It has been established earlier [1, 2] that electromigration in tungstates with the structure of scheelite is carried by ions O2− and WO4 2−. Since the polyanionic transfer is a rare and insufficiently studied phenomenon, that is of interest to investigate the type and the nature of the conductivity of trivalent metal tungstates, for example Gd2(WO4)3 and Ho2(WO4)3. This tungstates crystallizes in Eu2(WO4)3 structural type, so-called `defective scheelite', in which 1/3 of Me-sites are vacant, Me2/3[V M e ]1/3WO4 [3, 4]. The crystal structure of `defective scheelite' can be characterized by the presence of isolated tetrahedrons [WO4], which have a common oxygen apex with [MeO8] dodecahedrons [3, 4].
Gd2(WO4)3 was prepared by the solid-state method from the powders of Gd2O3 and WO3 of the `extra-pure grade' qualification in air. The oxides Gd2O3 and WO3 were previously calcinated for 6 hours to remove traces of moisture, Gd2O3at 1100∘C, WO3 at 800∘C. Synthesis was performed according to the equation:
(1) with the gradual increase of temperature (600-1100∘C) in six steps with intermediate grindings in the ethyl alcohol media; annealing time at each stage varied from 12 to 96 hours.
Ho2(WO4)3 was prepared by glycerole-nitrate method described in Ref. . The precursors for glycerole-nitrate method were (NH4)10W12O41, Ho2O3, C4H6O6, and 57% HNO3 (all of high purity grade). The phase purity of the samples was confirmed by X-ray diffraction (XRD) method (Bruker D8 ADVANCE, Cu-K α radiation, 40 kV, 40 mA, exposition 1 s, angles range 15 ≤ 2θ≤ 65). The unit cell parameters were refined by the FullProf Package.
Compacted briquettes of Ln2(WO4)3 with a diameter of 1 cm and 2 mm thickness were obtained by pressing of powders at 900 kg/cm2 followed by sintering at 1100∘C for 24 hours (Gd2(WO4)3) and 900∘C for 48 hours (Ho2(WO4)3). The relative densities of the ceramic samples were 85% for Gd2(WO4)3 and 60% for Ho2(WO4)3. Ln2(WO4)3 (Ln = Gd, Ho) samples resistance was measured by impedance spectroscopy with the Immittance Parameters Meter IPI1 (Trapeznikov Institute of Control Sciences, Moscow) at frequencies of 100 Hz - 1 MHz in the temperature range 450-880∘C.
The sum of ionic transference numbers was determined by the EMF method in the cell:
The effect of oxygen partial pressure P O 2 on conductivity was measured at fixed temperature in the temperature range 700-940∘C. The oxygen pressure was set and controlled by an oxygen pump and a sensor made of solid electrolyte on the basis of yttrium-stabilised zirconia ZrO2(Y2O3).
The nature of mass and charge transfer was determined by the Tubandt method in a two-piece cell:
The experiment was carried out at 850∘C; the voltage U = 300 V was applied to Cell with the current not exceeding 1 mA. The amount of electricity passing through the cell Q ranged from 10 to 86 C. The TG and DSC measurements were carried out with the simultaneous thermal analysis device NETZSCH STA 409 PC LUXX equipped with the Quadrupole Mass Spectrometer QMS 403 AËOLOS.
3.1. Phase identification
Gd2(WO4)3) and Ho2(WO4)3were confirmed to be a single phase by XRD analysis (Fig. 1). The unit cell parameters of the synthesised phases are in good agreement with the published earlier (Table 1) .
3.2. Thermogravimetric Analysis and DSC Ln2(WO4)3 Research
TG and DSC data are presented in Fig. 2. TG analysis showed no change in the weight of the Gd2(WO4)3 samples in the entire temperature range under study. Mass loss at 150 for tungstate holmium associated with dehydration due to its hygroscopicity. Endothermal effect at 1172∘C on the DSC curve of Gd2(WO4)3 and 926∘C on the DSC curve of Ho2(WO4)3 can be interpreted as the phase transformation from low-temperature defect scheelite Eu2(WO4)3 structure to the high-temperature Sc2(WO4)3 type structure .
3.3. Electrical Conductivity and Transference Numbers of Ln2(WO4)3
The resistance of Ln2(WO4)3 samples was measured by impedance spectroscopy. The temperature dependences of the conductivity of the Ln2(WO4)3 are shown in Fig. 3. The jump in conductivity Ho2(WO4)3 at 850∘C is probably due to the phase transition, the nature of which is not yet clear.
Conductivity value of both phases is almost identical (Fig. 3) below 850∘C, due to close radii values of Ln3+ (Gd3+ - 0.1193 nm, Ho3+- 0.1155 nm)  and due to the same phase structure. But above 850∘C Ho2(WO4)3 conductivity is 7-8 times higher than for Gd2(WO4)3. This fact can probably be explained by the Ho2(WO4)3 phase transition at 850∘C.
The conductivity isotherms of Ln2(WO4)3 versus oxygen partial pressure are shown in Fig. 4. The fact of independence of the value of total conductivity with oxygen pressure may confirm the ionic conductivity in Ho2(WO4)3 and Gd2(WO4)3 within the temperature range 700-940∘C. ∑t i o n of Gd2(WO4)3measured by EMF method is close to 1. This fact is in a good agreement with the independence of the Gd2(WO4)3electrical conductivity of the P o 2 (Fig. 4).
3.4. The Nature of Mobile Carriers in Ln2(WO4)3 (Tubandt method)
To clarify the type of charge carriers for Ln2(WO4)3, Tubandt experiments were carried out in a two-section cell at 850∘C. Mass loss of the cathode section and growth of the anode section weight were found (Fig. 5).
Reproducible mass reduction of the cathode section proved that negative ions were responsible for the charge transfer. Since the transfer of oxygen ions cannot lead to a change in mass of the samples, the experimentally observed change in mass of cathode and anode sections are due to the [WO4]2− transfer. If this supposition is correct, then a phase enriched by lanthanide and a phase enriched by tungsten should appear in the cathode space and in the anode space, respectively. The XRD results of the cathode and anode areas of Ho2(WO4)3 briquettes are shown in Fig. 6. Indeed, XRD pattern of the Ho2(WO4)3 cathode briquette surface with was in contact with the Pt(–) electrode reveals an appearance of a phase enriched by holmium (Ho6WO12). At the same time, a phase enriched by tungsten (WO3) was identified on the surface of the anode briquette Ho2(WO4)3in contact with the Pt(+) electrode.
Possible electrode reactions that can occur in the cathode and anode sections are:
By using the Faraday's law and assuming that the amount of mass loss of the cathode section Δm(−) is correspondent to the WO3 mass related to the migration of [WO4]2− ions, the [WO4]2− ion transference number was calculated according to the formula:
(6) where W O 3 is WO3 molar mass (g/mol), z = 2, Q is the amount of electricity passed through the system (C), F is Faraday's constant.
The molar mass of WO3 M W O 3 (instead of M W O 4 2− ) was used in the formula, because according to the equation of reaction (4), the mass loss of the cathode section caused by the disappearing of tungsten oxide is the following:
The transference numbers of tungstate ion calculated according to the Eq. (6) are about 4% for gadolinium tungstate and 11% for holmium tungstate.
The obtained results indicate that [WO4]2− makes no significant contribution to the charge transfer in the studied systems; therefore, the particles that do not affect the mass change play a predominant role. Thus, one can conclude that oxygen ions can serve as mobile species, i.e., t (o2−) >> t (wo4 2−).
This result is quite unexpected, because contribution of tungstate ion in the electric transport for isostructural divalent metal tungstates (CaWO4, SrWO4, BaWO4) according to the data , is much higher, and varies from 20 to 50%. The insignificant contribution of [WO4]2−transfer to the electrotransport of Ho2(WO4)3 and Gd2(WO4)3 with the defect scheelite structure (as compared with the divalent metal tungstates) can be due to the fact that structural vacancies V L n open channels for the O2− migration that increase the contribution of oxygen conductivity.
Thus, the set of the experimental results suggests that O2− ions are the main charge carriers in the tungstates Ln2(WO4)3 (Ln = Ho, Gd). Together with that, a minor (under 11%) contribution of polyanions [WO4]2− into the ion transport was detected.
The research results were obtained in the framework of the State Task of the Ministry of Education and Science of Russia and supported by the grants of the Russian Foundation for Basic Research RFBR 14-03-00804_a. The equipment of the Ural Center for Shared Use “Modern nanotechnology” UrFU was used.