Determination of the vibrational contribution to the entropy change at the martensitic transformation in Ni-Mn-Sn metamagnetic shape memory alloys: a combined approach of time-of-flight neutron spectroscopy and ab initio calculations.

The different contributions to the entropy change linked to the austenite-martensitic transition in a Ni-Mn-Sn metamagnetic shape memory alloy have been determined by combining different experimental techniques. The vibrational contribution has been inferred from the vibrational density of states of both the martensitic and austenite phases. This has been accomplished by combining time-of-flight neutron scattering measurements and ab initio calculations. Further, the electronic part of the entropy change has also been calculated. Since the martensitic transformation takes place between two paramagnetic phases, the magnetic contribution can be neglected and the entropy change can be reduced to the sum of two terms: vibrational and electronic. The obtained value of the vibrational contribution ([Formula: see text]) nearly provides the total entropy change measured by calorimetry ([Formula: see text]), the difference being the electronic contribution within the experimental error.

Chemistry and structure by design: ordered CuNi(CN)4 sheets with copper(ii) in a square-planar environment.

Layered copper-nickel cyanide, CuNi(CN)4, a 2-D negative thermal expansion material, is one of a series of copper(ii)-containing cyanides derived from Ni(CN)2. In CuNi(CN)4, unlike in Ni(CN)2, the cyanide groups are ordered generating square-planar Ni(CN)4 and Cu(NC)4 units. The adoption of square-planar geometry by Cu(ii) in an extended solid is very unusual.

Phonons and colossal thermal expansion behavior of Ag3Co(CN)6 and Ag3Fe(CN)6.

Recently colossal volume thermal expansion has been observed in the framework compounds Ag(3)Co(CN)(6) and Ag(3)Fe(CN)(6). We have measured phonon spectra using neutron time-of-flight spectroscopy as a function of temperature and pressure. Ab initio calculations were carried out for the sake of analysis and interpretation. Bonding is found to be very similar in the two compounds. At ambient pressure, modes in the intermediate frequency part of the vibrational spectra in the Co compound are shifted slightly to higher energies as compared to the Fe compound. The temperature dependence of the phonon spectra gives evidence for a large explicit anharmonic contribution to the total anharmonicity for low-energy modes below 5 meV. We have found that modes are mainly affected by the change in size of the unit cell, which in turn changes the bond lengths and vibrational frequencies. Thermal expansion has been calculated via the volume dependence of phonon spectra. Our analysis indicates that Ag phonon modes within the energy range 2-5 meV are strongly anharmonic and major contributors to thermal expansion in both systems. The application of pressure hardens the low-energy part of the phonon spectra involving Ag vibrations and confirms the highly anharmonic nature of these modes.

Quantum spin liquid in frustrated one-dimensional LiCuSbO4.

A quantum magnet, LiCuSbO4, with chains of edge-sharing spin-1/2 CuO6 octahedra is reported. While short-range order is observed for T<10 K, no zero-field phase transition or spin freezing occurs down to 100 mK. Specific heat indicates a distinct high-field phase near the 12 T saturation field. Neutron scattering shows incommensurate spin correlations with q=(0.47±0.01)π/a and places an upper limit of 70 µeV on any spin gap. Exact diagonalization of 16-spin easy-plane spin-1/2 chains with competing ferro- and antiferromagnetic interactions (J1=-75 K, J2=34 K) accounts for the T>2 K data.

Ground states, excited states, and metal-ligand bonding in rare earth hexachloro complexes: a DFT-based ligand field study.

Metal (4f)-ligand (Cl 3p) bonding in LnCl(6)(3-) (Ln = Ce to Yb) complexes has been studied on the basis of 4f-->4f and Cl,3p-->4f charge-transfer spectra and on the analysis of these spectra within the valence bond configuration interaction model to show that mixing of Cl 3p into the Ln 4f ligand field orbitals does not exceed 1%. Contrary to this, Kohn-Sham formalism of density functional theory using currently available approximations to the exchange-correlation functional tends to strongly overestimate 4f-3p covalency, yielding, for YbCl(6)(3-), a much larger mixing of Cl 3p-->4f charge transfer into the f(13) ionic ground-state wave function. Thus, ligand field density functional theory, which was recently developed and applied with success to complexes of 3d metals in our group, yields anomalously large ligand field splittings for Ln, the discrepancy with experiment increasing from left to the right of the Ln 4f series. It is shown that eliminating artificial ligand-to-metal charge transfer in Kohn-Sham calculations by a procedure described in this work leads to energies of 4f-4f transitions in good agreement with experiment. We recall an earlier concept of Ballhausen and Dahl which describes ligand field in terms of a pseudopotential and give a thorough analysis of the contributions to the ligand field from electrostatics (crystal field) and exchange (Pauli) repulsion. The close relation of the present results with those obtained using the first-principles based and electron density dependent effective embedding potential is pointed out along with implications for applications to other systems.