Intercalation Compounds: Dichalcogenides

Intercalation Compounds: Dichalcogenides Prof. Antonella Glisenti - Dip. ... between the chalcogen atoms in a layer, a H, divided by twice their ionic...

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Laurea Magistrale in Scienza dei Materiali

Materiali Inorganici Funzionali

Intercalation Compounds: Dichalcogenides

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

Lamellar Host Lattice and their Intercalates Metal dichalcogenides Metal oxyhalides Metal phosphorus trisulphides Metal oxides Metal phosphates and phosphonates Graphite Layered silicates, clay minerals and double hydroxides Other layered host lattices

Metal dichalcogenides ªTM dichalcogenides often possess layered structures (a). ª The lattices consist of two close-packed chalcogen layers between

which reside the metal ions. ª 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 dichalcogenides ª

The ability of the metal atom to adopt octahedral and trigonal prismatic coordination and for the X-M-X units to stack in different sequences gives rise to a wide variety of polymorphic and polytypic forms ª Brown and Beernsten notation

Polytype designation

Stacking sequence


Metal coordination



MX2 (M=Ti, Zr, Hf, V; X = S, Se, Te)




MX2 (M=Ta, Nb; X = S, Se)

Trigonal prismatic



TaSe2 , NbSe2

Trigonal prismatic



TaSe2 , TaS2

Octahedral, Trigonal prismatic

Chalcogen layer illustrating the stacking sequence notation ª A, B, C = different anions in the layer; a, b, c

= different metal sites; [a], [b], [c] = intercalated guest ions.

Metal dichalcogenides ª 1T = the simplest structure: all octahedral

metals and one X-T-X slab per unit cell; ª 2Ha and 2Hc = the two most common polytypes of the all-prismatic structures; two layers per unit cell. ª 2Ha (frequently referred to as 2H): the metal atoms lie directly above each other. ª 2Hc (frequently referred as 2H MoS2): the metal atoms are staggered. ª 4Hb = mixed octahedral/trigonal prismatic structure

(110) Projections of layered TM dichalcogenides

Organic intercalation compounds A wide range of organic molecules form intercalation compounds. ª 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. ª 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. ª

Generic class


Amines Phosphines Amides Amine oxides Phosphine oxides N-heterocycles Isocyanides

RNH2, R2N, R3N, H2N(CH2)nNH2 R3P RCONH2, CO(NH2)2 Pyridine N-oxide R3PO Pyridine, substituted pyridines RNC

Organic molecules that form IC compounds

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. ª In some cases reactions are facilitated by pretreatment of the dichalcogenide with ammonia or hydrazine. ª The progress of the reaction can be followed (qualitatively) by observing the volume expansion of the solid phase or (quantitatively) by means of XRD. ª The host lattice may be recovered unchanged by thermal deintercalation of the organic molecules at temperature higher than the initial reaction T (200-300°C). ª

2H TaS2

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. ª n ≤ 4: hydrocarbon chains parallel to the dichalcogenide layers; ª 5 ≤ n ≤ 11: ?; ª n ≥ 12: perpendicular orientation; composition = A2/3TaS2 (A = amine) NH2groups adjacent to the layers to interact with the Ta through the nitrogen lone pair. ª

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

n-Alkylamines: CnH2n+1NH2 2H TaS2 Increase in the interlayer spacing (triangles) and the onset temperature for superconduttivit y (circles) as a function of n in CnH2n+1NH2 for the n-alkylamine intercalation compounds of TaS2.

n-Alkylamines: CnH2n+1NH2 TaS2, TiS2, NbS2 n ≤ 9,10 ª Direct reaction 150°200°C for several days; ª Indirect reaction: dichalcogenide preintercalated with ammonia or hydrazine and then reacted for some hours at 100°C. ª n > 10 ª Displacement reactions of amine intercalation compound of lower C number. ª

d = dhost + 2[(n-1)1.26 + 1.25 + 1.5 + 2.0)]Å CH2




n-Alkylamines: CnH2n+1NH2 Groups VI dichalcogenides

IC compounds can be prepared by ion-exchange reactions of the hydrated sodium intercalation compound Na0.1(H2O)0.6MoS2 ª 1 ≤ n ≤ 5; c-spacing = constant; ª 6 ≤ n < 11: c increases linearly with ∆d/n = 2.3 per C, implying a bilayer tilted at 68°; ª n > 11 alkylammonium cations are perpendicular. ª

Organic guests: Pyridine ) The 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 dichalcogenide layers

Organic guests: Pyridine )

Neutron 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 (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 compounds The organic intercalation compounds have been described as chargetransfer 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. ª Amines, amides, amine oxides: σ donation from the nitrogen lone pair orbital to the conduction band. ª 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 pKa values less than 3.0 did not. ª 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. ª Isocyanides (and phosphines) are very weak bases but can intercalate because of a combination of σ-donor and π-acceptor properties. ª

Bonding in organic intercalation compounds ª Correlation was also observed between pyridine basicity and

intercalation capability. ª 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 dz2 orbital. ª Bonding is described as an electrostatic interaction between negatively charged layers and cations, analogous to alkali and organometallic intercalation compounds.

2 py Œbipy + 2H+ + 2 ex py + xH+ Œ xpyH+ xpyH+ + (0.5 – x)py + xe- + TaS2 Œ (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.

Synthesis 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 (alkali metal in liquid ammonia, butyllithium, sodium naphthalide); c) Electrochemical intercalation. z z

Cointercalation of the solvent is also possible with metods b) and c) (cointercalated ammonia, 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.

Example: NaxTiS2 Method

Stage 3

Stage 2

Stage 1 (trigonal prismatic)

Stage 1 (trigonal antiprismatic)


0.17 < x < 0.33

0.38 < x < 0.68

0.79 < x ≤ 1


? < x < 0.18

0.35 < x < 0.58

0.68 < x ≤ 1




0.46 < x < 0.70

0.80 < x ≤ 1


? < x < 0.11

0.12 < x < 0.25

0.50 < x < ?

0.81 < x ≤ 1

Ionicity of the transition metal-chalcogen bond ª fractional ionic character of a single bond (ionicity). fi = 1-exp[-(1/4)(XA-XB)2]

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


= (N/M)(1-fi)

XA, XB = electronegativities (Pauling) of A and B N = covalenza M = numero di legami

Plot of the nearestneighbour distance between the chalcogen atoms in a layer, aH, divided by twice their ionic radius, RX, versus the Pauling single-bond ionicity (or fractional charge transfer). Within the accuracy of fi (0.1) a good linear relationship is observed.

Trigonal Prismatic vs Octahedral coordination

trigonal prismatic Octahedral Covalent Ionic Contribution Contribution

Closed circles = octahedral, open circles = trigonal prismatic structures.

Intercalation and host structure modification Structure parameter plot including the data of the intercalation compounds Closed symbols = octahedral coordination; Open symbols = trigonal prismatic coordination.

Coordination of the alkali ions A d/a vs a/2Rx- plot, is equally successful if applied to the coordination of the alkali ions. Trigonal prismatic favored by large alkali ions Slope = Covalent bond contribution

Ordering of the intercalate ions ª At low enough temperatures the ions will order on superlattices for

certain fractional values of the composition; as the temperature increases, the disorder increases, and, at a critical temperature, le long-range order collapses and the system becomes disordered. ª 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.

Depending on the coordination of the intercalate ion, the sublattice in the van der Waals gap 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-range order at RT The stability of a particular ordered structure is determined by the interactions between the particle forming this structure.

Observed superstructure patterns and the corresponding unit cells in NaxTiS2.

Lithium intercalated lamellar metal dichalcogenides Host = TiS2 z A single homogeneous phase has been found for the entire stoichiometry range LixTiS2 (0 ≤ x ≤1) z Lithium occupies the octahedral interlayer sites and the final product, LiTiS2, is isostructural with LiVS2 and LiCrS2.

octahedral interlayer sites radii = ca 0.71Å; Li+ = 0.59 Å only a small expansion along the c-axis is required to accomodate this cation.

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









152 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.

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

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.

Mercury intercalated lamellar dichalcogenides × 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 incommensurate

sublattice. × The thermal reversibility of the Hg intercalation at relatively low

temperatures indicates unusually weak metal-host consistent with minimal Hg-TiS2 charge transfer.


Electronic Structure and Bonding Schematic representation of the band structures of the layered Group IVB, VB, and VIB TM dichalcogenides

The valence electrons of the alkali atoms are transferred to the TX2 sandwich filling the lowest unoccupied d-band levels. The dispersion and relative position of the d bands stay almost unchanged upon intercalation. The upper conduction and lower valence bands change considerably


Host Properties

Intercalate Intercalate Properties


wide band gap semiconductor



2H-NbSe2 metal superconductor


poor metal (x = 1) expect a semiconductor




diamagnetic semiconductor

Ammonia as a guest ) Anhydrous 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. (1+x/3) NH3 + TaS2 x/6 N2 + (NH4+)x(NH3)1-xTaS2 ) 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 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 to 130°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 compounds of layered dichalcogenides

Organometallic intercalation compounds of layered dichalcogenides

Organometallic guests and orientation > Second ring size > lattice expansion = principal axis parallel to the layers

Organometallic guests and orientation ) The 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

Van der Waals dimensions of cobaltocene

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

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

Organometallic guests and orientation

In general organometallic sandwich complexes always adopt a preferred orientation in which their metal-to-ring centroid axes lie parallel to the host layer planes.

Macromolecular guests ) Intercalation of poly(ethyleneoxide) – PEO – with an average

molecular weight of ca. 105 Daltons into MS2 (M = Mo, Ti) by means of two synthetic approaches: Ô


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 Lattice and their Intercalates Metal dichalcogenides Metal oxyhalides Metal phosphorus trisulphides Metal oxides Metal phosphates and phosphonates Graphite Layered silicates, clay minerals and double hydroxides Other layered host lattices

Intercalation chemistry of metal chalcogenohalides ª

Main structure types: FeOCl, AlOCl, SmSI, PbFCl

Intercalation chemistry of metal chalcogenohalides ª

Main structure types: FeOCl, AlOCl, SmSI, PbFCl

AlOCl 0.35 ≤

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

0.46 ≤

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

FeOCl SmSI 0.66 ≤

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

PbFCl 0.69 ≤

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 ª 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. ª each O2- is shared by three tetrahedra. ª The Cl- vertices are unshared and lie on alternating sides of the Al-O net along a given row of tetrahedra

ª Orthorhombic structure ª GaOCl

Metal oxyhalides with the FeOCl structure Stack of double sheets of cis-FeCl2O4 distorted octahedra linked together by shared edges within the crystallographic ac-plane. The neutral layers of FeOCl are orientated perpendicular to the b-direction, with chlorine forming the outermost atoms of each layer. ª


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 lanthanides with rM3+ ≤ 1.00 Å

Organometallic cations ª Well crystalline materials with an orthorhombic unit cell; a and c axes

nearly unchanged from the hosts; b axis expanded and doubled. ª Alternate layers of the host MOCl structure shift by one-half unit cell in

the (101) direction (along the a-c diagonal).

ª Metallocene intercalates

of oxyhalides

Intercalation in FeOCl and oxidative polymerisation Polyaniline (PANI) FeOCl

Polymer is intercalate in an ordered FeOCl(PANI)x fashion that is commensurate with the FeOCl lattice

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

ªPANI 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. ªThe hybrid organic-inorganic material is a p-type semiconduction in which reduced FeOCl host still dominates the electrical conduction.

Metal oxyhalides electronic properties ª The layered transition metal oxyhalides are semiconductors. ª 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. ª 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.

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

Polymerization of pyrrole in FeOCl: layered conducting polymer-inorganic hybrid materials FeOCl + excess pyrrole (60°C)

ª XRD: high crystallinity and an

increase in FeOCl interlayer spacing from 7.980 to 13.210 Å ª This 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 (C2axis paralles) to the layer planes.