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Powder Metallurgy and Metal Ceramics, Vol. 39, Nos. 3-4, 2000 INTERACTIONS

I N T H E LiPO3 - MOO3,

N a P O 3 - MOO3, A N D K P O 3 - M o O 3 S Y S T E M S

V. V. Lisnyak, M. S. Slobodyanik, and V. S. Sudavtsova UDC 546.185

The MP03 - MoO3 (M = Li, Na, K) pseudobinary systems have been examined by differential thermal analysis and x-ray phase analysis. The phase diagrams have been derived, and the temperatures and compositions of the eutectics have been determined. The compounds have been examined by x-ray phase analysis, infrared spectroscopy, and diffuse reflection spectroscopy. The conductivity and optical properties of the compounds MMoO2(P04) (M = Li, Na, K) have been determined. The coordinates of the liquidus lines have been used to calculate the activities of the components in the MP03 - MoO3 systems. Those activities show large negative deviations from ideal solutions. This indicates strong interaction between particles in the liquid state. Keywords: metal phosphates, phase diagrams, component activity, electrical conductivity.

Research has been done on the crystallization of compounds from melts in the M P O 3 - MoO3 pseudobinary systems, in which M = Li, Na, or K, in connection with the ferroelectric and nonlinear-optical properties of various materials based on metal phosphate-molybdate fluxes [1, 2]. One can make a sound choice of the best compositions for the initial mixtures or the ionic molten solution in order to synthesize materials with certain properties only if there is systematic evidence on the physicochemical properties of the initial systems. It is particularly important to have detailed data on the melting points of the mixtures and the compound crystallization temperatures, including the dependence on the composition of the initial mixtures. The phase equilibria in the binary systems have been determined by differential thermal analysis based on an NTR-64 pyrometer with a recording attachment. The thermal stability of the products was determined, and we recorded heating and cooling curves with a Q-1500 derivative recorder. The structure of the crystalline specimens, which were synthesized from pseudobinary systems, were examined with a DRON-3 diffractometer used with CuKa radiation. The diffuse reflection spectra were recorded with a Specord M-40 instrument. The magnetic susceptibility was measured with a magnetometer operating on the Faraday-Soxsmith principle at 295 K. The optical studies were made with a Bipolar PI polarizing microscope and MIM-8 scanning microscope. We determined the electrical conductivity, the dependence on temperature, and the other electrophysical parameters on disks prepared by crushing the solutions to powders and pressing them (d = 11.2 mrn, h = 2-4 mm) followed by sintering at temperatures 30-40°C below the melting points. The surfaces of the disks were ground and were coated with silver electrodes by cathode sputtering. The specific conductivities of these polycrystalline specimens were determined with a TR-145 teravoltmeter and P-4105 stabilizer, while the dielectric parameters were measured with an E8-2 bridge. The molybdenum and phosphorus contents of the crystalline specimens were determined by x-ray fluorescence with a Philips PW-1400 instrument, while the lithium contents were determined by flame photometry with an NPO-51 instrument fitted with an FEP-I attachment [3]. The samples were dissolved in an equimolar HNO3:H2SO4 mixture with the addition of concentrated HF solution. We used molybdenum oxide MoO3 (analytical grade) and the metaphosphates of the alkali metais: NaPO3 (chemically pure), KPO3 (chemically pure), and LiPO3 (pure). We also used the reaction of lithium carbonate with the calculated amount of orthophosphoric acid (85%) and then melted the product at 700°C. The melting point of the vitreous mass was 690 + 5°C, which corresponds to the melting point of LiPO3 [4]. Taras Shevchenko Kiev National University. Translated from Poroshkovaya Metallurgiya, Nos. 3-4(412), pp. 33-40, March-April, 2000. Original article submitted April 14, 1999. 1068-1302/00/0304-0139525.00 ©2000 Kluwer Academic/Plenum Publishers

139

"r,

T,

oc

°C

o

7oo[-

A

~

686c

I

750 70{

65( NN~ 600 ~

o

606°C 593°C 400

550 500

I I

LiPO3 0,1

I

I

0.3

I

I

0.5

i

i

0.7

350~ 300 ' I 0.9 MoO3 NaPO3 0.2 i

I

I

I

0.4 0.6 X Mo03

XMo03

I

I

0.8

I MoO

3

T,°C 750[~

684Oc~

::oF 6OO 55O 5OO 45O 4OO 350 300

I

I

KPO3 0.2

I

I

I

0.4

0.6

1

I

0.8

I

MoO~

X Mo03

Fig. 1. Phase diagrams for the MPO3 - MoO3 systems, in which M = Li, Na, or K.

The alkali metaphosphates were dried to constant mass at 120-200°C before use and were then heated to 350°C for 3 h. Melt specimens of mass 1-2 g were prepared by melting the alkali metaphosphates with the calculated amounts of molybdenum trioxide at temperatures 25-50°C above the melting points of the alkali metaphosphates for 1 h. The mixtures were homogenized by stirling. The resulting cooled homogeneous glass was annealed at 500-550°C for 60-80 h. The metaphosphates of the univalent metals when used with molybdenum trioxide in a molar ratio of 25% MPO3:75% MoO3 form liquids that readily crystallize. The crystallization of the small amount of glass in the specimens was accelerated by the treatment, and so the liquid was brought to complete crystallization. Figure 1 shows the phase diagrams for the MPO3 - MoO3 systems, in which M = Li, Na, or K. Compounds are formed that melt congruently with molar ratios of the initial materials of 1:1. We found that these systems contain alkali molybdenophosphates, which are formed by the reaction MPO3+ MoO3 = MMoO2(PO4), M = Li, Na, K. The phase diagrams for these systems may be triangulated on the basis of two binary systems: on the one hand, molybdic anhydride with alkali metal molybdenophosphate, and on the other, alkali metal molybdenophosphate with alkali metal

140

metaphosphate, each of which is of eutectic type. The eutectic horizontals in the molybdenum trioxide-alkali metal metaphosphate systems occur at 570 and 593°C, 594 and 610°C, and 616°C for the lithium, sodium, and potassium compounds respectively. The heating thermograms showed substantial exothermic effects in the temperature range 590620°C, which represent the crystallization of the vitreous fluxes. The thermodynamic parameters of the solutions enable one to establish the character of the particle interactions. However, those parameters have so far not been determined for melts in the MPO3 - MoO3 systems. We therefore forecast the component activities from the phase diagrams by several methods. Schr6der equation is the most appropriate for calculating component activities in the region of solid componentliquid equilibrium: A G i = ASmp(T- Tmp), in which ASnw and Tin0 are the melting entropy and melting point of component i, while T is the melting point of the liquid. As the refractory component in all these systems is molybdenum(Vl) oxide, we determined its activity at 1100 K. We used the Hauffe-Wagner equation for the region of coexistence of the compounds with the liquids: I xi ATdx l - xi Ar + (1- xi) I • A~t~(T,X) = t~Sr~¢omp~t xi _ Xi x~ (xi - Xi )-

in which Xi and x, are the molar fractions of MoO3 in the compound and in the liquid, with Ala~{T,X) the chemical potential of component i with respect to the state in the compound, and AT is the difference between the melting points of the compound and the solution in equilibrium with it. We referred the AP.i to the standard state (pure component) from the value of aMoo3 at the eutectic point, which was calculated from the Schr6der equation. The activity of MoO 3 in the region of the MPO 3 - liquid equilibrium was extrapolated from the values calculated by means of the SchrOder and Hauffe-Wagner equations. They also were used to integrate the Gibbs-Duhem equation. The resulting MoO3 activities for the entire composition range were fitted to polynomials and used in the numerical integration of the Gibbs-Duhem equation. Figure 2 shows the calculated activities of the components in the MPO3 - MoO3 liquids. The interaction between the components is strongest in the KPO3- MoO3 system. This is ascribed to the size factor, which has its most prominent effect in this system. The atomic radii of the ions for the interacting components that form the orthophosphates of molybdenum containing the alkali metal are as follows: lithium 0.068 rim, sodium 0.098 nm, potassium 0.133 nm, and molybdenum 0.139 nm [5]. We checked the component activities in the MPO3 - MoO3 systems as calculated from Schr6der equation and obtained by integrating the Gibbs-Duhem equation from the values of aMoO3 calculated from Schr6der equation by the use of the enthalpies of melting for MPO 3, in which M = Li, Na, or K. Figure 2 shows good agreement between the values of aMoo3 calculated by the two methods. Table 1 gives the standard enthalpies of formation for sodium and potassium metaphosphates and MoO3 [4]. The enthalpies of formation for compounds of MMoO2(PO4) type were estimated from data on AfHOMPo3, AfHOoo3 and the excess Gibbs energies of formation for the compounds derived from the phase diagrams (Table 1). We calculated the enthalpies and entropies of melting for the compounds, which melt congruently, from the values for the initial components on the basis of additivity. The entropies of melting of these compounds were calculated from A n ~ = AmpH/Tmp, and those entropies as derived by these methods are correlated. We determined A/H098 for LiMoO2(PO4) and KMoO2(PO4), which are similar one to another; the formation of NaMoO2(PO4) is accompanied by an appreciably smaller exothermic effect. The specific cyclic structure of the condensed phosphates undoubtedly affects the activities of the alkali-metal metaphosphates in their interactions with polyvalent-metal oxides. The polyphosphate groups are broken up in the liquids

141

a~

0.9 I 0.8, O.7

O.6 0.5!

i

4

0.4 i

0.31 0.2 0.1 0

MPO3 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9MOO3

XMo03 Fig. 2. Solution component activities in the MPO3 - MoO3 systems. (1-3) Activities of MoO3 in MPO3 - MoO3 liquids, in which M = Li, Na, or K as calculated for 1100 K from the Schrtider and Hauffe-Wagner equations; (4-6) activities of LiPO3, NaPO3, and KPO3.

TABLE 1. Thermodynamic Characteristics of Some Alkali Metal Metaphosphates and MMoO2(PO4) Compounds

Compound I

k,molI so ,Kmo I

O,mo

AmpS, J/K.mol

AmpSad, J/K.mol

Trap , ° C

N aPO3 [4]

- 1220.0

95.48

17.28

19.20

627.6

LiPO3 [4]

-1254.8

72.38

23.01

23.86

690

MoO3 [4]

-754.9

78.28

52.50

49.10

796

KPO 3 [4]

-1246.0

108.00

18.41

16.41

LiMoO2(PO4) [*]

-2029.1

150.60

75.51

86.00

72.96

606

NaMoO2(PO4) [*]

-1994.9

174.28

69.78

72.76

-2020.9

186.30

70.91

71.55

68.30 66.04

686

KMoO2(PO4) [*]

813

718

[*] Our data.

because of breakage of the P - O bonds in the P - O - P chains. The chemical activity of the metaphosphates is dependent on the noncentrosymmetric structure of the central groups, i.e., to the basic structure of the chain metaphosphates. The solution of molybdenum trioxide tends to accentuate the breakup in the liquid with the formation of strong Mo - O - P bonds, which are more favorable in energy. There is also an interaction of another type in the liquid: the transition of molybdenum trioxide in accordance with MoO3 ---) Mo O 2+ , which is accompanied by a change in the anionic composition of the liquid. The compositions of these compounds have been established by analysis for molybdenum and phosphorus by x-ray fluorescence and for the alkali metals by flame photometry (Table 2). These specimens were indicated by the optical studies as being single-phase polycrystalline compounds that did not contain any appreciable amounts of other unidentified phases. The x-ray patterns indicate that the potassium and lithium representatives of the MMoO2(PO4) series crystallize in the monoclinic system and are not isostructural one with the other. The phosphate that contains sodium has been identified

142

from the x-ray phase analysis (ACTM catalogs) as the ~x phase NaMoO2(PO4) [6]. Table 3 gives the unit-cell parameters of these compounds. The crystals of MMoO2(PO4) are of elongated prismatic habit, with NaMoO2(PO4) and KMoO2(PO4) crystallizing as hexagonal prisms. The interference colors are bright. The high birefringence is characteristic of the series of binary orthophosphates of lithium, sodium, and potassium: LiMoO2(PO4) - - ng = 1.656, np = 1.65; NaMoO2(PO4) - - ng = 1.732, np= 1.719; KMoO2(PO4) - - ng = 1.71, np= 1.70. The maximum difference between these indices occurs with the sodium compound. We examined the infrared spectra of specimens of the glass after annealing and cooling to room temperature and also of the polycrystalline compounds, which showed that the glass contained the pO34- grouping found in the polycrystalline specimens. The glass spectra were also accompanied by low-intensity bands due to the v~, P - O - P, vs P - O - P vibrations in the diphosphate anion. This indicates that the primary polyphosphate chains have been destroyed together with the (P - O - P)n bonds with the formation of diphosphates and orthophosphates. Figure 3 shows the IR spectra of polycrystalline specimens with the composition MMoO2(PO4); the frequencies and assignments of the absorption bands are given below for the spectra of molybdenum and potassium orthophosphates.

Assignment of the IR Spectra of Compounds with the Composition MMoO2(PO4) -I Absorption frequency, cm

Assignmentof characteristic vibrations LiMoO2(PO4)

430 510,563,625.650 935,955 1008,1030(S)*

6s(P04) 6as (PO4); kas and ks (Mo - O) in [MoO6] vs(PO4), Vs and Vas(Mo - O) in [MOO6] vs(PO4) and Vas (PO4)

NaMoO2(PO4) 410,430,465,480(S) 520,540,630,650 920(S).950 1010,1020(S)

ks(P04) kes(P04); 6asand 6s(Mo - O) in [MOO6]

Vs and Vs [MOO2] vs(PO4) and Vas(PO4) KMoO2(PO4)

465,480(S) 594,650 910(S),955

ks(P04) 8as( P04); kas and ks (Mo - O) in [MOO6] Vs and Vas(Mo - O) in [MOO2]

1008,1058,1100,1135(S)

v~(P04) and Vas (P04)

* Shoulder.

The specific conductivities of these compounds are 10-s-10-13 £2-I.cm-' at 25°C, that's why the materials belong to the class of insulators. The specific conductivities of these materials decrease monotonically as the size of the cation increases. The temperature dependence of the dielectric constant and conductivity indicate that the size and nature of the alkali-metal cations affect the dielectric parameters, and the same applies to the dielectric loss tangent in the range 25-350°C. Potassium molybdenyl orthophosphate is an insulator showing little variation with temperature. Compounds MMoO2(PO4) with M = Li or Na have higher specific conductivity than KMoO2(PO4); their dielectric constants increase in direct proportion 143

LiMoO2(P0 4) ICd~oO2(PO,)

NaMoO2(PO

,

J

=

~

=

t

1 = 1 = "1"100900 700 =

~)

-1

=

V,

Fig. 3. IR spectra of NaMoO2(PO4), LiMoO2(PO4), and KMoO2(PO4).

T A B L E 2. Compositions of Compounds in the M P O 3 - MoO3 Systems

Com.n0

I

M

I

Mo, mass %

P, mass %

LiMoO2(PO4)

3.044/2.98"

41.74/40.43

13.48/13.2

NaMoO.(PO4)

9.35/9.25

39.02/38.90

12.60/12.32

KMoO2(PO4)

14.92/14.90

36.62/36.53

11.82/11.52

* The value before the slash is the calculated one, while that after it is the analysis result.

T A B L E 3. Unit-Cell Parameters of Compounds with the Composition MMoO2(PO4)

Comun, Ispace,rouplonm KMoO2(PO4) ct-NaMoO2(PO4) LiMoO2(PO4)

brim ,n lY'e,

p,~cm 3

Ref

Monoclinic

1.080

0.785

1.130

111

3.67

[*]

P2 i/n

1.208

1.196

0.636

91.2

3.56

Monoclinic

1.154

0.805

1.232

110

2.73

[6] [*]

[*] Our data.

T A B L E 4. Electrophysical Parameters of MMoO~(PO4) Compounds lgo

-In(tg~i) Compound

tg8 (200°C)

Eact, eV 100 ° C

IO0°C

200° C

200°C

LiMoO2(PO4)

8.2

7.5

1.0

8.3

5.3

NaMoO2(PO4)

10.5

10.2

1.0

8.5

7.5

1.103

KMoO2(PO4)

11.0

11.0

0.86

9.04

8.1

2-102

144

4.103

to temperature. The relatively high conductivities in the compounds with small cations is due to the greater mobility of the carriers, while the high values for the activation energy are due to the strong bonds to the anion framework. All these compounds are diamagnetic, which shows that there are no atoms, particularly molybdenum in the +5 oxidation state, that make an electronic contribution to the conductivity. The diffuse reflection spectra do not indicate that the molybdenum atoms have degrees of oxidation less than six. The alternating-current conductivity at 100-200°C follows the Arrhenius equation ~ - = aoexp(-E=tlRT); on the whole, the values of a - , activation energy Eact, and conductivity ~0 decrease as one passes from lithium to potassium. The dielectric loss tangent tan ~5shows the same form of temperature dependence. All the compounds show appreciable dielectric losses and low conduction activation energies. The MMoO2(PO4) compounds are insulators with ionic conductivity. The high conductivity of the lithium compound is due to the carrier mobility, while the large activation energy is due to the strong bonds of the cations to the anion framework (Table 4). The properties of the compounds and components in the MPO 3 - MoO3 pseudobinary systems indicate that these compounds are similar, with any differences explained by the size factor.

REFERENCES I.

U. Peuchert, L. Bohaty, and R. Frohlic, "New nickel sodium and molybdyl phosphate with KTP structure," Acta

Crystallogr. C., 51, No. 9, 1719-1721 (1995).

3. 4.

U. Peuchert, L. Bohaty, and J. Schreuer, "Novel series of temperature sensors materials," in: 15th Eur. Crystallogr. Meet., Munich (1994), p. 535. V. A. Nazarenko (ed.), The Analytical Chemistry of the Elements [in Russian], Nauka, Moscow (1973). V. P. Glushko (ed.), Thermal Constants of Substances: Handbook, in 10 issues [in Russian], Issue 10, Part 1,

5.

Moscow ( 1981 ). V. F. Tikavii (ed.), General Chemistry [in Russian], Izd. Moscow Univ., Moscow (1987).

6.

P. Kierkegaard, "'Refinement of NaMoO2(PO4) structure," Arkiv Kemi, 284, No. 1, 31-35 (1956).

7. 8.

I. Schulz, "Uber crystallization fiir NaWO2(PO4) crystall," Zh. Anorg. Chem., 284, No. 1, 36-40 (1956). P. I. Fedorov, M. V. Mokhoseev, and V. I. Krivenko, "Interactions in molybdate-phosphate liquids," Zh. Neorg. Khim., VII, No. 1, 76-80 (1962).

2.

145