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Chemical science 9(14), 3555-3562 (2018). doi:10.1039/C8SC00163D, Published by RSC, Cambridge

The inelastic response of AV2Al20 (with A = Al, Ga and Y) was probed by high-resolution inelastic neutron scattering experiments and density functional theory (DFT) based lattice dynamics calculations (LDC). Features characteristic of the dynamics of Al, Ga and Y are established experimentally in the low-energy range of the compounds. In the stereotype 'Einstein-solid' compound AlV2Al20 we identify a unique spectral density extending up to 10 meV at 1.6 K. Its dominating feature is a peak centred at 2 meV at the base temperature. A very similar spectral distribution is established in GaV2Al20 albeit the strong peak is located at 1 meV at 1.6 K. In YV2Al20 signals characteristic of Y dynamics are located above 8 meV. The spectral distributions are reproduced by the DFT-based LDC and identified as a set of phonons. The response to temperature changes between 1.6 and ∼300 K is studied experimentally and the exceptionally vivid renormalization of the A characteristic modes in AlV2Al20 and GaV2Al20 is quantified by following the energy of the strong peak. At about 300 K it is shifted to higher energies by 300% for A = Al and 450% for A = Ga. The dynamics of A = Y in YV2Al20 show a minor temperature effect. This holds in general for modes located above 10 meV in any of the compounds. They are associated with vibrations of the V2Al20 matrix. Atomic potentials derived through DFT calculations indicate the propensity of A = Al and Ga to a strong positive energy shift upon temperature increase by a high quartic component. The effect of the strong phonon renormalization on thermodynamic observables is computed on grounds of the LDC results. It is shown that through the hybridization of A = Al and Ga with the V2Al20 dynamics the matrix vibrations in the low-energy range follow this renormalization.

We report on the inelastic response of AV2Al20 (with A = Sc, La and Ce) probed by high-resolution inelastic neutron scattering experiments. Intense signals associated with the dynamics of Sc, La and Ce are identified in the low-energy range at 6–14 meV in ScV2Al20 and at 8–16 meV in LaV2Al20 and CeV2Al20. Their response to temperature changes between 2 and 300 K reveals a very weak softening of the modes upon heating in LaV2Al20 and CeV2Al20 and a distinguished blue shift by about 2 meV in ScV2Al20. By means of density functional theory (DFT) and lattice dynamics calculations (LDC) we show that the unusual anharmonicity of the Sc-dominated modes is due to the local potential of Sc featured by a strong quartic term. The vibrational dynamics of ScV2Al20 as well as of LaV2Al20 and CeV2Al20 is reproduced by a set of eigenmodes. To screen the validity of the DFT and LDC results they are confronted with data from X-ray diffraction measurements. The effect of the strong phonon renormalization in ScV2Al20 on thermodynamic observables is computed on grounds of the LDC derived inelastic response. To set the data in a general context of AV2Al20 compounds and their physical properties we report in addition computer and experimental results of the binary V2Al20 compound.

International audience; The mononuclear complex (Bu4N)2[ReIVCl4(CN)2].2DMA (DMA =N,N-dimethylacetamide) displays intricate magnetization dynamics, implying Orbach, direct, and Raman-type relaxation processes. The Orbach relaxation process is characterized by an energy barrier of 39 K (27 cm-1) that is discussed based on high-field electron paramagnetic resonance (EPR), inelastic neutron scattering and frequency domain THz EPR investigations.

Determination of the electronic energy spectrum of a trigonalsymtnetry mononuclear Yb3+ single-molecule magnet (SMM) by highresolution absorption and luniinescente spectroscopies reveals that the first excited electronic doublet is placed nearly 500 cm(-1) above the ground one. Fitting of the paramagnetic relaxation times of this- SMM to a thermally activated (Orbach) model {tau = tau(0) X exp [Delta(Orbach)/(k(B)T)]} affords an activation barrier, Delta(Orbach), of only 38 cm(-1). This result is incompatible with the spectroscopic observations. Thus, we unambiguously demonstrate, solely on the basis Of experimental data, that Orbach relaxation cannot ci priori be considered as the main mechanism determining the spin dynamics of SM.Ms. This study highlights the fact that the general:synthetic approach of optimizing SMM behavior by maximization of the anisotropy barrier, intimately linked to the ligand field, 'as the sole paratneter to be tuned, is insufficient because of the complete neglect of the interaction of the magnetic moment of the molecule with its environment. The Orbach mechanism is expected dominant only in the cases in which the energy of the excited ligand field state is below the Debye temperature, which is typically low for molecular crystals and, thus, prevents the use of the anisotropy barrier as a design criterion, for the realization of high-temperature SM.Ms. Therefcire, consideration of additional design criteria that address the presence of alternative relaxation processes beyond the traditional double-well picture is required.

The nature of the magnetic interaction through fluoride in a simple, dinuclear manganese(III) complex (1), bridged by a single fluoride ion in a perfectly linear fashion, is established by experiment and density functional theory. The magnitude of the antiferromagnetic exchange interaction and the manganese(III) zero-field-splitting parameters are unambiguously determined by inelastic neutron scattering to yield J = 33.0(2) cm(-1) ((H) over cap = J (S-1) over cap.(S-2) over cap Hamiltonian definition) and single-ion D = -4.0(1) cm(-1). Additionally, high-field, high-frequency electron paramagnetic resonance and magnetic measurements support the parameter values and resolve vertical bar E vertical bar approximate to 0.04 cm(-1). The exchange coupling constant (J) is 1 order of magnitude smaller than that found in comparable systems with linear oxide bridging but comparable to typical magnitudes through cyanide, thus underlining the potential of fluoride complexes as promising building blocks for novel magnetic systems.

A facile synthesis of the [ReF6 ](2-) ion and its use as a building block to synthesize magnetic systems are reported. Using dc and ac magnetic susceptibility measurements, INS and EPR spectroscopies, the magnetic properties of the isolated [ReF6 ](2-) unit in (PPh4 )2 [ReF6 ]⋅2 H2 O (1) have been fully studied including the slow relaxation of the magnetization observed below ca. 4 K. This slow dynamic is preserved for the one-dimensional coordination polymer [Zn(viz)4 (ReF6 )]∞ (2, viz=1-vinylimidazole), demonstrating the irrelevance of low symmetry for such magnetization dynamics in systems with easy-plane-type anisotropy. The ability of fluoride to mediate significant exchange interactions is exemplified by the isostructural [Ni(viz)4 (ReF6 )]∞ (3) analogue in which the ferromagnetic Ni(II) -Re(IV) interaction (+10.8 cm(-1) ) dwarfs the coupling present in related cyanide-bridged systems. These results reveal [ReF6 ](2-) to be an unique new module for the design of molecule-based magnetic materials.

The term “frustration” in the context of magnetism was originally used by P. W. Anderson and quickly adopted for application to the description of spin glasses and later to very special lattice types, such as the kagomé. The original use of the term was to describe systems with competing antiferromagnetic interactions and is important in current condensed matter physics in areas such as the description of emergent magnetic monopoles in spin ice. Within molecular magnetism, at least two very different definitions of frustration are used. Here we report the synthesis and characterization of unusual nine-metal rings, using magnetic measurements and inelastic neutron scattering, supported by density functional theory calculations. These compounds show different electronic/magnetic structures caused by frustration, and the findings lead us to propose a classification for frustration within molecular magnets that encompasses and clarifies all previous definitions.

International audience; [(iC3H7)2NH2][Cr8NiF9(O2CCMe)18], or Cr8Ni, is a prominent example of an odd-membered antiferromagnetic "wheel." A detailed characterization of the magnetic properties of Cr8Ni has been conducted. Inelastic neutron scattering (INS) is used to investigate the energy and momentum transfer dependence of the low-lying spin excitations, including excited states inaccessible by other experimental techniques. The richness of the INS data, in conjunction with microscopic spin Hamiltonian simulations, enables an accurate characterization of the magnetic properties of Cr8Ni. Nearest-neighbor exchange constants of JCrCr = 1.31 meV and JCrNi = 3.22 meV are determined, and clear evidence of axial single-ion anisotropy is found. The parameters determined by INS are shown to fit magnetic susceptibility. The spectroscopic identification of several successive S=1 excited total spin states and lowest spin band excitations show that the rotational band picture, valid for bipartite AFM wheels, breaks down for this odd-numbered wheel. The exchange constants determined here differ from previous efforts based on bulk measurements, and possible reasons are discussed. The large JCrNi/JCrCr ratio in Cr8Ni puts this wheel into a regime with strong quantum fluctuations in which the ground state can be described with a valence bond solid state picture.

The low-temperature elementary spin excitations in the AFM molecular wheel Fe18 were studied experimentally by inelastic neutron scattering and theoretically by modern numerical methods, such as dynamical density matrix renormalization group or quantum Monte Carlo techniques, and analytical spin-wave theory calculations. Fe18 involves eighteen spin-5/2 Fe(III) ions with a Hilbert space dimension of 10^14, constituting a physical system that is situated in a region between microscopic and macroscopic. The combined experimental and theoretical approach allowed us to characterize and discuss the magnetic properties of Fe18 in great detail. It is demonstrated that physical concepts such as the rotational-band or L&E-band concepts developed for smaller rings are still applicable. In particular, the higher-lying low-temperature elementary spin excitations in Fe18 or AFM wheels in general are of discrete antiferromagnetic spin-wave character., 16 pages, 10 figures

The synthesis, magnetic and electron paramagnetic resonance (EPR) characterisation of isolated, discrete, {Cr III } antiferromagnetically coupled chain complexes is reported for n = 6 and 7. Previous studies had reported supramolecular linked {Cr III } x species. For n = 6, the lowest lying total spin state is diamagnetic with S = 1 and 2 first and second excited states, respectively; for n = 7, the lowest lying total spin state is S = 3/2 with S = 1/2 and 5/2 first and second excited states, respectively. The zero-field splittings of these states are well defined by low-temperature, multi-frequency EPR spectroscopy.

International audience; Inelastic neutron scattering has been used to determine the microscopic Hamiltonian describing two high-spin variants of the high-anisotropy Mn-6 nanomagnet. The energy spectrum of both systems is characterized by the presence of several excited total-spin multiplets partially overlapping the S = 12 ground multiplet. This implies that the relaxation processes of these molecules are different from those occurring in prototype giant-spin nanomagnets. In particular, we show that both the height of the energy barrier and resonant tunnelling processes are greatly influenced by low-lying excited total-spin multiplets.

International audience; The low thermal conductivity in filled skutterudites has been ascribed to rattling atoms inducing a phonon glass. Experimental evidence now shows that the phonon glass description is incorrect, and provides essential insight for the development of microscopic models aimed at describing the thermoelectric properties of these materials.

Energy resolved neutron diffraction elucidates the magnetic ordering process in the magnetic A-site thiospinel compound MnSc2S4, which reveals two magnetic transitions at TN1 = 2.3 K and K. The coexistence of long range magnetic order and remaining short range contributions with different propagation vectors for clarifies that the transition at K is first order. The ordering process can be described in the framework of single- and double-q short range order fluctuations around approaching the magnetic ordering transition at TN1 = 2.3 K from the high temperature paramagnetic state. The double-q short range contribution may serve as a precursor for the finally stabilized spiral structure with for . Anisotropic magneto-elastic coupling is suggested to induce the magnetic ordering and to lift a possible exchange frustration via an associated lattice distortion.

The metallic thiospinel CuV 2 S 4 shows an enhanced value of the low-temperature specific heat with γ = C / T ≈ 60 mJ / mol K 2 , suggesting magnetic or electronic correlations. To elucidate the nature of this low-temperature phase with respect to possible magnetic fluctuations, inelastic neutron scattering measurements were performed on polycrystalline CuV 2 S 4 and CuTi 2 S 4 . Both compounds show an inelastic response dominated by phonons. The low-energy phonon response of CuTi 2 S 4 is centered around 13 meV and depends only weakly on temperature. The corresponding phonon intensity in the low energy transfer region of CuV 2 S 4 is centered at around 6 meV ( T = 120 K ). The center of gravity of this intensity shifts to about 10 meV ( T = 1.8 K ) below the structural phase transition at T ≈90 K .The significant difference in the low-energy phonon response between both compounds, which display the same lattice symmetry at 120 K, and the strong renormalization of the phonon energies below the structural transition might reflect the influence of strong electron-phonon coupling in CuV 2 S 4 .

The generalized density of states of LaFe4Sb12 and CeFe4Sb12 has been determined by inelastic neutron scattering and its main features are found to be in agreement with recently published calculations (J. L. Feldman, D. L. Singh, C. Kendziora, D. Mandrus and B. C. Sales, Phys. Rev. B, 2003, 68, 094301). In both compounds a localized vibrational contribution appears superposed on the low-energy Debye response. The distinct inelastic response of La in LaFe4Sb12 is obtained by subtraction of the data for the Ce filled compound and it shows even more clearly the resolution limited peak at 7 meV, attributed to the localized mode of La-atoms.

The anisotropy splitting in the ground state of the single-molecule magnet [Mn4O3Br(OAc)3(dbm)3] is studied by inelastic neutron scattering as a function of hydrostatic pressure. This allows a tuning of the anisotropy and thus the energy barrier for slow magnetisation relaxation at low temperatures. The value of the negative axial anisotropy parameter $D_{\rm cluster}$ changes from -0.0627(1) meV at ambient to -0.0603(3) meV at 12 kbar pressure, and in the same pressure range the height of the energy barrier between up and down spins is reduced from 1.260(5) meV to 1.213(9) meV. Since the $\rm Mn-Br$ bond is significantly softer and thus more compressible than the $\rm Mn-O$ bonds, pressure induces a tilt of the single ion Mn$^{3+}$ anisotropy axes, resulting in the net reduction of the axial cluster anisotropy., 4 pages, 3 figures

An external magnetic field suppresses the spin-energy gap in singlet ground state S=1/2 TlCuCl3. The system becomes quantum-critical at Hc≈5.7 T, where the energy of the lowest Zeeman-split triplet excitation crosses the nonmagnetic ground state. Antiferromagnetic ordering is reported above Hc, which underlines the three-dimensional nature of the observed quantum phase transition. The intrinsic parameters of S=1/2 TlCuCl3 allow us to access the critical region microscopically by neutron scattering. A substantial study of the spin dynamics in the high-field phase of TlCuCl3 at T=1.5 K up to H=12 T was performed for the first time. The results possibly indicate two dynamical regimes, which can be understood within characteristically renormalized triplet modes and a low-lying dynamics of potentially collective origin.

The magnetic exchange interactions in the mixed-valence dodecanuclear polyoxovanadate cluster compound (NHEt3)(4)[V12As8O40(H2O)] x H2O were investigated by a detailed inelastic neutron scattering study using cold neutrons. The data show clear evidence for the presence of a magnetic anisotropy within the cluster. Exchange parameters are accurately determined.

Dynamic resistance, noise spectrum and electron diffraction studies on electron irradiated orthorhombic TaS3 were performed in the temperature range of 75 K-280 K. At low defect concentrations (< 10-4 displacement per Ta atoms) the threshold field of the nonlinear conductivity (ET) increases linearly with the irradiation dose in spite of the fact that there is hardly any effect on the Peierls transition temperature, Tp = 221 K. Concomitantly the sharp peaks in the noise spectrum are smeared out and the broad band noise level increases. At higher defect concentrations, Tp decreases and the transverse coherence of CDWs is lost. In this regime the sharp threshold disappears from the nonlinear characteristics.