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

Bogdan Sawicki,Sebastian Pawlus,Monika Oboz,Andrzej Nowok,Tadeusz Groń,Elżbieta Tomaszewicz,Henryk Duda,Marta Karolewicz,Zenon Kukuła

doi:10.3390/ma14133692

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.

Highlights

Scheelite-type molybdates and tungstates are attractive materials due their large efficient scintillation yield, X-ray absorption coefficient, and high thermal and chemical stability
New calcium molybdato-tungstates doped with cobalt and gadolinium ions with the chemical formula of Ca1−3x−yCoy xGd2x(MoO4)1−3x(WO4)3x (CCGMWO), where x = 0.0050, 0.0098, 0.0238, 0.0455, 0.0839, 0.1430, 0.1667, and 0.2000; y = 0.02 and denotes vacancies, were successfully synthesized via high-temperature annealing of ternary CoMoO4/Gd2(WO4)3/CaMoO4 mixtures with various content of initial reactants
We observed a slight mass loss for each obtained material. This loss did not exceed the value of 0.25%. This observation shows that a synthesis of doped samples runs practically without their mass change and a process between initial reactants occurred according to the general equation: (1−3x−y)·CaMoO4 + x·Gd2(WO4)3 + y·CoMoO4 = Ca(1−3x−y)Coy xGd2x(MoO4)(1−3x)(WO4)3, where homogeneity range of this solid solution is 0 < x ≤ 0.2 and 0 < y ≤ 0.02

Summary

Introduction

Scheelite-type molybdates and tungstates are attractive materials due their large efficient scintillation yield, X-ray absorption coefficient, and high thermal and chemical stability. Studies of Gd3+-doped lead molybdato-tungstates with the chemical formula of Pb1−3x xGd2x(MoO4)1−3x(WO4)3x (x = 0.0455, 0.0839, 0.1154, 0.1430, 0.1667, and 0.1774; ↓ represents vacancies) showed a paramagnetic state with characteristic superparamagnetic-like behavior. New calcium molybdato-tungstates doped with cobalt and gadolinium ions with the chemical formula of Ca1−3x−yCoy xGd2x(MoO4)1−3x(WO4)3x (CCGMWO), where x = 0.0050, 0.0098, 0.0238, 0.0455, 0.0839, 0.1430, 0.1667, and 0.2000; y = 0.02 and denotes vacancies, were successfully synthesized via high-temperature annealing of ternary CoMoO4/Gd2(WO4)3/CaMoO4 mixtures with various content of initial reactants. This loss did not exceed the value of 0.25% This observation shows that a synthesis of doped samples runs practically without their mass change and a process between initial reactants occurred according to the general equation: (1−3x−y)·CaMoO4 + x·Gd2(WO4)3 + y·CoMoO4 = Ca(1−3x−y)Coy xGd2x(MoO4)(1−3x)(WO4), where homogeneity range of this solid solution is 0 < x ≤ 0.2 and 0 < y ≤ 0.02.

Characterization of Methods

X-ray Diffraction Analysis and Particle Size Distribution

UV –Vis Spectra and Their Analysis

Magnetic Properties

Electrical Properties

Conclusions