Investigation of magnetic, optical and electrical properties of La3Ba3Cu6O14 synthesized by molten flux method

JOURNAL OF RARE EARTHS, Vol. 33, No. 12, Dec. 2015, P. 1341

Investigation of magnetic, optical and electrical properties of La3Ba3Cu6O14 synthesized by molten flux method K. Agilandeswari1, A. Ruban Kumar2,* (1. Department of Chemistry, Auxilium College, Vellore, Tamil Nadu 632006, India; 2. School of Advanced Sciences, VIT University, Vellore, Tamil Nadu 632014, India) Received 27 May 2015; revised 8 October 2015

Abstract: The great deal of novel rare earth based semiconducting lanthanum barium copper oxide La3Ba3Cu6O14 (LBC-336) were synthesized by low temperature molten salt synthesis (MSS) due to “dissolution precipitation mechanism”. Here, we reported one pot synthesis of product by direct precipitation from a molten KOH-NaOH mixture at 450 ºC and single phase La3Ba3Cu6O14 with tetragonal crystal system. The particle size of 140–200 nm were observed in both PXRD pattern and HRSEM micrographs and it showed a cubic morphology. The semiconducting nature was extracted from various parameters like optical band gap (1.8 eV), AC conductivity (0.70 eV), DC conductivity (0.70 eV), and also Hall effect parameters like the charge carrier concentration values n=6.0×1026 m–3 and it proved as a p-type semiconductor. The electrical phase trasition temperature from ferroelectric to antiferroelectric system (Tm=420 K) and anti-ferroelectric – paraelectric system (depolarization temperature Td=673 K) which attributed to the space charge polarization contributed to the conduction mechanism. The magnetic phase transitions were from ferromagnetic to ferrimagnetic system (Curie temperature (Tc=70 K)) and it led to soft magnetic material and also held good for superconductor application upto 70 K. Keywords: molten flux synthesis; band gap; phase transitions; semiconductor; superconductors; rare earths

Lanthanum-barium-copper oxygen deficient pervoskite attracted a great deal of interest in the recent years, due to the unusual metal-insulator transition properties. LBC336 system has efficient application as transport properties and also acts as a good catalyst for oxidation of carbon monoxide. La3Ba3Cu6O14 was first synthesized by solid state method in the air, for 24 h at 1000 ºC[1–3]. The electrical properties depend on structure, composition, oxygen partial pressure and it leads to both metallic, semiconducting behavior of the LBC-336[3,4]. La3Ba3Cu6O14 has one sixth of copper atoms in the ideal composition is in trivalent state. Additional oxygen is located at the centre of the unit cell, converting two neighboring Cu(II)O5 square pyramidal polyhedron into corner linked Cu(III)O6 octahedron. This may then inhibit movement of electrons along the chains suppressing superconductivity. Wide superconducting transitions may be related with the optimal preparation method[5,6]. Rare earth (RE) lanthanum based ceramic magnetic material has wide industrial applications like aerospace, automotive, electronics, medical, and military. The rare earth magnetic materials are essential ingredients in these high performance magnets. Based on intermetallic behavior is due to the extremely high magneto crystalline anisotropy made possible by unique 3d-4f interactions between transition metals and rare earths. In recent years, the fea-

sibility of a low temperature technique as molten salt synthesis has been investigated. MSS has been considered as a good technological process for preparing inorganic materials as well where the synthetic temperature can be greatly lowered, and the diffusion speeds of reacting constituents can be obviously accelerated due to the incorporation of salt medium. So products generally possess a good crystalline and different morphology[7–14]. In this work, the molten salt technique has been used to synthesize La3Ba3Cu6O14 at low temperature using easily available oxide raw materials and optical properties have been studied. To our knowledge La3Ba3Cu6O14 powder, prepared by this method has ever been reported. AC-conductivity observed for the function of temperature from room temperature to 673 K indicates that the space charge polarization contributes to the conduction mechanism. The AC conductivity data have been used to estimate the apparent activation energy and minimum hopping length[15]. The important parameters such as activation energy, carrier concentration, ionic mobility, etc., were determined by four probe resistivity and Hall effect measurement. The temperature dependent magnetic study proves the soft ferrimagnetic nature of the material. This molten hydroxide method represents a useful technique for the preparation of rare earth compounds like lanthanum based pervoskite.

* Corresponding author: A. Ruban Kumar (E-mail: [email protected]; [email protected]; Tel.: +91-04162202353) DOI: 10.1016/S1002-0721(14)60567-6

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1 Experimental

were indexed, and the sharpness of the diffraction peaks indicates that the materials have good crystalline nature and are in good agreement with that of the JCPDS file (74-0761). The PXRD pattern reveals that the crystal system belongs to tetragonal crystal system and its lattice parameters are a=b=0.55257 nm, c=1.17462 nm consistent with those reported in JCPDS (card No. 74-0761) as shown in Fig. 1(b). Hence, the La3Ba3Cu6O14 compound was successfully formed at low temperature compared to early reports. The synthesized powder particle size was estimated using the Scherrer Eq. (1) through PXRD diffraction pattern. D=Kλ/βcosθ (1) Where, D–crystalline size (nm); K–shape factor (K=0.9 constant value), λ–wavelength of X-ray, β–full width at half maximum (FWHM). The calculated average particle size lies between 140–200 nm.

The high purity analytical-grade starting materials were lanthanum oxide, barium hydroxide, copper oxide, potassium hydroxide, and sodium hydroxide taken in stoichiometric ratio (1.5 mol/L La2O3, 6 mol/L CuO and 3 mol/L Ba(OH)2·8H2O) and ground in an agate mortar for 4 h. The mixed fine powders were added into the 1:1 molar ratio of NaOH and KOH flux for reaction recrystallized alumina crucible for 2 h at 170 ºC in a silica carbide furnace (INDFUR furnace heating rate 10 ºC/min). Initially, we observed clear blue solution and future increasing the temperature slowly up to 450 ºC the blue color solution changed into black in color and kept the same environment to 6 h for homogenous reaction. The synthesized black powder was washed with deionized water and dried in a hot air oven at 60 ºC and the process was repeated for 6 times to harvest single phase La3Ba3Cu6O14 powder.

2.2 FTIR studies

2 Results and discussion 2.1 PXRD analysis The powder X-ray diffraction (PXRD) patterns were recorded on a Bruker D8 advanced instrument with a scan speed 0.3 s with Cu Kα (λ=1.5406). Fig. 1(a) shows that the PXRD pattern of La3Ba3Cu6O14 was obtained as pure single phase without any impurities, and this graph was exactly coincident with early reports. The peaks

Fig. 1 XRD patterns of La3Ba3Cu6O14 (a) and La3Ba3Cu6O14 (b) compared with standard JCPDS data

Since the inorganic compound has board and stretched peaks behaviour and it is strongly relates to the particle size, phase and morphology of the compounds. So, we study through the recorded FTIR spectrum, as shown in the Fig. 2. The synthesized compound was mixed with KBr powder in the 1.5:0.5 molar ratio and made into 12 mm circular pellet which were subjected to Fourier transform infra red (FTIR) analysis in range 400–4000 cm–1 using JASCO 400 spectrometer. The FTIR spectrum of La3Ba3Cu6O14 possesses two board and strong bands at 3493 and 3149 cm−1 assigned to the stretching vibrations of the bulk OH− ions group due to the presence of a small amount of water adsorbed on the surface. The small band at 1625 cm−1 is due to the OH bending of water in the surface and compensates the oxygen deficiency of synthesized compound. A sharp peak was observed near 1400 cm–1. It indicates that lanthanum (La2+) compounds are exposed to ambient condition and the process of carbonation occurs. It leads to be the formation of carbonates on the surface. The small intensity peak at 571 cm–1 reveals that group of La–O, Cu–O

Fig. 2 FTIR pattern of La3Ba3Cu6O14

K. Agilandeswari et al., Investigation of magnetic, optical and electrical properties of La3Ba3Cu6O14 synthesized by …

and Ba–O with the covalent band and oxidation state of the title compound affects the intensity in visible region. All these observed peaks confirm the presence of functional group in the title compound. 2.3 SEM and EDAX analysis The morphology and chemical composition of crystalline phases were obtained by using an FEI Quanta FEG 200-High resolution scanning electron microscope (HRSEM). Fig. 3 shows the HRSEM and EDX image of LBC-336 compound. From the image, we observed a same cubical morphology for various scale regions because La3Ba3Cu6O14 belongs to canted pervoskite type structure in which CuO6 octahedral and atoms are bonded through corner-sharing, so it leads to the formation of the cubic morphology in the particles. Hence, it proves that each particles have small cube in shape, size in micro region and these arranged in an irregular pattern. The EDAX spectrum of La3Ba3Cu6O14 is shown in Fig. 3(e). The data were collected at three different locations on the sample as arrow mark represented in Fig. 3. Fig. 3(e) shows one of the EDAX spectra and the result is tabulated in Table 1. The results of EDAX analysis exhibit that the weight ratio of entitled compound as per the intended stoichiometry composition which implies that the composition of La3Ba3Cu6O14 is synthesized at low temperature. 2.4 Diffuse reflectance spectroscopy The optical absorption spectrum of La3Ba3Cu6O14

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Table 1 EDX chemical composition Element

Calculated/wt.%

Experimental average/wt. %

OK

18.40

18.45

Ba L

27.30

27.35

La L

29.30

29.31

Cu K

25.00

24.89

100

100

powder was recorded in the range of 200–1100 nm using an Jasco V-670-UV-Visible diffused reflectance spectrometer and optical band gap were determined using tauc’s plot shown in Fig. 4(a) and inset Fig. 4(b). Absorbance is an important phenomena to reveal the information about structural and oxidation state and in visible region most of the canted pervoskite inorganic systems involves promotion of the electrons in π-π, π* and n orbital from the ground state to higher excitation states. The graph gives clear information about high absorbance nature in visible region and after visible region sudden drop in graph due to very little absorbance in IR region after 845 nm. The optical band gap (Eg) of La3Ba3Cu6O14 can be obtained in Tauc’s relation by Eq. (2). (αhv)n=A(hv−Eg) (2) Where, α–the absorption coefficient, A–constant, Eg– band gap and exponent n depends on the type of transition. For n is directly allowed transition, or indirect allowed transition or direct forbidden transition, it values are 1/2, or 2, or 3/2, respectively[16–18]. Inset Fig. 4(b) shows Tauc relation by plotting (αhν)2 against hν, by extrapolating the curve to photon energy axis and the optical band gap was calculated using fit straight line near linear changes in the plot. The optical absorption plot shows energy band gap as 1.8 eV because ligand to metal charge transfer (LMCT) takes place from O2– to Cu2+. Hence d-d charge transfer takes place in the lanthanum barium copper system. It lies a semiconductor region. 2.5 Electrical resistivity studies Electrical resistivity plays a major role in technical ap-

Fig. 3 HRSEM images of La3Ba3Cu6O14 (a–d) and EDX spectra of La3Ba3Cu6O14 (e)

Fig. 4 UV-visible absorbance pattern of La3Ba3Cu6O14 (a) and DRS band gap spectra of La3Ba3Cu6O14 (b)

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plications point of view and it is esteemed by common well-known DC Four probe method. It gives information about resistivity and DC conductivity of the material with respect to temperature. La3Ba3Cu6O14 powder is pressed into a pellet with a diameter of 12 mm and coated high temperature sliver paste and fired at 473 K for 10 min. The constant applied current (I) 8 mA was applied to the sample with respect to varing temperature range between 300–473 K the resistance were measured. This method has major advantage, it can eliminate the effects of contact resistance between the sample and electrical contacts and therefore is most suitable for low and accurate resistance measurements[19]. The resistivity of the material is obtained from the following Eq. (3) ρ=2πS(V/I) (3) Where, S is the distance between probe (S=0.1875 mm), V is the obtained voltage across the two inner contacts, and I is the current passing through the sample. Fig. 5(a) shows the resistivity depended with temperature graph. La3Ba3Cu6O14 is a well-known p-type semiconductor material so it must have a narrower band gap. Hence appreciable number of charge carriers have valance in conduction bands at room temperature. La3Ba3Cu6O14 exhibited maximum resistivity (0.26 mΩ·cm) near room temperature and linearly decreasing slope was observed for temperature increasing from 300 to 473 K manually. Hence the temperature increases, however thermal agitation increases, outermost valance electron gains high energy and it jumps from valance band to conduction band. These materials have been well adopted for piezoelectric and thermoelectric applications. The DC activation energy of the material is calculated using Eq. (4), where k is Boltzmann constant. Ea=(2.303×2k×Slope)/2 (4) Fig. 5(b) shows that the graph is plotted between lgρ versus T–1. It shows anomaly behavior of electrical resistivity with the temperature increase, which the resistivity had reverse slope due to hopping conduction mechanism.

The slope value is extracted from straight line relation in the graph and to substitute in Eq. (4). The activation energy of La3Ba3Cu6O14 is 0.70 eV and it resembles that in semiconductor regions. 2.6 Hall effect measurements Hall effect were measured at room temperature using a Four probe measurement setup with a maximum applied field of 8 T and a maximum current range upto 4 A. The pellet circular samples with 12 mm diameter and sliver coated were used for measurement. The contact resistance between the sample surface and each probe was less than 100 Ω manited throughout the measurements. Hall effect realtions were evaluted by vander Pauw[20] and through below relation the parametrs can be excuted one by one. The resistance of the material (R0) measured by using Eqs. (5, 6) R0=(π/ln2) f (R1+R2/2) (5) f=1–[ln2/2 (R1-R2/R1+R2)2] (6) Where, R1 is the resistance when voltage is measured across the terminal 3 and 4, then current is passed between terminal 1 and 2. R2 is the resistance when voltage is measured across the terminal 1 and 2, then current is passed between terminal 3 and 4. The hall mobility was measured by using Eq. (7) μH=ΔRx108/BR0 (7) Where B is the applied magnetic field, ΔR is the change in resistance and R0 is the resistance of the material. Resistivity ρ=R0T (8) Conductivity σ=1/ρ (9) Hall coefficient RH=μHρ (10) Carrier concentration n=1/RHe (11) Where T is the thickness of the pellet and e=1.6×10–19 is the charge of an electron. The excuted paramters are resistivity of material (ρ)–3×10–3 Ωm, hall mobility (μH)– 2×10–3 m2/Volt·s, hallcoefficient (RH)–0.4×10–4 2 m /columb and carrier concentration (n)–6.0×1026 m–3. The results of charge carrier concentration have the same order of early reported value[21] and hence, it proves as one of the p-type semiconductor materials. These results indicate that the electronic structure of La3Ba3Cu6O14 material contains donors creating as intrinsic oxygen vacancy and acceptors of copper ion in these oxidation states Cu2+ and Cu+ double layer interactions in the ceramics[21–23]. 2.7 Temperature dependence dielectric studies

Fig. 5 Electrical resistivity as a function of temperature for La3Ba3Cu6O14 (a) and lgρ versus 1000/T showing activation energy for La3Ba3Cu6O14 (b)

The temperature depended dielectric constant and loss were measured for La3Ba3Cu6O14 ceramic powder in circular pellet formed through full auto machine LCR meter (HIOKI 3532-50 LCR meter HITESTER) in the frequency range from 50 Hz to 5 MHz. The dielectric constant (ε′) is calculated from Eq. (12) ε′=Ct/ε0A (12) Where, C is the capacitance, t is the thickness of the pel-

K. Agilandeswari et al., Investigation of magnetic, optical and electrical properties of La3Ba3Cu6O14 synthesized by …

let, ε0 is the permittivity of the free space, A is the area of the pellet[24]. From Fig. 6(a), the intense peak were observed for all the frequency in the same temperature range (420 K) which corresponds to the phase transition temperature (Tm) i.e. Curie temperature. In this temperature range the structural phase transition takes place from polar to non-polar phase because of the polarization from two different types of charge carriers[25–27]. The strong relaxation behavior of La3Ba3Cu6O14 (ε′) was observed. For future investigation, the conduction process can be attributed to the presence of two types of charge carriers, that is, p type, as a hole exchange between Cu+ and Cu2+ due to transfer of O2− between filled side with vacant oxygen side. The following Eqs. (13) and (14) can explain the mechanism as follows. Cu2+→Cu+ + h* (13) 2* 2– 1/2O2 + V →O (14) The AC conductivity is calculated from the Eqs. (15, 16) σac=ωε0ε' tanδ (15) ω=2πf (16) where f is the applied AC frequency and activation energy is obtained from the graph σac versus inverse of temperature in Kelvin. σ=σ0 exp (Ea/kT) (17) where k is Boltzmann constant and T is the temperature. The inset Fig. 6(b) shows that the low dielectric loss upto 450 K after that loss can be observed due to the structural phase (anti-ferroelectric-paraelectric) changes and domain wall density increases. Hence, the resulting that dielectric constant also decreases and depolarization temperature (Td) was obaserved near 450 K. Fig. 7(a) shows the isomerism of temperature dependence of AC conductivity (σac) at 5 kHz frequency. The inset figure shows the diffuse phase transition at 5 kHz. The σac were gradually changed with temperature up to 423 K after which a rapid hike peak was obtained due to ferroelectric to anti-ferroelectric phase transitions of the material.

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Fig. 7 Variations of AC conductivity for La3Ba3Cu6O14 with temperature at a frequency of 5 MHz (a) and diffuse phase transition graph plot between ln(ε′m/ε′) vs ln (T–Tm) at a frequency of 5 MHz for La3Ba3Cu6O14 (b)

The reverse effect of conductivity veruse temperature was obtained hence ulitamte activation eneriges lies in between semiconductor region and it may be relaxor like ferroelectric nature[28]. The activation energies were calculated by curve fitting Eq. (17) the activation energy Ea=0.70 eV. The low activation energy suggests an intrinsic conduction due to the contribution of space charge carriers and holes. The inset Fig. 7(b) shows Vogel-Fulcher relation extracted plot. They are ε′m/ε′(f,T)=1+[T–Tm(f)]γ/2δ2γ (1≤γ≥2) (18) where, ε′m–maximum dielectric constant at Tm, Tm–the phase transition temperature, ε′–dielectric constant, γ–the degree of dielectric relaxation, δ–the degree of diffuseness, and T–the absolute temperature in kelvin. Hence, degree of dielectric relaxation can be obtained from the slope of the graph, (γ=1.96) is >1, expressing more relaxor ferroelectric behaviour of transition. The intercept of graph is the degree of diffuseness (δ=0.63) resulting in a lower degree of diffuseness and weaker relaxor behaviour[29]. 2.8 Low temperature dependent magnetic measurements

Fig. 6 Variations of dielectric constant (ε′) (a) and loss (tanδ) (b) as a function of temperature for La3Ba3Cu6O14

The temperature dependent magnetization below the ambient temperature in the range of 300–20 K measured under zero field cooling (ZFC) and field cooling (FC– 0.1 T) condition are shown in Fig. 8. The Curie temperature observed near 70 K of both the ZFC and FC curves signifies that there is a change in magnetic order from ferromagnetic ordering to ferrimagnetic ordering and sharp peak indicates that Curie temperature TC–70 K of our compound. The ZFC and FC both curves are not overlapping with each other and its FC is 2 times greater than ZFC in magnetic ordering, hence the material has exhibited meta magnetism properties. Similar result has been reported in the literatures at very earlier days but other materials from the same family[30,31]. Maxi-

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Fig. 8 ZFC & FC graph of magnetic moment versus temperature

mum magnetization of ~0.9 and ~0.6 memu/g were obtained for FC and ZFC respectively. This is in correlation with the M-H loop result as described in the next section. After Curie temperature further increased, the M-T loop saturated with small anomaly present in the curves due to oxygen deficiency in A-O octahedral site. However, there is a change in magnetic ordering with respect to the temperature where thermal activation is too high, and rectangular shaped crystal particles become pinning direction to superparamagnetic state due to their magnetization decreases according to the Langevin function theory. The ZFC magnetization was very lesser than FC curve due to thermal activation and reflection of the domain structure in the surface spin. The splitting of ZFC and FC curves in this La3Ba3Cu6O14 case can be attributed to the movement of magnetic domain walls since a well defined magnetic transition and magnetic saturation is observed which is not the characteristic of spin glass systems and these materials are Meta magnetism based magnetic family materials[30]. Fig. 9 shows the M-H loops obtained for the synthesized La3Ba3Cu6O14 sample at different temperatures 50–300 K. The magnetic field was applied perpendicular to the sample plane. At all the temperatures, the magnetization saturates low magnetic fields and exhibits a

Fig. 9 Magnetic moment (M) vs. magnetic field (H) recorded as a function of various temperatures for La3Ba3Cu6O14

JOURNAL OF RARE EARTHS, Vol. 33, No. 12, Dec. 2015

hysteresis at lower fields. These two features indicate ferrimagnetic behavior. When examined closely, it is found that the shapes of the hysteresis loops display a dual-loop characteristic at temperatures lower than 300 K. This is a clear indication of the fact that there is another ferrimagnetic and super paramagnetic signal superimposed on the ferromagnetic signal. There is no magnetic transitions observed in this temperature range in M-H loop. Hence, these materials have very low temperature and high temperature phase transitions but not near in and around room temperature. So there are no electrical phase transitions also similar to magnetic phase transitions at low temperature. Hence these materials have prominent to hold good superconductors behavior up to 70 K[30–33].

3 Conclusions Novel La3Ba3Cu6O14 powder was synthesized at a lower temperature as 450 ºC by the direct precipitation from molten alkali hydroxide mixtures. The advantage of this method is low cost, very little time consuming and environmental friendly. The single phase, structural and particle size (140–200 nm) were successfully confirmed by PXRD which was highly coincident with earlier reports. The speculation of the samples increased in the spectral range 4000–600 cm−1 up to 85%, due to the formation of mixed canting pervoskite structure. The microstructure of synthesized compound at low temperature showed agglomerated cubes and also had an average grain size lying between ~140 to 250 nm. The stoichiometric ratio of the product La3Ba3Cu6O14 were calculated theoretically and experimentally. It remained same through elemental analysis. The DC and AC conductivity remained good agreement and both led to hold same proving the semiconducting nature of La3Ba3Cu6O14 because space charge polarization contributes to the conduction mechanism. The optical band gap (1.8 eV) also behaved as a semiconducting material due to ligand to metal charge transfer in the visible region. Hall extracted parameters held good agreements in semiconductors and hence concluded that these materials were having nature relaxor like ferroelectrics. La3Ba3Cu6O14 exhibited meta magnetism behavior and curie temperature TC at 5 K and phase transition from ferromagnetic to ferricmagntism. The M-H loop showed that direct propositional behavior of saturation magnetization MS and coercive field HC which denoted that transformation to softer ferrimagnetisms with increasing temperature. The results demonstratd that molten hydroxide method represented a useful technique for the preparation of lanthanum based pervoskite. Acknowledgements: Authors are grateful to VIT University for providing major financial support and excellent research facilities. The authors thank IIT SAIF-Chennai for magnetic

K. Agilandeswari et al., Investigation of magnetic, optical and electrical properties of La3Ba3Cu6O14 synthesized by … measurement studies. Authors also thank Dr. S. Kalainathan, for providing the facilities to carry out dielectric studies.

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