Nuclear magnetic resonance in LiF: a single nuclear-dipolar-spin temperature in crystals with more than one spin species

Journal ArticleIn a crystal containing more than one species of nuclear spin in a large dc magnetic field, the secular dipolar interactions between all spins, unlike as well as like, form a single reservoir described by a single spin temperature. We demonstrated this concept with experiments on LiF. In particular, we showed that TXD of 7Li and 1 9F are equal. We also measured TX and TXP of 1 9F using 7Li detection. Finally we applied this concept'to the Provotorov theory of saturation, which we extended to the case of a two-spin system, and demonstrated experimentally the validity of the treatment

Nuclear double resonance: cross relaxation rates between two spin species

Journal ArticleA rotating-frame nuclear-double-resonance experiment is reported in which the cross-relaxation rates between 7Li and 6Li in powdered lithium metal were measured. The theory developed my McArthur, Hahn, and Walstedt (MHW) is applied to these data and good agreement is obtained. We also apply this theory to other published experimental data (LiF by Lang and Moran and adamantane by Pines and Shattuck) and find good agreement. We conclude that the assumption of a Lorentzian correlation function, which forms the basis of the theory of MHW is generally valid

NMR observations of molecular motions and Zeeman-quadrupole cross relaxation in 1,2- difluorotetrachloroethane

Journal ArticleWe report measurements of "F NMR relaxation times Tu Tlf, TID, and T2 in the plastic crystal CFCL2-CFCL2 . From the data near the melting point, we obtain the jump time for translational selfdiffusion. At lower temperatures, we observe on the cold side of the T, and Tif minima an unusual field dependence which is substantially less than the normal field-squared dependence. We also observe a reduction in T\ near 40 MHz due to cross relaxation between the Zeeman levels of the "F spins and quadrupole levels of the 3 5C1 and 3 7C1 spins. We measured the cross relaxation times TIS as a function of field and found good agreement with our theoretical calculation of TIS

Molecular reorientations in the ordered phases of KCN and NaCN studied by NMR

Journal ArticleWe have measured 13C and 23Na NMR spin-lattice relaxation times as a function of temperature in KCN and NaCN in order to study the head-to-tail reorientations of the CN- molecules in the two low-temperature ordered phases. We have combined our data with those of dielectric-response and ionic-thermal-conductivity measurements and have determined the correlation time rc of the reorientations over more than five decades

Evidence for elastic disorder in the elastically ordered phase of KCN

Journal ArticleWe have obtained evidence from 1 3C NMR measurements that the CN~ ion in the elastically ordered phase of KCN is misoriented slightly with respect to the orthorhombic b axis. This misorientation varies randomly over the lattice, averaging to zero on a macroscopic scale. The misorientations are manifested through small-angle CN" reorientations which contribute to the spin-lattice relaxation time Tx of the °C nuclei, via dipolar interactions and chemical shift anisotropy

Crystal and magnetic structures of hexagonal YMnO \textrm3

The available data on the structural and magnetic transitions in multiferroic hexagonal YMnO3 have been reviewed, first making use of the computer programs from the group theoretical ISOTROPY software suite to list possible crystal and magnetic structures, then taking into account the capability of neutron diffraction and other physical methods to distinguish them. This leads to a clear view of the transformation sequence, as follows. Hexagonal YMnO3 is paraelectric in P63/mmc at elevated temperatures, and undergoes a single structural transition on cooling through 1250 K to a ferrielectric phase in P63cm that is retained through room temperature. At a much lower temperature, 70 K, there is a magnetic transition from paramagnetic to a triangular antiferromagnetic arrangement, most likely with symmetry P63'cm'. Comment is made on the unusual coupling of ferroelectric and magnetic domains reported to occur in this material, as well as on the so-called `giant magneto-elastic' effect

An Exhaustive Symmetry Approach to Structure Determination: Phase Transitions in Bi2Sn2O7

The exploitable properties of many materials are intimately linked to symmetry-lowering structural phase transitions. We present an automated and exhaustive symmetry-mode method for systematically exploring and solving such structures which will be widely applicable to a range of functional materials. We exemplify the method with an investigation of the Bi2Sn2O7 pyrochlore, which has been shown to undergo transitions from a parent γ cubic phase to β and α structures on cooling. The results include the first reliable structural model for β-Bi2Sn2O7 (orthorhombic Aba2, a = 7.571833(8), b = 21.41262(2), and c = 15.132459(14) Å) and a much simpler description of α-Bi2Sn2O7 (monoclinic Cc, a = 13.15493(6), b = 7.54118(4), and c = 15.07672(7) Å, β = 125.0120(3)°) than has been presented previously. We use the symmetry-mode basis to describe the phase transition in terms of coupled rotations of the Bi2O′ anti-cristobalite framework, which allow Bi atoms to adopt low-symmetry coordination environments favored by lone-pair cations