M. Amami

Structural investigation of the phase transitions of [N(CH3)4]2HgCl4

Abstract The crystal structures of [N(CH3)4]2HgCl4 between 191 K and 350 K have been studied by single crystal X-ray diffraction. Below Tc1 = 277 K the structure is monoclinic P21/a with lattice parameters a = 12.320 (7), b = 9.034 (4), c = 15.402 (4) Å, and β = 89.64 (1)° at T = 191 K. Above Tc1 the structure is orthorhombic Pnma with lattice parameters 12.419 (7), b = 9.068 (4), and c = 15.684 (2) Å at room temperature, and with lattice parameters a = 12.421 (9), b = 9.125 (1), and c = 15.777(4) Å at T = 350 K. At room temperature each TMA cation disorderly occupies two orientations (TMA is tetramethylammonium). At low temperature the structure is fully ordered, with additional shifts and rotations of all ions, in accordance with the results of Asahi et al., J. Phys. Soc. Jpn. 60 (1991) 921. Refinements using restrictions of the non-crystallographic tetrahedral point group for the TMA cations and refinements without restrictions are used to show that TMA has tetrahedral symmetry at all temperatures. The HgCl4 anions are distorted, but the distortions are independent of the temperature. The phase transition at Tc1 is described as an order/disorder transition accompanied by major rotations and shifts of the ions. New evidence for a previously reported phase transition at Tc2 = 320 K (Zangar et al. Phase Transitions 12 (1988) 141) is obtained from NMR. This transition is shown to be a non symmetry breaking structural transition, that correspond to a gradual loss of the orientational order of the TMA cations.

Structural and Magnetic Properties of Substituted Delafossite-Type Oxides CuCr 1−x Sc x O 2

This work describes the scandium doping effect on the structural and magnetic properties of delafossite-type oxides CuCr1 － xScxO2. The lattice parameters were found to vary according to Vegard’s low. A reflection broadening is observed, that is ascribed to local lattice distortion due to the ionic radius difference between Cr3+ and the non-magnetic dopants. Magnetic susceptibility measurements show that the dominant interactions are antiferromagnetic (AFM) but that doping induces significant changes. The coupling between the local spins at the Cr sites and doped metal transition may enhance spin fluctuations at the Cr sites, which break the residual magnetic degeneracy as fluctuation-induced symmetry breaking in a highly magnetic degenerate ground state manifold of some frustrated systems.

Structural, magnetic and electric properties of Delafossite-type oxide, Cu1-xAgxCrO2 (0 < x < 0.5)

Polycrystalline samples of the delafossite Cr oxide Cu1−xAgxCrO2 were investigated to clarify its transport properties. The synthetized phases are found to contain a slight oxygen excess (δ = 0.06). The valence state of the Cr ions in the Cu1−xAgxCrO2 samples was identified as >3+ by analysing their TGA spectra, their lattice parameters and their electrical properties. The temperature dependence of zerofield-cooling magnetization was measured. All samples exhibits paramagnetic behaviour at high temperature. At low temperature they exhibit a clear ferromagnetic (FM) transition around 130 K. For x = 0.10 the samples exhibit an AFM transition at 25K. Clear hysteresis loops indicate that FM order exists in the Ag-doped samples at 2 K. All samples behave like semiconductors.

H-Bond Effect on the Phase Transitions in [NH(4-n)(CH3)n]2HgCl4 and Related Compounds

The structural instability of the [NH 4- n (CH 3 ) n ] 2 HgCl 4 group have been investigated. Consideration of different crystals of this family shows that the H-bonds, obviously, hinder the appearance of incommensurately modulation. The dependence of phase transition sequence from the sizes of cation and anion may be obtained only for crystal with simple cation. The appearance of H-bonds complicates the situation and, leads to gradual disappearance of IC phase in the A 2 BX 4 family.

Structure of incommensurate ammonium tetrafluoroberyllate studied by structure refinements and the maximum entropy method

Incommensurately modulated ammonium tetrafluoroberyllate (AFB) occurs in a narrow temperature interval between the paraelectric room-temperature phase with space group Pnma (Ti = 178 K) and the ferroelectric low-temperature phase with space group Pna2(1) (Tc = 173 K). The structure is determined from accurate single-crystal X-ray diffraction data collected with synchrotron radiation at 175 K. The superspace group of the structure is Pnma(alpha00)0ss with alpha = 0.4796 (4). Both structure refinements and the maximum entropy method lead to the same structure model, which involves only single harmonic modulations. The building units of the structure are BeF4(2-) and NH4+ complex ions with approximately tetrahedral point symmetry. They are relatively rigid and the modulations consist mainly of translations of the tetrahedra and their rotations around a fixed axis. The modulation is related to changes in the network of the hydrogen bonds. The low-temperature superstructure can be described as a commensurately modulated structure with the same superspace symmetry. The first harmonic modulations of the low-temperature and incommensurate phases are related by a scale factor with a value of approximately two. In addition, the low-temperature phase exhibits a second harmonic modulation that is responsible for shifts along c and the ferroelectricity in this phase. The experimental data of the incommensurate phase do not contain any evidence for the presence of a second harmonic in the modulation functions. This suggests that the development of the second harmonic, i.e. the development of the spontaneous polarization, is responsible for the lock-in transition.

Retraction: “Structure Determination of the TMA2HgCl4 by X-Ray Diffraction and Maximum Entropie Method”

It has been found that this paper is not an original paper, that is, its main part is a duplication of a part of Structural investigation of the phase transitions of [N(CH3)4]2HgCl4, M. Amami, S. van Smaalen, L. Palatinus, A. Ben Salah, X. Helluy and A. Sebald, Z. Kristallogr. 217 (2002) 532–541. This act is a violation of the copyright of the paper published in Zeitschrift fuer Kristallographie. Therefore, the paper by M. Amami and A. Ben Salah published in J. Phys. Soc. Jpn. is retracted.