STRUCTURAL ASPECTS OF INORGANIC SPIN PEIERLS COMPOUNDS
M. Braden1,2, G. Heger3, B. Hennion2, W. Reichardt1, G. Dhalenne4 and A. Revcolevschi4
1Forschungszentrum Karlsruhe, INFP,
Postfach 3640, Karlsruhe, Germany
2Laboratoire Leon Brillouin,
CE-Saclay, 91191 Gif-sur-Yvette, France
3Inst. für Kristallographie, RWTH-Aachen, Jägerstr. 17--19,
52056 Aachen, Germany
4Laboratoire de Chimie de Solides
Orsay, Univ. Paris-Sud, 91405 Orsay, France
A spin-Peierls (SP) transition is the magnetic analog of the electronic Peierls transition. The coupling of a one-dimensional magnetic system of half-integer spins with the three-dimensional phonons may lead to a combined transition characterized by a gap in the magnetic excitation spectrum and a lattice distortion. The structural distortion modulates the magnetic interaction in the chains thereby forming magnetic dimers. As in the electronic analog the gain in energy due to the finite gap overcomes the elastic energy induced by the structural deformation. However, in most cases a weak three-dimensional magnetic coupling favors a three-dimensional antiferromagnetic ordered structure preventing the occurrence of the SP transition. Until recently, there were only a few SP compounds known, all of them possess a complicated organic crystal-structure limiting the performance of microscopic studies like crystallography or neutron scattering. The discovery by Hase et al. [1] of a SP transition at 14.3 K in the inorganic and rather simple compound CuGeO3 opens new possibilities in this field on both the experimental and the theoretical side. More recently a second inorganic material, a'-NaV2O5, which shows a more complex structure, was reported to exhibit the SP transition [2]. We will review the structural aspects of these two compounds in relation to their magnetic properties.
CuGeO3, which so far has been studied most, shows a crystal structure related to the pyroxenes consisting of two types of chains : GeO4-tetrahedra and CuO6-octahedra, the latter implying the magnetism. For the first time it was possible to fully characterize the structural distortion in a SP phase [3]. At first sight it may appear astonishing that the strongest displacements are observed for the oxygen positions perpendicular to the magnetic chains. In addition one finds a - however smaller - shift for the Cu-site parallel to the chains. The deformation is characterized by a strong modulation of the bond angles and only minor shifts of the bond distances. The strongest effect is found for the Cu-O-Cu angle.
The magnetic interaction, J, between two neighboring spins of Cu+2 ( spin 1/2) results from a superexchange on a ~90o Cu-O-Cu bond. The magneto-elastic coupling in CuGeO3 may be understood on a microscopic scale by a quantum-mechanical model based on electronic band structure parameters. J is found to depend sensitively on the Cu-O-Cu bond angle and slightly on the connection between octahedra and tetrahedra [3]. Within this theory, the structural distortion in the SP phase may fully explain the magnetic features, in particular the modulation of the magnetic interaction within a chain. The magneto-elastic coupling manifests itself already above the SP transition in an anomalous thermal expansion, which again can be traced back to the Cu-O-Cu bond angle [4,5]. Furthermore, there is high sensitivity of the magnetic properties on external pressure; for example the SP transition temperature increases with 5 K/GPa. The structural shifts revealed by powder neutron diffraction studies under high pressure may quantitatively account for the observed magnetic effects.
An important question concerns the softening of the related phonon mode at the SP transition, which was considered to be necessary in early theories. The exact knowledge of the structural deformation combined with extensive lattice dynamical calculations allowed us to identify the involved phonon modes [6]. In contrast to a purely structural and continuous phase transition the structural deformation in CuGeO3 does not correspond to one Eigen-mode of the system. Instead at least two modes belonging to the same irreducible representation are involved in the transition. Due to their bond bending character these modes show intermediate frequencies (3.2 and 6.8 THz) which are favorable for the SP transition. Both involved modes do not exhibit frequency softening near the transition temperature, they even increase in frequency on cooling as a consequence of the spin phonon coupling. This behavior is explained by the frequencies which are large compared to the magnetic gap, yielding strong non-adiabatic conditions.
The second inorganic SP compound, a'-NaV2O5, is less studied. Its ambient structure was subject of controversy, but is now assumed to be described in the space group Pmmn [7]. This material should be considered as a ladder compound, with the spins being carried on V-O-V molecular orbitals forming the rungs of the ladder. The complicated structural arrangement suggests that again bond bending distortions are related to the dimerization. However, until now only the modulation vector of the distortion is known [8].