Synthesis and thermal, optical and magnetic properties of new Mn2+-doped and Eu3+-co-doped scheelites

Abstract

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.

New Gd3+ and Mn2+-Co-Doped Scheelite-Type Ceramics—Their Structural, Optical and Magnetic Properties

“…The crystal lattice expansion observed for CaMnGdMoWO materials when Gd 3+ ions concentration was increasing is probably caused by a strong defect of their structure, i.e., when three crystallographic positions of calcium ions are occupied by two Gd 3+ ones only and an appearance of cationic vacants takes place. An analogous phenomenon in other doped materials, e.g., RE 3+ -doped CdMoO 4 (RE = Eu, Dy) [ 23 , 24 ], Eu 3+ and Mn 2+ -co-doped calcium molybdato-tungstates [ 29 ] have already observed. The lattice parameter ratio c / a was also calculated.…”

“…But influence of Gd 3+ ions on the Mn 2+ magnetic system originates not only from the paramagnetic nature of three-valent gadolinium ions. As we mentioned in our previous work, where comparable scheelites Ca 1−3 x − y Mn y ▯ x Eu 2 x (MoO 4 ) 1−3 x (WO 4 ) 3 x were studied [ 29 ], disturbance of Mn 2+ is visible also under coexistence with non-magnetic Eu 3+ ions. This mechanism was explained by charge non-equilibrium between di- and three-valent ions where the creation of additional electronic vacancies (▯ x ) takes place.…”

“…99.95%, Alfa Aesar and without thermal pre-treatment). Calcium molybdate (CaMoO 4 ), manganese molybdate (MnMoO 4 ), and gadolinium tungstate (Gd 2 (WO 4 ) 3 ) were obtained analogously to our previous studies [ 26 , 29 ]. In the next step, two series of ternary mixtures comprising CaMoO 4 , MnMoO 4 , and Gd 2 (WO 4 ) 3 were prepared, i.e., when MnMoO 4 concentration was constant and equaled 3.00 mol% ( y = 0.0200) and gadolinium tungstate was variable and ranged from 0.50 ( x = 0.0050) to 50.00 mol% ( x = 0.2500).…”

“…The UV-vis absorption spectra of CaMoO 4 and CaMnGdMoWO ceramics obtained by solid-state reaction as well as combustion methods are shown in Figure 7 a and Figure 8 a, respectively. The intense and broad absorption bands within the spectral range of 200–375 nm can be ascribed to O 2– → W 6+ , O 2– → Mo 6+ as well as O 2– → Mn 2+ charge transfer bands [ 9 , 11 , 22 , 23 , 24 , 29 , 30 ]. The trivalent gadolinium ion, Gd 3+ has half-filled 4f orbitals (4f 7 configuration, ground state 8 S 7/2 ); therefore, it is expected that the intensity of absorption bands due to 4f-4f transitions would be very weak.…”

“…For materials with a direct band gap n = ½ [ 31 , 32 , 33 , 34 , 37 , 38 , 39 ]. The values of the optical band gap of CaMnGdMoWO samples were obtained by extrapolating a linear part of ( αhν ) 2 curve of each material to the photon energy axis as it is shown in Figure 7 b and Figure 8 b [ 24 , 29 , 30 , 31 , 32 , 33 , 36 ]. The determined E g values are given in Table A1 .…”