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Physica B 405 (2010) S6–S11

Contents lists available at ScienceDirect

Physica B

0921-45

doi:10.1

E-m

journal homepage: www.elsevier.com/locate/physb

Have we been here before? Inorganic precursors for collective electronicbehaviour of molecular crystals

Peter Daya Department of Chemistry, University College London, 20 Gordon Street, London WC1H 0AJ, UK

a r t i c l e i n f o

Keywords:

Structure–property correlation

Low-dimensional magnetism

26/$ - see front matter & 2009 Elsevier B.V. A

016/j.physb.2009.10.017

ail address: profpday@msn.com

a b s t r a c t

The community focused on collective electronic properties of organic and metal-organic molecular

crystals sometimes assumes that these are uniquely a consequence of the fact that the lattice is

composed of molecular building blocks. However, quantum mechanics has a wider horizon and

precursors for some of the phenomena currently occupying our field were observed and investigated a

while ago in various inorganic lattices. We recall some of the latter, which serve to highlight what really

is unique to the molecular solid state. We also recall a simplified classification scheme to correlate

crystal structures and physical properties. Examples from magnetism include one- and two-

dimensional ferromagnetism and complex magnetic lattice topologies; from electron transport we

mention low-dimensional superconductors incorporating localized magnetic moments

& 2009 Elsevier B.V. All rights reserved.

1. Introduction

One aim of the ‘evening’ talks at ISCOM 2009 is to put thedetailed scientific presentations of the rest of the programme intoa wider context, that is, to stand back and reflect about the placethat contemporary work on crystalline molecule-based materialsoccupies within the broad sweep of condensed-matter sciences.Since, increasingly, horizons are short, it may help to come froman older generation, to which category I plead guilty of belonging.Up to the middle of the last century condensed-matter physicsused to focus on the properties of the simplest prototypes for eachmajor bonding category: ionic, covalent, metallic. etc. and theirphysical properties were measured and explained in great detail.Then the chemists came on the scene and drew attention to theexistence of great swarthes of substances whose properties werenot described adequately by those simplistic paradigms.

This came about for two reasons. First, imaginative chemicalsynthesis created new structures of much greater complexitythan, say, MgO, Si or Al, sometimes exhibiting more than one ofthe extreme forms of bonding in the same lattice: ionic in onedirection and covalent in another for example, leading to whatbecame known as ‘low-dimensionality’. Second, playing with thefull palette of options offered by the Periodic Table, cases came tolight where, for instance, the structure suggested that inter-molecular Van der Waals forces were important but which werefound to be metallic conductors, or molecular organic solids thatbehaved as metals or ferromagnets. The latter, of course, provide

ll rights reserved.

the focus for the ISCOM meetings (International Symposium onCrystalline Organic Metals and Magnets) which I helped to found,along with Olivier Kahn, Philipp Guetlich, Dieter Schneider andothers. Originally conceived as a European forum, ISCOM hassince achieved world-wide status, as witness its current venue(the second time in Hokkaido).

Against this background, and from the viewpoint of aninorganic chemist, this contribution aims to recall some earlyexamples of unusual (that is, unusual for their time) physicalbehaviour in solids that nowadays would probably not be thoughtto lie within the ISCOM remit. I do so, not just to remember timespast, but to suggest that many of the phenomena we concentrateon today have their roots in a wider perspective of solid-statescience. I will also present a simple scheme relating structuresand properties that at least this inorganic chemist found helpful infocusing down on specific chemical systems.

2. Complex crystal structures

It may be salutary to remark that not only organic molecularcrystals have complex lattice structures. Back in 1975, thedistinguished solid-state chemist Jean Rouxel characterized someof the elaborate new structures he had discovered according tovarious styles of French architecture [1]. Thus an orderedintergrowth of linked tetrahedra in an ReO3 structure wasdesignated ‘Louis XIII style’ (what is sometimes called Roman-esque), while an elaborate succession of NdO6 octahedra betweenGaS4 tetrahedra in (NdO)4Ga2S5 could not be other than ‘Baroque’.More recently, as the high-Tc cuprates came under intense

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Fig. 1. (a) TEM lattice image of BISCO and (b) in-plane BiyBi spacings [2].

P. Day / Physica B 405 (2010) S6–S11 S7

scrutiny in the late—1980s, similar baroque structures emerged.In the BISCO material, for instance, TEM lattice-imaging revealed asinusoidally varying sequence of BiyBi separations (Fig. 1) [2].Both this and the previous baroque example have in common thatthe periodic repeat distances of their modulations areincommensurate with the underlying lattice.

Another point of similarity is that both the structures are ‘low-dimensional’, in this case layers. In fact the incommensuratemodulations almost certainly result from a competition betweendominant intra-layer bonding, which determines the preferredmetal–metal spacing in each layer, and the fact that the layers arejoined together in three dimensions with the stoichiometric ratiobetween the two components determined by chemical valence,e.g. [NdO] and [Ga2S5] being in the immutable ratio 4:1. Long agode Gennes pointed out that the incommensurate modulationsfound in the inter-molecular spacings in columnar discotic liquidcrystals like the phthalocyanines arose from just such a competi-tion [3]. In that case, which is actually one- rather than two-dimensional, the competition is between the preferred spacings ofthe aromatic rings and their long alkyl side-chains. Nevertheless itseems clear that analogous thermodynamic factors are involved.Another example of complex interaction geometry is the Kagomelattice, combining hexagons and triangles, but although there arewell-documented cases among organic molecular solids such asthe nitronyl–nitroxide magnets, the most comprehensivelystudied examples come from the inorganic world, such as themineral jarosite [4].

3. Low-dimensionality: structure–property correlations

The simplest approach to the thermodynamic properties ofaggregates is that of the ‘mean field’, which implies that the localinteractions are isotropic. As soon as we depart from the highestsymmetry space-groups then, it breaks down. Myriads ofinorganic crystals contain well developed planes or layers ofclosely spaced atoms and the recognition of this fact galvanizedtheorists from the 1960s onwards while the first practicalexamples, e.g. in magnetism, came from inorganic chemistry, asthe pioneering review of de Jongh and Miedema demonstrates [5].

Just because there are so many different kinds of compoundcontaining so many different elements—not to mention all thosecompounds that have not been synthesized yet—inorganicchemists have long been driven to seek correlations betweenstructures and properties at an empirical rather than theoreticallevel. Thus, the BCS theory of superconductivity tells us that highTc comes about when the density-of-states at the Fermi surface is

large and the electron–phonon coupling strong. That is certainlytrue of the examples known up to the 1980s but it is not muchhelp to practical inorganic chemists trying to decide what to makenext.

In the same spirit, the Hubbard model [6] provides an elegantexplanation why some transition-metal oxides are magnetically-ordered insulators while others are metallic and, furthermore,how temperature, pressure or composition can change one intothe other. The two parameters in the Hubbard model are the near-neighbour transfer integral t (roughly proportional to the overlapintegral between the frontier orbitals on neighbouring atoms ormolecules) and the repulsion U between two electrons of oppositespin on the same site. Broadly speaking when t dominates we arein the realm of conductors while if U dominates, a magneticinsulator results. More elaborate models bring into accountrepulsion between electrons on neighbouring sites, V, andelectron–phonon interactions (l).

For complex lattices the Hamiltonians taking account of allthese interactions are hard to solve but become manageable whenthe interactions are restricted to one- or two-dimensions [7].Nevertheless, like the BCS model, the Hubbard model is not veryapproachable for synthetic chemists. For that reason, I tried manyyears ago to find some simple structure–property correlationswhich, at least for low-dimensional solids, could guide such folktowards the categories of physical behaviour they were seeking[8]. Of course it oversimplifies matters grossly but may help tofind the physical wood through the chemical trees. The correla-tion concentrates attention on two features: the number ofbridging groups (atoms or molecules) separating neighbouringmetal ions and whether the metal ions are of single or mixedvalence. Table 1 [8b] lists some representative examples of singlevalence chains and layers and Table 2 [8b] has mixed valenceexamples, both classified according to the bridging geometries. Itturns out that clear patterns of optical, magnetic and electrontransport behaviour can be ascribed to each class, as set out inTable 3. In the remainder of this brief overview I will mentionproperties of a few examples that correspond to the categories ofTable 3.

4. Two contrasting Pt chain salts

There was much interest in metal-chain compounds in the1970s [9]. Two of the most famous examples from that timecontain Pt. The first is Krogmann’s salt (KCP), K2[Pt(CN)4]Br0.3 �3H2O [10], which is metallic along one axis at ambienttemperature but insulating along orthogonal axes. The second is

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Table 1Single-valence low-dimensional compounds with bridging groups [8b].

Geometry of

bridging

groups

Chains Layers

M—X—M Silicates (pyroxenes, amphiboles) Silicates (micas and clays,

e.g. montmorillonite)Sr(PbF5) FK2NiF4 and related

compounds (Rb2MnF4,

Cs2CrCl4, etc.,)

NbOCl3

Organic intercalated

structures related to

K2NiF4 ((RNH3)2MX4)

FeF3 �3H2O

High-oxidation states

ternary oxides (e.g

Ca2MnO4)

Nonmetal oxides (SeO2, Sb2O3)

Polymers of non-metals (e.g. SNx)

CrO3

4-Coordinate MX2

(a) tetrahedral (BeCl2, SiS2,

KFeS2)

(a) halides of 3d

elements (FeCl2, etc.)

(b) planar (PdCl2, PtS (b) chalcogenides of 4d

and 5d elements (TaS, etc.)

Oxyhalides (FeOCl)6-coordinate with terminal groups

MX2 �2Y(X=halogen, Y=H2O,

pyridine, 12 phenanthroline etc.)

Hexagonal perovskites –(a) halides AMX3 (A group IA or

organic cation; M=V, Cr, Mn, Fe,

Co, Ni, Ca)

(b) oxides (e.g. BaNiO3)

Planar d8 complexes Metal-rich phases: GdCl

(a) anions: A2[Pt(CN)4]

(b) neutal molecules: Nidmg2,

Ptbipy2 Cl2

(c) slats: [Pt(NH3)4][PtCl4]

Table 2Examples of mixed-valence low-dimensional compounds classified by numbers of

bridging groups [8b].

Geometry of

bridging groups

Chains (ID) Layers (2D)

M—X—M Pt(NH3)2Cl3, Wolfram’s Red Salt etc. Ca2–xYxMnO4

Magnetic phases oxides: MonO3n�1

B-subgroup oxides: Pb3O4, Sb2O4 KCu4S3

TIS

– –

K2Pt(CN)4Br0.30 �3H2O(KCP)

K0.60Ir(CO)2Cl2 �O �5H2O Gd2Cl3

GaS metal-rich

phases:

Ag2F, Hf2S

P. Day / Physica B 405 (2010) S6–S11S8

Wolfram’s red salt (WRS), [Pt(EtNH2)4][Pt(EtNH2)Cl2]Cl4 �4H2O[11]. The former consists of columns of identical square-planarcoordinated Pt atoms with a mean oxidation state of +2.3; thelatter likewise consists of chains of Pt atoms coordinatedequatorially by four ligands but with one Cl� between each pairof Pt-not, however, equidistant from both Pt but nearer one thanthe other forming a dimerised diatomic chain. In completecontrast to KCP, WRS is an insulator, albeit with stronglyanisotropic optical properties: dark red when the electric vectorof the incident radiation is parallel to the Pt chains but colourlesswhen it is perpendicular. WRS is an example of a deeply trappedcharge density wave, with alternating metal atoms well repre-sented by the oxidation states +2 and +4. In inorganic chemists’language it belongs to Class II in the Robin–Day classification ofmixed-valence compounds [12] whilst KCP belongs to Class III.

An alternative way of looking at WRS, more familiar tophysicists, is that as a manifestation of a dimerised one-dimensional N-site, N-electron system, it is an example ofnegative Hubbard U, i.e. the electron occupancies of the relevantPt orbitals are 2, 0, 2, 0y.. instead of 1, 1, 1, 1y.[13]. Anotherperhaps even more famous manifestation of a dimerised N-site,N-electron chain is poly-acetylene, (CH)x [14]. Fig. 2 highlights theisomorphism between these chemically disparate systems, firstpointed out in Ref. [15].

On doping with electron acceptors such as AsF5, (CH)x becomeshighly conducting, the charge transport being ascribed to solitons,i.e. discommensurations in the phase of the charge-density wave(Fig. 3). Something similar might be expected to occur in WRS andindeed, in the last few years, they have been observed directly by

scanning tunneling microscopy [16]. Unfortunately, however, nobulk charge transport has been detected though higher-orderoptical non-linearities are present [17].

5. Metal-chain magnets

The Pt chain salts are diamagnetic but many chain compoundsof the 3d elements are paramagnetic at ambient temperature,with intra-chain magnetic correlations becoming increasinglyimportant at low temperature. The best known and most wide-spread group are the hexagonal perovskites A+B2 +(X�)3 Thesecan be considered formally as related to the parent CdI2 structurein an analogous way to that in which the CdCl2 structure is relatedto cubic perovskites [18]. In the context of near-neighbourinteractions, this structure consists of chains of BX6 octahedrasharing opposite faces, the chains being separated by the Acations (Fig. 4).

If A is a large cation like [N(CH3)4]+, inter-chain interactionsare very weak. The earliest example of such a chain to be fullycharacterized as a one-dimensional antiferromagnet was[N(CH3)4]MnCl3 (TMMC) [19], where the extreme anisotropy ofthe magnetic interactions is illustrated by the dispersion of spin-wave propagating along and perpendicular to the chains (Fig. 5)[19]. An alternative measure would be to say that the mean-fieldTN derived from the near-neighbour interaction would be over60 K, while the observed TN is actually only 1 K.

More interesting than antiferromagnets are ferromagnets anda number of hexagonal perovskites share that property. In theparlance of the molecular magnets community they would becalled ‘single-chain magnets’. Near-neighbour ferromagneticexchange in insulators requires that the orbitals carrying thelocalized moments should be orthogonal [20]. Because suchorthogonality is very sensitive to the precise B-X-B angle, theoverall sign of the near-neighbour exchange may be altered bychanging either X or even the cation A. Table 4 presents this forvarious Fe and Ni examples [21]. The first single-chain magnet tobe thoroughly investigated was CsNiF3 [22] but the case of the Fecompounds is much more interesting and complicated. Spin–orbitcoupling splits the high-spin octahedral d6 ground-state 5T2g sothat a J=1 state lies lowest; trigonal distortion splits that intoMJ=0 and +/�1 with their relative energies depending onwhether the trigonal distortion is a flattening or elongation. Inthe hexagonal perovskites, the MJ=0 state lies lowest; hence adelicate balance arises.

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Table 3Correlation of structures and physical properties of low-dimensional compounds [8b].

Properties

Optical Magnetic Electron transport

Single valence

Bridged Discrete weak ligand field transitions Weak ferro- or ant

ferromagnetic

Insulators

Direct Ligand field bands strongly perturbed. Allowed bands strongly

shifted by dipolar coupling

superexchange

diamagnetic

Insulators conducting under pressure

Mixed valence

Bridged (Robin–

Day Class II)

Lowest transition is ionic exciton (M-M charge transfer) Diamagnetic Semiconductors metallic under high pressure

Direct (Robin–Day

Class III)

Plasma edge in visible reflectivity – Metals at 3001K become semiconducting at low

temperature (Peierls distortion)

Fig. 2. Two manifestations of the dimerised diatomic N-site, N-electron chain. (a)

WRS and (b) (CH)x. In each case the two possible phases of the charge-density

wave are shown [13].

Fig. 3. Two phases of charged defects (solitons) in WRS [13].

Fig. 4. BX6 chains in the hexagonal perovskite ABX3 structure.

Fig. 5. Spin-wave dispersion in TMMC parallel and perpendicular to the metal

chains [19].

Table 4Sign of intra-chain exchange in ABX3; (+) denotes FM, (–) denotes AFM [21].

Fe Ni

X= Cl Br F Cl Br

A=K + – –

Rb + – –

Cs + – + � –

NMe4 + +

P. Day / Physica B 405 (2010) S6–S11 S9

If the splitting of the two MJ states is large compared with theexchange-field, the ground-state remains a non-magnetic singlet;in the opposite case, a ferromagnetically-coupled chain arises. Thefirst situation is realized in CsFeCl3, which is actually non-magnetic at low-temperature. However, if field is applied there is

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Fig. 6. Magnetic neutron diffraction of CsFeCl3 at 0.7 K in applied field [23].

Fig. 7. The crystal structure of (BEDT-TTF)4[CuCl4] �H2O [29]. Green atoms are C,

yellow S, blue Cu, purple Cl and red O. (For interpretation of the references to the

color in this figure legend, the reader is referred to the web version of this article.)

P. Day / Physica B 405 (2010) S6–S11S10

a transition to long-range magnetic order, as shown by neutrondiffraction [23] but, as Fig. 6 shows, the initial ordering is actuallyincommensurate, only becoming commensurate at slightly higherfields. This unusual situation arises from competition betweentwo inter-chain interactions: dipolar, which increases with theintra-chain correlation length and exchange. On replacing Cs withthe smaller Rb, the exchange field increases so RbFeCl3 undergoesa transition to 3D ordering of the ferro-magnetic chains at finitetemperature in zero field, though even then, the ordering passesvia an incommensurate phase [24].

6. Inorganic and organic layers: combining physicalproperties

Some 25 years ago I pointed out the potential for inorganic–organic hybrid layer compounds to combine in one crystalproperties characteristic of the inorganic solid state, such asmagnetic ordering and those obtainable only in molecular organicsolids, like mesomorphism and polymerization [25]. Since thenmany other combinations have come to light, especially conduc-tion within the organic layers. Still, even that remains within thescope of the broad classification of Table 3 since the moleculesconcerned are all p-electron systems whose frontier orbitalsoverlap directly and whose mean charge is non-integral (+0.5 or+0.67), i.e. they are ‘mixed-valence’. Indeed the phenomenon of‘charge ordering’ in the organic metals is really another way forinorganic chemists to say that the lattice belongs to class II

instead of class III of the Robin–Day classification of mixedvalence compounds [12].

The greatest interest of the layer compounds containingsingle-valent metal ions bridged by ligands arises from theirmagnetic order. An early example is the series (CnH2n +1NH3)

2[CrX4] [26] which were the first bulk ferromagnets found to besoluble and optically transparent [27]. Their optical properties inparticular are of exceptional interest since ligand-field transitionsin the visible decrease in intensity at least one-hundredfold belowTc. Even up to ambient temperature the intensity relates directlyto the in-plane correlation length [28].

Much more recently attention turned towards layer com-pounds containing mixed-valence species, though the latter areorgano-chalcogen (e.g. BEDT-TTF) rather than metal-containingmolecules. In that context, what the inorganic chemists call‘mixed-valence’ class II the physicists call ‘charge ordering’, i.e.while the chemical stoichiometry tells us that the mean charge ofthe organic molecules is non-integral (+0.5 say), the crystal-lographic or Raman data point to different charges on differentmolecules (+1.0 and 0 in an extreme case). In contrastFig. 7 shows one example of a mixed-valence class III system: itis a paramagnetic metal down to 60 mK whilst the interleavedanions impart local moments showing evidence of short-rangeferromagnetic correlations at low temperature [29]. Morespectacular is the first example of a molecular superconductorwhose lattice contains paramagnetic metal ions and, indeed, thefirst superconductor of any kind incorporating paramagnetic 3dions [30]. In these ‘molecular’ examples, the presence of theinorganic ions is crucial to the combination of propertiesobserved.

In the last few years yet another way to combine organic andinorganic layers has come to light in the form of superlattices ofdifferent packing types [31]. While the normal sequence of layersin charge transfer salts is D+X�D+X�y intermediate layers canbe incorporated to give e.g. D+X�D0 +X�D+X�y. In that waylayer salts have been synthesized where D+ is BEDT�TTF and D0 +

is Na+ [32], opening the way to further novel combinations ofproperties such as electronic conductivity in the organic layer andionic conductivity in the inorganic one.

7. Conclusions

Especially when it comes to magnetic interactions and electrontransport, inorganic chemistry has much to teach those interestedby the organic molecular solid state. Of course there are someproperties such as mesomorphism, polymerization, compressi-bility and ambient-temperature solubility where organic moieties

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P. Day / Physica B 405 (2010) S6–S11 S11

play a crucial role. However, it is when we combine the organicand inorganic components that unique multi-functionality startsto appear.

Acknowledgements

My group has been supported by UK Research Councils (SRC,SERC, EPSRC) over the last 45 years. Of outstanding importance,however, have been our international collaborations, in particularwith European and Japanese colleagues. These have been financedby JSPS (Japan) and the European Commission (FP6 MAGMANetNetwork of Excellence).

References

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