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We have systematically studied the temperature-pressure diagram of ${\mathrm{C}}_{60}$ at pressures between 37 kbar and 80 kbar and from room temperature to 900 \ifmmode^\circ\else\textdegree\fi{}C. We show here the specific role played by the temperature in the phase transformations observed in the $P\ensuremath{-}T$ region covered by this study. As temperature is increased, we follow the continuous transformation of fcc fullerite to an orthorhombic phase of polymerized ${\mathrm{C}}_{60}$ chains that, at higher temperature, polymerize to two-dimensional ${\mathrm{C}}_{60}$ polymers. Above 800 -900 \ifmmode^\circ\else\textdegree\fi{}C the ${\mathrm{C}}_{60}$ cage collapses with formation of an $s{p}^{2}$ amorphous phase. The continuous evolution of the x-ray patterns during these phase transformations is explained on the basis of a homogeneous solid-state polymerization.

We show in this Letter that heating under high pressure drives ${\mathrm{C}}_{60}$ to new distorted crystalline phases that are metastable at room temperature and pressure. We report three different distortions that are essentially characterized by two nearest neighbor distances, ${d}_{0}^{\ensuremath{'}}\ensuremath{\sim}10$ and ${d}_{0}^{\ensuremath{'}\ensuremath{'}}\ensuremath{\sim}9.2$ \AA{}. The excellent accord with theoretical calculations supports the view that these new carbon phases can be understood as the long range order polymerization of ${\mathrm{C}}_{60}$ through cycloaddition reactions that are at the origin of the shorter intermolecular distances.

We measure the pressure dependence of the electrical resistivity and the crystal structure of iron superconductor Sr2VO3FeAs. Below ~10 GPa the structure compresses but remains undeformed, with regular FeAs4 tetrahedrons, and a constant Tc. Beyond 10 GPa, the tetrahedron strongly distorts, while Tc goes gradually to zero. Band structure calculations of the undistorted structure show multiple-nesting features that hinder the development of an antiferromagnetic (AF) ground state, allowing the appearance of superconductivity. The deformation of the tetrahedra that breaks band degeneracy degrades multiple nesting, thus favouring one particular AF state at the expense of Tc.

Presents a method of measuring the thermal conductivity of very tiny and fragile samples, based on the differential thermal conductivity technique. To illustrate the possibilities of this method the authors have measured the thermal conductivity of filaments of NbSe3 (10 mu m in diameter and 3 mm in length) in the temperature range 2 to 150K with good resolution.

The resistance of single crystals of NbSe3 under pressure, between 1.2 and 70K has been measured in magnetic fields up to 180 kG. The CDW which appears at ambient pressures at 59K, is totally suppressed for pressures greater than 5.5 kbar. At this pressure NbSe3 is fully superconducting as shown by a total Meissner effect. Fermi surface studies by Shubnikov-de Haas oscillations show that the Fermi surface is strongly affected by the CDW formation. At ambient pressure, when H is parallel to the plane (b, c) one frequency (0.3 MG) is detected, but when the pressure has suppressed the CDW, at least eight frequencies (up to 7 MG) were detected. The experiments are consistent with the model of different types of chains in NbSe3 on which the two CDW form.

We present the first results on the thermal conductivity of TiSe2. For this semimetal the heat transport is found to occur only through phonons within the investigated temperature range (1.65 - 150 K). At lower temperatures, possible origins for the nearly T2 observed law are analyzed. The other features of K(T) are also examined.

We report measurements of the resistive superconducting transitions of NbSe3 as a function of magnetic field for sufficient pressures to destroy the lower CDW. These indicate that NbSe3 is a bulk anisotropic superconductor with a total Meissner effect. The critical field anisotropies are well described by an effective mass model and the values obtained are in good agreement with those obtained by conductivity or Shubnikov-de Haas measurements. At ambient pressure we have measured zero-resistivity transitions for compacted samples, but do not find a total Meissner effect. Hence this superconductivity is not a bulk effect but is probably associated with barriers between domains in the sample.

We correlate s-shape resistivity in A-15, C-15 and charge density wave compounds with structural instabilities. We propose that the exponential term of the resistivity is caused by the scattering against thermally excited molecular states. We show that this interpretation is supported by a large number of experimental data and naturally explains the apparent violation of Mathiessen's rule in irradiation experiments. Connections with recent theoretical developments on strong electron-phonon coupling are discussed.

The superconducting oxide BaPb(1-x)Bi(x)O3, discovered in 1975 (ref. 1), is an exotic system having an unusually high transition temperature (T(c)) of ~12K, despite a relatively low density of states at the Fermi level. The subsequent prediction that doping the electronically inactive barium donor sites, instead of the bismuth sites, might induce superconductivity with a higher T(c) led to the discovery in 1988 of superconductivity in the Ba(1-x)K(x)BiO3 system (T(c) ~30 K for x = 0.4). But it remains an open question why many of the superconducting properties of these materials are similar to those of the well-known copper oxide superconductors, despite their pronounced structural differences: the former have a three-dimensional bismuth-oxygen framework, whereas the structures of the latter are predominantly two-dimensional, consisting of copper-oxygen planes. Understanding of the copper oxide superconductors has gained immensely from the study of many different superconducting systems, and so it might be expected that the identification of bismuth oxide superconductors beyond the substituted BaBiO3 compounds will prove to be similarly fruitful. Here we report the synthesis of a second family of superconducting bismuth oxides, based on SrBiO3. We show that partial substitution of potassium or rubidium for strontium induces superconductivity with: T(c) values of ~12 K for Sr(1-x)K(x)BiO3 (x= 0.45-0.6) and ~13K for Sr(1-x)Rb(x)BiO3 (x = 0.5).

The thermal conductivities of two chalcogenides - NbSe3 and TiSe2 - have been measured. A differential technique has been developed which is suitable for these small samples. In both materials phonons play the main role in thermal transport. At lower temperatures 2-D features are found to control the temperature dependence.

After 25 years of research on the superconducting cuprates, the discovery of high Tcsuperconductivity in related iron based pnictides in 2008 has stimulated the search of newsuperconductors. In particular, systems with 3d transition elements, antiferromagnet withhigh Néel temperature, moderate magnetic moments and with crystallographic 2D layerscould be adequate parent phases for new unconventional superconductors.In this context we have reinvestigated the physical properties of n=1, 2,3 and 1 membersof the Srn+1CrnO3n+1 Ruddlesden Popper (RP) series. These compounds had beenpoorly studied because of their synthesis conditions under high pressure and high temperature.We successfully synthesized Sr2CrO4 (n=1), Sr3Cr2O7(n=2), Sr4Cr3O10 (n=3)and SrCrO3 (n=1). We present results of several studies performed on these phasessuch as neutron powder diffraction, synchrotron X-ray diffraction under high pressureand low temperature, and electrical transport properties under high pressure. Even ifno superconductivity was observed, interesting properties had been discovered such as anunusual “anti-Jahn-Teller” in Sr2CrO4 at low temperature or an orbital ordering and ahuge magneto-structural coupling in Sr3Cr2O7.To explore new Cr-based systems where superconductivity could potentially be inducedby changing Cr-Cr interactions in the Sr2CrO4 (n=1) and Sr3Cr2O7(n=2) RP phases,new chromates have been synthesized under high pressure/temperature by substitutingSr2+ with other alkaline earth elements in order to “play” with the cation size effect. Forthe Ca2CrO4 RP (n=1) phase, substitution with Ca2+ as a replacing element lead to aweakening of both the insulating behavior and AFM ordering. Synthesis to form the n=2RP Ca-based lead to the formation of the unexpected and new n=3 phase Ca4Cr3O10. Asa replacing element, Ba2+ broke the RP structure configuration and led to a new phase,whose crystal structure was solved using electron diffraction tomography.In 2014, superconductivity has been discovered in CrAs (Tc=2.2 K at P=0.8 GPa),the first superconducting Cr-based system. In this context, we have look for other AFMCr-based system with relatively high Néel temperature and moderate Cr magnetic moment.In consequence, we investigated another system, the narrow-gap semiconductorCrSb2. We find a metal-insulating transition from transport measurement at P=10 GPafor both polycrystalline and single crystal samples we synthesized. At this pressure, wealso observed a phase transition on synchrotron XRD under pressure. Finally, pressuredependence of the AFM ordering had been established by neutron powder diffractionunder high pressure.; Après 25 années de recherche sur les cuprates, la découverte en 2008 de la supraconductivitéà haute Tc dans les composés à base de fer a stimulé la recherche de nouveauxcomposés supraconducteurs. Les composés d’éléments de transition 3d, antiferromagnétiquesà haute température de Néel, de moment magnétique modéré et de structure cristallographiquelamellaire sont particulièrement intéressants comme phases mères pour depotentiels nouveaux supraconducteurs à haute Tc.Dans ce contexte de recherche, nous nous sommes intéressés dans un premier temps auxpropriétés physiques de la famille Ruddlesden-Popper (RP) Srn+1CrnO3n+1. Ces composésà base de chrome n’ont été que peu étudiés à cause de leurs conditions synthèses,nécessitant d’atteindre les hautes pressions et hautes températures (HP-HT). Nous avonssynthétisé avec succès les phases Sr2CrO4 (n=1), Sr3Cr2O7 (n=2), Sr4Cr3O10 (n=3) etSrCrO3 (n=1). Des études en Grands Instruments et de transport sous pression ont étéeffectuées pour la première fois sur ces phases. Bien que la supraconductivité n’y ait pasété détectée ni sous pression (jusqu’à 20 GPa et 4.4K) ni par dopage en site de Cr, nousavons découvert au sein de ces phases d’intéressants phénomènes physiques tels qu’uneffet « anti-Jahn-Teller » inhabituel à basse température dans Sr2CrO4 ou l’instaurationd’un ordre orbital dans Sr3Cr2O7 associé à un fort couplage magnéto-structural.Pour tenter d’induire un régime métallique, voire supraconducteur, au sein des phasesSr2CrO4 et Sr3Cr2O7, des substitutions en site cationique Sr2+ ont été entreprises. Cessubstitutions, par un autre élément alcalino-terreux, ont pour objectif de modifier lesinteractions Cr-Cr en « jouant » avec l’effet de taille du cation (pression chimique). Denouveaux chromates ont ainsi été synthétisés sous HP-HT par substitution avec un cationde plus petit rayon ionique, Ca2+, et de plus grand rayon ionique, Ba2+. La substitution aucalcium donne naissance à une nouvelle phase RP Ca2CrO4 (n=1) dont le comportementsemiconducteur ainsi que l’ordre AFM semblent affaiblis comparés à la phase au strontium.La substitution au baryum, quant à elle, brise la configuration structurale RP pour formerune nouvelle phase dont la structure cristallographique a été résolue par une étude detomographie en diffraction électronique.En parallèle, en 2014, la supraconductivité a été découverte dans le composé CrAs,devenant ainsi le premier composé supraconducteur à base de chrome. En conséquence,nous nous sommes intéressés à un composé similaire, un semiconducteur à petit gap deTN´eel élevée ( 270K) : CrSb2. Nos mesures sous pression de transport électrique et deDRX en cellule à enclumes de diamant ont mis en évidence une transition métal-isolantet structurale à 10 GPa. Finalement, en complétant avec une expérience de diffraction deneutrons sous pression, nous avons établi la dépendance en pression de l’ordre AFM.

After 25 years of research on the superconducting cuprates, the discovery of high Tc superconductivity in related iron based pnictides in 2008[1] has stimulated the search of new superconductors. In particular antiferromagnet (AFM) with high Néel temperature, moderate magnetic moments and with crystallographic 2D layers could be adequate parent phases for new unconventional superconductors. In particular, other systems with 3d transition elements are very interesting systems for potential new high Tc superconductors.In this context we have reinvestigated the physical properties of n=1, 2,3 and + members of the Srn+1CrnO3n+1Ruddlesden Popper (RP) series. We have successfully synthesized at HP-HT Sr2CrO4 (n=1), Sr3Cr2O7(n=2), Sr4Cr3O10 (n=3) and SrCrO3 (n=infinite). We present some results of several studies performed on these phases such as neutron powder diffraction, Synchrotron X-ray diffraction under high pressure and low temperature, electrical transport properties under high pressure. Even if no superconductivity was observed, interesting properties had been discovered such as an unusual “anti-Jahn-Teller” in Sr2CrO4 at low temperature or an orbital ordering in Sr3Cr2O7.To explore new Cr-based systems where superconductivity could potentially be induced by changing Cr-Cr interactions in the Sr2CrO4 and Sr3Cr2O7RP phases, new chromates have been synthesized under HP-HT by substituting Sr2+ with other alkaline earth elements in order to “play” with the cation size effect. For the n=1 RP Ca2CrO4 phase, substitution with Ca2+ as a replacing element lead to a weakening of both the insulating behavior and AFM ordering. Synthesis to form the n=2 RP Ca-based lead to the formation of the unexpected and new n=3 phase Ca4Cr2O10.Ba2+ as a replacing element broke the RP structure configuration and led to a new phase, whose crystal structure was solved using electron diffraction tomography.In 2014, superconductivity has been discovered in CrAs (Tc=2.2K at P=0.8GPa), the first superconducting Cr-based system. In this context, we have look for other AFM Cr-based system with relatively high Néel temperature and moderate Cr magnetic moment. In consequence, we investigated another system, the narrow-gap semiconductor CrSb2. We find a metal-insulating transition from transport measurement at 10 GPa for both polycrystalline and single crystals samples we synthesized. At this pressure, we also observed a phase transition on synchrotron XRD under pressure. Finally, pressure dependence of the AFM ordering had been established by NPD under high pressure., Après 25 ans de recherche sur les oxydes de cuivres supraconducteurs, les cuprates, la découverte d'une supraconductivité à haute Tc dans les pnictures à base de fer en 2008 a stimulé la recherche de nouveaux composés supraconducteurs. Les systèmes possédant un élément de transition 3d sont réellement intéressants en tant que potentiel nouveaux supraconducteurs. De plus, un composé antiferromagnétique (AFM) à haute température de Néel, de moment magnétique modéré et de structure cristallographique lamellaire (2D) semble essentiel pour atteindre les hautes Tc.Dans ce contexte de recherche, nous avons réinvestigué les propriétés physiques des membres n=1,2,3 et infini de la famille d'oxyde de chrome Ruddlesden-Popper (RP) Srn+1CrnO3n+1. Les résultats de plusieurs nouvelles études sur ces phases sont présentés telles que des expériences de diffraction de neutron sur poudre (DNP), de la diffraction des rayons-X en synchrotron sous pression et basse température, et des mesures de transport électriques sous pression. Bien que la supraconductivité n'ait pas été observée dans ces phases, nous reportons certains phénomènes physiques exotiques et intéressants tels qu'un effet "anti-Jahn-Teller" dans Sr2CrO4 à basse température et l'instauration d'un ordre orbital dans Sr3Cr2O7. Pour explorer de nouveaux composés à base de Cr dans lesquels la supraconductivité pourrait être potentiellement induite en changeant les interactions entre Cr dans les phases RP Sr2CrO4 et Sr3Cr2O7, de nouveaux chromates ont été synthétisés sous HP-HT par substitution de Sr2+ par d'autres éléments alcalino-terreux dans le but jouer avec l'effet de taille des cations (pression chimique). Pour la nouvelle phase RP n=1 Ca214, la substitution par un cation plus petit Ca2+ amène à un affaiblissement aussi bien du comportement isolant et que de l'ordre AFM. Concernant la synthèse de Ca327, cette phase n'a pu être obtenue, donnant lieu à la place à la nouvelle phase n=3 Ca4310. La substitution par un cation de rayon ionique plus grand du Ba2+ casse la structure RP et mène vers une nouvelle phase cubique à grand paramètre dont la structure a été résolue par tomographie en diffraction électronique. En 2014, la supraconductivité a été découverte dans le composé CrAs (2.2K à 0.8 GPa), soit le premier composé à base de Cr supraconducteur. Nous avons alors cherché un autre composé à base de Cr avec un TNéel relativement haute et moment magnétique modéré. En conséquence, nous avons travaillé sur un autre système similaire, le semiconducteur à gap étroit CrSb2. Nous avons observé une transition métal-isolant à partir de mesures de transport à 10GPa aussi bien pour des échantillons polycristallins que monocristallins que nous avons synthétisé. Nous mesurons aussi à cette pression une transition de phase par DRX en synchrotron. Finallement, une étude de DNP sous pression confirmera la dépendance en pression de l'ordre AFM.