High-pressure phase transitions of LaF3 and CeF3

Article abstract of DOI:10.1023/A:1027361811994

The effect of high pressure on the structure of LaF₃ (tysonite), CeF₃, and nonstoichiometric LaF₃-based solid solutions (rhombohedral system, P3c1, Z = 6) is discussed. In the systems MF ₂-LaF₃ with M = Pb and Sr, fluorite-like M _1-xL_axF_2+x solid solutions (Fm3m, Z = 4) are obtained in the composition range 0.5 ≤ x ≤ 0.75 by quenching from 1500-1700 K at p = 10 GPa. By extrapolating the composition dependences of the lattice parameter for these cubic solid solutions, the lattice parameter of cubic LaF₃ is estimated at a = 5.86 ± 0.02 A. High-pressure x-ray diffraction studies indicate that CeF₃ undergoes a first-order phase transition similar to that revealed recently in LaF ₃. At p = 20.6 GPa, the lattice parameters of orthorhombic CeF ₃ (Cmma, Z = 8) are a = 8.062 ± 0.006 A, b = 8.457 ± 0.009 A, and c = 5.091 ± 0.006 A. The transition is accompanied by a 7.3% decrease in volume. The high-pressures structure of LaF₃ and CeF₃ can be thought of as a distorted modification of the structure of a hypothetical fluorite-like trifluoride. The lattice parameters of the orthorhombic and cubic cells are related by a _ortho ～ b_ortho ～ √2 a_cubic and c_ortho ～ a_cubic.

Full text of DOI:10.1023/A:1027361811994

Inorganic Materials, Vol. 39, No. 11, 2003, pp. 1198–1202. Translated from Neorganicheskie Materialy, Vol. 39, No. 11, 2003, pp. 1384–1388.
Original Russian Text Copyright © 2003 by Dyuzheva, Lityagina, Demishev, Bendeliani.
High-Pressure Phase Transitions of LaF and CeF
3
3
T. I. Dyuzheva, L. M. Lityagina, G. B. Demishev, and N. A. Bendeliani
Vereshchagin Institute of High-Pressure Physics, Russian Academy of Sciences,
Troitsk, Moscow oblast, 142190 Russia
e-mail: nab@ns.hppi.troitsk.ru
Received December 24, 2002
Abstract—The effect of high pressure on the structure of LaF (tysonite), CeF , and nonstoichiometric LaF -
3
3
3
based solid solutions (rhombohedral system, P3c1, Z = 6) is discussed. In the systems MF –LaF with M = Pb
2
3
and Sr, fluorite-like M1 – xLaxF2 + x solid solutions (Fm3m, Z = 4) are obtained in the composition range 0.5 ≤
x ≤ 0.75 by quenching from 1500–1700 K at p = 10 GPa. By extrapolating the composition dependences of the
lattice parameter for these cubic solid solutions, the lattice parameter of cubic LaF is estimated at a = 5.86 ±
3
0
.02 Å. High-pressure x-ray diffraction studies indicate that CeF undergoes a first-order phase transition sim-
3
ilar to that revealed recently in LaF . At p = 20.6 GPa, the lattice parameters of orthorhombic CeF (Cmma,
3
3
Z = 8) are a = 8.062 ± 0.006 Å, b = 8.457 ± 0.009 Å, and c = 5.091 ± 0.006 Å. The transition is accompanied
by a 7.3% decrease in volume. The high-pressures structure of LaF and CeF can be thought of as a distorted
3
3
modification of the structure of a hypothetical fluorite-like trifluoride. The lattice parameters of the orthorhom-
bic and cubic cells are related by aortho ~ bortho
~
2 acubic and cortho ~ acubic.
INTRODUCTION
text, it was also of interest to synthesize fluorite-like
solid solutions close in composition to LaF in fluoride
3
LaF (tysonite) and CeF have a rhombohedral
3
3
and oxyfluoride binary systems. In this paper, we
structure (P3c1, Z = 6 [1]) which is the most densely review the main results in this area of research, which
packed among the known structures of the trifluorides provide a general picture of the behavior of trifluorides
of elements stable at normal pressure. This structure under strong compression. We also report new experi-
can be described on the basis of the hexagonal close- mental data on fluorite-like solid solutions in the
packed lanthanum sublattice, with the fluorine atoms PbF −LaF and SrF –LaF systems and on the phase
2
3
2
3
residing in interstices: two-thirds of the anions occupy transition of CeF . Data on the structure and properties
3
tetrahedral sites, and the rest sit in the center position of of rare-earth fluorides, in particularYF and LaF , are of
3
3
the shared faces of the octahedra. The M X compounds
3
immense importance in both science and technology [3].
with M = alkali metal and X = Bi or Sb, isostructural
with tysonite, demonstrate that a more densely packed
configuration can be achieved if the M atoms occupy
interstices in the cubic close-packed X sublattice: two-
thirds of the M atoms occupy tetrahedral sites, as in the
tysonite structure, while the rest sit in octahedra. In the
case of trifluorides, it is convenient to represent such a
hypothetical configuration in terms of the fluorite struc-
EXPERIMENTAL
The starting reagents for this investigation were pre-
pared at the Shubnikov Institute of Crystallography,
Russian Academy of Sciences. Optical-quality SrF2
and PbF single crystals were grown in a fluorine
2
atmosphere. LaF and CeF containing ≤0.07 wt %
3
3
ture type (CaF , Fm3m, Z = 4), with additional fluoride
2
oxygen were also prepared in a fluorine atmosphere,
ions sitting in all of the octahedra (or, in other words, in
by melting La O and Ce O , respectively. Fluorite-
2
3
2
3
empty cubes of the anion sublattice) and compensating
1
like solid solutions were obtained by quenching at a
rate of ~1000 K/s under a pressure of 10 GPa after
0-min holding at 1500–1700 K in hermetically
sealed platinum capsules. The oxygen content of the
synthesis products was no higher than 0.4 wt %. The
phase purity of the samples was checked by powder
x-ray diffraction (XRD).
the larger charge on the cation.
The difference in density between tysonite- and flu-
orite-like phases is 9.6%, judging from the data on
3
Rb Sb, which exists in both forms [2]. This led a num-
3
ber of researchers to examine the possibility of a similar
polymorphism in LaF at high pressures. In this con-
3
1
The compressibility and phase transition of CeF3
In what follows, phases with such structures will be called, for
convenience, fluorite-like.
were investigated by XRD (MoK radiation) at room
α
0
020-1685/03/3911-1198$25.00 © 2003 MAIK “Nauka/Interperiodica”

HIGH-PRESSURE PHASE TRANSITIONS OF LaF AND CeF
1199
3
3
temperature, using a high-pressure diamond anvil cell.
a, Å
6.0
The pressure was calibrated using the R ruby fluores-
1
cence line. As the pressure-transmitting medium, we
used a 1 : 1 mixture of ethanol and methanol.
4
5
5
.9
.8
3
2
RESULTS AND DISCUSSION
The formation of fluorite solid solutions in CaF2–
RF (R - rare earth) systems was first analyzed by Gold-
3
5.7
schmidt et al. [4]. In their model, random substitution
of trivalent cations on the Ca site is accompanied, in
conformity with the electroneutrality condition, by the
incorporation of additional fluoride ions into some of
the 1/2 1/2 1/2 sites (the center position of the empty
cubes in the anion sublattice of the fluorite structure).
The stability of this configuration was questioned by
Belov [5], who supposed that the difference in ionic
5
5
5
.6
.5
.4
1
2
+
–
radius between Ca and F is too large for an addi-
tional fluoride ion to be accommodated in an empty
cube. Belov’s assumption that fluorides may be oxi-
dized to ROF, without changes in the total number of
anions in the parent fluorite structure, proved valid in
0
0.2
0.4
0.6
0.8
1.0
x
Fig. 1. Composition dependences of the lattice parameter
for fluorite-like solid solutions: (1) Ca1 – xLaxF2 + x (᭹ [8],
᭺
[9]), (2) LaO1 − xF1 + 2x (᭹ [12], ᭺ [11]), (3) Sr1 – xLaxF2 + x
᭹ [13], ᭺ this work), (4) Pb1 – xLaxF2 + x (᭹ [14], ᭺ this
work); ᭹ normal pressure, ᭺ high pressure.
the case of BiF . The data cited in many reports as evi-
3
(
dence for the existence of cubic BiF were shown to be
3
invalid [6]. Nevertheless, nonstoichiometric solid solu-
tions containing additional fluoride ions in the fluorite
structure are easily formed without oxidation, by a
more complicated mechanism. Structural analysis of
Ca0.607Ce0.393F2.393 single crystals [7] shows that,
instead of the conventional incorporation of an addi-
tional fluoride ion, one anion in position 1/4 1/4 1/4
crystals, we can do no more than assume that, in this
composition range, further increase in x is accompanied
by a decrease in the amount of anion vacancies and
changes in the anion position in the cube. A similar pat-
tern was reported for the LaOF–LaF system [11],
3
(cube corner) is replaced by two anions in general posi-
which was found to contain cubic LaO1 – xF1 + 2x solid
solutions with a large anion excess (x = 0.80) relative
to the fluorite-like compound LaOF, which belong to
space group Fm3m. The lattice parameter of hypothet-
tions on body diagonals. The two fluoride ions are dis-
placed by Ӎ1/6 from the center position to the common
unoccupied cube corner. Owing to the disordered dis-
tribution of the solute cations, additional anions, and
vacancies, the space group of fluorite (Fm3m) remains
unchanged. Simple calculations demonstrate that the
filling of all the empty cubes in the fluorite structure
leads to the saturation of solid solutions with the gen-
ical cubic LaF can be estimated by extrapolating the
3
composition dependences of the lattice parameter for
existing cubic solid solutions (a = 5.867 Å) (Fig. 1). It
is of interest to note that, at normal pressure, all of the
solid solutions in question have the tysonite (LaF3)
structure in the range 0.67 < x < 1. Thus, the high-
2
+
eral formula M RxF2 + x at x = 0.75.
1
– x
Under normal pressure, the highest trifluoride con- pressure phase transitions of tysonite-structure LaF ,
tent of such phases is x Ӎ 0.5 [8]. Since the formation CeF , and other fluorides are closely related to the
3
3
of solid solutions with 0.5 < x < 1 is accompanied by a observed polymorphism of nonstoichiometric solid
decrease in volume, the solid-solution range must solutions.
become broader with increasing pressure. Indeed, the
MF –LaF (M = Ca [9], Sr, Pb) systems were shown to
It seems likely that the extent of solid solutions that
can be quenched and stored under normal conditions
2
3
contain high-pressure fluorite-like solid solutions with
x as large as 0.85, 0.75, and 0.75, respectively. Detailed depends on the concentration of anion vacancies and
single-crystal structural analysis of Pb La F [10] the relative amounts of solute ions in the cation and
0
.3
0.7 2.7
anion sublattices. A random distribution of such defects
raises the entropy of the solid solution and favors the
formation of a metastable phase. From this viewpoint,
demonstrates that the cubic (Fm3m) structure of this
phase is formed by the mechanism described by
Otroshchenko et al. [7], with the generation of anion
vacancies. The structure of CaF -based solid solutions, the existence of metastable fluorite-like LaF or other
2
3
in spite of the change in the mechanism of the incorpo- trifluoride appears unlikely. For example, just like the
ration of additional fluoride ions for x > 0.75, belongs trifluorides, the RH compounds have no cubic poly-
to space group Fm3m up to x = 0.85. For lack of single morphs: the fluorite-like structures of LaH , CeH , and
3
3
3
INORGANIC MATERIALS Vol. 39 No. 11 2003

1
200
DYUZHEVA et al.
–
a, Å
Indeed, it is via partial substitution of OH ions for flu-
oride ions (more than 12%) in LaF that the stoichio-
3
5
5
5
5
5
5
.95
.93
.91
.89
.87
.85
metric phase can be stabilized. Cubic La[F (OH)
y
1 – y 3
]
solid solutions (sp. gr. Fm3m) were obtained in the
composition range 0.45 < y < 0.88. The lattice
parameter of cubic LaF evaluated by extrapolation in
3
this system (Fig. 2) is a = 5.855 Å. The average over
all the systems in question, a = 5.86 ± 0.02 Å, pro-
vides an estimate of the relative difference in volume
between tysonite and hypothetical fluorite-like LaF3:
∆V/Vtysonite = –0.082. Judging from the T–x phase dia-
gram of the LaF –La(OH) system, these phases are in
3
3
equilibrium starting at p = 10 GPa and T ~ 1900 K
16]. From the displacement of certain points in this
[
phase diagram upon an increase in pressure from 7.7
to 10 GPa, one can roughly estimate the slope of the
line representing this equilibrium in the p–T phase
diagram. In this way, we find that the equilibrium in
question (real or hypothetical) occurs in the range 15–
0
0.1
0.2
0.3
0.4
0.5
Mole fraction of La(OH)3
Fig. 2. Composition dependence of the lattice parameter for
La[(F1 – y(OH) ] fluorite-like solid solutions [6].
2
0 GPa.
y 3
As shown by in situ XRD, LaF compressed in a
3
high-pressure diamond anvil cell undergoes a first-
order phase transition accompanied by a 7.7% decrease
in volume [17]. At room temperature, the transition
occurs at 19 GPa, in accordance with theoretical predic-
tions, but the high-pressure phase has an orthorhomb-
ically distorted fluorite-like structure (Fig. 3). The lat-
tice parameters of the orthorhombic phase (Cmma, Z =
Hypothetical
cubic phase
LaF3
8
; a = 8.221(6) Å, b = 8.589(9) Å, and c = 5.231(6) Å
at 16.0 GPa) are related to the lattice parameter of the
cubic unit cell by aortho ~ bortho 2 acubic and cortho
~
~
High-pressure phase LaF3
High-pressure phase CeF3
acubic. The reverse transformation occurs at 14 GPa. The
hysteresis of the transition, large as it is (5 GPa), is
insufficient for the dense phase to persist down to nor-
mal pressure.
A similar transition in tysonite-structure CeF was
3
revealed in this work by the same method. The table
shows the indexing scheme for the high-pressure phase
(
20.6 GPa). The orthorhombic (Cmma) cell parameters
are a = 8.062 + 0.006 Å, b = 8.457 + 0.009 Å, and c =
.091 + 0.006 Å. At Z = 8, the pressure dependences of
5
the relative volumes for the tysonite and orthorhombic
phases were identical, to within experimental accuracy,
2
0
35
50
65
θ, deg
to those for LaF , which were determined more care-
3
2
fully (Fig. 4). This led us to conclude that the coeffi-
Fig. 3. Splitting of peaks in the XRD pattern of an orthor-
hombically distorted cubic phase.
cients of the Murnagan–Birch equation of state [18]
calculated for LaF can be assigned, as a first approxi-
3
mation, to the two phases of CeF : K = 147(10) GPa,
3
0
K' = 3 for tysonite and K = 207(6) GPa, K' = 2.1 for the
orthorhombic phase.
0
PrH are tetragonally distorted [5]. The above-men-
3
tioned intermetallic compounds form fluorite-like
phases either as a result of mutual substitutions in the
The existence of cubic La(F,OH) , whose stoichi-
3
ometry is identical to that of trifluorides, and some fea-
tures of the behavior of nonstoichiometric solid solu-
tions suggest that, by raising the temperature, the
two sublattices (Sb and Rb) of Rb Sb (which is unlikely
3
in predominantly ionic phases) or in the presence of
two alkali metals (Na KSb), which is equivalent to the orthorhombically distorted phases can be converted
2
presence of two kinds of anions in fluoride phases. into high-symmetry fluorite-like forms. Judging from
INORGANIC MATERIALS Vol. 39 No. 11 2003

HIGH-PRESSURE PHASE TRANSITIONS OF LaF AND CeF
1201
3
3
V/V0
(Ӎ20 GPa for LaF and CeF ) but also at high tempera-
3
3
tures.
1
0
0
0
0
0
0
.00
1
2
3
4
.96
.92
.88
.84
.80
.76
ACKNOWLEDGMENTS
We are grateful to Prof. B.P. Sobolev and Prof.
P.P. Fedorov for supplying the samples.
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2
3
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INORGANIC MATERIALS Vol. 39 No. 11 2003