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Register a new userThe role of the lattice in the $gamma to alpha$ phase transition of Ce: a high pressure neutron and x-ray diffraction study
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The temperature and pressure dependence of the thermal displacements and lattice parameters were obtained across the $gamma to alpha$ phase transition of Ce using high-pressure, high-resolution neutron and synchrotron x-ray powder diffraction. The estimated vibrational entropy change per atom in the $gamma to alpha$ phase transition, $Delta S^{gamma - alpha}_{rm vib} approx (0.75 pm 0.15)$k$_{rm B}$, is about half of the total entropy change. The bulk modulus follows a power-law pressure dependence which is well described using the framework of electron-phonon coupling. These results clearly demonstrate the importance of lattice vibrations, in addition to the spin and charge degrees of freedom, for a complete description of the $gamma to alpha$ phase transition in elemental Ce.
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We report on the most complete investigation to date of the 4f-electron properties at the gamma-alpha transition in elemental Ce by resonant inelastic x-ray scattering (RIXS). The Ce 2p3d-RIXS spectra were measured directly in the bulk material as a function of pressure through the transition. The spectra were simulated within the Anderson impurity model. The occupation number nf was derived from the calculations in both gamma- and alpha-phases in the ground state along with the f doubleoccupancy. We find that the electronic structure changes result mainly from band formation of 4f electrons which concurs with reduced electron correlation and increased Kondo screening at high pressure.
The pressure effects on the JT distortion of three representative compounds belonging to the LaMn_1-xGa_xO_3 (x= 0.2, 0.3, 0.4) family was widely investigated by means of X-ray diffraction and Raman spectroscopy. A compound with a fully JT distorted structure (x=0.2), one with regular octahedra (x=0.6) and one in an intermediate configuration (x=0.3) were selected. A pressure induced transitions from the orthorhombic Pbnm phase towards structures with higher symmetry were observed in all the samples. Both Raman and X-ray data confirm that the most important structural effect of pressure is that of reducing the octahedral distortion. The appearance of a feature in the lattice parameter behavior connected to a structural instability was also detected, pointing out the key role of the JT distortion in stabilizing the manganite structures. On the other hand, the complete suppression of the JT distortion in the high-pressure phases cannot be claimed. The Raman spectra collected from more distorted compounds (x=0.2, 0.3) reveal clearly the coexistence of domains of distorted and more regular octahedra in a certain pressure range. The first sketch of the Pressure vs. Ga-content phase diagram was drawn.
YbBiPt is a heavy-fermion compound possessing significant short-range antiferromagnetic correlations below a temperature of $T^{textrm{*}}=0.7$ K, fragile antiferromagnetic order below $T_{rm{N}}=0.4$ K, a Kondo temperature of $T_{textrm{K}} approx1$ K, and crystalline-electric-field splitting on the order of $E/k_{textrm{B}}=1,textrm{-},10$ K. Whereas the compound has a face-centered-cubic lattice at ambient temperature, certain experimental data, particularly those from studies aimed at determining its crystalline-electric-field scheme, suggest that the lattice distorts at lower temperature. Here, we present results from high-resolution, high-energy x-ray diffraction experiments which show that, within our experimental resolution of $approx6,textrm{-},10times10^{-5}$ AA, no structural phase transition occurs between $T=1.5$ and $50$ K. In combination with results from dilatometry measurements, we further show that the compounds thermal expansion has a minimum at $approx18$ K and a region of negative thermal expansion for $9<T<18$ K. Despite diffraction patterns taken at $1.6$ K which indicate that the lattice is face-centered cubic and that the Yb resides on a crystallographic site with cubic point symmetry, we demonstrate that the linear thermal expansion may be modeled using crystalline-electric-field level schemes appropriate for Yb$^{3+}$ residing on a site with either cubic or less than cubic point symmetry.
Through powder x-ray diffraction we have investigated the structural behavior of SmVO3, in which orbital and magnetic degrees of freedom are believed to be closely coupled to the crystal lattice. We have found, contrary to previous reports, that SmVO3 exists in a single, monoclinic, phase below 200 K. The associated crystallographic distortion is then stabilized through the magnetostriction that occurs below 134 K. The crystal structure has been refined using synchrotron x-ray powder diffraction data measured throughout the structural phase diagram, showing a substantial Jahn-Teller distortion of the VO6 octahedra in the monoclinic phase, compatible with the expected G-type orbital order. Changes in the vanadium ion crystal field due to the structural and magnetic transitions have then been probed by resonant x-ray diffraction.
We present the results of neutron diffraction studies on polycrystals of a metallic kagome lattice, Tb3Ru4Al12, reported recently to undergo reentrant magnetism, with the onset of long range antiferromagnetic order below (TN=) 22 K and spin-glass features below about 17 K. The present results reveal long-range antiferromagnetic order of an incommensurate type with the moments oriented along c-axis at all temperatures below TN. There are however notable changes in the T dependence of propagation vector along b-axis across 17 K. An observation of interest is that there is no decrease of intensity of magnetic Bragg peaks on entrance into the glassy phase (that is, below 17 K). This finding suggests that the magnetism of this compound is an exotic one and we wonder whether this compound is an example for dynamical spin-glass freezing phenomenon, as a consequence of geometrical frustration.
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