Lattice dynamics of piezoelectric copper metaborate CuB2O4

Tetragonal piezoelectric copper metaborate CuB2O4 (space group I (4) over bar 2d, Z = 12) became a subject of active research during the last decade because it demonstrates unique magnetic, optical, and magnetooptical properties. We report results of the optical studies of polarized infrared reflection and transmission spectra and Raman back-scattering spectra in the range from 20 to 2500 cm(-1). In accordance with the symmetry analysis, infrared studies allowed us to identify all polarB(2)(z), and E(x, y) normal vibrational modes. Frequencies of all longitudinal (LO) and transverse (TO) optical phonons were determined by fitting the observed infrared reflection spectra. Raman measurements revealed the frequencies of the majority of nonpolar A(1)(xx, yy, zz) and B-1(xx, yy) modes and polar B-2(z) and E(x, y) LO modes. We show that several groups of closely spaced spectral lines observed in the vibrational spectra of CuB2O4 can be considered as a result of the Davydov splitting, which takes place in the case of several equivalent molecular units in a primitive crystal cell. Multiphonon absorption in the range of 1200-2500 cm(-1) was analyzed and a tentative assignment of some features was given. Ab initio calculations of all zone-center polar and nonpolar modes were performed using density functional theory (DFT). The computed frequency distribution is in a good agreement with observed line positions in the infrared and Raman spectra. All these results allowed us to suggest reliable identification of all zone-center phonon modes thus providing a firm basis for further experimental and theoretical studies of this material with complex crystal, magnetic, and electronic structures and fascinating physical properties.