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Phase equilibria in the ternary system: MgONa2OP2O5
Solid State Sciences 2 (2000) 489 – 493
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Phase equilibria in the ternary system: MgONa2OP2O5 Partial system: Mg3(PO4)2Mg4Na(PO4)3Na4P2O7Mg2P2O7 Teresa Podhajska-Kaz´mierczak * Department of Inorganic Chemistry, Faculty of Engineering and Economics, Academy of Economics, Komandorska 118 /120, 53 -345 Wrocl*aw, Poland Received 22 July 1999; received in revised form 19 October 1999; accepted 15 March 2000
Abstract The partial system Mg3(PO4)2Mg4Na(PO4)3Na4P2O7Mg2P2O7 in the ternary system MgONa2OP2O5 was investigated using thermal and X-ray diffraction analyses and microscopy, and its phase diagram has been determined. In this range of composition, two binary phosphates occur: Mg4Na(PO4)3 and Mg6Na8(P2O7)5. The former melts incongruently (at 1155°C) and the latter does congruently (at 808°C). In the partial system of interest, the two sections Mg4Na(PO4)3Mg2P2O7 and Mg4Na(PO4)3Mg6Na8(P2O7)5 are studied, and their phase diagrams are established. The partial system is divided into three partial ternary systems in which two ternary eutectics and one ternary peritectic occur. © 2000 E´ditions scientifiques et me´dicales Elsevier SAS. All rights reserved. Keywords: Partial systems; Binary phosphates; Ternary systems
1. Introduction Investigation of the phase dependencies, which occur in the partial system Mg3(PO4)2Mg4Na(PO4)3Na4P2O7Mg2P2O7, is a successive stage of author’s work on the ternary system MgONa2OP2O5. The above range of composition is bounded by the four subsystems: (1) Mg3(PO4)2Mg2P2O7, (2) Mg3(PO4)2Mg4Na(PO4)3, (3) Mg4Na(PO4)3Na4P2O7, (4) Mg2P2O7Na4P2O7. The initial compounds such as Mg3(PO4)2 [1–7], Mg2P2O7 [1,8 – 18], Na4P2O7 [19 –22] and Mg4Na(PO4)3 [7,23 – 27] are widely described in the literature. Also, the phase diagrams of the above sub-systems are known. The first of the systems,
* Correspondence and reprints: Tel.: + 48-71-3672853; fax: + 48-71-3680292. E-mail address: [email protected] (T. PodhajskaKaz´mierczak).
Mg3(PO4)2Mg2P2O7, is a simple eutectic system [1]. The second compound, Mg3(PO4)2Mg4Na(PO4)3, is a small part of the quasi-binary system Mg3(PO4)2Na3PO4. Literature data concerning the phase equilibria in this system show essential discrepancies both on the stoichiometry of the intermediate compounds accompanying and on the means of their formation, as well as on polymorphism [7,24,25,28–30]. According to Ref. [7], an intermediate compound of formula Mg4Na(PO4)3 occurs in the Mg3(PO4)2-rich part of the Mg3(PO4)2Na3PO4 system. It melts incongruently at 1165°C giving Mg3(PO4)2 and a liquid, and reveals a polymorphic transition at 1005°C. The existence of this compound is confirmed by the authors of Ref. [23], however, according to their data, Mg4Na(PO4)3 melts congruently at 1180°C and it appears in the form of one polymorphic modification. Polymorphism of Mg4Na(PO4)3 was investigated by the authors of Ref. [24] who have found the phosphate to crystallise
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in the orthorhombic system (a = 9.875(2), b= 15.234(4), c =6.346(1) A, , but did not ascribe presented parameters to either high or low temperature modification). The discrepancies mentioned above inclined the author of this paper to attempt to verify the data. The result of this attempt was that the data in Ref.
Fig. 3. Phase diagram of the partial Mg3(PO4)2Mg4Na(PO4)3Na4P2O7Mg2P2O7. Fig. 1. Phase diagram of the system Mg4Na(PO4)3Mg2P2O7. , DTA heating.
system:
[7] were confirmed, and only small differences were found in temperatures of, (i) the peritectic decomposition reaction of phosphate Mg4Na(PO4)3 (1155°C) and (ii) the polymorphic transition (1000°C). The phase diagram of the third subsystem, Mg4Na(PO4)3Na4P2O7, has been determined in this laboratory [27]. The system shows eutectic features at lower temperatures. The eutectic melts at 745°C, and its composition is 63 wt.% Na4P2O7 and 37 wt.% Mg4Na(PO4)3. In the high temperature part of the system, which is Mg4Na(PO4)3-rich, and in the composition range 0–11 wt.% Na4P2O7, the peritectic reaction takes place at 1155°C: Mg4Na(PO4)3 Mg3(PO4)2 + L(11 wt.%)
Fig. 2. Phase diagram of the Mg4Na(PO4)3Mg6Na8(P2O7)5. , DTA heating.
system
Literature data concerning the phase equilibria in the fourth subsystem Mg2P2O7Na4P2O7 also show essential discrepancies. According to Ref. [31] two intermediate compounds exist in the system. These form at molar ratios of Mg2P2O7:Na4P2O7 equal to 1:1 and 2:3, hence, their ascribed formulae are MgNa2P2O7 and Mg4Na12(P2O7)5, respectively. Both double phosphates melt congruently at about 800°C.
T. Podhajska-Kaz´mierczak / Solid State Sciences 2 (2000) 489–493
The existence of the MgNa2P2O7 compound was suggested as early as 1962 [32]. According to Ref. [33], in the system Mg2P2O7Na4P2O7 an intermediate compound occurs, which has the formula 9Mg2P2O7·7Na4P2O7, and it melts congruently at 832°C. In our own investigation [34] of the Mg2P2O7Na4P2O7 system, we have confirmed the existence of only one compound. It forms at a molar ratio of initial phosphates equal to 3:2, with the formula Mg6Na8(P2O7)5 ascribed to it [34]. The compound melts congruently at 808°C and gives a very flat maximum for the liquidus curve. We have found that Mg6Na8(P2O7)5 appears in two polymorphic modifications. The temperature of the polymorphic transition is 735°C.
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The phase purity of the reagents and the phase structures of the products was examined by X-ray powder diffraction analysis. The X-ray powder analysis was performed on Siemens D 5000 and HZG-4 diffractometers (Guinier camera, Cu–Ka-radiation, Ni-filter). Samples for microscopy in reflected light were made by melting in a homemade resistive furnace with a Pt30Rh winding. Microsections were prepared from crystallised samples. The microscopic observation was helpful to estimate the phase purity of the substances under investigation, as well as the phase composition of the alloys.
3. Results 2. Experimental Samples for the investigation of the phase equilibria in the partial system Mg3(PO4)2Mg4Na(PO4)3Na4P2O7Mg2P2O7 were prepared from initial phosphates, i.e. from Mg3(PO4)2, Mg2P2O7, Na4P2O7 and Mg4Na(PO4)3. These phosphates were home-synthesised from the following initial materials of analytical grade: MgO, MgHPO4·3H2O, Na2HPO4·2H2O and Na3PO4·12H2O. MgO was annealed for 1 h at 1000°C. Mg2P2O7 was obtained by complete dehydration of MgHPO4·3H2O at 900°C for 1 h. Mg3(PO4)2 was synthesised from MgO and Mg2P2O7 by sintering at a temperature of 1200°C for 30 min. Na4P2O7 was obtained as a result of heating Na2HPO4·2H2O at 200°C for 1 h. Na3PO4 was produced by slow dehydration of Na3PO4·12H2O at temperatures of 200, 300 and finally 600°C. Mg4Na(PO4)3 was obtained as a result of 900°C/72 h-heating a mixture of Mg3(PO4)2 and Na3PO4 of the molar ratio 4:1. Phosphate Mg6Na8(P2O7)5 was obtained by heating a mixture of 3 mol Mg2P2O7 and 2 mol Na4P2O7 at 700°C for 70 h. The samples were investigated using the methods of differential thermal analysis (DTA) of heating, X-ray diffraction analysis, and microscopy in reflected light. The DTA of heating was carried out by using derivatographs of the type 3427 and C (MOM, Budapest). The heating rate was 5°C min − 1; the mass of the samples ranged from 0.2 to 0.9 g.
The investigation of the partial system Mg3(PO4)2Mg4Na(PO4)3Na4P2O7Mg2P2O7 has resulted in the identification of two quasi-binary sections, (i) Mg4Na(PO4)3Mg2P2O7 and (ii) Mg4Na(PO4)3Mg6Na8(P2O7)5. On the basis of the data obtained from DTA of heating, X-ray diffraction and optical microscopy, the respective phase diagrams are established in the entire range of composition and temperature. The procedure of preparing the samples for the investigation of the phase equilibria in the two quasibinary systems and in the whole partial system, was almost the same. The only differences were the conditions of presynthesis, i.e. the duration and temperature of the initial heating. Initial phosphates were used to prepare heteromolar mixtures. The mixtures were homogenised by mixing in a weighing bottle and grinding in an agate mortar, then pressed into pellets and heated to ensure equilibrium conditions for the samples. The samples for the system Mg4Na(PO4)3Mg2P2O7 were heated at 750°C for 40 h and those for Mg4Na(PO4)3Mg6Na8(P2O7)5 were heated at 500°C for 65 h. The sinters obtained were cooled down to room temperature and then thoroughly ground. The samples prepared in this way were analysed by DTA of heating. Microscopy in reflected light was carried out for melted and slowly cooled samples (2°C min − 1). Grafting at temperatures above the solidus line was employed during the cooling. This was helpful because the melted samples in the composition range of interest showed tendency to glaze upon setting.
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The phase diagram of the Mg4Na(PO4)3Mg2P2O7 system is given in Fig. 1. In its high temperature part it reveals a complicated character. Above 1007°C four phases are present: liquid L and the phosphates Mg3(PO4)2, Mg4Na(PO4)3 and Mg2P2O7. The ternary peritectic reaction: Mg4Na(PO4)3 +Mg2P2O7 LP +Mg3(PO4)2 proceeds at the temperature 1007°C. As a result of the reaction the liquid L reacts with Mg3(PO4)2 forming the double phosphates Mg4Na(PO4)3 and Mg2P2O7. Accordingly, only two phases remain in the subsolidus region: Mg4Na(PO4)3 and Mg2P2O7, and the system attains the binary character. As it was mentioned at the beginning, the compound Mg4Na(PO4)3 appears in two polymorphic modifications. The temperature of the transition is 1000°C. The DTA-heating curves of samples of the Mg4Na(PO4)3Mg2P2O7 system, near the transition temperature 1000°C, reveal only one large thermal effect. It can be inferred that the polymorphic transition a/b-Mg4Na(PO4)3 and the peritectic reaction overlap to give a joint thermal effect. Mg2P2O7 appears in three polymorphic modifications [12,13,18,34]. For the pure compound, the polymorphic transition b/g proceeds at a temperature of 68°C (b-Mg2P2O7 crystallizes in the monoclonic system with unit cell parameters a= 6.49, b=8.28, c= 4.51 A, ; a =104°; Dz =3.156 g cm − 3 [12]) and of a/b proceeds at 954°C [18,34]. In the Mg4Na(PO4)3Mg2P2O7 system, in the entire range of composition, the low temperature transition is manifested on the DTA-heating curves by a strong thermal effect. The high temperature transition can be recognised, however, from a very weak, diffuse thermal effect on the DTA curves. This effect is noticeable only in the Mg2P2O7-rich part of the system. The phase diagram of the Mg4Na(PO4)3Mg6Na8(P2O7)5 system is presented in Fig. 2. At low temperature the system has an eutectic character. The eutectic mixture e3 is composed of 72 wt.% Mg6Na8(P2O7)5 and 28 wt.% Mg4Na(PO4)3, and it melts at 790°C. In the high temperature part of the system, in the composition range 0–23 wt.% Mg6Na8(P2O7)5, a peritectic reaction takes place. It proceeds according to the equation: Mg4Na(PO4)3 L +Mg3(PO4)2
The phosphate Mg4Na(PO4)3 appears in two polymorphic modifications. For the pure compound, the transition a/b-Mg4Na(PO4)3 proceeds at 1000°C. In the Mg4Na(PO4)3Mg6Na8(P2O7)5 system, in the composition range 0–30 wt.% Mg6Na8(P2O7)5, the transition is observed in the form of clear thermal effects on the curves of DTA-heating. It is worth noting that when the samples become more and more Mg6Na8(P2O7)5-rich, the effect connected with the a/b transition gradually diminishes. Also the phosphate Mg6Na8(P2O7)5 appears in two polymorphic modifications. For the pure compound, the polymorphic transition a/b-Mg6Na8(P2O7)5 occurs at 735°C. In the Mg4Na(PO4)3Mg6Na8(P2O7)5 system the corresponding thermal effects are seen on the DTA curves for the samples containing 30–100 wt.% Mg6Na8(P2O7)5. The corresponding temperature is approximately 735°C. The phase diagram of the partial system Mg3(PO4)2Mg4Na(PO4)3Na4P2O7Mg2P2O7 is shown in Fig. 3.
4. Discussion The quasi-binary systems described above divide the composition range under investigation into three partial ternary systems: (i) Mg3(PO4)2Mg4 Na(PO4)3Mg2P2O7, (ii) Mg4Na(PO4)3Mg6Na8 (P2O7)5Mg2P2O7 and (iii) Mg4Na(PO4)3Na4P2O7 Mg6Na8(P2O7)5. According to the existence of five phosphates, five fields of primary crystallisation, easily detectable in Fig. 3, are in evidence. Three ternary invariants, corresponding to two ternary eutectics and one ternary peritectic, are denoted: (i) Ternary eutectic E1 at 780°C, corresponds to the equilibrium: Mg2P2O7 + Mg6Na8(P2O7)5 + Mg4Na(PO4)3 lliq. E1 Composition of E1: 25.3 wt.% MgO, 15.8 wt.% Na2O, 58.9 wt.% P2O5. (ii) Ternary eutectic E2 at 670°C, corresponds to: Na4P2O7 + Mg6Na8(P2O7)5 + Mg4Na(PO4)3 lliq. E2 Composition of E2: 14.4 wt.% MgO, 30.0 wt.% Na2O, 55.6 wt.% P2O5. (iii) Ternary peritectic P at 1007°C, corresponds to:
T. Podhajska-Kaz´mierczak / Solid State Sciences 2 (2000) 489–493
Mg4Na(PO4)3 +Mg2P2O7 lliq. P+ Mg3(PO4)2 Composition of P: 37.9 wt.% MgO, 7.7 wt.% Na2O, 54.4 wt.% P2O5. During the solidification of molten mixtures corresponding to points of the field Mg3(PO4)2 Mg4Na(PO4)3PMg2P2O7 (triple peritectic quadrangle) the above-mentioned ternary peritectic reaction proceeds. It ends the solidification of mixtures which have their compositions corresponding to points of the field Mg3(PO4)2Mg4Na(PO4)3Mg2P2O7. For molten mixtures of the field Mg4Na(PO4)3P Mg2P2O7 an excess liquid, LP, remains as a result of the peritectic reaction. Hence, the run of solidification consists, along the line PE1, of separation, at variable temperature (1007 – 780°C), in Mg4Na(PO4)3 and Mg2P2O7. After the point E1 is reached the ternary eutectic reaction starts and proceeds at a constant temperature (780°C), giving the three solids: Mg2P2O7, Mg6Na8(P2O7)5 and Mg4Na(PO4)3. In the partial system, Mg4Na(PO4)3Na4P2O7 Mg6Na8(P2O7)5, the components form pseudo-binary systems of eutectic type. The lines issued from the binary eutectics intersect at point E2 at 670°C. At this temperature the second ternary eutectic reaction starts, giving the three solids: Na4P2O7, Mg6Na8(P2O7)5 and Mg4Na(PO4)3. Finally, the line P – c – d intersects the two fields of primary crystallisation of Mg3(PO4)2 and Mg4Na(PO4)3. For molten mixtures, of the field Mg4Na(PO4)3 – P – c – d – p, the following binary peritectic reaction is evidenced: Mg3(PO4)2 + LPcdp Mg4Na(PO4)3 LPcdp denotes liquids of composition corresponding to points of line ‘P – c – d – p’.
References [1] J. Berak, Roczn. Chem. 32 (1958) 17. [2] J. Ando, J. Bull. Chem. Soc. Jpn. 31 (1958) 201.
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[3] J.F. Sarver, F.L. Kantack, F.A. Hummel, J. Electrochem. Soc. 105 (1959) 962. [4] A.G. Nord, P. Kierkegaard, Acta Chem. Scand. 22 (1968) 1466. [5] C. Calvo, Am. Min. 53 (1968) 742. [6] A. Siddique, Pac. J. Sci. Ind. Res. 25 (1973) 119. [7] J. Majling, Chem. Zvesti 27 (6) (1973) 732. [8] O. Andersen, J. Washington Acad. 4 (1914) 318. [9] F. Machatschki, Fortscher. Mineralog. 20 (1938) 48. [10] R. Roy, E.T. Middleswarth, F.A. Hummel, Am. Miner. 33 (1948) 458. [11] A.N. L * azarev, T.F. Teniszeva, Izw. A.N.SSR. 2 (1960) 242. [12] K. L * ukaszewicz, Roczn. Chem. 35 (1961) 31. [13] K. L * ukaszewicz, Prace Nauk. WSE-Wrocl*aw 15 (37) (1968) 206. [14] F.L. Oetting, R.A. McDonald, J. Phys. Chem. 67 (1963) 275. [15] M.N. Nabiev, J.A. Barakov, W.A. Suistmov, N.T. Saibova, Prikl*. ChimZh. 46 (11) (1973) 2377. [16] O.S. Novikova, N.A. Parpiev, W.S. Wyzgo, M.N. Nabiev, Dokl*. Akad. Nauk. Uz. SSR 6 (1961) 27. [17] D.J. Sutor, J.E. Wooley, J. Chem. Soc. 5 (1968) 1143. [18] J. Berak, T. Podhajska-Kaz´mierczak, Polish J. Chem. 65 (1991) 1151. [19] P. Partridge, V. Hicks, G.W. Smith, J. Am. Chem. Soc. 63 (1941) 454. [20] G.W. Morey, E. Ingerson, Am. J. Sci. 242 (1944) 1. [21] C.T. Turkdogan, W.R. Maddoks, J. Iron Steel Inst. (London) 172 (1952) 6. [22] J. Berak, T. Znamierowska, Roczn. Chem. 46 (1972) 1967. [23] V.M. Ustiantsev, M.G. Tretnikova, E.N. Kelareva, Izw. Akad. Nauk. SSSR Ser. Neorg. Mat. 12 (8) (1976) 1438. [24] J. Majling, F. Hanic, Chem. Zvesti 30 (2) (1976) 145. [25] V.M. Ustiantsev, M.G. Tretnikova, Ogneupory 5 (1976) 53. [26] T. Podhajska-Kaz´mierczak, T. Znamierowska, Prace Nauk. Akad. Ekonomicznej-Wrocl*aw 677 (1994) 364. [27] T. Podhajska-Kaz´mierczak, T. Znamierowska, Prace Nauk. Akad. Ekonomicznej-Wrocl*aw 792 (1998) 74. [28] V.M. Ustiantsev, M.G. Tretnikova, Izw. Akad. Nauk. SSSR Ser. Neorg. Mat. 12 (10) (1976) 1894. [29] V.M. Ustiantsev, M.G. Tretnikova, Ogneupory 6 (1976) 52. [30] T. Podhajska-Kaz´mierczak, T. Znamierowska, Polish J. Chem. 73 (1999) 279. [31] S.I. Berul, N.K. Voskresenskaya, Izw. Akad. Nauk. SSSR Ser. Neorg. Mat. 4 (12) (1968) 2129. [32] R. Klement, Chem. Ber. 93 (1960) 2314. [33] J. Majling, F. Hanic, J. Solid State Chem. 7 (1973) 370. [34] T. Podhajska-Kaz´mierczak, T. Znamierowska, Polish J. Chem. 66 (1992) 277.
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Phase equilibria in the ternary system: MgONa2OP2O5 Partial system: Mg3(PO4)2Mg4Na(PO4)3Na4P2O7Mg2P2O7 Teresa Podhajska-Kaz´mierczak * Department of Inorganic Chemistry, Faculty of Engineering and Economics, Academy of Economics, Komandorska 118 /120, 53 -345 Wrocl*aw, Poland Received 22 July 1999; received in revised form 19 October 1999; accepted 15 March 2000
Abstract The partial system Mg3(PO4)2Mg4Na(PO4)3Na4P2O7Mg2P2O7 in the ternary system MgONa2OP2O5 was investigated using thermal and X-ray diffraction analyses and microscopy, and its phase diagram has been determined. In this range of composition, two binary phosphates occur: Mg4Na(PO4)3 and Mg6Na8(P2O7)5. The former melts incongruently (at 1155°C) and the latter does congruently (at 808°C). In the partial system of interest, the two sections Mg4Na(PO4)3Mg2P2O7 and Mg4Na(PO4)3Mg6Na8(P2O7)5 are studied, and their phase diagrams are established. The partial system is divided into three partial ternary systems in which two ternary eutectics and one ternary peritectic occur. © 2000 E´ditions scientifiques et me´dicales Elsevier SAS. All rights reserved. Keywords: Partial systems; Binary phosphates; Ternary systems
1. Introduction Investigation of the phase dependencies, which occur in the partial system Mg3(PO4)2Mg4Na(PO4)3Na4P2O7Mg2P2O7, is a successive stage of author’s work on the ternary system MgONa2OP2O5. The above range of composition is bounded by the four subsystems: (1) Mg3(PO4)2Mg2P2O7, (2) Mg3(PO4)2Mg4Na(PO4)3, (3) Mg4Na(PO4)3Na4P2O7, (4) Mg2P2O7Na4P2O7. The initial compounds such as Mg3(PO4)2 [1–7], Mg2P2O7 [1,8 – 18], Na4P2O7 [19 –22] and Mg4Na(PO4)3 [7,23 – 27] are widely described in the literature. Also, the phase diagrams of the above sub-systems are known. The first of the systems,
* Correspondence and reprints: Tel.: + 48-71-3672853; fax: + 48-71-3680292. E-mail address: [email protected] (T. PodhajskaKaz´mierczak).
Mg3(PO4)2Mg2P2O7, is a simple eutectic system [1]. The second compound, Mg3(PO4)2Mg4Na(PO4)3, is a small part of the quasi-binary system Mg3(PO4)2Na3PO4. Literature data concerning the phase equilibria in this system show essential discrepancies both on the stoichiometry of the intermediate compounds accompanying and on the means of their formation, as well as on polymorphism [7,24,25,28–30]. According to Ref. [7], an intermediate compound of formula Mg4Na(PO4)3 occurs in the Mg3(PO4)2-rich part of the Mg3(PO4)2Na3PO4 system. It melts incongruently at 1165°C giving Mg3(PO4)2 and a liquid, and reveals a polymorphic transition at 1005°C. The existence of this compound is confirmed by the authors of Ref. [23], however, according to their data, Mg4Na(PO4)3 melts congruently at 1180°C and it appears in the form of one polymorphic modification. Polymorphism of Mg4Na(PO4)3 was investigated by the authors of Ref. [24] who have found the phosphate to crystallise
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T. Podhajska-Kaz´mierczak / Solid State Sciences 2 (2000) 489–493
in the orthorhombic system (a = 9.875(2), b= 15.234(4), c =6.346(1) A, , but did not ascribe presented parameters to either high or low temperature modification). The discrepancies mentioned above inclined the author of this paper to attempt to verify the data. The result of this attempt was that the data in Ref.
Fig. 3. Phase diagram of the partial Mg3(PO4)2Mg4Na(PO4)3Na4P2O7Mg2P2O7. Fig. 1. Phase diagram of the system Mg4Na(PO4)3Mg2P2O7. , DTA heating.
system:
[7] were confirmed, and only small differences were found in temperatures of, (i) the peritectic decomposition reaction of phosphate Mg4Na(PO4)3 (1155°C) and (ii) the polymorphic transition (1000°C). The phase diagram of the third subsystem, Mg4Na(PO4)3Na4P2O7, has been determined in this laboratory [27]. The system shows eutectic features at lower temperatures. The eutectic melts at 745°C, and its composition is 63 wt.% Na4P2O7 and 37 wt.% Mg4Na(PO4)3. In the high temperature part of the system, which is Mg4Na(PO4)3-rich, and in the composition range 0–11 wt.% Na4P2O7, the peritectic reaction takes place at 1155°C: Mg4Na(PO4)3 Mg3(PO4)2 + L(11 wt.%)
Fig. 2. Phase diagram of the Mg4Na(PO4)3Mg6Na8(P2O7)5. , DTA heating.
system
Literature data concerning the phase equilibria in the fourth subsystem Mg2P2O7Na4P2O7 also show essential discrepancies. According to Ref. [31] two intermediate compounds exist in the system. These form at molar ratios of Mg2P2O7:Na4P2O7 equal to 1:1 and 2:3, hence, their ascribed formulae are MgNa2P2O7 and Mg4Na12(P2O7)5, respectively. Both double phosphates melt congruently at about 800°C.
T. Podhajska-Kaz´mierczak / Solid State Sciences 2 (2000) 489–493
The existence of the MgNa2P2O7 compound was suggested as early as 1962 [32]. According to Ref. [33], in the system Mg2P2O7Na4P2O7 an intermediate compound occurs, which has the formula 9Mg2P2O7·7Na4P2O7, and it melts congruently at 832°C. In our own investigation [34] of the Mg2P2O7Na4P2O7 system, we have confirmed the existence of only one compound. It forms at a molar ratio of initial phosphates equal to 3:2, with the formula Mg6Na8(P2O7)5 ascribed to it [34]. The compound melts congruently at 808°C and gives a very flat maximum for the liquidus curve. We have found that Mg6Na8(P2O7)5 appears in two polymorphic modifications. The temperature of the polymorphic transition is 735°C.
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The phase purity of the reagents and the phase structures of the products was examined by X-ray powder diffraction analysis. The X-ray powder analysis was performed on Siemens D 5000 and HZG-4 diffractometers (Guinier camera, Cu–Ka-radiation, Ni-filter). Samples for microscopy in reflected light were made by melting in a homemade resistive furnace with a Pt30Rh winding. Microsections were prepared from crystallised samples. The microscopic observation was helpful to estimate the phase purity of the substances under investigation, as well as the phase composition of the alloys.
3. Results 2. Experimental Samples for the investigation of the phase equilibria in the partial system Mg3(PO4)2Mg4Na(PO4)3Na4P2O7Mg2P2O7 were prepared from initial phosphates, i.e. from Mg3(PO4)2, Mg2P2O7, Na4P2O7 and Mg4Na(PO4)3. These phosphates were home-synthesised from the following initial materials of analytical grade: MgO, MgHPO4·3H2O, Na2HPO4·2H2O and Na3PO4·12H2O. MgO was annealed for 1 h at 1000°C. Mg2P2O7 was obtained by complete dehydration of MgHPO4·3H2O at 900°C for 1 h. Mg3(PO4)2 was synthesised from MgO and Mg2P2O7 by sintering at a temperature of 1200°C for 30 min. Na4P2O7 was obtained as a result of heating Na2HPO4·2H2O at 200°C for 1 h. Na3PO4 was produced by slow dehydration of Na3PO4·12H2O at temperatures of 200, 300 and finally 600°C. Mg4Na(PO4)3 was obtained as a result of 900°C/72 h-heating a mixture of Mg3(PO4)2 and Na3PO4 of the molar ratio 4:1. Phosphate Mg6Na8(P2O7)5 was obtained by heating a mixture of 3 mol Mg2P2O7 and 2 mol Na4P2O7 at 700°C for 70 h. The samples were investigated using the methods of differential thermal analysis (DTA) of heating, X-ray diffraction analysis, and microscopy in reflected light. The DTA of heating was carried out by using derivatographs of the type 3427 and C (MOM, Budapest). The heating rate was 5°C min − 1; the mass of the samples ranged from 0.2 to 0.9 g.
The investigation of the partial system Mg3(PO4)2Mg4Na(PO4)3Na4P2O7Mg2P2O7 has resulted in the identification of two quasi-binary sections, (i) Mg4Na(PO4)3Mg2P2O7 and (ii) Mg4Na(PO4)3Mg6Na8(P2O7)5. On the basis of the data obtained from DTA of heating, X-ray diffraction and optical microscopy, the respective phase diagrams are established in the entire range of composition and temperature. The procedure of preparing the samples for the investigation of the phase equilibria in the two quasibinary systems and in the whole partial system, was almost the same. The only differences were the conditions of presynthesis, i.e. the duration and temperature of the initial heating. Initial phosphates were used to prepare heteromolar mixtures. The mixtures were homogenised by mixing in a weighing bottle and grinding in an agate mortar, then pressed into pellets and heated to ensure equilibrium conditions for the samples. The samples for the system Mg4Na(PO4)3Mg2P2O7 were heated at 750°C for 40 h and those for Mg4Na(PO4)3Mg6Na8(P2O7)5 were heated at 500°C for 65 h. The sinters obtained were cooled down to room temperature and then thoroughly ground. The samples prepared in this way were analysed by DTA of heating. Microscopy in reflected light was carried out for melted and slowly cooled samples (2°C min − 1). Grafting at temperatures above the solidus line was employed during the cooling. This was helpful because the melted samples in the composition range of interest showed tendency to glaze upon setting.
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The phase diagram of the Mg4Na(PO4)3Mg2P2O7 system is given in Fig. 1. In its high temperature part it reveals a complicated character. Above 1007°C four phases are present: liquid L and the phosphates Mg3(PO4)2, Mg4Na(PO4)3 and Mg2P2O7. The ternary peritectic reaction: Mg4Na(PO4)3 +Mg2P2O7 LP +Mg3(PO4)2 proceeds at the temperature 1007°C. As a result of the reaction the liquid L reacts with Mg3(PO4)2 forming the double phosphates Mg4Na(PO4)3 and Mg2P2O7. Accordingly, only two phases remain in the subsolidus region: Mg4Na(PO4)3 and Mg2P2O7, and the system attains the binary character. As it was mentioned at the beginning, the compound Mg4Na(PO4)3 appears in two polymorphic modifications. The temperature of the transition is 1000°C. The DTA-heating curves of samples of the Mg4Na(PO4)3Mg2P2O7 system, near the transition temperature 1000°C, reveal only one large thermal effect. It can be inferred that the polymorphic transition a/b-Mg4Na(PO4)3 and the peritectic reaction overlap to give a joint thermal effect. Mg2P2O7 appears in three polymorphic modifications [12,13,18,34]. For the pure compound, the polymorphic transition b/g proceeds at a temperature of 68°C (b-Mg2P2O7 crystallizes in the monoclonic system with unit cell parameters a= 6.49, b=8.28, c= 4.51 A, ; a =104°; Dz =3.156 g cm − 3 [12]) and of a/b proceeds at 954°C [18,34]. In the Mg4Na(PO4)3Mg2P2O7 system, in the entire range of composition, the low temperature transition is manifested on the DTA-heating curves by a strong thermal effect. The high temperature transition can be recognised, however, from a very weak, diffuse thermal effect on the DTA curves. This effect is noticeable only in the Mg2P2O7-rich part of the system. The phase diagram of the Mg4Na(PO4)3Mg6Na8(P2O7)5 system is presented in Fig. 2. At low temperature the system has an eutectic character. The eutectic mixture e3 is composed of 72 wt.% Mg6Na8(P2O7)5 and 28 wt.% Mg4Na(PO4)3, and it melts at 790°C. In the high temperature part of the system, in the composition range 0–23 wt.% Mg6Na8(P2O7)5, a peritectic reaction takes place. It proceeds according to the equation: Mg4Na(PO4)3 L +Mg3(PO4)2
The phosphate Mg4Na(PO4)3 appears in two polymorphic modifications. For the pure compound, the transition a/b-Mg4Na(PO4)3 proceeds at 1000°C. In the Mg4Na(PO4)3Mg6Na8(P2O7)5 system, in the composition range 0–30 wt.% Mg6Na8(P2O7)5, the transition is observed in the form of clear thermal effects on the curves of DTA-heating. It is worth noting that when the samples become more and more Mg6Na8(P2O7)5-rich, the effect connected with the a/b transition gradually diminishes. Also the phosphate Mg6Na8(P2O7)5 appears in two polymorphic modifications. For the pure compound, the polymorphic transition a/b-Mg6Na8(P2O7)5 occurs at 735°C. In the Mg4Na(PO4)3Mg6Na8(P2O7)5 system the corresponding thermal effects are seen on the DTA curves for the samples containing 30–100 wt.% Mg6Na8(P2O7)5. The corresponding temperature is approximately 735°C. The phase diagram of the partial system Mg3(PO4)2Mg4Na(PO4)3Na4P2O7Mg2P2O7 is shown in Fig. 3.
4. Discussion The quasi-binary systems described above divide the composition range under investigation into three partial ternary systems: (i) Mg3(PO4)2Mg4 Na(PO4)3Mg2P2O7, (ii) Mg4Na(PO4)3Mg6Na8 (P2O7)5Mg2P2O7 and (iii) Mg4Na(PO4)3Na4P2O7 Mg6Na8(P2O7)5. According to the existence of five phosphates, five fields of primary crystallisation, easily detectable in Fig. 3, are in evidence. Three ternary invariants, corresponding to two ternary eutectics and one ternary peritectic, are denoted: (i) Ternary eutectic E1 at 780°C, corresponds to the equilibrium: Mg2P2O7 + Mg6Na8(P2O7)5 + Mg4Na(PO4)3 lliq. E1 Composition of E1: 25.3 wt.% MgO, 15.8 wt.% Na2O, 58.9 wt.% P2O5. (ii) Ternary eutectic E2 at 670°C, corresponds to: Na4P2O7 + Mg6Na8(P2O7)5 + Mg4Na(PO4)3 lliq. E2 Composition of E2: 14.4 wt.% MgO, 30.0 wt.% Na2O, 55.6 wt.% P2O5. (iii) Ternary peritectic P at 1007°C, corresponds to:
T. Podhajska-Kaz´mierczak / Solid State Sciences 2 (2000) 489–493
Mg4Na(PO4)3 +Mg2P2O7 lliq. P+ Mg3(PO4)2 Composition of P: 37.9 wt.% MgO, 7.7 wt.% Na2O, 54.4 wt.% P2O5. During the solidification of molten mixtures corresponding to points of the field Mg3(PO4)2 Mg4Na(PO4)3PMg2P2O7 (triple peritectic quadrangle) the above-mentioned ternary peritectic reaction proceeds. It ends the solidification of mixtures which have their compositions corresponding to points of the field Mg3(PO4)2Mg4Na(PO4)3Mg2P2O7. For molten mixtures of the field Mg4Na(PO4)3P Mg2P2O7 an excess liquid, LP, remains as a result of the peritectic reaction. Hence, the run of solidification consists, along the line PE1, of separation, at variable temperature (1007 – 780°C), in Mg4Na(PO4)3 and Mg2P2O7. After the point E1 is reached the ternary eutectic reaction starts and proceeds at a constant temperature (780°C), giving the three solids: Mg2P2O7, Mg6Na8(P2O7)5 and Mg4Na(PO4)3. In the partial system, Mg4Na(PO4)3Na4P2O7 Mg6Na8(P2O7)5, the components form pseudo-binary systems of eutectic type. The lines issued from the binary eutectics intersect at point E2 at 670°C. At this temperature the second ternary eutectic reaction starts, giving the three solids: Na4P2O7, Mg6Na8(P2O7)5 and Mg4Na(PO4)3. Finally, the line P – c – d intersects the two fields of primary crystallisation of Mg3(PO4)2 and Mg4Na(PO4)3. For molten mixtures, of the field Mg4Na(PO4)3 – P – c – d – p, the following binary peritectic reaction is evidenced: Mg3(PO4)2 + LPcdp Mg4Na(PO4)3 LPcdp denotes liquids of composition corresponding to points of line ‘P – c – d – p’.
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