Synthesis, crystal structure and luminescent properties of pyrovanadates A2CaV2O7 (A=Rb, Cs)

The novel vanadium oxides Rb2CaV2O7 and Cs2CaV2O7 have been prepared by solid-state reaction and their crystal structures determined and refined using X-ray, neutron powder and electron diffraction data. Rb2CaV2O7 and Cs2CaV2O7 are isostructural, crystallizing in space group P21/n with unit cell parameters: a=13.8780(1), b=5.96394(5), c=10.3376(1)Å, β=104.960(1)° and a=14.0713(2), b=6.0934(1), c=10.5944(1)Å, β=104.608(1)°, respectively. Their crystal structures can be described as a framework of CaO6 octahedra and V2O7 pyrogroups with alkaline metals found in the tunnels formed. Photoluminescence (PL) and PL excitation spectra of the considered pyrovanadates have been studied in the vacuum ultraviolet (VUV) to visible light (Vis) range as well as their pulse cathode luminescence (PCL) spectra and the kinetic parameters of PCL. In the PL and the PCL spectra of both pyrovanadates recorded at T=300K a broad band with maxima at 2.2, 2.4eV and two shoulders (bands) at 2.0 and 2.58eV have been observed. At T=10K the band at 2.0eV becomes the main band in the spectra. Two types of luminescence centers for each pyrovanadate, with very similar excitation bands at 3.75, 4.84, 6.2, 7.3 and 9.1eV, have been found. The nature of the luminescence centers connected with the bands at 2.0, 2.2, 2.4 and 2.58eV is discussed.