Magnetic structures of non-cerium analogues of heavy-fermion Ce2RhIn8: case of Nd2RhIn8, Dy2RhIn8 and Er2RhIn8

R2RhIn8 compounds (space group P4/mmm, R is a rare-earth element) belong to a large group of structurally related tetragonal materials which involves several heavy-fermion superconductors based on Ce. We have succeeded to grow single crystals of compounds with Nd, Dy and Er and following our previous bulk measurements, we performed neutron-diffraction studies to determine their magnetic structures. The Laue diffraction experiment showed that the antiferromagnetic order below the N\'eel temperature is in all three compounds characterized by the propagation vector k = (1/2, 1/2, 1/2). The amplitude and direction of the magnetic moments, as well as the invariance symmetry of the magnetic structure, were determined by subsequent experiments using two- and four-circle diffractometers. The critical exponents were determined from the temperature dependence of the intensities below TN.Comment: 10 pages, 7 figures, submitted to PR

Anomalous dispersion of optical phonons at the neutral-ionic transition: Evidence from diffuse X-ray scattering

Diffuse X-ray data for mixed stack organic charge-transfer crystals approaching the neutral-ionic phase transition can be quantitatively explained as due to the softening of the optical phonon branch. The interpretation is fully consistent with vibrational spectra, and underlines the importance of electron-phonon coupling in low-dimensional systems with delocalized electrons.Comment: 4 pages, 4 figure

Ultra-low temperature structure determination of a Mn12 single-molecule magnet and the interplay between lattice solvent and structural disorder

We have determined the ultra-low temperature crystal structure of the archetypal single-molecule magnet (SMM) [Mn12O12(O2CMe)16(H2O)4]·4H2O·2MeCO2H (1) at 2 K, by using a combination of single-crystal X-ray and single-crystal neutron diffraction. This is the first structural study of any SMM in the same temperature regime where slow magnetic relaxation occurs. We reveal an additional hydrogen bonding interaction between the {Mn12} cluster and its solvent of crystallisation, which shows how the lattice solvent transmits disorder to the acetate ligands in the {Mn12} complex. Unusual quantum properties observed in 1 have long been attributed to disorder. Hence, we studied the desolvation products of 1, in order to understand precisely the influence of lattice solvent on the structure of the cluster. We present two new axially symmetric structures corresponding to different levels of desolvation of 1, [Mn12O12(O2CMe)16(H2O)4]·4H2O (2) and [Mn12O12(O2CMe)16(H2O)4] (3). In 2, removal of acetic acid of crystallisation largely resolves positional disorder in the affected acetate ligands, whereas removal of lattice water molecules further resolves the acetate ligand disorder in 3. Due to the absence of acetic acid of crystallisation, both 2 and 3 have true, unbroken S4 symmetry, showing for the first time that it is possible to prepare fully axial Mn12–acetate analogues from 1, via single-crystal to single-crystal transformations

Coupled spin cross-over and ferroelasticity: revisiting the prototype [Fe(ptz)6](BF4)2 material

ABSTRACTSpin-crossover (SCO) materials exhibit thermal conversion from low to high-spin states. We review different models developed to describe this entropy-driven process and the occurrence of cooperative conversions resulting from elastic interactions. There is a growing number of SCO materials exhibiting unusual thermal conversions when symmetry breaking occurs. To illustrate the importance of considering both phenomena, we review studies of the prototype [Fe(ptz)6](BF4)2 system, exhibiting at atmospheric pressure a single step thermal transition with hysteresis, where a ferroelastic distortion occurs from the high-spin high-symmetry (HShs) phase, towards the low-spin low-symmetry (LSls) phase. Under pressure, sequential conversions occur on cooling from the HShs phase towards a high-spin low-symmetry (HSls) phase, followed by a spin crossover towards the LSls phase. In addition, a metastable low-spin high-symmetry (LShs) state forms upon fast cooling. We revisit this coupling and decoupling of spin crossover and ferroelastic phase transition through the Landau theory model adapted by Collet, which provides qualitative agreement with the experimental data, such as the phase diagram and the evolution of spin transition curves or lattice deformations under pressure. This Ferroelastic Instability coupled to Spin Crossover (FISCO) approach should be generalized to many materials undergoing coupled spin transition and symmetry breaking

Multi-phase spin crossover in Fe(ptz)₆(BF₄)₂

International audienceTransition metal complexes present solid state phase transitions associated with a change of the spin state, low-spin at low temperature to high-spin at high temperature, often with a significant change of metal-ligand bond lengths. Elastic interactions between small low-spin molecules and large high-spin ones play an important role in the phase transition. Since the discovery of spin crossover phenomenon, the complex [Fe (ptz)6](BF4)2 has successfully attracted the interest of scientists because of its complete spin conversion and of its sensitivity to light. This very rich phase multi-stability also offers a unique possibility to investigate carefully the spin crossover phenomena in diverse fundamental cases (coupled with a ferroelastic transition, after quenching, after irradiation, ...), and in an unusual case where the metal ion lies on a high-symmetry site. On the basis of results obtained by neutron scattering under high pressure, we present here the (pressure–temperature) phase diagram of the complex [Fe (ptz)6](BF4)2, which appears as seriously more complex than expected from the earlier literature, with in particular at least two new phases, one corresponding to complexes in a high-spin state but stacked in low symmetry, the other arising from an unexpected reconstructive phase transition

Structural phase transition in the spin-crossover complex [Fe(ptz)₆](BF₄)₂ studied by x-ray diffraction

The spin-crossover system [Fe(ptz)6](BF4)2 has been studied for more than 25 years and is used as a model system to the understanding of the eponymous phenomenon in solid-state materials. However, the structural properties of the low-spin phase formed at low temperature after a slow cooling have never been elucidated due to a splitting of the Bragg peaks. We report here a reinvestigation of this low-spin phase by single-crystal x-ray diffraction. This study demonstrates the perfect matching between the structural and magnetic transitions temperatures and hysteresis width through careful unit-cell temperature dependence. The Bragg splitting is also unambiguously associated to the spin transition. Above all, this work reveals a reversible doubling of the unit-cell parameters a and b corresponding to the spin transition. A preliminary solution for the crystal structure of the low-spin phase notably shows the potential major role of the deformation of the n-propyl groups in the physical behavior of this material

Structural movies of the gradual spin-crossover in a molecular complex at various physical scales

The thermally induced Spin-CrossOver (SCO) undergone by the mononuclear iron(ii) complex [Fe(PM-AzA)2(NCS)2] (PM = N-2'-pyridylmethylene, AzA = 4-(phenylazo)aniline) is fully pictured by a quasi-continuous structural determination all along the spin-state modification within the sample. This large scale multi-temperature Single-Crystal X-Ray Diffraction (SCXRD) investigation leads to making structural movies. The latter reveal or confirm some features of the SCO that are subsequently validated by the same systematic investigation performed on a zinc isostructural analogue complex. Notably, the continuous views of the temperature dependencies of the unit-cell parameters, the dilatation tensors, the metal coordination sphere geometry and the intermolecular distances confirm a few of the structure-property relationships already known for SCO materials. In parallel, the examination of the temperature dependencies of the atomic coordinates and the atomic displacement parameters reveals unexpected behaviours in this gradual SCO material such as antagonistic atomic movements due to the single SCO and the pure thermal effects.Etude femtoseconde rayons X et optique de la dynamique ultrarapide de photocommutation de matériaux moléculaires magnétique

Probing photoinduced phase transition in a charge-transfer molecular crystal by 100 picosecond X-ray diffraction

International audienc

Mosaicity and structural fatigability of a gradual spin-crossover single crystal

The present Letter introduces a novel approach to test the efficiency of spin crossover materials with regard to structural fatigability. By measuring single-crystal mosaicity, structural fatigability is evidenced in a gradual SCO iron compound. The non fatigability of the analogue non-SCO zinc compound demonstrates the role of SCO in such observation. The mosaicity strongly increases during the first cycles. It is therefore clearly shown that fatigability can affect non-cooperative SCO systems. Magnetic properties appear however not altered by the observed structural fatigability which is thus related to mechanical aspects of SCO

Mosaicity of spin-crossover crystals

International audienceReal crystals are composed of a mosaic of domains whose misalignment is evaluated by their level of “mosaicity” using X-ray diffraction. In thermo-induced spin-crossover compounds, the crystal may be seen as a mixture of metal centres, some being in the high-spin (HS) state and others in the low spin (LS) state. Since the volume of HS and LS crystal packings are known to be very different, the assembly of domains within the crystal, i.e., its mosaicity, may be modified at the spin crossover. With little data available in the literature we propose an investigation into the temperature dependence of mosaicity in certain spin-crossover crystals. The study was preceded by the examination of instrumental factors, in order to establish a protocol for the measurement of mosaicity. The results show that crystal mosaicity appears to be strongly modified by thermal spin-crossover; however, the nature of the changes are probably sample dependent and driven, or masked, in most cases by the characteristics of the crystal (disorder, morphology …). No general relationship could be established between mosaicity and crystal properties. If, however, mosaicity studies in spin-crossover crystals are conducted and interpreted with great care, they could help to elucidate crucial crystal characteristics such as mechanical fatigability, and more generally to investigate systems where phase transition is associated with large volume changes