Effect of Ni doping on magnetic and electrical properties of CuCr2Se4 single crystals

Journal of Alloys and Compounds 593 (2014) 158–162

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Effect of Ni doping on magnetic and electrical properties of CuCr2Se4 single crystals Izabela Jendrzejewska a,⇑, Paweł Zajdel b, Tadeusz Gron´ b, Henryk Duda b, Tadeusz Mydlarz c a

University of Silesia, Institute of Chemistry, Szkolna 9, 40-006 Katowice, Poland University of Silesia, Institute of Physics, Uniwersytecka 4, 40-007 Katowice, Poland c International Laboratory of High Magnetic Fields and Low Temperatures, Gajowicka 95, 53-529 Wrocław, Poland b

a r t i c l e

i n f o

Article history: Received 13 September 2013 Received in revised form 2 January 2014 Accepted 3 January 2014 Available online 10 January 2014 Keywords: Semiconductors Crystal growth Magnetisation Electrical and magnetic measurements X-ray diffraction

a b s t r a c t The X-ray studies showed that the new CuCr2xNixSe4 single crystals crystallized in the spinel structure  Magnetic measurements revealed that both the long and short-range with the space group Fd3m. magnetic interactions are comparable and do not significantly depend on the Ni-content. Electrical measurements showed that p-type metallic conduction, thermopower and power factor decrease as the Ni-content increases. These results are considered in a framework of the structural defects, double exchange mechanism and mixed valence band of the chromium ions. Ó 2014 Elsevier B.V. All rights reserved.

1. Introduction The chemical compounds on base of CuCr2Se4 doped by nickel were obtained in polycrystalline form in the two systems: Cu1xNixCr2Se4 and CuCr2xNixSe4. In the first system, Cu1xNix Cr2Se4, the pure spinel phase is formed only for x = 0.1, 0.2. In the second system, CuCr2xNixSe4, the spinel phase forms throughout the entire range (x = 0.1–1.9). For x = 0.1–0.9 pure spinel phase is formed [1]. However, up till now, there are no studies conducted on CuCr2Se4 single crystalline materials with Ni content. CuCr2Se4 is a ferromagnet with metallic conductivity and has the cubic spinel structure with normal cation distribution and  The reported lattice parameter symmetry of the space group Fd3m. 0 is in the range 10.321–10.337 Å A [2–4]. The high Curie and paramagnetic Curie–Weiss temperatures (TC = 460 K [5,6] and h = 465 K [7]) have been attributed to the strong ferromagnetic interactions between the collinear spins of chromium ions. However, the saturation magnetic moment is 4.94 lB/f.u. instead of 6 lB/f.u. expected for two Cr3+ ions [2,7]. This finding has been rationalized by the presence of the magnetic moment of about 1 lB with an opposite orientation [8–13]. The metallic conductivity of CuCr2Se4 has been accounted for delocalised charge carriers

⇑ Corresponding author. Tel.: +48 323591503. E-mail address: [email protected] (I. Jendrzejewska). 0925-8388/$ - see front matter Ó 2014 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jallcom.2014.01.010

[14]. Its p-type character has been deduced from the positive value of the Seebeck coefficient [6,15–18]. Many experimental and theoretical studies, aimed at determining the electronic structure and magnetic behaviour of the parent phase CuCr2Se4 [2–18] and of the spinels admixed with Ga3+ [19], In3+ [20], Co2+ [21] and Zn2+ [22] have been published. In the light of earlier investigations, it is interesting to study how substitution of lower magnetic Ni ions influences on magnetic and electrical properties of this class of single crystals. We present here a family of single crystals of CuCr2Se4 doped of Ni with various concentrations of Ni2+ substituent. In this paper, we report structure of single crystals and the results of magnetic and electrical studies with an aim to show the effect of nickel ions on physical properties of these spinels. 2. Experimental 2.1. Samples preparation and chemical composition The single crystals of CuCr2Se4 doped by nickel have been prepared from polycrystalline CuCr2xNixSe4-compounds for x = 0.2 and 0.3 using a chemical transport with iodine (I2) at concentrations of 6 mg per cm3 of volume of silica ampoule [23,24]. The exact quantities (2 g) of polycrystalline samples (CuCr1.8Ni0.2Se4 and Cr1.7Ni0.3Se4) were transferred quantitatively to the two quartz ampoules, which were evacuated to a pressure of 104 Pa and sealed. The experiments were carried out in ampoules with an outer diameter of 20 mm and a length of about 160 mm. A horizontal furnace with a melting zone temperature of 1173 K and crystallization zone temperature of 973 K was used. After 14 days of heating, the furnace was cooled in one day. The obtained single crystals had regular octahedral shape and

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I. Jendrzejewska et al. / Journal of Alloys and Compounds 593 (2014) 158–162 well-formed regular (1 1 1) faces with edge lengths up to 3 mm. The presence of nickel in the obtained single crystals was checked by the electron microscope method (SEM). This helped to select off the samples containing the Ni admixtures. Three single crystals with differing in Ni2+ content were chosen for X-ray diffraction measurements (see Fig. 1). The results of SEM are presented in Table 1.

Table 1 Chemical composition of CuCr2xNixSe4 –single crystals. x

0.2 0.3 0.3

2.2. Single crystal structure determinations Three samples with different nominal nickel content were polished to a nearly spherical shape in order to facilitate the X-ray absorption correction as the linear absorption coefficient for MoKa radiation amounts about 30 mm1. The diameters of the crystals chosen for the measurements on a Kuma KM-4 diffractometer were: / = 0.28, 0.32 and 0.35 mm. For each compound the unit cell dimensions were calculated using 19 centred reflections of higher order (99° 6 2h 6 123° range) and monochromatic Cu Ka radiation. Within three standard deviations the unit cells remained cubic (Table 2). The same samples were used for intensity data collection on a Kuma KM-4 diffractometer with molybdenum radiation (graphite monochromator). The other details of the crystal data together with summary of the experimental conditions and structure calculations are given in Table 2. The structure was refined by the full-matrix least-squares method using SHEL of the XL’97 program [24]. The origin of the unit cell was taken at the point 3m  (No. 227 in the International Tables for X-ray Crystallography). space group Fd3m The tetrahedral, octahedral and selenium sites were in 8a: (1/8, 1/8, 1/8), 16d: (½, ½, ½) and 32e: (x, x, x), respectively. For each composition the Ni cations were refined with coupled site occupation factors, both at the tetrahedral and octahedral positions. The results of the best models are given in Table 3.

2.3. Electrical and magnetic measurements The electrical conductivity r(T) was measured with the aid of the DC method using a KEITHLEY 6517B Electrometer/High Resistance Meter. The thermoelectric power S(T) was measured in the temperature range 300–600 K with the aid of a Seebeck Effect Measurement System (MMR Technologies, Inc., USA). The electrical and thermal contacts were made by a silver lacquer mixture (Degussa Leitsilber 200). Magnetic susceptibility of the single crystals was studied in the temperature range of 4.2–300 K and in magnetic fields up to 1 T. Magnetisation isotherms were measured at liquid helium temperature using a vibrating sample magnetometer with a step motor in applied external fields up to 14 T [26]. The magnetisation measurements were carried out over the temperature range 1.8–300 K using a Quantum Design SQUID – based MPMSXL–5-type magnetometer with magnetic field of 0.1 T. The Curie temperature, TC, was determined as the temperature corresponding to the extreme of dM/dT versus temperature T and the Curie–Weiss temperature, h, – from the relation: v1 = (T  h)/C, where M is the magnetisation and C is the molar Curie constant.

3. Results and discussion 3.1. Crystal structure and cation distribution The unit cell dimensions of the three compositions of the 0 system appeared to be longer than a = 10.321 Å A reported for non-substituted CuCr2Se4 [27]. However, the unit cells of the three samples differ only slightly of each other. This observation can be justified taking in view small differences in the ionic radii of Cu+, Cu2+, Ni2+ and Cr3+ (see Table 4) [28]. The small change of the unit cells and value of EV indicated that Ni2+ would rather accommodate the octahedral sites substituting Cr3+, than the tetrahedral sites. According to the assumption, the model assumed Ni2+ on the

Cu0.99Cr1.84Ni0.16Se4

%weight

Chemical composition

Cu

Cr

Ni

Se

13.31 13.16 13.04

19.53 18.70 17.75

2.46 3.55 4.91

64.70 64.58 64.30

Cu1.02Cr1.83Ni0.20Se4.0 Cu1.02Cr1.75Ni0.30Se4.0 Cu1.04Cr1.69Ni0.41Se4.0

octahedral sites with the starting value of Ni/Cr ratio equal to 0.2/0.8 has been considered for each composition. The refinement based on the this model of the cation distribution converged with mixed occupancies of Cr3+ and Ni2+ at the octahedral sites. The resulting site occupation factors (SOF) for the particular compositions are given in Table 3, where other structural data are also shown. It can be observed that the anion position parameter, u, does not deviate much from the ideal value x = 0.275. The u parameter is not influenced by nickel concentration, as the small differences are within the range of standard deviation. The same is seen from the list of metal–metal and metal-selenium distances (Table 5) where the differences are insignificant. Based on the structural data the formula describing cation distribution in the system is: Cu[Cr2xNix]Se4, where x = 0.16, 0.36 and 0.44. However, the X-ray scattering powers of Cu, Ni and Cr are nearly the same, therefore the cation distributions resulting from X-ray diffraction are charged with relatively large standard deviations. The chemical compositions, which are obtained from SEM and XRD method, are insignificantly different. Since, we did not observe the satellite reflections, it can mean that the Ni2+ ions do not create the long-range order and they are randomly distributed in the unit cell, substituting the Cr3+ ions (Table 1). Such replacing increases the bond distances and the bond angles in comparison with an appropriate bond lengths and bond angles in the matrix CuCr2Se4 [4]. It is known that cation distribution in spinel structure depends on several conditions, two of which are stoichiometry and individual site preference of the cations. The Cr3+ ion has the highest octahedral site preference of all the 3d metals (69 kJ/mol) [29,30] and is capable of forcing Ni2+ to move to tetrahedral sites in e.g. NiCr2O4, where high Ni2+ concentration leads to the Jahn-Teller tetragonal deformation of the spinel structure [31]. Similarly, NiCr2S4 and NiCr2Se4 form the monoclinic defect structure of NiAs-type [32,33]. In the case of full stoichiometry of Cu, the Ni2+ ions are forced to share the octahedral positions with Cr3+, as Ni2+, after the Cr3+ions has the next strong preference for the octahedral sites (37.7 kJ/mol) [29,30].

3.2. Electrical properties The results for electrical conductivity (r), thermopower (S) and power factor (S2r) are collected in Table 6 and illustrated in Figs. 2–4. All single – crystalline spinels under study are metallic

Cu0.98Cr1.58Ni0.36Se4

Cu0.98Cr1.46Ni0.44Se4

Fig. 1. Single crystals of CuCr2xNixSe4-system obtained by chemical vapour transport.

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Table 2 Details of the crystal data and a summary of X-ray data collection and structure refinement. Crystal data

(1)

(2)

(3)

Chemical formula (XRD) Chemical formula (SEM) Temperature (K) Crystal system Space group a (Å) Volume (Å3) Z, Density calc.(Mg/m3) Absorption coeff. (mm1)

(Cu)[Cr1.84Ni0.16]Se4 Cu1.02Cr1.83Ni0.20Se4.0 297 Cubic  Fd3m

(Cu)[Cr1.58Ni0.36]Se4 Cu1.02Cr1.75Ni0.30Se4.0 297 Cubic  Fd3m

(Cu)[Cr1.46Ni0.44]Se4 Cu1.04Cr1.69Ni0.41Se4.0 297 Cubic  Fd3m

10.3224(2) 1099.87(4) 8 5.844 34.22

10.2956(2) 1091.33(4) 8 5.862 34.63

10.3205(2) 1099.26(4) 8 5.80 34.42

Mo, Ka, 0.71073

Mo, Ka, 0.71073

Mo, Ka, 0.71073

51.86 22, 14 16, 22 22, 17

52.2 15, 22 15, 19 22, 14

52.02 21, 18 18, 15 22, 11

9016/507 Numerical 0.153, 0.041

8040/503 Numerical 0.121, 0.039

8062/505 Numerical 0.106, 0.030

343

311

342

11 1.008 0.0282 0.0525 0.00307(16) 1.44, 1.43

11 1.266 0.0177 0.0426 0.00142(8) 2.13, 1.26

11 1.007 0.0296 0.0462 0.00144(9) 1.91, 1.69

Data collection Radiation, wavelength (Å) hmax for data collection Limiting indices h k l Reflection collected/unique Absorption correction R(int) before, after absorption correction

Refinement Refinement method

Full-matrix least-squares on F2

Reflections >2r(I) Number of refined parameters Goodness of fit on F2 Final R indices [I > 2r(I)] R1 wR2 Extinction coefficient Largest diff. peak and hole (eÅ3)

Table 3 Structural characteristics of the spinel system (Cu)[Cr2xNix]Se4. The atom positions are: (A) site: 8(a): (1/8, 1/8, 1/8); [B] site: 16(d): (1/2, 1/2, 1/2); Se site: 32(e): (x, x, x). Thermal displacement amplitudes are given as the isotropic Uiso. Uiso 103(A2)

Compound

Anion parameter

Site occupation

u

(A)

[B]

On (A) site

On [B] site

Se

(Cu)[Cr1.84Ni0.16]Se4 (Cu)[Cr1.58Ni0.36]Se4 (Cu)[Cr1.46Ni0.44]Se4

0.25745(18) 0.25764(14) 0.25752(16)

1.0 1.0 1.0

0.92:0.08(9) 0.79:0.18(15) 0.73:0.22(12)

11.66(21) 11.21(23) 11.81(34)

6.96(24) 7.41(37) 7.86(30)

7.64(14) 7.70(12) 9.83(18)

Table 4 The ionic radii and site-preference-energy EV for ions of copper, nickel and chromium [27,28]. Ion +

Cu Cu2+ Ni2+ Cr3+

Ionic radius [Å]

EV [kJ/mol K]

0.60 0.57 0.55 tetra 0.69 octa 0.62

36.0 0.4 – 37.7 69.5

conductors. In Fig. 2, ln r(103/T) exhibits a strong temperature dependence and a drop in conductivity as the Ni-content increases.

In comparison with the CuCr2Se4 matrix [18,34] the electrical conductivity of the studied crystals is two orders of magnitude lower. The sign of thermopower is positive for all examined crystals (Fig. 3), suggesting that the metallic p-type conduction appears to be associated with an excess of cation vacancies from one side and what is more probable with the appearance of the mixed valence of the chromium ions from the other. The mixed valence band (Cr3+, Cr4+) is the main source of the increase of the hole concentration which was observed in the CuCr2Se4 matrix [34]. However, strong decrease of the power factor with the rise in the content of nickel (Fig. 4) suggests rather the hopping transport, magnon scattering and lower covalence of the studied materials as compared to classical thermoelectric semiconductors [18].

Table 5 0 Selected interatomic distances in A Å, and bond angles. Distances and angles

(Cu)[Cr1.84Ni0.16]Se4

(Cu)[Cr1.58Ni0.36]Se4

(Cu)[Cr1.46Ni0.44]Se4

Cu – Se Cr/Ni – Se Se - Cr/Ni - Se Cr - Se - Cr Cu - Se - Cr

2.3665(6) 2.5035(4) 93.58(1) 93.48(1) 122.77(1)

2.3690(5) 2.5076(4) 93.56(1) 93.45(1) 122.78(1)

2.3685(6) 2.5070(4) 93.56(1) 93.45(1) 122.78(1)

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Table 6 Magnetic and electrical parameters for (Cu)[Cr2xNix] Se4 single crystals in comparison to pure CuCr2Se4 [5–7,18,34]. TC and h are the Curie and Curie–Weiss temperatures, respectively, C is the Curie constant, leff and lsat are the effective and saturation magnetic moments, respectively, r is the electrical conductivity, S is the thermopower and S2r is the power factor at 300 K. Compound

TC (K)

CuCr1.84Ni0.16Se4 CuCr1.58Ni0.36Se4 CuCr1.46Ni0.44Se4 CuCr2Se4 Free Cr3+ ion Free Ni2+ ion

h (K)

355 360 375 460

7

leff (lB/f.u.)

C (emu K/mol)

372 371 356 465

3.997 3.558 3.303 2.723

5.66 5.34 5.14 4.67

r300K (S/m) 2

4.31 3.90 3.59 4.94 3.0 2.0

3.95  10 0.67  102 0.99  102 3.57  103

S300K (lV/K)

S2r (lW cm1 K2)

9.24 6.24 5.18 25

0.034 0.003 0.003 2.231

0.040

Cu0 .99Cr 1.84Ni0.1 6Se4

Cu0.99Cr1.84Ni0.16Se4

0.035

Cu0 .98Cr 1.58Ni0.3 6Se4 Cu0 .98Cr 1.46Ni0.4 4Se4

S σ (μWcm K )

6

Cu0.98Cr1.58Ni0.36Se4

0.030

Cu0.98Cr1.46Ni0.44Se4

-1

-2

5 4 3

0.025 0.020 0.015

2

ln [σ (S/m)]

lsat (lB/f.u.)

0.010

2

0.005

1

0.000 2.5

3.0 3

3.5

300

350

-1

Fig. 2. Electrical conductivity (ln r) vs. reciprocal temperature (103/T) of CuCr2xNixSe4 crystals.

9.0

40

Cu 0.98Cr1.58 Ni 0.36Se4

7.5 35

χ (emu/mol)

Cu 0.98Cr1.46 Ni 0.44Se4 20 15

6.0

30 25

4.5 20 15

10

10

5

H = 100 Oe Cu0.99Cr1.84Ni0.16Se4.0

3.0

Cu0.98Cr1.58Ni0.36Se4.0

1.5

Cu0.98Cr1.46Ni0.44Se4.0

5 0

300

350

400

500

Fig. 4. Power factor S2r vs. temperature T of CuCr2xNixSe4 crystals.

Cu 0.99Cr1.84 Ni 0.16Se4

25

S ( µV/K)

450

45

30

0

400

T (K)

10 /T (K )

450

500

T (K) Fig. 3. Thermoelectric power S vs. temperature T of CuCr2xNixSe4 crystals.

Materials with a large power factor value (S2r) are usually heavily doped semiconductors, such as Bi2Te3 [18]. In this context a matrix CuCr2Se4 has the most promising thermoelectric properties. 3.3. Magnetic properties The results for magnetic studies are collated in Table 6 and illustrated in Figs. 5 and 6. The effective magnetic moments estipffiffiffi mated from equation: leff ¼ 2:83 C , where C is the molar Curie constant taken from experiment (see Table 5) correspond to the qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi effective number of Bohr magnetons peff ¼ ð1  xÞp2Cr3þ þ xp2Ni2þ ,

1/χ (mol/emu)

0 2.0

0

60

120

180

240

300

360

0.0 420

T (K) Fig. 5. Molar susceptibility v (full symbol) and reverse molar susceptibility 1/v (empty symbol) vs. temperature T at 100 Oe for CuCr2xNixSe4 crystals. A dotted (black) line indicates a Curie–Weiss behaviour.

pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi where p ¼ 2 SðS þ 1Þ for a Cr3+ ion (S = 3/2) with 3d3 and for a Ni2+ ion (S = 1) with 3d8 electronic configurations. The values of peff are 5.37, 5.16 and 5.04 for x = 0.16, 0.36 and 0.44, respectively. It means that the orbital moment of both magnetic ions is totally quenched due to strong crystalline electric field interactions. All single crystals under study are ferromagnets with the large and comparable values of Curie (TC) and Curie–Weiss (h) temperatures. It means that both the long and short-range magnetic interactions are comparable and do not significantly depend on the Ni-content in a sample. Therefore, the double exchange

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References

4.5 4.0

[1] [2] [3] [4]

M (μB/f.u.)

3.5 3.0

[5]

2.5 [6] [7]

2.0 1.5

T = 4.2 K Cu0,99Cr1,84Ni0,16Se4

[8] [9]

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Cu0,98Cr1,58Ni0,36Se4

[10]

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Cu0,98Cr1,46Ni0,44Se4

0.0

0

2

4

6

8

10

12

14

μ0H (T) Fig. 6. Magnetic isotherms for CuCr2xNixSe4 crystals.

mechanism, resulting in the jump of an electron between Cr3+ and Cr4+ ions [34], strongly contributes to the p-type metallic conduction in the spinel under study. Slight differences between leff and peff can also provide about the mixed valency of the chromium ions.

[11] [12] [13] [14] [15] [16] [17] [18] [19]

4. Conclusions

[20]

CuCr2xNixSe4 single crystals (where x = 0.16, 0.36 and 0.44) were examined for magnetic and electrical properties. All single crystals are the normal spinels, ferromagnets and p-type conductors. Substitution of nickel in place of chromium does not influence on magnetic properties indeed, however strongly to electric properties, leading to the increase of the hopping transport, magnon scattering and ionic bond of the studied materials.

[21]

Acknowledgments This work was partly supported by Ministry of Scientific Research and Information Technology (Poland) and funded from science resources: No. 1S-0300-500-1-05-06. Two of us (I.J. and P.Z.) are also indebted to the National Science Centre for support from science grant (Project No. N N204 151940).

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