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Spin-orbit coupling in manganese doped calcium molybdato-tungstates
Ceramics International 44 (2018) 3307–3313
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Spin-orbit coupling in manganese doped calcium molybdato-tungstates a
b
T. Groń , M. Pawlikowska , E. Tomaszewicz
b,⁎
a
a
, M. Oboz , B. Sawicki , H. Duda
T
a
a
Institute of Physics, University of Silesia, ul. Uniwersytecka 4, 40-007 Katowice, Poland Department of Inorganic and Analytical Chemistry, Faculty of Chemical Technology and Engineering, West Pomeranian University of Technology, Al. Piastów 42, 71-065 Szczecin, Poland b
A R T I C L E I N F O
A B S T R A C T
Keywords: A. Sintering D. Scheelites C. Electrical properties C. Magnetic properties
The manganese doped calcium molybdato-tungstates with the formula of Ca1-xMnx(MoO4)0.50(WO4)0.50 (x = 0.01, 0.03, 0.05, 0.10, 0.125, and 0.15) were successfully obtained by two-step synthesis using in both steps a solid state reaction route. All ceramics show scheelite-type tetragonal structure with space group I41/a. The electrical and magnetic studies within the temperature range of 2–300 K showed a weak p-type electrical conductivity and the paramagnetic state of Mn-doped ceramic materials. With increasing Mn content in samples under study, a change in the short-range interactions from ferromagnetic to antiferromagnetic as well as an increase in the orbital contribution to the magnetic moment, resulting in a strong spin-orbit coupling, were observed. The Brillouin procedure was used to estimate the Landé factor.
1. Introduction Alkaline earth metal molybdates and tungstates, AXO4 (A = Ca, Sr, and Ba; X = Mo, W) are members of important inorganic material families of a scheelite-type structure [1–8]. These compounds crystallize in the tetragonal symmetry and I41/a space group with four molecules in each crystallographic cell. The divalent A2+ and hexavalent ×6+ ions coordinate with eight and four oxygen anions, respectively [1–8]. The scheelite-type molybdates and tungstates have been used in various fields such as electro-optics, lasers, amplifiers, and microwave ceramics [9–17]. Scheelite-type calcium compounds, i.e. CaMoO4, CaWO4, have been a particular center of interest due to their wide application as phosphors in industrial radiology and medical diagnosis, solid-state lasers, storage applications, tunable fluorescence, sensors for dark matter search, and for the detection of γ-rays [18–23]. Tetragonal calcium molybdate and tungstate show favorable thermal and chemical stability, relatively low phonon energy and they have been proven to be perfect host materials for rare-earth metals luminescence or laser operation [1,6,9,10,15,17,24,25]. However, recently the research interests have been shifted from the rare-earth ions to transition metal ions. Wang et al. have demonstrated for the first time Mn2+ ions as luminescence centers for persistent phosphor [26]. Since that time, many studies have been reported on new phosphors doped with Mn2+ and codoped with rare earth ions micro- and nanoceramics, showing that modification of this type significantly increases the emission time [27–34]. The Mn2+ ions have [Ar]d5 electronic configuration. The main emission of this luminescent dopant corresponds to the 4T1 → 6A1 ⁎
transition and it is strongly dependent on ligand field and coordination number. It is known that the Mn2+ ions in inorganic oxide materials can be generally placed in fourfold or sixfold coordination sites. Increase in the coordination number of Mn2+ leads to a shift in their luminescence emission from green to far red [35]. Previously, we have obtained new manganese doped calcium molybdato-tungstates with the formula of Ca1-xMnx(MoO4)0.50(WO4)0.50, where 0 < x ≤ 0.15 [36]. X-ray diffraction measurements at room temperature showed that the solid solution under study crystallizes in tetragonal scheelite-type structure with space group of I41/a in a C 4h6 symmetry [36]. In these phases, the Mn2+ ions substitute the Ca2+ ones and are located in dodecahedral sites. Electron paramagnetic resonance (EPR) measurements showed that Mn2+ ions exist in the structure as isolated magnetic centers if Mn doping is low [36]. With increasing manganese concentration, the exchange and dipolar interactions lead to developing of the magnetic system, in which antiferromagnetic interactions dominate among Mn2+ ions [36]. The UV–vis studies showed that Mn-doped materials are insulators with a large direct energy gap Eg > 3.5 eV [36]. Their Eg values decreased nonlinearly with increasing content of manganese ions in the scheelite framework [36]. In this study, Ca1-xMnx(MoO4)0.50(WO4)0.50 (x = 0.01, 0.03, 0.05, 0.10, 0.125 and 0.15) microceramics were prepared, and their electrical conductivity and magnetic susceptibility were measured within the temperature range of 2–300 K. The Brillouin fit of the magnetization at 2 K was used to interpret the spin-orbit coupling.
Corresponding author. E-mail address: [email protected] (E. Tomaszewicz).
https://doi.org/10.1016/j.ceramint.2017.11.105 Received 12 September 2017; Received in revised form 14 November 2017; Accepted 15 November 2017 Available online 16 November 2017 0272-8842/ © 2017 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
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Fig. 1. XRD patterns of Ca1-xMnx(MoO4)0.50)(WO4)0.50 ceramics in the 2Θ range of 15–60° (1A) and 25–35° (1B).
2. Experimental procedure
10, and 300 K using a QD-PPMS device in applied external fields up to 75 kOe. The effective magnetic moment was determined using the
2.1. Materials and synthesis
following equation: μeff =
3kB C NA μB2
≅ 2.828 C [37,38], where kB is the
Boltzmann constant, NA is the Avogadro number, μB is the Bohr magneton and C is the molar Curie constant.
Samples of Ca1-xMnx(MoO4)0.50(WO4)0.50 ceramics with the compositions of x = 0.01; 0.03; 0.05; 0.10; 0.125 and 0.15 were successfully prepared by two-step synthesis using in both steps high-temperature solid state reaction route. As the initial reactants, the following materials were used: CaCO3, MnO, MoO3, and WO3 (all raw materials of high purity grade min. 99.95%, Alfa Aeasar or Fluka, and without thermal pre-treatment). For the synthesis of calcium compounds, i.e. CaMoO4, CaWO4, stoichiometric mixtures of CaCO3 with MoO3 or WO3 were heated in air, in several 12-h sintering stages and at temperatures in the range of 823–1373 K. Manganese molybdate (MnMoO4) was prepared by heating a stoichiometric MnO/MoO3 mixture in the temperature range of 823–1173 K. In the next step of the synthesis, CaMoO4, CaWO4 and MnMoO4 mixed at the appropriate molar ratios were heated according to the procedure described earlier [36].
3. Results and discussion 3.1. XRD analysis and morphological studies The XRD technique was used to investigate the structure and phase purity of the prepared ceramics. Fig. 1a shows the room temperature powder XRD patterns of pure and Mn-doped calcium molybdato-tungstates. XRD analysis indicated that a scheelite-type phase was formed within the composition range investigated in the present work, without any appearance of secondary phases. The formation of a tetragonal scheelite-type structure indicates that Mn2+ ions have diffused into the Ca(MoO4)0.50(WO4)0.50 lattice to form a solid solution of Ca1xMnx(MoO4)0.50(WO4)0.50 with a space group of I41a (No. 88). The most prominent peaks corresponded to [101, 112, 103, 211, 204, 116] and [132] lattice planes, and they shifted toward higher 2Θ angle with increasing Mn content (Fig. 1b). Sharp and very intense reflections indicated the crystalline nature of samples under study. One could expect a change in the lattice parameters of calcium molybdato-tungstate upon doping with Mn because the radius of manganese ion (Mn2+ − 96 pm) is much smaller than that of calcium one (Ca2+ − 112 pm). As we have shown in our previous studies, as the concentration of manganese ions was increased, both lattice constants, a and c, of tetragonal cell systematically decreased [36]. Both unit cell parameters show a linear dependence vs. x parameter within whole homogeneity range of solid solution (the Vegard law was satisfied, Fig. 2). The FESEM image of one of Ca1-xMnx(MoO4)0.50(WO4)0.50 ceramics, i.e. the sample for x = 0.05 is shown in Fig. 3. All materials under study exhibited well-defined and sharp grain boundaries. This fact suggests that these ceramic materials are well-crystallized. The pure matrix and each Mn-doped sample were composed of spherical or oval and only partially aggregated particles with average size of single grains ranging from ~ 10 to ~ 40 µm. It was also found that the grain size of ceramics decreased on increasing Mn concentration in the samples without any appreciable change in their microstructure.
2.2. Experimental methods The powder X-ray diffraction patterns were collected under ambient conditions by using an EMPYREAN II diffractometer (PANalytical) and CuKα1,2 radiation (2Θ range of 2–100° and the scanning step 0.013°). Obtained XRD patterns were analyzed with the HighScore Plus 4.0 software. Lattice parameters were calculated using a procedure described earlier and they were presented in our previous work on Ca1xMnx(MoO4)0.50(WO4)0.50 materials [36]. Scanning electron microscopy studies were carried out by us during earlier investigations and they are partially published in [36]. Applied equipment for SEM studies was described in our previous paper [36]. The electrical conductivity σ(T) and the I–V characteristics of doped microcermics were measured by the DC method using a KEITHLEY 6517B Electrometer/High Resistance Meter. The thermoelectric power S(T) was measured in the temperature range of 300–600 K using a Seebeck Effect Measurement System (MMR Technologies, Inc., USA). The static dc magnetic susceptibilities were measured in the temperature range of 2–300 K and in two different cooling modes. In the zero-field cooled (ZFC) mode, each sample was first cooled down in the absence of an external magnetic field, and then investigated while heating in a given magnetic field of Hdc = 1 kOe. Field cooled (FC) mode usually followed the ZFC run when the same magnetic field was set on at high temperatures and measurements were performed on decreasing temperature. Magnetization isotherms were measured at 2,
3.2. Electrical properties Results of the electrical measurements of Ca1−xMnx(MoO4)0.50(WO4)0.50 3308
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Fig. 4. Electrical conductivity (lnσ) vs. reciprocal temperature 103/T for Ca1(x = 0.01; 0.03; 0.05; 0.10; 0.125; 0.15).
xMnx(MoO4)0.50(WO4)0.50
Fig. 2. Linear dependences of both a and c lattice constants vs. x parameter.
Fig. 3. FESEM image of Ca1-xMnx(MoO4)0.50)(WO4)0.50 (x = 0.05).
(x = 0.01, 0.03, 0.05, 0.10, 0.125 and 0.15) solid solution showed insulating behavior with small values of the p-type electrical conductivity of σ ∼ 10−10 S/m independent of the manganese content (Figs. 4 and 5). No thermal activation of the current carriers was observed. Similar behavior was observed for R2WO6 tungstates (R = Nd, Sm, Eu, Gd, Dy and Ho) [39],
Fig. 5. Thermoelectric power S vs. temperature T for Ca1-xMnx(MoO4)0.50(WO4)0.50 (x = 0.05, 0.10 and 0.15).
Table 1 Magnetic parameters of the Ca1-xMnx(MoO4)0.50(WO4)0.50 solid solution: C is the Curie constant, θ is the Curie-Weiss temperature, μeff is the effective magnetic moment, M is the magnetization at 2 K and in the magnetic field of 70 kOe, peff is the effective number of Bohr magnetons, M0 is the magnetization at the highest value of H/T and g is the Landé factor. x
C (emu K/mol) Experiment
θ (K)
μeff (μB/f.u.)
M(2K) (μB/f.u.)
peff
M0 (μB/f.u.) Brillouin fit
g
0.01 0.03 0.05 0.10 0.125 0.15
0.0325 0.0795 0.206 0.378 0.523 0.595
52 73 5 12 −3 −3
0.510 0.797 1.297 1.739 2.044 2.181
0.049 0.148 0.245 0.477 0.542 0.659
0.592 1.025 1.323 1.871 2.092 2.291
0.049 0.150 0.250 0.510 0.590 0.740
1.75 1.42 1.22 0.85 0.76 0.66
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Fig. 6. ZFC and FC magnetic susceptibility χ and 1/χZFC vs. temperature T for Ca1-xMnx(MoO4)0.50(WO4)0.50 for x = 0.01 (a); x = 0.03 (b); x = 0.05 (c); x = 0.10 (d); x = 0.125 (e); x = 0.15 (f).
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(caption on next page)
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Fig. 7. Magnetization M vs. H/T at 2, 10 and 300 K for Ca1-xMnx(MoO4)0.50(WO4)0.50 for x = 0.01 (a); x = 0.03 (b); x = 0.05 (c); x = 0.10 (d); x = 0.125 (e); x = 0.15 (f). Inset: Landé factor fit (solid black line) to the experimental data at 2 K.
CdRE2W2O10 tungstates (RE = Y, Pr, Nd, Sm, Gd–Er) [40,41] and Cd13xGd2x⌷xMoO4 molybdates (x = 0.005, 0.01, 0.025, 0.04, 0.10, 0.20 1.00) [42]. The residual electrical conduction of the p-type in the molybdatotungstates under study seems to be connected with the cationic vacancies. Another explanation may be related to the fact that at thermal equilibrium structural defects (n) are always present in the lattice even in the crystal which is ideal in other respects. A necessary condition for free energy minimalization gives: n ≅ Nexp(-EV/kT) for n « N, where N is the number of atoms in the crystal and EV is the energy required to transfer the atom from the bulk of the crystal on its surface [43]. Therefore, we expect deep trap levels localized in the energy gap of 3.85 eV [36], hindering the electron transport.
For the samples studies the effective angular momentum: J = S = 5/2 was assumed. The Brillouin functions together with the experimental data of magnetic moments are shown in the insets of Figs. 7a-7f. The g-values given in Table 1 decrease from g = 1.75 for x = 0.01 to g = 0.66 for x = 0.15. This suggests that the orbital contribution to the magnetic moment increases with increasing manganese content in the sample, and thus the spin-orbit coupling becomes stronger. The consequence of this is the change in short-range interactions from ferromagnetic to antiferromagnetic in the nonconductive solid solution under study.
3.3. Magnetic properties
We have synthesized the Mn2+ doped Ca(MoO4)0.50(WO4)0.50 ceramic materials using the two steps solid state reaction method. The room temperature XRD data confirm the formation of tetragonal scheelite-type structure with space group I41/a. The electrical and magnetic measurements showed that Ca1-xMnx(MoO4)0.50(WO4)0.50 (x = 0.01, 0.03, 0.05, 0.10, 0.125, and 0.15) ceramics are p-type insulators and paramagnets with the short-range FM and AFM interactions. The Brillouin fit procedure showed an increase in the orbital contribution to the magnetic moment as Mn-content increases in the samples, resulting in a strong spin-orbit coupling.
4. Conclusions
The results of magnetic susceptibility measurements of Ca1(x = 0.01, 0.03, 0.05, 0.10, 0.125 and 0.15) solid solution are depicted in Table 1 and in Figs. 6a–6f. All studied molybdato-tungstates are paramagnetic in the temperature range of 2–300 K. The samples poorer in manganese show short-range ferromagnetic interactions whose occurrence is confirmed by positive values of the Curie-Weiss temperature. On the other hand, manganese-rich samples exhibit short-range antiferromagnetic effects confirmed by negative values of the Curie-Weiss temperature (Table 1). EPR studies corroborate the dominance of the antiferromagnetic short-range interactions with increasing manganese content in the sample [36]. There was no split between the ZFC and FC magnetic susceptibilities in any phase, which means no spin frustration as well as the absence of longrange magnetic interactions in the examined temperature range (Figs. 6a-6f). The effective magnetic moment, μeff, is slightly lower than the effective number of Bohr magnetons, peff, for the Mn2+ ion with the effective spin of S = 5/2, given by the 2[S(S+1)]1/2 expression [44]. This may mean that a small amount of Mn2+ ions is present in the sample. The results of magnetic moment measurements of Mn-doped molybdato-tungstates are shown in Table 1 and in Figs. 7a–7f. Magnetic isotherms do not show hysteresis, coercive field and remanence. In terms of the reduced coordinate (H/T) the magnetic isotherms coincide with the universal Brillouin curve only for x = 0.01. Such behavior is characteristic of superparamagnetic particles. However, with increasing manganese content in the samples, the isotherms show increasing deviations from the universal Brillouin curve. The reason for this may be the appearance of an orbital contribution to the magnetic moment, and consequently spin-orbit coupling. xMnx(MoO4)0.50(WO4)0.50
Acknowledgements The authors are grateful to the team of the workshop at the Institute of Physics (University of Silesia) and the Department of Inorganic and Analytical Chemistry (West Pomeranian University of Technology, Szczecin) for providing practical and technical assistance. References [1] P. Yang, Ch. Li, W. Wang, Z. Quan, S. Gai, J. Lin, Uniform AMoO4:Ln (A = Sr2+, Ba2+; Ln = Eu3+, Tb3+) submicron particles: solvothermal synthesis and luminescent properties, J. Solid State Chem. 182 (2009) 2510–2520. [2] A.P. de Azevedo Marques, V.M. Longo, D.M.A. de Melo, P.S. Pizani, E.R. Leite, J.A. Varela, E. Longo, Shape controlled synthesis of CaMoO4 thin films and their photoluminescence property, J. Solid State Chem. 181 (2008) 1249–1257. [3] S. Vidya, S. Solomon, J.K. Thomas, Synthesis, sintering and optical properties of CaMoO4: a promising scheelite LTCC and photoluminescent material, Phys. Status Solidi A 209 (2012) 1067–1074. [4] J.C. Sczancoski, L.S. Cavalcante, N.L. Marana, R.O. da Silva, R.L. Tranquilin, M.R. Joya, P.S. Pizani, J.A. Varela, J.R. Sambrano, M.S. Li, E. Longo, J. Andrés, Electronic structure and optical properties of BaMoO4 powders, Curr. Appl. Phys. 10 (2010) 614–624. [5] J.C. Sczancoski, L.S. Cavalcante, M.R. Joya, J.A. Varela, P.S. Pizani, E. Longo, SrMoO4 powders processed in microwave-hydrothermal: synthesis, characterization and optical properties, Chem. Eng. J. 140 (2008) 632–637. [6] F. Lei, B. Yan, Hydrothermal synthesis and luminescence of CaMoO4:RE3+ (M = W, Mo; RE = Eu, Tb) submicro-phosphors, J. Solid State Chem. 181 (2008) 855–862. [7] E. Gurmen, E. Daniels, J.S. King, Crystal structure refinement of SrMoO4, SrWO4, CaMoO4, BaWO4 by neutron diffraction, J. Chem. Phys. 55 (1971) 1093–1097. [8] F.A. Rabuffetti, S.P. Culver, L. Suescun, R.L. Brutchey, Structural disorder in AMoO4 (A = Ca, Sr, Ba) Scheelite nanocrystals, Inorg. Chem. 53 (2014) 1056–1061. [9] F. Lei, B. Yan, Hydrothermal synthesis and luminescence of CaMoO4:RE3+ (M = W, Mo; RE = Eu, Tb) submicro-phosphors, J. Solid State Chem. 181 (2008) 855–862. [10] J.H. Kim, H. Choi, E.O. Kim, H.M. Noh, B.K. Moon, J.H. Jeong, Li doping effects on the upconversion luminescence of Yb3+/Er3+-doped ABO4 (A = Ca, Sr; B = W, Mo) phosphors, Opt. Mater. 38 (2014) 113–118. [11] S.K. Ghosh, S.K. Rout, A. Tiwari, P. Yadav, J.C. Sczancoski, M.G.R. Filho, L.S. Cavalcante, Structural refinement, Raman spectroscopy, optical and electrical properties of (Ba1−xSrx)MoO4 ceramics, J. Mater. Sci. 26 (11) (2015) 8319–8335. [12] N. Khobragade, E. Sinha, S.K. Rout, M. Kar, Structural, optical and microwave dielectric properties of Sr1−xCaxWO4 ceramics prepared by the solid state reaction route, Ceram. Int. 39 (2013) 9627–9635. [13] G.-K. Choi, S.-Y. Cho, J.-S. An, K.S. Hong, Microwave dielectric properties and sintering behaviors of scheelite compound CaMoO4, J. Eur. Ceram. Soc. 26 (2006) 2011–2015. [14] G.-K. Choi, J.-R. Kim, S.H. Yoon, K.S. Hong, Microwave dielectric properties of scheelite (A = Ca, Sr, Ba) and wolframite (A = Mg, Zn, Mn) AMoO4 compounds, J.
3.4. Brillouin fit In order to get estimates of the atomic moments containing orbital contribution in Ca1-xMnx(MoO4)0.50(WO4)0.50 (0 < x ≤ 0.15) solid solution, the Brillouin procedure, which does not include any cluster interactions, was used. Saturation magnetization at 2 K in the paramagnetic region was only reached for the samples poorer in manganese. Their experimental virgin magnetization curves, M(H/T), can be easily fitted by the following expression:
M = M0 BJ (x)
(1)
where M0 is the magnetization at the highest value of H/T, x = gJμBH/ kT, g is the fitted Landé factor and the Brillouin function BJ is given by [44]:
BJ (x ) =
2J + 1 2J + 1 1 x ⎞x − coth ⎛ coth 2J 2J 2J ⎝ 2J ⎠
(2) 3312
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Spin-orbit coupling in manganese doped calcium molybdato-tungstates a
b
T. Groń , M. Pawlikowska , E. Tomaszewicz
b,⁎
a
a
, M. Oboz , B. Sawicki , H. Duda
T
a
a
Institute of Physics, University of Silesia, ul. Uniwersytecka 4, 40-007 Katowice, Poland Department of Inorganic and Analytical Chemistry, Faculty of Chemical Technology and Engineering, West Pomeranian University of Technology, Al. Piastów 42, 71-065 Szczecin, Poland b
A R T I C L E I N F O
A B S T R A C T
Keywords: A. Sintering D. Scheelites C. Electrical properties C. Magnetic properties
The manganese doped calcium molybdato-tungstates with the formula of Ca1-xMnx(MoO4)0.50(WO4)0.50 (x = 0.01, 0.03, 0.05, 0.10, 0.125, and 0.15) were successfully obtained by two-step synthesis using in both steps a solid state reaction route. All ceramics show scheelite-type tetragonal structure with space group I41/a. The electrical and magnetic studies within the temperature range of 2–300 K showed a weak p-type electrical conductivity and the paramagnetic state of Mn-doped ceramic materials. With increasing Mn content in samples under study, a change in the short-range interactions from ferromagnetic to antiferromagnetic as well as an increase in the orbital contribution to the magnetic moment, resulting in a strong spin-orbit coupling, were observed. The Brillouin procedure was used to estimate the Landé factor.
1. Introduction Alkaline earth metal molybdates and tungstates, AXO4 (A = Ca, Sr, and Ba; X = Mo, W) are members of important inorganic material families of a scheelite-type structure [1–8]. These compounds crystallize in the tetragonal symmetry and I41/a space group with four molecules in each crystallographic cell. The divalent A2+ and hexavalent ×6+ ions coordinate with eight and four oxygen anions, respectively [1–8]. The scheelite-type molybdates and tungstates have been used in various fields such as electro-optics, lasers, amplifiers, and microwave ceramics [9–17]. Scheelite-type calcium compounds, i.e. CaMoO4, CaWO4, have been a particular center of interest due to their wide application as phosphors in industrial radiology and medical diagnosis, solid-state lasers, storage applications, tunable fluorescence, sensors for dark matter search, and for the detection of γ-rays [18–23]. Tetragonal calcium molybdate and tungstate show favorable thermal and chemical stability, relatively low phonon energy and they have been proven to be perfect host materials for rare-earth metals luminescence or laser operation [1,6,9,10,15,17,24,25]. However, recently the research interests have been shifted from the rare-earth ions to transition metal ions. Wang et al. have demonstrated for the first time Mn2+ ions as luminescence centers for persistent phosphor [26]. Since that time, many studies have been reported on new phosphors doped with Mn2+ and codoped with rare earth ions micro- and nanoceramics, showing that modification of this type significantly increases the emission time [27–34]. The Mn2+ ions have [Ar]d5 electronic configuration. The main emission of this luminescent dopant corresponds to the 4T1 → 6A1 ⁎
transition and it is strongly dependent on ligand field and coordination number. It is known that the Mn2+ ions in inorganic oxide materials can be generally placed in fourfold or sixfold coordination sites. Increase in the coordination number of Mn2+ leads to a shift in their luminescence emission from green to far red [35]. Previously, we have obtained new manganese doped calcium molybdato-tungstates with the formula of Ca1-xMnx(MoO4)0.50(WO4)0.50, where 0 < x ≤ 0.15 [36]. X-ray diffraction measurements at room temperature showed that the solid solution under study crystallizes in tetragonal scheelite-type structure with space group of I41/a in a C 4h6 symmetry [36]. In these phases, the Mn2+ ions substitute the Ca2+ ones and are located in dodecahedral sites. Electron paramagnetic resonance (EPR) measurements showed that Mn2+ ions exist in the structure as isolated magnetic centers if Mn doping is low [36]. With increasing manganese concentration, the exchange and dipolar interactions lead to developing of the magnetic system, in which antiferromagnetic interactions dominate among Mn2+ ions [36]. The UV–vis studies showed that Mn-doped materials are insulators with a large direct energy gap Eg > 3.5 eV [36]. Their Eg values decreased nonlinearly with increasing content of manganese ions in the scheelite framework [36]. In this study, Ca1-xMnx(MoO4)0.50(WO4)0.50 (x = 0.01, 0.03, 0.05, 0.10, 0.125 and 0.15) microceramics were prepared, and their electrical conductivity and magnetic susceptibility were measured within the temperature range of 2–300 K. The Brillouin fit of the magnetization at 2 K was used to interpret the spin-orbit coupling.
Corresponding author. E-mail address: [email protected] (E. Tomaszewicz).
https://doi.org/10.1016/j.ceramint.2017.11.105 Received 12 September 2017; Received in revised form 14 November 2017; Accepted 15 November 2017 Available online 16 November 2017 0272-8842/ © 2017 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
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Fig. 1. XRD patterns of Ca1-xMnx(MoO4)0.50)(WO4)0.50 ceramics in the 2Θ range of 15–60° (1A) and 25–35° (1B).
2. Experimental procedure
10, and 300 K using a QD-PPMS device in applied external fields up to 75 kOe. The effective magnetic moment was determined using the
2.1. Materials and synthesis
following equation: μeff =
3kB C NA μB2
≅ 2.828 C [37,38], where kB is the
Boltzmann constant, NA is the Avogadro number, μB is the Bohr magneton and C is the molar Curie constant.
Samples of Ca1-xMnx(MoO4)0.50(WO4)0.50 ceramics with the compositions of x = 0.01; 0.03; 0.05; 0.10; 0.125 and 0.15 were successfully prepared by two-step synthesis using in both steps high-temperature solid state reaction route. As the initial reactants, the following materials were used: CaCO3, MnO, MoO3, and WO3 (all raw materials of high purity grade min. 99.95%, Alfa Aeasar or Fluka, and without thermal pre-treatment). For the synthesis of calcium compounds, i.e. CaMoO4, CaWO4, stoichiometric mixtures of CaCO3 with MoO3 or WO3 were heated in air, in several 12-h sintering stages and at temperatures in the range of 823–1373 K. Manganese molybdate (MnMoO4) was prepared by heating a stoichiometric MnO/MoO3 mixture in the temperature range of 823–1173 K. In the next step of the synthesis, CaMoO4, CaWO4 and MnMoO4 mixed at the appropriate molar ratios were heated according to the procedure described earlier [36].
3. Results and discussion 3.1. XRD analysis and morphological studies The XRD technique was used to investigate the structure and phase purity of the prepared ceramics. Fig. 1a shows the room temperature powder XRD patterns of pure and Mn-doped calcium molybdato-tungstates. XRD analysis indicated that a scheelite-type phase was formed within the composition range investigated in the present work, without any appearance of secondary phases. The formation of a tetragonal scheelite-type structure indicates that Mn2+ ions have diffused into the Ca(MoO4)0.50(WO4)0.50 lattice to form a solid solution of Ca1xMnx(MoO4)0.50(WO4)0.50 with a space group of I41a (No. 88). The most prominent peaks corresponded to [101, 112, 103, 211, 204, 116] and [132] lattice planes, and they shifted toward higher 2Θ angle with increasing Mn content (Fig. 1b). Sharp and very intense reflections indicated the crystalline nature of samples under study. One could expect a change in the lattice parameters of calcium molybdato-tungstate upon doping with Mn because the radius of manganese ion (Mn2+ − 96 pm) is much smaller than that of calcium one (Ca2+ − 112 pm). As we have shown in our previous studies, as the concentration of manganese ions was increased, both lattice constants, a and c, of tetragonal cell systematically decreased [36]. Both unit cell parameters show a linear dependence vs. x parameter within whole homogeneity range of solid solution (the Vegard law was satisfied, Fig. 2). The FESEM image of one of Ca1-xMnx(MoO4)0.50(WO4)0.50 ceramics, i.e. the sample for x = 0.05 is shown in Fig. 3. All materials under study exhibited well-defined and sharp grain boundaries. This fact suggests that these ceramic materials are well-crystallized. The pure matrix and each Mn-doped sample were composed of spherical or oval and only partially aggregated particles with average size of single grains ranging from ~ 10 to ~ 40 µm. It was also found that the grain size of ceramics decreased on increasing Mn concentration in the samples without any appreciable change in their microstructure.
2.2. Experimental methods The powder X-ray diffraction patterns were collected under ambient conditions by using an EMPYREAN II diffractometer (PANalytical) and CuKα1,2 radiation (2Θ range of 2–100° and the scanning step 0.013°). Obtained XRD patterns were analyzed with the HighScore Plus 4.0 software. Lattice parameters were calculated using a procedure described earlier and they were presented in our previous work on Ca1xMnx(MoO4)0.50(WO4)0.50 materials [36]. Scanning electron microscopy studies were carried out by us during earlier investigations and they are partially published in [36]. Applied equipment for SEM studies was described in our previous paper [36]. The electrical conductivity σ(T) and the I–V characteristics of doped microcermics were measured by the DC method using a KEITHLEY 6517B Electrometer/High Resistance Meter. The thermoelectric power S(T) was measured in the temperature range of 300–600 K using a Seebeck Effect Measurement System (MMR Technologies, Inc., USA). The static dc magnetic susceptibilities were measured in the temperature range of 2–300 K and in two different cooling modes. In the zero-field cooled (ZFC) mode, each sample was first cooled down in the absence of an external magnetic field, and then investigated while heating in a given magnetic field of Hdc = 1 kOe. Field cooled (FC) mode usually followed the ZFC run when the same magnetic field was set on at high temperatures and measurements were performed on decreasing temperature. Magnetization isotherms were measured at 2,
3.2. Electrical properties Results of the electrical measurements of Ca1−xMnx(MoO4)0.50(WO4)0.50 3308
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Fig. 4. Electrical conductivity (lnσ) vs. reciprocal temperature 103/T for Ca1(x = 0.01; 0.03; 0.05; 0.10; 0.125; 0.15).
xMnx(MoO4)0.50(WO4)0.50
Fig. 2. Linear dependences of both a and c lattice constants vs. x parameter.
Fig. 3. FESEM image of Ca1-xMnx(MoO4)0.50)(WO4)0.50 (x = 0.05).
(x = 0.01, 0.03, 0.05, 0.10, 0.125 and 0.15) solid solution showed insulating behavior with small values of the p-type electrical conductivity of σ ∼ 10−10 S/m independent of the manganese content (Figs. 4 and 5). No thermal activation of the current carriers was observed. Similar behavior was observed for R2WO6 tungstates (R = Nd, Sm, Eu, Gd, Dy and Ho) [39],
Fig. 5. Thermoelectric power S vs. temperature T for Ca1-xMnx(MoO4)0.50(WO4)0.50 (x = 0.05, 0.10 and 0.15).
Table 1 Magnetic parameters of the Ca1-xMnx(MoO4)0.50(WO4)0.50 solid solution: C is the Curie constant, θ is the Curie-Weiss temperature, μeff is the effective magnetic moment, M is the magnetization at 2 K and in the magnetic field of 70 kOe, peff is the effective number of Bohr magnetons, M0 is the magnetization at the highest value of H/T and g is the Landé factor. x
C (emu K/mol) Experiment
θ (K)
μeff (μB/f.u.)
M(2K) (μB/f.u.)
peff
M0 (μB/f.u.) Brillouin fit
g
0.01 0.03 0.05 0.10 0.125 0.15
0.0325 0.0795 0.206 0.378 0.523 0.595
52 73 5 12 −3 −3
0.510 0.797 1.297 1.739 2.044 2.181
0.049 0.148 0.245 0.477 0.542 0.659
0.592 1.025 1.323 1.871 2.092 2.291
0.049 0.150 0.250 0.510 0.590 0.740
1.75 1.42 1.22 0.85 0.76 0.66
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Fig. 6. ZFC and FC magnetic susceptibility χ and 1/χZFC vs. temperature T for Ca1-xMnx(MoO4)0.50(WO4)0.50 for x = 0.01 (a); x = 0.03 (b); x = 0.05 (c); x = 0.10 (d); x = 0.125 (e); x = 0.15 (f).
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Fig. 7. Magnetization M vs. H/T at 2, 10 and 300 K for Ca1-xMnx(MoO4)0.50(WO4)0.50 for x = 0.01 (a); x = 0.03 (b); x = 0.05 (c); x = 0.10 (d); x = 0.125 (e); x = 0.15 (f). Inset: Landé factor fit (solid black line) to the experimental data at 2 K.
CdRE2W2O10 tungstates (RE = Y, Pr, Nd, Sm, Gd–Er) [40,41] and Cd13xGd2x⌷xMoO4 molybdates (x = 0.005, 0.01, 0.025, 0.04, 0.10, 0.20 1.00) [42]. The residual electrical conduction of the p-type in the molybdatotungstates under study seems to be connected with the cationic vacancies. Another explanation may be related to the fact that at thermal equilibrium structural defects (n) are always present in the lattice even in the crystal which is ideal in other respects. A necessary condition for free energy minimalization gives: n ≅ Nexp(-EV/kT) for n « N, where N is the number of atoms in the crystal and EV is the energy required to transfer the atom from the bulk of the crystal on its surface [43]. Therefore, we expect deep trap levels localized in the energy gap of 3.85 eV [36], hindering the electron transport.
For the samples studies the effective angular momentum: J = S = 5/2 was assumed. The Brillouin functions together with the experimental data of magnetic moments are shown in the insets of Figs. 7a-7f. The g-values given in Table 1 decrease from g = 1.75 for x = 0.01 to g = 0.66 for x = 0.15. This suggests that the orbital contribution to the magnetic moment increases with increasing manganese content in the sample, and thus the spin-orbit coupling becomes stronger. The consequence of this is the change in short-range interactions from ferromagnetic to antiferromagnetic in the nonconductive solid solution under study.
3.3. Magnetic properties
We have synthesized the Mn2+ doped Ca(MoO4)0.50(WO4)0.50 ceramic materials using the two steps solid state reaction method. The room temperature XRD data confirm the formation of tetragonal scheelite-type structure with space group I41/a. The electrical and magnetic measurements showed that Ca1-xMnx(MoO4)0.50(WO4)0.50 (x = 0.01, 0.03, 0.05, 0.10, 0.125, and 0.15) ceramics are p-type insulators and paramagnets with the short-range FM and AFM interactions. The Brillouin fit procedure showed an increase in the orbital contribution to the magnetic moment as Mn-content increases in the samples, resulting in a strong spin-orbit coupling.
4. Conclusions
The results of magnetic susceptibility measurements of Ca1(x = 0.01, 0.03, 0.05, 0.10, 0.125 and 0.15) solid solution are depicted in Table 1 and in Figs. 6a–6f. All studied molybdato-tungstates are paramagnetic in the temperature range of 2–300 K. The samples poorer in manganese show short-range ferromagnetic interactions whose occurrence is confirmed by positive values of the Curie-Weiss temperature. On the other hand, manganese-rich samples exhibit short-range antiferromagnetic effects confirmed by negative values of the Curie-Weiss temperature (Table 1). EPR studies corroborate the dominance of the antiferromagnetic short-range interactions with increasing manganese content in the sample [36]. There was no split between the ZFC and FC magnetic susceptibilities in any phase, which means no spin frustration as well as the absence of longrange magnetic interactions in the examined temperature range (Figs. 6a-6f). The effective magnetic moment, μeff, is slightly lower than the effective number of Bohr magnetons, peff, for the Mn2+ ion with the effective spin of S = 5/2, given by the 2[S(S+1)]1/2 expression [44]. This may mean that a small amount of Mn2+ ions is present in the sample. The results of magnetic moment measurements of Mn-doped molybdato-tungstates are shown in Table 1 and in Figs. 7a–7f. Magnetic isotherms do not show hysteresis, coercive field and remanence. In terms of the reduced coordinate (H/T) the magnetic isotherms coincide with the universal Brillouin curve only for x = 0.01. Such behavior is characteristic of superparamagnetic particles. However, with increasing manganese content in the samples, the isotherms show increasing deviations from the universal Brillouin curve. The reason for this may be the appearance of an orbital contribution to the magnetic moment, and consequently spin-orbit coupling. xMnx(MoO4)0.50(WO4)0.50
Acknowledgements The authors are grateful to the team of the workshop at the Institute of Physics (University of Silesia) and the Department of Inorganic and Analytical Chemistry (West Pomeranian University of Technology, Szczecin) for providing practical and technical assistance. References [1] P. Yang, Ch. Li, W. Wang, Z. Quan, S. Gai, J. Lin, Uniform AMoO4:Ln (A = Sr2+, Ba2+; Ln = Eu3+, Tb3+) submicron particles: solvothermal synthesis and luminescent properties, J. Solid State Chem. 182 (2009) 2510–2520. [2] A.P. de Azevedo Marques, V.M. Longo, D.M.A. de Melo, P.S. Pizani, E.R. Leite, J.A. Varela, E. Longo, Shape controlled synthesis of CaMoO4 thin films and their photoluminescence property, J. Solid State Chem. 181 (2008) 1249–1257. [3] S. Vidya, S. Solomon, J.K. Thomas, Synthesis, sintering and optical properties of CaMoO4: a promising scheelite LTCC and photoluminescent material, Phys. Status Solidi A 209 (2012) 1067–1074. [4] J.C. Sczancoski, L.S. Cavalcante, N.L. Marana, R.O. da Silva, R.L. Tranquilin, M.R. Joya, P.S. Pizani, J.A. Varela, J.R. Sambrano, M.S. Li, E. Longo, J. Andrés, Electronic structure and optical properties of BaMoO4 powders, Curr. Appl. Phys. 10 (2010) 614–624. [5] J.C. Sczancoski, L.S. Cavalcante, M.R. Joya, J.A. Varela, P.S. Pizani, E. Longo, SrMoO4 powders processed in microwave-hydrothermal: synthesis, characterization and optical properties, Chem. Eng. J. 140 (2008) 632–637. [6] F. Lei, B. Yan, Hydrothermal synthesis and luminescence of CaMoO4:RE3+ (M = W, Mo; RE = Eu, Tb) submicro-phosphors, J. Solid State Chem. 181 (2008) 855–862. [7] E. Gurmen, E. Daniels, J.S. King, Crystal structure refinement of SrMoO4, SrWO4, CaMoO4, BaWO4 by neutron diffraction, J. Chem. Phys. 55 (1971) 1093–1097. [8] F.A. Rabuffetti, S.P. Culver, L. Suescun, R.L. Brutchey, Structural disorder in AMoO4 (A = Ca, Sr, Ba) Scheelite nanocrystals, Inorg. Chem. 53 (2014) 1056–1061. [9] F. Lei, B. Yan, Hydrothermal synthesis and luminescence of CaMoO4:RE3+ (M = W, Mo; RE = Eu, Tb) submicro-phosphors, J. Solid State Chem. 181 (2008) 855–862. [10] J.H. Kim, H. Choi, E.O. Kim, H.M. Noh, B.K. Moon, J.H. Jeong, Li doping effects on the upconversion luminescence of Yb3+/Er3+-doped ABO4 (A = Ca, Sr; B = W, Mo) phosphors, Opt. Mater. 38 (2014) 113–118. [11] S.K. Ghosh, S.K. Rout, A. Tiwari, P. Yadav, J.C. Sczancoski, M.G.R. Filho, L.S. Cavalcante, Structural refinement, Raman spectroscopy, optical and electrical properties of (Ba1−xSrx)MoO4 ceramics, J. Mater. Sci. 26 (11) (2015) 8319–8335. [12] N. Khobragade, E. Sinha, S.K. Rout, M. Kar, Structural, optical and microwave dielectric properties of Sr1−xCaxWO4 ceramics prepared by the solid state reaction route, Ceram. Int. 39 (2013) 9627–9635. [13] G.-K. Choi, S.-Y. Cho, J.-S. An, K.S. Hong, Microwave dielectric properties and sintering behaviors of scheelite compound CaMoO4, J. Eur. Ceram. Soc. 26 (2006) 2011–2015. [14] G.-K. Choi, J.-R. Kim, S.H. Yoon, K.S. Hong, Microwave dielectric properties of scheelite (A = Ca, Sr, Ba) and wolframite (A = Mg, Zn, Mn) AMoO4 compounds, J.
3.4. Brillouin fit In order to get estimates of the atomic moments containing orbital contribution in Ca1-xMnx(MoO4)0.50(WO4)0.50 (0 < x ≤ 0.15) solid solution, the Brillouin procedure, which does not include any cluster interactions, was used. Saturation magnetization at 2 K in the paramagnetic region was only reached for the samples poorer in manganese. Their experimental virgin magnetization curves, M(H/T), can be easily fitted by the following expression:
M = M0 BJ (x)
(1)
where M0 is the magnetization at the highest value of H/T, x = gJμBH/ kT, g is the fitted Landé factor and the Brillouin function BJ is given by [44]:
BJ (x ) =
2J + 1 2J + 1 1 x ⎞x − coth ⎛ coth 2J 2J 2J ⎝ 2J ⎠
(2) 3312
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