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Register a new userCrystal structures of dodecaborides: complexity in simplicity
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Analysis of the intriguing physical properties of the dodecaborides, $R$B$_{12}$, requires accurate data on their crystal structure. We show that a simple cubic model fits well with the atomic positions in the unit cell but cannot explain the observed anisotropy in the physical properties. The cooperative Jahn-Teller (JT) effect slightly violates the ideal metric of the cubic lattice and the symmetry of the electron density distribution in the lattice interstices. Theoretical models of the JT distortions of the boron framework are presented. Their correspondence to the electron-density distribution on the maps of Fourier syntheses obtained using x-ray data and explaining the previously observed anisotropy of conductive properties is demonstrated. The effect of boron isotope composition on the character of the lattice distortions is shown. We also discuss the application of the Einstein model for cations and the Debye model for the boron atoms to describe the dynamics of the crystal lattice.
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We have investigated the crystal and magnetic structures of the trigonal iron-boracite Fe3B7O13X with X = OH by neutron diffraction. Neutron diffraction enables us to locate the hydrogen atom of the hydroxyl group and determine the magnetic ground state of this member of the multiferroic boracite family. No evidence was found for a monoclinic distortion in the magnetic ordered state. The magnetic symmetry allows for magnetoelectric and ferroelectric properties. The N/eel tempera- ture TN of 4.86(4) K confirms the general trends within the boracites that TN decreases from X = I > Br > Cl > OH. Surprisingly while Fe3B7O13OH exhibits the largest frustration with $|theta/T_N| = 5.6$ within the Fe3B7O13X series, no reduction of the magnetic moment is found using neutron diffraction.
Crystal and magnetic structures of a series of novel quantum spin trimer system Ca3Cu3xNix(PO4)4 (x=0,1,2) were studied by neutron powder diffraction at the temperatures 1.5-290 K. The composition with one Ni per trimer (x=1) has a monoclinic structure (space group P 21 /a, no. 14) with the unit cell parameters a = 17.71 A, b = 4.89 A, c = 8.85 A and = 123.84 deg at T=290 K. The (x=2) composition crystallizes in the C 2/c space group (no. 15) with the doubled unit cell along c-axis. Each trimer is formed by two crystallographic positions: one in the middle and the second one at the ends of the trimer. We have found that the middle position is occupied by the Cu2+, whereas the end positions are equally populated with the Cu2+ and Ni2+ for (x=1) while in the (x=2) the trimers were found to be of only one type Ni-Cu-Ni. Below TN = 20 K the (x=2) compound shows an antiferromagnetic ordering with propagation vector star {[1/2,1/2, 0], [-1/2,1/2, 0]} The magnetic structure is very well described with the irreducible representation tau22 using both arms of the star {k} with the magnetic moments 1.89(1) muB and 0.62(2) muB for Ni2+ and Cu2+ ions, respectively. We note that our powder diffraction data cannot be fitted by a model containing only one arm of the propagation vector star. The Cu/Ni-spins form both parallel and antiparallel configurations in the different trimers, implying substantial effect of the inter-trimer interaction on the overall magnetic structure.
Neutron powder diffraction studies of the crystal and magnetic structures of the magnetocaloric compound Mn1.1Fe0.9(P0.8Ge0.2) have been carried out as a function of temperature, applied magnetic field, and pressure. The data reveal that there is only one transition observed over the entire range of variables explored, which is a combined magnetic and structural transformation between the paramagnetic to ferromagnetic phases (Tc~255 K for this composition). The structural part of the transition is associated with an expansion of the hexagonal unit cell in the direction of the a- and b-axes and a contraction of the c-axis as the FM phase is formed, which originates from an increase in the intra-layer metal-metal bond distance. The application of pressure is found to have an adverse effect on the formation of the FM phase since pressure opposes the expansion of the lattice and hence decreases Tc. The application of a magnetic field, on the other hand, has the expected effect of enhancing the FM phase and increasing Tc. We find that the substantial range of temperature/field/pressure coexistence of the PM and FM phases observed is due to compositional variations in the sample. In-situ high temperature diffraction measurements were carried out to explore this issue, and reveal a coexisting liquid phase at high temperatures that is the origin of this variation. We show that this range of coexisting phases can be substantially reduced by appropriate heat treatment to improve the sample homogeneity.
Inelastic neutron scattering measurement is performed on a breathing pyrochlore antiferromagnet Ba3Yb2Zn5O11. The observed dispersionless excitations are explained by a crystalline electric field (CEF) Hamiltonian of Kramers ion Yb3+ of which the local symmetry exhibits C3v point group symmetry. The magnetic susceptibility previously reported is consistently reproduced by the energy scheme of the CEF excitations. The obtained wave functions of the ground state Kramers doublet exhibit the planer-type anisotropy. The result demonstrates that Ba3Yb2Zn5O11 is an experimental realization of breathing pyrochlore antiferromagnet with a pseudospin S = 1/2 having easy-plane anisotropy.
This paper has two main parts. The first one presents a direct path from microscopic dynamics to Debye screening, Landau damping and collisional transport. It shows there is more simplicity in microscopic plasma physics than previously thought. The second part is more subjective. It describes some difficulties in facing plasma complexity in general, suggests an inquiry about the methods used empirically to tackle complex systems, discusses the teaching of plasma physics as a physics of complexity, and proposes new directions to face the inflation of information.
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