Lattice vibration spectra. LIX. Single crystal infrared and Raman studies of spinel type oxides

Lutz, H. D.; Müller, Beate; Steiner, H.‐J.

doi:10.1016/0022-4596(91)90171-d

Cited by 153 publications

(75 citation statements)

References 26 publications

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“…Table II lists the energies of the phonon modes at a Γ point. The values of energies are consistent with the previous phonon studies on other spinel compounds 14,15 . Here, at a Γ point the (F 2u and F 1u ) modes with 0 < hω < 22 meV involve mainly vibrations of the heavy Cd ions, the (F 1u , F 2g , and E u ) modes with 22 <hω < 45 meV involve mainly vibrations of the Cr ions, while the modes withhω > 45 meV involve mainly O ions except modes withhω ∼ 60 meV which have dominant Cr vibrations.…”

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confidence: 92%

Synchrotron X-ray Study of Lattice Vibrations in CdCr₂O₄

et al. 2011

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Using inelastic x-ray scattering we have investigated lattice vibrations in a geometric frustrated system CdCr2O4 that upon cooling undergoes a spin-Peierls phase transition at TN = 7.8 K from a cubic and paramagnetic to a tetragonal and Neel state. Phonon modes measured around Brillouin zone boundaries show energy shifts when the transition occurs. Our analysis shows that the shifting can be understood as the ordinary effects of the lowering of the crystal symmetry.PACS numbers: 63.20.-e, 78.70.Ck, A spinel AB 2 O 4 system is an excellent model to study the physics of frustration. The octahedral B sites surrounded by oxygen ions form a three dimensional network of corner sharing tetrahedra, called a pyrochlore lattice. Since oxygen octahedra form an edge sharing network, if the B site is occupied by a magnetic ion with unpaired t 2g electrons, the nearest neighbor interactions become dominant, which yields strong frustration. In spite of the theoretical predictions that the pyrochlore system should not order at any temperature, spinels usually undergo phase transitions into ordered states at nonzero temperatures 1 . This is because the spin degree of freedom can be coupled with other degrees of freedom, such as orbital and lattice, to lift the magnetic degeneracy 2 .One example is a spin-Peiels phase transition that occurs in ACr 2 O 4 . The Cr-based spinels in the absence of the orbital degree of freedom, ACr 2 O 4 , remain paramagnetic to temperatures far below the Curie-Weiss temperature, -390 K and -88 K for A = Zn 3 and Cd 4 , respectively. Upon further cooling the system undergoes a first order spin-Peierls-like phase transition from a cubic paramagnet to a tetragonal Neél state at T N = 12.5 K and 7.8 K for A = Zn 3 and Cd 4 . The tetragonal lattice distortion induces an exchange anisotropy that lifts the frustration and allows the system to select a particular spin configuration as its ground state. The nature of the phase transition and that of the ground state depend on the delicate balancing act between the lattice energy cost for the distortion and the magnetic energy gain due to the spin ordering. Previous studies have shown that the ionic size of the A ion has a crucial role in the selection process. In the case of ZnCr 2 O 4 with smaller Zn 2+ ions, the phase transition involves a tetragonal contraction along the c-axis with I4m2 symmetry 5 and commensurate Neél ordering 3 . In the case of CdCr 2 O 4 with larger Cd 2+ ions, however, the transition yields a tetragonal elongation along the c-axis with I4 1 /amd symmetry and an incommensurate Neél ordering 4 . Their microscopic mechanisms are not to be understood yet. A theory based on Dzyaloshinskii-Moriya interactions was proposed to explain the static spin-lattice coupling in CdCr 2 O 4 6 . This theory, however, does not provide an accurate account of the one-to-one correspondence between the tetragonal distortion and the Neél state that were experimentally observed, and the microscopic mechanism of the static spin-lattice coupling is yet to be u...

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confidence: 92%

Synchrotron X-ray Study of Lattice Vibrations in CdCr₂O₄

et al. 2011

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“…M (6) of the transverse modes are found at 209, 299, 394 and 536cm -1, with an extraordinary mode located at 647 cm-1 The four main frequencies are those predicted by group theory for the spinel structure, as observed in some of the previously studied ferrites (Katsnelson et al 1989;Lutz et al 1991;Shirai et al 1982): MnFeO4 (-, 300, 357, 530cm-t), ZnFe20 4 (163, 303, 373, 532 cm-1) and NiFe20 4 (151,189, 376, 562 cm-1). However, these frequencies are far from those obtained for the stoichiometric iron ferrite Fe30 4 or magnetite (Degiorgi et al 1987), which occur at 97, 259, 343, and 550 cm -t. Ishii ct al.…”

Section: Resultsmentioning

confidence: 86%

The infrared dielectric properties of maghemite, ?-Fe2O3, from reflectance measurement on pressed powders

1995

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Abstract. The infrared complex permittivity functions of three varieties of maghemite, V-Fe203, having different degrees of vacancy ordering, have been determined from their IR reflectance spectra, measured at near to normal incidence on pressed powder pellets. The optical constants therefrom obtained have been verified by using them in the simulation of the corresponding absorption spectra for KBr-diluted pellets, and these are in excellent agreement with the experimental spectra. All calculations are based on a procedure for the estimation of the effective dielectric function of a mixture, which incorporates percolation features, recently developed by the authors.

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“…As for IR studies of spinels, we can mention the early works of Waldron (10), Preudhomme and Tarte (11)(12)(13), and Onari (14). More recently, Lutz et al (15) and Baraton et al (20) discussed both IR and Raman spectra whereas McCarty and Boehme (16), Graves et al (17), and Degiori et al (18) presented a Raman study. The Raman spectra of rare earth orthoferrites have been discussed by Venugopalan et al (21,22) and Koshizuka and Ushioda (23).…”

Section: Introductionmentioning

confidence: 93%