Sr(PO3F)
T /K
293(2)
203.61
monoclinic
P21/c, 14
7.4844(2)
7.0793(2)
8.4265(2)
108.6960(10)
422.913(19)
4
293(2)
185.59
M /g mol–1
Crystal system
Space group, no.
a /Å
MAS NMR Spectroscopy: The 19F and 31P MAS NMR spectra were
recorded with a Varian 500 NMR spectrometer with Larmor frequen-
cies of 470.6 (19F) and 202.3 MHz (31P) at a spinning speed of 35 kHz
and a Varian 1.6 mm MAS NMR probe. A π/2 pulse length of 2 (19F)
or 2.5 μs (31P) was used. The 19F spectra were referenced to CFCl3,
and the 31P spectra were referenced to H3PO4. For the 31P{19F}
CPMAS NMR experiment, contact times of 5 ms were employed.
Relaxation delays of 1200–1800 s were used for the acquisition of
the spectra.
monoclinic
P21/c, 14
6.71689(9)
7.11774(11)
8.66997(13)
128.0063(7)
326.605(8)
4
b /Å
c /Å
ꢀ /°
V /Å3
Z
ρX-ray /g cm–3
Crystal size/ mm3
Colour / shape
μ /mm–1
F(000)
3.198
3.774
0.10 × 0.08 × 0.05
colourless block
7.054
Vibrational Spectroscopy: A Bruker Tensor 27 FTIR spectrometer
384
combined with a Harrick MVP2 Series diamond ATR accessory was
Radiation (λ /Å)
Diffractometer
Ag-Kα (0.56087)
Bruker D8 Advance
Cu-Kα
Bruker D8 Ad-
vance
used to record spectra in the ν = 4000–370 cm–1 region. The Raman
˜
spectra were recorded with a Renishaw RM 1000 micro-Raman sys-
tem in the spectral range ν = 4000–40 cm–1. The 632.8 nm excita-
˜
Absorption correction
Min./max. transmission
Index range
multiscan
tion line of a 17 mW HeNe laser was focused on the sample surface
with a 50×/0.75 objective lens. The Raman intensities were collected
with a thermoelectrically cooled CCD array detector. The resolution
0.8441/1.000
–13/–12/–14–12/12/14
2.27–29.50
Θ range /°
2.5–70.0
1278
33
of the system was 4 cm–1, the wavenumber accuracy was 1 cm–1
,
Collected reflections
28333
both calibrated with the Rayleigh line and the 520.5 cm–1 line of a
Si standard. Owing to the very high fluorescence of anhydrous
Sr(PO3F) under the red laser light, this sample was additionally
Independent data/restraints/ 2391/2 /70
parameters
Observed reflections
(I > 2σ), Rint
1947, 0.045
47 background
points
RP = 0.010
Rwp = 0.015
measured with a blue laser in the spectral region ν = 1600–40 cm–1,
˜
R (all data)
R1 = 0.036
but a spectrum with similar very high fluorescence was obtained
(Figure S2).
wR2 = 0.049
w–1 = [σ2(Fo2) + (0.0224P)2 RF = 0.030
+ 0.1278P]
2
P = (Fo + 2Fc2)/3
1.054
RBragg = 0.038
Differential Scanning Calorimetry (DSC): For the DSC measure-
ments, a NETZSCH DSC 200 F3 Maia system was employed; alumin-
ium pans with pierced lids were filled with the samples and heated
at a heating rate of 2.5 °C/min in a flowing argon atmosphere
(20 mL/min) in the temperature range 30–230 °C.
GooF
Min./max. residual
density /e Å–3
ꢀ2
–0.54/1.11
8.94
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2002, 628, 1749–1755.
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Keywords: Main group elements · Layered compounds ·
Solid-state structures · Structure elucidation · Topotactic
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Received: October 3, 2015
Published Online: February 8, 2016
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