A series of Mn 2? -doped and Eu 3? -co-doped calcium molybdato-tungstates, i.e., Ca 1−3x−y Mn y ⌷ x Eu 2x (MoO 4 ) 1−3x (WO 4 ) 3x (0 \ x ≤ 0.2222 when y = 0.0200 and 0 \ y ≤ 0.0667 when x = 0.1667, ⌷ represents vacancy) materials were successfully synthesized via high-temperature annealing. XRD results confirmed the formation of single, tetragonal scheelite-type phases (space group I4 1 /a). A change in both lattice constants (a and c), lattice parameter ratio c/a and progressive deformation of MoO 4 /WO 4 tetrahedra with increasing Eu 3? as well as Mn 2? contents were observed. The melting point of doped materials is lower than the melting point of pure matrix, i.e., CaMoO 4 . New materials exhibit strong absorption in the UV range. They are insulators with the optical direct band gap (E g ) higher than 3.50 eV. The E g values nonlinearly change with increasing dopants concentrations. EPR measurements allowed to establish the nature of magnetic interactions among Mn 2? ions. Additionally, EPR spectra were sensitive on both parameters: Mn 2? and Eu 3? concentration.
Scheelite-type Ca1−xMnxMoO4 (x = 0.0, 0.01, 0.05, 0.10 and 0.15) nanomaterials were successfully synthesized via a combustion route. Dielectric studies showed a weak n-type electrical conductivity characteristic for insulators and low relative permittivity (εr < 15) decreasing with increasing Mn2+ content. CaMoO4 and Mn2+-doped nanomaterials are chemically compatible with Al and Ag electrodes and promising for low-temperature co-fired ceramic applications. Magnetic studies showed, at room-temperature diamagnetism for pure CaMoO4, the balance between diamagnetism and paramagnetism for Ca1−xMnxMoO4 (x = 0.01) and paramagnetic behaviour when 0.05 ≤ x ≤ 0.15 as well as the short-range antiferromagnetic interactions growing in strength as Mn2+ content increases. The Landé factor fitting procedure showed a spin-only contribution to the magnetic moment. CaMoO4 matrix unexpectedly revealed the residual paramagnetism at low temperatures derived probably from the molybdenum ions having unpaired 4d electrons as well as a paramagnetic-diamagnetic transition at 70 K.
A series of Co2+-doped and Gd3+-co-doped calcium molybdato-tungstates, i.e., Ca1−3x−yCoy▯xGd2x(MoO4)1−3x(WO4)3x (CCGMWO), where 0 < x ≤ 0.2, y = 0.02 and ▯ represents vacancy, were successfully synthesized by high-temperature solid-state reaction method. XRD studies and diffuse reflectance UV–vis spectral analysis confirmed the formation of single, tetragonal scheelite-type phases with space group I41/a and a direct optical band gap above 3.5 eV. Magnetic and electrical measurements showed insulating behavior with n-type residual electrical conductivity, an almost perfect paramagnetic state with weak short-range ferromagnetic interactions, as well as an increase of spin contribution to the magnetic moment and an increase in the power factor with increasing gadolinium ions in the sample. Broadband dielectric spectroscopy measurements and dielectric analysis in the frequency representation showed a relatively high value of dielectric permittivity at low frequencies, characteristic of a space charge polarization and small values of both permittivity and loss tangent at higher frequencies.
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