Structural investigation of quaternary copper oxides with low dimensional magnetic properties

The aim of this work is the study of ternary and quaternary copper oxides with low-dimensional magnetic properties, mainly by means of powder and single crystal X-ray diffraction and single crystal neutron diffraction experiments, at low and high temperatures. Two of these compounds, SrCu2(BO3)2 and BaCuSi2O6, are two-dimensional spin gap systems, in which the Cu2+ ions (S=1/2) are antiferromagnetically coupled to each other, but they don’t exhibit a three-dimensional long range ordered magnetic structure. The magnetic properties in such compounds are very often extremely sensitive to even small structural distortions. An accurate knowledge of the Cu-Cu and Cu-O bond lengths and Cu-O-Cu angles is therefore important to get a better understanding of their magnetic properties. The structure of SrCu2(BO3)2 (I-42m) consists of corrugated Cu2(BO3)2-layers, magnetically separated along the c axis by Sr-layers. Within the Cu2(BO3)2-layers, the Cu2+ ions form a triangular array, topologically equivalent to the Shastry-Sutherland model. This leads to spin frustration and the formation of a spin gap at low temperatures. SrCu2(BO3)2 is the first known compound that realizes an exact dimer ground state. The temperature evolution of the structure of SrCu2(BO3)2 has been determined between 9K and 590 K. We have found a second order structural phase transition at Ts=395K, from the low temperature space group I-42m to the high temperature space group I4/mcm. The main structural change upon the phase transition concerns the Cu2(BO3)2-layers: The Cu, B and O atoms are distributed above and below the plane z=1/4 in the low temperature phase; they all lie in the same plane above Ts, and the Cu-Cu interlayer distances become equivalent. A change in the behaviour of the magnetic susceptibility was also observed around Ts, thus pointing to the existence of weak but significant magnetic interactions between the Cu2(BO3)2-layers, which had previously been neglected in the literature. We observed that the thermal ellipsoids of the atoms within the Cu2(BO3)2-layers are very elongated in the c direction at room temperature, and the anisotropy of the thermal ellipsoids increases strongly with the temperature until Ts. We thus performed single crystal neutron diffraction experiments in order to search for anharmonicities in the material. Since SrCu2(BO3)2 lies close to the critical point between the dimer and Néel states in the magnetic phase diagram of the Shastry-Sutherland model, studies of doping effects on the magnetic properties of this compound were performed, in order to change the values of the magnetic Cu-Cu interactions and to possibly come close to the critical point. We studied (Sr,M)Cu2(BO3)2, with M=Ca,Ba. However, the substitution of Sr by Ca or Ba seems to have very few impacts on the structure and properties of the material, so that a qualitative change of the magnetic ground state does not occur. BaCuSi2O6 is one of the few silicates with isolated Si4O12-rings. It contains Cu2Si4O12-layers, magnetically separated along the c axis by Ba-layers. Within the Cu2Si4O12-layers, the Cu2+ ions build dimers parallel to c, separated from each other by SiO4 tetrahedra. Like SrCu2(BO3)2, BaCuSi2O6 shows at low temperatures no sign of magnetic ordering and possesses a spin gap. The structure has been studied at low as well as high temperatures, by means of single crystal X-ray diffraction methods. We discovered at room temperature the presence of superstructure reflections, and therefore redetermined more accurately the structure of BaCuSi2O6 in the space group I41/acd. The Raman spectroscopy results published earlier, aimed at a better understanding of the structure of amorphous SiO2, need to be reinterpreted taking into account this superstructure. A structural phase transition occurs in BaCuSi2O6 at 610 K, from the room temperature space group I41/acd to the high temperature space group I4/mmm. This phase transition leads to a high temperature unit cell volume four times smaller than the room temperature one, but its order is not clear yet. During the synthesis of (Sr,Ba)Cu2(BO3)2 under LiBO3-flux, we discovered the first quaternary compound in the Li-Cu-B-O system: Li6CuB4O10. It crystallizes in the space group P-1. Its structure consists of CuO4 distorted squares, surrounded by two B2O5 groups and two LiO4 tetrahedra. Because of the large nearest neighbour distance between the Cu2+ ions, Li6CuB4O10 is expected to present very weak magnetic interactions. Between room temperature and the melting point, Li6CuB4O10 undergoes three structural phase transitions which are still to be investigated by means of single crystal X-ray diffraction.

Grantor dc:publisher

Publikationsserver der RWTH Aachen University

Year dc:date

2003

Author dc:creator

• Sparta, Karine

Contributors dc:contributor

• Roth, Georg

• info:eu-repo/classification/ddc/530
• Kupferoxide
• Quaternäre Verbindungen
• Magnetische Eigenschaft
• Kristallstrukturanalyse
• Temperaturabhängigkeit
• Physik
• copper oxide
• low dimensional magnetic properties
• spin gap

Statement dc:rights

• info:eu-repo/semantics/openAccess

Language dc:language

eng

Identifier

https://publications.rwth-aachen.de/search?p=id:%22RWTH-CONV-123569%22

OAI identifier oai:identifier

oai:publications.rwth-aachen.de:61965