Recent advances in intercalation compounds physics

2 Intercalation of TiSe 2 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 E F 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.

3 Mn TiSe 2 Single crystals of Mn intercalated in TiSe 2 have been obtained for a first time. Measurements of resistivity carried out on 2 x 2 mm 2 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.

4 Red curve – Mn TiSe 2, in basal plane Black curve – TiSe 2 On the inset – Mn TiSe 2, in normal to layers direction

5 Ag x TiSe 2 Deposition of 2 monatomic layers of Ag (6 Å) on TiSe 2 surface in chamber of spectrometer with consequent annealing ( C, 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 TiSe 2 (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.

6 Ag x TiSe 2 LEED TiSe 2 TiSe ML (1.5 Å) Ag TiSe ML (6 Å) Ag, before annealing TiSe ML Ag, after annealing

7 XPS of TiSe 2 with deposited Ag Appearance of additional peaks after annealing leads to conclusion that Ag dissolves in TiSe 2

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9 The intercalation of TiSe 2 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 Ag x TiTe 2 – on the unit cell volume versus temperature curve there is a local minimum at T PBC ~ C. Moreover, at this temperature the drastic change of conductivity is observed. Unit cell volume versus temperature for Ag x TiTe 2 near PBC at Ag concentrations x = 0.65 ( curve 1) и 0.75 (curve 2) Conductivity versus temperature for Ag x TiTe 2 with various concentration of Ag near PBC (~150  C ore 423 K).

10 Fe x TiSe 2 Electroconductivity versus quenching temperature for Fe x TiSe 2. One can see that samples quenched from high temperatures have high electroconductivity. DTA data revealed two anomalies DTA for Fe 0.25 TiSe 2 (sample was quenched after annealing at 300  C for 1 week)

11 Fe x TiSe 2 Lattice parameter c and unit cell volume versus temperature for Fe 0.25 TiSe 2 Elastic neutron scattering experiment was carried out on High Resolution Fourier Difractometer in Joint Institute for Nuclear Research, Dubna, Russia in April, Diffraction reflexes width versus temperature for Fe 0.25 TiSe 2

12 Fe x TiSe 2 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 Fe x TiSe 2 (x = 0  0.5) samples, obtained with different thermal conditions. Slowly cooled samples, enriched with iron, revealed ordering with space group I12/m1

13 Modification of phonon spectra of TiSe 2 by intercalation Phonon DOS of M 0.25 TiSe 2.