Effect of Gd3+ Substitution on Thermoelectric Power Factor of Paramagnetic Co2+-Doped Calcium Molybdato-Tungstates

Abstract

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.

Magnetic and Electrical Characteristics of Nd3+-Doped Lead Molybdato-Tungstate Single Crystals

“…However, its value significantly increases with increasing temperature, i.e., in the intrinsic region above 300 K for a sample richer in neodymium ions, regardless of the crystallographic direction. Similar behavior of S 2 σ(T) with only a few fW/(cmK 2 ) was observed in ceramic Gd 3+ and Co 2+ -co-doped calcium molybdato-tungstates [ 46 ] as well as in Nd 3+ and Mn 2+ -co-doped calcium molybdato-tungstates [ 47 ]. The above mentioned studies show that even in ion-bonded materials containing a constant content of 3 d transition metal ions, thermoelectric efficiency can be improved by doping them with 4 f rare-earth ions.…”

Electrical Properties of Yb2-xVxO3+x Nanomaterials Obtained Mechanochemically from Yb2O3 and V2O5 Oxides

“…Therefore, the size effect, in this case, has a decisive influence on the increase of the power factor, since the volume of the unit cell (V ) increases with the increase of the vanadium content (x) in the sample (Table I) despite the fact that the vanadium ion (V 5+ ) substituted for ytterbium one (Yb 3+ ) has a smaller ion radius [24]. A similar size effect on the thermoelectric power factor was observed in the case of cobalt and gadolinium-doped calcium molybdato-tungstates [25]. It is well known that the materials with a large power factor value have the most promising thermoelectric properties, especially if they are heavily doped semiconductors, such as Bi 2 Te 3 [26].…”

Magnetic and Electrical Characteristics of Nd3+ and Mn2+-Co-doped Calcium Molybdato-Tungstates Single Crystals

“…No intrinsic region in the relation, σ(10 3 /T), was also observed among others in Ca 1−x Mn x (MoO 4 ) 0.50 (WO 4 ) 0.50 ceramics, 13 Ca 1−x Mn x MoO 4 nanomaterials, 12 CdRE 2 W 2 O 10 tungstate mi crocrystals (RE = Y, Nd, Sm, and Gd−Er), 42 and Ca 1−3x−y Co y □ x Gd 2x (MoO 4 ) 1−3x (WO 4 ) 3x molybdato-tungstate micropowders. 14 Residual electrical conductivity of the ptype up to 230 K and the n-type above this temperature (Figure 7) in the extrinsic region seem to be related to the presence of a small amount of cationic and anion vacancies in the studied single crystals. Another explanation may be related to the fact that at a state of thermal equilibrium structural defects (n) are always present in the lattice even in a crystal which is ideal in other respects.…”

“…Our previous studies of calcium molybdate nanomaterials doped with Mn 2+ ions 12 and calcium molybdato-tungstate microcrystals doped with Mn 2+ 13 or Co 2+ and Gd 3+ 14 ones have generally shown that they are nonconductive diamagnets or paramagnetics with a spin-only contribution for molybdates and a spin–orbit coupling for molybdato-tungstates. In these materials, a relatively low value of dielectric permittivity at low frequencies was observed, which is characteristic for space charge polarization.…”