Système cristallin orthorhombique

En cristallographie, le système cristallin orthorhombique est l'un des sept systèmes cristallins dans lesquels on classe les cristaux selon leurs propriétés de symétrie. Tout cristal orthorhombique possède comme opération de symétrie une rotation binaire ou une réflexion, voire les deux, selon trois directions perpendiculaires qui sont choisies comme axes du référentiel. Classes cristallines Le tableau ci-dessous donne les numéros des groupes d'espace des tables internationales de cristallographie du système cristallin orthorhombique, les noms des classes cristallines, les notations Schoenflies, internationales, et des groupes ponctuels, des exemples, le type et les groupes d'espace. Voir aussi *Structure cristalline *Système réticulaire orthorhombique *Présentation de tous les types de groupes d'espace Références

Orthorhombique

Insights into the Origin of Cooperative Effects in the Spin Transition of [Fe(NH(2)trz)(3)](NO3)(2): the Role of Supramolecular Interactions Evidenced in the Crystal Structure of [Cu(NH(2)trz)(3)](NO3)(2)center dot H2O

Christine Neuhausen

The thermally induced hysteretic spin transition (ST) that occurs in the polymeric chain compound Fe(NH(2)trz)(3)(2) (1) above room temperature (T-c(sic) = 347 K, T-c(sic) = 314 K) has been tracked by Fe-57 Mossbauer spectroscopy, SQUID magnetometry, differential scanning calorimetry (DSC), and X-ray powder diffraction (XPRD) at variable temperatures. From the XRPD pattern indexation, an orthorhombic primitive cell was observed with the following cell parameters: a = 11.83(2) angstrom, b = 9.72(1) angstrom, c = 6.361(9) angstrom at 298 K (low-spin state) and a = 14.37(2) angstrom, b = 9.61(4) angstrom, c = 6.76(4) angstrom at 380 K (high-spin state). The enthalpy and entropy variation associated to the ST of 1, have been evaluated by DSC as Delta H = 23(1) kJ mol(-1) and Delta S = 69.6(1) J mol(-1) K-1. These thermodynamic data were used within a two-level Ising like model for the statistical analysis of First Order Reversal Curve (FORC) diagram that was recorded for 1, in the cooling mode. Strong intramolecular cooperative effects are witnessed by the derived interaction parameter of J = 496 K. The crystal structure of Cu(NH(2)trz)(3)(2) center dot H2O (2) was obtained thanks to high quality single crystals prepared by slow evaporation after hydrothermal pretreatment. The catena poly[mu-tris(4-amino-1,2,4-triazole-N1,N2) copper.(II)] dinitrate monohydrate (2) crystallizes in the monoclinic space group C2/c, with a = 16.635(6) angstrom, b = 13.223(4) angstrom, c = 7.805(3) angstrom, beta = 102.56(3)degrees, Z = 4. Complex 2 is.a 1D infinite chain containing triple N1,N2-1,2,4-triazole bridges with an intra-chain distance of Cu center dot center dot center dot Cu = 3.903(1) angstrom. A dense H-bonding network with the nitrate counteranion involved in intra-chain and inter-chain interactions is observed. Such a supramolecular network could be at the origin of the unusually large hysteresis loop displayed by 1 (Delta T similar to 33 K), as a result of an efficient propagation of elastic interactions through the network. This hypothesis is strengthened by the crystal structure of 2 and by the absence of crystallographic phase transition for 1 over the whole temperature range of investigation as shown by XRPD.

2010

Competing Jahn-Teller distortions and ferrimagnetic ordering in the geometrically frustrated system Ni1-xCuxCr2O4

Philip Pattison

Competing Jahn-Teller distortions combined with geometrical frustration give rise to a rich phase diagram as a function of x(Cu) and temperature in the spinel system Ni1-xCuxCr2O4. The Jahn-Teller distortion of the end members acts in opposite ways, with an elongation of the NiO4 tetrahedra resulting in a structural transition at T-S1 = 317K in NiCr2O4, but a flattening in the CuO4 tetrahedra at T-S1 = 846K in CuCr2O4. In both cases the symmetry is lowered from cubic (Fd (3) over barm) to tetragonal (I4(1)/amd) on cooling. In order to follow the influence of Jahn-Teller active Ni2+ and Cu2+ ions on the structural and magnetic properties of chromium spinels, we have investigated a series of samples of Ni1-xCuxCr2O4 by x-ray and neutron powder diffraction. In the critical range 0.10 < x(Cu) < 0.20, strong orthorhombic distortions were observed, where competing Jahn-Teller activities between the Cu2+ and Ni2+ ions result in distortions along both the a and c axes. For Ni0.85Cu0.15Cr2O4, the orthorhombic structure (Fddd) is stabilized up to T-S2 = 368(2) K, close to the first structural phase transition at T-S1 = 374(2) K. A ferrimagnetic spin alignment of the Ni/Cu and chromium atoms sets in at much lower temperature TC = 95K in this compound. The end members NiCr2O4 and CuCr2O4 undergo this ferrimagnetic transition at TC = 74 and 135 K, respectively. These transitions are accompanied by the structural change to the orthorhombic symmetry which relieves the frustration. NiCr2O4 and Ni0.85Cu0.15Cr2O4 undergo a second magnetic transition at TM2 = 24 and 67K due to a superimposed antiferromagnetic ordering of the Cr moments resulting in a noncollinear magnetic structure. In the system Ni1-xCuxCr2O4, the magnetic transitions TC and TM2 merge with increasing copper content up to x(Cu) similar to 0.5. For the Ni-rich chromites, geometrical frustration causes a strong reduction of the chromium moments, where magnetic long-range order coexists with a disordered spin-liquid-like or a reentrant-spin-glass-like state. This paper provides insight into the interplay between the Jahn-Teller effect, geometrical frustration, and long-range magnetic order in these complex systems.

Amer Physical Soc2015

Christine Neuhausen

2010

Competing Jahn-Teller distortions and ferrimagnetic ordering in the geometrically frustrated system Ni1-xCuxCr2O4

Philip Pattison

Amer Physical Soc2015