Lattice dynamics and electronic transitions in a structurally complex layered copper borate Cu3(BO3)2

Abstract

Copper borate Cu3(BO3)2 is a complex compound with a layered crystallographic structure in which the Jahn -Teller active and magnetic copper Cu 2+ ions occupy sixteen nonequivalent positions in the unit cell displaying controversial magnetic behavior. In this paper, we report on the infrared and Raman spectroscopic studies of the lattice dynamics and the electronic structure of 3d 9 copper states below the fundamental absorption band. The lattice dynamics is characterized by a large number of phonons due to a low P1 space group symmetry and a large unit cell with Z = 10. Unusually rich set of phonons was found in the low-energy part of the infrared and Raman spectra below 100 cm −1 , which we tentatively assign to interlayer vibrations activated by a crystal superstructure and/or to weak force constants for modes related to some structural groups. Several phonons show anomalous behavior in the vicinity of the magnetic phase transition at T N =10 K thus evidencing pronounced magnetoelastic interaction. No new phonons were found below T N , which excludes the spin -Peierls type of the magnetic transition. In the region of electronic transitions, a strong broad absorption band centered at ∼1.8 eV is observed, which we assign to overlapping of transitions between the 3d 9 states of Cu 2+ ions split by the crystal field in nonequivalent positions. The fundamental charge-transfer absorption band edge has a complex structure and is positioned around ∼2.8-3.0 eV.

Sintering, Microstructure, and Dielectric Properties of Copper Borates for High Frequency LTCC Applications

“…The bond lengths reported for CuB 2 O 4 are 1.999 Å for prevailing Cu–O shorter bonds, 2.864 Å for Cu–O longer bonds, and 1.444–1.487 Å for B–O bonds [ 26 ]. For Cu 3 B 2 O 6 , the average Cu–O bond length is 2.1 Å [ 28 ] . The analysis of ppm , va , and blm values for CuB 2 O 4 and Cu 3 B 2 O 6 leads to the conclusion that these parameters are close to the ranges indicated in [ 41 ] for low permittivity candidate materials.…”

“…Peaks on the dielectric permittivity/dissipation factor versus frequency plots that occur above 0.7 THz are supposed to be attributed to phonon modes related to vibrations in Cu–O complexes [ 26 , 28 ]. Due to the large number of atoms that form the unit cells of both copper borates (42 atoms for CuB 2 O 4 , 110 atoms for Cu 3 B 2 O 6 [ 26 , 28 ]), phonon modes for these compounds are numerous, which was confirmed using infrared and Raman spectroscopic studies [ 26 , 27 , 28 , 29 ].…”

“…Cu 3 B 2 O 6 has a more complex structure and a lower symmetry [ 28 ] as compared with CuB 2 O 4 . The best known is Cu 3 B 2 O 6 with a triclinic structure, although the crystallization of this compound in the monoclinic and orthorhombic structures was also reported.…”

“…For this compound, the calculated average effective coordination number is close to four for the triclinic structure. In Cu 3 B 2 O 6 , boron also shows different coordination numbers—(BO 3 ) and (B 2 O 5 ) groups occur with shorter B–O bonds than for tetrahedral (BO 4 ) groups in CuB 2 O 4 [ 28 ].…”

Anomalous boron isotope effects on electronic structure and lattice dynamics of CuB2O4

“…These peaks originated from the electronic transitions from the 2p states of the oxygen ions to the 3d states of the copper ions. 22,23 The direct band gap E g of Cu 11 B 2 O 4 , estimated from the absorption coefficient a(E), was determined to be 3.16 AE 0.07 eV, as displayed in the inset of Fig. 2.…”