Suppression and inducement of the charge-density-wave state in Cr x TiSe2.

The x-ray diffraction, electrical resistivity and thermal expansion measurements have been employed to study how the intercalation of Cr atoms into TiSe2 matrix affects the crystal structure, formation of the charge density wave (CDW) and electrical properties. The intercalation of a small amount of Cr atoms (up to x ~ 0.03) is observed to suppress the CDW formation. The electrical resistivity of Cr x TiSe2 compounds with the Cr concentrations 0.03 ⩽ x ⩽ 0.20 shows a metallic-type behavior; while in the concentration range 0.25 ⩽ x ⩽ 0.5, the resistivity shows an anomalous behavior indicating the reappearance of the transition to a CDW-like state; further growth of the Cr content up to x = 0.6 again leads to the metallic-type resistivity. For the compound Cr0.25TiSe2, the phase transition below 160 K together with abnormal change in the electrical resistivity is found to be accompanied by anomalies in the lattice parameters and thermal expansion behavior; this transition is classified as first-order type. It has been found that despite the intercalation of Cr atoms some Ti-Se bonds in the Se-Ti-Se tri-layers of Cr x TiSe2 with x ⩽ 0.5 have nearly the same lengths as in the host lattice TiSe2, which apparently allows the transition to be returned to the CDW-like state.

Magnetic order, field-induced phase transitions and magnetoresistance in the intercalated compound Fe0.5TiS2.

Measurements of the magnetic susceptibility, magnetization, electrical resistivity and neutron diffraction have been performed for the compound Fe(0.5)TiS(2) in which Fe atoms are intercalated between S-Ti-S tri-layers. It has been shown that this compound with a monoclinic crystal structure exhibits an antiferromagnetic (AF) ground state below the Néel temperature T(N) ≈ 140 K. Small deviations from the stoichiometry and some disordering effects caused by the additional low-temperature heat treatment do not affect substantially the AF state in Fe(0.5)TiS(2). According to neutron diffraction data the magnetic structure at 2 K is described by the propagation vector k = (1/4,0,1/4). The Fe magnetic moments with a value of (2.9 ± 0.1) µ(B) are directed at an angle of (78.5 ± 1.8)° to the layers. Application of the magnetic field at T < T(N) induces a metamagnetic phase transition to the ferromagnetic (F) state, which is accompanied by the large magnetoresistance effect (|Δρ/ρ| up to 27%). Below 100 K, the field-induced AF-F transition is found to be irreversible, as evidenced by magnetoresistance and neutron diffraction measurements. The magnetization reversal in the metastable F state is accompanied at low temperatures by substantial hysteresis (ΔH ~ 100 kOe) which is associated with the Ising character of Fe ions.

Ferromagnetism and structural transformations caused by Cr intercalation into TiTe(2).

Crystal structure investigations, electrical resistivity, and magnetic measurements have been performed for polycrystalline samples of intercalated compounds Cr(x)TiTe(2) with a Cr concentration up to x = 0.65. According to the room-temperature x-ray diffraction study of Cr(x)TiTe(2), the initial hexagonal crystal structure transforms to a monoclinic one with increasing Cr content up to x≥0.5 due to the ordering of Cr ions. The intercalation results in the change of the resistivity behavior in Cr(x)TiTe(2) from metal-like at x = 0 to insulator-like above x = 0.33 and leads to ferromagnetic ordering of Cr magnetic moments at x≥0.5. For the compound Cr(0.25)TiTe(2), structural transformations and anomalous resistivity behavior are observed around 230 K, which cannot be explained only by the order-disorder transition within the subsystem of intercalated Cr ions. Structural changes within Te-Ti-Te sandwiches associated with charge density wave instability are suggested to be involved in this phase transition as well.