J.S.O. Evans – GR/N00524/01 NTE IGR 20032
Introduction/Background
Under GR/N00524/01 we were awarded funding to investigate the synthesis and properties of framework materials which show the unusual property of negative thermal expansion and oxygen mobility at low temperature.
This work has focussed on two main families of materials, those with general formula AM
2
O
8
(principally A=Zr, Hf;
M=W, Mo) and AM
2
O
7
(A=4+ metal, M=P, V).
The AM
2
O
8
family has been shown to exhibit negative thermal expansion (NTE) over an unprecedented temperature range of 2-1050 K.
1,2
This contraction is isotropic, reversible and comparable in magnitude ( α =-9x10
-6
K
-1 around room temperature for ZrW
2
O
8
) to the positive expansion of many “normal” ceramics. NTE materials have a range of potential applications, the most obvious being as components of composite materials with controllable positive, negative or even zero expansion coefficients. In addition to this one unusual property, the AM
2
O
8
family have also been shown to undergo a phase transition involving dynamic oxygen disorder at remarkably low temperatures
(ZrW
2
O
8
: 450 K; ZrWMoO
8
: 200 K). There are also a large number of different polymorphs of AM
2
O
8
materials which undergo a wealth of phase transitions as a function of temperature and pressure.
3
The structural complexity exhibited by materials with such a simple empirical formula is remarkable.
The cubic or pseudo-cubic AM
2
O
7
phases are a second family where a simple formula hides a wealth of structural subtlety.
3
Several of these materials had been shown to undergo phase transitions as a function of temperature, often from a high to low expansion form, with some solid solutions showing a negative coefficient of thermal expansion in their high symmetry phase.
4 An understanding of these structural phase transitions is crucial to understanding the properties of these materials.
Through this funding we have employed a PDRA (Dr Richard Gover) to study the properties of these materials. We have also been able to associate a PhD student (Simon Allen) with the project. We have enjoyed extremely profitable collaboration with experts in solid state NMR in the department (Professor R.K. Harris, Dr P.
Hodgkinson and Dr I.J. King) and (via a Marie-Curie collaboration) with Franck Fayon and Dominique Massiot of the
CNRS, Orleans. We have also collaborated with Ray Withers (ANU, Canberra) to perform electron diffraction studies.
This has allowed a multi-disciplinary approach to investigate the structures and properties of key members of these two families and the dynamic oxygen disorder that occurs at low temperatures in the AM
2
O
8
family of materials.
In later sections of this report we will highlight some of the key materials and methodology developments enabled by this funding. Owing to the page limitations of this document the descriptions given are brief, and the reader is referred to more detailed information in the literature and/or via the web where appropriate. A colour pdf file of this report is also on the web at http://www.dur.ac.uk/john.evans/webpages/grant_reports.html. The key studies and discoveries enabled by this funding are:
Cubic AM
2
O
8
Phases:
New AM
2
O
8
Materials:
• An in depth study of the kinetics of oxygen migration from 205 K in ZrWMoO
• The discovery of static to dynamic oxygen disorder in ZrMo
2
O
8
from 200 K
• The mechanism of oxygen exchange in ZrW
2
O
8
from 2D
17
O NMR
8
• In situ diffraction studies to probe the mechanism of formation of cubic ZrMo
2
O
8
from precursor material
• Total scattering experiments to probe the mechanism of NTE in ZrW
2
O
8
• The discovery of new polymorphs of AM
2
O
8
(A=Zr, Hf; M=Mo, W) – the LT phases
• Negative Thermal Expansion (NTE) properties of the LT phases
1

J.S.O. Evans – GR/N00524/01 NTE IGR 20032
Trigonal AM
2
O
8
: • Negative thermal expansion in one dimension in trigonal AM
2
O
8
phases
• The structure of a new high temperature form of trigonal AM
2
O
8
phases
AM
2
O
7
Materials: • Significant new insight into the structural complexity of AM
2
O
7
materials showing many of the literature assumptions concerning their structures to be incorrect
• 2D
31
P NMR studies on ZrP
2
O
7
showing its room temperature structure to contain 27 unique P environments
• Structure solution of ZrP
2
O
7
from powder data – 136 atoms in asymmetric unit
• Incommensurate superstructure in e.g. HfP
2
O
7
and PbP
2
O
7
• Phase transitions and structural complexity in SnP
2
O
7
, MoP
2
O
7
and NbP
2
O
7
• Structural complexity of GeP
2
O
7
– 1080 crystallographically unique atoms
Methodologies: • New methods of data analysis for extracting kinetic information from time/temperature resolved laboratory powder diffraction experiments
• Complex 2D
31
P experiments
• The first 2D
17
O EXSY experiments
• New methods for solving complex superstructures based on simultaneous use of Xray and neutron powder diffraction data
• Use of spherical harmonics in multi-pattern refinements to relate anisotropic peak broadening to structural features
Cubic AM
2
O
8
Materials
In addition to their unusual thermal expansion properties, many members of the cubic AM
2
O
8
family undergo an order-disorder transition at relatively low temperatures to a structure with dynamic oxygen disorder (the α → β phase transition for ZrW
2
O
8
). We have shown this phase transition to occur at ~450 K in ZrW
2
O
8
.
5
In the simplest picture the terminal oxygen atom (blue in Fig 1a) can be thought of as disordering over its site at ~(¼,¼,¼) and a related site at
~(¾,¾,¾). Alternatively this can be viewed as a concerted reversal of the direction in which tetrahedra point along the
<111> direction. At the beginning of this grant period
6
we gained evidence that in ZrWMoO
8
this process occurs at temperatures as low as 200 K – a remarkably low temperature for oxygen dynamics in the solid state. In contrast to
ZrW
2
O
8
we are able to quench the high temperature disordered structure to low temperatures and show that its volume is considerably lower than that of the ordered form (a rare example of dS/dV=-ive). We have also been able to study the kinetics of oxygen migration in this phase by a number of novel diffraction based techniques.
7
By quenching the material from a high to low temperature in a cryofurnace mounted on a laboratory powder diffractometer we have been able to trap the disordered structure at low temperature and follow the evolution of its diffraction pattern as a function of time as it anneals towards the more stable ordered phase at different temperatures (Fig 1b). The 892 diffraction patterns collected in this study have been analysed in a number of ways to extract the maximum amount of crystallographic and non-crystallographic information, and we’ve shown how either raw peak intensity changes or changes in Rietveld refinement derived quantities like cell parameters or site fractional occupancies can yield independent high quality kinetic information, the activation energy for the process (34(5) kJmol
-1
), and even information about the temperature dependence of coherent domain growth. Novel experimental protocols using temperature-time sweeps have also been used to measure oxygen dynamics in a different way (Fig 1c). This work has recently been highlighted as a “hot article” by the Royal Society of Chemistry and further details are available via the www.
7-10
2

J.S.O. Evans – GR/N00524/01 NTE IGR 20032
In the case of ZrMo
2
O
8
both our
6
and others
11
experiments have shown no evidence for oxygen ordering – the material appearing to remain in its disordered state at all temperatures despite careful thermal annealing. By performing high resolution neutron diffraction experiments at a series of closely spaced temperatures (4 K steps 2 to
502K; 7 K steps 502 to 3 K) on quench and slow cooled samples we’ve been able to show extremely subtle but significant changes in cell parameters and thermal expansion coefficient at ~200 K (Fig 1d).
7
We interpret this as being due to a static to dynamic disorder transition at this temperature – again remarkably detailed “non-crystallographic” information only obtainable using these multi-temperature methodologies.
0.8
(c)
0.6
0.4
0.2
0
0
-0.2
100 200 300 400 500
Fig 1a The structure of cubic AM materials. A O
2
O
8
6
octahedra share corners with M O
4 tetrahedra. At high temperatures
“terminal” oxygens (blue) are dynamically disordered over blue and white sites.
9.1500
9.1450
9.1400
9.1350
9.1300
9.1250
9.1200
9.1150
9.1100
0 quench-warm slow-cool
100 200 300
Temperature (K)
400 500
0.0020
0.0018
0.0016
0.0014
0.0012
0.0010
0.0008
0.0006
0.0004
0.0002
600
0.0000
Fig 1d The cell parameters of a quench cooled and slow cooled sample of
ZrMo
2
O
8
. The excess cell of the slow cooled sample reveals a dynamic to static disorder transition at 200 K.
Fig 1b Diffraction data collected on a quenched sample of ZrWMoO
8
in 13 minute intervals at 215 K. The growth of marked reflections allows the kinetics of oxygen ordering to be followed.
-0.4
O1 / O2
O3
O1 / O2 spinning sideband
O4
Temperature (K)
Fig 1c Excess occupancy of terminal oxygen sites for a slow cooled and quenchheated sample of ZrWMoO
8
. A least squares fit to the data (green) allows the activation energy to be derived.
O4
O1 / O2 spinning sideband O3 O1 / O2
750 700 650 600 500 450 400
Fig 1e 17 O EXSY spectrum of ZrW
2
O
8
at
313 K. Cross peaks are seen between all oxygen sites suggesting exchange between all crystallographically different oxygen sites on the 50 ms timescale.
A number of possible mechanisms for these order-disorder phase transitions have been proposed, though these have only been speculations based on average long range diffraction structures. To address this issue we have developed routes to produce highly enriched samples of
17
O labelled ZrW
2
O
8
. 1D variable temperature solid state
17
O
NMR studies have shown that at temperatures approaching the phase transition peaks broaden, and then coalesce at higher temperatures.
12
The chemical shift information suggests that all 4 crystallographic O sites are in dynamic exchange. Perhaps more interestingly, at 313 K (140 K below T
C
) 2D EXSY experiments have shown that all oxygen sites are in dynamic exchange with a rate on the order of 10’s of Hz or faster (Fig 1e). To the best of our knowledge these are the first such spectra to be published. The NMR data have led us to suggest that the disordering process occurs by a ratcheting motion of the WO
4
tetrahedra and that the motion is correlated over a reasonable length scale.
12
Total scattering studies on ZrW
2
O
8
to probe the origin of NTE in these materials, which will tie up some of the controversy in the literature about the subtle details of its mechanism, have also been performed in collaboration with
Martin Dove (Cambridge) and Dave Keen (ISIS/Clarendon Lab) and will be published shortly.
Early in the project we became aware of significant research in other groups regarding aliovalent doping in
AM
2
O
8
phases, some of which has now appeared.
13 We therefore did not expend significant research effort in this area.
3

J.S.O. Evans – GR/N00524/01 NTE IGR 20032
New AM
2
O
8
Phases and Mechanistic Insight Into Their Formation
One of the inherent difficulties with cubic AM
2
O
8
phases is that whilst they are kinetically stable over wide temperature ranges they are not thermodynamically stable at room temperature or, in some cases, under normal synthetic conditions. This makes their preparation difficult. In the case of cubic AMo
2
O
8
phases the best way to synthesise them is by careful decomposition of a precursor phase AM
2
O
7
(OH)
2
.2H
2
O.
14
Even this is a complex process with apparently crystallographically identical precursors made by different routes yielding cubic ZrMo
2
O
8
in differing narrow temperature windows or not at all.
We’ve studied the complex series of time and temperature dependent phase transitions in this system by insitu diffraction studies (Fig 2b). One key to successful synthesis seems to be a new intermediate polymorph of
ZrMo
2
O
8
, the so called LT phase. Despite the inherent low quality of diffraction data we’ve managed to solve the structure of this phase (Fig 2a) using a combination of X-ray and neutron powder diffraction data.
9,15
We’ve also developed methodologies for describing the anisotropic peak broadening of both X-ray and neutron peak widths simultaneously using spherical harmonic functions to allow successful Rietveld refinement (Fig 2d). To our knowledge this was the first publication of such an approach. We’ve been able to suggest a topotactic mechanism for the formation of the LT phase (Fig 2c), rationalise the anisotropic peak broadening and, from simple structural considerations, explain its facile transformation to the cubic NTE phase. The identification of the LT phase also allows quantitative Rietveld analysis of the reaction pathway from the precursor material allowing the ready identification of ideal preparation conditions for cubic materials (Fig 2e), and confirming that all transformations that occur are crystalline to crystalline. We’ve now prepared a range of LT materials
16
and have shown that they have an intrinsic volume contraction with temperature and are themselves a new family of NTE materials.
2
1
0
-1
-2
3
2 1 0 b
-1 -2 -3
-3 -2
-1 0
1 2 a
3
Fig 2a LT-AM
2
O
8
contains a network of corner sharing AO
6
octahedra and MO
4 tetrahedra. The framework topology can be described as a decorated rutile structure.
Fig 2d Rietveld refinement of structure of
LT-AM
2
O
8
The spherical harmonic required to describe the hkl dependent peak broadening is shown in the same orientation as the structure in Fig 2a. c a c b
Precursor → → → → → → → LT phase
Fig 2c The structure of precursor
ZrMo
2
O
7
(OH
2
).2H
2
O and the LT phase are topotactically related. The structural pathway between the two phases is shown schematically in the above figure.
Fig. 2b Diffraction data collected on warming ZrMo
2
O
7
(OH)
2
.2H
2
O from 300 to
900 K. Transitions to LT , cubic then trigonal ZrMo
2
O
8
can be readily followed as a function of time/temperature.
100
90
80
70
60
50
40
30
20
10
0
300
Precursor
LT
Cubic
Trigonal
400 500 600 700
Temperature (K)
800 900 1000 1100
Fig 2e Identification of the LT phase allows quantitative analysis of the phase transitions in this system. For the precursor studied in 2b pure cubic ZrMo
2
O can be made from 750 to 850 K.
8
Trigonal AM
2
O
8
Phases
Given the importance of phase transitions and polyhedral disorder in cubic AM
2
O
8
and other materials we have also investigated the structure of trigonal AM
2
O
8
phases, which had been reported as having somewhat unusual
4

J.S.O. Evans – GR/N00524/01 NTE IGR 20032 thermal expansion properties in the literature.
17
By variable temperature X-ray and neutron diffraction studies we have shown clearly the anisotropic nature of thermal expansion in these phases – they contract continuously in the ab plane over a wide temperature range yet expand along their c axis (Fig 3a) – and shown for the first time that they undergo a phase transition to a new form, α ’-AM
2
O
8
, at high temperature (Fig 3b).
18
The structure of the high T phase has been solved and refined for the first time (Fig 3c).
10.30
12.20
10.25
12.10
12.00
10.20
11.90
10.15
10.10
11.80
11.70
10.05
11.60
10.00
0 200 400 600
Temperature (K)
800
11.50
1000
Fig 3a Thermal expansion properties of
ZrMo
2
O
8
(open points) and HfMo
2
O
8
(closed points). Both phases show contraction along a and expansion along c . a b
Fig 3b The structure of the high temperature form of trigonal α ’-ZrMo
2
O
8
.
Fig 3c Rietveld refinment of the high temperature form of trigonal α ’-ZrMo
2
O
8 using back scattering data collected at
647 K on HRPD.
Pseudo-Cubic AM
2
O
7
Phases
The psuedo-cubic AM
2
O
7
phases can be structurally related to cubic AM
2
O
8
phases by the formal replacement of 2*MO
4
tetrahedra by a M
2
O
7
pyrophosphate or pyrovanadate group. The structure is thus made up of a network of corner sharing octahedra and tetrahedra with tetrahedra themselves sharing one corner. This, however, introduces a degree of structural frustration in that the resulting cubic structure requires the M-O-M linkage to be linear, an energetically unfavourable situation (P-O-P bonds, for example, being most common with bond angles of 124 to
144 ° ). This leads to the materials showing complex phase transitions as a function of temperature and unusual (in certain cases negative) thermal expansion behaviour.
Prior to this project these materials were all believed to relieve structural frustration by undergoing a phase transition to a 3x3x3 cubic superstructure with space group Pa¯3. We provided the first definitive evidence for the fallibility of this assumption for ZrP
2
O
7
using 2D through-space and through-bond
31
P solid state NMR studies which have shown definitively that there are 14 crystallographically distinct P
2
O
7
units present leading to 27 unique P sites
(Fig 4a).
19
Armed with this information we have developed techniques for solution and refinement of the complex superstructure using a simulated annealing approach applied simultaneously to neutron and X-ray data to solve the structure. The true symmetry of this material is Pbca with 136 crystallographically unique atoms in the asymmetric unit
(Fig 4a). It is one of the most complex materials studied to date by powder methods. Electron diffraction studies have revealed that ZrP
2
O
7
has an incommensurate structure at certain temperatures,
20
though no evidence for this is visible in X-ray measurements. We’ve shown that HfP
2
O
7
is isostructural at room temperature though, in this case, variable temperature X-ray experiments show clear evidence for the incommensurate region (Fig 4b).
16
We have also shown that other phases are more complex still. Variable temperature X-ray and neutron experiments on SnP
2
O
7
21
have revealed an unusual series of pseudo-cubic → triclinic → rhombohedral phase transitions with increasing temperature (Figs 4c and 4d), the first time such subtle symmetry changes had been identified. Solid state 31 P NMR studies have shown the room temperature structure to be more complex even than that of ZrP
2
O
7
, with 108 crystallographically independent P atoms in the asymmetric unit (Fig 4e).
22
The direct resolution of
98 of these independent P atoms (in 49 different P
2
O
7
groups) is a remarkable demonstration of the power of 2D solid state NMR in helping to unravel complex inorganic structures.
23
Using descent of symmetry arguments NMR data allow the room temperature symmetry of SnP
2
O
7
to be defined as either P2
1
or Pc. The structural methodologies developed for ZrP
2
O
7
should allow more detailed structural information to be derived. This work is in progress but is slowed by the computational difficulties of a low symmetry, large volume, 540 atom structure!
5

J.S.O. Evans – GR/N00524/01 NTE IGR 20032
8.04
I I
8.02
8.00
7.98
7.96
Pseudo
Cubic
Triclinic
Rhombohedral
7.94
C
B
(ppm)
-42
Fig 4a 31 P 2-D MAS double quantum spectrum of ZrP
2
O
7
. Cross-correlation peaks A-L reveal 14 P
2
O
7
groups in the structure, each coloured differently in the structure on the right.
0.34
8.30
0.33
0.32
0.31
0.30
0.29
0.28
0.27
0.26
0.25
300 400 500 600
Temperature (K)
700 800
8.29
8.28
8.27
8.26
8.25
8.24
8.23
8.22
8.21
900
8.20
Fig 4b The cell parameter of HfP
2
O
7 through the high to low expansion phase transition. An incommensurate region can be seen from the change in modulation of the type q~1/3<110> at 480 K.
Fig. 4c Selected diffraction data collected on warming SnP
2
O
7
from 300 to 1173 K.
Transitions from pseudo-cubic to triclinic to rhombohedral can be seen from e.g. subtle splitting of reflections at ~70 ° 2 θ .
7.92
100 300 500 700
Temperature (K)
900 1100 1300
Fig 4d The temperature dependence of the unit cell of SnP
2
O
7
on warming and cooling . Subtle phase transitions from a pseudo-cubic to triclinic to rhombohedral subcell can be clearly seen.
-28 -32 -36 -40
Single Quantum MAS dimension, ω
-44
/2 π (ppm)
Fig 4e Sheared and symmetrized experimental 31 P 2D refocused
INADEQUATE spectrum of SnP
2
O
7
. This can only be simulated using 98 distinct 31 P resonances corresponding to at least 49
P
2
O
7
units with two inequivalent P sites.
We have revealed similar levels of complexity in other materials such as MoP
2
O
7
and NbP
2
O
7
24
and in
GeP
2
O
7
have convincing preliminary evidence for a triclinic room temperature structure with 1080 crystallographically unique atoms in the pseudo-cubic cell – a complexity approaching that of small proteins. Other phases, such as
PbP
2
O
7
, have been shown to have incommensurate structures at room temperature, with aperiodic modulations leading to a continuum of local P environments. The work funded by this grant represents a significant leap forward in describing the response of these materials to their inherent structural frustration, which is in turn related to their thermal expansion properties. There is still, however, considerable work still required to fully understand these fascinatingly complex materials.
References
12
13
14
15
16
17
18
19
20
21
22
23
24
1
2
3
4
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5
6
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8 www.rsc.org/.
9 www.dur.ac.uk/john.evans/webpages/highlights_main.html.
10
11
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, 2003, submitted.
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, 2001, submitted.
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J. S. O. Evans; R. K. B. Gover; C. J. Crossland, ISIS Annual Report , 2003.
6