R. Moret

Single-Crystal X-Ray Study and Phase Diagram of the Tetragonal-Orthorhombic Transition in the High-Tc Superconductor La2-xSrxCuO4

The high-temperature tetragonal to low-temperature orthorhombic phase transition of La1.88Sr0.12CuO4 has been studied by single-crystal X-ray diffraction and diffuse scattering. The temperature dependence of superstructure reflections below TT-O = 215 K, precursor diffuse scattering above TT-O and the orthorhombic lattice parameter distortion is reported. Supplementary powder diffraction data allow to construct the structural phase diagram in the Sr concentration range 0 ≤ x ≤ 0.12. It shows that suppression of the orthorhombic distortion by Sr substitution is not a necessary condition for the occurrence of superconductivity.

High-pressure synthesis, structural and Raman studies of a two-dimensional polymer crystal of C60.

Abstract:Two-dimensional polymerisation of a C60 single crystal has been obtained under high-pressure high temperature conditions (700 K - 2 GPa). Crystalline order is preserved but the crystal splits into variants (orientational domains). The analysis of X-ray diffraction and Raman spectroscopy data reveals that the polymer crystal is primarily tetragonal with some admixture of rhombohedral phase. Furthermore, Raman spectroscopy gives evidence for additional C60-C60 dimers, which are probably disordered. For the tetragonal phase, it is shown that successive polymer layers are rotated by about the stacking axis, according to the P42/mmc space group symmetry. The structure of the rhombohedral phase is also clarified. The role of the interlayer interactions in stabilising the two-dimensional polymer phases of C60 is discussed.

Structures, phase transitions and orientational properties of the C60 monomer and polymers

The high symmetry and round shape of the fullerene molecules induce orientational phenomena that play an important role in the properties of the fullerene compounds. This is illustrated in this review which is focused on the orientational properties of C60 monomer and polymer structures. First, the orientational state of C60 at room temperature, its peculiar orientational ordering phase transition and the associated glassy state are described, together with the corresponding temperature-pressure phase diagram. Then, a summary of the polymerization processes and of the varied structures obtained under high-pressure high-temperature conditions or via doping by alkali metals is proposed. Finally, some information is given on the orientational properties of C60 layers and surfaces.

Structural instabilities in molecular conductors: Silver and copper salts of dicyanoquinonediimine, (DCNQI)2X (X = Ag or Cu)

Abstract An X-ray diffuse scattering and diffraction study of the low temperature structural properties of silver and copper salts of dicyanoquinonediimine (DCNQI) 2 X, X = Ag or Cu is reported. In the Ag salts (2,5 DM-DCNQI) 2 Ag, (2,5 MCl-DCNQI) 2 Ag and (2,5 MBr-DCNQI) 2 Ag (M: methyl, DM dimethyl) one observes quasi 1-D 2k F and 4k F diffuse scattering. In the case of DM, transitions occur at T 4k F = 100 K and T 2kF = 83 K leading to a dimerized intermediate state and a tetramerized spin-Peierls ground state, respectively. For MCl and MBr there is no transition which is presumably due to disorder effects. In the Cu salts no quasi-1D scattering is observed. While (2,5 DM-DCNQI) 2 Cu shows no structural instability, in (2,5 MCl-DCNQI) 2 Cu satellite reflections with qrmq c ± 1 3 c ∗ appear below about 200 K, in agreement with previous reports. These results strongly support the view that the silver compounds are quasi-one-dimensional materials in contrast with the copper ones where the mixed valency of the copper ions may play an important role.

Structural phase transitions in (TMTSF)2X and related compounds

Abstract Structural studies of the anion ordering phase transitions of some (TMTTF) 2 X and (TMTSF) 2 X salts are presented, with special attention on the Ising order parameter and on its kinetics. (TMTSF) 2 X salts containing dipolar anions (SCN − , PF 2 O 2 − ) exhibit unexpected behaviours, such as incommensurability or the occurrence of two successive phase transitions. Finally, the structural origin at the 77 K phase transition of the first sulphur-based organic superconductor (BEDT-TTF) 2 ReO 4 is established.

Tetragonal-orthorhombic phase transition in La2−xSrxCuO4−y: Symmetry analysis, pressure dependence and (T,P,x) phase diagram

It is reminded that the symmetry of the tetragonal-orthorhombic distortion is such that there is no splitting of the Van Hove singularity at the transition. High-pressure low-temperature X-ray single crystal studies show that the orthorhombic phase is suppressed under pressure. A preliminary (T,P,x) phase diagram is presented and correlations are made with the pressure dependence of T c .

Morphology of C60 decagonal crystals grown from n-hexane

Abstract Ten-sided prismatic crystals of C 60 grown by evaporation of n-hexane solutions have been characterized by scanning electron microscopy and X-ray diffraction. Various unusual morphologies have been observed. X-ray precession photographs confirm the decagonal symmetry and they are analyzed by means of twinning of monoclinic variants related by π/5 rotations. Possible twinning mechanisms are discussed.

Structural and physical properties of pressure polymerized C60

We discuss the structural and dynamic properties of C60 polymerized under low-P, low-T conditions, and suggest that the disordered mixed orthorhombic-tetragonal-rhombohedral phases produced under t ...

Polymer chain orientations in KC60 and RbC60: structural analysis and relation with electronic properties

Abstract First single crystal X-ray diffraction studies of polymerized KC 60 and RbC 60 are presented. While powder diffraction studies showed no definite structural differences, the single crystal diffraction data analysis reveals different relative chain orientations in the two compounds. They are discussed in relation with semi-empirical energy calculations. The structural difference between KC 60 and RbC 60 may be a key feature for the understanding of their contrasted conducting and magnetic properties.

The decagonal twinning in C60 crystals grown from n-hexane

The authors report results which unambiguously show that C60 crystals grown from n-hexane solutions (denoted C60-hexane) form twins and that the twinning mechanism may generate a tenfold symmetry. They also compare the characteristics of the C60-hexane crystals with those of C60-pentane