Synthesis and high temperature transport properties of newquaternary layered selenide NaCuMnSe2V. Pavan Kumar, U.V. Varadaraju n

Department of Chemistry, Indian Institute of Technology Madras, Chennai 600036, India

a r t i c l e i n f o

Article history:Received 24 October 2013Received in revised form8 January 2014Accepted 12 January 2014Available online 20 January 2014

Keywords:Quaternary selenideVariable range hoppingThermoelectricity

a b s t r a c t

Synthesis and high temperature transport properties of NaCu1þxMn1�xSe2, (x¼0�0.75) a newquaternary layered selenide, are reported. NaCuMnSe2 crystallizes in a trigonal unit cell with spacegroup of P-3m1 (a¼4.1276 Å, c¼7.1253 Å). The isovalent substitution of Mn2þ by Cu2þ is carried out. Allthe compositions show semiconducting nature, whereas the Seebeck coefficient increases gradually overthe entire measured temperature range. Compositions with x¼0 and 0.025 follow thermally activatedbehavior. With increase in copper concentration the conduction mechanism transforms to 2D variablerange hopping (VRH) for x¼0.05 and 0.075.

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1. Introduction

Thermoelectric modules are solid state devices which arecapable of converting heat into electricity and vice versa. Theperformance of a thermoelectric material is governed by figure ofmerit ZT¼S2sT/k where S, s,k and T are Seebeck coefficient(thermopower), electrical conductivity, thermal conductivity andabsolute temperature respectively [1,2]. Intense efforts are on tosearch for new thermoelectric materials which have high figure ofmerit (ZT) by maximizing the power factor (S2s) and/or minimiz-ing the thermal conductivity k [3–6]. Compounds with CaAl2Si2-type structure, e.g., CaZn2Sb2, are known to be good thermo-electric materials. The structure is amenable to crystal chemicalsubstitution at both cationic as well as anionic sites, and thusprovides a handle to tune the electronic transport properties.Appropriate substitution of ytterbium at the calcium site leads to amaximum ZT of 0.55 at 773 K [7]. This motivated us to look fornew compounds with CaAl2Si2-type structure and to study trans-port properties.

It is known that insertion of alkali metal ions into the threedimensional chalcopyrite CuFeS2 gives new layered quaternarysulfides ACuFeS2 (A¼Liþ , Naþ , Kþ , Rbþ , Csþ). Two crystallographicstructures have been observed depending on the size of the alkalimetal ion. The insertion of smaller alkali metal ions like Liþ and Naþ

leads to the formation of trigonal CaAl2Si2-type structure. Thestructure consists of hexagonal close-packed sulfur anions withcopper and iron atoms occupying the tetrahedral sites and the alkali

ions occupying the octahedral sites in the van der Waals gap [8]. Theinsertion of larger alkali metal ions like Kþ , Rbþ and Csþ leads to theformation of ThCr2Si2-type structure. LiCuFeS2 is well studied forelectrochemical applications as cathode material mainly because ofits two dimensional layered structure from which lithium can bereversibly intercalated and deintercalated [9]. Oledzka et al. havereported the synthesis of new layered quaternary sulfides withthe general formula NaCuMS2 (M¼Mn, Fe, Co, Zn) crystallizing inCaAl2Si2-type structure with trigonal unit cell; Cu and M aredistributed randomly in the tetrahedral site and they also studiedtheir electronic properties. All the compositions showed semicon-ducting behavior with the resistivity values varying from 6.2�10�1

to 5�10�2Ω cm at RT [10]. Other than these compounds, very fewchalcogenides are knownwith CaAl2Si2-type structure like AMnMTe2(A¼Liþ , Naþ and M¼Cuþ , Agþ) and Cu1.45Er0.85S2, which is adouble cation deficient compound [11,12]. This prompted us to lookfor new selenide analogs. In the present study, we have synthesized anew quaternary selenide, NaCuMnSe2, and studied the high tem-perature electronic transport properties. We have also varied the Cu:Mn ratio to study the influence on the transport properties. Ourattempts to synthesize the Fe and Co based analogs lead to theformation of impure phases which are moisture sensitive.

2. Experimental section

2.1. Synthesis

NaCu1þxMn1�xSe2 (0rxr0.075) compounds were synthe-sized by solid state reaction of stoichiometric amounts of Na(99.8 %, Alfa Aesar), Cu (99.9 %, LobaChemie), Mn (99.9 %, Alfa

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0022-4596/$ - see front matter & 2014 Elsevier Inc. All rights reserved.http://dx.doi.org/10.1016/j.jssc.2014.01.016

n Corresponding author. Tel.: þ91 4422574201.E-mail address: varada@iitm.ac.in (U.V. Varadaraju).

Journal of Solid State Chemistry 212 (2014) 64–68

Aesar) and Se (99.99 %, Alfa Aesar) powders in carbon coatedevacuated fused silica quartz ampoules. The ampoules were slowlyheated up to 1073 K at a heating rate of 2 K min�1, held for 48 hand furnace cooled to room temperature. The powders werefinally densified in a spark plasma sintering furnace at 550 K in agraphite die (12 mm) under a pressure of 50 MPa, holding for5 min in vacuum. The spark plasma sintering resulted in compactswith bulk densities 497%.

2.2. Physical characterization

Room temperature powder X-ray diffraction (PXRD) data werecollected on a D8 Bruker X-ray diffractometer equipped with CuKαradiation (λ¼1.5405 Å) employing a scan speed of 0.031 per secondin the 2θ range from 51 to 801 for all the synthesized bulk samples tocheck the phase purity and identity. Differential scanning calorimetry(DSC) studies were performed using a Netzsch DSC 204. Opticalabsorption spectra of samples were carried out at room temperatureusing a UV–visible spectrophotometer with a 150 mm integratingsphere attachment (V-560, JASCO).

2.3. Electrical transport properties

Thermopower and electrical resistivity were measured simulta-neously under helium atmosphere using a ZEM-3 Seebeck coeffi-cient/electrical resistivity measurement system (ULVAC-RIKO, Japan).Samples for transport measurements were cut into rectangularbars of size 8�3.5�3 mm3 using a low speed diamond wheelsaw (South Bay Technology, USA). The bars were placed verticallybetween two platinum electrodes with two Pt/Pt–Rh thermocouplesattached on one side. Transport properties were measured fromroom temperature to 700 K under an atmosphere of helium. Roomtemperature Hall measurement is done by using a LakeshoreHall effect system (model no 7604) under a reversible magnetic fieldof 5 kG.

3. Results and discussion

3.1. Phase formation and stability

Phases with nominal compositions NaCu1þxMn1�xSe2; x¼0,0.025, 0.05 and 0.075 were synthesized. All the synthesized phasesare black in color with a reddish tint and are stable in air. ThePXRD patterns (Fig. 1a) indicate that all the phases crystallize inthe CaAl2Si2-type structure. The patterns were indexed on thebasis of a trigonal unit cell with space group P-3m1. We observe aminor impurity of MnSe which is marked as n in the PXRD patternof the stoichiometric composition. The expanded PXRD pattern inthe 2θ range of 32º–34º is shown in Fig. 1b. It is interesting to notethat this impurity is removed on increasing the concentration ofcopper. Even with a slight increase i.e. for x¼0.025, a pure phase isobtained. The lattice parameters obtained from pattern matchingare a¼4.1276 Å, c¼7.1653 Å for the parent phase. The values donot vary significantly for the other compositions because of thelow dopant concentrations. It is worth pointing out that the latticeparameters for the corresponding sulfide phase viz., NaCuMnS2are a¼3.9485 Å, c¼6.959 Å [10]. The pattern matching profile forthe composition NaCu1.075Mn0.925Se2 shown in Fig. 2 (inset showsthe crystal structure of NaCuMnSe2) indicates the phase purity.

The thermogravimetric analysis reveals that significant weightloss occurs above 873 K. We have, therefore, restricted our studies to700 K; the thermopower and resistivity measurements have beencarried out from room temperature to 700 K. Representative differ-ential scanning calorimetry (DSC) data for second cycle heating forNaCu1.075Mn0.925Se2 is presented in Fig. 3. The studies reveal two

transitions near 400 K and 500 K as well as endothermic peaks at590 K and 623 K. The reasons for these transitions are not clear atthis point. Fig. 4 shows a plot of normalized (1�R)2/2R vs photonenergy for NaCuMnSe2 at room temperature, where R is thereflectance. The optical band gap Eg of NaCuMnSe2 is found to be2.09 eV, which indicates the wide band gap nature.

3.2. Transport properties

Fig. 5(a) and (b) displays the temperature dependence of resis-tivity ρ and Seebeck coefficient S, for NaCu1þxMn1�xSe2 (0rxr0.075). All the compositions show p-type conduction, indicating thatholes are the majority charge carriers. The room temperatureresistivity and the Seebeck coefficient at the parent phase composi-tion are relatively high and remain high over the entire temperaturerange over which the measurements were carried out. The roomtemperature resistivity and the Seebeck coefficient values decreasewith increase in copper concentration. The phase with x¼0.075composition, however, has marginally higher values. At RT, theresistivity for x¼0.05 composition is found to be 0.13Ω cm whichis lesser compared to that of the parent phase (0.63Ω cm). Asmentioned earlier the Cu and Mn atoms are present at the same site(tetrahedral coordination with respect to Se), albeit distributedrandomly. The excess copper (x) is expected to be present in theþ2 oxidation state to maintain overall charge neutrality of thecompound. This could probably be the reason for the observedincrease in carrier concentration with increase in ‘x’. However, thesudden decrease in carrier concentration and increase in mobility forx¼0.075 is puzzling. Further studies on Hall effect measurements asa function of temperature and/or high temperature XRD studies areneeded.

The Seebeck coefficient value also decreases from 84 mV K�1

(x¼0) to 42 mV K�1 (x¼0.05). NaCuMnSe2 shows a maximum See-beck coefficient value of 297 mV K�1 at 675 K. All the phases exhibitsemiconducting behavior in the temperature range of investigation,as evidenced from the ρ vs T plots. An interesting feature is that eventhough all the phases show semiconducting behavior, the Seebeckcoefficient value is found to increase with increasing temperature.Such behavior is noticed earlier in some cobalt oxides with misfitstructures [13–15] and more recently Bhattacharya et al. have obser-ved a similar kind of behavior in CuCrSe2 [16]. From the Seebeckcoefficient vs temperature plots, it is clear that there is a change inslope at nearly the same temperatures as observed in the DSC data.We have carried out room temperature Hall measurements toascertain the carrier concentration and mobility which is given inTable 1. The measurements indicate that the parent NaCuMnSe2 hascarrier concentration of 2.8�1018 cm�3 and the mobility value is2.85 cm2 V�1 s�1. The carrier concentration increases and mobilitydecreases with increase in copper concentration i.e. for compositionswith x¼0.025 and x¼0.05; for x¼0.075, however the carrierconcentration decreases and mobility increases by nearly one orderof magnitude. Fig. 5(c) shows the temperature dependence of powerfactor values for all the compositions. The lowest resistivity, observedfor NaCu1.05Mn0.95Se2, is 0.05Ω cm at 540 K leading to a maximumpower factor of 6.4�10�5 Wm�1 K�2 at 700 K.

3.3. Analysis of the conduction mechanism

For all the doped compositions the resistivity shows almosttemperature independent behavior above 500 K, indicating a heavilydoped semiconductor behavior. A linear fit for the ln s vs 1000/T,shown in Fig. 6. for the pristine NaCuMnSe2 in the temperature rangefrom RT to �490 K, indicates that the bulk conduction is thermallyactivated. The phase with x¼0.025 also shows a thermally activatedbehavior. Phases with higher copper content (x¼0.05, 0.075) exhibitdeviations from the straight line behavior in the ln s vs 1000/T plots.

V. Pavan Kumar, U.V. Varadaraju / Journal of Solid State Chemistry 212 (2014) 64–68 65

The data has power law dependence with temperature and can beanalyzed by following the variable range hopping conductionmechanism. In disordered materials, when carriers are localized nearthe Fermi level the carriers can hop from initial state to final statewhich is spatially separated by different distances.

Such variable range hopping conduction is given by [17]

s¼s0e�ðT0=TÞP ð1Þwhere s0 is a constant, T0 is the characteristic temperature and theexponent P is 1/4, 1/3, and 1/2 for 3D, 2D and 1D systems,respectively. Fig. 7. shows the linear fit of ln s vs 1=T1=3 for thecompositions x¼0.05 and x¼0.075 in the temperature range fromRT to �490 K. The good fit of the data vis a vis thermally activatedhopping (ln s vs. 1000/T) implies that the electronic transportfollows the 2D VRH mechanism. From the slope of linear fit ofln s vs 1=T1=3, we can deduce T0. The deduced values of T0 forx¼0.05 is 2.59�104 K and for x¼0.075 is 4.3�104 K. The increasein slope T0 with increasing x from 0.05 to 0.075 indicates thedisorderliness [18,19].

The characteristic temperature T0 is related to the electronicdensity of states at the Fermi level N(EF) and the radius of thelocalized states “a” (¼α�1), also known as localization length; fora two dimensional variable range hopping, it is given by therelation

T0 ¼ 3=a2 kBNðEFÞ� � ð2Þ

where kB is the Boltzmann constant. Localization length is eval-uated by using the ionization energy Ei and intrinsic permittivity κ.Ei obtained from high temperature activation energy of theintrinsic conduction of NaCuMnSe2 which is about 0.065 eV [20].The intrinsic permittivities of many chalcopyrites are in the rangeof 7–15. We take the intrinsic permittivity value of CuAlSe2 which

Fig. 1. (a) Powder X-ray diffraction pattern for NaCu1þxMn1�xSe2 (0rxr0.075) and (b) their magnified region in the 2θ range of 32–341.

Fig. 2. Pattern matching profile of powder X-ray diffraction pattern forNaCu1.075Mn0.925Se2 (inset shows the crystal structure of NaCuMnSe2).

Fig. 3. DSC data for NaCu1.075Mn0.925Se2, indicating phase transitions.

Fig. 4. Electronic absorption spectra for NaCuMnSe2.

V. Pavan Kumar, U.V. Varadaraju / Journal of Solid State Chemistry 212 (2014) 64–6866

is about 8.5, since it is structurally related to the pristinecompound [21].

a¼ 7:2=Eiκ ¼ 13:0 A ½17;20� ð3Þ

The calculated value of N(EF) is 5.3�1013 cm�2 eV�1 forx¼0.05 and 3.2�1013 cm�2 eV�1 for x¼0.075.

The most probable hopping distance R is given by

R¼ a=3 T0=T� �1=3 ð4Þ

The hopping distance R increases from 16.3 to 19.0 Å forx¼0.05, and for x¼0.075 it increases from 19.3 to 22.6 Å as

temperature decreases from 492 to 303 K. The activation energyW for VRH can be derived by using the equation Ea¼�d(ln sÞ/d(1/kT) and by substituting for s from Eq. (1):

W ¼ 0:33kBT1=30 T2=3 ð5Þ

Fig. 5. Temperature dependence of (a) resistivity, (b) Seebeck coefficient and (c) power factor (PF) for NaCu1þxMn1�xSe2 (0rxr0.075) phases over the temperature rangeof 300–700 K.

Table 1Room temperature Hall measurements for NaCu1þxMn1�xSe2 (0rxr0.075).

Composition(NaCu1þxMn1�xSe2)

Carrier density(cm�3)

Mobility(cm2 V�1 s�1)

x¼0 2.8�1018 2.85x¼0.025 7.0�1018 2.18x¼0.05 3.6�1019 1.15x¼0.075 5.7�1017 11.6

Fig. 6. Plot of ln s vs 1000/T for x¼0 and x¼0.025 over the temperature of 300–500 K.

V. Pavan Kumar, U.V. Varadaraju / Journal of Solid State Chemistry 212 (2014) 64–68 67

W and R are calculated at different temperatures as shown inFig. 8. for the compositions x¼0.05 and x¼0.075. The activationenergyW increases from 37 to 51 meV for x¼0.05 and for x¼0.075

also W increases from 44 to 61 meV as temperature increases from303 to 492 K. The systematic increase in activation energy forhopping implies that due to the increase in disorder more energyis needed for the carriers to hop from initial state to the final state.The calculated values of R and W at different temperatures followthe necessary criteria for VRH i.e. W4kBT and αR41. With incre-asing dopant concentration from x¼0.05 to x¼0.075, the hoppingdistance R and the activation energy increase, and hence resistivityvalue for x¼0.075 also increases when compared with x¼0.05.This indicates that with increase in copper concentration thecarrier concentration decreases (Hall measurement) and the com-pound becomes more disordered which causes the increase inresistivity for x¼0.075.

4. Conclusions

In conclusion, we have synthesized a new quaternary selenide andstudied its high temperature transport properties. We have achievedformation of pure phase starting with an off-stoichiometric composi-tion. The phase exhibits high thermopower, satisfying the primaryqualification to be a good thermoelectric. We have attemptedimprovements in electrical conductivity by altering the carrier con-centration through crystal chemical substitution and achieved limitedsuccess. We also observed a compositionally induced change in theconduction mechanism from thermally activated behavior to 2Dvariable range hopping (VRH). Though all the compositions showedhigh Seebeck coefficient values, the calculated power factor is low dueto low electronic conductivity of the phases. Further studies are under-way to create ordering at Cu/Mn site which could lead to the reductionin the resistivity values there by increasing the ZT.

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Fig. 7. Plot of ln s vs 1=T1=3 for x¼0.05 and x¼0.075 over the temperature range of300–500 K.

Fig. 8. Plot of activation energy for hopping (W) and hopping distance (R) for(a) NaCu1.05Mn0.95Se2 and (b) NaCu1.075Mn0.925Se2.

V. Pavan Kumar, U.V. Varadaraju / Journal of Solid State Chemistry 212 (2014) 64–6868