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The intriguing, earlier observed, temperature shift of the X-band ESR magnetic field H res was studied for powder samples of linear Cu 3 EtBz·2MeOH and Cu 3 BuNaEs·2MeOH trimeric Cu(II) complexes. For Cu 3 BuNaEs·2MeOH, H res changes dramatically at ∼100 K, where a crystallographic phase transition was found by X-ray diffraction. For Cu 3 EtBz·2MeOH, ESR measurements were performed under high pressure. It was observed that H res decreases with increasing pressure but the linewidth increases. Far-infrared absorption measurements were performed as a function of temperature for Cu 3 EtBz·2MeOH and Cu 3 BuNaEs·2MeOH. No clear change of vibration frequencies with temperature was observed. These results agree with the earlier interpretation that the temperature shift of H res is caused by the thermal averaging of ESR lines due to the short spin-lattice relaxation time.

Superstructure reflections due to charge ordering have been observed in single crystals using X-ray diffraction. The critical temperatures for the ordering process are 150 K and 180 K in samples with Sr concentrations of and , respectively. For x =0.125 the charge order transition correlates with a drastic resistivity enhancement, while a much smaller change of the resistivity is found for x = 0.15. It is argued that the onset of charge ordering in the sample with x = 0.125 is related to the suppression of co-operative Jahn-Teller distortions which is initiated by rising ferromagnetism.

We have studied the thermal conductivity κ of stripe ordering La2–xSrxNiO4. In particular, long range stripe order is well established for x = 1/3. For this composition κ is enhanced in the charge ordering phase and exhibits a clear maximum at low T. Even small deviations from this stochiometry cause a strong change of κ: the low T-maximum is suppressed and no anomaly is observed at high T. Our data confirm that the stripe phase is most stable for x = \(\tfrac{{\text{1}}}{{\text{3}}}\). Moreover, we find clear differences in the thermal conductivity for x > \(\tfrac{{\text{1}}}{{\text{3}}}\)and for x < \(\tfrac{{\text{1}}}{{\text{3}}}\), respectively, which indicates different properties of the stripe ordered phase in these two regions of the phase diagram.

The magnetoelastic coupling in Ca$_{1.8}$Sr$_{0.2}$RuO$_4$ and in Ca$_{1.5}$Sr$_{0.5}$RuO$_4$ has been studied combining high-resolution dilatometer and diffraction techniques. Both compounds exhibit strong anomalies in the thermal-expansion coefficient at zero and at high magnetic field as well as an exceptionally large magnetostriction. All these structural effects, which are strongest in Ca$_{1.8}$Sr$_{0.2}$RuO$_4$, point to a redistribution of electrons between the different $t_{2g}$ orbitals tuned by temperature and magnetic field. The temperature and the field dependence of the thermal-expansion anomalies in Ca$_{1.8}$Sr$_{0.2}$RuO$_4$ yield evidence for a critical end-point lying close to the low-temperature metamagnetic transition; however, the expected scaling relations are not well fulfilled., Comment: 17 pages, 7 figures

The crystal structure of ${\mathrm{Ca}}_{2\ensuremath{-}x}{\mathrm{Sr}}_{x}{\mathrm{RuO}}_{4}$ with $0.2\ensuremath{\le}x\ensuremath{\le}1.0$ has been studied by diffraction techniques and by high resolution capacitance dilatometry as a function of temperature and magnetic field. Upon cooling in zero magnetic field, the crystal structure and the octahedra shrink along the $c$ direction and elongate in the $a$ and $b$ planes, whereas the opposite occurs upon cooling at high field ($x=0.2$ and 0.5). These findings yield evidence for an orbital rearrangement driven by temperature and magnetic field, which accompanies the metamagnetic transition at low temperature. The temperature and magnetic-field dependencies are found to be governed by the same energy scale.

Strong resonant enhancements of the charge-order and spin-order superstructure-diffraction intensities in La1.8Sr0.2NiO4 are observed when x-ray energies in the vicinity of the Ni L2,3 absorption edges are used. The pronounced photon-energy and polarization dependences of these diffraction intensities allow for a critical determination of the local symmetry of the ordered spin and charge carriers. We found that not only the antiferromagnetic order but also the charge-order superstructure resides within the NiO2 layers; the holes are mainly located on in-plane oxygens surrounding a Ni2+ site with the spins coupled antiparallel in close analogy to Zhang-Rice singlets in the cuprates.

We studied the crystal and magnetic structure of Ca2RuO4 by different diffraction techniques under high pressure. The observed first order phase transition at moderate pressure (0.5 GPa) between the insulating phase and the metallic high pressure phase is characterized by a broad region of phase coexistence. The following structural changes are observed as function of pressure: a) a discontinuous change of both the tilt and rotation angle of the RuO6-Octahedra at this transition, b) a gradual decrease of the tilt angle in the high pressure phase (p>0.5 GPa) and c) the disappearance of the tilt above 5.5GPa leading to a higher symmetry structure. By single crystal neutron diffraction at low temperature, the ferromagnetic component of the high pressure phase and a rearrangement of antiferromagnetic order in the low pressure phase was observed., 7 pages, 5 figures

The crystal and magnetic structure of ${\mathrm{La}}_{1\ensuremath{-}x}{\mathrm{Sr}}_{1+x}\mathrm{Mn}{\mathrm{O}}_{4}$ $(0\ensuremath{\leqslant}x\ensuremath{\leqslant}0.6)$ has been studied by diffraction techniques and high resolution capacitance dilatometry. There is no evidence for a structural phase transition related to octahedron tilting like those found in isostructural cuprates or nickelates, but there are significant structural changes induced by the variation of temperature and doping which we attribute to a rearrangement of the orbital occupation.

The phase-diagram of Ca2-xSrxRuO4 has been studied by several diffraction techniques as function of concentration, temperature, pressure and magnetic field. The substitution of Sr through the smaller Ca induces a series of structural phase transitions with a strong impact on the physical properties. The spin triplet superconductor Sr2RuO4 exhibits an undistorted crystal structure, and the Mott-insulator Ca2RuO4 strong structural distortions. For intermediate structural distortions samples stay metallic but with outstanding properties. Throughout the phase diagram we find a close coupling between the crystal structure on one side and magnetic and electronic behavior on the other side.

The phase diagram of ${\mathrm{Ca}}_{2\ensuremath{-}x}{\mathrm{Sr}}_{x}{\mathrm{RuO}}_{4}$ has been studied by neutron diffraction on powder and single-crystalline samples. The experiments reveal antiferromagnetic order and structural distortions characterized by tilts and rotations of the ${\mathrm{RuO}}_{6}$ octahedra. There is strong evidence that the structural details of the isovalent samples tune the magnetic as well as the electronic behavior. In particular we observe for low Sr concentration a metal-insulator transition associated with a structural change and magnetic ordering.

We have studied the influence of a magnetic field on the thermodynamic properties of Ca 2 - x Sr x RuO 4 in the intermediate metallic region with tilt and rotational distortions ( 0.2 ⩽ x ⩽ 0.5 ) . We find strong and anisotropic thermal expansion anomalies at low temperatures, which are suppressed and even reversed by a magnetic field. The metamagnetic transition of Ca 1.8 Sr 0.2 RuO 4 is accompanied by a large magnetostriction. Furthermore, we observe a strong magnetic-field dependence of c p / T and that can be explained by magnetic fluctuations.

Superstructure reflections at low temperatures in La1−xSr x MnO3 single crystals indicating charge order in samples containing 12.5% and 15% of Sr have been observed. The positions of the reflections in reciprocal space suggest a fourfold unit cell referring to the cubic perovskite structure. The onset temperatures for charge ordering are 150 K and 180 K for x=0.125 and x=0.15, respectively. Especially in the sample with the lower Sr concentration, the onset is correlated to a jump in resistivity measured on the same sample.