The Mo-F System 67 2 Molybdenum and Fluorine Older data are given in "Molybdän", 1935, pp. 150/3. 2.1 The Molybdenum-Fluorine System Fig. 38 shows the phase diagram of the Mo-F system at 1 atm pressure and temperatures above 150°C. It results from a eritieal evaluation of the available experimental observations and the results of thermodynamie ealeulations reported up to 1978 [1]. wt % tluorlne Mo 30 50 70 F ~,LI __-L __ L--L~I~~-LI ~I_I~I~I~I~II lZ00 ~ Mo+gas 1100! ZOO 1000 c- 800 I- 1 atm gas c -2::: 600 I- w 0.. E Mo +MoFl w ~ m u- D ::E 400 I- Fig.38. Phase diagram of the Mo-F 300! 40' system at 1 atm [1]. ........ Z67! 10' 200 c- u:' MoF4 D + ::E ~:F'L+gas L Mo 16 ~ I Mo 70 80 90 ot % tluorlne At low temperatures, phase equilibria have been studied in the eomposition range MoFr MoF6 by thermal methods, X-ray diffraetion, and measurements of magnetie sus­ eeptibility [2]. The study of the system in the MoFrMoFs eomposition range was eomplieated by the small temperature dependenee of the solubility of MoF4 and the diffieulty of reaehing equilibrium. Therefore the speeimens were kept at 100 to 150°C for 10 to 12 h. MoFs also forms a glass. The equilibrium diagram of the MoFrMoFs subsystem, see Fig. 39, p. 68, shows no eompounds. The euteetie is alm ost degenerate, the euteetie temperature is elose to the melting point of MoFs, and the line for the periteetie eomposition of MoF4 is at 300°C [2]. Gmelin Handbook Mo Suppl. Vol. B 5 5· H. Jehn et al., Mo Molybdenum © Springer-Verlag Berlin Heidelberg 1989 68 The Mo-F System JojrCd9 280 I 1 Fig. 39. The MoFc MoF6 subsystem [2]. Speeimens in the eomposition range MoF5-MoF6 when fused give yellow liquids. Slow eooling leads to their eomplete erystallization but when eooled rapidly they form glasses. The MoF5-MoF6 equ;librium diagram (see Fig.39) reveals a simple euteetie system with the euteetie point at 6SC, 85.0 wt% MoF6, and with a line of the polymorphie transition of MoF6 at -10.5°C throughout that range of eomposition [2]. In an earlier study of the MoF5-MoF6 subsystem the formation of two eompounds MoF5 • MoF6 and MoF5 • 3 MoF6 and the simulta­ neous presenee of a line of polymorphism throughout the whole eomposition range has been stated [3]. However, fram equilibrium investigations of the MoF5-MoOF4 system (see Fig. 50, p. 204) and magnetie suseeptibility measurements in the MoF4-MoF5 subsystem it was shown (for diseussion see the paper) that this eontradietory result was obviously eaused by the presenee of small amounts of MoOF4 impurity whieh signifieantly affeet the strueture of the MoF5-MoF6 diagram [2]. The eritieal evaluation of the data of [4, 5] show that for liquid MoF5 with an exeess of MoF4 over MoF6 the relationship ln(p[MoF61· x) = 5.827 - 6400fT ± 0.03 holds in the range 340 to 540 K (p[MoF6] in atm, x in mole fraetion of MoF4) [1]. The 1-atm gas in equilibrium with MoF3(s) and Mo(s) at 1100 ±1 OO°C is ealeulated to eontain 75±5mol% MoF4(g) and 20±2mol% MoF5(g) as shown in Fig.40. Eaeh of MoF6(g) and Mo2F,o(g) is ealeulated to be 3 ± 2 mol%. These speeies are not shown in Fig.40 whieh 1.0,----------------, ::: ~ 0.6 ::> V> V> ~ 04 Fig. 40. Composition of molybdenum­ fluorine 1-atm gas in equilibrium with solid molybdenum [1]. 2000 2500 T In K Gmelin Handbook Mo Suppl. Vol. B 5 Molybdenum Fluorides 69 indicates the variation with temperature of the gaseous species MoF2, MoF3, MoF4, and MoF5 in the 1-atm gas in equilibrium with molybdenum metal. MoF4 is seen to be the major species over a wide temperature range. MoF does not become significant until higher temperatures in the liquid molybdenum range [1]. References: [1] Brewer, L.; Lamoreaux, R. H. (At. Energy Rev. Spec. Issue No. 7 [1980]195/356, 241/4). [2] Khaldoyanidi, K. A.; Yakovlev, I. 1.; Ikorskii, V. N. (Zh. Neorgan. Khim. 26 [1981] 3067/9; Russ. J. Inorg. Chem. 26 [1981] 1639/40). [3] Popov, A. P.; Tsvetnikov, A. K.; Goncharuk, V. K. (Zh. Neorgan. Khim. 23 [1978]236/9; Russ. J. Inorg. Chem. 23 [1978] 132/3). [4] Krause, R. F., Jr. (Proc. Electrochem. Soc. 78-1 [1978]199/209; C. A. 89 [1978] No. 66346). [5] Krause, R. F., Jr.; Douglas, T. B. (J. Chem. Thermodyn. 9 [1977] 1149/63). 2.2 Molybdenum Fluorides Survey. Stable fluoride phases exist for molybdenum in each of the oxidation states 3 through 6. The hexafluoride is known since 1905 when it was prepared for the first time by the direct combination of the elements, cf. "Molybdän", 1935, pp. 150/1. The existence of a stable molybdenum trifluoride was for a long time in doubt. The pure compound was obtained for the first time in 1949 by reacting the tribromide with HF at high temperatures. In 1957, MoF4 and MoF5 have been isolated from the product of the reaction between MO(CO)6 and fluorine. Fluorides MoF n with n ~ 2 are metastable. The existence of a fluoride of composition M02Fg as an individual phase could not be confirmed. At room temperature, MoF3, MoF4, and MoF5 are sOlids, whereas MoF6 forms a colorless liquid above ~17°C. Studies of the physical and chemical properties were complicated by the high sensitivity of the fluorides to traces of water resulting in the formation of oxide fluorides. Even very small amounts of the oxide fluorides affect the properties and lead to wrong conclusions; e. g., a wrong melting point for MoF5 and the apparent existence of MoF4-MoF5 phases were simulated by oxide fluoride contaminations. 2.2.1 Molybdenum Fluorides MoFn with n~1 Solid fluorides MoF n with n ~1 have been found to form (in addition to MoF3) when molybdenum metal was exploded by electric discharge in gaseous PF5 at 515 Torr. The relative proportions of lower fluorides and MoF3 in the reaction products depend strongly upon the imparted electric energy, e. g. at 840 J imparted energy about 54% of the exploded metal has been converted to lower fluorides of composition MoFo.33 to MoFo4g . At an energy of 530 J the solid, insoluble (hot concentrated NaOH solution) residues formed besides traces of MoF3 have empirical formulas MoFo74 to MoF1.00• At higher energy levels MoF3 forms at the expense of the lower fluorides, thus at 2190 J only 19% of the metal was converted to an insoluble residue of empirical composition MoFo.25• It was assumed that the MoFn products were mixtures of noncrystalline MoF with molybdenum fluorides of very low fluorine content [1]. Undoubtedly these phases are metastable [2]. Gmelin Handbook Mo Suppl. Vol. B 5 70 Molybdenum Fluorides References: [1] Johnson, R. L.; Siegel, B. (J. Inorg. Nucl. Chem. 31 [1969] 955/63, 958, 960). [2] Brewer, L.; Lamoreaux, R. H. (At. Energy Rev. Spec. Issue NO.7 [1980]195/356, 241). 2.2.2 Molybdenum(1) Fluoride MoF The gaseous species MoF (in addition to higher fluorides) has been detected by high­ temperature mass spectrometry in the effusion beams generated by the reaction of molybde­ num wire with MoF6 at 1700 K and with SF6 at 1500 to 2000 K [1,6], see also p.169. The following gives (estimated) molecular constants and (mostly estimated) thermo­ dynamic properties of gaseous MoF. The Mo-F distance in the molecule is assumed as r=1.84 [1] or 1.89A [2] resulting in moments of inertia (in 10-40 g. cm2) of 89.2 [1] and 93.9 [2], respectively. The vibration frequency of the Mo-F bond is estimated at 700 [1] and 645 [2] cm-1. For the electronic ground state a quartet is assumed in [2], while a doublet is assumed in [1]. The effective charge q on the Mo atom has been calculated in [3] by the method given in [4]. For r =1.84 and 1.89 A, q = 0.85 e and 0.83 e, respectively [3]. By multiple scattering Slater exchange calculations, the electron affinity, EA, and the ionization potential, IP, of the molecule have been calculated. Disregarding spin pOlarization, self-consistent field Hartree-Fock-Slater calculations yield IP = 6.80 and 6.84 eV for quartet and doublet states, respectively, and EA=0.14 eV. With spin polarization included into the calcula­ tions, IP = 7.55 eV and EA=1.51 eV for the doublet and IP = 7.69 eV for the quartet state [5]. The standard enthalpy of formation of gaseous MoF was determined from gas phase equilibria in the MoF6-Mo and SF6-Mo systems; i1H,,29S(MoF, g) = + 65.0 ± 2.2 kcaUmol [1,6]. From these equilibria the bond dissociation energies D29S(Mo-F) = 111 kcaUmol [1,6] and Dg(Mo-F) =110.3 ± 2.2 kcaUmol have been derived [1]. The heat of dissociation is estimated at 120 kcaUmol (based on a review of literature data) [8]. The standard entropy at 298 K is given as 58.86 cal' mol-1. K-1. Between 400 and 5000 K the esti mated Sr ranges from 61.24 to 83.41 cal· mol-1 . K-1 [2]. The free energy function - (F-H29S)IT (in cal' mol-1. K-1) has been estimated at 56.7 for 298 K, and for 400 to 2400 K it is estimated to range from 57.0 to 67.0 [1].