Infrared phonon anomaly and magnetic excitations in single-crystal Cu3Bi(SeO3)(2)O2Cl

Infrared reflection and transmission as a function of temperature have been measured on single crystals of Cu3Bi(SeO3)(2)O2Cl. The complex dielectric function and optical properties along all three principal axes of the orthorhombic cell were obtained via Kramers-Kronig analysis and by fits to a Drude-Lorentz model. Below 115 K, 16 additional modes [8(E parallel to(a) over cap)+ 6(E parallel to(b) over cap)+ 2(E parallel to(c) over cap)] appear in the phonon spectra; however, powder x-ray diffraction measurements do not detect a new structure at 85 K. Potential explanations for the new phonon modes are discussed. Transmission in the far infrared as a function of temperature has revealed magnetic excitations originating below the magnetic ordering temperature (T-c similar to 24 K). The origin of the excitations in the magnetically ordered state will be discussed in terms of their response to different polarizations of incident light, behavior in externally applied magnetic fields, and the anisotropic magnetic properties of Cu3Bi(SeO3)(2)O2Cl as determined by dc susceptibility measurements.