Subsolidus phase relations in CuWO4–Gd2WO6 system

Solid State Sciences 9 (2007) 43e51 www.elsevier.com/locate/ssscie

Subsolidus phase relations in CuWO4eGd2WO6 system E. Tomaszewicz a,*, A. Worsztynowicz b, S.M. Kaczmarek b a

Department of Inorganic and Analytical Chemistry, Szczecin University of Technology, Al. Piastow 42, 71-065 Szczecin, Poland b Institute of Physics, Szczecin University of Technology, Al. Piastow 17, 70-310 Szczecin, Poland Received 18 September 2006; received in revised form 6 November 2006; accepted 14 November 2006 Available online 16 January 2007

Abstract Reactivity in the solid state between CuWO4 and Gd2WO6 was investigated up to solidus line. Two new compounds with the formulas Cu3Gd2W4O18 and CuGd2W2O10 were synthesized. Cu3Gd2W4O18 and CuGd2W2O10 melt incongruently at 1173 and 1248 K, respectively. Cu3Gd2W4O18 crystallizes in the triclinic system. Its anion lattice is built by isolated groups [(W4O16)8]. CuGd2W2O10 crystallizes in the monoclinic system and its anion lattice is formed by structure elements [(W2O9)6]N. The asymmetric EPR signals of Cu3Gd2W4O18 and CuGd2W2O10, characteristic for concentrated, conducting magnetic systems are the same (Gd3þ exchange interactions) but the antiferromagnetic interaction between Gd3þ ions is lower as compared to Gd2WO6. Resonance line attributed to Cu2þ ions is not observed in EPR spectra of Cu3Gd2W4O18 and CuGd2W2O10. Ó 2006 Elsevier Masson SAS. All rights reserved. Keywords: Copper tungstate; Gadolinium tungstate; DTAeTG; IR; EPR

1. Introduction Rare-earth compounds are used extensively not only in luminescent materials as solid state lasers but also in e.g. magnetic materials [1,2]. The Cu2OeRE2O3eWO3 system has been examined by many authors [3e15]. Mu¨ller-Buschbaum et al. [3e10] prepared family of copper rare-earth oxytungstates with the formula of CuREW2O8, where RE ¼ Y, La, Nd, SmeLu. The compounds were synthesized by the solid state reactions in argon atmosphere between Cu2WO4, RE2O3 and WO3[3e10]. The CuREW2O8 compounds, where RE ¼ LaeDy, crystallize in the triclinic system and are isostructural with a-LiPrW2O8[3e10]. Isolated (W4O16)8 groups can be separated in structures of these phases [3e 10]. The CuREW2O8 compounds with small rare-earth cation (RE ¼ HoeLu) crystallize in the monoclinic system and are isostructural with b-LiYbW2O8[3e10]. Structures of these compounds consist of WO6 octahedra linked in chains [3e 10]. The CuREW2O8 phases, where RE ¼ Y, Nd, Dy and Er, * Corresponding author. E-mail address: [email protected] (E. Tomaszewicz). 1293-2558/$ - see front matter Ó 2006 Elsevier Masson SAS. All rights reserved. doi:10.1016/j.solidstatesciences.2006.11.010

show polymorphism [5,7,10]. The high-temperature modifications crystallize in the triclinic system and are isostructural with b-CuNdW2O8[5,7,10]. Szillat and Mu¨ller-Buschbaum [11] obtained single crystals of Cu0.25Ho1.25W2O8 by recrystallization from melt with initial sample comprising Cu2O, Ho2O3 and WO3 mixed at the molar ratio of 1:5:16. The Cu0.25Ho1.25W2O8 crystallizes in the monoclinic system (space group C2/c, a ¼ 1.9123, b ¼ 0.5613, c ¼ 1.1479 nm, b ¼ 111.44 , Z ¼ 8). This compound represents a new structure type characterized by (W4O18)8 ions and Cuþ/Ho3þ at one point position in statistical distribution. Cuþ and Ho3þ ions show an octahedral coordination [11]. Another point position is occupied by Ho3þ ions with coordination number 7 [11]. During slow cooling of melted b-CuDyW2O8 in closed copper tubes and Ar atmosphere, Gressling and Mu¨ller-Buschbaum [12] obtained single crystal of the CuDy5(WO4)8 compound. It shows monoclinic symmetry with space group C2/ c (a ¼ 1.9118, b ¼ 0.5612, c ¼ 1.1518 nm, b ¼ 111.32 , Z ¼ 2). The crystal structure of CuDy5(WO4)8 is characterized by (W4O18)8 groups which are connected to layers. Dy3þ shows one sided capped trigonal prism and Cuþ/Dy3þ with statistical distribution and octahedral oxygen surrounding

44

E. Tomaszewicz et al. / Solid State Sciences 9 (2007) 43e51

[12]. In reaction occurring in the solid state between Cu2O, RE2O3 and WO3 mixed at the molar ratio of 1:1:4, the compounds (analogous to these obtained by Mu¨ller-Buschbaum et al.) with the formula CuRE(WO4)2 were prepared by Klevtsov et al. [13]. The authors suggest that CuRE(WO4)2 compounds, where RE ¼ LaePr, Nd, SmeDy, were triclinic with the structure of a-LiPr(WO4)2 type [14]. The CuRE(WO4)2 phases, where RE ¼ Y, HoeLu, crystallize in the monoclinic system with the structure of b-LiYb(WO4)2 type [15]. CuRE(WO4)2 started to decompose in air, in the solid state, in the temperature range of 903e1033 K [13]. The decomposition process of CuRE(WO4)2 is accompanied by intensive oxidation of decomposition products, particularly Cu2WO4 to CuWO4x [13]. In this work, simultaneous DTAeTG and XRD methods were used to investigate the subsolidus phase relations in the CuWO4eGd2WO6 system. Particular emphasis was paid on the determination of basic properties of newly obtained compounds. 2. Experimental procedure 2.1. Sample preparation The starting materials were CuWO4 and Gd2WO6. Copper tungstate was prepared by heating equimolar CuO (99.99%, Aldrich)/WO3 (99.9%, Fluka) mixture in the following cycles: 973 K (12 h), 1023 K (12 h), 1073 K (12 h) and 1123 K (2  12 h). Gd2WO6 was obtained by the solid state reaction between Gd2O3 (99.9%, Aldrich) and WO3 as described previously [16,17]. The mixtures of CuWO4 with Gd2WO6 were prepared with the range of copper tungstate from 20.00 to 90.00 mol%. The CuWO4/Gd2WO6 mixtures were heated in air in the following cycles: 1023 K (12 h), 1073 K (2  12 h) and 1123 K (2  12 h). After each heating cycle, the samples were cooled slowly to room temperature, weighted, grounded and examined for their contents by XRD method and afterwards heated until an equilibrium state had been established. After the final heating cycle, samples were examined by DTAeTG, IR and EPR methods. 2.2. Characterization methods Routine phase analysis was conducted with DRON-3 diffractometer using Co Ka radiation (l ¼ 0.179021 nm). Diffraction patterns were collected within the 2Q range of 12e52 at the stepped scan rate of 0.02 per step and the count time of 1 s per step. The DTAeTG examinations were carried out using a TA Instruments SDT 2960 apparatus. These measurements were carried out within the temperature range of 293e1373 K, in air atmosphere, using corundum crucibles and at the heating rate of 10 K/min. IR spectra were recorded on a Specord M-80 spectrometer. The samples were pressed in pellets with KBr in the weight ratio of 1:100. The EPR measurements were performed with a conventional X-band Bruker ELEXSYS E500 CW-spectrometer operating

at 9.5 GHz with 100 kHz magnetic field modulation. Powdered samples of all compounds with a weight of 30 mg were placed into 4 mm diameter quartz tubes. The first derivate of the power absorption spectra has been recorded as a function of the applied magnetic field. Temperature dependence of the EPR spectra was registered using an Oxford Instruments ESP helium-flow cryostat in the 8e295 K temperature range. 3. Results and discussion 3.1. Reactivity in the solid state between CuWO4 and Gd2WO6 The contents of the initial mixtures and the results of XRD analysis for samples obtained after the last heating cycle of CuWO4/Gd2WO6 mixtures are presented in Table 1. The data in Table 1 indicate that the initial components of Gd2WO6 and CuWO4 react with each other in the solid state. The XRD analysis of the samples, the initial mixtures of which contained up to 25.00 mol% of Gd2WO6, showed that two solid phases were found in the treated samples, viz. the following compounds: CuWO4 and up to then unknown Cu3Gd2W4O18. At the molar ratio 3:1 of the CuWO4/Gd2WO6 mixtures, both reactants reacted to produce a new compound: 3CuWO4ðsÞ þ Gd2 WO6ðsÞ ¼ Cu3 Gd2 W4 O18ðsÞ

ð1Þ

On the other hand, the composition of samples at equilibrium, the initial mixtures of which contained over 25.00 mol% of Gd2WO6, shows that in the CuWO4eGd2WO6 system, apart from Cu3Gd2W4O18 other, up to now unknown compound e CuGd2W2O10, is formed. The presence of

Table 1 Contents of CuWO4/Gd2WO6 mixtures and results of XRD analysis of samples obtained after the last heating cycle No.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 a b

Compositions of samples CuWO4 (mol%)

Gd2WO6 (mol%)

90 85 80 77 76 75 74 72.5 70 69 67.5 65 60 55 50 45 40 33.33 20

10 15 20 23 24 25 26 27.5 30 31 32.5 35 40 45 50 55 60 66.67 80

Phases identified

CuWO4, Cu3Gd2W4O18 CuWO4, Cu3Gd2W4O18 Cu3Gd2W4O18, CuWO4 Cu3Gd2W4O18, CuWO4a Cu3Gd2W4O18, CuWO4a Cu3Gd2W4O18 Cu3Gd2W4O18, CuGd2W2O10b Cu3Gd2W4O18, CuGd2W2O10b Cu3Gd2W4O18, CuGd2W2O10 Cu3Gd2W4O18, CuGd2W2O10 Cu3Gd2W4O18, CuGd2W2O10 CuGd2W2O10, Cu3Gd2W4O18 CuGd2W2O10, Cu3Gd2W4O18 CuGd2W2O10, Cu3Gd2W4O18 CuGd2W2O10 CuGd2W2O10, Gd2WO6 CuGd2W2O10, Gd2WO6 Gd2WO6, CuGd2W2O10 Gd2WO6, CuGd2W2O10

CuWO4 was identified in a small amount. CuGd2W2O10 was identified in a small amount.

E. Tomaszewicz et al. / Solid State Sciences 9 (2007) 43e51

CuGd2W2O10 in those samples proves that other reaction takes place too: CuWO4ðsÞ þ Gd2 WO6ðsÞ ¼ CuGd2 W2 O10ðsÞ

45

of 2:1. These mixtures were heated under the same conditions as those applied to the preparation of samples obtained from CuWO4/Gd2WO6 mixtures. XRD analysis made for the samples containing initially CuWO4/CuGd2W2O10 as well as Gd2WO6/Cu3Gd2W4O18 showed that in the CuWO4e Gd2WO6 system other reactions run, too:

ð2Þ

The run of reaction (2) implies that Cu3Gd2W4O18 and CuGd2W2O10 were in equilibrium in the concentration range of 25.00e50.00 mol% of Gd2WO6. The concentration of a sample obtained after heating an initial mixture composed of 50.00 mol% of CuWO4 and 50.00 mol% of Gd2WO6confirms the quantitative course of reaction (2). In the other concentration range, i.e. over 50.00 mol% of Gd2WO6, the compounds to remain in an equilibrium within the subsolidus area are CuGd2W2O10 and Gd2WO6 (Table 1). Additionally, two independent CuWO4/CuGd2W2O10 and Gd2WO6/ Cu3Gd2W4O18 mixtures were prepared. CuWO4 was mixed with CuGd2W2O10 at the molar ratio of 2:1 as well as Gd2WO6 was mixed with Cu3Gd2W4O18 at the molar ratio

CuGd2 W2 O10ðsÞ þ 2CuWO4ðsÞ ¼ Cu3 Gd2 W4 O18ðsÞ

ð3Þ

Cu3 Gd2 W4 O18ðsÞ þ 2Gd2 WO6ðsÞ ¼ 3CuGd2 W2 O10ðsÞ

ð4Þ

3.2. Characteristics of Cu3Gd2W4O18 and CuGd2W2O10 3.2.1. Crystallography (from XRD data) Powder diffraction patterns of Cu3Gd2W4O18 and CuGd2 W2O10 were subjected to indexing by POWDER program [18,19]. Table 2 shows the results of indexing these compounds.

Table 2 Results of indexing Cu3Gd2W4O18 and CuGd2W2O10 powder diffraction patterns No.

dobs/nm

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40

Cu3Gd2W4O18 0.60985 0.56968 0.56380 0.54233 0.53645 0.50351 0.48956 0.47530 0.43734 0.39851 0.39140 0.35980 0.35510 0.33707 0.31797 0.30800 0.30338 0.29473 0.28545 0.27634 0.27307 0.27156 0.26612 0.26127 0.25212 0.24482 0.23741 0.23539 0.23098 0.22833 0.22304 0.21904 0.21777 0.21520 0.21079 0.20873 0.20336 0.20078 0.19512 0.19275

dcal/nm 0.61032 0.56927 0.56314 0.54271 0.53632 0.50303 0.48963 0.47583 0.43719 0.39832 0.39141 0.35968 0.35478 0.33724 0.31803 0.30830 0.30338 0.29432 0.28542 0.27643 0.27260 0.27136 0.26615 0.26125 0.25214 0.24481 0.23738 0.23540 0.23101 0.22832 0.22308 0.21908 0.21754 0.21521 0.21087 0.20872 0.20339 0.20080 0.19513 0.19276

I/I0 6 3 12 3 12 4 3 29 18 3 3 2 28 4 100 16 70 6 38 2 8 30 9 53 3 11 3 5 4 1 7 3 2 5 4 6 3 3 6 40

h 0 1 0 0 1 1 0 1 1 0 1 1 1 1 0 1 1 1 1 2 0 0 2 2 1 0 1 2 2 2 2 2 1 1 1 0 2 2 1 0

k 0 0 1 2 1 2 2 2 2 4 3 2 4 3 6 5 1 5 6 2 7 4 3 3 7 4 3 5 5 3 3 6 5 8 8 6 7 5 8 7

l

dobs/nm

dcal/nm

I/I0

h

k

l

1 0 1 1 0 0 1 0 1 1 1 1 0 1 0 0 2 1 0 1 0 2 0 1 0 2 2 0 1 1 2 0 2 0 1 2 0 2 2 2

CuGd2W2O10 0.57850 0.54568 0.48496 0.48227 0.43766 0.39711 0.39121 0.35153 0.33064 0.32421 0.31475 0.30300 0.29747 0.29502 0.28916 0.27829 0.27283 0.26818 0.25981 0.25646 0.25186 0.24474 0.24243 0.24048 0.23738 0.23304 0.23192 0.23048 0.22360 0.22240 0.22048 0.21833 0.21586 0.21154 0.20695 0.20154 0.19871 0.19547 0.19481 0.19294

0.57850 0.54561 0.48421 0.48267 0.43780 0.39692 0.39135 0.35144 0.33105 0.32415 0.31478 0.30256 0.29746 0.29496 0.28925 0.27867 0.27280 0.26811 0.25971 0.25624 0.25172 0.24547 0.24275 0.24025 0.23756 0.23261 0.23166 0.23024 0.22334 0.22214 0.22039 0.21824 0.21566 0.21112 0.20666 0.20139 0.19876 0.19542 0.19476 0.19284

10 6 4 12 4 2 4 6 100 82 78 6 10 50 72 4 12 2 21 6 12 5 14 2 2 29 17 6 2 16 2 8 10 4 6 6 19 26 34 7

0 2 0 1 2 2 1 0 3 1 2 3 3 0 0 2 4 3 0 1 2 2 2 4 3 1 0 4 1 3 5 5 1 1 1 3 3 5 3 0

1 0 1 1 0 1 0 1 0 1 1 0 1 0 2 1 0 1 2 2 1 0 0 1 0 2 1 1 2 2 0 0 2 0 2 1 2 1 2 3

0 0 2 1 2 0 4 4 3 4 3 4 1 6 0 5 0 4 3 3 5 6 7 1 5 4 7 2 5 1 1 0 5 8 6 6 5 4 4 0

E. Tomaszewicz et al. / Solid State Sciences 9 (2007) 43e51

46

Table 3 The parameters of Cu3Gd2W4O18 and CuGd2W2O10 unit cells and the values of experimental and theoretical density

Cu3Gd2W4O18 CuGd2W2O10

a [nm]

b [nm]

c [nm]

a [ ]

b [ ]

g [ ]

Z

Experimental density [g/cm3]

Theoretical density [g/cm3]

0.58948(1) 1.1021(1)

1.9216(9) 0.57850(5)

0.63478(7) 1.7874(1)

95.757(2)

104.61(4) 98.062(7)

92.031(3)

2 6

7.30 7.98

7.35 8.00

The parameters of Cu3Gd2W4O18 and CuGd2W2O10 unit cells and the values of experimental (obtained by degassing of samples and hydrostatic weighing in pycnometric liquideCCl4) and theoretical density are tabulated in Table 3. 3.2.2. Thermal properties Fig. 1 shows DTA curves of Cu3Gd2W4O18 (curve A), CuGd2W2O10 (curve B) and samples obtained after the last heating cycle of CuWO4/Gd2WO6 mixtures comprising initially: 85.00 mol% of CuWO4 (curve C), 60.00 mol% of CuWO4 (curve D) as well as 40.00 mol% of CuWO4 (curve E). On each DTA curve one or two endothermic effects were recorded up to 1373 K. No mass losses were recorded on the TG curves (not presented) up to the onsets of the first observed effects on the DTA curves. Separate samples of Cu3Gd2W4O18 and CuGd2W2O10 were heated in a furnace for 2 h after the onset of first effects observed on DTA curves, i.e. at 1188 K and 1263 K, respectively. After heating, the

A

samples were quickly quenched to 263 K and next examined by XRD method. Observation of the residues in crucibles shows that both samples were melted. On the basis of the XRD method it was found that Cu3Gd2W4O18 sample after heating contained CuGd2W2O10, while the CuGd2W2O10 sample contained Gd2WO6. Thus, the incongruent melting of Cu3Gd2W4O18 at 1178 K and CuGd2W2O10 at 1248 K can be described by Eqs. (5) and (6), respectively: Cu3 Gd2 W4 O18ðsÞ / CuGd2 W2 O10ðsÞ þ liquid

ð5Þ

CuGd2 W2 O10ðsÞ / Gd2 WO6ðsÞ þ liquid

ð6Þ

Fig. 2 shows the diagram of phase equilibria in the CuWO4e Gd2WO6 system up to the solidus line. The solidus line temperatures have been determined based on the onset of first endothermic effects recorded on the DTA curves (Fig. 1, curves CeE).

B

C

1168

1248 1178

1178

exo>

exo>

exo>

1240

1123 1173 1223 1273 1323

1173 1223 1273 1323

1123 1173 1223 1273

Temperature [K]

Temperature [K]

Temperature [K]

D

E

1248

1178

exo>

exo>

1248

1173 1223 1273 1323

1173 1223 1273 1323

Temperature [K]

Temperature [K]

Fig. 1. DTA curves of (A) e Cu3Gd2W4O18; (B) e CuGd2W2O10; (C) e sample obtained after heating CuWO4/Gd2WO6 mixture comprising initially 85 mol% of CuWO4; (D) e sample obtained after heating CuWO4/Gd2WO6 mixture comprising initially 60 mol% of CuWO4; (E) e sample obtained after heating CuWO4/ Gd2WO6 mixture comprising initially 40 mol% of CuWO4.

E. Tomaszewicz et al. / Solid State Sciences 9 (2007) 43e51

T [K]

47

T [K]

Points indicate DTA

1273

1248

Cu3Gd2W4O18(s) + CuWO4(s)

973

10

20

5K

1173

Cu3Gd2W4O18(s) + CuGd2W2O10(s)

30

40

1073

CuGd2W2O10

1073

CuWO4

1178

5K

Cu3Gd2W4O18

1168

1173

1273

5K

CuGd2W2O10(s) + Gd2WO6(s)

973

50

60

70

80

90

Gd2WO6

mol % Gd2WO6 Fig. 2. Diagram of phase equilibria in the CuWO4eGd2WO6 system up to the solidus line.

3.2.3. IR spectra Fig. 3 shows IR spectra of Cu3Gd2W4O18 and CuGd2 W2O10. Based on the literature information concerning binary and ternary lanthanide tungstates with tetrahedral and octahedral coordination of W ions by oxygen ions [20e25], the absorption bands with their maxima at 948 and 940 cm1 (Fig. 3, curve A for Cu3Gd2W4O18) can be assigned to the symmetric stretching modes of WeO bonds in isolated groups of octahedra (W4O16)8. The stretching vibrations of short terminal WeO bonds in (W4O16)8 groups were observed in IR spectra of KRE(WO4)2, where RE ¼ LaeNd (the region of vibration frequencies 950e940 cm1), and the RbRE(WO4)2 compounds, where RE ¼ GdeYb (the region of vibration frequencies 975e950 cm1) [20]. On the other hand, the absorption band with its maximum at 880 cm1 (Fig. 3, curve B for CuGd2W2O10) can be due to the stretching modes of WeO bonds in joint WO6 octahedra forming structure elements [(W2O9)6]N [20]. The stretching vibrations of WeO bonds in structure elements [(W2O9)6]N were observed in IR spectra of the RE2W2O9 compounds (RE ¼ PreGd, the region of vibration frequencies 885e867 cm1) [20]. The several absorption bands in the 870e512 cm1 region (for Cu3Gd2W4O18, curve A) as well as in the 802e520 cm1 region (for CuGd2W2O10, curve B) can be due to the asymmetric stretching vibrations of WeO bond in joint WO6 octahedra and also to the oxygen double bridge bonds WOOW [20e25]. The absorption bands found in the IR spectra of both compounds below 500 cm1 can be assigned to the symmetric and also asymmetric deformation modes of WeO bonds in joint WO6 octahedra as well as to the deformation modes of the oxygen bridges WOOW [20e25]. 3.2.4. EPR spectra Figs. 4 and 5 show the resonance spectra of CuWO4, Gd2WO6, Cu3Gd2W4O18 and CuGd2W2O10 compounds (the first derivative of the microwave power absorption) as a function

of a magnetic field at selected temperatures. As one can see the temperature dependence of CuWO4 resonance lines reveals symmetric behaviour (Fig. 4A) due to Cu2þ ions [26,27], while the resonance spectra of Gd2WO6 (Fig. 4B) show asymmetric, dysonian lineshapes due to Gd3þ ions, both containing single resonance line. As one can see from Fig. 5A and B the dominant feature of the resonance spectrum in the Cu3Gd2W4O18 and CuGd2W2O10 phases is a single asymmetric resonance line due to Gd3þ ions. Resonance line attributed to Cu2þ ions is not observed in the EPR spectra of these compounds. The temperature dependencies of the EPR signal intensity for the all considered compounds, as determined by the area under the absorption curve, are shown in Fig. 6. Besides the dependence for CuWO4 (Fig. 6A), all others show clear CurieeWeiss behaviour taking Tc ¼ 20, 2.4 and 2 K for Gd2WO6, Cu3Gd2W4O18 and CuGd2W2O10, respectively. CuWO4 becomes antiferromagnetically ordered below 20 K, which was established from a disappearance of EPR signal, being in good agreement with the Neel point at 23 K as reported in [27]. Moreover, the characteristic maximum of the intensity near 90 K is clearly seen which corresponds to a proper maximum reported in the paper [27] for magnetic susceptibility measurements. The temperature dependencies of a g-factor are presented in Fig. 7. As one can see, beside g-factor for Cu3Gd2W4O18 (Fig. 7C), g-factor for other compounds shows distinct increase with increasing temperature from 2.05 to 2.13 K for CuWO4 (Fig. 7A), 1.74 to 2.04 K for Gd2WO6 (Fig. 7B) and 4.5 to 8 K for CuGd2W2O10 (Fig. 7D). The temperature dependencies of a peak-to-peak linewidth shown in Fig. 8 exhibit usual decrease with increasing temperature for all the compounds. The peak-to-peak linewidth for CuWO4 over 90 K slowly increases with increasing temperature. Very well known single symmetric EPR spectrum of Cu2þ in CuWO4 can be observed in Fig. 4A. It is due to the presence of Cu2þ pairs [27]. Broad maximum in the temperature

E. Tomaszewicz et al. / Solid State Sciences 9 (2007) 43e51

48

100K 161K

A

260K 40K 35K 32K

EPR signal

A

27K

422

200

300

400

500

600

700

800

306 272

512 574

632

100

B

276K

692

100K

EPR signal

758 738

800

870 844

0

Magnetic Field [mT]

476 468

940 948

B

356

390

CuWO4

52,4K 27,8K

13,6K 7,4K 4,3K Gd2WO6

432

324

0

600

800

1000

1200

1400

Magnetic Field [mT]

336

464

520

604 584 600

400

Fig. 4. Resonance lines of CuWO4 (A) and Gd2WO6 (B) compounds for several selected temperatures.

366

404 800

756 740 704

802

392

880 1000

200

400

Wavenumber [cm-1] Fig. 3. IR spectra of Cu3Gd2W4O18 (A) and CuGd2W2O10 (B) compounds.

dependence of the intensity seen in Fig. 6A around 90 K can be associated with a stronger antiferromagnetic coupling of Cu2þ ion pairs. From Fig. 6A it can be concluded that the copper tungstate is antiferromagnetically ordered below 20 K. The temperature dependence of the g-factor resembles measurements of the static bulk susceptibility, which is determined by the spin fluctuations. Fig. 7A gives the evidence on the

above mentioned facts showing minimum of the g-factor observed near 20 K and the increase of the factor with increasing temperature from 2.05 for 20 K to 2.13 for 180 K. Change in a behaviour of the peak-to-peak linewidth observed at about 90 K (Fig. 8A) confirms a change in the mechanism of interactions between Cu2þ ions from short-range type to longrange one [27]. The width DH of the EPR absorption line reflects the mechanism of relaxation of the EPR probe. Above 90 K, the linewidth of CuWO4 increases linearly that reflects the spinelattice relaxation via delocalized charge carriers [28]. The steep increase of the linewidth below 90 K is a common feature of all high-Tc superconductors. The line broadening at low temperatures is attributed also to spin-glass-like freezing which is connected to a charge carrier localization [29,30]. Contrary to CuWO4, Gd2WO6 single, wide, featureless EPR resonance line shows good known asymmetric shape which clearly changes with a temperature, being an effect

E. Tomaszewicz et al. / Solid State Sciences 9 (2007) 43e51

49

230 K

Integral intensity [arb. units]

100 K 33 K

EPR signal

19 K 13 K 9K 6,6 K 4,4 K

0

200

400

600

800

Cu3Gd2W4O18

A

1000

1400

1200

Area CuWO4

Magnetic Field [mT] 0

50

100

150

200

A

250

300

Temperature [K] 230 K 120 K 68 K

Gd2WO6 Curie-Weiss fit

Integral intensity [arb. units]

EPR signal

44 K

28 K

17 K 11 K 6.5 K CuGd2W2O10 0

200

400

600

B

800

1000

1200

B 1400

Magnetic Field [mT]

0

50

100

150

200

250

Fig. 5. Resonance lines of Cu3Gd2W4O18 (A) and CuGd2W2O10 (B) compounds for several selected temperatures.

Cu3Gd2W4O18 CuGd2W2O10

C D

Curie-Weiss fit

Integral intensity [arb. units]

of Gd3þ exchange interactions (Fig. 4B) in concentrated magnetic system [31,32]. The single resonance line of the powder spectra is very broad and extends over the wide range from zero to more than 1000 mT. The linewidth of the signal (>100 mT) is the same order of a magnitude as the center field (w330 mT). Then the informative parameters that one can extract from the spectrum are only a line position ( g-value), a linewidth, a lineshape and their dependence on a temperature. From the temperature dependence of the line there result strong antiferromagnetic interactions between Gd3þ ions manifesting itself in a large Tc value (20 K). The g-factor, similarly as in case of antiferromagnetic CuWO4, increases with increasing temperature (Fig. 7B). One can distinguish two ranges: steep increase up to about 30 K and slow increase over 30 K. It means there are at least two different mechanisms of spin fluctuations in Gd2WO6. The peak-to-peak linewidth decreases with increasing temperature do not show any changes in the type of the exchange interactions. Dysonian lineshape observed in case

300

Temperature [K]

0

50

100

150

200

250

300

Temperature [K] Fig. 6. Intensity of the EPR signal for CuWO4 (A), Gd2WO6 (B), Cu3Gd2 W4O18 (C) and CuGd2W2O10 (D) compounds.

E. Tomaszewicz et al. / Solid State Sciences 9 (2007) 43e51

50 2,15

2400

2,10 2,05

linewidth [Gs]

2000

2,00

g-factor

A

width of CuWO4

2200

1,95 1,90

1800 1600 1400

1,85

1200

1,80

1000

A B

gfactor of CuWO4 gfactor of Gd2WO6

1,75

800 0

1,70 0

50

100

150

200

250

50

100

150

200

250

300

Temperature [K]

300

Temperature [K] 5200 8,5

B

Width Gd2WO6

5000

8,0

4800

linewidth [Gs]

7,5 7,0

g-factor

6,5 6,0 5,5 5,0

4600 4400 4200 4000 3800

4,5

3600

4,0

gfactor of Cu3Gd2W4O18

3,5

gfactor of CuGd2W2O10

C D

3400 0

3,0

50

100

150

200

250

300

Temperature [K]

2,5 0

50

100

150

200

250

6600

Temperature [K]

width of Cu3Gd2W4O18

6500

Fig. 7. g-Factors for CuWO4 (A), Gd2WO6 (B), Cu3Gd2W4O18 (C) and CuGd2W2O10 (D) compounds.

width of CuGd2W2O10

6400

C D

of the Gd2WO6 sample is probably a combination of the absorption (being symmetric around B ¼ 0) and dispersion (being asymmetric around B ¼ 0) components of symmetric Lorentzian, characteristic for conducting samples due to the skin depth effects [31]. Based on the comparison of the single asymmetric EPR signal of Gd2WO6 (Fig. 4B) and the single asymmetric EPR signals of Cu3Gd2W4O18 (Fig. 5A) and CuGd2W2O10 (Fig. 5B), the authors suggest the origin of the signal is the same (Gd3þ signal) but the range of the antiferromagnetic interaction between Gd3þ ions is lower as compared to Gd2WO6 (lower Tc). Moreover, the shape of the g-factor temperature dependence is something different among the two investigated compounds. It exponentially decreases in case of Cu3Gd2 W4O18, while for CuGd2W2O10 it decreases with increasing temperature up to 90 K and then slowly increases. It indicates different mechanisms of spin fluctuations and may be due to the spinelattice relaxation of the Gd3þ ions coupled via double exchange interaction with a participation of Cu2þ ions or

linewidth [Gs]

6300 6200 6100 6000 5900 5800 5700 5600 0

50

100

150

200

250

300

Temperature [K] Fig. 8. Peak-to-peak linewidth for CuWO4 (A), Gd2WO6 (B), Cu3Gd2W4O18 (C) and CuGd2W2O10 (D) compounds.

pairs. The peak-to-peak linewidth dependence for both compounds shows the same behaviour indicating domination of Gd3þ double exchange interactions. It decreases with increasing temperature, so spinelattice relaxation time of Gd3þ ions increases with a temperature.

E. Tomaszewicz et al. / Solid State Sciences 9 (2007) 43e51

by joint WO6 octahedra forming structure elements [(W2O9)6]N. The authors suggest the origin of the EPR signal of Gd2WO6, Cu3Gd2W4O18 and CuGd2W2O10 is the same (Gd3þ double exchange interaction) but the range of the antiferromagnetic interaction between Gd3þ ions is lower as compared to the Gd2WO6 (lower Tc).

100 CoDy2W2O10

Intensity I/I0 [%]

80

60

40

20

References

0 15

20

25

30

2Θ

35

40

45

50

CoKαaver. λ=0.179021 nm

100 CuGd2W2O10

Intensity I/I0 [%]

51

80

60

40

20

0 15

20

25

30

2Θ

35

40

45

50

CoKαaver. λ=0.179021 nm

Fig. 9. Powder diffraction patterns of CoDy2W2O10[33] and CuGd2W2O10.

4. Conclusions The experimental results concerning the reactivity in the solid state CuWO4 with Gd2WO6 showed that Cu3Gd2W4O18 and CuGd2W2O10 unknown up to now were formed. Cu3Gd2W4O18 crystallizes in the triclinic system and the latter compound crystallizes in the monoclinic system. Fig. 9 shows powder diffraction patterns of CuGd2W2O10 and one phase from the series of the isostructural compounds CoRE2W2O10 (RE ¼ Y, Dy, Ho and Er) [33]. As it is seen from Fig. 9 the number and positions of the diffraction lines recorded within 2Q angle range 12e52 for CuGd2W2O10 are very different in comparison to the number and positions of the diffraction lines observed on the CoDy2W2O10 diffraction pattern. In spite of an identical type of chemical formula the authors suggest that CuGd2W2O10 was not isostructural with the CoRE2W2O10 compounds. Cu3Gd2W4O18 and CuGd2W2O10 melt incongruently at 1178 and 1248 K, respectively. The anion lattice of Cu3Gd2W4O18 is built by isolated groups of octahedra (W4O16)8, while the anion lattice of CuGd2W2O10 is built

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