Recent advances in intercalation compounds physics

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Intercalation of TiSe2 with Ag and transition metals leads to formation of polarons which are the deformation centers of lattice and traps for

electrons. Corresponding band of impurity states with binding energy ~ 0.3 eV was observed with ARPES. Behavior of these polarons is

determined by competition of two effects: energy gain as a result of decrease of EF because of the appearing of

impurity polaron band energy loss because of deformation of lattice

Constants of elastic forces are temperature dependent, which leads to non-monotone dependence of localization degree of charge carriers.

State of localization centers and lattice deformation affect on interaction between centers and their ordering.

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Mn0.145TiSe2

Single crystals of Mn intercalated in TiSe2 have been obtained for a first time.

Measurements of resistivity carried out on 2 x 2 mm2 large crystals using a standard 4-contact methodic.

One can see a difference between a resistivity along layers and a resistivity in normal to layers direction.

Presence of charge-density-wave transition at ~ 100 K.

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0 50 100 150 200

1,0

1,5

1

2

3

0 50 100 150 200 250 3006,0x10-3

9,0x10-3

1,2x10-2

/ 4

,2

T, K

3

00

T, K

R,

Oh

m Red curve – Mn0.145TiSe2, in basal plane

Black curve – TiSe2

On the inset – Mn0.145TiSe2, in normal to layers direction

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AgxTiSe2

Deposition of 2 monatomic layers of Ag (6 Å) on TiSe2 surface in chamber of spectrometer with consequent annealing ( 250 0C, 2 hours) let us to see the changes in XPS spectra and in LEED before and after annealing.

LEED photographs show the appearance of superstructure after deposition of Ag

Valence band spectra shows that Ag dissolves in TiSe2 (appearance of Se after annealing)

Spectroscopy experiments were carried out on ESCALAB-5 spectrometer in Solid State Physics Institute, Chernogilovka, Russia, in April and July, 2006.

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AgxTiSe2 LEED

TiSe2

TiSe2 + 0.5 ML (1.5 Å) Ag

TiSe2 + 2 ML (6 Å) Ag, before annealing

TiSe2 + 2 ML Ag, after annealing

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XPS of TiSe2 with deposited Ag

Appearance of additional peaks after annealing leads to conclusion that Ag dissolves in TiSe2

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360 368 376

0

80

160

Ag 3d before annealing Ag 3d after annealing

Inte

nsi

ty, a

rb. u

nit

Binding energy, eV450 460 470

0

80

160

Ti 2p before aanealingTi 2p after annealing

Inte

nsi

ty

Binding energy, eV

48 56 64 72

0

80

160

Se 3d before annealingSe 3d after annealing

Inte

nsi

ty

Binding energy, eV150 160 170 180

0

80

160

Se 3p before annealing Se 3p after annealing

I

Kinetic energy, eV

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The intercalation of TiSe2 with transition metals and Ag leads to formation of polarons, which localization degree is temperature dependent. At specified temperature the highest localization degree (polaron band collapse, PBC) is observed. This phenomena is conducted by the high distortion of the lattice and the low mobility of polarons. This effect was earlier observed in compounds AgxTiTe2 – on the unit cell volume versus temperature curve there is a local minimum at TPBC ~ 150 0C. Moreover, at this temperature the drastic change of conductivity is observed.

Unit cell volume versus temperature for AgxTiTe2

near PBC at Ag concentrations x = 0.65 ( curve 1) и 0.75 (curve 2)

Conductivity versus temperature for AgxTiTe2 with various concentration of Ag near PBC (~150C ore 423 K).

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FexTiSe2

Electroconductivity versus quenching temperature for FexTiSe2. One can see that samples quenched from high temperatures have high electroconductivity.

DTA data revealed two anomalies

DTA for Fe0.25TiSe2 (sample was

quenched after annealing at 300C for 1 week)

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FexTiSe2

Lattice parameter c and unit cell volume versus temperature for Fe0.25TiSe2

Elastic neutron scattering experiment was carried out on High Resolution Fourier Difractometer in Joint Institute for Nuclear Research, Dubna, Russia in April, 2006.

Diffraction reflexes width versus temperature for Fe0.25TiSe2

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FexTiSe2

Bond length “iron - titan”МО – slowly cooled samples; зак. – quenched samples

It were researched interactions between centers Ti – Fe - Ti, leading to ordering of intercalant and FexTiSe2 (x = 00.5) samples, obtained with different thermal conditions.

Slowly cooled samples, enriched with iron, revealed ordering with space group I12/m1

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Modification of phonon spectra of TiSe2 by intercalation

Phonon DOS of M0.25TiSe2.