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Content archived on 2024-04-16



The Peierls transition in the blue bronzes (K0.3Mo03 and Rb0.3Mo03) has been probed by thermal conductivity and thermopower measurements. The results are satisfactorily described within the Wiedeman Franz law. Some discrepancies between the samples, mainly due to stoichiometry defects, are corroborated by Auger analysis. From Seebeck coefficients, 2 kinds of samples have been identified, but no clear correlations with nonlinear transport are found. The transition at 40 K between rigid and deformable charge density waves (CDW) is marked by hysteresis in the thermopower, rapid changes in the electron paramagnetic resonance (EPR) lines and changes in the lattice parameters. Also, the temperature interval above 100 K where the wave vector changes is marked by hysteresis. The modulated structure at 100 K has been determined with high accuracy. From the amplitude of the modulation of molybdenum atomic displacements, the calculated EPR molybdenum(V) linewidth studied is found to be in good agreement with the experimental values, establishing that the linewidth is dominated by inhomogeneous broadening. The CDW despinning has been investigated by EPR, proton channelling and transient response. All the results give strong evidence for CDW deformations prior to despinning in the whole range of temperatures explored. A model based on an analogy with plastic yielding to CDW defects (phase dislocations ), Snoeck effect in metallurgy has been proposed. A CDW domain wall configuration consistent with a large amount of data has been proposed. The divergence near threshold for CDW despinning in some relaxation time involved in the transient response suggests a critical behaviour.

To check whether also from structural perspectives some anomaly can be observed around 40 K, diffraction experiments using new helium cooling equipment were performed at 6 different temperatures, ranging from 145 K to 20 K. Following accurately the unit cell parameters at these 6 temperatures, indeed in a specific direction perpendicular to the monoclinic b-axis an anomaly around 40 K in the thermal expansion is observed. From the intensity variations it can be concluded that over the range 145 K to 20 K the structure undergoes quite specific structural changes (ie in the direction perpendicular to the Mo06 slabs).

Structural determinaitons establish that the purple bronzes (A Mo6017) are stoichiometric with no alkaline vacancies. Space groups have been redetermined. For A=potassuim, thalluim, the space groups are P3m1 and C2/m for A=sodium. For A=lithium, 2 modifications have been identified in relation to the preparation method. From magnetotransport data on KMo6017 a magnetic phase diagram has been obtained for fields up to 35 Tesla. The anomalous transport properties below 20 K suggest the existence of a spin density wave instability at low temperature. The dramatic magnetic fiels dependence of the thermopower seems to indicate that a phonon drag mechanism is not dominant. The large pressure dependence of the Peierls transition temperature is unusual and explained in terms of intraplanes and interplanes interactions. The lattice contribution to the thermal conductivity is predominant. This has to be related to the commensurability of the CDW state. The anomalies at 25 K in LiMo6017 suggest a CDW instability.

The quasi 2-dimensional oxides Mo4011 were investigated by thermal conductivity and Mossbauer measurements. The thermal conductivity is larger than in the blue bronzes but they show a smaller anomaly. This reflects a more imperfect nesting of the Fermi surface. The Mossbauer studies show that the CDW develops mainly on the sites Mo3, Mo4. The Peierls transition temperature is found to be time dependent with time scales of several days. This effect was ascribed to defect density wave. The modulated structure of Mo8023 and of the related compound Mo2S3 has been determined. A determination of the incommensurately modulated structure of Mo8023 showed large modulaton amplitudes of the oxygen atoms, up to 0.33 angstroms. The overall modulation picture reveals a breathing mode of the structure where the largest part, about two thirds, of the magnitudes of the displacements of the atoms can be attributed to a rotation of the Mo06 octahedra and where the remaining of the magnitudes in accounted for by distortion of the octahedra. Pressure dependent X-ray crystallographic studies show the compound to have a first order phase transition at about 1.5 GPa.
The proposal concerns the charge density wave instability in the low dimensional bronze and oxides. Several complementary techniques will be involved to study linear and non linear transport as well as the Peierls transition which occur in theses materials. Single crystals will be prepared at LEPES. Modulated structure determinations will be performed at Groningen. Linear and non linear conductivity , Hall effect, magnetoresitance, EPR, specific heat, magnetic susceptibility measurements will be performed at LEPES. The LNETI will complement the studies of the transport properties of these materials by measurements of thermoelectric power and thermal conductivity.


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