Journal
J. Am. Ceram. Soc., 80 [7] 1910–14 (1997)
Characterization of the Cotunnite-Type Phases of Zirconia and
Hafnia by Neutron Diffraction and Raman Spectroscopy
Julian Haines and Jean Michel Léger
Laboratoire de Physico-Chimie des Matériaux, C.N.R.S., 92190 Meudon, France
Steve Hull
ISIS Facility, Rutherford Appleton Laboratory, Chilton, Didcot, OX11 OQX, United Kingdom
Jean Pierre Petitet
Laboratoire d’Ingéniérie des Matériaux et des Hautes Pressions, C.N.R.S., 93430 Villetaneuse, France
Altair S. Pereira
Escola de Engenharia and Instituto de Fı́sica, UFRGS, 91501-970 Porto Alegre, RS, Brazil
Claudio A. Perottoni and João A. H. da Jornada
Instituto de Fı́sica, UFRGS, 91501-970 Porto Alegre, RS, Brazil
The crystal structures of the cotunnite-type phases (space
group, Pnam, Z = 4) of pure zirconia and hafnia prepared
under high-temperature, high-pressure conditions in a
multianvil device were refined by time-of-flight neutron
powder diffraction. The structures of both compounds are
very similar and the nine polyhedral metal–oxygen distances range from 2.133(1) to 2.546(1) Å in ZrO2 and from
2.121(1) to 2.535(2) Å in HfO2. The Raman spectra of both
phases resemble one another strongly and are consistent
with the cotunnite-type structure. These results confirm
that ZrO2 and HfO2 undergo transitions to the same phase
at high pressure.
I.
with a doubled a parameter. In the case of HfO2, refinement of
X-ray and neutron diffraction data from quenched samples confirm that OI-type HfO2 is isostructural.10,11 It can be noted that
neutron diffraction data allowed for a significant improvement
in the structure refinement of OI-type HfO2 with a shift of up
to 0.01 in the oxygen positions relative to the values obtained
by X-ray diffraction and an up to 20-fold reduction in their
estimated standard deviations.
A second high-pressure transition is observed in both compounds to another orthorhombic phase (OII), which was first
identified by X-ray diffraction from samples quenched from 20
GPa and 1000°C.12 The reported unit cell dimensions are consistent with a cotunnite (PbCl2) structure, Pnam, Z 4 4. At this
transition the cation coordination number increases from 7 in
the lower-pressure phases to 9. In the case of zirconia, the
structure of the cotunnite-type phase has been refined both in
situ at high pressure and on a quenched sample yielding atomic
positions typical of a cotunnite-type structure.1
The cotunnite-type phases of both ZrO2 and HfO2 are present over a large range of P–T space extending up to at least 70
GPa at ambient temperature.2 The P–T boundary between the
OI and OII phases of ZrO2 has been determined by in situ
electrical conductivity measurements13 and by calorimetric and
diffraction studies of quenched samples14,15 and calculations
based on the obtained data yield at ambient temperature, a
transition pressure of 7–8 GPa with the slope of the phase
boundary lying between 0.0087 and 0.0061 GPa/K. The OII
phase is stable up to 1000°C at 20 GPa, above which temperature it transforms to an unquenchable high-temperature, highpressure phase.16 Cotunnite-type phases have also been observed for yttrium-14 and calcium-doped17 zirconia at high
pressures.
Cotunnite-type ZrO2 and HfO2 are both highly incompressible with a bulk modulus of 3321 and 340 GPa,2 respectively.
It has recently been shown that stishovite, a high-pressure polymorph of silica, which similarly is incompressible with a bulk
modulus of 298 GPa,18 has a very high hardness.19 These cotunnite-type dioxides are thus candidates for hard materials.
The oxygen sublattice plays an important role in the low compressibility of these phases. It is of particular interest to refine
the structures of these cotunnite-type phases using neutron dif-
Introduction
T
has been a great deal of controversy concerning the
high-pressure phase transition sequences in pure ZrO2 and
pure HfO2. Recent papers, however, present a clearer picture of
the behavior of these dioxides1–3 and indicate that ZrO2 and
HfO2 undergo the same sequence of phase transitions. The
marked similarity between these two compounds is not surprising as the Zr4+ and Hf4+ cations are very close in size and
belong to the same group of the periodic table.
Both ZrO2 and HfO2 adopt monoclinic baddeleyite-type
structures,4–6 P21/c, Z 4 4, at ambient temperature and pressure. Phase transitions to an orthorhombic phase, termed OI,
are observed in both ZrO2 and HfO2 at pressures above 3 GPa.
The transition pressure is particularly sensitive to the state of
the sample. In the case of ZrO2, the structure of this phase has
been refined in situ by single-crystal X-ray diffraction7 using
the Pbcm space group with Z 4 4. Neutron powder data8,9
from quenched samples indicated that the space group is Pbca
HERE
O. Ohtaka—contributing editor
Manuscript No. 191201. Received February 14, 1997; approved April 29, 1997.
1910
July 1997
Communications of the American Ceramic Society
fraction data due to the much greater oxygen contribution to the
overall scattered intensity as compared to X-ray diffraction. Up
to the present, the structure of cotunnite-type ZrO2 has only
been refined from X-ray diffraction data1 and no structure refinements have been performed for cotunnite-type HfO2.
II.
Experimental Procedure
Powdered ZrO2 and HfO2 (Alfa Products) were used for the
synthesis experiments. The samples were sealed in 1.8–2.0 mm
long, 1.4 mm i.d. gold sample capsules. Each capsule was
placed in a cylindrical LaCrO3 furnace contained in a 10 mm
edge length MgO octahedral sample cell.20 The MgO octahedron was then assembled between eight WC cubes with 32 mm
edge lengths and a truncated edge length (TEL) of 5 mm using
preformed 5 mm × 2 mm pyrophyllite gaskets. The resulting
cube was positioned between the outer six anvils of the 1000
tonne, split-cylinder, MA-8 multianvil device. In all runs, the
pressure was increased gradually to 20 GPa over a period of 2
h. Zirconia samples were heated at temperatures between 750°
and 800°C for 1 to 3 h and hafnia samples were heated at
800°C for 2.5 to 5 h. Temperatures were measured using Pt–
PtRh thermocouples. The samples were quenched and pressure
was then gradually released over a period of about 12 h. The
volumes of the recovered ZrO2 and HfO2 pellets were of the
order of 1 mm3. These pellets were not well sintered and different regions could be distinguished visually. Knoop hardness
measurements21 yielded values ranging from 11 to 17 GPa for
the ZrO2 samples and from 6 to 13 GPa for the HfO2 samples.
The difference in measured hardness between the ZrO2 and
HfO2 samples reflects the relative degree of sintering.
Neutron diffraction measurements were performed on the
Polaris medium-resolution diffractometer at the ISIS spallation
source on samples placed in 1.5 mm diameter glass capillary
tubes. Data were collected at 2u 4 145° over a time-of-flight
(tof) range of 2480–19500 ms (d 4 1.6185 × 10−4 Å z ms−1 ×
tof) using a bank of 3He ionization counters. Because of the
small size of the sample, acquisition times were of the order of
1911
30 h. A pattern was also collected from an empty glass tube and
this was subtracted from the sample diffraction patterns prior to
further data analysis. Rietveld refinements were performed using the programme TF12LS.22
Raman spectra with a typical resolution of 1 cm−1 were
obtained in backscattering geometry from 2 to 5 mm diameter
regions of the samples using the 50× objective of a metallographic microscope coupled to a 0.32 m monochromator and a
liquid-nitrogen-cooled CCD detector. The 632.8 nm line of a
He–Ne laser was used for excitation and the acquisition times
were 100 s. A supernotch filter was used to reduce the intensity
of the Rayleigh line. No significant differences were observed
in the Raman spectra of the visually distinct sample regions. In
order to access the region below 110 cm−1, additional spectra
(2 cm−1 resolution) were obtained using a Dilor XY spectrometer equipped with a diode array detector using the 514.5 nm
line of an argon ion laser or the 676.4 nm line of a krypton ion
laser for excitation. In all cases, the Raman peaks were fitted to
Lorentzians in order to obtain accurate peak positions. No
traces of the lower-pressure monoclinic or orthorhombic-I
phases were detected either by neutron diffraction or by Raman
spectroscopy.
III.
Results and Discussion
(1) Neutron Diffraction
The refined neutron diffraction patterns of ZrO2 and HfO2
are shown in Figs. 1 and 2 and the resulting structural parameters and agreement factors are listed in Table I. In addition to
the cell constants, atomic positions and isotropic thermal parameters, the background polynomial coefficients, scale factor,
and line shape parameters were varied for both ZrO2 and HfO2.
In the case of ZrO2, a preferred orientation parameter was also
refined. A significant improvement to the fit was obtained by
including preferred orientation along the [010] direction. It can
be noted that preferred orientation is commonly observed for
cotunnite-type structures and for cotunnite-type ZrO2, in par-
Fig. 1. Experimental (+) and calculated (solid-line) profiles from Rietveld refinement of neutron diffraction data for cotunnite-type ZrO2. Intensity
is in arbitrary units and the difference profile is on the same scale. Vertical bars indicate the calculated positions of diffraction lines.
1912
Communications of the American Ceramic Society
Vol. 80, No. 7
Fig. 2. Experimental (+) and calculated (solid-line) profiles from Rietveld refinement of neutron diffraction data for cotunnite-type HfO2. Intensity
is in arbitrary units and the difference profile is on the same scale. Vertical bars indicate the calculated positions of diffraction lines.
ticular.1 In contrast, no indication of preferred orientation was
observed for HfO2. This difference with respect to the behavior
of ZrO2 is linked to the degree of sintering of the sample. The
pellet of HfO2, which was less well sintered, better approximates a randomly oriented powder. It can be noted that the R
factors for the refinement of HfO2 are greater than those for
ZrO2. This is due to the lower signal-to-noise ratio for HfO2,
which is a result of the much larger neutron absorption coefficient of the hafnium cation. The present results represent a
significant improvement in the determination of the atomic
Table I. Cell Constants, Atomic Positions, Isotropic
Thermal Parameters and Agreement Factors (%) Obtained
from Rietveld Refinement of Neutron Diffraction Data from
Cotunnite-Type ZrO2 and HfO2 Quenched from
High-Pressure and High-Temperature Conditions, Space
Group Pnam, Z = 4, M, O1, and O2 on 4c Sites (x, y, 1/4)
a (Å)
b (Å)
c (Å)
x(M)
y(M)
z(M)
B(M) (Å2)
x(O1)
y(O1)
z(O1)
B(O1) (Å2)
x(O2)
y(O2)
z(O2)
B(O2) (Å2)
RBragg
Rp†
Rwp‡
†
ZrO2
HfO2
5.5873(2)
6.4847(2)
3.3298(1)
0.2459(2)
0.1108(1)
1/4
0.10(1)
0.3599(2)
0.4248(2)
1/4
0.20(1)
0.0250(2)
0.3388(2)
3/4
0.34(2)
4.7
6.1
2.9
5.5544(1)
6.4572(2)
3.3070(1)
0.2461(2)
0.1104(2)
1/4
0.11(1)
0.3591(3)
0.4256(2)
1/4
0.18(2)
0.0245(2)
0.3388(3)
3/4
0.34(2)
5.7
12.0
4.1
p 4 unweighted profile, ‡wp 4 weighted profile.
positions in ZrO2 with respect to those refined from X-ray data
on a sample recovered from a diamond anvil cell1 and this is
the first time the structure of cotunnite-type HfO2 has been
refined.
The refined cell constants of ZrO2 and HfO2 are very similar
with those of HfO2 being slightly smaller. The cell volume of
HfO2 is 1.7% less than that of ZrO2 because of the slightly
smaller size of the Hf4+ cation. It can be noted that for the
ambient-pressure baddeleyite-type5,6 and the high-pressure orthorhombic-I-type phases,8,10,11 the cell volumes of HfO2 are
1.4% and 1.9% smaller, respectively, than those of ZrO2. The
atomic positions in both dioxides are very similar and hence the
polyhedral interatomic distances are all slightly smaller for
HfO2 (Table II). The cation coordination polyhedron in the
cotunnite structure (Fig. 3) is an elongated tricapped trigonal
prism giving a cation coordination number of 9 with the polyhedral metal–oxygen distances ranging from 2.133(1) to
2.546(1) Å in ZrO2 and from to 2.121(1) to 2.535(2) Å in HfO2.
Table II. Polyhedral Interatomic Distances for
Cotunnite-Type ZrO2 and HfO2†
Interatomic distance (Å)
(M–O1)
(M–O2)
(M–O3)
(M–O4)
(M–O5)
(M–O6)
(M–O)av
(O1–O2)
(O2–O3)
(O2–O4)
(O3–O4)
(O4–O6)
(O2–O6)
(O5–O6)
†
ZrO2
HfO2
2.133(1)
2.546(1) × 2
2.169(2)
2.139(1) × 2
2.180(2)
2.304(1) × 2
2.273
2.566(1)
2.558(1)
2.760(2)
2.485(1)
2.641(2)
3.022(2)
2.687(1)
2.130(2)
2.535(2) × 2
2.162(2)
2.121(1) × 2
2.168(2)
2.288(1) × 2
2.261
2.550(2)
2.548(2)
2.745(2)
2.471(2)
2.618(2)
3.005(2)
2.673(2)
Anions are numbered as in Fig. 3.
July 1997
Communications of the American Ceramic Society
Fig. 3. Crystal structure of cotunnite-type ZrO2. The cations and
anions are shaded dark and light, respectively.
The minimum O–O distance is rather short in both cases; however, this is commonly observed for dense metal dioxide structures. These short distances in conjunction with the large number of O–O contacts and the high cation and anion coordination
numbers play an important role in the low compressibility of
these phases.
(2) Raman Spectroscopy
Group theory indicates that cotunnite-structured crystals
should have 18 Raman-active modes: GRaman 4 6Ag + 6B1g +
3B2g + 3B3g of which 6 modes are essentially due to the heavy
metal cations and 12 to the anions. It can be seen that the
overall profiles of the spectra of ZrO2 and HfO2 are very similar (Fig. 4) and in both cases at least 17 bands are observed
1913
(Table III). The six lower-frequency modes (modes 1–6) exhibit very large shifts of between 40 to 64 cm−1 to lower values
upon passing from ZrO2 to HfO2. The ratio nZrO2/nHfO2 varies
from 1.37 to 1.30, which can be compared to the mass ratio:
(mZr/mHf)−1/2 4 1.40. These modes can thus be assigned to the
cations as the observed shifts are due to the much greater mass
of Hf4+. The other 12 modes (modes 7–18) exhibit significantly
smaller shifts (4–21 cm−1) in the opposite direction. These
vibrations can be assigned to the oxygen ions and the changes
in frequency can be understood in terms of the higher force
constants in HfO2 resulting from the shorter interatomic distances. Three additional Raman peaks were observed only in
the spectrum of HfO2 and because of their low intensity could
be either overtones or combinations. It can be noted that the
spectra obtained for ZrO2 and HfO2 are similar to those of the
high-pressure phases23,24 of CeO2 and TeO2, for which the
cotunnite structure has been proposed.
Apart from the much greater resolution in the present spectra, they are essentially identical to those obtained from
samples of ZrO2 and HfO2 quenched from above 50 GPa in
diamond anvil cells.25,26 These phases were proposed to be
tetragonal based on the small number of observed peaks due to
residual strain and lower resolution. Upon comparing the spectra, it is obvious that cotunnite-type ZrO2 and HfO2 were obtained in these studies. This is further confirmation that the
ambient-temperature, high-pressure sequence of phases in
ZrO2 and HfO2 is monoclinic baddeleyite → orthorhombic-I →
orthorhombic-II cotunnite and that, at present, there is no compelling evidence for a further transition in these dioxides up to
the highest pressure at which they have been investigated.
IV.
Summary
Rietveld refinement using neutron diffraction data of the
structures of cotunnite-type ZrO2 and HfO2, quenched from
high-temperature high-pressure conditions, confirm the strong
similarity between these phases. All distances in HfO2 are
slightly smaller than those in ZrO2 because of the smaller size
of the Hf4+ cation. The coordination number of the metal cation
is 9 in the cotunnite-type structure and the polyhedral metal–
Fig. 4. Raman spectra of cotunnite-type ZrO2 (bottom) and HfO2 (top) obtained using 632.8 nm excitation. * indicates plasma lines from the laser.
The broad bands in the background are due to sample fluorescence. × indicates unassigned peaks in the spectrum of HfO2 which are possibly
overtones or combinations. Insert (A) contains the spectrum of HfO2 in the 50–250 cm−1 region using 676.4 nm excitation.
1914
Table III.
Communications of the American Ceramic Society
Raman Spectral Data for Cotunnite-Type ZrO2
and HfO2
n (cm−1)†
ZrO2
1
2
3
4
5
6
7
8
9
10
149 m
163 s
167 m
201 vvw
239 w
279 w
341 vw
362 m
378 m
387 m
11
12
413 m
436 s
13
14
15
16
17
18
561 m
569 m
601 m
622 m
661 m, br‡
HfO2
nZrO2/nHfO2
109 m
121 s
125 m
151 w
180 w
215 w
345 vw
371 m
383 m
391 m
412 vw§
430 m
454 s
518 vw§
578 m
581 w
620 m
643 m
665 w
675 m
704 w§
1.37
1.35
1.34
1.33
1.33
1.30
0.99
0.98
0.99
0.99
0.96
0.96
0.97
0.98
0.97
0.97
0.98
†
s 4 strong, m 4 medium, w 4 weak, v 4 very, br 4 broad. ‡This broad peak
corresponds to modes 17 and 18. § 4 unassigned, possible overtone or combination.
Note: In the spectrum of ZrO2 shown in Fig. 4, mode 4 coincides with a laser line.
This mode was also detected using 514.5 nm excitation for which there is no interference from laser lines at this Raman shift.
oxygen distances range from 2.133(1) to 2.546(1) Å in ZrO2
and from 2.121(1) to 2.535(2) Å in HfO2. The Raman spectra
of both compounds are consistent with a cotunnite-type structure and bear a strong resemblance to one another with the
peaks in the low-frequency region being shifted to lower values
in HfO2 due to the greater mass of the hafnium cation, and with
those peaks in the higher-frequency region of the spectrum,
which arise principally from the oxygen sublattice, shifted to
higher values due to the smaller volume of cotunnite-type
HfO2.
Acknowledgment:
We would like to thank Dr. M. W. Schmidt for assistance with the multianvil experiments, which were performed at the Bayerisches Geoinstitut under the EC ‘‘Human Capital and Mobility—Access to
Large Scale Facilities’’ programme.
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