C. Carel

Structural evolution, thermal expansion anomaly and the Grüneisen constants of the superconductor YBa2Cu3O7

Abstract The analyses of the continuous structural evolutions of ceramics presenting a transition can be carried out using low temperature X-ray and neutron diffraction. A specific method is recalled here for evaluating the anisotropic elastic and Gruneisen constants required for the phase study. It also provides continuous characterization of the entire evolution of the thermo-elastic properties through the superconducting transition. This method is applied to the superconductor YBa 2 Cu 3 O 7 . The compressibility coefficients of the orthorhombic phase have been determined by neutron diffraction at 300 K between 0 and 10 kbar. Using structural approximations, average elastic constants and Gruneisen parameters are then fitted to experimental thermal expansion data: C 11 + C 12 =4.83; C 13 =1.24, C 33 =2.87 in 10 11 Pa units; γ 0 =2.7 at T >200 K. These parameters are then used for interpreting thermal expansion anomalies through the Gruneisen parameter variations. Microstructure might involve local stresses thus affecting the thermal expansion anomalies in the transition range.

Anisotropic thermal expansion and Grüneisen coefficients of the bismuth cuprate 2-2-1-2

Abstract The structural evolution of the superconductor Bi 2 Sr 2 CaCu 2 O 8 has been analyzed by X-ray diffraction measurements of the cell parameters a and c between 5 and 220 K. A tetragonal modeling already applied to YBa 2 Cu 3 O 7 is used in order to characterize the evolution of the anisotropic thermo-elastic properties of this bismuth cuprate by means of the Debye model and Gruneisen coefficients. Similarly to the case of YBa 2 Cu 3 O 7 , a strong peak γ a ≈ 2.8 at 45 K for the Gruneisen coefficient in the a , b plane is evidenced ( γ c = 0.85). The mean value γ a = 1.2 is determined at 220 K. The evolution of γ a ( T ) indicates that microstructural effects might induce an anomalous expansion below T c .

Anisotropic compressibility of the orthorhombic phase Bi2Sr2Ca1Cu2O8

Abstract The anisotropic compressibility factors of the superconducting phase Bi 2 Sr 2 Ca 1 Cu 2 O 8 (2212) are measured at 300 K by means of powder neutron diffraction under pressure up to 10 kbar. The values (in Pa −1 ) χ a =(4.9±1.0)×10 −12 , χ b =(2.0±0.4)×10 −12 and χ c =(9.8±2.0)×10 −12 have been found. The resulting volumic coefficients is χ v =17×10 −12 Pa −1 . This result is to be compared with the results concerning Y 1 Ba 2 Cu 3 O 7 : χ v =5.3×10 −12 Pa −1 . A comparison between the Gruneisen parameters of the superconductors is made.

Nucleation and stability of defect clusters: New energetic model in the case of substituted wustites

Abstract For wustite Fe1−z O (z < 0.08) an energetic model accounts for the stability of cubic defect clusters (m/n) which are partly ordered in the crystal. The Gibbs energy GT (N) associated with clusters, including their distorted envelope, is expressed as a sum of a volume term in N 3 and of surface terms in N 4; N is the number of bonds characteristic of the cluster size. In the case of a (10/4) type cluster, this energy is negative and minimum for Nm ranging between 4 and 5, when the volume and surface energies range between specific values. Using simple assumptions, a volume energy −0.80 eV per vacancy is found in accordance with the value of stabilization energy calculated by theorists for the (10/4) cluster. The substitution of Fe2+ by Ca2+ should lead to a decrease of cluster size; this has been recently suggested by neutron diffraction studies.

Magnetic and structural characterization of new alkaline copper iron sulfide Rb2Cu3FeS4

Abstract Rb2Cu3FeS4 has been studied by Mossbauer spectroscopy and NPD, as a function of temperature. The Mossbauer spectra show two components corresponding to two different iron environments. The first one orders around 150 K and is assigned to an isotropic Fe(III) surrounding, suggesting that all four of the nearest tetrahedral positions are occupied by iron ions. The second component orders around 50 K and is assigned to an inhomogeneous occupation of the neighboring sites. As a contrast to the Mossbauer data, no chemical order between Cu and Fe could be observed via NPD owing to the low contrast between the scattering amplitudes of those two elements. The crystal structure, refined with a mean transition element, is of ThCr2Si2-type (space group I4/mmm) consistent with the previous X-ray characterization. Antiferromagnetic ordering is observed below TN≈150 K, with a magnetic unit cell 4a×4a×c, whereas no anomaly could be detected in the neutron powder diagrams around 50 K.

Two Quaternary Sulfides Rb2Cu3FeS4 and Rb2Cu3MnS4 Magnetic Interactions and Electrical Behavior

The two quaternary transition metal chalcogenides Rb2Cu3 MS4 (M = Fe, Mn) are based on a common structural pattern consisting of two-dimensional (2D) arrangements of sulfur tetrahedra linked by edges, with the ThCr2Si2-type structure. They are prepared from alkali metal carbonate Rb2CO3 and transition metal oxides CuO, Fe2O3 or MnO by CS2/Ar sulfurization at 850°C  T  900°C. The magnetic susceptibilities are directly measured as a function of the temperature. Rb2Cu3MnS4 presents a magnetic transition from a high spin (Mn2+) to the low spin state (μeff ≍ 1.5 μ B ), while the iron analog Rb2Cu3FeS4 orders antiferromagnetically at T N ≍ 6K under an applied field of one kGauss.