Synthesis, crystal structure and electrophysical properties of triple molybdates containing silver, gallium and divalent metals

I. Yu. Kotova, A. A. Savina*, A. I. Vandysheva, D. A. Belov, S. Yu. Stefanovich Baikal Institute of Nature Management, Siberian Branch, Russian Academy of Sciences, 6 Sakh’yanova St., Ulan-Ude, 670047, Buryat Republic, Russian Federation Buryat State University, 24a Smolin St., Ulan-Ude, 670000, Buryat Republic, Russian Federation Lomonosov Moscow State University, Leninskie Gory, 1, Moscow 119991, Russian Federation *E-mail: alex551112@mail.ru


Introduction
A synthesis and studying of complex oxide compounds, the development of new materials with functionally significant properties based on those are among the main areas of the materials science. An important place in the study and obtaining of new phases with valuable physicochemical properties belongs to mo-lybdates, in particular triple ones, which are among the fastest-growing groups of complex oxide compounds containing a tetrahedral anion and three different cations. One of the largest families of these compounds is molybdates containing 1-, 2-and 3-charged cations. In particular, silver-containing NASICON-like phases  5 and AgFe II 3 Fe III (MoO 4 ) 5 single crystals were obtained and their crystal structures were determined [1][2][3][4][5][6][7][8][9]. The purpose of this work is to study the possibility of forming triple molybdates Ag 1-x A 1-x Ga 1+x (MoO 4 ) 3 and AgA 3 Ga(MoO 4 ) 5 (A = Mn, Co, Zn, Ni) and investigate crystal structure and electrophysical properties of the obtained compounds.

Experimental
The initial materials were simple molybdates of silver, manganese, cobalt, zinc, nickel, MoO 3 4 ) in the air with intermittent grindings every 15 hours for better sample homogenization. Power X-ray diffraction (PXRD) patterns of the prepared compounds do not contain reflections of starting or impurity phases. PXRD and thermal characteristics of all prepared compounds agree well with corresponding data reported in [10−15].
Sample compositions Ag 1-x A 1-x Ga 1+x (MoO 4 ) 3 (0 ≤ x ≤ 0.7, Δx = 0.1) and AgA 3 Ga(MoO 4 ) 5 were prepared by the annealing of appropriate stoichiometric mixtures of Ag 2 MoO 4 , АMoO 4, MoO 3 and Ga 2 O 3 . The initial mixtures were annealed starting at 300 °C followed by raising the temperature by 20-50 °C (in some cases, 5-10 °C) with intermittent grindings every 20-30 hours for sample homogenization. The calcination time at each temperature was 30-70 h. The phase composition of the obtained products was controlled by the PXRD analysis before each increasing of the annealing temperature.
PXRD patterns were collected at room temperature on a Bruker D8 ADVANCE diffractometer using Cu Kα radiation in the 2θ range from 5° to 100° with a step of 0.02076°. Possible impurity phases were checked by comparing their PXRD patterns with those in the Powder Diffraction File. The crystal structure refinement was carried out with the GSAS [16] program suite using PXRD data. Lattice parameters and individual scale factors were established, and five common peak-shape parameters of the pseudo-Voigt function (No. 2), one asymmetry parameter and one parameter for the zero-point correction were used to describe the powder patterns. The background level was described by a combination of 15-order Chebyshev polynomials. Isotropic displacement parameters (Uiso) were refined, and grouped by chemical similarity by used constrains.
Thermoanalytic studies were carried out on a STA 449 F1 Jupiter NETZSCH thermoanalyser (Pt crucible, heating rate of 10 °С / min in Ar stream).
Ceramic disks for dielectric investigations were prepared by the calcination of pressed powder at 600 °С for 2 h. The disks were of 9-10 mm in diameter and 1-2 mm thick, the electrodes were deposited by painting the disk bases with colloid platinum followed by subsequent one hour annealing at about 580 °С. The direct current (DC) electric conductivity was measured with a V7-38 microammeter.
To study the ion transfer, electrical conductivity was measured on an alternating current (AC) by the two-contact method in the frequency range 1 Hz-1 MHz in the temperature range 25-560 °C at the rate of 4 °C / min at both heating and cooling using a Novocontrol Beta-N impedance analyzer. The activation energy of electrical conductivity was calculated from the slope of the straight lines corresponding to the Arrhenius dependence in lg (σT) -(10 3 / T) coordinates.

PXRD characteristics
The presence of NASICON-like phases in the Ag 2 MoO 4 -AMoO 4 -Ga 2 (MoO 4 ) 3 systems was determined according to PXRD analysis of samples Ag 1−x A 1−x Ga 1+x (MoO 4 ) 3 (0 ≤ x ≤ 0.7, Δх = 0.1) which were annealed in the temperature range from 300 °C to melting point. The final annealed temperature was 550-700 °C and depended on both the composition of the reaction mixtures and the nature of the divalent metal. It was established that, despite the close values of the Al 3+ (0.53) and Ga 3+ (0.62 Å [17]) radii, gallium containing triple molybdates with NASICON-like structure, apparently, do not exist. All our attempts to obtain rhombohedral phases Ag 1−x A 1−x Ga 1+x (MoO 4 ) 3 by solid state synthesis did not lead to a positive result, probably this is due to the low reactivity of gallium in the molybdate systems. Thus, the simple gallium molybdate Ga 2 (MoO 4 ) 3 has not yet been obtained by ceramic techno logy, and only recently it was synthesized by the sol-gel method [18]. Besides, silver-gallium double molybdate is not synthesized either by ceramic technology or by co-precipitation. In [19] this compound was obtained by the calcining of mixtures of AgNO 3 , Ga 2 O 3 , MoO 3 (in ratio 2:1:4) at 350-400 °C for 8-10 h, followed by cooling, homogenization, and the repeated 12-20 hours annealing at 500-550 °C, but the PXRD data of the product are not given by the authors. It should be noted that in none of the later publications (including those of the same authors) additional information about this compound was found.
The powder XRD patterns of asprepared single-phase compounds AgA 3 Ga(MoO 4 ) 5 are similar and show that these oxides are isostructural to triclinic NaMg 3 In(MoO 4 ) 5 (sp. gr. P1, Z = 2) [20]. The diffractograms of the AgA 3 Ga(MoO 4 ) 5 (A = Mn, Co, Zn) were indexed with taking into account our data obtained earlier in the course of single-crystal structure de-termination of AgMg 3 R(MoO 4 ) 5 , R = Fe, Cr [7]. The result of indexing the PXRD patterns for AgA 3 Ga (MoO 4 ) 5 (A = Mn, Co, Zn) are given in Table 1. Unit-cell parameters are listed in Table 2.
Crystal structure of AgZn 3 Ga(MoO 4 ) 5 T h e c r y s t a l s t r u c t u r e of AgZn 3 Ga(MoO 4 ) 5 was refined ac-cording to the Rietveld method [21], starting with the atomic coordinates of AgMg 3 Fe(MoO 4 ) 5 structure [7]. Crystal data, data collection and structure refinement details are summarized in Table 3. Experimental, theoretical, and difference PXRD patterns for the AgZn 3 Ga(MoO 4 ) 5 are shown in Figure 1. The fractional  Tables 4 and 5 (2) Crystal system, space group (#) Triclinic, P1 (2) Unit-cell parameters: 6.9035 (5) 6.9643 (5)  M(4)O 6 octahedra, which are linked by the common vertices to form a 3D framework (Fig. 2). In the large framework cavities, the silver cations are disordered on three close positions with the distances Ag-Ag 0.595(4) Å and 1.101(2) Å.
Such a disordering is also typical of other compounds of this isostructural series [7,9], suggesting a possible mobility of the Ag + cations in the compounds. This is favored not only by defects in Ag positions along with their irregular coordina-    (2) tion, but also a rather flexible polyhedral framework of the NaMg 3 In(MoO 4 ) 5 structure type, which involves interconnected cavities.

Electrophysical properties
As was noted in the previous section, the structural features of the obtained molybdates allow us to expect these compounds to have the increased ionic conductivity. This was already confirmed by us in the case of AgMg 3 Al(MoO 4 ) 5 (σ = = 2.5 × 10 −2 S / cm) and AgMn 3 Al(MoO 4 ) 5 (σ = 7.1 × 10 −3 S / cm) at 500 °C [7]. In this work as an example, the results of studying electrophysical properties for AgMn 3 Ga (MoO 4 ) 5 are presented.
It was found that the DC conductivity of ceramic sample AgMn 3 Ga(MoO 4 ) 5 , measured with the V7-38 device, is negligible as compared to the ac conductivity (Fig. 3) in temperature region of 100-560 °C. As the platinum electrodes are blocking in the DC conductivity measurement mode, the DC conductivity of AgMn 3 Ga(MoO 4 ) 5 corresponds to the electronic one. Therefore, it can be concluded that the AC conductivity is almost equal to the ionic one.
It is seen that near room temperature the conductivity is as small as 10 -7 S / cm but quickly rises with temperature to va lues of about 10 -2 S / cm. It is noteworthy that the conductivity in AgMn 3 Ga(MoO 4 ) 5 increases with temperature in non-monotonic way showing distinct breaks on lgσ = f(1 / T) curves at 310 °C. Above these temperature the lgσ = f(1 / T) dependences are almost linear with the small activation ener gy va lue Е а = 0.26 eV. Above 310 °С, the ionic conductivity of AgMn 3 Ga(MoO 4 ) 5 increases up to 2.03•10 -2 S / сm at 500 °С, which is close to the corresponding characte ristics of the known ionic conductors.

Conclusions
The possibility of the formation of silver-containing gallium triple molybdates with Mn, Co, Zn, Ni, analogous to the phases Ag 1-x A 1-x R 1+x (MoO 4 ) 3  Co, Zn) were synthesized and characterized. AgNi 3 Ga(MoO 4 ) 5 was not obtained in the single-phase state. It was established that the obtained compounds incongruently melt and belong to the structural type of triclinic NaMg 3 In(MoO 4 ) 5 (sp. gr. P1, Z = 2). The structure of the obtained compounds was refined by the Rietveld method using the powder diffraction data for AgZn 3 Ga(MoO 4 ) 5 . The structural features of the obtained molybdates allow us to expect these compounds to have the increased ionic conductivity. This was confirmed by studying electrophysical properties of AgMn 3 Ga(MoO 4 ) 5 . It was shown that the high-temperature electrical conductivity of this compound reaches 10 -2 S / cm at E a = 0.26 eV, which is close to the corresponding characteristics of the known ionic conductors.