Structural complexity in Prussian blue analogues
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Structural complexity in Prussian blue analogues
John Cattermull,1, 2 Mauro Pasta∗ ,2 and Andrew L. Goodwin∗1
1
Department of Chemistry, University of Oxford, Inorganic Chemistry Laboratory, South Parks Road, Oxford OX1 3QR, U.K.
2
Department of Materials, University of Oxford, Parks Road, Oxford OX1 3PH, U.K.
(Dated: July 16, 2021)
We survey the most important kinds of structural complexity in Prussian blue analogues, their implications for
materials function, and how they might be controlled through judicious choice of composition. We focus on six
particular aspects: octahedral tilts, A-site ‘slides’, Jahn–Teller distortions, A-site species and occupancy, hexa-
cyanometallate vacancies, and framework hydration. The promising K-ion cathode material Kx Mn[Fe(CN)6 ]y
serves as a recurrent example that illustrates many of these different types of complexity. Our article concludes
with a discussion of how the interplay of various distortion mechanisms might be exploited to optimise the
arXiv:2107.07448v1 [cond-mat.mtrl-sci] 15 Jul 2021
performance of this and other related systems, so as to aid in the design of next-generation PBA materials.
I. INTRODUCTION
Since their crystal structure was solved half a century ago,1
Prussian blue analogues (PBAs) have been the focus of many
different areas of research; e.g. as battery materials,2 molec-
ular magnets,3 gas storage media,4 and more.5 In many ways
this functional diversity has its roots in the compositional and
structural versatility of the PBA family. PBAs are ‘hybrid’
double perovskites (general formula A2 BB0 X6 ), in which tran-
sition metals P and R occupy the alternating B- and B0 -
sites of a simple cubic lattice, are connected by cyanide link-
ers on the X-site to form a framework structure, with A-site
cations and water occupying the pore-space within.6 A signif- FIG. 1: Common types of structural complexity in PBAs: (a) octa-
icant feature of PBA chemistry is the incorporation of hexa- hedral tilts, (b) correlated ‘slides’ of A-site cations, (c) Jahn–Teller
cyanometallate, [Rn (CN)6 ](6−n)− , vacancies in the structure. distortions, (d) A-site occupancy (dis)order, (e) hexacyanometallate
These vacancies, along with variable oxidation states of the vacancies and the resulting connected pore-network structure, and (f)
P- and R-site metals, are electronically balanced by intercala- framework hydration.
tion of A-site cations. So the universal composition is given as
Ax Pm [Rn (CN)6 ]y · zH2 O, subject to the charge-balance con-
straint x + m = y(6 − n). Hereafter we will use the shorthand freedom, and where possible relate our observations to the
notation Ax P[R]y . specific example of potassium manganese hexacyanoferrate,
The crystallographic details of PBAs are known to have a Kx Mn[Fe]y .
significant effect on functionality; this is particularly true in
the case of battery materials.7 By way of example, potassium
manganese hexacyanoferrate – a promising K-ion cathode II. OCTAHEDRAL TILTS
material favoured for its high redox potential and low cost8
— undergoes two structural phase transitions during every Perhaps the most fundamental of the structural degrees of
charge/discharge cycle.9 These transitions are thought to im- freedom in PBAs are the octahedral tilts, which are widely
pact mechanical stability, resulting in capacity loss.9 Hence, studied in perovskites more broadly.12–17 Tilts are volume re-
a good understanding of the structural complexities at a fun- ducing distortions, since by bending the R–C≡N and P–N≡C
damental level will be vital for the design of next generation bonds, the overall distance between neighbouring transition
PBA materials, capable of competing with current state of the metals is reduced, as shown in Fig. 2. They can be activated
art lithium-ion technologies, for example. by external pressure18 or by internal (chemical) pressure.19
In this Focus article we survey the key types of structural The tilting of one octahedron necessarily affects that of its
complexity in PBAs, and explore how they might be con- neighbours, and at a critical external pressure, these tilts be-
trolled through judicious choice of composition. The char- come cooperative in a long-range sense, reducing the crys-
acteristic parent fcc structure — so famously associated with tal symmetry — usually to rhombohedral. This is seen,
PBAs — is common,10 but there are many distortions that can for example, in both stoichiometric Mn[Pt] and defective
break the cubic symmetry, locally or globally,11 and here we Mn[Co]2/3 .18 The crucial difference observed between the two
will discuss the most important of these: namely, octahedral is that Mn[Co]2/3 shows a greater volume contraction prior
tilts, A-site ‘slides’, Jahn-Teller activity, cation and vacancy to long-range symmetry breaking. Vacancies in a PBA sys-
order, and hydration [Fig. 1]. In our discussion we will ad- tem reduce connectivity, allowing volume contraction to orig-
dress the experimental sensitivity to these various degrees of inate from local distortions — rather than involving large do-
2
III. A-SITE SLIDES
One of the less obvious crystallographic consequences of
tilts is that they allow the A-site cations to off-centre. Doing so
can enhance further the interaction with neighbouring cyanide
ions, whilst also reducing cationic repulsion. Off-centering
necessarily creates an effective dipole in each cavity, and one
might expect that, at sufficiently high A-site occupancies, the
dipoles would interact in a collective way. This seems to be
the case in the monoclinic structure of K2 Mn[Fe] (a relatively
common structure type for PBAs9 ): the dipoles are large21
and arrange themselves in precisely the manner expected for
interacting dipoles on the cubic lattice26 [Fig. 3a]. We refer to
this collective displacement as a ‘slide’ degree of freedom (to
mirror the language of tilts12 and shifts20 in perovskites and
their analogues).
Just how significant is this degree of freedom in other mon-
oclinic PBAs? Here crystallography helps because the slide
distortion has a specific signature in the monoclinic unit cell
dimensions. In a technical sense, there are three contributions
to the monoclinic distortion: one is this slide component, a
FIG. 2: Approximate link between magnitude of correlated tilts second relates to a tilt distortion, and the third captures a vari-
and volume reduction in the PBA structure type. The solid line ation in the unit cell lengths (see SI for further discussion.)
illustrates the geometric result for volume reduction −∆V /V '
Using crystallographic data from a recent study of 24 differ-
6r[1 − cos(φ)], where the ‘tilt angle’ φ is the average distortion of
P. . .R–C and R. . .P–N angles and r is the average ratio of the P– ent Kx Mn[Fe]y samples21 we find the slide distortion to be at
N/R–C bond lengths to the P. . .R distance.11,18,21,22 Included in the least an order of magnitude more important than the other two
bottom left is an illustration of one representative tilt distortion. We (see Fig. 3b). Moreover the variation in slide distortion scales
include the positions of a variety of PBAs discussed in the text. Note, with potassium content (see Fig. 3c). The same analysis of
for example, that decreasing A-site cation radius favours larger tilts, other monoclinic PBAs yields similar results, and we provide
as seen for the family A2 Mn[Mn] (A = Cs, Rb, K, Na). a more comprehensive survey as Supporting Information (SI).
The key result is that A-site slides are most important in low
vacancy, high alkali-metal ion content PBA systems.27
On activation of an A-site slide, the A-site cation is trapped
in a lower symmetry site with a likely steeper potential en-
mains — with columns of octahedra able to compress into ergy well. The pore-size itself is smaller too, which in turn
the vacancies.20 Vacancies also increase the free pore space, is likely to affect ion mobility. Both factors will inhibit dif-
hence enhancing compressibility.20 fusion kinetics of the A-site cation.28 Furthermore, if the aim
is to maximise the A-site cation content for a higher specific
capacity active material then structural phase changes during
Substantial tilts > 15◦ are seen in PBA systems with high
electrochemical cycling will result in strains of the order of a
alkali-metal ion content, such as K2 Mn[Fe].21 Tilts reduce the
few percent.
size of the A-site cavity, and in doing so reduce the distance
between the intercalated cation and the anionic cyanide link- Just as tilts have a temperature dependence, so too does
ers. In this way one might intuitively expect smaller, more po- the degree of A-site sliding decrease at higher temperatures.
larising cations to more readily activate tilts. This is precisely For example the monoclinic distortion vanishes altogether at
what one finds in the series of manganese hexacyanoman- 400K in Na1.96 Cd[Fe]0.99 where the system returns to the par-
ganate systems A2 Mn[Mn] (A = Na, K, Rb, Cs) [Fig. 2].11,22 ent cubic structure.29 It follows that such phase transitions oc-
cur at lower temperatures when the A-site occupancy is re-
duced, since the magnitude of the dipole is smaller.30 In fact,
The fact that tilts are activated so easily suggests they are
phase transitions can be observed by changing the A-site oc-
the lowest-energy volume-reducing mode accessible to the
cupancy alone31 — an extreme version of the compositional
PBA structure type. This is consistent with our understand-
dependence discussed above.
ing of the phonon spectrum of PBAs20 and of their role in
negative thermal expansion.23,24 In principle, PBAs — even The phase change is a result of a cooperative distortion,
more so than double-perovskites25 — support a large variety something we don’t see in higher vacancy systems, because
of different tilt distortions. But it is the very simplest tilts (e.g. they can’t reach a high enough A-site concentration.32 A sim-
the rhombohedral distortion discussed above) that prevail in ilar theme emerges when Jahn-Teller active metals occupy the
practice.11,18,21 P-site.
3
FIG. 3: A-site slide distortions in monoclinic PBAs. (a) In K2 Mn[Fe], the K+ ions (green spheres) are displaced from the aristotypic (high-
symmetry) A-sites (white spheres). These displacements induce a local polarisation that couples such that successive layers ‘slide’ in opposite
directions. (b) The unit-cell parameters a, b, c, β of the monoclinic PBAs Kx Mn[Fe]y can be used to calculate the three fundamental symmetry-
adapted strains involved in the distortion away from cubic symmetry. The strain associated with A-site slides is an order of magnitude more
significant than the two other strains, and varies throughout the compositional series. The ellipsoid illustrates the spread of data points and
is a guide to the eye. (c) Other than for a single outlier (shown in red), the magnitude of A-site slide strain correlates well with K+ -ion
concentration.21 Further details and discussion are given in the SI.
IV. JAHN-TELLER DISTORTIONS duced to JT-inactive Mn(II) and cooperative JT order dis-
solves. There are a number of implications of JT distortions in
The Jahn-Teller (JT) theorem states that any non-linear this system. First, capacity loss is caused by structural dam-
molecule with a spatially degenerate electronic ground state age to the cathode from repeated cycling between cubic and
will undergo a geometric distortion.33 In the context of PBAs tetragonal phases.41 Second, the strong vibronic coupling re-
it’s typically the P-site metals which are JT-active; e.g. Mn3+ quired for electron transfer at the Mn3+ site limits rate ca-
or Cu2+ .24,34 Even in the presence of local JT distortions, pability when JT distortions are strongly coupled (this is the
however, the PBA structure may itself remain undistorted on effect referred to as ‘JT-release’ in Ref. 42). Third, the JT dis-
average as a result of the presence of vacancies and/or the tortion lowers the reduction potential itself.42 Hence, address-
many other mechanisms that impart flexibility.18,35 ing each of these issues will be key to optimising the electro-
chemical performance of this material, a point to which we
A good example of the effect of vacancies on collective JT
will return in due course.
order is given by the pair Cu[Pt] and Cu[Co]2/3 . Stoichiomet-
ric Cu[Pt] has tetragonal symmetry owing to the cooperative
JT distortion from Cu2+ ions.36 Vacancies typically disrupt
long-range JT order; hence the more familiar cubic structure
seen in Cu[Co]2/3 .18 A combination of single-crystal X-ray
diffuse scattering measurements and Monte Carlo simulations
have shown that the arrangement of vacancies can be pre-
dicted based upon crystal-field effects with Cu[Co]2/3 show-
ing a stronger preference for centrosymmetric Cu geometries
with vacancies aligned trans to one another.37 This tendency
means that JT-driven axial distortion can be accommodated
by aligning the longer bonds with the vacancies to coordinate
with structurally flexible water rather than the cyanide frame-
work [Fig. 4]. Once again, vacancies are disrupting connec-
tivity, meaning distortions are local and not long-range; hence
there is no splitting of Bragg reflections in the corresponding
powder diffraction patterns.35 The problem of the long-range
symmetry breaking in dilute JT systems has been extensively FIG. 4: Interplay amongst cooperative JT order, hexacyanometal-
studied in conventional perovskites such as KCu1−x Mx F3 (M late vacancies, and lattice strain. (a) In low-vacancy PBAs such as
Cu[Pt], the axial distortion of Cu2+ coordination environments (light
= Mg, Zn).38,39
blue octahedra) is strongly coupled, resulting in a tetragonal strain.
Returning to our leitmotif of the Kx Mn[Fe]y system, col- Here, for example, the cell axis denoted by the red arrows aligns
lective JT order has a strong effect on its performance as a with the JT axis and is longer than the orthogonal (black) axis. (b)
cathode material. The key problem is that the low vacancy The incorporation of hexacyanometallate vacancies allows for the JT
(y → 1) system in the fully oxidised state (x → 0) is tetrag- distortion axis to vary throughout the crystal such that a cubic metric
onal, as a consequence of the JT-active high-spin Mn(III) is retained on average. In this schematic, the central vacancy allows
3
(t2g eg1 ) configuration.40 During discharge, the Mn(III) is re- the JT axis to switch between vertical and horizontal orientations.4
Rb- and Cs-containing PBAs [Fig. 5]. Cation ordering can re-
duce repulsions and increase electrostatic attraction with the
cyanide ligands.48 It can be affected also by the reduction in
symmetry associated with other degrees of freedom. For ex-
ample, in Cs0.97 Cu[Fe]0.99 the Cs-ions fit well in the A-site
cavity and adopt a ‘dot-like’ arrangement47 (R-type order) to
minimise repulsions. In Rb0.85 Cu[Fe]0.95 , the activation of
tilts divides the A-site cavities into two types, one of which
is preferentially occupied by the smaller Rb ions forming a
‘rod’-like arrangement (M -type order).47 Although harder to
characterise using X-ray diffraction for the lighter alkali metal
FIG. 5: Two representative types of long-range cation order in PBAs. ions, their ordering and corresponding impact on lattice strain
(a) In Rb0.85 Cu[Fe]0.95 , correlated tilts give rise to two symmetry- would be helpful to understand when it comes to working with
distinct extraframework channels; the Rb+ ions (shown as green PBA electrodes, as is the case for Li-ion cathodes.49 Prelimi-
spheres) preferentially occupy the narrower channels so as to max- nary computational studies have shown cation ordering should
imise electrostatic interactions with the anionic framework. (b)
be prevalent in PBAs.48
In Cs0.97 Cu[Fe]0.99 , the absence of any long-range tilt distortions
means that all channels are equivalent by symmetry. The Cs+ ions
(green spheres) now occupy alternating A-sites so as to minimise
electrostatic repulsion between one another. Figure adapted from VI. HEXACYANOMETALLATE VACANCIES
Ref. 47.
It is difficult to decouple the dual effects of
[Rn (CN)6 ](6−n)− vacancies and A-site cations, since as
V. A-SITE CATIONS
the number of vacancies is reduced, A-site cations are
necessarily intercalated to charge-balance. In the Ax Co[Fe]y
The nature, position, and spatial ordering of A-site cations system discussed above, for example, it is not only the
in PBAs also has a large effect on their properties. A-site cations but also the vacancies that affect crystal-field
The impact of A-site cation type on the redox poten- stabilisation energies — because increasing hexacyanoferrate
tial of the P- and R-site metals is well exemplified by the content means Co–OH2 bonds are replaced by Co–NC
Ax Co[Fe]y system.43 Cation exchange of Na+ for K+ in bonds. Hence we observe CoII [FeIII ]0.67 and CoIII [FeII ]0.97
Na0.31 CoII [FeIII ]0.77 drives a change in charge order to give to be the dominant configurations across the Csx Co[Fe]y
K0.31 CoIII II II 44
0.77 Co0.23 [Fe ]0.77 . Although the effect of chang- family.50 There is a functional consequence of these different
ing A-site ion is clear, the physical origin of this change in electronic configurations. Whereas the former is a candidate
charge order is not well understood.44 Drawing on our analy- for photoinduced charge transfer, the latter is not: it lacks
sis above, we expect the smaller Na+ to interact more strongly sufficient flexibility to accommodate the associated change in
with surrounding cyanide ions hence promoting framework bond lengths.51
distortion and varying the P–N≡C–R angles. Hence the These dual effects of vacancies — namely, variation in both
crystal-field stabilisation energies at P- and R-sites should crystal-field stabilisation energies and framework flexibility
change as Na+ is exchanged for K+ . Furthermore, it’s un- — seem impossible to ignore when seeking to reduce the
derstood that Na+ , unlike K+ , can sit in the square windows number of vacancies in Kx Mn[Fe]y .9 Reducing vacancy con-
of the anionic framework,45 hence interacting more strongly centration should improve capacity, but will do so at the ex-
with the cyanide ligand at the expense of the crystal field of pense of the framework flexibility that improves cycle life.52
Co. The combination of these effects would reduce the mag- Hence, one anticipates some compromise must be found53 —
nitude of the Co crystal-field splitting in the Na+ -containing in which case we have to consider also the potential for va-
system, so stabilising the CoII (t2g5
eg2 )–NC–FeIII (t2g
5
) ground- cancies themselves to order.
state configuration. The tendency for vacancies to order was established in
Unlike the transition-metal octahedra, which are essen- Prussian blue itself (FeIII [FeII ]0.75 ), where they were found
tially fixed on the B and B0 sites, the A-site cations are in to avoid one another.54 It’s now known that vacancies are
principle free to displace across a valley of extra-framework strongly correlated — if not long-range ordered — across
sites, with the particular site adopted depending both upon the a wide range of PII [RIII ]2/3 PBAs.37 The form of these va-
ion/nanopore size, A-site type, and its concentration.46 Tradi- cancy correlations primarily depends on the nature of the P-
tional arguments based upon cation size can be used to explain site metal and the particular synthesis conditions used. Since
which site is occupied, with only those bigger than Na+ con- it has been demonstrated that larger cations — Rb+ and pos-
sistently occupying the largest interstitial site at the centre of sibly K+ — preferentially migrate via vacancy channels, the
the nanopore.45 Occupancy of this site comes at the expense vacancy content and pore-network geometry has important
of electrostatic interaction with the cyanide ligands; hence we consequences for diffusivity in a Kx Mn[Fe]y electrode.28 The
observe tilts and slides — as discussed in the previous sections nature of pore-networks in PBAs and the interplay with struc-
— in PBAs with high K-ion concentrations.9 tural distortions and hexacyanometallate vacancy distributions
Positional ordering of A-site cations has been observed in are illustrated in Fig. 6.5
FIG. 6: Effect of structural complexity on pore-networks in PBAs. (a) The activation of correlated tilt distortions, such as for the monoclinic
PBA shown here, isolates the cavities centred on each A-site. Here the void volume is shown as isosurfaces with a blue exterior and white
interior; note that neighbouring volumes do not connect. (b) In the aristotypic structure type, these cavities connect via narrow windows at the
cube faces to form a pore-network structure with simple cubic topology. (c) The incorporation of hexcyanometallate vacancies dramatically
opens up the pore-network, which now includes channels that run along the cube face diagonals. These channels are necessarily lined by the
water molecules (red spheres) that cap the vacant coordination sites of neighbouring transition-metal cations. All isosurfaces are generated
using the same probe radius.
VII. HYDRATION and crystal-field stabilisation energy mean the redox poten-
tials of Fe3+/2+ and Mn3+/2+ overlap to produce a single
It would be easy to view water as a spectator in PBA chem- plateau in the charge/discharge profile, distinct from the two
istry since the compounds are so insoluble, but it is now clear plateaux observed for the hydrated system.62
that the hydration environment of a PBA influences both struc- Ligand water is more strongly bound to the PBA lattice, but
tural complexity and functionality.22,55 It has long been known its removal can also have significant structural and electronic
that there are essentially two distinct kinds of water in PBAs: consequences. If there are two vacancies surrounding the P-
ligand water, which bonds tightly to the P-site metals that site transition-metal such that its octahedral coordination is
neighbour a hexacyanometallate vacancy, and interstitial wa- [PN4 O2 ] then the waters can arrange either cis or trans. In a
ter which occupies the nanopore.56 When ligand water neigh- trans arrangement, removal of bound water leaves the P-site
bours an interstitial water molecule it moves ‘off-axis’ to form in a (geometrically stable) D4h environment, but removal of
a hydrogen bond, but if the interstitial water is removed by cis water-pairs leaves the P-site in a polar C2v arrangement.
heating, then the off-axis water appears to return to the stan- The P-site metal necessarily relaxes along the polar C2 axis
dard ligand water site.57 to acquire tetrahedral coordination, which in turn affects both
In the case of Li+ and Na+ , which don’t necessarily oc- structure and electronics [Fig 7].63
cupy the body centre of the nanopore, interstitial water can Ligand water becomes reactive at high cell potentials, caus-
accompany the A-site cation58 to which it is strongly bound.59 ing unwanted side reactions with the electrolyte,7 exposing
The hydrophilicity of the Li- and Na-ions raises the Gibbs free the P-site metal to dissolution, and leading to capacity loss.21
energy of insertion, which is why one finds lower Na+ occu- Given that ligand water cannot be removed entirely by dry-
pancy compared with K+ .58 Conversely, larger cations replace ing before cell assembly, the only viable method for exclud-
interstitial water: hence the limiting stoichiometry K2 Mn[Fe] ing it from the PBA structure seems to be to replace it with
is anhydrous.60 hexacyanometallate anions.65 This strategy must be tensioned
Since interstitial water is relatively labile on heating, the against the potential benefits of some vacancy inclusion, as
drying conditions used in PBA synthesis can by itself al- discussed above.
ter their structure.61 For example, the system Na2 Mn[Mn]
switches between an open monoclinic form and a dense rhom-
bohedral form with the loss of two formula units of H2 O VIII. Kx MN[FE]y : A TRULY COMPLEX CATHODE
[Fig. 2].22 Sodium-ion PBAs are in general particularly MATERIAL
sensitive to drying conditions, with high temperature vac-
uum drying favouring a dense rhombohedral structure,7 in To illustrate the functional implications of the various types
the same way that PBAs collapse under external pressure.18 of structural complexity covered in our Focus article, we con-
These structural transitions effect the crystal-field stabilisa- clude by returning to our core example, Kx Mn[Fe]y . We have
tion energy of the P- and R-site metals. In the dehydrated already seen that this system displays many of the complexi-
Na1.89 Mn[Fe]0.97 system the competing ionisation energy ties covered in our article, and we now argue that understand-6
ing the interplay of these various aspects will be key to opti- distortion show improved electrochemical performance.27,65 If
mising its performance as a K-ion cathode material. We con- this is the case, then the distortion should be studied further to
sider in turn the two key strategies currently proposed in the gain a fundamental understanding that can better inform the
literature: namely, eradicating vacancies altogether,60 and ac- design of low vacancy Kx Mn[Fe]y .
tively including vacancies in concentrations as large as 15%.53 If the optimal solution is a PBA that includes vacancies
The first strategy is based on the assumption that, by for superior rate capability and structural stability,2,53 then the
removing vacancies from the structure, one can at once biggest problem is the presence of water in the structure.65
both maximise capacity and remove water entirely from the As well as careful choice of electrolyte to minimise damag-
framework.60 Doing so would solve the problem of side reac- ing reaction with water,8 alterations should be made to the
tions in a non-aqueous electrolyte at high potentials65 driv- structure to prevent water from escaping into the cell, since it
ing dissolution of the active material and causing capacity appears impossible to remove prior to assembly.61 Coating the
fade.21 The drawback of this strategy is that there remain particles could certainly be a promising solution since it also
two significant phase changes on cycling,41 and the reduced prevents dissolution of Mn ions.68,69 Vacancy network corre-
vacancy concentration lowers ionic conductivity.53 Taken to- lation could also play a role since they appear to improve ionic
gether, these two points mean the theoretical capacity is very conductivity.28,53
hard to achieve in practice, particularly at high rates.8
Doping on the P-site is an obvious strategy to overcome
the tetragonal distortion caused by JT-active Mn3+ in low va- IX. CONCLUDING REMARKS
cancy MnIII [FeIII ]y . As little as 12% Ni doping in sodium-
ion Mn[Fe]0.98 was enough to prevent tetragonal distortion.66 If there is one take-home message from our brief survey it
Avoiding global distortion to the structure activates the Mn re- is surely that PBAs support an extraordinarily rich diversity
duction reaction (Mn3+ / Mn2+ ), raising the specific energy.67 of structural complexity. Based on the collective experience
The drawback is that substitution with Ni does sacrifice of studying similarly complex families — e.g. the manganite
some capacity since it is electrochemically inert, but it seems perovskites, where the interplay of charge, spin, lattice, and
to give some security to the framework, improving cycling orbital degrees of freedom gives rise to a variety of unexpected
stability.66 Such a loss in capacity can be avoided by dop- and important physical properties (e.g. colossal magnetoresis-
ing with electrochemically-active Fe2+ . One third doping in tance) — one expects that learning both how to control the
Kx Fe0.33 Mn0.67 [Fe]0.98 suppresses the tetragonal distortion, various individual distortion types we discuss here and how
also improving cycling stability.41 Furthermore, the rate capa- to exploit the interactions between different distortions will
bility is improved by decreasing the band gap and lowering the together be crucial for developing the next generation of func-
K-ion diffusion activation energy.41 So it seems P-site doping tional PBA materials. Hence there is a clear motivation for
is an excellent strategy to supress the JT distortion, but what fundamental structural studies that systematically explore the
about the monoclinic distortion? It doesn’t appear to be avoid- vast parameter space accessible experimentally to PBAs.21
able in the same way, but the evidence suggests the highly While our emphasis here in terms of applications has been
crystalline samples with a strongly cooperative monoclinic on the use of PBAs as battery materials, there is every reason
to expect that structural complexity plays an important role in
the many other fields in which PBAs find application. We have
already highlighted one or two examples in the context of pho-
tosensitivity and charge order, but the implications for spin-
crossover,70,71 proton conductivity,72 gas storage,4 and coop-
erative magnetism73,74 are all straightforward to envisage.
Looking forward, one of the most difficult challenges to
be overcome is for the community to develop a robust un-
derstanding of the local structure of high-symmetry PBAs,
and the mechanistic importance of local distortions for func-
tion. In the various examples given here, we have (under-
FIG. 7: P-site hydration geometries and their implications. (a) The standably) relied on long-range symmetry breaking to iden-
trans-[PN4 O2 ] configuration, e.g. as favoured by Cu2+ , is struc- tify the structural degrees of freedom at play. Yet the intu-
turally stable to dehydration since both hydrated and dehydrated co-
ition is that these same degrees of freedom are just as im-
ordination geometries are non-polar and so constrain the P-site po-
sition. (b) By contrast, the cis-[PN4 O2 ] configuration observed in portant in the absence of long-range symmetry breaking. In
some Zn- and Mn-containing PBAs has C2v symmetry. On dehy- related fields, such as that of the disordered rocksalt cathode
dration, the P-site cation displaces along the local C2 axis (shown materials, computation is crucial in developing an atomistic
here as an arrow) such that its coordination geometry becomes ap- picture of local structure.75,76 However, the more open frame-
proximately tetrahedral. (c) A fragment of the network structure of work structure of PBAs relative to the dense lattice of rocksalt
rhombohedral (non-PBA) polymorph of Zn3 [Co(CN)6 ]2 , in which oxides, the importance of H2 O, and that of vacancies and their
Zn2+ ions are tetrahedrally coordinated by four hexcyanocobaltate correlations collectively mean that the computational chal-
anions.64 The geometry at the Zn2+ site is similar to that achieved on lenge here is substantial indeed. The development of efficient
dehydration of the cis-N4 O2 coordination environment shown in (b). force-fields, a current priority in the field of metal–organic7
frameworks,77–79 may be particularly useful in allowing sim- Acknowledgements
ulation on the requisite ∼ 10 nm scale. Experimentally, there
is an obvious motivation for further systematic local-probe in-
vestigations, exploiting e.g. total scattering, nuclear magnetic
resonance spectroscopy, Mössbauer spectroscopy, and/or X-
ray absorption spectroscopy.
Will it be worth it? We argue, emphatically, yes: despite
the obvious challenges posed by the many sources of com- We gratefully acknowledge financial support from the
plexity in PBAs, this diversity of structural, compositional, E.R.C. (Grant 788144), the ISCF Faraday Challenge projects
and electronic degrees of freedom presents an almost unparal- SOLBAT (grant number FIRG007) and LiSTAR (grant num-
leled opportunity to exploit complexity in functional materials ber FIRG014) as well as the Henry Royce Institute (through
design. UK Engineering and Physical Science Research Council grant
EP/R010145/1) for capital equipment. The authors would
like to thank Hanna Boström (Stuttgart), Arkadiy Simonov
Conflicts of interest (Zurich), Chris Howard (Newcastle), Trees De Baerdemaeker
(BASF), and Samuel Wheeler (Oxford) for many useful dis-
There are no conflicts to declare. cussions.
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