Phase diagrams for the M2MoO4–Ln2(MoO4)3–Hf(MoO4)2 systems, where M = Li–Cs, Tl and Ln = La–Lu 224 Bazarova Zh. G., Grossman V. G., Bazarov B. G., Tushinova Yu. L., Chimitova O. D., Bazarova Ts. T. Chimica Techno Acta. 2017. Vol. 4, No. 4. P. 224–230. ISSN 2409–5613 D O I: 1 0. 15 82 6/ ch im te ch /2 01 7. 4. 4. 03 Zh.G. Bazarova1,2, V.G. Grossman1, B.G. Bazarov1,2, Yu.L. Tushinova1,2, O.D. Chimitova1, Ts.T. Bazarova1 1Baikal Institute of Nature Management, Siberian Branch of Russian Academy of Sciences, 8 Sakh’yanovoi St., Ulan-Ude, 670047, Russian Federation 2Buryat State University, 24a SmolinaSt., Ulan-Ude, 670000, Russian Federation e-mail: jbaz@binm.ru Phase diagrams for the M 2 MoO 4 –Ln 2 (MoO 4 ) 3 –Hf(MoO 4 ) 2 systems, where M = Li–Cs, Tl and Ln = La–Lu In this paper, the results of systematic studies of complex molybdate sys- tems M 2 MoO 4 –Ln 2 (MoO 4 ) 3 –Hf(MoO 4 ) 2 (M = Li–Cs, Tl; Ln = La–Lu) are presented. Subsolidus phase diagrams of ternary systems were constructed and new triple molybdates were obtained. The optimum synthesis conditions for poly- and monocrystalline form were determined. According to single-crystal data, the structure of one of the representatives of triple molybdates was determined. Keywords: phase equilibria, synthesis, systems, lithium, sodium, potassium, rubidium, ce- sium, thallium, lanthanides, hafnium, crystal structure. Received: 17.11.2017; accepted: 06.12.2017; published: 25.12.2017. © Bazarova Zh. G., Grossman V. G., Bazarov B. G., Tushinova Yu. L., Chimitova O. D., Bazarova Ts. T., 2017 Introduction The molybdates containing tet- rahedrally coordinated anions MoO4 2– are among the most exciting objects in inor- ganic and crystal chemistry. Special at- tention has been paid to the molybdates that include luminescent elements, such as Ce, Pr, Eu, Tb, Tm, in their composi- tion. These compounds can be used as phosphors for white light emitting diodes. Currently, numerous studies had been undertaken that aim to search for new effective phosphors for the creation of WLED. These phosphors should possess high stability, compactness, high lumines- cence efficiency when excited by near UV, long lifetime and low cost. Luminescent materials containing rare-earth elements are also within the scope of such studies [1–3]. In this paper, we present results on the study of phase equilibria in ternary molybdate systems containing single-, three- and tetravalent elements, as well as data on the crystal structure of complex molybdate structural types. Experimental Reagents Li2MoO4 (“pure” grade), Na2MoO4·2H2O (“pure for anal- ysis” grade), K2MoO4 (“pure” grade), Cs2MoO4 (“pure” grade), Rb2CO3 (“chemi- cally pure” grade), Tl2O3 (“chemically pure” grade), MoO3 (“chemically pure” grade), HfO2 (“chemically pure” grade), and rare earth oxides of 99.9% purity 225 were used as starting materials. Hf(MoO4)2 and Ln2(MoO4)3 were synthesized by the solid state method. The annealing had started at 400–500  °C with the follow- ing temperature increase up to 800  °C in order to prevent MoO3 losses due to its high volatility. The total calcination time was 100–150 h. Rb2MoO4 was prepared within the temperature range 350–650 °C during 100 h. Tl2MoO4 was synthesized during 50 h firing starting at 400 °C with final temperature equal to 550  °C. The samples were regrinded after each 24 h in the course of annealing in order to reach better homogeneity. The phase formation inside a sub- solidus region of the ternary salt systems M2MoO4–Ln2(MoO4)3–Hf(MoO4)2 (M  = Li–Cs, Tl; Ln = La–Lu) in air were stu- died using the intersecting joins method. The phase composition of the samples was monitored by X-ray diffraction (XRD) measurements using Bruker D8 Ad- vance diffractometer (Cu  Kα radiation, VANTEC-1, maximum angle 2θ=100°, scan step 0.01–0.02°). Primary fitting of the diffraction patterns was made using PROFAN software from the CSD package. Results and discussion The data concerning phase equilib- rium for the side systems of studied tri- angles (quasi-ternary systems) M2MoO4– Ln2(MoO4)3–Hf(MoO4)2 (M  = Li–Cs, Tl; Ln = La–Lu) are taken from the litera- ture. The phase formation in the quasi- binary Li2MoO4–Ln2(MoO4)3 systems was described in paper [4] in details. The aforementioned systems can be divided into two groups. The first group includes systems with Ln = La–Tb in which two intermediate phases LiLn5(MoO4)8 and LiLn(MoO4)2 possess a significant homo- geneity range. The second group includes the systems with Ln = Dy–Lu. Inside these systems two intermediate compounds that did not exhibit any noticeable homogenei- ty ranges were detected: LiLn(MoO4)2 and Li7Ln3(MoO4)8. Two intermediate compounds with the constituents mole ratio of 5:1 and 1:1 were detected inside the quasi-binary Na2MoO4–Er2(MoO4)3 systems similarly to the K2MoO4–Ln2(MoO4)3 (Ln = La, Tb, Dy, Er), M2MoO4–Ln2(MoO4)3 (M  = Tl, Rb; Ln = La–Lu) systems [4–8]. Addition- ally to those two one more intermediate phase with the constituents mole ratio 1:5 was found to exist inside the Na2MoO4– Ln2(MoO4)3 (Ln = Nd, Sm) and K2MoO4– Sm2(MoO4)3 systems [4, 5]. The phases formed inside the cesium- containing rare-earth molybdate systems included those with the constituents mole ratio 3:1 and 1:1 with Ln = Nd, whereas the systems with Ln = Sm, Tb, Er revealed only 1:1 compound [4, 5]. Lithium hafnium molybdate with the significant homogeneity range Li10–4xHf2+x(MoO4)9 (0.21 ≤ x ≤ 0.68) was formed in the Li2MoO4–Hf(MoO4)2 system [9], but in the oxide systems with larger alkali earth metals M2MoO4–Hf(MoO4)2 (M  = K, Tl, Rb, Cs) two types of com- pounds were confirmed – M8Hf(MoO4)6 and M2Hf(MoO4)3 [10–13]. It should be noted that Na-containing system slightly differs from the aforementioned ones. It was found that along with phases with the constituents mole ratios equal to 4:1 and 1:1 one more compound is formed with the 3:1 composition. The phases that were isolated in the Ln2(MoO4)3–Hf(MoO4)2 systems 226 are Ln2Hf3(MoO4)9 (Ln  = La–Tb), Ln2Hf2(MoO4)7 (Ln  = Sm–Ho), and Ln2Hf(MoO4)5(Ln = Er–Lu) [14]. The phase diagrams for various mo- lybdate systems are shown in Fig. 1–6, and the corresponding phase composi- tions are listed in Table 1. No new compounds were detected in the lithium-containing and sodium- containing systems [15, 16]. In contrast, the new compounds listed in Table 1 and shown in Fig. 3–6 were confirmed in the M2MoO4–Ln2(MoO4)3–Hf(MoO4)2 (M  = K, Rb, Tl, Cs; Ln = La–Lu) systems [14, 17, 18]. The molybdates with the composition K5LnHf(MoO4)6 were identified in the sys- tems with Ln = Sm–Lu. Two types of compounds, name- ly M5LnHf(MoO4)6 (5:1:2) and M2LnHf2(MoO4)6.5 (2:1:4), were isolated in the Tl2MoO4–Ln2(MoO4)3–Hf(MoO4)2 (Ln = Ce–Lu) and Rb2MoO4–Ln2(MoO4)3– Hf(MoO4)2 (Ln  = Ce–Lu) systems. In case of the Tl-containing systems, for Ln = Ce–Nd one more phase except those mentioned above had been obtained – TlLnHf0.5(MoO4)3 (1:1:1). The phase with composition Cs2LnHf2(MoO4)6.5 (2:1:4) was found to Fig. 1. Subsolidus phase diagrams of the Li2MoO4–Ln2(MoO4)3–Hf(MoO4)2 systems (T - Li10–4xHf2+x(MoO4)9, 0.21 ≤ x ≤ 0.68; shaded double-phase equilibrium region) [15] Fig. 2. Subsolidus phase diagrams of the Na2MoO4–Ln2(MoO4)3–Hf(MoO4)2 systems; shaded double-phase equilibrium region [16] 227 exist in the Cs-containing rare earth mo- lybdates with Ln = Pr–Lu. The systems M2MoO4–Ln2(MoO4)3– Hf(MoO4)2 (M  = K, Rb, Tl, Cs; Ln = La– Lu) are characterized by the formation of a different number of phases with vary- ing homogeneity ranges with respect to the lanthanide elements. One can see that M5LnHf(MoO4)6 (M  = K, Tl, Rb) com- pounds were formed if the size difference for the single charged M+ cation and rare earth element cation Ln3+ lies in the range of 0.682 Å ≤ r(M+)(CN=12)–r(Ln 3+)(CN=6) ≤ 0.859 Å [19]. In the case of M+ = Li+, Na+, Cs+ such radii difference lies out- side this range. As a result, formation of M5LnHf(MoO4)6 compounds for these cations is impossible. If the size difference for the single charged M+ cation and rare earth ele- ment cation Ln3+ lies in the range 0.839 Å ≤ r(M+)(CN=12) – r(Ln 3+)(CN=6) ≤ 1.019 Å, it Fig. 3. Subsolidus phase relations in the K2МоО4–Ln2(MoO4)3–Hf(MoO4)2 systems (S1–K5LnHf(MoO4)6 (5:1:2); shaded double-phase equilibrium region) [14] Fig. 4. Subsolidus phase diagrams for the Tl2MoO4–Ln2(MoO4)3–Hf(MoO4)2 systems where Ln = La–Lu. Notations: S1–Tl5LnHf(MoO4)6 (5:1:2), S2–Tl2LnHf2(MoO4)6.5 (2:1:4), and S3– TlLnHf0.5(MoO4)3 (1:1:1); shaded double-phase equilibrium region [17] 228 Fig. 6. Subsolidus phase relations in the Cs2МоО4–Ln2(MoO4)3– Hf(MoO4)2 systems (S2–Cs2LnHf2(MoO4)6.5 (2:1:4); shaded double-phase equilibrium region) Fig. 5. Subsolidus phase diagrams for the Rb2MoO4–Ln2(MoO4)3–Hf(MoO4)2 systems where Ln = La–Lu. Notations: S1–Rb5LnHf(MoO4)6 (5:1:2) and S2–Rb2LnHf2(MoO4)6.5 (2:1:4); shaded double-phase equilibrium region [18] Table 1 Compositions of triple molybdates in the M2МоO4–Ln2(МоO4)3–Hf(MoO4)2 (M = Li–Cs, Tl; Ln = La–Lu) systems La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu LiHf NaHf KHf K5LnHf(MoO4)6 TlHf Tl5LnHf(MoO4)6 Tl2LnHf2(MoO4)6.5 TlLnHf0.5(MoO4)3 RbHf Rb5LnHf(MoO4)6 Rb2LnHf2(MoO4)6.5 CsHf Cs2LnHf(MoO4)6.5 229 makes a formation of the M2LnHf2(MoO4)6.5 molybdates pos- sible. The complex molybdates M5LnHf(MoO4)6 (M  = K, Tl, Rb) are isostructural to the earlier grown sin- gle crystals of Rb5LnHf(MoO4)6 (Ln  = Nd, Eu, Er), which possess the trigonal structure with R3c space group [18]. Three-dimensional framework of this structure is built of the consequently alternating MoO4 tetrahedra and (Ln, Hf )O6 octahedra linked with each other through the common O-vertices. 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Phase diagrams for the M2MoO4–Ln2(MoO4)3–Hf(MoO4)2 systems, where M = Li– Cs, Tl and Ln = La–Lu. Chimica Techno Acta. 2017;4(4):224–30. DOI:10.15826/chim- tech/2017.4.4.03.