Physical-Chemical Properties of Bi13-XMexMo5O34±δ (Me = Mg, Ca, Sr, Ba) Solid Solutions

Single phase samples of Bi_13-xMe_xMo₅O_34±δ, Me = Mg, Ca, Sr, Ba (0.0 £ x £ 1.0) series were indexed according to triclinic Bi₁₃Mo₅O_34.5 structure (if the concentration of the dopant was small) or monoclinic Bi₁₃Mo₅O_34.5 structure (if the concentration of dopant was large). These ranges are various for different dopants because of ratio difference of ionic radii of the dopants and the Bi ion: r_{Mg <<} r_Bi+3; r_Ca, r_Sr≈ r_Bi+3; r_Ba> r_Bi+3 (the values of ionic radii with coordination number = 8 are: r_Mg= 0.89Å, r_Ca= 1.12 Å, r_Sr = 1.26 Å, r_Ba= 1.42 Å; r_Bi+3= 1.17 Å. For all substituting metals the increase of dopant concentration results in the rise of symmetry and in stabilization of the monoclinic form at room temperature.

HTXPRD was used for investigation of thermal properties of Bi_13-xMe_xMo₅O_34±δ, Me = Mg, Ca, Sr, Ba solid solutions. The dependences of volume and unit cell parameters vs. temperature were obtained. The phase transition to the monoclinic modification was detected for the samples corresponding to the triclinic modification at room temperature. This phase transition causes stepwise change of the unit cell parameters and unit cell compression in the moment of transition. No impurities were detected within the whole temperature range, the unit cell parameters curves coincided at heating and cooling. The unit cell parameters vs. temperature curves are satisfactorily fitted by the linear equation. The decrease of dopant concentration leads to the phase transition temperature (T_tàm) decrease, for example T_tàm=~593 K for Bi_12.9Ca_0.1Mo₅O_34±δ and T_tàm=~523 K for Bi_12.6Ca_0.4Mo₅O_34±δ.

The EDX- and AAS- analysis detected that the concentration of dopants is close to theoretical; concentration of bismuth and molybdenum couldn’t be determined because of analytical peaks overlapping. The concentration of all dopants confirms the theoretical formula within the experimental errors.

Hydrostatic weighting showed high density of the sintered pellets, experimental density reaches values more than 97% of theoretical (X-ray) density.

The impedance spectroscopy was used for investigation of electro-conductive properties of the ceramic samples of substituted Bi₁₃Mo₅O_34±δ. For analysis of impedance plots the equivalent electrical circuits method was used (Zview software, Version 2.6b, Scribner Associates, Inc.).

At high temperatures (higher than~ 823-873 K) the impedance plots of all investigated complex oxides correspond to non-central semicircle or two separated semicircles (Fig.8 a), the low intercept is non-equal to zero. The equivalent electrical circuits for high temperature region is shown in Fig.8 a. It can be described as R1 – R2(CPE1) – R3(CPE2) serial connection, where R2 (CPE1) and R3 (CPE2) fragments are parallel connections of resistor (R) and constant phase element (CPE). The “capacitance” of CPE1 and CPE2 is about 10^-5-10^{-6 F, which is typical for oxide systems. Therefore R2(CPE1) and R3(CPE2) parallel connections correspond to electrochemical processes at the electrodes, and R1 describes total resistance of the sample.}

At low temperatures the complex plane plot and the equivalent electrical circuits change (Fig.8 b). The impedance plot in this case consists of one separated and two adjacent semi-circles. The low intercept of the left (high-frequency) semicircle is equal to zero. The equivalent electrical circuits can be described as R1(C1)–R2(CPE2)–R3(CPE3) serial connection, where R2 (CPE2) and R3 (CPE3) fragments are parallel connections of resistor (R) and constant phase element (CPE) and R1(C1) is a parallel connection of resistor and capacitor. The R1(C1) connection describes the smallest left semi-circle, the C1 capacitance value is about 10^{-11 F}, therefore R1 can be attributed to the total conductivity of the sample. R2(CPE2) element can be attributed to electrochemical processes at the electrodes (the “capacitance” value of CPE2 is about 10^{-6 F}), R3 (CPE3) can describe complicated diffusion processes at low temperatures (the “capacitance” value of CPE2 is about 10^{-5 F}).

The present work concerns investigation of the Bi_13-xMe_xMo₅O_34±δ, Me=Mg, Ca, Sr, Ba complex oxides series. The solid solutions ranges and modifications limits for mentioned solid solutions are defined by ratio of ion radii of isolated Bi and dopant ions. The increase of the dopant concentration leads to the unit cell compression and change of crystal system from triclinic to monoclinic. The deviation from linear function of unit sell parameters and volume vs temperature are observed above 850-900 K. It agrees with activation energy changing at ~850 K detected for samples with unchanging monoclinic structure. In general the modification of electroconductive properties depend on radius of the dopant ion. The assumption of the presence of two forms of monoclinic modifications with different oxygen sublattice structure was suggested.

The work was financially supported by Russian Fund of Fundamental Research, grant 14-03-92605.