| 1. |
- Macovez, Roberto, et al.
(författare)
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Hybridization, superexchange, and competing magnetoelastic interactions in TiOBr
- 2007
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 76:20, s. 205111-
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Tidskriftsartikel (refereegranskat)abstract
- A crystalline sample of TiOBr is probed at room temperature by a combination of electron spectroscopies and the results are compared to theoretical embedded-cluster calculations. Resonant photoemission of the valence band confirms that the lowest binding energy feature arises from the singly occupied Ti 3d orbital. The polarization dependence of this orbital in nonresonant photoemission is consistent with the expected dominant d(y)(2)-z(2) character. The analysis of the Ti L-2,L-3 x-ray absorption spectra confirms the complete splitting of the Ti 3d shell. X-ray absorption and resonant photoemission at the O 1s edge provide direct evidence for hybridization between the transition metal orbitals and the O 2p levels, which leads to superexchange interactions between the Ti ions. The existence of a mixing of O and Ti states and of strong superexchange interactions is supported by calculations of the ground-state electronic and magnetic properties. The calculated superexchange interchain interaction is one fifth in strength of the total magnetic coupling along the chain, and is antiferromagnetic in character. This O-mediated interchain interaction is frustrated in the room temperature phase of TiOBr and thus couples strongly to distortions of the soft lattice. The competition between the interchain magnetoelastic coupling and the spin-Peierls interaction might be at the origin of the complex TiOX phase diagram.
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| 2. |
- Arulraj, Anthony, et al.
(författare)
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Strain effects in perovskite manganites
- 2007
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Ingår i: Progress in Solid State Chemistry. - : Elsevier BV. - 1873-1643 .- 0079-6786. ; 35:2-4, s. 367-377
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Tidskriftsartikel (refereegranskat)abstract
- Colossal magnetoresistance (CMR) - the dramatic reduction of electrical resistivity in a magnetic field and charge ordering (CO) in rare earth manganites of the type Ln(1-x)A(x)MnO(3) [Ln: rare earth, A: divalent cation] are manifestations of the intricate relation between orbital, spin, charge and lattice degrees of freedom. Recent studies indicate that the inhomogeneous state of mixed-valence manganites - evidenced by the presence of texture and multiple phase coexistence - is important for the CMR property of manganites. Theoretical models that explain the multiphase coexistence are based on quenched disorder or strain. Here we show that lattice strain due to the Jahn-Teller (IT) distortions of MnO6 octahedra and their tilt rotations are not sufficient to provide a unique structure-property relation. We present evidence that the science of manganites should take into account shear distortions of the MnO6 octahedra as well. Pressure evolution of the lattice strain of Nd0.5Ca0.5MnO3 shows a minimum around 7 GPa, with the same lattice strain above and below this pressure achieved by shear- and JT-type distortions, respectively. In general, a particular lattice strain of manganites can be achieved by different combinations of JT-type and shear-type distortions of the MnO6 octahedra, together with their tilts, which suggest a plausible description of the inhomogeneous state in manganites as one where phases with differently distorted states having the same lattice strain are preserved. (C) 2007 Published by Elsevier Ltd.
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| 3. |
- Jönsson, Johan, et al.
(författare)
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Inverse pressure-induced Mott transition in TiPO4
- 2019
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Ingår i: Physical Review B. - : AMER PHYSICAL SOC. - 2469-9950 .- 2469-9969. ; 99:24
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Tidskriftsartikel (refereegranskat)abstract
- TiPO4 shows interesting structural and magnetic properties as temperature and pressure are varied, such as a spin-Peierls phase transition and the development of incommensurate modulations of the lattice. Recently, high-pressure experiments for TiPO4 reported two structural phases appearing at high pressures, the so-called phases IV and V [M. Bykov et al., Angew. Chem. Int. Ed. 55, 15053 (2016).]. The latter was shown to include the first example of fivefold O-coordinated P atoms in an inorganic phosphate compound. In this work, we characterize the electronic structure and other physical properties of these phases by means of ab initio calculations and investigate the structural transition. We find that the appearance of phases IV and V coincides with a collapse of the Mott insulating gap and quenching of magnetism in phase III as pressure is applied. Remarkably, our calculations show that in the high-pressure phase V, these features reappear, leading to an antiferromagnetic Mott insulating phase, with robust local moments.
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| 4. |
- Mondal, Swastik, et al.
(författare)
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Electron-Deficient and Polycenter Bonds in the High-Pressure gamma-B-28 Phase of Boron
- 2011
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Ingår i: PHYSICAL REVIEW LETTERS. - : American Physical Society. - 0031-9007. ; 106:21, s. 215502-
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Tidskriftsartikel (refereegranskat)abstract
- The peculiar bonding situation in gamma boron is characterized on the basis of an experimental electron-density distribution which is obtained by multipole refinement against low-temperature single-crystal x-ray diffraction data. A topological analysis of the electron-density distribution reveals one-electron-two-center bonds connecting neighboring icosahedral B-12 clusters. A unique polar-covalent two-electron-three-center bond between a pair of atoms of an icosahedral cluster and one atom of the interstitial B-2 dumbbell explains the observed charge separation in this high-pressure high-temperature polymorph of boron.
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| 5. |
- Wunschel, Markus, et al.
(författare)
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Influence of the molecular structures on the high-pressure and low-temperature phase transitions of plastic crystals
- 2003
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Ingår i: Acta Crystallographica. Section B: Structural Science. - 0108-7681. ; 59:1, s. 60-71
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Tidskriftsartikel (refereegranskat)abstract
- The crystal structures of tert-butyl-tris(trimethylsilyl)silane, Si[C(CH3)3]1[Si(CH3)3]3 (Bu1), and di-ferr-butyl-bis(trimethylsilyl)silane, Si[C(CH3) 3]2[Si(CH3)3]2 (Bu2), at room temperature and at 105 K have been determined by X-ray powder diffraction; the high-pressure behavior for pressures between 0 and 5 GPa is reported. The room-temperature structures have cubic Fm3→m symmetry (Z = 4) with a = 13.2645 (2) Å, V= 2333.87 (4) Å3 for Bu1 and a = 12.9673 (1) Å, V = 2180.46 (3) Å3 for Bu2. The molecules are arranged in a cubic close packing (c.c.p.) and exhibit at least 48-fold orientational disorder. Upon cooling both compounds undergo a first-order phase transition at temperatures Tc = 230(5)K (Bu1) and Tc = 250(5)K (Bu2) into monoclinic structures with space group P21/n. The structures at 105 K have a = 17.317 (1), b = 15.598 (1), c = 16.385 (1) Å, γ = 109.477 (4)°, V =4172.7 (8) Å3 and Z = 8 for Bu1 and a = 17.0089(9), b = 15.3159 (8), c = 15.9325 (8) Å, γ = 110.343 (3)°, V= 3891.7 (5) Å3 and Z = 8 for Bu2. The severe disorder of the room-temperature phase is significantly decreased and only a two- or threefold rotational disorder of the molecules remains at 105 K. First-order phase transitions have been observed at pressures of 0.13-0.28 GPa for Bu1 and 0.20-0.24 GPa for Bu2. The high-pressure structures are isostructural to the low-temperature structures. The pressure dependencies of the unit-cell volumes were fitted with Vinet equations of state and the bulk moduli were obtained. At still higher pressures further anomalies in the pressure dependencies of the lattice parameters were observed. These anomalies are explained as additional disorder-order phase transitions.
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