TbMnO3 is an orthorhombic insulator where incommensurate spin order for temperature T(N)<41 K is accompanied by ferroelectric order for T<28 K. To understand this, we establish the magnetic structure above and below the ferroelectric transition using neutron diffraction. In the paraelectric phase, the spin structure is incommensurate and longitudinally modulated. In the ferroelectric phase, however, there is a transverse incommensurate spiral. We show that the spiral breaks spatial inversion symmetry and can account for magnetoelectricity in TbMnO3.

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We present a detailed powder and single crystal neutron diffraction study of the spin chain compound Ca3Co2O6. Below 25 K, the system orders magnetically with a modulated partially disordered antiferromagnetic structure. We give a description of the magnetic interactions in the system which is consistent with this magnetic structure. Our study also reveals that the long-range magnetic order co-exists with a shorter range order with a correlation length scale of ∼ 180Å in the ab plane. Remarkably, on cooling, the volume of material exhibiting short range order increases at the expense of the long-range order.PACS numbers: 75.30. Gw, 75.25.+z, 75.30.Fv, 75.50.Ee The appearance of plateaux in the magnetization curves of low dimensional quantum spin systems (e.g. NH 4 CuCl 3 [1] and SrCu 2 (BO 3 ) 2 [2]) has generated considerable attention from both an experimental and a theoretical point of view. In this respect the spin chain system Ca 3 Co 2 O 6 is very interesting because at low temperatures several steps, equally spaced in magnetic field, appear in the magnetization [3,4,5], a behavior reminiscent of quantum tunneling of magnetization in molecular magnets [6]. The origin of this intriguing phenomenon is still an open question [7,8] and the magnetic properties of Ca 3 Co 2 O 6 have been intensely studied in the past decade using many techniques, including x-ray [9,10,11] and neutron [12,13,14] Ca 3 Co 2 O 6 is a rare example of a material where ferromagnetic (FM) 1D Ising spin chains are coupled through a much weaker antiferromagnetic (AF) exchange on a triangular lattice. The spin chains in Ca 3 Co 2 O 6 are made up of alternating face-sharing octahedral (CoI) and trigonal prismatic (CoII) CoO 6 polyhedra, running along the c axis and arranged in a triangular lattice in the ab plane [17]. The different Co environments leave the Co 3+ ions on the CoI sites in a low-spin (S=0) state, and those on the CoII sites in the high-spin (S=2) state [9,11,15]. Crystalline electric fields also lead to a very strong anisotropy with the moments preferentially aligned along the c axis [3,4]. The zero-field magnetic structure of Ca 3 Co 2 O 6 has yet to be described unambiguously. Most theoretical descriptions of the magnetic structure center around the two stable configurations for 1D Ising chains coupled antiferromagnetically on a 2D triangular lattice, the ferrimagnetic structure (M, M, −M ) and the partially disordered antiferromagnetic (PDA) structure (M, −M, 0) [18] (the labels in parenthesis indicate the relative magnetizations on the triangular lattice). The step-like magnetization has been described using the Ising model on a triangular lattice [7]. There are, however, experimental data which suggest a more complex magnetic structure for Ca 3 Co 2 O 6 . A pronounced drop in the intensity of the magnetic peaks on cooling below 18 K in zero field has been observed in powder [12,13] and single crystal diffraction studies [14]. Recent resonant x-ray scattering (RXS) studies [10] showed a small (∼ 0.01Å −1 ) incommensu...

87191-41-1; TeF5N(I)CF3, 98720-72-0; TeF5N(CF3)C(0)CH3, 98720-73-1; (TeF,),NCF,, 87191-42-2; TeFSN(SF5)CF3, 98720-74-2; TeF5N-(CF,)CH,CH,Br, 98720-75-3; TeF,N=C(CI)OCH,, 98720-76-4; Te-24, Mg2CoHS and its deuteride have been prepared as black crystalline solids by a sintering technique at temperatures between 620 and 770 K and hydrogen (deuterium) pressures between 40 and 60 bar. X-ray and neutron powder diffraction data recorded at room temperature suggest a tetragonally distorted CaF,-type metal atom structure (for the deuteride a = 4.463 (4) A, c = 6.593 (6) A, space group P 4 / n m m , and 2 = 2). The D atoms surround the Co atoms in an ordered square-pyramidal configuration (d[Co-D] = 1.590 (17) A (apical) and 1.515 (3) A (basal)). The structure transforms at 488 (5) K into a disordered cubic modification ( a = 6.453 (6) A, at 498 K). The heat of dissociation of the hydride as measured from pressure-composition isotherms is 86 (5) kJ/mol of H,. The electrical resistivity suggests nonmetallic behavior.

By means of magnetic susceptibility and specific heat measurements, x-ray and unpolarised neutron diffraction investigations on powder and single-crystal samples, simultaneous long-

The properties of sodiumnitroprusside (Na2[Fe(CN)5NO]*2H2O) are investigated by density functional theory. The calculated results both for the free anions as well as for the solid show that the ground-state Born–Oppenheimer surface has local minima for sidewards bonded and inverted NO. The calculated properties for the local minima: very long lifetime because of large barrier, diamagnetism, optical excitation energies, vibrational, and Mössbauer properties are in essential agreement with experiment. The present findings elucidate the possibility of population transfers by illumination with light of different wavelengths.

We report neutron-diffraction measurements of the magnetic phase diagram of the colossal magnetoelectric DyMn 2 O 5. Our measurements reveal the magnetic origin of the complex dielectric anomalies in this material. Frustrated magnetic interactions result in a large density of low-energy magnetic states which can be selected by the application of relatively small magnetic fields. The unusual combination of magnetic frustration and strong magnetoelastic coupling is responsible for the remarkable changes of the dielectric properties of DyMn 2 O 5 in small applied magnetic fields.