Phase transition and compressibility of LaF3 under pressures up to 40 GPa

Article abstract of DOI:10.1016/S0925-8388(01)01829-1

The compressibility of LaF₃ (tysonite) has been studied up to 40 GPa using X-ray diffraction in a diamond anvil cell. A phase transition from tysonite to an orthorhombic phase (Cmma, No. 67) was observed at a pressure of 19 GPa at room temperature. The volume discontinuity was ΔV/V₀ = -0.077 at the transition point. The cell parameters of the high-pressure phase are a=8.221(6) A?, b=8.589(9) A?, c=5.231(6) A?, Z=8 at p=16 GPa, which is accepted conventionally as equilibrium. The bulk moduli and pressure derivatives calculated by fitting V(p) data to the Birch-Murnagan equation of state are presented for both phases.

Full text of DOI:10.1016/S0925-8388(01)01829-1

Journal of Alloys and Compounds 335 (2002) 59–61
L
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Phase transition and compressibility of LaF3 under pressures up to 40 GPa
*
T.I. Dyuzheva , L.M. Lityagina, G.B. Demishev, N.A. Bendeliani
L.F. Vereschagin Institute of High Pressure Physics, Russian Academy of Sciences, 142190 Troitsk, Moskovskaya Oblast, Russian Federation
Received 6 August 2001; accepted 21 August 2001
Abstract
The compressibility of LaF3 (tysonite) has been studied up to 40 GPa using X-ray diffraction in a diamond anvil cell. A phase
transition from tysonite to an orthorhombic phase (Cmma, No. 67) was observed at a pressure of 19 GPa at room temperature. The
˚
volume discontinuity was DV/V0 5 2 0.077 at the transition point. The cell parameters of the high-pressure phase are a58.221(6) A,
˚
˚
b58.589(9) A, c55.231(6) A, Z58 at p 5 16 GPa, which is accepted conventionally as equilibrium. The bulk moduli and pressure
derivatives calculated by fitting V( p) data to the Birch–Murnagan equation of state are presented for both phases.
2002 Elsevier
Science B.V. All rights reserved.
Keywords: High pressure; Phase transition; Compressibility; Tysonite (LaF3)
1. Introduction
appearance at room temperature could be expected at
15–20 GPa. The present work was carried out to detect the
corresponding structural phase transition by X-ray diffrac-
tion under high pressure at room temperature.
]
Lanthanum trifluoride LaF3, tysonite (Z56, P3c1 No.
165), has a structure which is characteristic for the
majority of rare-earth metal trifluorides. Stoichiometric
phases with such structures are the most dense among the
large family of known MIIIF3 compounds. This allows us
to consider these compounds as the most suitable subjects
in the search for, and for studying, new states that are
realized under high compression conditions. Previously,
information about possible structural pressure-induced
transformations has been obtained by studying phase
relations in the LaF3 –La(OH)3 system at 7.7 and 10 GPa
[1]. The cubic phase of variable composition,
La[F12y(OH)y]3 (0.12 , y , 0.5, Fm3m, Z54), which is
metastable under normal conditions, has been recovered by
quenching. This cubic structure may be represented as the
fluorite structure (CaF2) with additional anions in positions
2. Experimental procedure
Lanthanum triofluoride (tysonite) was prepared by melt-
ing La2O3 in a fluorine atmosphere. Chemical analysis
indicated that the maximum oxygen concentration did not
exceed 0.07 wt.%.
High-pressure X-ray powder diffraction experiments
were carried out using a diamond anvil cell. A methanol–
ethanol (4:1) mixture was used as the pressure-transmitting
medium. A preliminary run performed using a bulk
volume high-pressure chamber and a hermetically sealed
platinum ampoule showed the absence of any chemical
interaction between LaF3 and the above-mentioned mix-
ture under 9 GPa at room temperature during 6 h.
The pressure was determined from the wavelength of the
ruby R1 fluorescence line. The diffraction patterns con-
tained eight to nine reflections for each phase. The X-ray
data were recorded during both increasing and decreasing
pressure runs. The X-ray powder diffraction measurements
required 8–16 h exposure with photographic film using Mo
Ka radiation from a rotating-anode and fine-focus tube.
Details of the experiment are described in Ref. [2].
1
(xxx). The cubic phase has not been observed for y , 0.12,
but an analysis of the phase diagram shows that, if the
LaF3 composition is reached, this phase should be in
equilibrium with tysonite at 10 GPa and 1900 K. Its
*
Corresponding author. Tel.: 17-095-334-0733; fax: 17-095-334-
0012.
E-mail address: tid@ns.hppi.troitsk.ru (T.I. Dyuzheva).
1For convenience we will call this structure with Fm3m symmetry a
fluorite-like structure.
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PII: S0925-8388(01)01829-1
2002 Elsevier Science B.V. All rights reserved.

60
T.I. Dyuzheva et al. / Journal of Alloys and Compounds 335 (2002) 59–61
Table 1
3. Results
Powder diffraction data for the high-pressure phase of LaF3 at p 5 16
GPa
The LaF3 sample was transparent and colorless over the
whole pressure interval. Therefore, one could assume the
absence of any significant change in electronic structure
for both LaF3 phases up to 40 GPa. Curve 1 in Fig. 1
shows the relative volume change of tysonite under
pressures up to 18 GPa. The volume data were approxi-
mated by the Birch–Murnagan equation of state [3]:
hkl
I/I0
dmeas
dcalc
021
201
220
002
040
400
222
421
023
100
100
38
27
19
38
47
19
23
3.319
3.230
2.968
2.622
2.148
2.056
1.963
1.747
1.616
3.319
3.232
2.970
2.616
2.147
2.055
1.963
1.747
1.616
P 5 3/2 K0(x27 / 3 2 x25 / 3)[1 2 3/4(4 2 K0 )(x22 / 3 2 1)]
9
(1)
9
˚
where x 5V/V0, and K0 and K0 are the bulk modulus and
fluorite-like LaF3 phase (Fm3m, Z54, ac 5 5.855 A at
normal pressure [1]). Then, the number of formula units
per orthorhombic cell Z58 and the pressure dependence of
the relative volume are determined by the position of curve
2 in Fig. 1. Extrapolation of this dependence to normal
pressure gives the value V/V0 5 0.902, which is close to the
value for the above-mentioned cubic phase (V/V0 5 0.916).
Naturally, extrapolation over the interval of 14 GPa causes
some error in the determination of the volume of the
orthorhombic phase. Nevertheless, possible deviations
from the obtained value leave no doubt as to the structural
relationship between the orthorhombic and cubic phases.
A more significant uncertainty arises when estimating
the pressure derivative of the volume. Therefore, the
its pressure derivative. The fit of experimental data to Eq.
(1) for tysonite gives values K0 5 147(10) GPa and K0 5 3
9
at normal pressure. The diffraction patterns of tysonite did
not change qualitatively up to 18 GPa. The ratio of
reflection intensities and the value c/a 5 1.02, which is
typical for the tysonite structure type, remained unchanged.
Disappearance of the (002) reflection was observed at 18
GPa. Special diffraction and optical studies are required in
a narrow pressure interval just before the transition to
explain this phenomenon.
The diffraction lines of tysonite practically disappeared
at 19 GPa. All reflections of the high-pressure phase were
interpreted on the basis of an orthorhombic unit cell with
extinction conditions corresponding to the space group
Cmma, No. 67 (Table 1). One can readily see that the
recorded diffraction pattern is the result of splitting of the
diffraction reflections determining the f.c.c. structure (Fig.
2). The relationships between the lattice parameters for the
orthorhombic and cubic cells are ao | bo | acœ2 and co |
ac, where ao, bo and co are the orthorhombic lattice
parameters and ac the cubic lattice parameter. It may be a
good guess that the structure of the new phase is a
distorted variant of the structure of the hypothetical
9
values K0 5 207(6) GPa and K0 5 2.1 for the orthorhom-
bic phase, determined by fitting the experimental points to
Eq. (1), should be considered as nothing more than the
coefficients of the Birch–Murnagan equation of state.
Fig. 1. Pressure dependence of the relative volume for LaF3. (s)
Increasing pressure; (d) decreasing pressure.
Fig. 2. A schematic diagram of LaF3 phases.

T.I. Dyuzheva et al. / Journal of Alloys and Compounds 335 (2002) 59–61
61
Table 2
Lattice parameters, bulk moduli and their pressure derivatives for phases of LaF3 at p 5 16 GPa
˚
˚
˚
Phase
a (A)
b (A)
c (A)
Z
V/Z
K
K9
]
P3c1
(tysonite)
Cmma
6.968(6)
8.221(6)
7.131(7)
5.231(6)
6
8
49.97
46.17
191(10)
237(6)
2.6
1.6
8.589(9)
Useful information is contained in the data for the behavior
of the volume and the bulk modulus of LaF3 at the
transition point to the dense phase. Table 2 presents data
for both phases at 16 GPa. The value obtained by
averaging the pressures of the direct and reversed transi-
tions can be taken as the equilibrium pressure to a first
approximation.
even for intermetallic compounds. Nevertheless, the prob-
lem of the possible existence of the cubic phase of LaF3
remains open because one cannot ignore the influence of
pressure on the character and energy of chemical bonds
defining the atomic structure. Quite possibly, the stability
region of the cubic phase is located at higher temperature
and will be detected in the same pressure interval using an
X-ray diamond anvil cell with constant heating.
4. Discussion
Acknowledgements
Two more classes of compounds are also known for
which, under normal conditions, not only the tysonite type,
but also distorted fluorite-like structures, are characteristic.
Cubic phases are absent in the LnH3 system (Ln;La,
rare-earth metal). The structures of La, Ce and Pr trihy-
drides are tetragonal distorted [4]. On the contrary, cubic
phases exist for MX3 compounds (M;alkaline metal,
X;Bi, Sb) [5]. Rb3Sb is dimorphous and forms a tysonite-
like as well as a cubic modification with a density
difference of |9.6%. However, in this case the Fm3m
symmetry is not realized. The symmetry is raised to Fd3m
because of redistribution of the atoms on different posi-
tions. Only the structure of the ternary Na2KSb compound
has Fm3m symmetry. An additional atom in a quasi-
symmetrical environment of eight atoms of the same kind
seems to become unstable in the fluorite-like structure,
The authors wish to thank Prof. P.P. Fedorov for
supplying the samples. We are also grateful to Dr. T.N.
Kolobyanina for help and useful discussions.
References
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[2] T.N. Kolobyanina, G.B. Demishev, T.I. Dyuzheva, V.G. Losev, S.S.
Kabalkina, Prib. Tekh. Eksper. 2 (1984) 194.
[3] P. Birch, J. Geophys. Res. 57 (1952) 227.
[4] P. Vaida, in: K.A. Gschneidner Jr., L. Eyring (Eds.), Handbook on
the Physics and Chemistry of Rare-Earths, Vol. 20, Elsevier,
Amsterdam, 1995, p. 214, Chapter 137.
[5] R.W.G. Wyckoff (Ed.), Crystal Structures, Vol. 2, Wiley–Inter-
science, New York, 1960, p. 109, 111.