Low temperature synthesis of LnOF rare-earth oxyfluorides through reaction of the oxides with PTFE

Article abstract of DOI:10.1016/j.materresbull.2011.12.014

A low temperature solid-state synthesis route, employing polytetrafluoroethylene (PTFE) and the rare-earth oxides, for the formation of the LnOF rare-earth oxyfluorides (Ln = Y, La, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er), is reported. With the exception of LaOF, which forms in a tetragonal variant, rhomobohedral LnOF is found to be the major product of the reaction. In the case of PrOF, a transition from the rhombohedral to the cubic fluorite phase is observed on heating in air to 500 °C. X-ray diffraction shows the expected lanthanide contraction in the lattice parameters and bond lengths. Magnetic susceptibility measurements show antiferromagnetic-like ordering in TbOF, T_m = 10 K, with a metamagnetic transition at a field μ₀H_t = 1.8 T at 2 K. An antiferromagnetic transition, T_N = 4 K, is observed in GdOF. Paramagnetic behavior is observed above 2 K in PrOF, NdOF, DyOF, HoOF and ErOF. The magnetic susceptibility of EuOF is characteristic of Van Vleck paramagnetism.

Full text of DOI:10.1016/j.materresbull.2011.12.014

Materials Research Bulletin 47 (2012) 714–718
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Materials Research Bulletin
journal homepage: www.elsevier.com/locate/matresbu
Low temperature synthesis of LnOF rare-earth oxyfluorides through reaction of
the oxides with PTFE
*
S.E. Dutton , D. Hirai, R.J. Cava
Department of Chemistry, Princeton University, Princeton, NJ 08544, USA
A R T I C L E I N F O
A B S T R A C T
Article history:
A low temperature solid-state synthesis route, employing polytetrafluoroethylene (PTFE) and the rare-
earth oxides, for the formation of the LnOF rare-earth oxyfluorides (Ln = Y, La, Pr, Nd, Sm, Eu, Gd, Tb, Dy,
Ho, Er), is reported. With the exception of LaOF, which forms in a tetragonal variant, rhomobohedral LnOF
is found to be the major product of the reaction. In the case of PrOF, a transition from the rhombohedral
to the cubic fluorite phase is observed on heating in air to 500 8C. X-ray diffraction shows the expected
lanthanide contraction in the lattice parameters and bond lengths. Magnetic susceptibility measure-
ments show antiferromagnetic-like ordering in TbOF, Tm = 10 K, with a metamagnetic transition at a field
Received 7 November 2011
Accepted 9 December 2011
Available online 17 December 2011
Keywords:
A. Inorganic compounds
B. Chemical synthesis
C. X-ray diffraction
D. Crystal structure
D. Magnetic properties
m0Ht = 1.8 T at 2 K. An antiferromagnetic transition, TN = 4 K, is observed in GdOF. Paramagnetic behavior
is observed above 2 K in PrOF, NdOF, DyOF, HoOF and ErOF. The magnetic susceptibility of EuOF is
characteristic of Van Vleck paramagnetism.
ß 2011 Elsevier Ltd. All rights reserved.
1. Introduction
Typically rare-earth oxyfluorides, LnOF, have been made using a
high temperature solid-state synthesis from the sesquioxides,
The Ln3+ cations have generated much interest due to their
unique luminescent properties and potential applications in lasers
[1,2], phosphors [3], and biological markers [4]. In phosphors some
of the most successfulhost lattices are rare-earth oxyfluorides, LnOF,
which have been extensively studied [5–7]. Current work on these
compoundsinvolvesusingnewsyntheticroutestocontrol the shape
and size of the nanoparticles to manipulate the fluorescence [8–14].
The luminescence of Ln3+ materials arises due to crystal electric field
(CEF) effects in the 4f orbitals. These same 4f orbitals are responsible
for the magnetism in Ln3+ containing materials. Rare-earth
magnetism is of particular interest in the pyrochlore system [15–
18]where, atlow temperature, the frustratedlattice has beenshown
to exhibit both spin-ice [15–17] and spin-liquid [18] behavior. In the
Ln3+ pyrochlores, the non-magnetic cations and oxygen displace-
ment have a significant effect on the properties observed [17,18]. In
rhomobohedral LnOF the Ln3+ lattice is a distorted array of face-
sharing Ln3+ tetrahedra that can be considered as a three-
dimensional analogue of the highly geometrically frustrated
edge-sharing triangular planar lattice. The low temperature
magnetism of LnOF may thus be of interest. Only the magnetic
susceptibility of NdOF has been previously reported [19,20].
However, given the sensitivity of the magnetism of the Ln3+ cations
to changes in the crystal structure, the behavior may differ
significantly across the LnOFs.
Ln2O3, and rare-earth fluorides, LnF3 [21,22]. Ceramic synthesis of
LnOF has also been carried out by reacting Ln2O3 and a small excess
of ammonium fluoride, NH4F, at T ꢀ 900 8C [19,20,23,24]. Other
routes to LnOFs include using a sol–gel method [10,11] or
precipitation from solvents [9,14]. Mechanochemical synthesis
of LnOF has also been reported [12], and thin films have been made
using chemical vapour deposition (CVD) [8,13]. Critically these
routes focus on formation of the high temperature fluorite phase
rather than the low temperature rhombohedral one. In this paper
we report a method for the formation of LnOF at low temperatures.
By using stoichiometric amounts of polytetrafluoroethylene
(PTFE), CF2, as the fluorination agent, LnOF can be formed from
Ln2O3 by a simple solid-state reaction. The reaction temperature,
T ꢁ 650 8C, allows for isolation of the low temperature rhombohe-
dral phase. By tuning the reaction temperature we demonstrate
that this simple method allows for the synthesis of the majority of
the rare-earth oxyfluorides. From magnetic measurements we
demonstrate that TbOF has an antiferromagnetic-like ordering
transition at Tm = 10 K with a metamagnetic transition observed at
m
0Ht = 1.8 T at 2 K. We also show antiferromagnetic (AFM)
ordering in GdOF, TN = 4 K. In the other LnOF studied no magnetic
ordering is observed for T > 2 K, although the magnetic suscepti-
bility of EuOF is characteristic of Van Vleck paramagnetism.
2. Experimental
Rare-earth oxyfluorides were synthesized using a solid-state
reaction from their oxides and PTFE powder (CF2). Stoichiometric
* Corresponding author. Tel.: +1 609 258 5556; fax: +1 609 258 6746.
E-mail address: sdutton@princeton.edu (S.E. Dutton).
0025-5408/$ – see front matter ß 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.materresbull.2011.12.014

S.E. Dutton et al. / Materials Research Bulletin 47 (2012) 714–718
715
Table 1
¯
Reaction conditions and lattice parameters for LnOF, obtained from X-ray diffraction measurements at room temperature. Phases have rhombohedral symmetry (R3m) except
where noted.
˚
˚
Ln
Synthesis temp. (8C)
Color
a (A)
c (A)
Impurity phases
Particle size (nm)
Y
500
500
White
White
3.79528 (5)
4.0516 (2)
18.8631 (3)
20.177 (3)
2.12 (10) wt% Y2O3
Tetragonal LaOF main
phase 92 (2) wt%
70 (7)
31 (7)
La
˚
¯
Pr
400
1000
500
550
500
500
400
1000
600
650
500
Cream
White
Blue
Cubic phase, Fm3m, a = 5.66828 (14) A
56 (11)
98 (3)
29 (4)
˚
¯
Pr
Cubic phase, Fm3m, a = 5.66847 (14) A
3.9528 (2) 19.6995 (10)
Nd
Sm
Eu
Gd
Tb
Tb
Dy
Ho
Er
–
White
White
White
Tan
Sm4O3F6 impurity phase, structure not known, no quantitative analysis
3.88286 (8)
3.86469 (11)
3.83959 (12)
3.83966 (8)
3.82052 (9)
3.80085 (6)
3.78125 (7)
19.3274 (5)
19.2516 (7)
19.1134 (7)
19.1158 (5)
18.9988 (5)
18.9000 (4)
18.8075 (4)
–
103 (13)
85 (11)
55 (4)
–
3.0 (3) wt% Tb2O3
White
White
Pink
2.4 (2) wt% Tb2O3
101 (15)
66 (9)
–
–
–
90 (10)
34 (2)
Pink
amounts of the sesquioxide, Ln2O3 (Ln = Y, La, Nd, Sm, Eu, Gd, Tb,
Dy, Ho, Er), and PTFE were intimately mixed and pressed into
pellets. In the case of PrOF and TbOF mixed valence Pr6O11 and
Tb4O7 were used as starting materials rather than the correspond-
ing Ln2O3. The lighter Ln2O3 (Ln = La, Nd, Sm, Eu, Gd) and Pr6O11
oxides were pre-dried at 800 8C prior to reaction. The pellets were
sealed into evacuated quartz tubes and heated for 24 h. The exact
temperature of the reaction depended on Ln and is detailed in Table
1. To remove the residual carbon present after heating, samples
were heated in air at 500 8C for 2 h. To ensure complete reduction
of Ln4+, samples of TbOF and PrOF were then heated in an
evacuated quartz tube for 2 h at 1030 8C.
Tb4O7 were reduced to LnOF, there is no apparent trend in the
temperature of formation of LnOF in the reaction with PTFE.
In all cases, the product was found to be black after the initial
heating. Since LnOF is expected to have a pale color or be white, this
dark shade is ascribed as due to residual carbon. On heating to
500 8C in air a color change was observed for all of the LnOF. With
the exception of PrOF, no significant changes were observed in the
X-ray diffraction patterns. On heating PrOF to 500 8C in air, a phase
¯
transition from the rhombohedral [30] R3m phase to the cubic [31]
¯
Fm3m fluorite structure was observed. The structures of the two
phases are shown in the inset of Fig. 1. In both, the Ln3+ cations are
cubically coordinated, with the cubes sharing edges to form a three
Data for quantitative analysis of the crystal structures were
collected over the angular range 5 ꢁ 2
= 0.028, using a Bruker D8 Focus X-ray diffractometer operating
with Cu K radiation and a graphite diffracted beam monochro-
u
ꢁ 908 with a step size of
2u
a
mator. Rietveld analysis [25] was carried out using the FULLPROF
suite of programs [26]. Backgrounds were fit using a linear
interpolation of data points and then fixed. The peak shape was
modeled using a pseudo-Voigt function. Peak broadening was
modeled using a symmetry dependent Scherrer analysis [27]. This
phenomenological model employs a spherical harmonic expansion
consistent with the Laue class of the crystals [28,29]. The size of the
crystals can then be inferred by comparing the peak widths with
those of a crystalline alumina standard.
Magnetization measurements on selected samples of LnOF
were made on a Quantum Design Physical Properties Measuring
System (PPMS). Magnetic susceptibility,
x = M/H, measurements
were made after cooling in zero field (ZFC) in a 1 T measuring field
2 ꢁ T ꢁ 300 K; for all but TbOF, M was linearly proportional to H,
m
oH ꢁ 1 T, in the measured temperature range. Due to the
presence of a field-induced transition at low temperatures in
TbOF, ZFC and field cooled (FC) magnetic susceptibility measure-
ments were also made in a 0.1 T measuring field. Additionally,
isothermal magnetization measurements, À9 ꢁ
m
oH ꢁ 9 T, were
carried out on TbOF, at selected temperatures, after cooling to the
measuring temperature in a 0 T field using a Quantum Design
PPMS.
3. Results and discussion
For all of the rare-earth oxyfluoride syntheses, formation of
LnOF was observed, and even for YbOF and LuOF, not discussed
here, significant quantities of LnOF were observed in addition to
unreacted Ln2O3. Only the reaction of CeO2 with PTFE resulted in
the formation of a lanthanide fluoride, LnF3. The exact temperature
of the reaction is highly dependent on the Ln element present.
Beyond the relative ease with which mixed valence Pr6O11 and
Fig. 1. X-ray diffraction for (a) cubic PrOF and (b) rhombohedral ErOF. The difference
curve is also shown; reflection positions are indicated by the vertical lines. The
structures for(a)cubicand(b) rhombohedral LnOFareinset. A comparison ofthe peak
width in X-ray diffraction of ErOF and the Al2O3 standard is also inset in panel (b).

716
S.E. Dutton et al. / Materials Research Bulletin 47 (2012) 714–718
dimensional array. The cubic phase of PrOF has the fluorite
structure in which the O2À and FÀ anions are randomly distributed
across the anion sites rather than forming the ordered layers
observed in the rhombohedral phase. The cation ordering in the
rhombohedral structure is accompanied by a small distortion from
the ideal cubic fluorite lattice [22]. This structural transition has
been observed at high temperatures in other LnOF [21]. When the
temperature of the second firing step was reduced to 400 8C for
PrOF, a mixture of both the high and low temperature phases was
observed. Further reduction in the temperature of the second firing
did not remove the residual carbon. In previous studies, the
temperature of transition from the rhombohedral to the cubic
structure in PrOF has been reported to be at ꢀ800 8C [21–23]. This
value is similar to that reported for other LnOF systems [21–23].
The anomalously low temperature of the transition to the fluorite
phase in the current study may be facilitated by the presence of
carbon or Pr4+; alternatively it could be a result of the instability of
the fluorite phase for the larger Ln3+ cations. The latter is supported
by the fact that tetragonal LaOF as the major phase in our synthesis
of the La variant. Like rhombohedral LnOF, this phase has an
ordered anion array but in the absence of a structural distortion has
the tetragonal P4/nmm space group.
Following removal of the residual carbon, samples of TbOF had
a dark tan color that suggested incomplete reduction of Tb4+. This
was confirmed by the change in color to white on heating to
1030 8C. Although a dramatic increase in the particle size was
observed, no significant changes to the crystal structure were seen
following this additional heating (Table 1). On heating to 1030 8C a
less dramatic color change is also observed in PrOF; the cubic phase
of PrOF is maintained although an increase in the particle size is
observed after the additional heating. For simplicity, only the
structure and properties of fully reduced PrOF and TbOF will be
discussed below.
rhombohedral LnOF was not previously reported, the structure of
the closest LnOF was used for a starting point and the lattice
parameters taken from the literature [22,24]. In the case of SmOF
the presence of a Sm4O3F6 impurity phase [22] with an unknown
structure was observed and thus, due to significant overlap
between peaks from the two phases, a quantitative analysis of the
SmOF structure was not attempted. Due to the relatively small
contribution of the anions to the X-ray diffraction pattern, the
isotropic temperature factor, Biso, for the anion sites had no
significant effect on the refinement and Biso for both sites was
2
˚
therefore fixed at the arbitrary value of 0.7 A . The expected
lanthanide contraction is observed in the size of the unit cell for
rhomobohedral LnOF (Fig. 2). Consideration of the Ln–Ln distances
also indicates the expected lanthanide contraction (Fig. 2). Some
deviations from the expected behavior are observed in the Ln–O
and Ln–F bonds, this is most likely due to the large error in
refinement of the position of the lighter O2À and FÀ ions using X-
ray diffraction, rather than the result of deviations from the
anticipated trend. In all cases significant broadening of the
diffraction peaks is observed. This is due to the small particle
size of the LnOF product and is a consequence of the low
temperature of the reaction. A significant range in the size of the
LnOF particles is observed, from 103 nm in EuOF to 29 nm in NdOF.
From our experiments it appears that the LnOF phases are
relatively stable, and thus it might be possible to tune the size
of the particles using either the reaction temperature or a post
formation anneal step for greater control of the particle size. For
example, in NdOF a size range from 29 to 86 nm particles can be
produced depending on the exact reaction conditions; the larger
particles form in reactions with PTFE at 800 8C.
The magnetic susceptibility of selected samples of LnOF (Ln = Pr,
Nd, Gd, Eu, Dy, Ho, Er) is shown in Fig. 3. In the case of PrOF NdOF,
DyOF, HoOF and ErOF the expected paramagnetic behavior is
observed above 2 K. EuOF is also paramagnetic but due to low lying
excited states has a temperature dependendent susceptibility
characteristic of Van Vleck paramagnetism. The behavior of the
A quantitative structural analysis of the rhombohedral LnOF
formed was carried out based on powder X-ray diffraction data,
and is detailed in Tables 1 and 2. For cases where the structure of
Table 2
Atomic positions and selected bond lengths for rhombohedral LnOF obtained from room temperature X-ray diffraction.
Ln
Y
Ln 6c 00z
z
O 6c 00za
z
F 6c 00za
z
Rwp
x2
Ln–Ln
Ln–O
Ln–F
2
˚
Biso (A )
0.7418 5 (6)
2.00 (5)
0.6211 (4)
0.8685 (3)
14.9
25.8
20.6
26.1
29.2
27.5
24.7
13.3
11.4
1.18
1.23
1.06
1.05
1.05
1.07
1.08
1.31
1.20
3.79528 (4) Â 6
3.5842 (13) Â 3
4.0882 (14) Â 3
3.9282 (12) Â 6
3.751 (3) Â 3
4.254 (3) Â 3
3.88286 (6) Â 6
3.682 (2) Â 3
4.174 (2) Â 3
3.86469 (8) Â 6
3.661 (4) Â 3
4.163 (4) Â 3
3.83959 (8) Â 6
3.633 (3) Â 3
4.137 (3) Â 3
3.83966 (6) Â 6
3.627 (3) Â 3
4.144 (3) Â 3
3.82052 (9) Â 6
3.610 (2) Â 3
4.117 (2) Â 3
3.80085 (4) Â 6
3.591 (2) Â 3
4.095 (2) Â 2
3.78125 (5) Â 6
3.5717 (11) Â 3
4.0762 (11) Â 3
2.278 (8)
2.389 (6)
2.261 (2) Â 3
2.435 (3) Â 3
Nd
Eu
Gd
Tb
Tbb
Dy
Ho
Er
0.74221 (12)
0.74224 (11)
0.74205 (16)
0.74195 (14)
0.74176 (12)
0.74186 (10)
0.74187 (6)
0.74182 (5)
2
0.82 (6)
0.04 (6)
0.10 (8)
1.57 (8)
0.71 (7)
0.40 (5)
1.5 (4)
0.6217 (13)
0.6199 (11)
0.6198 (15)
0.6197 (12)
0.6190 (11)
0.6185 (10)
0.6208 (6)
0.6194 (5)
0.8697 (11)
0.8700 (10)
0.8704 (14)
0.8669 (10)
0.8690 (9)
0.8729 (9)
0.8688 (5)
0.8684 (5)
2.37 (2)
2.51 (2)
2.360 (6) Â 3
2.524 (9) Â 3
2.36 (2)
2.471 (19)
2.310 (5) Â 3
2.475 (8) Â 3
2.35 (3)
2.47 (2)
2.298 (7) Â 3
2.463 (11) Â 3
2.34 (2)
2.39 (2)
2.282 (5) Â 3
2.477 (9) Â 3
2.35 (2)
2.43 (2)
2.278 (5) Â 3
2.462 (8) Â 3
2.34 (2)
2.48 (2)
2.265 (5) Â 3
2.419 (7) Â 3
2.290 (11)
2.399 (10)
2.263 (3) Â 3
2.436 (4) Â 3
0.95 (3)
2.301 (9)
2.381 (9)
2.246 (2) Â 3
2.428 (4) Â 3
a
˚
Biso fixed at 0.7 A .
b
TbOF sample heated to 1000 8C for 2 h.

S.E. Dutton et al. / Materials Research Bulletin 47 (2012) 714–718
717
Fig. 2. Variation of (a) the lattice parameters and (b) the Ln–Ln bonds in
rhombohedral LnOF as derived from X-ray diffraction. The Ln framework in
rhombohedral LnOF is inset, short (green) intermediate (red) and long (black) Ln–Ln
bonds are shown. Lines are a guide to the eye; unless shown error bars are smaller
than the data points. (For interpretation of the references to color in this figure
legend, the reader is referred to the web version of the article.)
magnetic susceptibility of Eu2O3 and Eu-doped Y2O3 has also been
attributed to Van Vleck paramagnetism [32–35] and so this is not
unexpected for Eu3+ cations. At high temperatures the suscepti-
bility of GdOF is similar to that of the other LnOF measured,
however at low temperatures a maximum in the susceptibility is
observed. We attribute this to the AFM ordering of the Gd3+ cations
at TN = 4.0 (5) K. This transition temperature, though high, is of the
correct order for an AFM transition of a 4f7 Ln3+ ion. The effective
Fig. 3. Magnetic susceptibility,
x
, as a function of temperature for (a) cubic PrOF,
NdOF and EuOF and (b) GdOF, DyOF, HoOF and ErOF. The inverse susceptibility is
inset. The antiferromagnetic transition in GdOF is shown in more detail in the inset
of panel (b).
magnetic moment,
from the measured magnetic susceptibilities using the Curie-
Weiss law, ). To minimize the contribution of CEF
effects to the susceptibility, only the low temperature data,
T < 20 K, were fit; in the case of GdOF only data above TN were
considered. The values obtained are shown in Table 3, for all LnOF
m
eff, and Weiss temperature,
u
, were obtained
unrealistically high,
u
< À20 K, which we attribute to CEF effects
rather than the strength of the true magnetic interactions between
Ln3+ ions; these have thus been omitted from the tabulated values.
The paramagnetic susceptibility in NdOF and its relationship to
changes in the crystal structure and CEF have previously been
discussed in some detail [19,20].
x
À
x
o = C/(T À
u
the effective magnetic moments,
m
eff, are close to the theoretical
Unlike the other LnOF studied, the magnetic susceptibility of
TbOF shows evidence for two magnetic transitions; a sharp peak
at 10 K and a change in the gradient of the magnetic susceptibility
at 5 K (Fig. 4a). At higher temperatures the magnetic susceptibility
of TbOF obeys the Curie-Weiss law with the fitted parameters
(Table 3) consistent with those expected for Tb3+ ions. At low
temperatures, the sharp transition and the absence of hysteresis
between the ZFC and FC susceptibility indicates the presence of
long-range three-dimensional ordering, Tm = 10 K. Three-dimen-
sional ordering at such a high temperature is unusual in a Ln3+
magnetic lattice. In TbOF, this may be due to a significant FM
component in the interactions, elevating the ordering tempera-
ture. The large spin of the Tb3+, 4f 8, cations could also raise Tm.
Isothermal magnetization measurements indicate complex mag-
netic behavior below the transition(s) (Fig. 4b) with maxima in
values and within the range observed in other Ln3+ containing
systems. It should be noted that, since the measurements were
made on powder samples of LnOF, the Curie constant and Weiss
temperature are powder averaged values; as such any anisotropy
in the magnetism cannot be detected [36]. No attempts to fit the
Van Vleck paramagnetism of EuOF have been attempted. In the
case of NdOF and cubic PrOF, the obtained Weiss temperature is
Table 3
Magnetic characterization of LnOF.
Ln
meff
(
mB per fu)
u
(K)
a
a
Pr (cubic)
3.34
3.53
Nd
Gd
Tb
Dy
Ho
Er
7.84
À2.4
À6.4
À6.5
À7.8
À0.08
dM/d
m
0H observed at Æ1.8 T at both 2 and 8 K (Fig. 4c). At higher
9.61
fields, the isothermal magnetization below 10 K saturates to ꢀ7.5 mB
per Tb, lower than the expected saturation magnetization for Tb3+ of
8.5 mB. The negative Weiss temperature, u = À6.4 K and the magnetic
susceptibility in 0.1 T suggest that in low fields the magnetic
ordering has a dominant AFM component which on application of a
10.56
10.71
9.59
a
Significant CEF effects result in a large AFM Weiss temperature; the values
obtained have hence been omitted.

718
S.E. Dutton et al. / Materials Research Bulletin 47 (2012) 714–718
experiments are required to further explore the nature of the
magnetic ordering in TbOF.
Our results demonstrate the use of a new solid-state route for
the formation of LnOF by reaction of the rare-earth oxides and
PTFE. By tuning the temperature of reaction, we show that LnOF
can be produced for nearly all of the rare-earth elements.
Furthermore, unlike other synthetic routes, reaction with PTFE
results in the formation of the anion ordered low temperature
rhombohedral phase.
Acknowledgments
The authors would like to acknowledge helpful discussions
with Shuang Jia, Ni Ni, Esteban Climent-Pascual and Collin
Broholm. This research was supported by the U.S. Department
of Energy, Office of Basic Energy Sciences, Division of Materials
Sciences and Engineering under Award DE-FG02-08ER46544.
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