Thermoelectric power of CuCrxVySe4 p-type spinel semiconductors

Journal of Physics and Chemistry of Solids 73 (2012) 262–268

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Thermoelectric power of CuCrxVySe4 p-type spinel semiconductors E. Malicka a,n, H. Duda b, T. Gron´ b, A. Ga˛gor c, S. Mazur d, J. Krok-Kowalski b a

University of Silesia, Institute of Chemistry, ul. Szkolna 9, 40-006 Katowice, Poland University of Silesia, Institute of Physics, ul. Uniwersytecka 4, 40-007 Katowice, Poland ´lna 2, 50-950 Wroc!aw, Poland Institute of Low Temperature and Structure Research, Polish Academy of Sciences, ul. Oko d ´ ski Institute of Nuclear Physics, Polish Academy of Sciences, ul. Radzikowskiego 152, 31-342 Krako ´w, Poland The Henryk Niewodniczan b c

a r t i c l e i n f o

a b s t r a c t

Article history: Received 24 July 2011 Received in revised form 22 September 2011 Accepted 24 October 2011 Available online 3 November 2011

Structural, electrical and magnetic measurements of polycrystalline CuCrxVySe4 spinels with x ¼1.79, 1.64 and 1.49 and y¼ 0.08, 0.22 and 0.45, respectively, are presented. The compounds under study crystallize in regular system of a normal spinel type MgAl2O4 structure with the space group symmetry Fd3m. The chromium spins are coupled ferromagnetically and show both strong long- and short-range magnetic interactions evidenced by the large values of the Curie (TC) and Curie–Weiss (yCW) temperatures, decreasing from TC ¼ 407 K and yCW ¼415 K for y¼ 0.08, via TC ¼349 K and yCW ¼ 367 K for y ¼0.22 to TC ¼ 283 K and yCW ¼ 293 K for y¼ 0.45, respectively. In all the studied spinels a change of the electrical conductivity character from the semiconductive into the metallic one above 230 K was observed. A detailed thermoelectric power analysis showed a domination of diffusion thermopower component, maximum of phonon drag component at 230 K, a decrease of impurity component with increasing V content, as well as the weak magnon excitations at 40 K. & 2011 Elsevier Ltd. All rights reserved.

Keywords: A. Chalcogenides B. Crystal growth D. Electrical properties D. Magnetic properties

1. Introduction Materials with high thermoelectric efficiency in the vicinity of room temperature are being intensively sought on account of their potential applications in electronic refrigerators, air conditioners and power generators. Thermoelectric materials are able to generate the electric energy directly from the warmth or to serve as the electric heat pump. Thermoelectric cooling is an environmentally friendly method applied on a large scale in computers, infrared detectors, electronics and optoelectronics. The thermoelectric properties investigated in the spinel materials revealed a lot of interesting results. For example, in CdCr2Se4 and HgCr2Se4 semiconductors a room temperature (RT) power factor of about 1 mW/cm K2 was found [1], while in the insulating chromium spinels for low Cu content such as CdxCu1  xCr2Se4 [2] and GaxCu1  xCr2Se4 [3] the maximum RT power factor of 10  4 mW/cm K2 was observed [4]. In the parent ferromagnet, CuCr2Se4 being a p-type metallic conductor with Curie temperature TC ¼416 K and Curie–Weiss temperature yCW ¼436 K [5], the chromium spins are coupled ferromagnetically via a double exchange interaction involving the electrons jumping between Cr3 þ and Cr4 þ ions [6,7]. X-ray photoelectron spectroscopy and polarized neutron diffraction

n

Corresponding author. Tel.: þ48 323591627. E-mail address: [email protected] (E. Malicka).

0022-3697/$ - see front matter & 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.jpcs.2011.10.032

studies for CuCr2Se4 provide direct evidence for the presence of monovalent copper atoms with a 3d10 configuration [8,9]. In CuCr2Se4, however, it was not possible to measure the magnetic splitting of the Cr 3s levels, because they overlapped with the Cu 3p levels [8]. Positron annihilation studies carried out on polycrystalline CuCr2Se4 metallic conductors sintered in air and in vacuum revealed that the oxidation and adsorption processes as well as the effects of structural defects play a significant role considering the positron trapping and the corresponding linear temperature decrease of conductivity [10]. The longer bulk lifetime of positrons and the lower values of the electrical conductivity observed with the CuCr2Se4 sample sintered in air have been attributed to the scattering on the oxidation layer (surface scattering) [10]. From literature it is known, that in a case of manganese–zinc ferrites the conductivity of polycrystals is usually much lower than the conductivity of monocrystals of the same composition [11], suggesting an influence of oxidized grain boundaries. The second end composition of the CuCr2 xVxSe4 system, i.e. CuV2Se4 is not known, as yet. In the isostructural CuCr2  xVxS4 spinel system, a rapid decrease in TC from 267 K for x¼ 0.25 to 8 K for x¼1.75 was observed [12]. This fact has been explained by weaker Cr–S–V superexchange than the original Cr–S–Cr interactions, as well by a weakening of the nearest neighbor ferromagnetic Cr–S–Cr interactions due to the initial lattice expansion [12]. Recently, the electron spin resonance (ESR), electrical and magnetic investigations carried out on the single-crystalline CuCr1.6V0.4Se4 spinel showed the p-type semiconducting behavior, negative magnetoresistance, the

E. Malicka et al. / Journal of Physics and Chemistry of Solids 73 (2012) 262–268

ESR change from paramagnetic to ferromagnetic resonance at TC ¼ 193 K, the large paramagnetic Curie–Weiss temperature yCW ¼ 207 K and magnon excitations at low temperatures derived from thermopower analysis [13]. Getting to know the mechanism of generating charge carriers forced by thermal gradient in the polycrystalline CuCrxVySe4 spinels with reference to the single-crystalline CuCr1.6V0.4Se4 one [13] is a main motivation of this work. For that the thermoelectric power analysis was used.

263

The magnetic susceptibility measurements were performed in the temperature range 77–600 K and in an applied external magnetic field of 600 Oe using a Faraday-type Cahn RG automatic electrobalance. Electrical conductivity s was measured with the 4-point dc method using the apparatus with Keithley K181 digital multimeters. The maximal error ds/s was less than 71%. Thermoelectric power was measured via a differential method using the temperature gradient DT of  2 K. The accuracy of the value of thermopower was estimated to be better than 3 mV/K.

3. Results and discussion 2. Experimental details

3.1. Crystallographic properties

Polycrystalline CuCrxVySe4 samples were prepared by annealing stoichiometric mixtures of high purity ( Z99.99%) elements: Cu, Cr, V and Se. The mixtures were pulverized in agate mortar and sintered three times in evacuated quartz ampoules at temperature 1023 K for 10 days. The chemical composition of the polycrystals was determined using an energy-dispersive X-ray spectrometry (EDXRF). The X-ray spectra from the samples were collected by thermoelectrically cooled Si-PIN detector (Amptek, USA) with resolution of 145 eV at 5.9 keV. The quantitative analysis was performed using fundamental parameters method with and without reference samples. Fig. 1 shows an exemplary EDXRF spectrum of CuCr1.49V0.45Se4 collected in air atmosphere using 1000 mm pinhole collimator. Table 1 presents the EDXRF analysis of CuCrxVySe4 polycrystalline spinels. Powder diffraction data were collected at X’Pert PRO X-ray Diffraction system equipped with the PIXcel line detector, focusing mirror and Soller slits for CuKa radiation. The diffraction patterns were collected at room temperature in reflection mode in the range of 101r951 using step scans of 0.0261. Refinement of the X-ray diffraction patterns was done using Rietveld method implemented in Jana2000 program package [14]. All the compounds were obtained in a spinel MgAl2O4 structure, however, in CuCr1.79V0.08Se4, traces of CuSe2 were found and in CuCr1.64V0.22Se4 as well as CuCr1.49V0.45Se4 traces of hexagonal selenium [15] were detected.

CuCrxVySe4 crystallize in the regular system with a spinel Fd-3m structure. In the normal spinel structure Cu þ ions are located at 8a (1/8, 18, 1/8) (A site) with tetrahedral coordination whereas Cr3 þ ions that are known for strong octahedral site preference [16] occupy octahedrallycoordinated position 16d: (1/2, 1/2, 1/2) (B site). Accordingly, due to the very small difference in ionic radii V3 þ and Cr3 þ ions were refined sharing octahedral B site with coupled site ˚ r 3 þ ¼0.615 A˚ [17]). At first occupancy factors (SOF) (r V3 þ ¼ 0.64 A, Cr SOFs from chemical analysis were implemented in the structure model and constrained in order to avoid correlations with displacement parameters of ions. In the last refinement runs, we calculated SOFs for chromium and vanadium independently. Since, the differences between the refined values and those from chemical analysis were meaningful the final model contains the latter ones. The copper Cu þ ions were assumed to adopt A lattice sites with tetrahedral coordination. Table 2 summarizes crystal data, data collection and refinement results for CuCrxVySe4 spinel system. The final structure parameters and interatomic distances for CuCrxVySe4 are presented in Tables 3 and 4. Cr3 þ deficit in all samples may indicate fraction of V4 þ ions in the octahedral environment (r V4 þ ¼ 0.58 A˚ [17]). There is no evidence that the presence of vanadium influences the lattice parameters in CuCrxVySe4, mainly due to the fact, that ionic radii of Cr3 þ and V3 þ are very similar. Correspondingly, almost all distances vary only within the error limit, see Table 4. 3.2. Magnetic and electrical properties

Fig. 1. EDXRF spectrum of CuCr1.49V0.45Se4 pressed into pellets of diameter 1 cm.

All the spinels under study are ferromagnets. The characteristic behavior for this state is seen in the magnetic isotherm for CuCr1.79V0.08Se4 with saturation magnetization of 4.74 mB/f.u at 4.2 K (Fig. 2) as well as in the magnetic susceptibility temperature dependences in Figs. 3–5. The temperatures TC and yCW decrease as the V content increases, i.e. from TC ¼407 K and yCW ¼415 K for y¼0.08, via TC ¼349 K and yCW ¼367 K for y¼0.22 to TC ¼283 K and yCW ¼ 293 K for y¼0.45, respectively, indicating a strong influence of the V substitution on the long and short-range magnetic interactions. The electrical measurements depicted in Figs. 6–9 revealed the change of the electrical conductivity (s) character from the semiconductive into the metallic one above 230 K and the p-type

Table 1 Results in % (m/m) of the EDXRF analysis for the CuCrxVySe4 polycrystals. Chemical formula

CuCr1.79V0.08Se4 CuCr1.64V0.22Se4 CuCr1.49V0.45Se4

Determined concentration of elements Cu

Cr

V

Se

13.69 ( 70.07) 13.19 ( 70.07) 13.30 ( 70.07)

19.44 (7 0.28) 17.93 (7 0.27) 16.14 (7 0.26)

0.84 (7 0.03) 2.33 (7 0.04) 4.82 (7 0.06)

66.03 (70.19) 66.55 (70.19) 65.75 (70.19)

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E. Malicka et al. / Journal of Physics and Chemistry of Solids 73 (2012) 262–268

Table 2 Crystal data and structure refinement results for CuCrxVySe4.

CuCr1.79V0.08Se4 476.5 Cubic, Fd-3m 10.3386 (4)

CuCr1.65V0.22Se4 476.5 Cubic, Fd-3m 103371 (3)

CuCr1.49V0.45Se4 480.3 Cubic, Fd-3m 10.3371 (5)

V (A˚ 3) Z, radiation d (g/cm3)

1105.04

1104.56

1104.57

8, Cu Ka 5.727

8, Cu Ka 5.729

8, Cu Ka 5.774

Refinement R factors Profile function No. of parameters (D/r)max

Rp ¼0.031, Rwp ¼ 0.051, Rexp ¼0.006 Pseudo-Voit 21 0.05

Rp ¼ 0.044, Rwp ¼ 0.074, Rexp ¼0.004 Pseudo-Voit 21 0.01

Rp ¼ 0.012, Rwp ¼0.024, Rexp ¼0.005 Pseudo-Voit 20 0.04

Compound

CuCr1.79V0.08Se4

CuCr1.65V0.22Se4

CuCr1.49V0.45Se4

Cu

0.004 (4) 1 0.001 (1) 0.89 0.001 (1) 0.04 0.006 (2) 1 0.2574 (3)

0.005 (4) 1 0.002 (1) 0.83 0.002 (1) 0.11 0.004 (2) 1 0.2574 (3)

0.005 (4) 1 0.007 (3) 0.75 0.007 (3) 0.22 0.002 (2) 1 0.2571 (3)

Cr V Se

Uiso SOF Uiso SOF Uiso SOF Uiso SOF

u

χ (emu/mol)

Table 3 Isotropic displacement parameters (Uiso), site occupation factors (SOF) and anion parameter (u) for CuCrxVySe4.

3.0

540

2.5

450 360

2.0 TC

270

1.5 χ

1.0

180

1/χ (T-θ)/C

0.5

90

CuCr1.79V0.08Se4

0.0

Atomic positions are given in standard setting for space group Fd3m (No. 227): Cu 8a (1/8.1/8.1/8), Cr/V 16d (1/2.1/2.1/2), Se 32e (u, u, u).

0

100

200

300

1/χ (mol/emu)

Crystal data Chemical formula M (g/mol) Cell setting, space group ˚ a (A)

400

500

0 600

T (K) Fig. 3. Magnetic susceptibility w and inverse susceptibility 1/w vs. temperature T for CuCr1.79V0.08Se4.

Table 4 ˚ for CuCrxVySe4. Interatomic distances (A) CuCr1.65V0.22Se4

CuCr1.49V0.45Se4

Cu1-Se1 Cr1/V-Se1

4  2.3704 (7) 6  2.5108 (7)

4  2.370 (3) 6  2.510 (3)

4  2.366 (3) 6  2.513 (3)

5

30

70

25

60 50

20

40 15 10

1/χ (T-θ)/C

4

M (µB/f.u.)

5

3

30

TC

χ

20 10

CuCr1.64V0.22Se4

0 0

90

180

1/χ (mol/emu)

CuCr1.79V0.08Se4

χ (emu/mol)

Compound

270 360 T (K)

450

0 540

2 Fig. 4. Magnetic susceptibility w and inverse susceptibility 1/w vs. temperature T for CuCr1.64V0.22Se4.

1

CuCr1.79V0.08 Se4 0

0

9

18

27

36 45 H (kOe)

54

63

72

Fig. 2. Magnetization M vs. magnetic field H for CuCr1.79V0.08Se4 at 4.2 K.

conduction. Below 230 K (in the semiconductive region) s strongly decreases as the V content increases while above 230 K (in the metallic region) this dependence is weaker. In case of the

single-crystalline CuCr1.6V0.4Se4 spinel only the semiconducting properties in the temperature range 6–290 K were observed [13]. All the spinels under study have both similar values and the temperature dependences of thermopower S(T) (see Figs. 7–9). 3.3. Thermoelectric power analysis Conventional theory of metals [18] predicts that the thermoelectric power (TEP) consists of a diffusion component, which according to the Mott formula [19] is proportional to temperature

E. Malicka et al. / Journal of Physics and Chemistry of Solids 73 (2012) 262–268

120

3.2

24 Sexp

2.8

60

1.6 1.2

χ

0.8

1/χ (T-θ)/C

Sph

16

S (μV/K)

TC

Sdiff

1/χ (mol/emu)

χ (emu/mol)

2.0

CuCr1.64V0.22Se4

Stheor

20

90

2.4

265

30

Simp Smag

12 8

CuCr1.49V0.45Se4

0.4

4

0

0.0 100

200

300

400

T (K)

0 0

Fig. 5. Magnetic susceptibility w and inverse susceptibility 1/w vs. temperature T for CuCr1.49V0.45Se4.

90

180

270

360

450

T (K) Fig. 8. Thermopower S vs. temperature T for CuCr1.64V0.22Se4 where Sexp is the experimental curve, Stheor. is the theoretical curve and Sdiff, Sph, Simp and Smag are diffusion, phonon drag, impurity and magnon drag components of thermopower, respectively.

5

4 V content y 0.08 0.22 0.45

3

24 Sexp

CuCr1.49V0.45Se4

Stheor

20

Sdiff

2

Sph

16

S (μV/K)

ln[σ(Ω-1m-1)]

TC

1

Simp Smag

12 8

0 0

2

4

6

8

10

12

14

4

103 (K-1) Fig. 6. Electrical conductivity (lns) vs. reciprocal temperature (103/T) for CuCrxVySe4.

0 0

24 Sexp

CuCr1.79V0.08Se4

Sdiff Sph

S (μV/K)

16

180 270 T (K)

360

450

Fig. 9. Thermopower S vs. temperature T for CuCr1.49V0.45Se4 where Sexp is the experimental curve, Stheor. is the theoretical curve and Sdiff, Sph, Simp and Smag are diffusion, phonon drag, impurity and magnon drag components of thermopower, respectively.

Stheor

20

90

Simp Smag

12 8 4 0 0

90

180 270 T (K)

360

450

Fig. 7. Thermopower S vs. temperature T for CuCr1.79V0.08Se4 where Sexp is the experimental curve, Stheor. is the theoretical curve and Sdiff, Sph, Simp and Smag are diffusion, phonon drag, impurity and magnon drag components of thermopower, respectively.

and a phonon drag component, which rises as T3 below yD/10 (where yD is the Debye temperature) and falls as T  1 above approximately yD/2 [18]. This theory also predicts that the phonon drag is often

diminished by impurities [18]. The so-called the impurity component of TEP is described by the T1/2 law [20]. The term T1/2 comes from the variable range hopping between localized electronic states near Fermi level [20]. In magnetic materials the magnetic component of TEP may result from a coupling of the phonon system to the electron gas via the spin lattice (magnon gas) [21], or from the transfer of the magnon momentum to the electron gas (magnon drag) [22]. This magnetic contribution is described by the Bloch law, i.e. the spontaneous magnetization decreases with temperature at T3/2 when T is small [23]. The experimental curve shape of thermopower S(T) of the spinel polycrystals under study (Figs. 7–9) can be easily fitted by the modified Matoba, Anzai and Fujimori semiempirical formula [24] including the magnetic contribution [13] as follows  3=2 T I T 3 J aa FðyD Þ ef f 3 1=2 SðTÞ ¼ DUT þEUT þ þ HUT þ ð1Þ  5=2 G þðyTD Þ4 T K þ Jaa ef f

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E. Malicka et al. / Journal of Physics and Chemistry of Solids 73 (2012) 262–268

Table 5 ab Calculated magnetic parameters: X, yse, yde, B, Wd, Jaa, Jab, baa, bab, J aa ef f and J ef f of the CuCrxVySe4 spinel system. y/2 is the normalized V content connected with the following

normalization condition: x3 þ x4 þy/2¼ 1, x3 and x4 are the portions of the Cr3 þ and Cr4 þ ions, respectively, X is the mixture of spins due to the presence of Cr3 þ , Cr4 þ and V3 þ ions, yse is the superexchange contribution to yCW, yde is the double exchange contribution to yCW, B is the total hopping integral for the first and second coordination ab spheres, Wd is the mixed valence (Cr3 þ ,Cr4 þ ) bandwidth, Jaa and Jab, baa and bab, J aa ef f and J ef f are the superexchange, double exchange and effective integrals for the first two

coordination spheres, respectively. y/2 (V3 þ )

x3 (Cr3 þ )

x4 (Cr4 þ )

X

ySe (K)

yde (K)

B (K)

Wd (eV)

Jaa (K)

Jab (K)

baa (K)

bab (K)

Jaa ef f (K)

J ab ef f (K)

0 0.04 0.11 0.225

0.512 0.594 0.594 0.412

0.488 0.366 0.296 0.363

2.896 3.087 3.039 2.721

90 185 182 163

340 230 185 130

4391 2056 2077 1917

0.76 0.36 0.36 0.33

63.90 64.13 55.38 45.17

 9.65  8.63  7.21  5.72

332.65 155.76 157.35 145.23

65.53 31.15 31.47 29.05

466.51 418.64 359.95 292.74

 83.09  90.02  75.45  59.95

Values for the CuCr2Se4 matrix (y/2 ¼0.0) are taken from Ref. [28] for comparison.

Table 6 Fitting parameters: D, E, F, G, H, I and K in Eq. (1) of the thermoelectric power analysis of the CuCrxVySe4 spinel system. R is the agreement index and yD is the Debye temperature taken from Ref. [13]. y

D (mV/K2)

E (mV/K4)

F (mV/K)

G

H (mV/K1,5)

I (mV/K)

K

R (%)

yD (K)

0.08 0.22 0.45

0.03461 0.03588 0.03848

8  10  9 1  10  12 2.9  10  12

5.02882 4.59188 4.06066

0.25732 0.16506 0.14657

0.13776 0.10713 0.0456

0.42 0.4304 0.4383

0.084 0.06137 0.055

99.95 99.95 99.97

280 280 280

18

V content y 0.08 0.22 0.45

Sdiff (μV/K)

15 12 9 6 3 0 0

90

180 270 T (K)

360

450

Fig. 10. Diffusion component of thermopower Sdiff vs. temperature T for CuCrxVySe4.

4.5 4.0 3.5

Sph (μV/K)

where yD is the Debye temperature taken from Ref. [13], and J aa ef f is the effective exchange integral for the first coordination sphere. J aa ef f was estimated using the exchange Hamiltonian including the superexchange and double exchange parts and the high temperature expansion of the magnetic susceptibility described in detail in Ref. [13] and collected in Table 5. Others calculated magnetic parameters displayed in Table 5 will be also further discussed. First and second terms in Eq. (1) is the diffusion component where the small correction E is the temperature dependence of D [25]. In the case of strongly magnon–electron coupled systems, and so we characterize our CuCr1.6V0.4Se4 single crystal [13], we might have a linear magnon diffusive contribution to the first term of Eq. (1) [21], third term in Eq. (1) is the phonon drag component, fourth term in Eq. (1) is the impurity component, and the last term in Eq. (1) is the magnon drag component. D, E, F, G, H, I and K in Eq. (1) (listed in Table 6) are the curve-fitting parameters. The agreement index R between the experimental (Sexp) and the theoretical (Stheor.) thermopowers is high (over 99%, see Table 6). We have run fitting programs up to TC only and found that the results are that close to those covering the whole data range, that they are within the systematic error margin. From the results of the magnetic considerations presented in Table 5 it follows that with increasing vanadium content y: (1) contribution of the superexchange interaction, yse, to the Curie–Weiss temperature yCW shows maximum for y¼0.08, and (2) contribution of the double exchange interaction, yde, to the Curie–Weiss temperature yCW and the total hopping integral, B, as well as the Wd bandwidth of the 3d t2g band due to Cr3 þ and Cr4 þ ions decrease. It means that the presence of the V3 þ ions in the octahedral sites narrows the Wd bandwidth of the mixed valence [Cr3 þ ,Cr4 þ ] band, finally weakening the double exchange mechanism. It is also visible in more quickly diminishing values of integrals of the double exchange (baa and bab) than of the superexchange (Jaa and Jab). The effective exchange constants J aa ef f and Jab ef f for the first two coordination spheres are positive and negative, respectively, and the absolute value of J aa ef f is almost five times larger than Jab ef f . For comparison, the relevant calculated magnetic parameters [13] of the parent CuCr2Se4 metallic ferromagnet [6] are collected in Table 5. The results of TEP analysis of the CuCrxVySe4 polycrystalline spinels depicted in Figs. 10–13 show that the intensity of the magnon drag component (Smag) is the smallest in comparison

3.0 2.5 2.0 1.5 V content y 0.08 0.22 0.45

1.0 0.5 0.0 0

90

180

270

360

450

T (K) Fig. 11. Phonon drag component of thermopower Sph vs. temperature T for CuCrxVySe4.

E. Malicka et al. / Journal of Physics and Chemistry of Solids 73 (2012) 262–268

more vacancies than polycrystals. The phonon (Sph) and impurity (Simp) components play a smaller role. Sph shows a peak of 4 mV/K at 263 K for CuCr1.79V0.08Se4 and the Sph maximum slightly shifts to lower temperatures as V content increases (Fig. 11). The Sph peak is close to the theoretical temperature range yD/10–yD/2 predicted by the Debye theory [18]. Almost two times smaller value of the Sph maximum for all polycrystals than for CuCr1.6V0.4Se4 single crystal [13] suggests that the spin–phonon coupling seems to be weaker for the polycrystalline samples. The so-called impurity component of the thermopower (Simp) described by the T1/2 law [20] generates a maximum value of 2.83 mV/K at 420 K, which strongly decreases as the V content increases (Fig. 12). The Simp(T) dependence well correlates with the cation deficiencies (x þyo2) of the spinels under study.

3.0 V content y 0.08 0.22 0.45

Simp (μV/K)

2.5

267

2.0 1.5 1.0 0.5 0.0 0

90

180

270

360

450

T (K)

0.4

We have shown that in the CuCrxVySe4 ferromagnets the semiconductive-metallic transition is mainly caused by the carrier diffusion and phonon excitations accordingly to the spinphonon coupling, where the exchange subsystem and conduction electrons subsystem act as the intermediate reservoirs, although, in a case of CuCr1.6V0.4Se4 single crystal [13] this coupling was stronger. The grain boundaries and oxidation layers [10] in the spinel polycrystals under study dump the magnon excitations, which are usually connected with a double exchange mechanism coexisting with the superexchange interaction. For this reason the magnon excitations in the single-crystalline CuCr1.6V0.4Se [13] are ten times stronger than in the CuCrxVySe4 polycrystals. It was also observed, that the higher non-stoichiometry (xþ ya2), the higher impurity thermopower.

0.2

Acknowledgments

0.0

This work is partly founded from science Grant no. N N204 145938 and no. N N202 260939.

Fig. 12. Impurity component of thermopower Simp vs. temperature T for CuCrxVySe4.

1.0 V content y 0.08 0.22 0.45

0.8

Smag (μV/K)

4. Conclusions

0.6

0

90

180

270 T (K)

360

450

References Fig. 13. Magnon drag component of thermopower Smag vs. temperature T for CuCrxVySe4.

with other components. For the CuCr1.79V0.08Se4 spinel Smag reaches a peak of 0.72 mV/K at 119 K. As the V content increases the intensity of Smag increases and its maximum shifts to lower temperatures (Fig. 13). It means that the magnon excitations, usually driven by the double exchange mechanism, are observed only at low temperatures, where the transfer of the magnon momentum to the electron gas is more efficient. This effect is almost ten times stronger in the case of the single-crystalline CuCr1.6V0.4Se4 [13]. Contribution to the total TEP originates mainly from the diffusion component (Sdiff), which increases linearly with temperature, accordingly to the Mott formula [19] and reaches a maximum value of 16 mV/K at 430 K, independently on the V substitution (Fig. 10). Comparing with CuCr1.6V0.4Se4 single crystal the diffusion TEP for CuCr1.49V0.45Se4 polycrystal is different by a factor of 2 at 300 K. This difference is surprisingly large having weighed, that for the drag components one might assume a loss of transport efficiency due to the grain boundaries and eventual even a different one, as the structural and spin structural boundaries might be different. Partly it could be explained with the fact, that the positron annihilation studies carried out on single and polycrystals of the Zn1  xCuxCr2Se4 spinel series [26,27] showed the largest positron lifetime when dealing with the single crystals suggesting that they contained

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