intercalation compounds: dichalcogenides

IntercalationCompounds:

Dichalcogenides

Prof. Antonella Glisenti - Dip. Scienze Chimiche - Università degli Studi di Padova

Laurea Magistrale in Scienza dei MaterialiMateriali Inorganici Funzionali

Lamellar Host Latticeand their Intercalates

Metal dichalcogenidesMetal oxyhalidesMetal phosphorus trisulphidesMetal oxidesMetal phosphates and phosphonatesGraphiteLayered silicates, clay minerals and doublehydroxidesOther layered host lattices

Metal dichalcogenidesTM dichalcogenides often possess layered structures (a).

The lattices consist of two close-packed chalcogen layers betweenwhich reside the metal ions.

metal ions can be found in sites of trigonal prismatic (b) or octahedral(c) symmetry.

Intralayer bond: strong and largely ionic

Interlayer bond: van der Waals

Metal dichalcogenidesThe ability of the metal atom to adopt octahedral and trigonal prismaticcoordination and for the X-M-X units to stack in different sequences

gives rise to a wide variety of polymorphic and polytypic formsBrown and Beernsten notation

Chalcogen layer illustrating the stacking sequencenotation

A, B, C = different anions in the layer; a, b, c = different metal sites; [a], [b], [c] = intercalated

guest ions.

Polytype designation Stacking sequence Examples Metal coordination

1T aB/Ca MX2 (M=Ti, Zr, Hf, V; X = S, Se, Te)

Octahedral

2Ha BaB/CaC MX2 (M=Ta, Nb; X = S, Se) Trigonal prismatic

2Hb BcB/CbC TaSe2 , NbSe2 Trigonal prismatic

4Hb aB/CaC/BaC/BaB/Ca TaSe2 , TaS2 Octahedral, Trigonal prismatic

Metal dichalcogenides

1T = the simplest structure: all octahedral metals and one X-T-X slab per unit cell;

2Ha and 2Hc = the two most common polytypes of the all-prismatic structures; two

layers per unit cell.2Ha (frequently referred to as 2H): the

metal atoms lie directly above each other.2Hc (frequently referred as 2H MoS2): the

metal atoms are staggered. 4Hb = mixed octahedral/trigonal prismatic

structure

(110) Projections of layered TM dichalcogenides

Organic intercalation compoundsA wide range of organic molecules form intercalation compounds.All the reactions are characterized by an expansion of the crystal lattice along the c direction to an extent that may be correlated with the molecular dimensions of the guest and the stoichiometry.Stabilities vary and depend on the nature of guest and host; highest stabilities = 2H TaS2, 2H NbS2, 1T TiS2; 2H NbSe2 does not form organic IC compounds with the exception of ethylendiamine.

Organic molecules that form IC compounds

Generic class Examples

AminesPhosphines

AmidesAmine oxides

Phosphine oxidesN-heterocycles

Isocyanides

RNH2, R2N, R3N, H2N(CH2)nNH2

R3PRCONH2, CO(NH2)2

Pyridine N-oxideR3PO

Pyridine, substituted pyridinesRNC

Organic intercalation compounds: synthesis

Intercalation reactions with organic compounds are usually carried out by direct reaction of the dichalcogenide in powder form with the organic compound or with a benzene or toluene solution for high molecular weight systems.In some cases reactions are facilitated by pretreatment of the dichalcogenide with ammonia or hydrazine.The progress of the reaction can be followed (qualitatively) by observing the volume expansion of the solid phase or (quantitatively) by means of XRD.The host lattice may be recovered unchanged by thermal deintercalationof the organic molecules at temperature higher than the initial reaction T (200-300°C).

n-Alkylamines: CnH2n+1NH2

A complete series of samples for n = 1 to 18 was prepared by direct reaction with the amine or amine in benzene solution for n > 12 at 25°C for 30 days.n ≤ 4: hydrocarbon chains parallel to the dichalcogenidelayers;5 ≤ n ≤ 11: ?;n ≥ 12: perpendicular orientation; composition = A2/3TaS2 (A = amine) NH2-groups adjacent to the layers to interact with the Ta through the nitrogen lone pair.

2H TaS2

Schematic representation of the structure of (octadecylamine)2/3TaS2


2H TaS2

Increase in the interlayer spacing (triangles) and the onset temperature for superconduttivity (circles) as a function of n in CnH2n+1NH2 for the n-alkylamineintercalation compounds of TaS2.


TaS2, TiS2, NbS2

n ≤ 9,10Direct reaction 150°-200°C for several days;Indirect reaction: dichalcogenidepreintercalated with ammonia or hydrazine and then reacted for some hours at 100°C.

n > 10Displacement reactions of amine intercalation compound of lower C number.

d = dhost + 2[(n-1)1.26 + 1.25 + 1.5 + 2.0)]Å

CH2 C-N NH2 CH3


Groups VI dichalcogenides

IC compounds can be prepared by ion-exchange reactions of the hydrated sodium intercalation compound Na0.1(H2O)0.6MoS2

1 ≤ n ≤ 5; c-spacing = constant;

6 ≤ n < 11: c increases linearly with ∆d/n = 2.3 per C, implying a bilayer tilted at 68°;

n > 11 alkylammoniumcations are perpendicular.

Organic guests: PyridineThe reactivity of pyridine is closely analogous to that exhibited by ammonia. Direct reaction of 2H-TaS2 with pyridine leads to the formation of the first stage phase with limiting composition The reaction proceeds until the limiting first stage composition TaS2(py)0.5.

Three models for the orientation of

pyridine molecules between

dichalcogenidelayers

Organic guests: PyridineNeutron diffraction studies on TaS2(py-d5)0.5 have determined that the nitrogen lone pair is directed parallel to the layers.

Pyridine sublattice is ordered at RT in both (py)1/2TaS2 and (py)1/2NbS2: rectangular superlattice 2a√3 x 13a.

Schematic representation of the packing and orientation of the guests in

TaS2(Py)0.5

Bonding in organic intercalation compoundsThe organic intercalation compounds have been described as charge-transfer or donor-acceptor compounds in which charge is transferred from the organic molecule to the empty (Ti, Zr, Hf) or half-filled (Nb, Ta) dz2 band of the dichalcogenide. Amines, amides, amine oxides: σ donation from the nitrogen lone pair orbital to the conduction band.

Some ligand basicity is required: an empirical correlation with basicity was found: Molecules with pKa values greater than 4.0 formed stable compounds with 2H TaS2 whereas molecules with pKavalues less than 3.0 did not. Failure to intercalate 4-tert-butylpyridine after 24 days at 200°C even though its pKa = 6.0 is due to steric effects on the kinetic of intercalation.

Isocyanides (and phosphines) are very weak bases but can intercalate because of a combination of σ-donor and π-acceptor properties.

Bonding in organic intercalation compoundsCorrelation was also observed between pyridine basicity and intercalation capability. Difficulty with the lone pair donor model because NH3 and py IC compounds have the nitrogen lone pair midway between and parallel to the layers precluding a direct interaction with the dz2orbital.

Bonding is described as an electrostatic interaction between negatively charged layers and cations, analogous to alkali and organometallic intercalation compounds.

2 py bipy + 2H+ + 2 e-

x py + xH+ xpyH+

xpyH+ + (0.5 – x)py + xe- + TaS2 (pyH+)x(py)0.5-xTaS2

Metal Ion Guests

1959 – Rudorff and Sick

Alkali metals in liquid ammonia + TiS2

1965 - Rudorff

Alkaline earth ions, Eu2+, Yb2+ + TiS2

Whittingham and Gamble - 1975

Rouxel et al. 1979

Hydrated metal intercalates: AxMX2(H2O)y

Guest species are inserted into empty sites between the layers.

Small guests (metal cations) = occupy the octahedral, tetrahedral or trigonal prismatic sites; in general the coordination of the transition metal is preserved on intercalation but it cannot be assumed that the X-M-X stacking will remain the same.

The final structure is determined by guest-host bonding as well as by the steric requirements of the guest and guest-guest interaction.

Staging is occasionally observed.

The layered dichalcogenides can all incorporate excess metal between the layers to give nonstoichiometric, metal-rich phases M1+xX2. The metal rich phases are poor hosts for intercalation of other than smallest ions.

Layered compounds of TM ions having a high d configuration show exclusively the octahedral coordination whereas the low d transition metal atoms of Groups IVB, VB, and VIB occur in both structures.

a) High-temperature synthesis from the host material and the metal of from the elements;

b) Intercalation of the host material with a solution of the metal (alkalimetal in liquid ammonia, butyllithium, sodium naphthalide);

c) Electrochemical intercalation. Cointercalation of the solvent is also possible with metods b) and c) (cointercalatedammonia, as an example, has to be removed by heat treatment);Methods b) and c) are RT methods so metastable phases may be produced and equlibration may take long time.

Synthesis

Method Stage 3 Stage 2 Stage 1(trigonal prismatic)

Stage 1(trigonal antiprismatic)

b 0.17 < x < 0.33 0.38 < x < 0.68 0.79 < x ≤ 1b ? < x < 0.18 0.35 < x < 0.58 0.68 < x ≤ 1c ? ? 0.46 < x < 0.70 0.80 < x ≤ 1c ? < x < 0.11 0.12 < x < 0.25 0.50 < x < ? 0.81 < x ≤ 1

Example: NaxTiS2

Ionicity of the transition metal-chalcogen bondfractional ionic character of a single

bond (ionicity). fi = 1-exp[-(1/4)(XA-XB)2]XA, XB = electronegativities

(Pauling) of A and B

covalent character for a complicated molecule or a crystal: the resonating-bond ionicity

1-fi’ = (N/M)(1-fi)N = covalenza

M = numero di legami

Plot of the nearest-neighbour distance

between the chalcogenatoms in a layer, aH,

divided by twice theirionic radius, RX, versusthe Pauling single-bond

ionicity (or fractionalcharge transfer). Withinthe accuracy of fi (0.1) a good linear relationship

is observed.

Trigonal Prismatic vs Octahedral coordination

Closed circles = octahedral, open circles = trigonal prismatic

structures.

trigonal prismaticCovalent

Contribution

OctahedralIonic Contribution

Intercalation and host structuremodification

Structure parameter plot including the

data of the intercalation compounds

Closed symbols = octahedral

coordination;Open symbols = trigonal prismatic

coordination.

A d/a vs a/2Rx- plot, is equally successful if applied to the

coordination of the alkali ions.

Coordination of the alkali ions

Slope =Covalent bond

contribution

Trigonal prismatic favored by large

alkali ions

At low enough temperatures the ions will order on superlattices for certain fractional values of the composition; as the temperatureincreases, the disorder increases, and, at a critical temperature, le long-range order collapses and the system becomes disordered.At “low” temperatures these ordered phases will have a compositional range. The stoichiometry can be varied within certain limits by creating vacancies or adding interstitial atoms.

Ordering of the intercalate ions

Depending on the coordination of the intercalate ion, the sublattice in the van der Waalsgap is (1) a honeycomb lattice (trigonal prismatic coordination, TP), (2) a triangular lattice (octahedral coordination, O), or (3) a puckered honeycomb lattice (tetrahedral coordination, T).

NaxTiS2: long-rangeorder at RT

The stability of a particular ordered

structure isdetermined by the

interactionsbetween the

particle formingthis structure.

Observed superstructure patterns and the corresponding unit cells

in NaxTiS2.

octahedral interlayer sites radii = ca0.71Å; Li+ = 0.59 Åonly a small expansion along the c-axis

is required to accomodate this cation.

Host = TiS2A single homogeneous phase has been found for the entirestoichiometry range LixTiS2 (0 ≤ x ≤1)Lithium occupies the octahedral interlayer sites and the final product, LiTiS2, is isostructural with LiVS2 and LiCrS2.

Lithium intercalated lamellar metal dichalcogenides

Selected examples of intercalation compounds formed by the metal disulphides with different guest ions

and molecules

CRD Li+ K+ Cs+

4 73 151

6 90 152 181

Ionic radii

Alkali metal intercalated lamellar dichalcogenides

The structures adopted by intercalation compounds formed with other alkali metals are much more varied, as these larger ions can occupy either octahedral or trigonal prismatic interlayer sites.

At low alkali metal concentrations (except lithium) staging results in the formation of compounds with alternating sequences of filled and empty van

der waals gaps.

Phase relations for the alkali metal intercalates of TiS2 and ZrS2. I, II and IV indicate 1st, 2nd and 4th stage intercalates, respectively.

Mercury vapour has been shown to reversibly intercalate into nearly stoichiometric TiS2 to give HgxTiS2 (x ≤ 1.29). The fully intercalated phase exhibits a 2.9 Å interlayer expansion.

Structural studies suggest that the guests form an incommensuratesublattice.

The thermal reversibility of the Hg intercalation at relatively lowtemperatures indicates unusually weak metal-host interactionsconsistent with minimal Hg-TiS2 charge transfer.

Mercury intercalated lamellar dichalcogenides

The valence electrons of the alkali atoms are transferred to the TX2sandwich filling the lowest unoccupied d-band levels.

Electronic Structure and Bonding

The dispersion and relative position of the d bands stay almost unchanged upon intercalation. The upper conduction and lower valence bands change considerably

Schematic representation of the band structures of the

layered Group IVB, VB, and VIB TM dichalcogenides

PROFOUND CHANGES IN THE ELECTRONIC PROPERTIES OF THE HOST

Host Host Properties Intercalate Intercalate Properties

1T-HfS2 wide band gap semiconductor

KxHfS2 metal

2H-NbSe2 metalsuperconductor

KxNbSe2 poor metal (x = 1)expect a semiconductor

2H-MoS2 diamagneticsemiconductor

KxMoS2 superconductor

Ammonia as a guestAnhydrous ammonia + layered metal dichalcogenide at – 78°C followed by warming to RT leads to a rapid reaction. The onset of intercalation is marked by swelling of the sample and often a slight colouration of the solution. The reaction proceeds until the limiting first stage composition MX2NH3 is achieved.readily loses NH3 to go to the second stage material MX2(NH3)0.5.

NH3 redox is involved in the reaction; these materials contain NH4+

solvated by neutral molecules.Ammonia orientation seems to be determined by the ion-dipole

interactions with the NH4+ cations

(1+x/3) NH3 + TaS2 x/6 N2 + (NH4+)x(NH3)1-xTaS2

Schematic representation of the packing and orientation of the guests in

TaS2(NH3)

Organometallic intercalation compounds: synthesis

Direct reaction. Toluene solutions of the organometallic guests are heated with the solid hosts in sealed tubes at temperature up to130°C. Kinetics are slow particularly for bulky guests; temperature increment causes the organometallic compound decomposition. Cobaltocene reacts more readily.

Ion-exchange reactions in aqueous solution; successfully used for Na1/3(H2O)TaS2 at RT. The hydrated sodium IC compound is prepared by reaction of TaS2 with aqueous sodium dithionite. Ion-exchange typically lead to lower stoichiometries and powder diffraction patterns of poorer quality than do direct reactions. The reaction may be more difficult in different solvents.

Organometallic cations may be intercalated from aqueous or non-aqueous solutions electrochemically.

Organometallic intercalation compoundsof layered dichalcogenides

Organometallic guests and orientation

> Second ring size > lattice expansion = principal axis

parallel to the layers

Organometallic guests and orientationThe lattice expansion of ca 5.3 Å observed for all simple metallocene intercalates does not immediately reveal the orientation of the guest. These molecules have almost a spherical van der Waals surface

X-ray and neutron diffraction techniques applied to MS2{Co(Cp)2}x

(M = Zr, Sn, Ta; x = 0.25-0.30):

Van der Waals dimensions of cobaltocene

Organometallic guests and orientation

In general organometallic sandwich complexes always adopta preferred orientation in which their metal-to-ring centroid

axes lie parallel to the host layer planes.

Macromolecular guestsIntercalation of poly(ethyleneoxide) – PEO – with an average molecular weight of ca. 105 Daltons into MS2 (M = Mo, Ti) by means of two synthetic approaches:

Delamination of the metal sulphides in aqueous suspension with an acetonitrile solution of PEO/LiClO4 followed by reconstitution of the lamellar structure upon drying.Treatment of the lithium intercalate LiMS2 with an aqueous solution of PEO and LiClO4.

These materials behave as semiconductors with reduced band gaps.

Lamellar Host Latticeand their Intercalates

Metal dichalcogenidesMetal oxyhalidesMetal phosphorus trisulphidesMetal oxidesMetal phosphates and phosphonatesGraphiteLayered silicates, clay minerals and doublehydroxidesOther layered host lattices

Intercalation chemistry of metal chalcogenohalides

Main structure types: FeOCl, AlOCl, SmSI, PbFCl

Intercalation chemistry of metal chalcogenohalides

Main structure types: FeOCl, AlOCl, SmSI, PbFCl

AlOCl0.35 ≤ rM3+/rO2- ≤ 0.44 Metal ion is four-coordinated

FeOCl0.46 ≤ rM3+/rO2- ≤ 0.58 Metal ion is six-coordinated

SmSI0.66 ≤ rM3+/rO2- ≤ 0.69 Metal ion is seven-coordinated

PbFCl0.69 ≤ rM3+/rO2- ≤ 0.86 Metal ion is eight or nine-coordinated

Synthetic routes:Sealed-tube reaction of chloride with a variety of chalcogenides

Metal oxyhalides with AlOCl structure

Orthorhombic structureGaOCl

Each metal ion is located in a slightly distorted tetrahedron coordinated to three O2- and one Cl-; the O2- vertices of the tetrahedra are linked together in puckered layers perpendicular to the b axis.each O2- is shared by three tetrahedra.

The Cl- vertices are unshared and lie on alternating sides of the Al-O net along a given row of tetrahedra

Metal oxyhalides with the FeOCl structureStack of double sheets of cis-FeCl2O4 distorted octahedra linked together by

shared edges within the crystallographic ac-plane. The neutral layers of FeOClare orientated perpendicular to the b-direction, with chlorine forming the

outermost atoms of each layer.

The b-direction cell parameter increases on intercalation.

Structure of MOCl (M = Ti, V, Cr)

and InOX (X = Cl, Br, I)The thiochlorides and thiobromides of Y and

all the lanthanideswith rM3+ ≤ 1.00 Å

Organometallic cationsWell crystalline materials with an orthorhombic unit cell; a and c axes nearly unchanged from the hosts; b axis expanded and doubled.

Alternate layers of the host MOCl structure shift by one-half unit cell in the (101) direction (along the a-c diagonal).

Metallocene intercalates of oxyhalides

Intercalation in FeOCl and oxidativepolymerisation

PANI can be extracted from the host lattice by dissolving the FeOCl in acid; molecular weight ca. 3500 (ca. 7700 for bulk prepared) but the recovered polymer shows a narrower

length distribution.The hybrid organic-inorganic material is a p-type semiconduction in which reduced FeOCl

host still dominates the electrical conduction.

FeOClPolyaniline (PANI)

FeOCl(PANI)x

Polymer is intercalate in an ordered fashion that is commensurate with

the FeOCl lattice

Schematic representation of the structure of the polymer (PANI) in

FeOCl

Metal oxyhalides electronic properties

The layered transition metal oxyhalides are semiconductors.

Intercalation of organic and organometallic guest molecules increases the electrical conductivity of the host lattice: FeOCl single crystal = 10-7

S cm-1; FeOCl(py)0.33, FeOCl(ET)0.25 (ET = bis(ethyleneditio)tetrathiafulvalene), FeOCl(PANI)x = 0.15-0.25 Ω-1cm-1.

FeOCl(py)0.33, FeOCl(ET)0.25, FeOCl(PANI)x, FeOCl{Fe(Cp)2}0.16: Temperature dependence of the conductivity is indicative of

semiconducting behaviour.

FeOCl(PPY)0.34 (PPY = polypyrrole) = p-type metallic behaviour.

Polymerization of pyrrole in FeOCl: layeredconducting polymer-inorganic hybrid materials

XRD: high crystallinity and an increase in FeOCl interlayer spacing

from 7.980 to 13.210 ÅThis 5.23 Å expansion is

comparable to that observed in the formation of (pyridine)0.33FeOCl and

(pyridine)0.5TaS2 where pyridine molecular plane is perpendicular (C2-

axis paralles) to the layer planes.

FeOCl + excess pyrrole(60°C)

intercalation compounds: dichalcogenides

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