Network Of Corner-sharing Tetrahedra Research Articles

LiYbSe2: Frustrated Magnetism in the Pyrochlore Lattice.

Three-dimensionally (3D) frustrated magnets generally exist in the magnetic diamond and pyrochlore lattices, in which quantum fluctuations suppress magnetic orders and generate highly entangled ground states. LiYbSe2 in a previously unreported pyrochlore lattice was discovered from LiCl flux growth. Distinct from the quantum spin liquid (QSL) candidate NaYbSe2 hosting a perfect triangular lattice of Yb3+, LiYbSe2 crystallizes in the cubic pyrochlore structure with space group Fd3m (No. 227). The Yb3+ ions in LiYbSe2 are arranged on a network of corner-sharing tetrahedra, which is particularly susceptible to geometrical frustration. According to our temperature-dependent magnetic susceptibility measurements, the dominant antiferromagnetic interaction in LiYbSe2 is expected to appear around 8 K. However, no long-range magnetic order is detected in thermomagnetic measurements above 70 mK. Specific heat measurements also show magnetic correlations shifting with applied magnetic field with a degree of missing entropy that may be related to the slight mixture of Yb3+ on the Li site. Such magnetic frustration of Yb3+ is rare in pyrochlore structures. Thus, LiYbSe2 shows promise in intrinsically realizing disordered quantum states like QSL in pyrochlore structures.

Fluctuation-Driven Selection at Criticality in a Frustrated Magnetic System: The Case of Multiple-kPartial Order on the Pyrochlore Lattice

We study the problem of partially ordered phases with periodically arranged disordered (paramagnetic) sites on the pyrochlore lattice, a network of corner-sharing tetrahedra. The periodicity of these phases is characterized by one or more wave vectors k={1/21/21/2}. Starting from a general microscopic Hamiltonian including anisotropic nearest-neighbor exchange, long-range dipolar interactions, and second- and third-nearest neighbor exchange, we use standard mean-field theory (SMFT) to identify an extended range of interaction parameters that support partially ordered phases. We demonstrate that thermal fluctuations ignored in SMFT are responsible for the selection of one particular partially ordered phase, e.g., the "4-k" phase over the "1-k" phase. We suggest that the transition into the 4-k phase is continuous with its critical properties controlled by the cubic fixed point of a Ginzburg-Landau theory with a four-component vector order parameter. By combining an extension of the Thouless-Anderson-Palmer method originally used to study fluctuations in spin glasses with parallel-tempering Monte Carlo simulations, we establish the phase diagram for different types of partially ordered phases. Our results elucidate the long-standing puzzle concerning the origin of the 4-k partially ordered phase observed in the Gd2Ti2O7 dipolar pyrochlore antiferromagnet below its paramagnetic phase transition temperature.

Electronic structure and interatomic bonding in Al10V

On the basis of ab initio calculations we analysed the electron densitydistribution in the elementary cell of the compound Al10V.We found covalent bonding between certain atoms. The Al–V bondsof enhanced covalency are linked into –Al–V–Al–V–chains that extend over the whole crystal. The chains intersect ateach V site and together form a Kagomé network of corner-sharingtetrahedra. The large voids of this network are filled by Z16Friauf polyhedra consisting of Al atoms only. The skeleton of the Friauf polyhedronhas the form of a truncated tetrahedron and consists of 12 strongly bonded Al atoms.These Al–Al bonds also have covalent character. The bonding is dominated by sp2hybridization. The centre of the Friauf polyhedron may be empty or occupied by anAl atom. The thermodynamic stability of the phase is investigated. The Al21V2phase with occupied voids is at low temperatures less stable than Al10V. TheAl10Vstructure can be considered as a special case of the Al18Cr2Mg3structural class. We have found the same picture of bonding as we report here for Al10Vfor several other aluminium-rich alloys belonging to the Al18Cr2Mg3structural class also.

First-principles analysis of the Raman spectrum of vitreous silica: comparisonwith the vibrational density of states

The HH and HV Raman spectra of vitreous silica are calculated from firstprinciples for a model structure consisting of a disordered network ofcorner-sharing tetrahedra, for which the vibrational properties were obtainedpreviously. We analyse the contribution of specific atomic motions to the Ramanspectra and perform a detailed comparison with respect to the vibrationaldensity of states. We find that the HV spectrum closely resembles thevibrational density of states. By comparison, the HH spectrum showssignificant differences and arises almost exclusively from oxygen vibrations.

Cooperative Paramagnetism in the Geometrically Frustrated Pyrochlore AntiferromagnetTb2Ti2O7

Neutron scattering, muon spin relaxation, and dc susceptibility studies have been carried out on polycrystalline ${\mathrm{Tb}}_{2}{\mathrm{Ti}}_{2}{\mathrm{O}}_{7}$, a pyrochlore antiferromagnet in which the ${\mathrm{Tb}}^{3+}$ moments reside on a network of corner-sharing tetrahedra. Unlike other geometrically frustrated systems, ${\mathrm{Tb}}_{2}{\mathrm{Ti}}_{2}{\mathrm{O}}_{7}$ remains paramagnetic down to $\ensuremath{\sim}0.07\mathrm{K}$, rather than ordering into a conventional N\'eel or spin-glass-like state, despite the fact that short-range antiferromagnetic correlations (AFC) develop at $\ensuremath{\sim}50\mathrm{K}$. At the first AFC wave vector, its low-lying, relatively flat magnetic excitation spectrum softens partially below 30 K.

Single crystal growth by the floating-zone method of a geometrically frustrated pyrochlore antiferromagnet, Tb 2Ti 2O 7

Large, high quality, single crystals of Tb 2Ti 2O 7, have been successfully grown by the floating zone method under a controlled atmosphere. Crystals, approximately 3–5 mm in diameter and up to 20 mm in length, were grown from a small seed crystal. Several crystals have been characterised to determine their crystal quality, chemical structure and magnetic properties and these results were compared to those of sintered polycrystalline pellets. These materials were shown to have the face centred cubic structure, Fd–3m, with a room temperature lattice parameter a=10.12(7) Å, when grown in oxygen, and a=10.13(6) Å, when grown in argon. They are currently of interest as they are comprised of antiferromagnetically coupled magnetic moments residing on a network of corner-sharing tetrahedra, a system known to display the effects of geometrical frustration. Magnetic susceptibility measurements, with the applied field parallel to the growth direction were performed on both crystals. Both single crystal and polycrystalline samples showed very similar Curie-Weiss like behaviour down to 5 K, with Curie–Weiss temperatures, θ cw of approximately −19 K.

Neutron scattering studies of geometrically frustrated pyrochlore antiferromagnets

Elastic and inelastic neutron scattering work on the cubic pyrochlore antiferromagnets Y 2Mo 2O 7, Tb 2Ti 2O 7, and Tb 2Mo 2O 7 is reviewed. These materials are characterized by antiferromagnetically coupled moments residing on a network of corner-sharing tetrahedra. Geometrical frustration manifests itself strongly in such systems in that two of them display a spin-glass-like transition in the apparent absence of chemical disorder, while the other remains paramagnetic to at least 0.1 K, despite developing antiferromagnetic short-range order at 50 K.

Geometrically-frustrated magnetism on triangular and tetrahedral lattices

The combination of simple, nearest neighbor antiferromagnetic interactions and certain lattice symmetries leads to phenomena known broadly as frustration. In such systems, the formation of a collinear magnetic structure cannot occur at low temperature. The material may undergo a phase transition to an unusual magneticallyordered state, or it may not undergo a conventional transition at all-entering a glass-like state as the temperature is lowered. This paper will review recent work, mostly from neutron scattering studies, on three such magnetic materials. CsMnBr3 and CsCoBr3 are XY-like and Ising-like stacked-triangular lattice antiferromagnets, respectively. Both undergo magnetic phase transitions with interesting properties due to the presence of geometricalfrustration within the triangular layers. TD2Mo2O7 is a pyrochlore antiferromagnet, in which the magnetic moments reside on a network of corner-sharing tetrahedra. This material enters a magnetic glass-like state at low temperature.