Post-print of: Physics of the Earth and Planetary Interiors volumes 190–191, January 2012,
pages 87–94
Aluminum incorporation in α-PbO2 type TiO2 at pressures up to 20 GPa
Alberto Escudero, Falko Langenhorst
Bayerisches Geoinstitut, Universität Bayreuth, D-95440 Bayreuth, Germany
Abstract
Aluminum incorporation into the high pressure polymorph of TiO2 with the structure of αPbO2 has been studied from 10 to 20 GPa and 1300 °C by XRD, high-resolution 27Al MAS-NMR
and TEM. Al-doped α-PbO2 type TiO2 can be recovered at atmospheric pressure. Al2O3
solubility in α-PbO2 type TiO2 increases with increasing the synthesis pressure. The α-PbO2
type TiO2 polymorph is able to incorporate up to 35 wt.% Al2O3 at 13.6 GPa and 1300 °C,
being the substitution of Ti4+ by Al3+ on normal octahedral sites the mechanism of solubility.
The transition to the higher pressure TiO2 polymorph with the ZrO2 baddeleyite structure,
Akaogiite, has not been observed in the quenched samples at room pressure. The
microstructure of the recovered sample synthesized at 16 GPa and 1300 °C points to the
existence of a non-quenchable aluminum titanium oxide phase at these conditions.
Keywords
TiO2-II; Alumina; High-pressure; TEM; 27Al MAS-NMR;
solutions; Stacking faults
Non-quenchable phases; Solid
1. Introduction
Titanium dioxide TiO2 has been intensively studied for a few decades due to both basic and
applied interests in geology and material science. Rutile is a common minor mineral in
metamorphic and plutonic igneous rocks. Rutile transforms to a high pressure TiO2 polymorph
with the structure of α-PbO2 at about 9 GPa and 1300 °C, despite experiments on the phase
boundary between rutile and the α-PbO2 type TiO2 polymorph yielded quite controversial
results (Akaogi et al., 1992, Olsen et al., 1999 and Withers et al., 2003). This last polymorph has
been used as a geobarometer to indicate ultra high pressure metamorphism (UHPM) (Hwang
et al., 2000 and Wu et al., 2005), despite the suitableness of using nano-structured α-PbO2
type TiO2 as an indicator of ultra high pressure is still under discussion (Chen and Fu, 2006,
Escudero et al., 2012 and Wu et al., 2006). At about 17 GPa and 1300 °C TiO2 transforms into a
polymorph with the structure of ZrO2 baddeleyite (Sato et al., 1991). This mineral has been
recently encountered in heavily shocked garnet-cordierite-sillimanite gneiss in the Suevite
breccia of the Ries meteorite impact crater in Germany and has been named as Akaogiite (El
Goresy et al., 2010). Higher pressure TiO2 polymorphs have also been reported in the
literature (Dubrovinskaia et al., 2001).
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Aluminum is one of the trace elements in rutile that has been suggested to provide
information on the P-T conditions of the rutile-bearing metamorphic rocks. In fact, exsolved
corundum lamellae have been reported in diamondiferous eclogites from South Africa
(Sobolev and Yefimova, 2000). Recent studies have shown that Al solubility in TiO2 rutile
increases drastically with pressure, with two different mechanism of solubility. Enhanced
aluminum concentration in TiO2 rutile as well as (1 1 0) twinned CaCl2-type structure TiO2
grains are thus a clear indication of high-pressure conditions (Escudero et al., 2011b).
However, in order to evaluate the possibility of using the Al incorporation into TiO2 phases as
a possible indicator of high pressure conditions, the effect of the nature of the different TiO2
polymorphs at higher pressure on the Al incorporation into TiO2 phases should be determined.
We present in this paper a chemical and microstructural study of the Al incorporation into
TiO2 phases at pressures between 7 and 20 GPa, where the high pressure polymorphs of TiO2
are expected to be stable.
2. Experiments and methods
2.1. Synthesis
High-pressure experiments were performed from commercial Al2TiO5 (Sigma–Aldrich
nanopowder, <25 nm particle size (BET), 98.5% trace metals basis) in the range from 7 to 20
GPa were carried out in a 1200 ton MA8 Kawai-type multi-anvil press, using 18, 14, and 10 mm
Cr2O3-doped MgO octahedra with a stepped LaCrO3 furnace, depending on the pressure. The
high pressure octahedral assembly was compressed using 8 tungsten carbide anvils with
corner truncations of 11, 8, and 5 mm lengths, also depending on the pressure. Samples were
placed in a Pt metal foil capsule and inserted into the center of the furnace inside an MgO
sleeve. All ceramic parts of the high pressure cell were fired at 1000 °C for 30 min prior to
assembling. A W97Re3–W75Re25 thermocouple was inserted to measure the temperature at
the top surface of the capsule. Samples were first pressurized to the target pressure and then
heated at 1300 °C for 3 h. A summary of the synthesized samples is shown in Table 1.
An extra sample with a 85% TiO2 + 15% Al2O3 (wt.%) composition was also prepared in order
to carry out 27Al MAS-NMR measurements without the interference of the signal
corresponding to corundum, which overlaps with the signals corresponding to Al in TiO2. TiO2
(rutile nanopowder, 99.5% sigma) and Al2O3 (corundum nanopowder, 99.97%, Chempur)
were mixed in a ball mill at 600 rpm during 60 min. This precursor material was later submitted
to high pressure treatments at 14 GPa and 1400 °C in a 5000 ton MA8 Kawai-type multi-anvil
press during 1 h. This multi anvil press allows synthesizing sample volumes 10 times larger
than the obtained in conventional systems.
2.2. Characterization techniques
Powder diffraction patterns of the quenched samples were obtained at atmospheric pressure
with a Stoe STADI-P diffractometer operating in transmission mode, using Co Kα1 radiation
selected with a focusing germanium monochromator and a linear position-sensitive detector.
High quality XRD patterns (recorded from 2θ 20° to 120° with 0.016° steps and 1000 s counting
time) were analyzed using the Le Bail algorithm of the program package GSAS (Larson and Von
2
Dreele, 1994) to determine the unit cell parameters. Refined parameters were background
coefficients, lattice constants, line widths, asymmetry parameters, and zero error.
High-resolution 27Al magic angle spinning nuclear magnetic resonance (MAS NMR)
spectroscopy was carried out in a Bruker DRX800 (18.8 T) spectrometer installed at the
University of Lille, France. The spectrometer is equipped with a multinuclear probe operating
at 208.49 MHz for 27Al. Powdered samples were packed in 2.5 mm zirconia rotors and spun at
30 kHz. A single-pulse sequence was used with an observation frequency for 27Al of 208.49
MHz, and a pulse width of 0.5 μs (π/2 pulse length of 5 μs). The delay time was optimized to
500 ms. The chemical shifts are reported in parts per million from 0.1 M AlCl3 solution. The
spectrum was simulated using the DMFit program (Massiot et al., 2002) handling finite
spinning speeds and mixed quadrupolar/CSA interaction in MAS experiments. 1H MAS-NMR
spectroscopy was used to determine the possible presence of H species in the samples.
An analytical transmission electron microscope (ATEM, Philips CM-20FEG) operating at 200 kV
and equipped with an energy-dispersive X-ray spectrometer (EDXS, NORAN Ge detector) was
used to observe the microstructures of the TiO2 grains. Samples were thinned in an Ar ion
milling machine at 4.5 kV and 1 mA until electron transparency was reached. ELNES spectra
were collected with a Gatan PEELS 666 parallel electron spectrometer attached to the Philips
CM20 FEG TEM. For the calibration of energy scale, the C K-edge of amorphous carbon was
measured simultaneously. Aluminum content in TiO2 has been quantified by EDX analysis. The
quantification included an absorption correction and the calibration of Cliff–Lorimer factors for
aluminum and titanium with respect to oxygen, using standard samples (Langenhorst et al.,
1995). At least ten Al-doped α-PbO2 type TiO2 single crystals have been analyzed at each
pressure to calculate mean compositions. A counting time from 100 to 300 s has been used in
order to accumulate enough X-ray counts.
3. Results
3.1. X-ray Diffraction data
Representative XRD patterns of the samples synthesized at 7, 10, 13.6, 16, and 20 GPa and
1300 °C are shown in Fig. 1. The diagram of the sample synthesized at 7 GPa shows reflections
corresponding to CaCl2 – structured rutile (Escudero et al., 2011b), and corundum (α-Al2O3,
PDF 10-0173). The diagrams of the samples synthesized from 10 upto 20 GPa show the same
reflections of corundum (α-Al2O3), and new peaks corresponding to the TiO2 high pressure
polymorph with the structure of α-PbO2 (PDF 21-1236). The further transformation of TiO2
into the baddeleyite-type TiO2 is not observed in the quenched samples. Increasing the
synthesis pressure produces very similar α-PbO2 TiO2 patterns. There are, however, small
variations in the peak positions, which shift slightly towards larger angles until 16 GPa.
However, the signals corresponding to the α-PbO2 type TiO2 observed in the sample
synthesized at 20 GPa shift to lower values of 2θ, respect to the signals of the 16 GPa sample
(see peaks under the vertical guide lines). The position of the corundum signals does not
change appreciably with the synthesis pressure. Two differences can be observed in the XRD
diagram corresponding to the sample synthesized at 16 GPa. On one hand, some low intensity
reflections situated at 2θ = 28.3°, 38.5° and 46.5° appear on the XRD diagram. The reflections
have been marked with asteriks on the Figure. On the other hand, the intensity of the peaks
3
corresponding to α-PbO2 type TiO2, especially those situated at high 2θ angles is much higher
than those corresponding to the corundum phase, if the intensity radio is compared to the
lower pressure samples.
To refine the lattice parameters of α-PbO2 type TiO2 recovered from different pressures, the
XRD patterns have been analyzed with the Le Bail method using the GSAS software (Larson and
Von Dreele, 1994), as described in the experimental section. The starting parameters have
been taken from Grey, Li, Madsen and Braunshausen for α-PbO2 type TiO2 (Grey et al., 1988),
and from Toebbens, Stuesser, Knorr, Mayer and Lampert for corundum (Tobbens et al., 2001).
All the reflections corresponding to the TiO2 phase of the samples synthesized from 10 to 20
GPa could be fitted on the basis of an orthorhombic unit cell with space group Pbcn, which is
the crystal structure of α-PbO2 type TiO2.
Fig. 2 shows the variation of the unit cell parameters of for α-PbO2 type TiO2 as a function of
synthesis pressure. All the lattice parameters (a, b, c as well as the unit cell volume) decrease
linearly when increasing the synthesis pressure up to 16 GPa. The lattice parameters of the
recovered α-PbO2 type TiO2 phase after the treatment at 20 GPa do not follow this trend.
These parameters are higher than it would be expected, according to the behavior of the
samples synthesized at lower pressure, and are similar to the ones of an Al-doped α-PbO2 type
TiO2 phase synthesized at about 13 GPa. Lattice parameters of the recovered samples are
shown in Table 1.
3.2. High-resolution 27Al MAS-NMR and 1H MAS-NMR
Fig. 3 shows the 27Al MAS-NMR spectrum of the 85% TiO2 + 15% Al2O3 sample synthesized at
14 GPa and 1400 °C. The spectrum consists of two different signals centered at 15 and 5.5
ppm, and correspond to hexacoordinated Al environments. No signals corresponding to other
octahedral or tetrahedral Al environments are observed. The XRD pattern of this recovered
sample (not shown) indicates the presence of α-PbO2 type TiO2 as a very major phase with
some very small reflections of α-Al2O3 corundum. Thus, the first signal can be attributed to
remaining α-Al2O3 corundum, and the second signal must correspond to an octahedral Al
environment in α-PbO2 type TiO2. This corresponds to Al3+ replacing a Ti4+ on the normal
octahedral Ti sites of the α-PbO2 type TiO2 polymorph. This signal can be readily fitted with CQ
= 6.3 MHz, quadrupolar asymmetry parameter η = 0.5, and isotropic chemical shift δiso = 10.3
ppm. Single-pulse MAS NMR carried out at high external magnetic field has demonstrated to
be very sensitive to the different Al environment in Al-doped TiO2 rutile synthesized at high
pressure (Escudero et al., 2011a). This indicates that despite other Al species have been
observed in Al-doped rutile, only one octahedral Al environment exists in α-PbO2 type TiO2, at
least at relative low pressures (c.a. 10–14 GPa). The 1H MAS-NMR spectrum (not shown) did
not show significant signals corresponding to H and thus the samples can be considered to be
anhydrous.
3.3. Electron microscopic observations
Fig. 4 shows two micrographs of the sample synthesized at 10 GPa. The microstructure consists
of α-PbO2 type TiO2 grains decorated with lamellae which present different thickness. Relative
wide lamellae of this secondary phase can be observed in Fig. 4A, as well as the corresponding
4
electron diffraction pattern. The observed extra reflections on the SAED pattern not
corresponding to the α-PbO2 type TiO2 are compatible with CaCl2-type TiO2, which is an
orthorhombic distortion of Al-doped TiO2 rutile produced when quenching from high pressure
(Escudero et al., 2011b). The high resolution TEM image of this sample as well as its SAED
shown in Fig. 4B reveal that the lamellae also exist in a nanometric level, and are parallel to the
(0 0 1) α-PbO2 type TiO2 plane.
Samples synthesized at 12 and 13.6 GPa (not shown) exhibit normal α-PbO2 type TiO2 grains
with no extra phases or defects, and corundum grains. Fig. 5 shows a TEM micrograph of an αPbO2 type TiO2 grain of the sample synthesized at 16 GPa. A microstructure consisting of
ordered and homogeneous lamellae can be observed. The HRTEM image shown in Fig. 5B
indicates that the lamellae are parallel to the (3 View the MathML source1¯ 1) plane of the αPbO2 type TiO2 polymorph. The lamellae are about 5 or 6 lattice planes wide, being the d
spacing around 6.7 Å.
This observation suggests the presence of another phase, almost no detectable by XRD, and
that could be responsible for the low intensity reflections observed in its XRD diagram. To
clarify the nature of this phase, many SAED were collected in order to detect extra diffraction
spots which could correspond to this secondary phases. Only along three different zone axes
some extra and generally weak spots not corresponding to the α-PbO2 type TiO2 polymorph
could be observed (Fig. 5C, D, and E). The extra spots are compatible with a material
possessing an orthorhombic symmetry, as it will be discussed in the next section. The ELNES
spectrum of the Al-doped sample synthesized at 16 GPa is shown in Fig. 6. Spectra
corresponding to both Al-doped and Si-doped TiO2 synthesized at 12 GPa have been added to
the Figure for comparative reasons. ELNES spectra have demonstrated to be sensitive to the
valence state of Ti (Stoyanov et al., 2007). There are no appreciable changes between the
different spectra, which consist of four bands centered at 458.4, 460.5, 463.7 and 465.7 eV.
The signals correspond to the typical Ti L3 and L2 edges, whose positions are the expected for
Ti4+ (Stoyanov et al., 2007). This indicates that no changes in the valence state of Ti have been
produced during the high pressure experiments.
Fig. 7 shows a TEM micrograph of an α-PbO2 type TiO2 grain of the sample synthesized at 20
GPa. The microstructure of the TiO2 is different to all previous samples and consists of stacking
faults.
Fig. 8 shows the change in the Al2O3 solubility in TiO2 with pressure. In general terms the
effect of high pressure is to enhance the Al2O3 solubility in TiO2, but two drops at 10 and 20
GPa can be observed. The TiO2 grains are able to accommodate up to at least 35% Al2O3
(wt.%) at 13.6 GPa and 1300 °C. EDX data from the sample synthesized at 16 GPa have not
been taken into account for reasons that will be explained below. Al2O3 contents observed in
the TiO2 grains are shown in Table 1.
4. Discussion
4.1. Phase stability of the Al-doped α-PbO2 type TiO2. Absence of Akaogiite at higher pressures
in the quenched samples
5
XRD data indicate that CaCl2-structured TiO2 is the stable Al-doped TiO2 phase in the
quenched samples up to at least 7 GPa, transforming into the α-PbO2 type TiO2 between this
pressure and 10 GPa. This last is the stable phase above 10 GPa until at least 13.6 GPa. These
results are in good agreement with the previous studies of Withers (Withers et al., 2003) and
Akaogi (Akaogi et al., 1992) for pure TiO2. Thus, the Al-doped α-PbO2 type TiO2 can be
recovered at atmospheric pressure and does not transform back into rutile or CaCl2-type TiO2
when releasing the pressure. The transformation of pure α-PbO2 type TiO2 to the baddeleyite
phase, Akaogiite, has been reported at about 17 GPa at 1300 °C (Olsen et al., 1999). However,
this last phase is not observed in the quenched sample synthesized at 20 GPa and 1300 °C,
being the pure α-PbO2 type TiO2 the detected polymorph by XRD. Two explanations can be
given to this observation. On one hand, the presence of Al3+ in the TiO2 structure may
stabilize the α-PbO2 type TiO2 phase, thus shifting the phase boundary with the baddeleyite
phase toward higher pressures. On the other hand, the baddeleyite phase may be the stable
one at higher pressures but cannot be observed at atmospheric pressure, transforming back to
the α-PbO2 type TiO2 when releasing the pressure. Based on the behavior of the lattice
parameters with the synthesis pressure, the second hypothesis seems to be more reasonable.
All the lattice parameters, a, b, c and the unit cell volume decrease linearly with increasing
pressure up to 16 GPa, suggesting the formation of a solid solution between TiO2 and Al2O3 in
which Al3+ replaces Ti4+ on normal α-PbO2 type TiO2 octahedral sites. This mechanism of
solubility is consistent with the NMR data, which do not show the presence of interstitial Al3+,
at least at relatively low pressures (up to 14 GPa). A similar behavior has been observed in the
lattice parameters of Al-doped rutile at pressures up to 2 GPa at 1300 °C. This was associated
with a substitutional solid solution in which oxygen vacancies compensate the charge
difference between Al3+ and Ti4+ ( Escudero et al., 2011b and Slepetys and Vaughan, 1969).
Due to the similar ionic radii (0.61 Å for Ti4+ and 0.53 Å for Al3+ in octahedral coordination)
(Shannon, 1976) aluminum can occupy regular cation positions giving a substitutional solid
solution. The slightly different ionic radii of the cations explains the decrease of the unit cell
parameters when this substitution takes place. However, the change of the Al-doped α-PbO2
type TiO2 lattice parameters at increasing pressures is larger than the observed in the rutile
structure at pressures in which the subsitutional solid solution is the main solubility
mechanism. This indicates that the substitution is more effective at higher pressures. Taking
into account that interstitial Al was found in the TiO2 rutile structure at much lower pressures
and above 3.2 wt.% of Al2O3 (Escudero et al., 2011b), that no other Al environments have
been detected by high-resolution 27Al MAS-NMR, that the 1H MAS-NMR data strongly suggest
that the samples are anhydrous and that the ELNES spectra indicate the absence of other Ti
species such as interstitial Ti3+, it seems that the main mechanism of Al incorporation into αPbO2 type TiO2 with increasing pressure consists of the substitution of Ti4+ by Al3+, with the
formation of oxygen vacancies to compensate the charge, at least until about 14 GPa.
Although the formation of oxygen vacancies would not be favored at high pressure, the αPbO2 type TiO2 seems to be able to accommodate them up to a certain level. Above 14 GPa,
the structure is not able to accommodate more oxygen vacancies and thus a phase transition
takes place, as it would be discussed in the 4.3 section. The substitution of Ti4+ by Al3+ would
thus energetically compensate the instability produced by the presence of oxygen vacancies at
6
relatively lower pressures (10–14 GPa). The same substitutional mechanism has been reported
for Fe3+ doped α-PbO2 type TiO2 at high pressure (Bromiley et al., 2004), and only
hexacoordinated Si has been observed in Si-doped α-PbO2 type TiO2 at the same pressures
(Mosenfelder et al., 2010).
As it was commented above, the trend in the behavior of the Al-doped α-PbO2 type TiO2
lattice parameters with increasing the synthesis pressure is not observed in the sample
synthesized at 20 GPa, which shows larger lattice parameters than expected. This indicates a
minor Al incorporation into the TiO2 structure and suggests that another TiO2 polymorph,
which presents a different affinity for Al, is the stable one at 20 GPa and 1300 °C. In fact,
according to the TiO2 phase diagram (Olsen et al., 1999), the baddeleyite polymorph is the
expected for pure TiO2 at this conditions and probably it is the one formed in the conditions of
the experiment. However, TiO2 baddeleyite type could not be observed experimentally at
room pressure after decompression (Dubrovinskaia et al., 2001, Sato et al., 1991 and Tang and
Endo, 1993). Thus, this polymorph reverts to α-PbO2 type TiO2 when releasing the pressure,
being this last one the observed at ambient conditions and keeping the Al content existing in
the baddeleyite phase. The Al2O3 content determined from the recovered TiO2 grains should
reflect the solubility of Al2O3 in TiO2 baddeleyite, Akaogiite, at 20 GPa. The increase of the
recovered α-PbO2 type TiO2 lattice parameters in the sample synthesized at 20 GPa compared
to the ones determined at 16 GPa indicates that the solubility of Al2O3 in TiO2 baddeleyite is
lower than in the α-PbO2 type TiO2 phase. The lattice parameters of the of the recovered αPbO2 type TiO2 of the sample synthesized at 20 GPa are close to those corresponding to an Aldoped sample synthesized between 12 and 14 GPa and 1300 °C. This comparison suggests that
the solubility of Al2O3 in TiO2 baddeleyite at 20 GPa is similar than the one shown by Al2O3 in
α-PbO2 type TiO2 at about 12–14 GPa.
4.2. Solubility of Al2O3 in α-PbO2 type TiO2
In general terms the solubility of Al2O3 in TiO2 phases increases with increasing pressure. This
behavior has been observed in Al-doped rutile (Escudero et al., 2011b). Al2O3 solubility in αPbO2 type TiO2 increases drastically with the synthesis pressure. The α-PbO2 type TiO2
polymorph is able to accommodate up to at least 35% Al2O3 (wt.%) at 13.6 GPa and 1300 °C,
compared to 1.22% at atmospheric pressure, in the rutile polymorph (Slepetys and Vaughan,
1969). However, two drops in the solubility of Al2O3 in TiO2 can be observed in Fig. 8. Both
decreases in the Al incorporation in TiO2 phases can be associated with phase transitions. On
one hand, TiO2 incorporates less Al at 12 GPa than at 10 GPa. This last sample showed
inhomogeneous nanometer-size rutile lamellae in the α-PbO2 type TiO2 grains that can give
rise to higher global Al2O3 content in TiO2. In fact, the uncertainty of the measurements is
higher for this sample. This indicates that the solubility of Al2O3 in rutile in this range of
pressure is higher than in the α-PbO2 type TiO2. This is consistent with the absence of
interstitial Al3+ in the α-PbO2 type TiO2, which does not present a second mechanism of
solubility to enhance the global Al2O3 solubility in TiO2.
On the other hand, the Al2O3 content of the TiO2 grains of the sample synthesized at 20 GPa is
much lower than the expected, suggesting again a TiO2 phase transition between 16 and 20
GPa. As it was indicated by the behavior of the lattice parameters, the baddeleyite phase
7
seems to be the stable polymorph at 20 GPa, transforming into α-PbO2 type TiO2 when
releasing the pressure but keeping its original Al content. The Al2O3 measured in this sample
can be compared to the one shown by an Al-doped α-PbO2 type TiO2 synthesized between 12
and 14 GPa and 1300 °C. All these observations support the non-quenchable character of the
TiO2 baddeleyite phase, and are consistent with the XRD data. The different microstructure
shown by the recovered 20 GPa sample must be thus connected with this reversible phase
transition from the TiO2 baddeleyite polymorph to the α-PbO2 type TiO2.
Some crystal-chemical explanations can be given to explain the different affinity of Al for the
different TiO2 polymorphs. Except around the phase boundary between rutile and α-PbO2
type TiO2, Al solubility in α-PbO2 type TiO2 is markedly higher than in TiO2 rutile. Taking into
account the geometry of the Ti4+ octahedral site in both rutile and α-PbO2 type TiO2, the
octahedral site in the α-PbO2 type TiO2 structure is more distorted that the one of the rutile
structure. Three different Ti–O bond distances of 1.87, 1.96, and 2.06 Å are found in α-PbO2
type TiO2 (Grey et al., 1988), whereas two almost identical distances of 1.96, and 1.97 Å are
reported for TiO2 rutile (Bokhimi et al., 2002). This suggests that the more distorted octahedral
Ti site in the α-PbO2 type TiO2 polymorph might facilitate the accommodation of an atom of
different nature in this structure, making thus the incorporation of dopants into this high
pressure polymorph of TiO2 more effective. The coordination number of Ti increases from six
to seven across the α-PbO2 type TiO2 to baddeleyite transition (Sato et al., 1991). Given the
radius radio criterion (Müller, 2007), the incorporation of a relatively small cation such as Al3+
into heptacoordinated sites would be less favored than into hexacoordinated sites, even at
very high pressures. This explains the observed lower affinity of Al for the Akaogiite
polymorph.
4.3. Possible non-quenchable phase at 16 GPa
The microstructure of the Al-doped TiO2 grains in the sample synthesized at 16 GPa suggests
the existence of a non-quenchable phase at this pressure and temperature, which reverts to αPbO2 type TiO2 when decreasing temperature and pressure. However, some rest of this phase
are observed in the quenched sample. EDX analyses of the different TiO2 grains indicate a
AlTi0.64O2.8 composition. This composition reflects very likely the original composition of a
possible non-quenchable aluminum titanium oxide phase, and it is close to a ABO3
stoichiometry. Two important mineral structures are well-known for this stoichiometry:
ilmenite and perovskite. In fact, the extra diffractions observed in the XRD pattern of this
sample (Fig. 1d) are compatible with both the structures of FeTiO3 ilmenite (space group
RView the MathML source3¯, PDF 01-071-1140) and with a family of orthorhombic rare earth
titanates perovskites (NdTiO3, PDF 29-922; GdTiO3, PDF 29-0614, space group Pnma). Despite
the cation size difference between the rare earths and Al, this perovskite-type structure is also
known for FeTiO3 at high pressure ( Leinenweber et al., 1991, Ming et al., 2006 and Wilson et
al., 2005). However, experimental studies have shown that it reverts to lower pressure
polymorphs when decompressing, being thus not observable at atmospheric conditions (Ming
et al., 2006). The extra weak electron diffraction spots observed in Fig. 5C, D, and E have been
tried to be indexed considering both possible structures. The planes could not be indexed
considering a hexagonal symmetry but are compatible with the orthorhombic structure shown
by the rare earth titanate perovskites. Taking into account that Fe3+ and Al3+ show a similar
8
size (Shannon, 1976), the non-quenchable character of the perovskite-type high pressure
phase of FeTiO3 is consistent with the similar behavior of the supposed perosvkite-type
structure of Al-doped TiO2 at 16 GPa. However, the formation of this supposed nonquenchable aluminum titanium oxide phase with an orthorhombic perovskite structure would
imply changes in the valence state of one of its elements in order to keep the electroneutrality
of the structure. ELNES spectra have shown that Ti remains as Ti4+. This indicate that the
supposed perovskite structure should be cation-deficient. In fact, cation-deficient rare earth
titanates with the perovskite structure have been reported in the literature. These compounds
are A-site deficient perovskite-type and exhibit an orthorhombic or tetragonal symmetry with
an orthorhombic distortion observed at room temperature due to octahedral tilting ( Howard
and Zhang, 2004, Jung, 2005 and Yoshii, 2000). They show a RE2/3TiO3 stoichiometry. The rare
earth cation is the bigger one and occupies the A perovskite site. In case of the aluminum
titanium oxide phase, the bigger cation should occupy the A site, giving thus a Ti2/3AlO3
compound, which is exactly the composition experimentally determined by EDX analyses on
the TEM. Indeed, both the extra and low intensity reflections observed on the XRD of the
sample synthesized at 16 GPa are also compatible with this structure (Nd0.66TiO3, PDF 00049-0244) and the extra weak electron diffraction spots appearing on the electron diffraction
patterns can also be indexed taking into account the orthorhombic structures shown by the
cation deficient rare earth titanates perovskites.
The observed microstructure of the Al-doped TiO2 grains recovered at room temperature and
pressure, the extra spots appearing on the SAED patterns, the composition determined by EDX
and the studies existing in the literature for similar compounds, suggest that a new nonquenchable phase with a cation – deficient perovskite structure is formed in the Al2O3–TiO2
system at 16 GPa and 1300 °C. However, in situ measurements at high pressure and high
temperature are demanded in order to clarify the nature of this possible new phase.
5. Conclusions
Aluminum incorporation in α-PbO2 type TiO2 increases with increasing the synthesis pressure.
Al-doped CaCl2-type TiO2 transforms into the α-PbO2 type TiO2 polymorph between 7 and 10
GPa in the quenched samples. The further transition into the baddeleyite type polymorph,
Akaogiite, has not been directly observed. The microstructure of the TiO2 grains with
coexisting rutile and α-PbO2 type TiO2 consists of α-PbO2 type TiO2 grains decorated with
CaCl2-type TiO2 lamellae which are parallel to the [0 0 1] α-PbO2 type TiO2 plane. The
transition from rutile to the α-PbO2 type TiO2 polymorph produces a decrease of the Al2O3
solubility in TiO2, given the lower affinity of Al for the α-PbO2 type TiO2. A second decrease in
the Al2O3 incorporation in α-PbO2 type TiO2 exists between 16 and 20 GPa. This has been
ascribed to the non-quenchable character of the Akaogiite polymorph, which reverts to αPbO2 type TiO2 when decompressing. The α-PbO2 type TiO2 grains recovered from the sample
synthesized at 20 GPa are decorated with stacking faults, which are produced during this
reversible phase transition. The affinity of Al for the Akaogiite polymorph is lower than for αPbO2 type TiO2. The microstructure of the Al-doped sample recovered from 16 GPa and 1300
°C points to the existence of a non-quenchable aluminum titanium oxide compound at this
conditions. This phase might show a cation-deficient orthorhombic perosvskite structure.
However, new in situ experiments are demanded in order to clarify the nature of this phase.
9
Acknowledgments
Uwe Dittmann, Laurent Delevoye, and Nobuyoshi Miyajima are gratefully acknowledged for
help with thin section samples preparation, MAS-NMR, and ELNES measurements,
respectively. This work was supported by the European Union VI Framework Programme as an
HRM Activity (Contract number MRTN-CT-2006-035957), the Spanish Ministerio de Ciencia e
Innovación (Postdoctoral Fellowship MICINN-FECYT), the Visitors Programme of the
Bayerisches Geoinstitut, and the Leibniz program of the Deutsche Forschungsgemeinschaft (LA
830/14-1 to FL).
10
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16
Figure captions
Figure 1. Selected portions of the X-ray diffraction patterns of Al2TiO5 calcined at 1300 °C at
(a) 7 GPa, (b) 10 GPa, (c) 13.6 GPa, (d) 16 GPa, and (e) 20 GPa. R = TiO2 rutile, C = α-Al2O3
corundum, α = α-PbO2 type TiO2. The vertical guide lines are a help for the eye.
Figure 2. Al-doped α-PbO2 type TiO2 unit cell parameters (a, b, c and volume) plotted as a
function of the synthesis pressure. The error bars are approximately the size of the symbols.
Figure 3. High-resolution 27Al MAS-NMR spectrum of the 85% TiO2 + 15% Al2O3 sample
synthesized at 14 GPa and 1400 °C (black crosses). The fit of the spectrum (red line) as well as
the individual contributions (gray lines) are also included. (For interpretation of the references
to color in this figure legend, the reader is referred to the web version of this article.)
Figure 4. (A) TEM micrograph of an Al-doped α-PbO2 type TiO2 grain synthesized at 10 GPa
and 1300 °C. Zone axes: [0 View the MathML source1¯ 2] α-PbO2 type TiO2; [0 View the
MathML source1¯ 1] CaCl2-type TiO2. (B) High resolution TEM image. Zone axis [View the
MathML source1¯ 1 0] α-PbO2 type TiO2.
Figure 5. (A) TEM micrograph of an Al-doped α-PbO2 type TiO2 grain of the sample synthesized
at 16 GPa and 1300 °C. Zone axis [1 2 5]. (B) High resolution image. (C, D, and E) SAED patterns
of the grains taken along different zone axes. α = α-PbO2 type TiO2; Pv = Perovskite phase.
Zone axes α = α-PbO2 type TiO2 [3 2 View the MathML source7¯], [2 View the MathML
source5¯ 1], and [0 View the MathML source1¯ 2], respectively. The extra and weak diffraction
spots not corresponding to α-PbO2 type TiO2 have been indexed taking into account the
RETiO3 perovskite structure (NdTiO3, orthorhombic symmetry, space group P nma (Amow and
Greedan, 1996)), zone axes [View the MathML source2¯ 1 0], [1 View the MathML source1¯ 0],
and [1 0 1], respectively, but are also compatible with the cation-deficient structures
(Nd0.66TiO3, orthorhombic symmetry, space group P mmm (Yoshii, 2000)), zone axes [View
the MathML source2¯ 2 1], [View the MathML source1¯ 0 1], and [0 1 0], respectively.
Figure 6. ELNES spectra of Al-doped α-PbO2 type TiO2 grains synthesized at 16 GPa and 1300
°C (a), and 12 GPa and 1300 °C. The spectrum shown in (c) corresponds to a Si-doped α-PbO2
type TiO2 grain synthesized at 12 GPa which shows 1.6 wt.% SiO2 (Escudero and Langenhorst,
2011).
Figure 7. TEM micrograph of an recovered Al-doped α-PbO2 type TiO2 grain synthesized at 20
GPa and 1300 °C. Zone axis [1 2 1¯].
Figure 8. Solubility of Al2O3 in TiO2 versus pressure at 1300 °C. Data have been obtained from
EDX analyses on single crystals carried out on the TEM. The standard deviation of the
measurements is also shown. The straight lines are only a help for the eye. The dotted line
between 14 and 16 GPa indicates that the composition shown by the TiO2 grains synthesized
at 16 GPa may not correspond to Al2O3 in TiO2 but to another aluminum titanium phase.
Vertical guides indicate the reported transitions with pressure between the different TiO2
polymorphs. The α-PbO2-type TiO2 polymorph is the unique TiO2 phase observed in this study
at synthesis pressures above 12 GPa.
17
Table 1
Table 1. Summary of the Al-doped α-PbO2 type TiO2 samples synthesized at
high pressure at 1300 °C. The lattice parameters obtained from the Le Bail
analysis and the Al2O3 compositions in TiO2 determined by TEM-EDX are also
included. Numbers in brackets indicate standard deviations.
GPa
10
12
13.6
16
20
a (Å)
4.4662
4.4454
4.4394
4.4128
4.4408
b (Å)
5.5527
5.5313
5.5170
5.5001
5.5254
c (Å)
4.9325
4.9102
4.9004
4.8914
4.9080
Unit cell volume (Å3)
122.3234
120.7361
120.0224
118.7184
120.4286
wt.% Al2O3
18(2)
15.5(6)
35(1)
50(1)
26.2(7)
18
Figure 1
19
Figure 2
20
Figure 3
21
Figure 4
22
Figure 5
23
Figure 6
24
Figure 7
25
Figure 8
26