Metal materials have excellent conductivity, but their thermal expansion property could induce thermal stress and even function failure. Meanwhile, ceramic materials have low thermal expansion coefficients but their conductivities are usually poor. Recent investigations suggest that negative thermal expansion materials could be used to prepare low thermal expansion materials by partial ion substitution, doping, or compounding with positive thermal expansion materials,^[1–3] which could also result in high conductivity.^[4–8]

ZrV₂O₇ is one of the isotropic negative thermal expansion materials but its negative thermal expansion is only present after a phase transition.^[9] P⁵⁺ ion has been used to substitute V⁵⁺ in ZrV₂O₇ to decrease the phase transition temperature and realize low thermal expansion. ZrV_2−xP_xO₇ ( ) showed lower phase transition temperature at 340 K for x = 0.4.^[10] The heterovalent ion substitution of Mo⁶⁺ or W⁶⁺ for V⁵⁺ in ZrV₂O₇ could not reduce the phase transition temperature obviously.^[11,12] Dual-ion substitutions of Cu²⁺/P⁵⁺ or Fe³⁺/P⁵⁺ for Zr⁴⁺/V⁵⁺ in ZrV₂O₇ have also been investigated but it is difficult to reach a high solubility of the dopants.^[13,14] However, dual-ion Fe³⁺/Mo⁶⁺ or Al³⁺/Mo⁶⁺ for Zr⁴⁺/V⁵⁺ in ZrV₂O₇ could lead to a low thermal expansion material.^[15,16] Even near zero thermal expansion can be obtained (Zr_0.1Fe_0.9V_1.1Mo_0.9O₇) by dual-ion substitutions of Fe³⁺/Mo⁶⁺ for Zr⁴⁺/V⁵⁺ in ZrV₂O₇.^[15]

ZrV₂O₇ has poor conductivity at room temperature (RT), which is increased with increasing temperature (σ = 3.56×10⁻³ S/cm at 573 K).^[13,17] The introduction of Cs in ZrV₂O₇ could enhance the ionic conductivity. Charge transport in VO₄ could provide the observed high ionic conductivity.^[18,19] In this paper, dual-ion substitutions of Fe³⁺/Mo⁶⁺ for Zr⁴⁺/V⁵⁺ in ZrV₂O₇ are investigated to increase the conductivity of ZrV₂O₇. Zr_0.1Fe_0.9V_1.1Mo_0.9O₇ is found with electrical conductivity σ = 8.2 ×10⁻⁵ and 9.41 ×10⁻⁴ S/cm at 291 K and 623 K, and activation energy E_a = 0.160 eV and 0.233 eV for 291–413 K and 533–623 K, calculated from the Arrhenius plots for the total conductivity.

Zr_0.1Fe_0.9V_1.1Mo_0.9O₇ was synthesized by a solid state method with raw materials Fe₂O₃, ZrO₂, MoO₃, and V₂O₅. The raw materials were mixed according to stoichiometric amounts of Zr_0.1Fe_0.9V_1.1Mo_0.9O₇ (with excess 5 mol% V₂O₅) and ground in a mortar for 3 h. The homogenized raw materials were pressed into 10 mm ×2.5 mm (diameter ×height) cylindrical pellets, sintered in a tubular furnace at 5 K/min heating rate, held at 953 K for 2–5 h, and then cooled down slowly to RT.

X-ray diffraction (XRD, Bruker D8 Advance Diffractometer) was used to analyze the crystal phase of the sample. Microstructures were checked using scanning electron microscopy (SEM, JSM-6700 F) and elements were analyzed with energy dispersive spectrum (EDS). Pellet electrical conductivity was measured after polishing and coating both surfaces with Ag paste. Complex impedance was measured using a frequency response analyzer (1 Hz to 4 MHz) in the temperature range from RT to 623 K. The linear thermal expansion coefficients were measured on dilatometers (LINSEIS DIL L75), with heating and cooling rates of 5 K/min.

Figure 1(a) shows the temperature dependent XRD patterns of Zr_0.1Fe_0.9V_1.1Mo_0.9O₇ from RT to 873 K. The lattice parameters of Zr_0.1Fe_0.9V_1.1Mo_0.9O₇ are calculated using the least-square method with PowderX program,^[20] which decrease with increasing temperature (Fig. 1(b)). The detailed linear coefficients of thermal expansion of Zr_0.1Fe_0.9V_1.1Mo_0.9O₇ along the a, b, and c axes are −0.4×10⁻⁶ K⁻¹, −0.97×10⁻⁶ K⁻¹, and −1.22×10⁻⁶ K⁻¹, respectively (RT–873 K). However, by the dilatometer measurement, the coefficient of thermal expansion of Zr_0.1Fe_0.9V_1.1Mo_0.9O₇ is determined to be 0.72×10⁻⁶ K⁻¹ from 140 K to 673 K (Fig. 1(c)). The different results from temperature dependent XRD and dilatometer measurement could relate to the microstructure and thermal hysteresis of ceramics Zr_0.1Fe_0.9V_1.1Mo_0.9O₇. The sample for temperature dependent XRD is much smaller than that for dilatometer. The XRD measurement begins after holding temperature for 5 min. However, the dilatometer measurement is continuous with temperature.

Fig. 1.

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	Fig. 1. (a) XRD patterns of Fe0.9Zr0.1V1.1Mo0.9O7 heated from RT to 873 K. (b) Dependence of lattice constants on temperature of orthorhombic Zr0.1Fe0.9V1.1Mo0.9O7. (c) Relative length change of ZrV2O7 and Zr0.1Fe0.9V1.1Mo0.9O7 from 140 K to 673 K. (d), (e) SEM images and (f) EDS of orthorhombic Zr0.1Fe0.9V1.1Mo0.9O7.

Figures 1(d)–1(f) show the SEM images and EDS of Fe_0.9Zr_0.1V_1.1Mo_0.9O₇, respectively. The microstructure presents irregular particles and tiny pores. The pores could result in the above different coefficients. The EDS shows that the elements in the sample include Zr, Fe, Mo, V, and O. From the EDS, the atomic percentages of Zr, Fe, Mo, V, and O in the sample are 1.10%, 8.60%, 10.02%, 14.31%, and 67.97%, respectively, which indicate that the atomic ratio of Zr, Fe, Mo, V, and O is close to .

Figure 2(a) shows the impedance of Fe_0.9Zr_0.1V_1.1Mo_0.9O₇ from 291 K to 413 K. The Nyquist plots present a series of arc, where the real axis (Z) corresponds to the contact resistance. With increasing temperature, the radius of the arc decreases, indicating a decrease of resistance. This is similar to the property of semiconductors due to the increase of carrier concentration with heating. At higher temperatures (413–503 K), the Nyquist plots (Fig. 2(b)) show a single semicircle at high frequency, where the real axis (Z) corresponds to the bulk resistance related to the superposition of grain and grain boundary effects. The Nyquist plots present little tails near low frequency, indicating ionic conductivity. These results are in consistent with the results on other ionic conductor materials.^[21–23] The Nyquist plots also suggest the increase of resistance with increasing temperature, just like conductor resistance increases with temperature. We use Zsimwin 3.30 program^[24] to study the Nyquist curve after stabilization. The impedance of Fe_0.9Zr_0.1V_1.1Mo_0.9O₇ is calculated by fitting the experimental data (Figs. 2(d) and 2(e)) to the equivalent circuits comprising of resistances R_i, capacitances C_j, and constant phase element CPE, in series and/or parallel, as shown in Fig. 2(c). The total impedance and admittance are

Fig. 2.

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	Fig. 2. Representative Zr0.1Fe0.9V1.1Mo0.9O7 AC impedance spectra for (a) 291–413 K and (b) 413–473 K. (c) Equivalent circuit: Rs, R1, R2, and R3 are the electrical resistances of grain, grain boundary, and ion/electrode interface, respectively. Impedance spectra at 383 K (d) and 503 K (e).

Figure 3(a) shows the impedance from 503 K to 623 K. The Nyquist plots present a single semicircle at high frequency, and have little tails at low frequency. The impedance of Fe_0.9Zr_0.1V_1.1Mo_0.9O₇ is calculated by fitting the experimental data (Figs. 3(c)–3(e)) to the equivalent circuits comprising of R_i, C_j, and CPE, in series and/or parallel, as shown in Fig. 3(b). The Nyquist plots show that the total resistance of the material does not change and then decreases as the temperature increases, which could relate to the increase of carrier concentration by intrinsic excitation with heating.

Fig. 3.

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	Fig. 3. (a) Representative Zr0.1Fe0.9V1.1Mo0.9O7 AC impedance spectra for 503–623 K. (b) Equivalent circuit: Rs, R1, R2, and R3 are the electrical resistances of grain, grain boundary, and ion/electrode interface, respectively. Impedance spectra at (c) 533 K, (d) 563 K, and (e) 623 K.

To obtain appropriate resistance dependence on temperature, we measured direct-current (DC) current–voltage (I–V) characteristics of Zr_0.1Fe_0.9V_1.1Mo_0.9O₇ at different temperatures (Fig. 4). The conductance is calculated using the least square method. The total resistance is calculated by . The results are consistent with the impedance spectra.

Fig. 4.

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	Fig. 4. (a)–(g) The DC I–V curves from 291 K to 623 K of Zr0.1Fe0.9V1.1Mo0.9O7.

The conductivity σ can then be calculated from^[25]

Fig. 5.

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	Fig. 5. Zr0.1Fe0.9V1.1Mo0.9O7 total electrical resistance (a) and (b) conductivity, and (c) Arrhenius plots for the total conductivity.

Arrhenius plots for Zr_0.1Fe_0.9V_1.1Mo_0.9O₇ in normal atmosphere. The values of σ are calculated to be 8.2 ×10⁻⁵ S/cm, 3.80 ×10⁻⁴ S/cm, 4.16 ×10⁻⁴ S/cm, and 9.41 ×10⁻⁴ S/cm at 291 K, 383 K, 473 K, and 623 K, respectively. The linear relationship of conductivity with temperature (Fig. 5(c)) could relate to no phase transitions at 291–413 K. The relaxation process activation energy E_a of Zr_0.1Fe_0.9V_1.1Mo_0.9O₇ at higher temperatures and RT could be calculated to be 0.233 eV and 0.160 eV using Arrhenius expression , where α is the pre-exponential factor, k is Boltzmann’s constant, and T is the absolute temperature.^[14]

Carrier concentration could be controlled by temperature.^[26–28] The conductivity of Zr_0.1Fe_0.9V_1.1Mo_0.9O₇ is larger than that of ZrV₂O₇.^[14] This could be related to the substituting effect of Fe³⁺/Mo⁶⁺ for Zr⁴⁺/V⁵⁺ in ZrV₂O₇, like p type doping ( and n type doping ( ) in semiconductor. Fe/Mo co-doping induces the band gap of ZrV₂O₇ to narrow down. Thermal excitation in Zr_0.1Fe_0.9V_1.1Mo_0.9O₇ (0.160 eV) is easier than that in ZrV₂O₇ (0.319 eV). Meanwhile, the strong interaction between the formed ( and ( results in low thermal expansion in Zr_0.1Fe_0.9V_1.1Mo_0.9O₇. The conductivity presents an obvious change at 413–533 K, which could result from high scattering probability and slowing-down increase of conductive particle number. However, the conductivity increases with increasing temperature (533–623 K), which may be related to the intrinsic thermal excitation of semiconductor.

Zr_0.1Fe_0.9V_1.1Mo_0.9O₇ shows near zero thermal expansion and high conductivity. The conductivities of Zr_0.1Fe_0.9V_1.1Mo_0.9O₇ are 8.2×10⁻⁵ S/cm, 3.80×10⁻⁴ S/cm, 4.16×10⁻⁴ S/cm, and 9.41×10⁻⁴ S/cm at 291 K, 383 K, 473 K, and 623 K, respectively. The electrical conductivity is linear for 291–413 K, and the activation energy is 0.160 eV. At 533–623 K, the activation energy is 0.233 eV, indicating that Zr_0.1Fe_0.9V_1.1Mo_0.9O₇ acts like a classical doped semiconductor. Thermal excitation leads to carrier concentration increase from RT to 413 K, which leads to a rapid resistance decrease. The unchanged conductivity during 413–533 K is due to high scattering probability and slowing-down increase of carrier concentration. The intrinsic thermal excitation leads to rapid increase of conductivity at 533–623 K.