JOURNAL OF SOLID STATE CHEMISTRY ARTICLE NO. 133, 545—554 (1997) SC977528 New Defective Brannerite-Type Vanadates II. Synthesis and Study of Mn12x2y AgyUxV222x2y Mo2x1y O6 Solid Solutions: Ag–Mn-stabilized Hexagonal MoO3 Bogna Napruszewska, Piotr Olszewski, and Jacek Ziółkowski1 Institute of Catalysis and Surface Chemistry, Polish Academy of Sciences, ul. Niezapominajek, 30-239 Krako& w, Poland Received January 3, 1997; in revised form June 26, 1997; accepted June 29, 1997 1. INTRODUCTION MnV2O6 (below 540°C) and AgVMoO6 are monoclinic C2/m brannerite-type structures. They show complete miscibility, forming MnAg 5 Mn12y AgyV22yMo11yO6 solid solutions. MnAg is able to incorporate an excess of MoO3 and form MnAgU 5 Mn12x2yUx AgyV222x2y Mo2x1yO6 phases (U 5 cation vacancy in the original Mn/Ag site) belonging to the pseudoternary MnV2O6 –AgVMoO6 –MoO3 system. The system can be visualized as an equilateral triangle with X 5 100x and Y 5 100y coordinates along the MnV2O6 –MoO3 and MnV2O6 –AgVMoO6 arms, respectively. The limit of the existence of MnAgU is almost linear and extends between the X–Y points S1 5 (45, 00) and S2 5 (12, 88). As the ionic radii of dopants (Ag1/Mn21, Mo61/V 51) are larger than those of the mother cations, the lattice constants in both series—MnAg and almost saturated MnAgU (located along the S1S2 curve—rise systematically with an increase in Y. Behind the S1S2 curve, saturated MnAgU has been observed accompanied by ‘‘hexagonal MoO3’’ (h-MoO3), Y phase 5 Mn 0·15V0.3 Mo0.7O6 , by (MoxV12x)2O5, and sometimes by orthorhombic MoO3 and MnMoO4 . According to the literature, so-called h-MoO3 is a defective tunnel structure stabilized by the inserted free or hydrated L 5 K1, Rb1, Cs1, NH1 4 ions. The formula of dry h-MoO3 is L0.13V0.13Mo0.87O3 (I). Ag was believed to be too small to stabilize the peculiar h-MnO3 . Despite that, we observed Ag-containing h-MoO3 in the field of the MnV2O6 –AgVMoO6 –MoO3 triangle. Attempts to obtain a pure Ag-containing h-MoO3 have failed so far; apparently the presence of Mn (beside Ag) stabilizes the h-MoO3 structure and the formula is more complex as compared with I. The phase diagram of the MnV2O6 –AgVMoO6 system reveals great similarity to those containing Li and Na instead of Ag. All three diagrams involve a narrow lens-type solidus–liquidus gap at high Y and two peritectic meltings at lower Y, yielding, besides liquid, the hightemperature b-MnAg polymorph and Mn2V2O7 . ( 1997 Academic Press 1To whom correspondence should be addressed. About 30 years ago Ruh and Wadsley (1) resolved the structure of the mineral ThTi O , containing some dopants. 2 6 The structure, called brannerite, has been identified as monoclinic, C2/m (although C2 and Cm symmetries are not excluded among the brannerite-type family, described below). In this layered AB O structure (visualized in Fig. 1) 2 6 distorted BO octahedra sharing edges form the anionic 6 sheets parallel to (001), with packing tight along [010] and loose along [100]. The edge-sharing, distorted AO oc6 tahedra make isolated [010] pillars, linking the anionic layers in the [001] direction. The pillars are tight in the [001] direction and bulky along [010]. The c sin b parameter expresses the distance between the anionic layers of the structure. Soon it was proved that either a number of bivalent metal vanadates Me2`V 5`O (Me"Mg, Mn, Co, Cu, Zn, Hg, 2 6 Cd) adapt this structure or the structure is exhibited by at least one of their polymorphs (2—9). Further progress was made when it was reported that the same structure is possessed by compounds of formula ¸1`V 5`Mo6`O (¸"Li, 6 Na, Ag) (10, 11). Finally, the structure appeared to be amenable to random isomorphous substitutions including in some cases the cation vacancies ' in the Me/¸ sublattice, and the following solid solutions were identified (X"100x, ½"100y): — ¸'"¸ ' V Mo O with 04X416 for 1~x x 1~x 1`x 6 ¸"Li, 04X430 for Na, 04X412 for Ag, and 184X424 for K [although pure KVMoO (12) has never 6 been synthesized in the brannerite form] (10). — Me'"Me ' V Mo O with 04X445 1~x x 2~2x 2x 6 for Mn, and 04X422 for Co, 04X415 for Zn (8, 13, 14). — Me¸"Me ¸ V Mo O with 04½4100 for 1~y y 2~y y 6 Mn/Li, Mn/Na, Zn/Li, and Mg/Li and 54½4100 for Co/Li, (8, 15—18). 545 0022-4596/97 $25.00 Copyright ( 1997 by Academic Press All rights of reproduction in any form reserved. 546 NAPRUSZEWSKA, OLSZEWSKI, AND ZIÖŁKOWSKI FIG. 1. Idealized outline of the brannerite-type MeV O structure [after (1)]. (a) A sheet of VO octahedra parallel to the (001) plane [nonequivalent 2 6 6 oxygens linked with one, two, or three vanadium atoms are labeled O(1), O(2), and O(3), respectively]. (b) Projection of the structure on the (010) plane with marked Me cations and VO groups on two different levels [0 and 0.5 of (b)]. 6 — Me¸'"Me ¸'V Mo O (Mn/Li, 1~x~y y x 2~2x~y 2x`y 6 Co/Li, Zn/Li, Mg/Li, Mn/Na, Cu`/Cu2` with variable X ranging up to 45 and ½ "100 (14—19). .!9 .!9 It may be relevant to recall that the stoichiometry of Me¸' may be expressed as Me ¸'V 1~x~y y x 2~2x~y Mo O "(1!x!y)MeV O #y¸VMoO #2xMoO . 2x`y 6 2 6 6 3 Me¸' compounds thus are pseudoternary MeV O — 2 6 ¸VMoO —MoO systems and their composition may 6 3 be represented using an equilateral triangle (cf. Fig. 2) with the composition variables X"100x marked along the MeV O —MoO and ¸VMoO —MoO arms (Me' and 2 6 3 6 3 ¸' solid solutions), ½"100y marked along the MeV O — 2 6 ¸VMoO arm (Me¸ solutions), and Me¸' solutions occu6 pying a part of the triangle area. In view of the above brief summary of the literature data one can conclude that the layered AB O brannerite-type 2 6 structure of vanadates or vanadomolybdates is composed of the following: i. VO /MoO octahedra, making (001) sheets and con6 6 taining ions of almost the same size. The octahedral ionic radii of V5` and Mo6` are 0.476 and 0.498 As , respectively [the selected examples of radii compiled by one of the authors (20) and by Shannon (21) are gathered later in Table 5]. All these octahedra are filled with cations. ii. AO [010] pillars linking the above-mentioned sheets, 6 which may contain either monovalent or bivalent cations or cation vacancies in variable amounts, the size of monovalent cations ranging from 0.728 (Li`) to 1.440 (K`) and that of bivalent cations from 0.707 (Mg2`) to 1.020 As (Hg2`) (20). iii. The stability of the structure apparently depends on the filling of the [010] pillars [strains resulting from the ion FIG. 2. Area of stability of the MnAg' solid solutions. Solid circles correspond to the studied samples composed of single brannerite-type phase. Open circles, behind the S S boundary, represent studied samples 1 2 contaminated with other compounds; ‘‘h’’ marks the samples containing Ag(Mn)-stabilized MoO . The stability ranges of MnLi' (15) and MnNa' 3 (18) are recalled in the upper part of the figure. 547 DEFECTIVE BRANNERITE-TYPE VANADATES sizes simultaneously present, and cation cavities and bond energy distribution (22) dependent on the variable amounts of Me2`, L`, and ']. The aim of our former work (8, 9, 13—19) and contemporary work has been to determine how far the brannerite-type structure is resistant to the consequences of doping indicated in point iii. Partial answers have been offered in the quoted papers. Now we intend to extend the studies on the Ag—Mn—V—Mo—O system (present work) and K—Mn—V— Mo—O system (23). 2. EXPERIMENTAL The compositions of all samples prepared and studied by XRD and partly by DTA are indicated in Fig. 2. Samples were synthesized by the amorphous citrate method (24), adapted empirically for the present systems (15). Reactants were AgNO , MnCO , NH VO , (NH ) Mo O · 4H O, 3 3 4 3 46 7 24 2 0.1 M HNO , 0.1 M ammonia, and citric acid, all of p.a. 3 grade. The procedure has been described (15). The final thermal treatment was carried out in air at 520—530°C for 24 h (or more, up to 96 h, if doubt arises whether the reaction had been completed); thereafter, the samples were quenched. XRD patterns were obtained with a DRON-2 diffractometer using CuKa radiation and in some cases with an internal standard of Al (a"4.0492 As at 25°C). Data were collected on a floppy disk and processed with the SMOK (Elector Co., Kraków, Poland) and LATCON (CERN library) programs. Reflections of 5°(2h(80° were used to determine the lattice constants. Phase identification was based on the published patterns of AgVMoO (10), 6 MnV O (8), MnMoO (25), V MoO (26), orthorhombic 2 6 4 2 8 MoO (27), hexagonal MoO (28—32), V O (33), 3 3 2 5 (Mo V )O (34), and Mn V Mo O (35). DTA x 1~x 5 0.15 0.3 0.7 3 (SETARAM M5 microanalyzer, 10°/min, Pt crucibles, samples of about 12 mg, air) and treatment of DTA curves were performed as described previously (13, 18). 3. RESULTS AND DISCUSSION 3.1. MnV O —MoO Arm of the MnV O —AgVMoO — 2 6 3 2 6 6 MoO Diagram 3 This arm, including Mn' solid solution [04X442 at room temperature or at most X"45 at elevated (530°C) temperatures], has been described in (8) and reconsidered in (35). In view of the recent data a new phase ½"Mn V M O "Mn V Mo O appears at 0.15 0.3 0.7 3 0.3 0.6 1.4 6 formal X"0.7. Therefore, we deal with Mn' within 04X442, with Mn'#½ at 42(X(70, ½ phase at X"70, and ½#o-MoO at 70(X(100 and finally with 3 orthorhombic MoO at X"100. [As mentioned in (35), 3 due to the kinetic hindrances a pure ½ phase has never been obtained]. The lattice constants of the ½ phase (35) along TABLE 1 Comparison of the Lattice Constants of the Y Phase, Mn0.15V0.3 Mo0.7O3 , in the Almost Pure State [26 Reflections, after (35)] and Average Values for Samples in the Multiphasic Area of the MnV2O6 –AgVMoO6 –MoO3 Triangle (11 Reflections, Present Paper) Parameter a (As ) b (As ) c (As ) b [deg] » (As 3) Pure ½ Average for the Mn—Ag—V—Mo—O field 11.832(2) 3.654(1) 10.335(3) 101.53(2) 437.8(3) 11.835(7) 3.660(4) 10.334(8) 101.51(5) 438.7(9) Note. The ½ phase is absent for X—½"30—70. For all of the remaining samples of the multiphasic area, the 2h values of the ½ phase are almost the same, proving that no solid solutions are formed. The average 2h values were thus used for XRD data analysis and the average lattice constants coincide with those of the pure ½ phase in the limit of error. Standard deviations are given in parentheses. with those belonging to the multiphasic area of the MnV O —AgVMoO —MoO system (except for X—½" 2 6 6 3 30—70) are given in Table 1. The 2h values of the ½ phase are almost the same for all the samples belonging to the triangle, proving that no solid solutions in the ½ phase are formed; therefore, the average 2h values are used for further analysis. 3.2. MnV O —AgVMoO Arm of the 2 6 6 MnV O —AgVMoO —MoO Diagram 2 6 6 3 As will be described further in detail MnV O and 2 6 AgVMoO are completely miscible and form MnAg solid 6 solutions in the entire compositions range (04½4100). The XRDs data are gathered in Tables 2 and 3. 3.3. AgVMoO —MoO Arm of the 6 3 MnV O —AgVMoO —MoO Diagram 2 6 6 3 As determined in (10) and confirmed in our laboratory Ag' solid solutions are formed within 04X412, and we deal with the coexistence of the saturated Ag' and o-MoO 3 within 12(X(100. The lattice constants of these phases are gathered in Table 4. 3.4. Range of Stability of MnAg' as Compared with MnNa' and MnLi' The compositions of all the MnAg' samples studied are indicated in Fig., 2 (solid and open circles). X-ray phase analysis revealed that all samples of the compositions marked by solid circles give the diffraction pattern of pure brannerite, entirely indexable in a monoclinic system with 548 NAPRUSZEWSKA, OLSZEWSKI, AND ZIÖŁKOWSKI TABLE 2 Lattice Parameters of MnAg 5 Mn12yAgyV22yMoyO6 Solid Solutions and Relative Changes Dp% between the End Members of the Series Sample X—½ c (As ) c sin b (As ) a (As ) b (As ) b (deg) MnV O " 2 6 00—00 00—10 00—20 00—30 00—40 00—50 00—60 00—70 00—80 00—90 00—100 "AgVMoO 6 9.315(3) 9.342(4) 9.360(6) 9.385(6) 9.414(4) 9.428(2) 9.449(3) 9.471(4) 9.484(3) 9.499(3) 9.506(4) 3.536(1) 3.549(1) 3.562(1) 3.572(1) 3.587(1) 3.603(1) 3.610(1) 3.621(1) 3.631(1) 3.647(1) 3.656(1) 6.754(2) 6.791(2) 6.829(3) 6.869(2) 6.916(3) 6.949(2) 7.007(3) 7.061(4) 7.120(2) 7.184(2) 7.249(2) *p% Mn/Ag 2.05 3.39 7.33 9.430(4) 1.24 111.01(1) *p% Mn/Na NaVMoO 6 3.656(1) 7.257(4) 3.39 7.37 6.775(4) 233.6(3) 8.38 13.48 3.650(1) 3.22 LiVMoO 6 6.637(1) !1.73 111.65(1) *p% Mn/Li 9.346(2) 0.33 6.168(1) 210.4(1) !1.04 2.50 112.66(2) 112.52(2) 112.41(3) 112.32(1) 112.29(2) 112.15(1) 112.02(1) 111.84(2) 111.50(1) 111.11(1) 110.71(2) 6.233(3) 6.273(3) 6.313(4) 6.354(4) 6.399(3) 6.436(2) 6.496(3) 6.554(4) 6.625(2) 6.702(2) 6.781(3) 8.79 TABLE 4 AgVMoO6 –MoO3 Arm of the Diagram: Lattice Constants of AgVMoO6 (0–100), Saturated AgU (both marked as B), and Orthorhombic MoO3 (o-M) X—½ » (As 3) 0—100 205.3(2) 207.0(2) 210.5(3) 213.0(3) 216.1(3) 218.6(2) 221.6(2) 224.8(3) 228.1(2) 232.2(2) 235.7(3) 12—88 30—70 40—60 60—40 74—26 90—10 100—0 9.506 3.659 7.225 110.48 235.41 9.512 3.658 7.217 110.47 235.27 9.484 3.657 7.205 110.37 234.18 9.488 3.658 7.211 110.41 234.55 9.539 3.663 7.247 110.67 236.99 t — — — — — — — — — 3.969 13.863 3.695 203.67 3.961 13.907 3.700 203.84 3.969 13.863 3.689 203.02 3.962 13.877 3.693 203.04 B a 9.506 b 3.656 c 7.249 b 110.71 » 235.66 o-M a b c » — — — — 3.962 3.963 13.864 13.850 3.701 3.696 203.28 202.90 Note. Parameters a, b, c in As , » in As 3, b in degrees. Standard deviations are comparable to those in Tables 1 and 2. t, traces. 14.79 as briefly recalled in the upper part of Fig. 2). This range extends between the MnV O —AgVMoO arm 2 6 6 of the triangle and the S S curve which could be approx1 2 imated by two straight (X—½) segments between (45, 00), (20, 50), and (12, 88). Behind the S S curve we deal with the 1 2 multiphasic area as described in detail in Section 3.7. The stability diagram of MnAg' strongly resembles those for MnLi' (15) and MnNa' (18) recalled in the upper part of Fig. 2. The following features are common to all three diagrams: (i) complete miscibility of MnV O and ¸VMoO , 2 6 6 Note. Parameters and *p% values for the MnNa and MnLi series are included for comparison. Standard deviations are given in parentheses. systematic extinctions for h#k"2n#1. The stability range of MnAg' thus constitutes about 40% of the MnV O —AgVMoO —MoO triangle area (as compared 2 6 6 6 with about 50% for MnLi' and about 60% for MnNa', TABLE 3 Dependence of the Lattice Parameters ( p) of MnLi, MnNa, and MnAg Solid Solutions on the Composition Parameter y and the Correlation Coefficient (corr). Parameter: p 5 a01a1 y1a2 y21a3 y3 Lattice parameter a b c c sin b b » Solid solution a 0 MnLi MnNa MnAg 9.315]10`00 9.313]10`00 9.314]10`00 3.193]10~04 2.018]10~03 2.663]10~03 !8.614]10~06 !7.027]10~06 0.991 0.998 0.999 MnLi MnNa/MnAg 3.535]10`00 3.536]10`00 1.108]10~03 1.338]10~03 !1.398]10~06 0.998 0.999 MnLi MnNa/MnAg 6.755]10`00 6.755]10`00 !1.176]10~04 3.305]10~03 1.636]10~05 0.998 0.999 MnLi MnNa/MnAg 6.232]10`00 6.240]10`00 !6.325]10~04 2.802]10~03 2.444]10~05 0.996 0.998 MnLi MnNa MnAg 1.126]10`02 1.126]10`02 1.126]10`02 !7.560]10~03 !6.123]10~04 !1.519]10~02 !3.005]10~05 !3.907]10~05 2.592]10~04 MnLi MnNa MnAg 2.052]10`02 2.053]10`02 2.054]10`02 5.103]10~02 2.301]10~01 2.252]10~01 4.700]10~04 7.574]10~04 Note. a, b, c, c sin b in As , » in As 3, b in degrees. a 1 a 2 a 3 !1.148]10~06 !3.078]10~06 corr 0.999 0.995 0.999 0.998 0.999 0.999 549 DEFECTIVE BRANNERITE-TYPE VANADATES FIG. 3. Lattice parameters of MnAg, MnNa, and MnLi as a function of composition (cf. Table 1). Squares, MnLi series; open circles, MnNa series; solid circles, MnAg series; half-solid circles are common to MnNa and MnAg. (ii) large stability area of Mn¸' (40—60% of the triangle area), (iii) slightly concave boundary of the saturated Mn¸' solutions. The main difference is that behind the S S line of 1 2 the saturated solid solutions we deal with the monophasic (MoO ) area for MnLi' [however, in view of (35) this con3 clusion is to be reconsidered due to the recently discovered presence of the ½ phase on the MnV O —MoO arm] and 2 6 3 with the multiphasic areas for MnNa' and MnAg'. It should be recalled here that MnV O — AgVMoO —MoO is only 2 6 6 3 a pseudoternary section of the MnO—Ag O—V O —MoO 2 2 5 3 system, which allows the presence of phases of composition not belonging to the triangle (15). 3.5. Lattice Parameters of MnAg as Compared with MnNa and MnLi The determined lattice parameters of MnAg solid solutions are listed in Table 2 along with their relative changes *p% between the end members of the series (p"a cell parameter a, b, 2 , and *p%"100*p/p ). The paray/0 meter—composition plots for MnAg compared with those for MnNa and MnLi are presented in Fig. 3. Exact comparison of the parameter—composition dependencies is crucial; therefore, the numerical, analytical data are gathered in Table 3. There are mainly two factors influencing the lattice constants of this structure: (i) the anisotropic tightness of packing of AO and BO octahedra and (ii) the size (radii) of the 6 6 ions. The relevant octahedral ionic radii (20, 21) are gathered in Table 5. The radii of Ag` taken from two references differ markedly. This problem has already been discussed in (17); the conclusion was that the radius of Ag` determined in (20) seems to be underestimated as compared with the remaining radii and in particular with Na` because of the unusual or uncertain structure of Ag O. It has been 2 concluded that both radii are comparable and close to those TABLE 5 Octahedral Ionic Radii r6 (As ) of the Considered Ions and the Relative Changes Dr% between the End Members of the MnLi, MnNa, and MnAg Series of Solid Solutions V5` Mo6` 1/2*r% (Mo/V) Mn2` Li` *r% (Li/Mn) Na` *r% (Na/Mn) Ag` *r% (Ag/Mn) Ref. 0.465 0.54 0.498 0.59 3.5 4.6 0.823 0.83 0.728 0.76 !11.5 !8.4 1.092 1.02 32.7 22.9 0.927a 1.15 12.6 38.6 (20) (21) aIn view of (17) it is rather close to 1.05 As . 550 NAPRUSZEWSKA, OLSZEWSKI, AND ZIÖŁKOWSKI DEFECTIVE BRANNERITE-TYPE VANADATES in (20). Table 5 also contains the relative changes in the ionic radii *r% between the end members of the MnLi, MnNa, and MnAg solid solutions considered. *r% values are defined as r `!r 2` M/ . *r%(Ag/Mn)"100 A' r 2` M/ [1] An increase in the ½ coordinate along the series of MnLi, MnNa, or MnAg solutions is equivalent to the rising extent of substitution of Mo for V (from 0 to 0.5) and Li, Na, or Ag for Mn (from 0 to 1). As the above-mentioned cations are ‘‘diluted’’ in the structure with oxygens of constant size, the *r% values can be used only to identify the trends in changes of lattice parameters along the series of solid solutions. The striking observation is that the linear (or almost) b (½) changes and the *b% are nearly the same for all three series of solid solutions studied. This means that b is sensitive solely to Mo/V substitutions. Indeed, BO octahedra 6 are tight in [010] and receptive to the substitution of a bit larger Mo6` for a smaller V5`. On the contrary, AO solids 6 are very bulky in this direction and have enough space to accept cations even as large as Ag` for Mn2`. However, tilting of the AO [010] chains is not excluded. 6 As for the c and c sin b parameters, their values diminish along the MnLi series, which is equivalent to the narrowing distance between the BO layers and due to the substitution 6 of smaller Li` for the larger Mn2` in AO octahedra, being 6 tight in [001]. An exactly (and strongly) opposite effect, despite the minor and nonlinear b(½) dependencies, is observed for MnNa and MnAg series of solutions where Na` and Ag` are more bulky as compared with Mn2`. c(½) and c sin b(½) overlap in the limit of error. The analytical expressions are common (Table 3). This seems to suggest again that the ionic radii of Ag` and Na` are nearly the same and equal to about 1.1 As . The lattice parameter a slightly increases along all three series of solid solutions studied and the slope increases in the order MnLi(MnNa(MnAg. This is surprising because the AO pillars have no link in this direction. The 6 observed fact may be ascribed to the increasing tilting of the AO pillars, forcing the BO layers to be more and more 6 6 loose in the [100] direction. As a consequence the unit cell volume » increases in the same order, although the differ- 551 ences between the MnNa and MnAg series are almost negligible. 3.6. Phase Diagram of the MnV O —AgVMoO System 2 6 6 as Compared with MnV O —NaVMoO and 2 6 6 MnV O —LiVMoO 2 6 6 The phase diagram of the pseudobinary MnV O — 2 6 AgVMoO system has been determined with DTA, as 6 shown in Fig. 4a. The temperatures of onset of endothermal effects (solid circles) and the final temperatures (open circles) have been determined accurately ($2°C). Positions of other effects, manifesting as shoulders or poorly separated peaks, were estimated tentatively (crosses). The identification of areas is based partly on the coincidence of the present diagram and that of the MoO —MnV O , studied pre3 2 6 viously (8) and recalled on the left side of Fig. 4a. The characteristic points of the diagram are indicated by their (½, temperature) coordinates. It should be noted that in view of the discovery of the ½ phase"Mn V M O 0.3 0.6 1.4 6 (35) at formal X"70 the MoO —MnV O phase diagram 3 2 6 (8) remains valid up to X+40 and requires revision at higher X. Correction of the rest of the diagram is intended to be done after the synthesis of the ½ phase in a pure form. So far, the coincidence along the ¹ axis above MnV O is 2 6 solely considered. MnV O is known (8, 15) to undergo a reversible phase 2 6 transformation at ½, ¹ (0, 540). The low-temperature amodification crystallizes in the brannerite-type structure, while the structure of the high-temperature b-polymorph is not known [nonindexed powder X-ray pattern is given in (8)]. At (0, 825), b-MnV O melts incongruently, decompo2 6 sing to Mn V O and liquid. The liquidus line in the 2 2 7 MnO—V O system over MnV O is attained at (0, 935). 2 5 2 6 On increasing ½, i.e., on doping with AgVMoO , MnAg 6 solid solutions are formed in both a and b matrices of MnV O . The solubility in b is at most (5, 644). There is 2 6 a narrow ‘‘triangular’’ field of a-MnAg and b-MnAg coexistence between (0, 540), (5, 644), and (20, 644), being a simple consequence of the phase rule. Brannerite-type a-MnV O and AgVMoO show misci2 6 6 bility in the entire composition range. Incorporation of dopant stabilizes a-MnAg against the aPb transformation. At (20, 644), a-MnAg, revealing the highest thermal stability among the a-type solid solutions, melts incogruently, FIG. 4. (a) Phase diagram of the pseudobinary MnV O —AgVMoO system based on DTA and X-ray investigations. Solid circles, temperatures of 2 6 6 onset of endothermal effects; open circles, final temperatures of endothermal effects; crosses, estimated positions of shoulders or poorly separated peaks. On the left the coincidence is shown with the previously determined (8) diagram of the MoO —MnV O systems (to be corrected for X(40, cf. 3.6). a and 3 2 6 b denote low-temperature (brannerite) and high-temperature polymorphs of MnAg, respectively; L, liquid. (b) Previously determined (16) phase diagram of the MnV O —NaVMoO system. (c) Previously determined (15) phase diagram of the MnV O —LiVMoO system. 2 6 6 2 6 6 552 NAPRUSZEWSKA, OLSZEWSKI, AND ZIÖŁKOWSKI decomposing to b-MnAg and liquid, represented by (5, 644) and (60, 644), respectively. On heating the samples of 5(½(40 above 644°C, and before total melting, one crosses the areas of coexistence of Mn V O #b-MnAg# 2 2 7 liquid and Mn V O #liquid. On heating the samples of 2 2 7 40(½(60, one directly crosses the liquidus. In the range of 20(½4100 and 610(¹(644, there are two areas of coexistence of a-MnAg#liquid with a shallow minimum of solidus—liquidus at (70, 596). The previously determined (16) phase diagram of the MnV O —NaVMoO system, recalled in Fig. 4b, has exact2 6 6 ly the same shape; even the characteristic points of the diagram almost overlap, which remains analogous to the X-ray pictures of both systems. Figure 4c recalls the phase diagram of the MnV O —LiVMoO system (15), which is 2 6 6 highly analogous to both formerly described. There are two minor differences: (i) the areas involving Mn V O are 2 2 7 smaller, (ii) a minimum solidus line at about ½"70 does not exist for MnLi because the *¹ between the peritectic temperature and melting point of LiVMoO is much higher 6 as compared with both remaining systems. 3.7. Multiphasic Area of the Mn V O —AgVMoO —MoO 2 2 6 6 3 Diagram Table 6 contains the lattice parameters of the almost saturated MnAg' solid solutions. In accord with MnAg the lattice parameters increase systematically with ½, which again is a consequence of the larger ionic radii of the dopants as compared with the mother cations. As shown in Tables 7 and 8, behind the S S curve 1 2 we deal with the multiphasic area in which the saturated brannerite-type (B) phases coexist almost everywhere with the Ag—Mn-stabilized hexagonal MoO (h-M) (to be 3 commented on below), ½"Mn V Mo O and (Mo 0.15 0.3 0.7 6 x V )O (VM), and sometimes with orthorhombic MoO 1~x 3 3 (o-M) and MnMoO (MM); the XRD reflections of the 4 latter usually overlap with those of other phases which makes it impossible to determine the lattice parameters. As TABLE 6 Lattice Parameters for the Almost Saturated MnAgU 5 Mn12x2y AgyUxV222x2y Mo2x1yO6 Solid Solutions X—½ 40—00 35—10 30—20 30—30 20—40 20—50 20—60 10—80 12—88 a 9.378 9.411 9.415 9.468 9.481 9.444 9.490 9.513 9.506 b 3.613 3.617 3.617 3.631 3.626 6.633 3.644 3.654 3.659 c 6.751 6.821 6.850 6.905 6.950 7.004 7.070 7.169 7.225 b 112.18 112.08 112.06 111.90 111.92 111.51 111.25 110.98 110.48 » 211.8 215.1 216.2 220.2 221.6 223.6 227.9 232.7 235.4 Note. Parameters a, b, c are given in As , » in As 3, b in degrees; standard deviations are at most 3 at the last decimal indicated. TABLE 7 Lattice Parameters of the Mn–Ag-Stabilized Hexagonal MoO3 Sample X—½ a (As ) c (As ) » (As 3) 30—60 40—30 50—10 50—40 60—10 60—20 70—10 70—20 80—10 Average 9.863(14) 9.864(08) 9.860(03) 9.914(12) 9.864(08) 9.886(11) 9.839(07) 9.885(19) 9.867(07) 9.871(10) 11.000(12) 10.929(09) 10.951(05) 10.886(18) 10.963(12) 10.957(11) 10.920(18) 11.012(19) 10.955(06) 10.953(12) 927(4) 920(2) 922(1) 927(4) 924(3) 927(3) 915(3) 932(5) 924(2) 924(3) Note. Standard deviations in parentheses. already mentioned the lattice constants of ½ are gathered in Table 1 and those of the remaining phases are given in Table 8. In view of identifying the presence of the Ag-containing h-MoO it seems relevant to recall the following. h-MoO 3 3 belongs to the family of defective phases containing dopants (29—31, 33). In all these structures the hexagonal molybdate framework (partly doped with V) consists of zig-zag chains of edge-shared octahedra parallel to the c axis; these chains share corners to create large [001] tunnels which are suited to accommodate large alkali metals (¸"K, Rb, Cs, or ammonium, alone or hydrated), which in turn stabilize a series of isomorphous compounds. According to the quoted literature the range of possible changes in composition is very narrow and in practice (a dry) h-MoO" ¸ (V Mo )O ("Q; ¸"monovalent element). The 0.13 0.13 0.87 3 dimensions of their unit cells (dry phases) depend slightly on the chemical composition. For ¸"K, i.e., for h-MoO (K), 3 they are a"10.481 As , c"3.701 As , and »"352.1 As 3. The doubled Q formula belongs to our MeV O —¸VMoO — 2 6 6 MoO triangle with coordinates X—½"76—24. h-MoO 3 3 with ¸"Ag has never been mentioned in the literature; Ag was believed to be too small to stabilize the peculiar hMoO structure. Indeed, along the AgVMoO —MoO axis 3 6 3 of the system and beside X—½"12—88 we deal with mixtures of saturated Ag' and orthorhombic MoO . However, 3 inside the MnV O —AgVMoO —MoO triangle we doubt2 6 6 3 less deal with the h-MoO phase, apparently stabilized by 3 both Ag and Mn, i.e., h-MoO (Ag, Mn), as summarized in 3 Table 7. The lattice constants of h-MoO (Ag, Mn) are on 3 average a"9.871 As and c"10.953 As with »"942 As 3. It is not strange that a is a bit smaller as compared with h-MoO (K) because the ionic radii of Ag` and Mn2` are 3 smaller as compared with that of K`. The parameter c corresponding to the direction of the [001] tunnels is three times higher for h-MoO (Ag, Mn) as compared with 3 553 DEFECTIVE BRANNERITE-TYPE VANADATES TABLE 8 Lattice Constants of Compounds Present in the X–Y Samples Belonging to the Multiphasic Area of the MnV2O6 –AgVMoO6 –MoO3 Triangle Pure Mn—B Phase 30—60 40—30 50—10 50—40 60—10 60—20 70—10 70—20 75—20 80—10 Pure Ag—B B a b c b » 9.315 3.546 6.754 112.66 205.3 V O 2 5 9.50 3.65 7.14 110.9 231.5 9.43 3.63 6.90 111.8 219.0 9.41 3.63 6.82 112.0 215.8 9.49 3.66 7.04 110.9 228.7 9.41 3.63 6.84 112.1 216.5 9.35 3.63 6.92 112.8 216.2 9.40 3.63 6.84 111.8 216.8 9.39 3.61 6.95 112.9 217.1 ? 9.36 3.61 6.92 112.9 215.3 9.506 3.656 7.249 110.71 235.66 VM a b c » 11.519 3.563 4.373 179.2 MoO 3 11.75 3.66 4.40 187.1 11.54 3.57 4.34 178.8 11.50 3.57 4.33 178.8 11.69 3.57 4.39 183.0 11.49 3.57 4.34 177.8 11.59 3.56 4.34 178.9 11.54 3.58 4.35 179.5 11.54 3.54 4.35 178.4 11.53 3.54 4.38 181.1 11.58 3.56 4.37 179.2 — 3.963 13.855 3.696 202.9 — — — — — — 3.96 13.87 3.69 202.7 3.97 13.86 3.69 203.1 3.97 13.87 3.69 203.2 3.97 13.87 3.69 203.2 3.963 13.855 3.696 202.9 — See Table 7 See Table 1 ? ? # # ? # # — — — — o-M MM h-Mo Y a b c » MoO 3 Note. B"brannerite, VM"(Mo V )O , o-M"orthorhombic MoO , MM"MnMoO , Mn—B"MnV O , Ag—B"AgVMoO . Parameters x 1~x 5 3 4 2 6 6 a, b, c are given in As , » in As 3, and b in the degrees and are rounded according to the worst standard deviation in each set. #, present; —, absent; ?, perhaps present. Parameters for the almost saturated Mn' (X—½"40—00) and Ag' (X—½"12—88) are given in Table 6. h-MoO (K), which may be ascribed either to the presence of 3 two dopants or to minor changes in the (MoO ) host xn lattice. Further attempts to obtain a pure h-MoO (Ag, Mn) 3 are necessary. 4. CONCLUSIONS MnV O of the brannerite-type structure (below 540°C), 2 6 referred to as a, doped with MoO and Ag O forms isomor3 2 phous solid solutions MnAg'"Mn Ag ' V 1~x~y y x 2~2x~y Mo O (', cation vacancy in the original Mn position). 2x`y 6 They belong to the pseudoternary MnV O —AgVMoO — 2 6 6 MoO system which may be visualized as an equilateral 3 triangle with coordinates X"100x and ½"100y marked along its MnV O —MoO and MnV O —AgVMoO arms, 2 6 3 2 6 6 respectively. The particular cases are MnAg"Mn 1~y Ag V Mo O (x"0), Mn'"Mn ' V Mo O y 2~y y 6 1~x x 2~2x 2x 6 (y"0), and Ag'"Ag ' V Mo O (x#y"1). 1~x x 1~x 1`x 6 MnV O and AgVMoO show miscibility in the entire 2 6 6 composition range (MnAg). The opposite boundary of the MnAg' phases, S S , passes through the X, ½ points 1 2 (45, 00), (20, 50), and (12, 88). As the ionic radii of dopants (Ag`/Mn2`, Mo6`/V 5`) are larger than those of the mother cations the lattice constants of both MnAg and almost saturated MnAg' (along S S ) rise systematically 1 2 with ½. Behind the S S boundary we deal with the 1 2 saturated MnAg' accompanied by Ag—Mn-stabilized hexagonal MoO , the ½-phase Mn V Mo O , and 3 0.15 0.3 0.7 3 (Mo V )O , and in some areas by MnMoO and orthox 1~x 5 4 rhombic MoO . The relation between the ¸ size and the 3 stability of Mn¸' is not clear at present. From the determined phase diagram of the pseudobinary MnV O — 2 6 AgVMoO system, the limit of thermal stability of a-MnAg 6 extends between the ½, ¹ points (0, 540), (20, 644), (70, 596), and (100, 610). The phase diagram shows (i) a narrow double-lens-type solidus—liquidus gap at high values of ½, with a shallow minimum at ½, ¹"(70, 596); (ii) two peritectic meltings at lower ½ (yielding the high-temperature bMnAg polymorph at 644°C and Mn V O at 726°C); and 2 2 7 (iii) a small area of b-MnAg within 540(¹(825 with ½ +5 at 644°C. Close analogy of this diagram with those .!9 of MnV O —LiVMoO and MnV O —NaVMoO has been 2 6 6 2 6 6 observed. REFERENCES 1. R. Ruh and A. D. Wadsley, Acta Crystallogr. 21, 974 (1996). 2. J. Angenault and A. Rimsky, C.R. Acad. Sci. Paris 267, 227 (1968). 554 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. NAPRUSZEWSKA, OLSZEWSKI, AND ZIÖŁKOWSKI J. C. Buloux and J. Galy, Bull. Soc. Chim. Fr. 3, 736 (1969). J. Angenault, Rev. Chim. Miner. 7, 651 (1970). H. N. Nag and C. Calvo, Canad. J. Chem. 50, 3619 (1972). E. E. Sauerbrei, M.Sc. thesis, McMaster University, Ontario, Canada, 1972. C. Calvo and D. Manolescu, Acta Crystallogr. Sect. B 29, 1743 (1973). R. Kozłowski, J. Ziółkowski, K. Mocała, and J. Haber, J. Solid State Chem. 35, 1 (1980); erratum 38, 138 (1981). K. Mocała and J. Ziółkowski, J. Solid State Chem. 69, 299 (1987) and quotations. J. Galy, J. Darriet, and B. Darriet, C.R. Acad. Sci. Paris Ser. C 264, 1477 (1967). B. Darriet and J. Galy, Bull. Soc. Fr. Mineral. Cristallogr. 91, 325 (1968). B. Darriet and J. Galy, C.R. Acad. Sci. Paris. Ser. C 266, 1698 (1968); JCPDS 21-710. K. Mocała, J. Ziółkowski, and L. Dziembaj, J. Solid State Chem. 56, 84 (1985). K. Mocała and J. Ziółkowski, J. Solid State Chem. 71, 426 (1987). J. Ziółkowski, K. Krupa, and K. Mocała, J. Solid State Chem. 48, 376 (1983). B. Masłowska and J. Ziółkowski, J. Solid State Chem. 110, 74 (1994). K. Mocała and J. Ziółkowski, J. Solid State Chem. 71, 522 (1987). 18. B. Masłowska and J. Ziółkowski, J. Solid State Chem. 87, 208 (1990). 19. T. Machej, R. Kozłowski, and J. Ziółkowski, J. Solid State Chem. 38, 97 (1981). 20. J. Ziółkowski, J. Solid State Chem. 57, 269 (1985). 21. R. D. Shannon, Acta Crystallogr. Sect. A 32, 751 (1976). 22. J. Ziółkowski and L. Dziembaj, J. Solid State Chem. 57, 291 (1985). 23. B. Napruszewska, P. Olszewski, and J. Ziółkowski, in preparation. 24. P. Courty, H. Ajot, and C. Marcilly, Powder ¹echnol. 7, 21 (1973). 25. S. C. Abrahams and J. M. Reddy, J. Chem. Phys. 43, 2533 (1965). 26. M. A. Eick and L. Kihlborg, Acta Chim. Scand. 20, 1659 (1966). 27. JCPDS 9-387. 28. JCPDS 21-569. 29. B. Darriet and J. Galy, J. Solid State Chem. 8, 189 (1973). 30. Y. Hu and P. K. Davies, J. Solid State Chem. 105, 489 (1993). 31. Y. Hu and P. K. Davies, J. Solid State Chem. 119, 176 (1995). 32. N. A. Caiger, S. Crouch-Baker, P. G. Dickens, and G. S. James, J. Solid State Chem. 67, 369 (1987). 33. H. G. Bachman, F. R. Ahmed, and W. H. Barnes, Z. Kristallogr. 115, 110 (1961). 34. F. Y. Robb, W. S. Glaunsinger, and P. Courtine, J. Solid State Chem. 30, 171 (1979). 35. J. Ziółkowski, P. Olszewski, and B. Masłowska-Napruszewska, submitted for publication. .