Pauling Entropy, Metastability, and Equilibrium in Dy_{2}Ti_{2}O_{7} Spin Ice.

Determining the fate of the Pauling entropy in the classical spin ice material Dy_{2}Ti_{2}O_{7} with respect to the third law of thermodynamics has become an important test case for understanding the existence and stability of ice-rule states in general. The standard model of spin ice-the dipolar spin ice model-predicts an ordering transition at T≈0.15 K, but recent experiments by Pomaranski et al. suggest an entropy recovery over long timescales at temperatures as high as 0.5 K, much too high to be compatible with the theory. Using neutron scattering and specific heat measurements at low temperatures and with long timescales (0.35 K/10^{6} s and 0.5 K/10^{5} s, respectively) on several isotopically enriched samples, we find no evidence of a reduction of ice-rule correlations or spin entropy. High-resolution simulations of the neutron structure factor show that the spin correlations remain well described by the dipolar spin ice model at all temperatures. Furthermore, by careful consideration of hyperfine contributions, we conclude that the original entropy measurements of Ramirez et al. are, after all, essentially correct: The short-time relaxation method used in that study gives a reasonably accurate estimate of the equilibrium spin ice entropy due to a cancellation of contributions.

H/D isotope effect on the molar volume and thermal expansion of benzene.

Time-of-flight neutron powder diffraction data have been collected from C6H6 and C6D6, as a function of temperature between 10 and 276 K using the High Resolution Powder Diffractometer at the ISIS Neutron and Muon Source. In order to achieve high accuracy and precision both in the determination of lattice parameters and in the control of the sample temperature, data were acquired using the same instrumental conditions for each isotopologue with an internal NIST silicon standard and in situ-heated sample holders. In contrast with the work from J. D. Dunitz and R. M. Ibberson, Angew. Chem. Int. Ed., 2008, 47, 4208-4210, we find that the difference in molar volume between the two isotopologues is almost invariant with temperature and that the anisotropic linear expansivities of the crystallographic axes are nearly identical in C6H6 and C6D6.

Novel high-efficiency 2D position-sensitive ZnS:Ag/6LiF scintillator detector for neutron diffraction.

Scintillator-based ZnS:Ag/6LiF neutron detectors have been under development at ISIS for more than three decades. Continuous research and development aim to improve detector capabilities, achieve better performance and meet the increasingly demanding requirements set by neutron instruments. As part of this program, a high-efficiency 2D position-sensitive scintillator detector with wavelength-shifting fibres has been developed for neutron-diffraction applications. The detector consists of a double scintillator-fibre layer to improve detection efficiency. Each layer is made up of two orthogonal fibre planes placed between two ZnS:Ag/6LiF scintillator screens. Thin reflective foils are attached to the front and back scintillators of each layer to minimize light cross-talk between layers. The detector has an active area of 192 × 192 mm with a square pixel size of 3 × 3 mm. As part of the development process of the double-layer detector, a single-layer detector was built, together with a prototype detector in which the two layers of the detector could be read out separately. Efficiency calculations and measurements of all three detectors are discussed. The novel double-layer detector has been installed and tested on the SXD diffractometer at ISIS. The detector performance is compared with the current scintillator detectors employed on SXD by studying reference crystal samples. More than a factor of 3 improvement in efficiency is achieved with the double-layer wavelength-shifting-fibre detector. Software routines for further optimizations in spatial resolution and uniformity of response have been implemented and tested for 2D detectors. The methods and results are discussed in this manuscript.

A thermal spin transition in [Co(bpy)3][LiCr(ox)3] (ox = C2O4(2-); bpy = 2,2'-bipyridine).

In the three-dimensional oxalate network structures [M(II)(bpy)3][M(I)-M(III)(ox)3] (ox= C2O4(2-); bpy = 2,2'-bipyridine) the negatively charged oxalate backbone provides perfect cavities for tris-bipyridyl complex cations. The size of the cavity can be adjusted by variation of the metal ions of the oxalate backbone. In [Co(bpy)3][NaCr(ox)3], the [Co(bpy)3]2 + complex is in its usual 4T1(t2g5e(g)2) high-spin ground state. Substituting Na+ by Li+ reduces the size of the cavity. The resulting chemical pressure destabilises the high-spin state of [Co(bpy)3]2+ to such an extent that the 2E(t2g6e(g)1) low-spin state becomes the actual ground state. As a result. [Co(bpy)3][LiCr(ox)3] becomes a spin-crossover system, as shown by temperature-dependent magnetic susceptibility measurements and single-crystal optical spectroscopy, as well as by an X-ray structure determination at 290 and 10 K.