Synthesis, structure and electrical properties of Mg-, Ni-codoped bismuth niobates

Mg-, Ni-codoped bismuth niobates Bi1.6Mg0.8–xNixNb1.6O7–δ (x = 0; 0.2; 0.4; 0.6; 0.8) were obtained by conventional solid-state reaction method. It was shown that the Mg atoms are distributed at the Nb sites while the Ni atoms are distributed over the Biand the Nb-sites, according to the results of comparison of pycnometric and X-ray density of the Bi1.6Mg0.4Ni0.4Nb1.6O7–δ pyrochlore. In this case, about 15–20% of the vacancies are formed at the Bi sites. The obtained compounds are stable up to their melting point based on the DSC analysis data. Real dielectric permittivity ε' of the Bi1.6Mg0.8–xNixNb1.6O7–δ samples decreases from 80 to 65 with the temperature decrease from 25 to 700 °C and practically does not depend on frequency in the range of 1–1000 kHz. Oxides Bi1.6Mg0.8–xNixNb1.6O7–δ behave like insulators up to 280 °C, their conductivity increases with temperature (Ea,dc ≈ 1.3 eV, dc) and with the Ni content at a given temperature.


Introduction
Ceramics in the Bi 2 O 3 -M x O y -Nb 2 O 5 ternary system are interesting from the perspective of their dielectric properties.The most attention has been paid to the Zn-, Mg-containing bismuth niobates, which possess high dielectric constant (170-180) and low dielectric loss (~10 -4 ) at 1 MHz (at room temperature) [1][2][3][4][5][6][7][8][9].To search for the same properties Fe- [10], Mn- [11][12], Co- [13], Ni- [12,[14][15], Cu- [12] and the mixed Zn-M (M -Sr [16], Ca [16][17], Mn [16,18], Ti [19][20][21][22], Sn [19,22], Zr [19,[21][22], Ce [19,22], Gd [21], Ta [23], La [24]), Mg-M (M -Sr [25], Nd [26], Cu [27]) bismuth niobates and other ones were synthesized.The improved permittivity was achieved by Ti doping of the Nb sites in the pyrochlore structure [21][22] and by Cu doping in Bi 1.5 Cu x Mg 1-x Nb 1.5 O 7 (x = 0.075) [27].In most cases, doping leads to the permittivity decrease and to the tangent loss increase.However, electrical properties of several systems were investigated in the high temperature range (up to 700 °C) only in order to determine their conductivity mechanism [3,9,[19][20]27].In our previous work [28][29] we have determined that the dielectric constant of the Bi 1.6 Cu x Mg 0.8-x Nb 1.6 O 7-δ pyrochlores behave unusually passing through a maximum (250-350 °C) with temperature increasing.The value of the dielectric constant at the maximum is very high: ~10 6 (100 Hz).Second-type phase transition was found at 200 °C.To establish the reasons for such behavior, the distribution of doped metals in the cation (A-, B-sites) positions in the pyrochlore structure (A 2 B 2 O 6 O' , the space group Fd3m (No 227)) was studied by X-ray diffraction pattern refinement (Rietveld analysis), and by comparison of pycnometric density with the calculated one.It has been determined that the elec-tronegativity plays the crucial factor for the distribution of the Mg atoms in the Nb sites and the Cu atoms -in the Bi and the Nb sites in equal ratios.In any case, there are 10-15% of vacancies in the Bi sites.In accordance with the other systems' investigations, the vacancy concentration always remains at about 5-10% in the Bi sites in the pyrochlore structure [4, 10-11, 14, 30].In this work we have a goal to determine a distribution of Ni and Mg dopants in the pyrochlore structure and investigate the temperature dependence of electrical properties of the Bi 1.6 Mg 0.8-x Ni x Nb 1.6 O 7-δ .
The phase composition of the samples was examined by powder X-ray diffraction method on a SHIMADZU XRD-6000 diffractometer using Cu Kα emission within the angle range 10-80° (the step -0.05°).Distribution of nickel and magnesium atoms in the Bi 1.6 Mg 0.4 Ni 0.4 Nb 1.6 O 7-δ pyrochlore was determined by Rietveld analysis (FullProf software package [33]).Scanning electron microscopy (SEM) was carried out on a TESCAN VEGA 3 SBU microscope.The local composition of the samples was studied on polished pellets by energy dispersion spectroscopy (EDS).Differential scanning calorimetry (DSC) and thermogravimetric analysis (TG) of Bi 1.6 Mg 0.4 Ni 0.4 Nb 1.6 O 7-δ powder were carried out in the air in platinum crucibles with heating up to 1300 °C and a heating rate of 5 °C/min (NETZSCH STA 409 PC/ PG).The electrical measurements were performed on the pellets, both sides of which were coated uniformly with a silver paste.Capacitance and dielectric loss tangent were measured by MT-4090 LCR meter in different gases (air, p(O 2 ) = 0.21 atm and oxygen, p(O 2 ) = 0.99 atm) at four frequencies (1,10,100,200 kHz) in the temperature range of 25-750 °C.The impedance plots were measured by immittance meter E7-28 at 0.5 V in the temperature and frequency ranges 25-700 °C and 24 Hz -10 MHz, respectively.The electrical data were collected after 10 min after the thermal equilibrium was reached.The thermoelectric effect -Seebeck coeffi-cient -was determined in the temperature range 130-330 °C in a temperature gradient of 30-40 °C across the material.[14].Various models were studied to determine the distribution of doped atoms in the cation (Bi, Nb) sites of the pyrochlore structure.Among the alternative models that were considered there are [Bi In these models 5% of vacancies remain at the Bi sites.The distribution of dopant atoms in equal ratio among two different cation sites causes formation of about 2.5% vacant sites both in the Bi and Nb sublattices.It is not typical for the pyrochlore structure.The best agreement between theoretical and observed X-ray patterns was obtained for the model designated as   3 and is in agreement with the pycnometric density value.Thus assumed amount of about 15-20% vacant sites in Bi sublattice seems to be in agreement with the experimental results obtained in the present study.

Electrical properties
Complex impedance plots of the Bi 1.6 Mg 0.8-x Ni x Nb 1.6 O 7-δ ceramics were drawn from impedance spectroscopy data.The data were obtained during cooling from 700 to 160 °C to exclude proton conductivity.Perfect semicircles are traced in   the temperature range 320-700 °C.At the temperature less than 320 °C half semicircles may be observed.In Fig. 5 impedance plots for the Bi 1.6 Mg 0.8-x Ni x Nb 1.6 O 7-δ (x = 0.4; 0.6) ceramics are presented.The plots are well fitted by a single parallel RC element (inset of Fig. 5) where R and C belong to bulk resistance and capacitance, respectively [36][37][38].The measured parameters are listed in Table 2.
Permittivity recalculated from the capacitance values for the samples with Ni content x = 0.20, 0.40, and 0.60 is (70-81), (70-81), and (65-76), respectively, for the temperature range of 700-280 °C.Calculated permittivity does not depend on the frequency in the range of 1-1000 kHz and is close to the dc permittivity values.At room temperature, the permittivity is around 80 in the frequency range of 1-1000 kHz.All ceramics under investigation behave like a dielectric (tan δ ≈ 0.002) up to 280 °С.
Calculated from Arrhenius direct conductivity plots activation energy values are close to 1.2 eV (the third column in Table 3).These values are almost equal to ones at 1 kHz (the second column in Table 3).The corresponding Arrhenius con-  For all ceramics, an electrical modulus (M") maximum is detected on the logarithmic scale of frequency (Fig. 7), indicating the presence of a polarization process.These relaxation effects are characterized by the full width at half maximum (FWHM) peaks of M"(f) being ~ 1.2 decades.This value is close to an ideal Debye response (1.14 decades) that characterizes the ceramics as electrically homogenous.At frequencies of the M" maximum value the relaxation time was calculated (Fig. 7).Frequency values at M" maxima were plotted vs temperature in an Arrhenius-type fashion.Obtained accordingly values of activation energy are

Conclusions
Mixed Mg-, Ni-containing bismuth niobates Bi 1.6 Mg 0.8-x Ni x Nb 1.6 O 7-δ (0 ≤ x ≤ 0.8) were synthesized by the conventional solid-state reaction method.For all samples the main crystal phase is the pyrochlore.The Bi 1.6 Mg 0.4 Ni 0.4 Nb 1.6 O 7-δ ceramic is a single-phase compound and is stable up to its melting point (1261 ºC).Based on structural analysis and the comparison of pycnometric and calculated densities of the Bi

6
Mg 0.8-x Ni x Nb 1.6 O 7-δ (0 ≤ x ≤ 0.8) are shown in Fig. 1.The pyrochlore structure is formed for the Bi 1.6 Mg 0.4 Ni 0.4 Nb 1.6 O 7-δ composition only.The small amounts of second phases, identified as MgNb 2 O 6 (Pbcn space group) and as NiNb 2 O 6 (Pbcn space group), were found in the samples with x = 0; 0.2 and with x = 0.6; 0.8, respectively.The surfaces of the Bi 1.6 Mg 0.8-x Ni x Nb 1.6 O 7-δ (0 ≤ x ≤ 0.6) polished pellets after the last calcination are shown in the SEM images (Fig. 2a-2c).According to the EDS data, the presence of additional phases such as MgNb 2 O 6 (at x = 0) or as mixed Mg-Ni containing niobates (at x = 0.2; 0.6) can be seen.The amount of impurities is around 5%.The local compositions of the main and second phases are presented in the caption to Fig. 2. The composition of the Bi 1.6 Mg 0.4 Ni 0.4 Nb 1.6 O 7-δ ceramic determined by EDS is Bi 1.60 Mg 0.38 Ni 0.45 Nb 1.6 O 7-δ , which is close to the desired composition.The porosity of the pellets was around 35-40%, as estimated from SEM micrographs.DSC and TG curves of the Bi 1.6 Mg 0.4 Ni 0.4 Nb 1.6 O 7-δ powder are shown in Fig. 3.The endothermal effect was found on the DSC curve at 1261 °C.This effect may be associated with the melting of the sample.The reason for the weight rise during the heating process has not been established yet.It may be related to the partial oxidation of Ni +2 to Ni +3 .The Rietveld refinement of the XRD pattern of Bi 1.6 Mg 0.4 Ni 0.4 Nb 1.6 O 7-δ was carried out.The occupations of atom sites were fixed in accordance with the quantitative composition of the compound.The possibility of displacement of the bismuth atoms (from 16c sites to 96h or 96g sites) and the oxygen atoms O′ associated with bismuth (from 8a sites to 32e sites) were considered, like in [Bi 0.833 Mg 0.11 □ 0.04 ] 2 [Mg 0.24 Nb 0.76 ] 2 O 7 and in [Bi 0.833 Ni 0.125 □ 0.04 ] 2 [Ni 0.25 Nb 0.75 ] 2 O 7 pyrochlores

Table 3
[37]38]ion energies of (dc, ac) conductivity and relaxation process of the substituted bismuth niobate pyrochlores close to the ones obtained from the Arrhenius conductivity plots (Table3).It points out that the hopping-type conductivity is typical for the Bi 1.6 Mg 0.8-x Ni x Nb 1.6 O 7-δ ceramics, like for Bi 1.5 ZnNb 1.5 O 7[36,38]and Bi 3.55 Mg 1.78 Ta 2.67 O 13.78[37]pyrochlores with E a (relaxation) are 0.94 eV and 1.37 eV, respectively.