Triple molybdates one-, one-and three(two)valence metals

D O I: 10 .1 58 26 /c hi m te ch .2 01 5. 2. 4. 03 2 G. E. Khaikina1,2, S. F. Solodovnikov3,4, O. M. Basovich1, Z. A. Solodovnikova3, Y. M. Kadyrova1, A. A. Savina1,2, E. S. Zolotova3, V. N. Yudin3,4, T. S. Spiridonova1,2 1 FSBUN Baikal Institute of nature management SB RAS, 670047, Ulan-Ude, Sakhyanovoy str., 6 2 FSBGU HPE “Buryat state University”, 670000, Ulan-Ude, Smolina str., 24a 3 Institute of inorganic chemistry Sibiryan Branch RAS, 630090, Novosibirsk, prospect Akademika Lavrentyeva, 3 4 Federal state Autonomous educational institution “Novosibirsk national research state University”, 630090, Novosibirsk, Pirogov str., 2 egkha@mail.ru, solod@niic.nsc.ru

The molybdates and tungstates are among the most popular objects of inorganic chemistry, crystal chemistry and solid state chemistry, as well as a base for developing of functional materials for various purposes, which maintains a constant interest in these compounds and explains a significant number of publications on this subject. In 1960-80 the focus of the scientists was double molybdates and tungstates phases with the general formula A x B y (XO 4 ) z , on the basis of which laser, ferroelectric, scintillation, nonlinear optical and other materials were later developed [1][2][3][4][5]. The main contribution to the formation of this group of compounds and their comprehensive study was made of the Russian scientific school: professor Kovba L. M., professor Trunov In the last two decades there has been a shifting of the centre gravity of studies from double molybdates and tungstates on triple molybdates. To date, this group of compounds has more than 550 individuals and is the fastest growing of complex oxide phases containing tetrahedral anion and cation. The large part of triple molybdates is prepared and is characterized by the employees of the Baikal Institute of nature management SB RAS (Ulan-Ude) and the Institute of inorganic chemistry named A. V. Nikolaev SB RAS (Novosibirsk). A brief overview of the different types of triple molybdates, different combinations of the charges of their constituent cations is earlier presented in [6]. The aim of this work is a detailed consideration of the phase formation, structure and properties of triple molybdates, containing two different singly mono-charged cation along with triple-charged (type 1-1-3) or doubly charged (type 1-1-2) cation.

Triple molybdates of the type 1-1-3
The first systematic searching researches of triple molybdates of one-, one-, and trivalent metals were conducted for lithium-containing systems Li 2 MoO 4 - phases in a series of REE vary significantly and with increasing size of singly charged cations move in the direction to the light lanthanides (Fig. 2).
The analysis of experimental data allows to draw a conclusion about the decisive influence of dimensional factor on the possibility of the formation of monoclinic triple molybdates of this family: LiMLn 2 (MoO 4 ) 4 are formed, if the difference in sizes of ions of large singly charged cation and rare earth element lies in the interval. 0.48 Ǻ ≤ r(M + ) -r(Ln 3+ ) ≤ 0.60 Ǻ.
At lower values of Dr in the cut of LiLn(MoO 4 ) 2 -MLn(MoO 4 ) 2 there is the formation of solid solutions. When Dr > 0.60 Å the consider phase is either not formed or its formation is so complicated that the connection cannot be allocated in single-phase condition using conventional methods of solid-phase synthesis [11].
Within the prescribed time interval the isothermally and isostructural copper compounds CuKLn 2 (MoO 4 ) 4 with Gd, Tb, Ho are prepared and characterized in [12,13] are stacked. The closeness of r(Cu + ) and r(Li + ) with a high degree of probability allows to predict a significant expansion of the triple molybdates M'M''R 2 (MoO 4 ) 4 due to containing Cu(I) phases of this type with K, Tl, Rb, and those of trivalent elements, the difference in dimensions which will satisfy the proposed criterion.
The structure of triple molybdates LiMR 2 (MoO 4 ) 4 is defined by the exam- is not a quasi-threefold.  4 , the only compound of this family which congruently melting [14]. Other compounds LiMR 2 (MoO 4 ) 4 decompose in the solid phase at the corresponding double molybdates [7,15] and their structure (for example LiMNd 2 (MoO 4 ) 4 , M = K, Tl, Rb) was refined by the Rietveld method for powder data [16]. The structures LiMR 2 (MoO 4 ) 4 are close to the structure of triple molybdates Li 3 Ba 2 Ln 3 (MoO 4 ) 8 [17] and are derived from the structural type BaNd 2 (MoO 4 ) 4 [18]. A characteristic features of structures LiMR 2 (MoO 4 ) 4 are the laced layers of the RO 8 polyhedron and connected to them through common vertices MoO 4 -tetrahedra. The neighbouring layers are interconnected by octahedra and LiO 6 polyhedra MO10 (Fig. 3).
The presence in compounds LiMR 2 (MoO 4 ) 4 ions Li + , filling the interstitial voids of the structural type BaNd 2 (MoO 4 ) 4 suggests that they have lithium ionic conductivity. The results [19,20] indicate the possibility of using these triple molybdates as sensitive elements of sensors of sensor systems for operational environmental monitoring. Spectral-luminescent characteristics LiMLn 2 (MoO 4 ) 4 : Eu 3+ (Nd 3+ ) give the basis to speak about the possibility of the application of triple molybdates of this family to create luminophors with high contrast colors, as well as active media of lasers [8,21]. The data obtained in [22] show the availability of using LiKGd 2 (MoO 4 ) 4 : in the capacity of: Eu 3+ is as a red phosphor for WLED.
As in the previous case, the possibility of formation of other isostructural series of triple molybdates Li 2 M 3 R(MoO 4 ) 4 (MR = CsFe, CsGa, RbGa, CsAl, RbAl, TlAl) is largely determined by a dimensional fac-tor: compounds are formed by small cations Fe 3+ , Ga 3+ , Al 3+ with tetrahedral coordination and quite major ions Tl + , Rb + and Cs + . The absence Li 2 M 3 Cr(MoO 4 ) 4 is apparently due to the high preference of Cr 3+ in octahedral coordination. These tetragonal compounds have a frame structure and are ordered derivatives of the cubic Cs 6 Zn 5 (MoO 4 ) 8 [23,24]. With the increasing of size of R 3+ , the region of existence of these phases shifts towards larger singly charged cations M + , which can be explained by the compliance of the sizes of the tetrahedral framework and the size of the extra framework cation. Obviously with namely dimensional discrepancy the crystallization Li 2 K 3 Al(MoO 4 ) 4 is bound in a different structural type [10].
Made in recent years the researches of systems M 2 MoO 4 -Cs 2 MoO 4 -R 2 (MoO 4 ) 3 (M = Na, Ag) allowed significantly to fill the group of triple molybdates of one-, one-and trivalent metals due to the sodium and silver-containing phases. The compositions and the fields of the existence of thus obtained compounds are shown in table. 2, the data of the RSA of the obtained single crystals are presented in table. 3.
Studied sodium compounds have, as a rule, difficult structures and frame struc-tures ( Fig. 5, 6), different in structure from the triple molybdates formed in the systems Li 2 MoO 4 -M 2 MoO 4 -R 2 (MoO 4 ) 3 (M = K-Cs, Tl). In the structures of the sodium-containing triple molybdates МоO 4tetrahedra and RO 6 -octahedra are present and sodium has an octahedral or trigonalprismatic coordination or generates polyhedra with lower CN. In these structures the Na + and R 3+ quite often jointly occupy one crystallographic position; along with them there are positions which partially filled with sodium cations that leads to the deviation of composition from stoichiometry. The phases of variable composition are widely distributed among the complex (double and triple) sodium molybdates [33,34], due to the proximity of sizes of ions Na + and и A 2+ or R 3+ .
According to the data of RSA, all triple molybdates found in the systems Ag 2 MoO 4 -Cs 2 MoO 4 -R 2 (MoO 4 ) 3 are isoformular to sodium analogs and are built on the same structural basis [32].  24 fields of compounds, based on common structural basis, equally shaded * -the resulting crystals and structure was determined on single crystal data by method RSA; ◆ -the resulting crystals and settings of cells were determined on single crystal data. In practical terms, triple molybdates Na 25 Cs 8 R 5 (MoO 4 ) 24 are the most interesting which the closely related structures are solved by single crystal data in the framework of pr. gr. P2 1 /c (In), P2 1 2 1 2 1 (Sc), P`1 (Fe) [38,39]. The Mo atoms in all three structures are coordinated tetrahedral, trivalent metal is octahedral, all or some of them occupy their positions together with the atoms of sodium. The remaining Na atoms have rather distorted oxygen coordination (CN = 5 and 6); the atoms of cesium are CN = 9-10 (In), 11 (Sc), 10-12 (Fe); some positions of the sodium cations may be partially settled. In all structures it is possible to allocate polyhedral layers which formed by pairs of articulated along edges of the octahedra (R, Na)O 6 and (R, Na)O 6 (or RO 6 ) that are connected by vertices with bridging MoO 4 -tetrahedra (Fig. 6, a-c). The layers contact bridging MoO 4 -tetrahedra in the  [40]. The rhombic or pseudorhombic metric of cells of triple molybdates occurs due to some mutual shift of the layers in comparison with monoclinic Na 5 Sc(MoO 4 ) 4 and alluaudite (pr. gr. C2/c), which may be due to the presence of cesium cations between the layers. Structural features of this group of triple molybdates suggests that this is not the kind of structural type of alluaudite and a separate, let closely related structural family.
The study of alluaudite-like ion-conductive properties of triple molybdates showed that these compounds undergo reversible phase transitions of type I, fol-lowed by an abrupt increase of conductivity. Above the temperatures of phase transitions, the electrical conductivity reaches values of 10 -2 -10 -3 sm/Sm, which gives an opportunity to consider Na 25 Cs 8 R 5 (MoO 4 ) 24 (R = In, Sc, Fe) as the promising objects for the development of new materials with high ionic conductivity [38,39].
The structural features of the other described above triple molybdates also allow to expect the existence of them increased the sodium (silver)-ionic conductivity and improve their conductive characteristics that apparently it is possible to achieve by suitable heterovalent substitutions with replacing part of the sodium (silver) or other cation in the structure on more high strength field and education vacancies.  [42,43]. They crystallized in the structural type II-Na 3 Fe 2 (AsO 4 ) 3 [49], in which the cations are distributed as follows: (Na 5 £) IX (M1) VI (M2) VI (M3) 3 VI (AsO 4 ) 6 = (Na 5 £)(Na) (Fe 3+ )(Fe 3+ ) 3 (AsO 4 ) 6 (here the Roman numerals denote the CN of the cations in the positions M1, M2 and M3). In the structures of the triple molybdates cations Li, A 2+ and K + are placed at the positions M1, M2 and M3 (Fig. 8), and the main part of the potassium is in a position with CN = 9, busy half due to short contacts C-C. The presence of potassium in the same position with the cations Mg 2+ , Mn 2+ , Co 2+ , Li + is rare case for crystal chemistry. Found on the structural data the compositions of the crystals are confirmed by good convergence of the local balance of valence efforts. The basis of all structures are three dimensional frames from octahedra around M1, M2 and M3 and tetrahedra In the structures of these compounds the positions M1, M2 and M3 are occupied by the cations Na + , A 2+ and A 2+ , respectively, and the positions of potassium, as in the previous case are occupied only half. The data on these triple molybdates are given in table. 4. The isostructurality of considered triple molybdates to sodium-ion conductor II-Na 3 Fe 2 (AsO 4 ) 3 gives reason to expect the presence of increased ionic conductivity. It is assumed that the ways of transport of ions in these phases are similar to found in the structure of II-Na 3 Fe 2 (AsO 4 ) 3 , where Na + cations are moved through the defective positions of sodium with CN = 9 and octahedral site M1, in the neighbouring coordination polyhedra and form three-dimensional network.
In In systems with Rb and Zn (Fig. 9, a) and cesium-containing systems with Co and Zn (Fig. 9, b) Rb 3 LiZn 2 (MoO 4 ) 4 и Cs 3 LiA 2 (MoO 4 ) 4 (A = Co, Zn) are found [41][42][43], the isostructural cubic Cs 6 Zn 5 (MoO 4 ) 8 [23,24]. The uniqueness of the composition and structure Cs 6 Zn 5 (MoO 4 ) 8 is connected with the incompleteness of the tetrahedral positions of the zinc, where the sixth part is vacant. The filling of vacancies by ions Li + and other singly charged cations M + according to scheme Zn 2+ + ☐ → 2M + creates the conditions for the synthesis of new compounds. An introduction to the structure of cubic Cs 6 Zn 5 (MoO 4 ) 8 of singly charged cations M + = Na, Ag with close to Zn 2+ ionic radius obtained cubic phases Cs 3 MZn 2 (MoO 4 ) 4 with disordered distribution of cations M + on the positions of the Zn 2+ . The features Cs 3 MZn 2 (MoO 4 ) 4 (M = Na, Ag) are given in table. 4. According to our data [43,50], between Cs 3 MZn 2 (MoO 4 ) 4 (M = Li, Na) and Cs 6 Zn 5 (MoO 4 ) 8 there are continuous solid solutions (Fig. 9, b) with the gradual filling of the cationic vacancies in the It should be noted that the substitution and the simultaneous introduction into the position of zinc in the structure Cs 6 Zn 5 (MoO 4 ) 8 different valent cations with very different ionic radius on the scheme 5Zn 2+ + ☐ → 2R 3+ + 4Li + leads to the formation of the group of triple molybdates Li 2 M 3 R(MoO 4 ) 4 (MR = CsFe, CsGa, RbGa, CsAl, RbAl, TlAl) described above. In the latter case, the cations Li + and R 3+ are distributed orderly in structure, which leads to a tetragonal distortion of the structure of the prototype.
The basis of the structures of the triple molybdates of both series, as structures Cs 6 Zn 5 (MoO 4 ) 8 are delicate three-dimensional frames. In phases with divalent metals they are formed by tetrahedrons of two sorts -around the molybdenum  [51].
In the study of solution-melt crystallization (solvent -Cs 2 Mo 2 O 7 ) in the systems Na 2 MoO 4 -Cs 2 MoO 4 -AMoO 4 (A = Ni, Co, Mn) the crystals CsNa 5 M 3 (MoO 4 ) 6 [47] related to the type alluaudite were isolated and structurally were investigated. The oxygen octahedra around the cations A 2+ and Na + are connected with common edges and faces, and then by common vertices with the MoO 4 tetrahedra into a three-dimensional frame, which is parallel (100) is divided into two kinds of layers (Fig. 11). In one of these layers (Fig.  11) the wide channels filled with cesium ions pass parallel to the axis c, which occupy half their positions and have CN = 8. The comparison of eludicating structures CsNa 5 A 3 (MoO 4 ) 6 and Na 4-2x A 1+x (MoO 4 ) 3 (A = Ni, Co, Mn) shows that in the triple molybdates part of the cations Na + in the channels was replaced with Cs + , significantly increased the parameters of the cell along the axis a and accordingly the width of the channels were significantly increased. View as along these channels the transport of sodium ions may be, it may increase the ionic conductivity. The close relationship of phases CsNa 5 A 3 (MoO 4 ) 6 and Na 4-2x A 1+x (MoO 4 ) 3 (A = Ni, Co, Mn) can indicate the formation of solid solutions between them, which requires additional researches. The features CsNa 5 A 3 (MoO 4 ) 6 (A = Ni, Co, Mn) are given in table. 4. In the systems Na 2 MoO 4 -Cs 2 MoO 4 -AMoO 4 (A = Co, Mn) also highlighted the triple molybdates of composition Cs 4 Na 10 A 5 (MoO 4 ) 12 [46,48] (Fig. 12, table  4) also were highlighted, which were very similar in structure to the above compounds Na 25 Cs 8 R 5 (MoO 4 ) 24 (R = In, Sc, Fe), forming together with them obviously the single family of phases with similar metrics of cells and different symmetry. The structure Cs 4 Na 10 Co 5 (MoO 4 ) 12 (pr. gr. Pbca) is most symmetrical, which can be regarded as the ancestor of this family. The symmetry the other compounds may be raised at phase transitions, which must be accompanied by disordering of the structure and the possible increase in the mobility of sodium cations.
In this regard, we can expect high ionic conductivity at triple molybdates Cs 4 Na 10 A 5 (MoO 4 ) 12 (A = Co, Mn, as this is the case for Na 25 Cs 8 R 5 (MoO 4 ) 24 (R = In, Sc, Fe).

Concluding remarks
Our carried studies of triple molybdates of the type 1-1-2 and 1-1-3 show that among them there are several families of isostructural or closely related in structure phases. The systems with trivalent metals have higher phase-forming ability, the large stoichiometric and structural diversity in which the triple molybdates belonging to 14 structural types (families) form, whereas triple molybdates of the type 1-1-2 belong only to four isostructural series. One from these explanations for this may be the wider range of cations R 3+ and their sizes compared to the ions A 2+ in the phase-forming sys-