Ante Bilušić

Structural (XRD and HRTEM) investigations of fullerite C60 and C70 samples

Abstract Since the carbon states on the basis of C 60 and C 70 exhibit very interesting physical properties such as hardness exceeding diamond, we focused our investigations on the comparison of structures of the C 60 and C 70 hard phases synthesized at high pressure and high temperatures. X-ray diffraction (XRD) and high-resolution transmission electron microscope (HRTEM) investigations have revealed that two states (amorphous and nanocrystalline) co-exist in samples of both kinds of fullerites. The difference between the diffraction patterns of C 60 and C 70 has been ascribed mainly to the grade of disorder. The XRD and HRTEM results have been correlated with the transport properties (electrical and thermal conductivity) of the samples.

High-pressure study of transport properties in Co0.33NbS2

This is the first study of the effect of pressure on transition metal dichalcogenides (TMDs) intercalated by atoms that order magnetically. Co0.33NbS2 is a layered system where the intercalated Co atoms order antiferromagnetically at T-N = 26 K at ambient pressure. We have conducted a detailed study of dc-resistivity (rho), thermoelectric power (S), and thermal conductivity (kappa). At ambient pressure the magnetic transition corresponds to a well-pronounced peak in dS/dT, as well as to a kink in the dc-resistivity. The effect of ordering on the thermal conductivity is rather small but, surprisingly, more pronounced in the lattice contribution than in the electronic contribution to kappa. Under pressure the resistivity increases in the high-temperature range, contrary to all previous measurements in other layered TMDs. In the low-temperature range the strong dependences of thermopower and resistivity on pressure are observed below T-N, which, in turn, also depends on pressure at rate of dT(N)/dp similar to -1 K/kbar. Several possible microscopic explanations of the reduction of the ordering temperature and the evolution of the transport properties with pressure are discussed.

Thermal and electrical conductivities in Al-based complex metallic alloys

A study is reported of the heat transport of AlPd(Mn, Fe, Co, Rh) complex metallic alloys that belong to an interesting class of Al-based alloys characterized by giant unit cells with quasicrystal-like cluster substructure. The electrical and thermal conductivity were investigated in the temperature range 8–300 K, in order to see how the exceptional structural complexity and the coexistence of two competing physical length scales affect the transport properties of these materials. The alloys have electrical conductivity typical of metallic alloys, and low thermal conductivity which is at room temperature comparable to that of thermal insulators, e.g. amorphous SiO2 and Zr/YO2 ceramics.

Anomalous thermal conductivity of single crystal Cu2Te2O5Cl2

A strong increase of the thermal conductivity is observed at the phase transition (Tc=18.2 K) in Cu2Te2O5Cl2 single crystal. This behavior is compared with that of the spin-Peierls system NaV2O5, where a similar experimental observation has been made, and the conventional spin-Peierls system CuGeO3, where a modest kink in the thermal conductivity curve has been observed. The strong increase of the thermal conductivity atTc in Cu2Te2O5Cl2 could be partially attributed to the opening of the energy gap in the magnetic excitation spectrum evident from the magnetic susceptibility measurements. However, the main reason for the anomaly of the thermal conductivity could be explained by a strong spin-lattice coupling in this system, which what is in agreement with the preliminary X-band electron spin resonance measurement.

Effects of doping on the transport properties of the quasi-one-dimensional system (TaSe4)2I

Abstract Studies of the effect of uncharged niobium impurities on the transport properties in (TaSe 4 ) 2 show that the Peierls transition T p ≈250–260K in the pure specimens is smeared out and suppressed in the alloys. The temperature of the Peierls transition T p and the low temperature energy gap Δ decrease linearly by increasing impurity concentration. In addition, the contribution to the thermal conductivity from the thermally assisted phason motion and charge-density-wave fluctuation lessen by increasing the impurity concentration.

Thermal conductivity of Taylor phase T-Al73Mn27 complex metallic alloy

Thermal conductivity, κ, of Taylor phase T-Al73Mn27-xPdx (x = 0 ; 2 ; 4 ; 6) complex metallic alloys (CMAs) has been studied in the temperature interval from 2 K to 300 K. Characteristic of κ are typical for CMAs: relatively small in magnitude, change of slope at about 50 K and increase above 100 K. At room temperature the magnitude of κ is between 2.7 W/mK and 3.7 W/mK. Such a low thermal conductivity originates in complex structure: aperiodic in short length scale, that leads to frequent electron scattering (i.e. to low electronic contribution to the thermal conductivity), while large lattice constant defines a small Brillouin zone that enhances Umklapp scattering of extended phonons. Above 100 K non-extended (localized) lattice vibrations are thermally excited, the hopping gives a new heat carrying channel resulting in the typical increase of the thermal conductivity with temperature.

Transport properties of fullerite samples

We have studied the transport properties of hard C 60 and hard C 70 fullerites. The samples from pristine C 60 and C 70 powder were synthesised at high pressure and high temperatures. Their electrical resistivity at room temperature is 35μ Qm and 85 μΩm, respectively, and is weakly dependent on temperature. X-ray diffraction show lack of the long range crystalline order. The room temperature value of the thermal conductivity is 2Wm -1 K -1 for both samples. Thermal conductivity of hard C 70 fullerite increases with temperature and becomes almost temperature independent in the temperature range 10 K-30 K. Contrary, thermal conductivity of hard C 60 fullerite increases with temperature in whole investigated temperature range, which is a consequence of the competing influences of the higher conductivity of crystalline regions and of the thermal resistance which arises at the junctions between amorphous and crystalline regions.

Anisotropy of the transport properties of (TaSe4)2I

The thermoelectric power, electrical and thermal conductivity of the charge-density-wave compound (TaSe 4 ) 2 I have been investigated in the chain and perpendicular to chain directions. A strong anisotropy of free carriers and lattice contribution to the transport properties is observed in the temperature range 100-320 K. The results of thermal conductivity are in favour to the our previous explanation that there are residual anomalies in thermal conductivity resulting from the contribution of the charge density wave excitations.

Transport properties of the quasi-one-dimensional crystal Nb4Te17I4

We report an investigation of the transport properties of the quasi-one-dimensional crystals Nb 4 Te 17 I 4 , and we compare them with the properties of (NbSe 4 ) 3 I which has more regular structure in the chain direction. Electrical resistivity at room temperature is ρ=5 Ωm, and no transition was observed in the range 100K<T<340K. By lowering temperature ρ(T) follows an activation law. Thermal conductivity κ(T) shows an exponential behavior at low temperatures. By increasing temperature around 180K the measured thermal conductivity is very closed to the minimum possible thermal conductivity for this system.

Phonon thermal conductivity of the inorganic quasi-one-dimensional conductors

The scattering mechanisms involved in heat transport of the inorganic quasi-one-dimensional conductors exhibiting charge density waves (CDW) may be investigated through thermal conductivity measurements. In particular, measurements of the thermal conductivity K(T), in the range 1.5–350 K, on (TaSe4)2I, (NbSe4)3.3I, K0.3MoO3 can give information on different types of scatterings: by the soft Kohn-Peierls phonons above the Peierls temperature Tc, by the excitations of the CDW just below Tc and by defects at low temperatures in the vicinity of K(T) maximum in addition to the umklapp scattering. In order to interpret K(T) specific for CDW ground state, we show for comparison the results obtained for (NbSe4)3I, which is structurally similar to (TaSe4)2I and (NbSe4)3.3I, but it does not exhibit CDW.