Pressure-induced amorphization of cubic ZrMo2O8 studied in situ

by X-ray absorption spectroscopy and diffraction

Tamas Vargaa, Angus P. Wilkinsona,*, Cora Lindb,William A. Bassettc, Chang-Sheng Zhad

aSchool of Chemistry and Biochemistry, Georgia Institute of Technology, Atlanta, GA 30332-0400, USAbDepartment of Chemistry, The University of Toledo, Toledo, OH 43606-3390, USA

cGeological Sciences, Department of Earth and Atmospheric Sciences, Snee Hall, Cornell University, Ithaca, NY 14853-1504, USAdCHESS, Wilson Lab, Cornell University, Ithaca, NY 14853, USA

Received 27 March 2005; received in revised form 16 May 2005; accepted 30 May 2005 A.K. Sood

Available online 13 June 2005

Abstract

A combination of in situ high-pressure X-ray diffraction and Mo K-edge XANES was used to examine the changes in local

structure that occur as the negative thermal expansion material cubic zirconium molybdate becomes amorphous on compression

and, hence, provide insight into the mechanism of amorphization. Amorphization started at w1.7 and by 4.1 GPa the sample

was glass like by diffraction. The XANES data shows that the pressure induced amorphization at 4.1 GPa does not involve a

complete change of molybdenum coordination from tetrahedral, in the starting phase, to approximately octahedral as would be

expected if the amorphous material were a metastable intermediate well advanced along the pathway towards decomposition

into a mixture of MoO3 and ZrO2. Recrystallization of the amorphous material at 600 8C and 4.1 GPa produced a sample that

was not a simple mixture of known MoO3 and ZrO2 polymorphs nor a known polymorph of ZrMo2O8.

q 2005 Elsevier Ltd. All rights reserved.

PACS: 62.50Cp; 61.10Ht; 61.43Kj; 61.10NZ

Keywords: D. Phase transitions; D. Thermal expansion; E. High pressure

1. Introduction

Negative thermal expansion (NTE) materials are of

considerable recent interest [1–4], due to scientific curiosity

and their potential for application in controlled thermal

expansion composites [5–9]. The low densities and highly

flexible frameworks of many NTE materials, combined with

the presence of lattice modes that soften on volume

reduction [10–16] predispose them to interesting high-

0038-1098/$ - see front matter q 2005 Elsevier Ltd. All rights reserved.

doi:10.1016/j.ssc.2005.05.041

* Corresponding author. Tel.: C1 404 894 4036; fax: C1 404 894

7452.

E-mail address: angus.wilkinson@chemistry.gatech.edu (A.P.

Wilkinson).

pressure behavior. Crystalline to crystalline phase tran-

sitions have been observed at high pressure in cubic ZrW2O8

[11,12,17–20], cubic HfW2O8 [13,21], cubic ZrMo2O8 [22,

23], cubic HfMo2O8 [22], ZrV2O7 [24], Sc2W3O12 [25],

Sc2Mo3O12 [26,27] and Al2W3O12 [27–29]. Additionally,

pressure induced amorphization (PIA) has been seen for

ZrMo2O8 [22,23], ZrW2O8 [30], Sc2W3O12 [31],

Sc2Mo3O12 [32] and Lu2W3O12 [33] at low pressures. PIA

is known in many different types of materials [33–41], and is

not uncommon for framework structures. Several different

microscopic mechanisms have been proposed for pressure

induced amorphization, some of which can be operating in

concert [39,40]. Typically, the amorphous phase produced

on compression is viewed as a metastable intermediate

between the low pressure crystalline phase and the

Solid State Communications 135 (2005) 739–744

www.elsevier.com/locate/ssc

T. Varga et al. / Solid State Communications 135 (2005) 739–744740

high-pressure thermodynamic equilibrium products [39,40].

Pressure induced transformations in NTE materials are not

purely of academic interest, the changes in expansion

characteristics that accompany them can be detrimental to

the material’s applications as the processing and use of

composites can involve high pressures [5–7].

ZrMo2O8 is polymorphic with trigonal [42], monoclinic

[43], orthorhombic [44] and cubic forms [45,46] accessible

at ambient temperature using ambient pressure syntheses.

There are also a variety of high-pressure ZrMo2O8 phases

known [24,47,48]. However, only the low density cubic

polymorph shows pronounced volume negative thermal

expansion; the orthorhombic phase shows weak NTE. Cubic

ZrMo2O8 displays isotropic NTE with a linear thermal

expansion coefficient of wK4.9!10K6 KK1 between 11

and 573 K, and it adopts a disordered structure related to

that of b-ZrW2O8 at all temperatures where it is kinetically

stable [45]. However, recent very precise measurements of

its lattice constant as a function of temperature suggest a

subtle change from static to dynamic oxygen disorder on

warming through 200 K [49]. The crystal structure consists

of a flexible network of corner-sharing ZrO6 octahedra and

MoO4 tetrahedra. The lack of a structural phase transition

within its range of thermal stability is potentially beneficial

from an applications perspective as the thermal expansion

coefficient displays no discontinuities in this range.

Additionally, unlike cubic ZrW2O8 [17], it does not undergo

any phase transformations under hydrostatic conditions at

pressures below 0.6 GPa [45], but it has been reported to

undergo a fully reversible 1st order phase transition between

0.7 and 2.0 GPa at room temperature under quasi-

hydrostatic conditions [22]. At higher pressure or under

non-hydrostatic conditions, ZrMo2O8 becomes amorphous

[22]. Grzechnik et al. [23] have reported that when

simultaneously heating and applying pressure, up to the

onset of amorphization at 1.3 GPa and up to w400 8C the

recovered products are a mixture of monoclinic [50] and

trigonal ZrMo2O8 [42,44,50]. This is consistent with prior

ambient pressure experiments, where cubic ZrMo2O8 was

observed to irreversibly transform to the trigonal polymorph

at w390 8C [51]. At pressures between 1.3 and 4 GPa and up

to 790 8C, and also at 1.3 GPa above 545 8C, the recovered

ZrMo2O8 was monoclinic [23]. These observations suggest

that a kinetically hindered transformation to the denser

monoclinic polymorph might be responsible for PIA at

!4 GPa. Quenching from temperatures in the range

740–825 8C and pressures above 4 GPa led to a mixture of

products, including the high-temperature high-pressure form

of MoO3 [52,53]. This suggests that a kinetically hindered

decomposition may be important in PIA at higher pressures.

In order to directly probe the mechanism of pressure

induced amorphization in cubic ZrMo2O8, an in situ local

structure measurement is desirable. This approach avoids

structural relaxation on pressure release [54,55]. High-

pressure XAS (EXAFS and XANES)[56] can provide some

of the desired information. While such studies have been

undertaken for many years [57], there are still considerable

experimental problems with measurements of this type. In

particular, diffraction from diamonds, if a diamond anvil

cell (DAC) is used, makes a large contribution to the

measured attenuation of the sample and DAC combination

for some X-ray energies and orientations of the diamonds

[58–60]. Diamond diffraction leads to unwanted peaks,

‘glitches’, in the measured absorption spectra, that can

interfere with data interpretation. This glitch problem is

sufficiently formidable that there have only been a relatively

small number of high-pressure XAS studies performed [56].

Here we report an in situ high-pressure, combined

XANES and diffraction study of cubic ZrMo2O8 in a DAC.

Diffraction probes long range order in the sample and XAS

provides information on the corresponding changes in local

structure. Using this approach, the presence or absence of

long range order in the sample examined by XAS is known.

However, if different samples/cells are used for the two

measurement types, the state of the sample examined by

XAS is somewhat uncertain. To the best of our knowledge,

there are no prior examples of high-pressure combined

monochromatic X-ray diffraction and absorption studies of

any material in the literature.

2. Experimental

2.1. Sample preparation

2.1.1. Cubic ZrMo2O8

ZrMo2O7(OH)2$2H2O was produced by the reaction of

aqueous solutions of ZrO(ClO4)2$xH2O and (NH4)6Mo7-

O24$4H2O in an acid medium by 3 days of refluxing. Then

the ZrMo2O7(OH)2$2H2O was dehydrated by a series of low

temperature heat treatment steps. Further details of the

preparation can be found in the literature [46].

2.1.2. Sc2Mo3O12

Stoichiometric quantities of Sc2O3 (Strem Chemicals,

Newburyport, MA) and MoO3 (Baker, Phillipsburg, NJ) were

thoroughly mixed, ground and heated together at 700 8C for

5 h and then, after regrinding, at 1100 8C for 12 h.

All other Mo-containing reference compounds were

commercial products: Na2MoO4$2H2O (Fisher Scientific,

Fair Lawn, NJ), MoO3 (Baker, Phillipsburg, NJ) and

MoO2(acac)2 (Strem Chemicals, Newburyport, MA).

2.2. Diamond anvil cell

A ‘hydrothermal diamond anvil cell’ (HDAC) [61,62]

was used with NaCl as a pressure calibrant, pressure

transmitting medium and sample diluent. 1.7 mm thick

diamonds with 500 mm culet faces were employed along

with a pre-indented rhenium gasket with a w300 mm hole.

The cubic ZrMo2O8 sample was uniformly mixed and

ground with NaCl and packed into the HDAC

Fig. 1. Diffraction patterns of ZrMo2O8 at room temperature as a

function of pressure in a diamond anvil cell. The data were collected

with 19.95 keV (lZ0.6215 A) X-rays.

T. Varga et al. / Solid State Communications 135 (2005) 739–744 741

(ZrMo2O8:NaCl w1:4). Pressure calibration made use of

the Birch equation of state for NaCl [63].

2.3. X-ray data collection and processing

All the measurements were carried out at the B-2 beam

line, Cornell High Energy Synchrotron Source (CHESS),

Ithaca, NY, USA, and employed a Si(220) double-crystal

monochromator. Diffraction patterns were recorded at

19.95 keV (lZ0.62148 A) using imaging plates. High-

pressure diffraction and Mo K-edge XAS data were

collected for ZrMo2O8 at 0.0, 0.24, 0.5, 0.71, 0.99, 1.28,

1.67 and 4.10 GPa using a 130 mm diameter incident beam

collimator. The sample at 4.1 GPa was heated to 600 8C, and

then cooled to room temperature prior to the collection of

further diffraction and spectroscopic data at ambient

pressure. Mo K-edge X-ray absorption spectra were also

recorded for cubic ZrMo2O8, Sc2Mo3O12, Na2MoO4$2H2O,

MoO3 and MoO2(acac)2 on samples that were diluted with

boron nitride under ambient conditions. XAS data were

collected using three scan regions with 5.0 eV steps in the

pre-edge region, 1.0 eV steps in the near-edge region and

3.0 eV steps in the post-edge region.

XAS spectra for the sample in the DAC were collected at

several slightly different orientations of the cell so that a

composite spectrum largely free from glitches due to

diamond diffraction could be subsequently constructed; as

the cell was reoriented the energies at which the glitches

appear change facilitating the reconstruction. 8–10 different

cell orientations were used at each pressure with one scan in

each orientation. The number of diamond glitches appearing

in an absorption spectrum recorded in a DAC depends upon

the energy of the measurement, with the glitch density

increasing as the energy goes up [56]. In the current case, the

glitch density was so high that we were unable to produce a

useable reconstruction of the EXAFS part of the XAS

spectrum, but we were able to produce useable data in the

XANES region as described in the next paragraph.

Spectra recorded at different orientations of the DAC

were compared to one another point by point in energy

space, and the lowest absorption value that was seen for all

of the orientations at a given energy was taken to be the true

absorption. This procedure works because the presence of a

diamond glitch at some energy/DAC orientation always

leads to an increase in the measured attenuation. In the event

that all the scans have a contribution from a diamond glitch

at some energy, the composite spectrum will contain a

residual artifact from the diamond glitches.

3. Results and discussion

3.1. Diffraction data

The integrated high-pressure diffraction patterns are

shown in Fig. 1. On compression to 1.7 GPa there is

progressive peak broadening and the appearance of

significant tailing on the high angle side of the ZrMo2O8

Bragg peaks. The broadening probably indicates the onset of

amorphization that has previously been reported to occur at

O1.3 GPa [22,23]. Alternatively, it could be a consequence

of pressure inhomogeneity/non-hydrostatic conditions

within the DAC. However, as the NaCl peaks do not show

a pronounced broadening like that seen for the ZrMo2O8,

and NaCl has a lower bulk modulus (w25 GPa [63,64]) than

cubic ZrMo2O8 (w45 GPa [45]), it is likely that the

broadening is not just a consequence of non-hydrostatic

conditions. The high angle asymmetry seen on the ZrMo2O8

peaks is most likely due to a non-uniform pressure

distribution within the illuminated sample volume, where

some relatively small fraction of the sample experiences

higher pressure than the majority of the material, perhaps

due to grain to grain contacts. Alternatively, it could

indicate the onset of a phase transformation where the

second phase has slightly smaller lattice constants than the

original material, but the current data provide no clear

evidence of the previously reported crystalline to crystalline

phase transition [22]. This may be due to the stress state of

our sample, as in the prior work [22] the transition was only

seen under quasi-hydrostatic conditions. On further com-

pression to 4.1 GPa, all traces of Bragg peaks from the

ZrMo2O8 are gone and the sample is glass like.

Heating the amorphous material to 600 8C for 1 h at

4.1 GPa led to re-crystallization. The diffraction pattern of

this material after cooling and decompression suggested that

there were some decomposition products present, possibly

T. Varga et al. / Solid State Communications 135 (2005) 739–744742

including the high-pressure, high-temperature form of

MoO3 (monoclinic space group P21/m) [53]. However,

there were peaks that could not be accounted for by any

known ZrO2, MoO3 or ZrMo2O8 polymorphs in the 2004

ICDD PDF-4 database. In particular, there was no evidence

of any monoclinic ZrMo2O8 [43] in the recovered sample.

These observations are generally in agreement with

previous work where ZrMo2O8 was recovered from high

pressure and temperature in a multi-anvil device [23] and

suggest to us the formation of a new phase or phases in the

ZrO2–MoO3 system.

3.2. X-ray absorption near edge spectroscopy (XANES) data

XANES spectra provide information on the coordination

geometry and oxidation state of the absorbing atom [65].

The pre-edge peak seen in many K-edge spectra is sensitive

to the site symmetry of the absorber, as the transition that

gives rise to this peak is dipole forbidden for centrosym-

metric species. In Fig. 2, Mo K-edge XANES are shown for

a series of compounds containing molybdenum in different

coordination environments. The compounds with tetrahed-

rally coordinated molybdenum, cubic ZrMo2O8, Na2-

MoO4$2H2O and Sc2Mo3O12, all have pronounced pre-

edge peaks (at w20.02 keV) that are resolved from the edge

consistent with a non-centrosymmetric coordination

environment. This pre-edge feature is not resolved from

the edge for either MoO3 or molybdenum oxide bis-

acetylacetonate (MoO2(C5H7O2)2 or MoO2(acac)2) and in

both cases it is weaker than in the compounds containing

tetrahedral molybdenum and much weaker in the case of the

acac complex. This is consistent with the distorted

octahedral coordination found in a-MoO3 [66,67], and the

Fig. 2. Ambient pressure Mo K-edge XANES for a series of

compounds containing molybdenum in different coordination

environments. (A) MoO2(acac)2, distorted octahedral coordination,

(B) MoO3, distorted octahedral, (C) Sc2Mo3O12, tetrahedral, (D)

Na2MoO4$2H2O, tetrahedral and (E) cubic ZrMo2O8, tetrahedral.

more regular, but still distorted octahedral coordination in

MoO2(acac)2; the oxygenyl groups on this later compound

are cis to one another [68]. In a-MoO3, the Mo–O bond

lengths range from 1.67 to 2.33 A [66] and in MoO2(acac)2,

they fall between 1.66 and 2.21 A [68].

The XANES data for ZrMo2O8 as it is compressed in the

DAC are shown in Fig. 3. As we were unable to remove all

traces of diamond glitches from these spectra, there are still

some spurious features present, most notably the attenuation

at w20.00 keV in the 0.71 GPa data, and the feature

between the pre-edge peak at 20.02 keV and the main edge

in the decompressed sample, almost certainly arise from

residual diamond glitches. However, the data are sufficiently

good to draw reliable qualitative conclusions. Over the

pressure range where the sample is still crystalline (0–

1.7 GPa), the intensity of the pre-edge feature and its

separation from the absorption edge does not change very

much, although there may be a slight drop in magnitude of

this feature at 1.67 GPa. Additionally, the shape of the

absorption maximum itself (20.04–20.07 keV) is almost

constant over this pressure range. However, on compressing

to 4.1 GPa, where the sample is X-ray amorphous, the shape

of the absorption maximum itself changes and the pre-edge

peak becomes noticeably less pronounced. The heated and

decompressed sample shows a similar pre-edge peak to the

amorphous 4.1 GPa sample, but the shape of the absorption

maxima are very different.

The presence of a pre-edge peak in the XANES for the

amorphous sample at 4.1 GPa that is resolved from the

absorption edge demonstrates that the molybdenum does not

fully transform from 4 coordination, in cubic ZrMo2O8, to

approximately octahedral coordination on amorphization.

This suggests that the amorphous material formed at

Fig. 3. Mo K-edge XANES data as a function of pressure for cubic

ZrMo2O8 compressed in a diamond anvil cell.

T. Varga et al. / Solid State Communications 135 (2005) 739–744 743

4.1 GPa should not be viewed as a metastable intermediate

that is well advanced along a pathway from crystalline cubic

ZrMo2O8 to a simple mixture of the binary oxide

decomposition products ZrO2 and MoO3. MoO3, even the

known high-pressure form of this compound, would be

expected to have a XANES signature similar to that shown

for a-MoO3 in Fig. 2, with a very weak pre-edge peak, as the

molybdenum coordination environments in these two forms

of MoO3 are almost identical [53]. The XANES indicate that

this sample contains some molybdenum in an environment

that is far from centrosymmetric, perhaps similar to that

found in monoclinic ZrMo2O8 [43,69], and suggest to us

that the amorphous material might be an intermediate on the

pathway to a higher density zirconium molybdate, although

not necessarily one with a 1:2 metal ratio. The XANES data

for the sample that was recovered to room temperature after

heating at 600 8C and 4.1 GPa for one hour still show a well

defined pre-edge peak. This is inconsistent with the

recrystallized sample only containing molybdenum as the

known high-pressure form of MoO3, as the XANES

spectrum would be similar to that of the a-MoO3 sample

shown in Fig. 2. However, it does not exclude the presence

of some 6-coordinate molybdenum. The XANES and

diffraction data taken together suggests that the recovered

sample contained a new zirconium molybdate that we were

unable to identify, perhaps, along with some MoO3. The

formation of a new high-pressure MoO3 polymorph with

coordination that is not close to octahedral seems unlikely. It

should be noted that the change in shape of the absorption

maximum after recrystallization tells us that the molyb-

denum in this sample is in a different coordination

environment from that in the starting amorphous material.

These observations suggest that the amorphous phase should

be viewed as a metastable intermediate on the way to a

denser zirconium molybdate, not as an intermediate on the

pathway to a mixture of MoO3 and ZrO2 decomposition

products.

4. Conclusions

Cubic ZrMo2O8 was observed to amorphize starting at

w1.7 and by 4.1 GPa there was no evidence for any residual

crystalline component in the sample. The combination of

in situ high-pressure diffraction and XANES demonstrates

that the pressure induced amorphization of cubic ZrMo2O8

at 4.1 GPa does not involve a change of molybdenum

coordination from purely tetrahedral, in the starting phase,

to purely octahedral or distorted octahedral as would be

expected if the amorphous material were a metastable

intermediate well along the pathway towards decomposition

into a mixture of MoO3 and ZrO2. However, the glass may

contain some six coordinate molybdenum, and the average

coordination number for the molybdenum in the glass may

increase on going to higher pressures. The recrystallized

material that was amorphized at 4.1 GPa was not a simple

mixture of known MoO3 and ZrO2 polymorphs, it probably

contained a zirconium molybdate phase that we were unable

to identify. The amorphous material at 4.1 GPa might

perhaps be viewed as a metastable intermediate on the

pathway to another crystalline zirconium molybdate.

Acknowledgements

This work is based upon research conducted at the

Cornell High Energy Synchrotron Source (CHESS), which

is supported by the National Science Foundation and the

National Institutes of Health/National Institute of General

Medical Sciences under award DMR-0225180. We are

grateful to the staff of CHESS, particularly K. Finkelstein,

for help and guidance. APW is grateful for support under

National Science Foundation grant DMR-0203342.

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