Cerium Tetrafluoride: Sublimation, Thermolysis, and Atomic Fluorine

Jul 13, 2015 - It was observed that CeF3 accumulates near the edge of the effusion orifice and near the junction of the lid and the body of the cell, ...
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Cerium Tetrafluoride: Sublimation, Thermolysis, and Atomic Fluorine Migration N.S. Chilingarov,*,† A.V. Knot’ko,† I.M. Shlyapnikov,‡ Z. Mazej,‡ M. Kristl,§ and L.N. Sidorov† †

Faculty of Chemistry, M.V. Lomonosov Moscow State University, Leninskie gory 1-3, Moscow 119992, Russia Department of Inorganic Chemistry and Technology, Jožef Stefan Institute, Jamova cesta 39, Ljubljana 1000, Slovenia § Faculty of Chemistry and Chemical Technology, University of Maribor, Smetanova ulica 17, Maribor 2000, Slovenia ‡

ABSTRACT: Saturated vapor pressure p° and enthalpy of sublimation (ΔsH°) of cerium tetrafluoride CeF4 were determined by means of Knudsen effusion mass spectrometry in the range of 750−920 K. It was discovered that sublimation of cerium tetrafluoride from a platinum effusion cell competes with thermal decomposition to CeF3 in the solid phase, but no accompanying release of fluorine to the gas phase occurs. Thus, fluorine atoms migrate within the surface layer of CeF4(s) to the regions of their irreversible drain. We used scanning electron microscopy to study the distribution of the residual CeF3(s) across the inner surface of the effusion cell after complete evaporation of CeF4(s). It was observed that CeF3 accumulates near the edge of the effusion orifice and near the junction of the lid and the body of the cell, that is, in those regions where the fluorine atoms can migrate to a free platinum surface and thus be depleted from the system. Distribution of CeF3(s) solid particles indicates the ways of fluorine atoms migration providing CeF3(s) formation inside the CeF4(s) surface layer.

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INTRODUCTION Tetrafluorides of rare earth elementsCeF4, PrF4, TbF4have been considered as thermally unstable compounds for a long time. The least stable is PrF4, which decomposes above 360 K; meanwhile, CeF4 and TbF4 decompose at 470−670 K. For all three of them it was proposed that their decompositions proceed with elimination of molecular fluorine according to equation: MF4 (s) → MF3(s) + 0.5F2

During investigation of CeF4(s) thermal stability, the fluorinating activity must be taken into consideration. In only one work 4 authors distinguish two ways of CeF 3 (s) formationthermal decomposition of CeF4(s) according to the Reaction 1 and solid-state fluorination of the material of the reactor, used for the sample evaporation. At temperatures below 1114 K platinum reactor has been found to be resistant to solid-state reactions with CeF4(s), while in monel reactor tetrafluoride is reduced to trifluoride. Moderate decomposition of CeF4(s) in preliminary fluorinated nickel reactor at 1050 K and higher temperatures is, most probably, caused by sample reduction by the reactor material. Complete evaporation of CeF4(s) samples at 800−1120 K has been achieved using a cell, manufactured from LaF3.7 A while ago, existence of two different phases of cerium tetrafluoride was reported.13 It is stated that fluorination of CeF3(s) by molecular fluorine F2(g) at temperatures lower than 540 K leads to metastable CeF4−I phase that totally decomposes at heating according to the Reaction 1, while high-temperature fluorination (T > 540 K) results in thermally stable phase CeF4−II. Such considerable differences in properties of those two phases are not explained by any adequate peculiarities of their structures. Those results have been later disproved in a patent published by French authors.14

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1−3

where M = Ce, Pr, Tb. Nowadays, there is no evidence contradicting the proposed mechanism of decompositions of PrF4(s) and TbF4(s). On the contrary, thermal decomposition of CeF4(s) with formation of CeF3(s) is accompanied by CeF4(s) transition to the gaseous phase that has been experimentally proved by means of chemical analysis and Xray powder diffraction.4 This fact has been additionally proved by mass spectrometric investigations of CeF4 saturated vapor.5−7 Considerably higher thermal stability of CeF4(s) in comparison with TbF4(s) is also confirmed by experimentally determined enthalpies of formation of cerium and terbium fluorides.8−10 According to those data decomposition of 1 mol of CeF4(s) according to the Reaction 1 requires 170−180 kJ more than decomposition of 1 mol of TbF4(s). In spite of considerably higher stability, cerium tetrafluoride is used as fluorinating agent. For example, reaction between fullerene C60(s) and CeF4(s) at T ≈ 520 K selectively yields C60F36 (∼90−95 mol %);11 iron trifluoride is fluorinated by CeF4(s) with formation of iron tetrafluoride in gaseous phase.12 © XXXX American Chemical Society

Received: April 29, 2015 Revised: July 10, 2015

A

DOI: 10.1021/acs.jpca.5b04105 J. Phys. Chem. A XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry A Attempts to determine a pressure of fluorine during the process of CeF4(s) thermolysis have been already made in early works.1−3 The total pressure, measured by manometers during heating of reactors with samples of CeF4(s), has been considered as fluorine pressure. Obviously, those results are not reliable without knowledge about vapor composition. Detection of fluorine by chemical analysis methods resulted in fluorine pressure estimation p(F2) < 0.5 mmHg (66.5 Pa) at T = 773 K.4 During mass spectrometric investigation of saturated vapor above CeF4(s) only weak signals on the mass number m/ e = 38 (F2+) have been detected.5,7 According to estimations, fluorine partial pressure p(F2) does not exceed 1.3 Pa at T = 1213 K, and the ratio p(CeF4)/p(F2) ≈ 10 at T = 970 K.7 It became possible to estimate equilibrium pressure of fluorine in Reaction 1 after the enthalpy of formation of CeF4(s) had been determined by solution calorimetry.8 In our recent investigation of cerium tetrafluoride evaporation from the platinum effusion cell by Knudsen effusion mass spectrometry (KEMS), saturated vapor pressure and enthalpy of sublimation of CeF4 were calculated: p°(CeF4) = 0.39 Pa (873 K); ΔsH°(CeF4) = 252 ± 5 kJ/mol (845 K).15 Saturated vapor pressure of CeF4 has been determined for the first time, while values of enthalpies of sublimation, which could be found in literature, are not in agreement with each other (Table 1). It has also been shown that evaporation of CeF4(s)

chemical analysis. (Cerium content was determined by complexometric titration; fluorine content was determined by using a fluoride ion selective electrode after total alkali decomposition of the sample with alkali fusion using NaKCO3.) 2. Investigation of Thermal Stability by Differential Scanning Calorimetry. Thermal stability of cerium tetrafluoride was studied by differential scanning calorimetry (DSC) using Mettler. Aluminum containers were filled with CeF4(s) samples in an argon atmosphere in a glovebox and in air. Temperature was increased at rate of 10 K/min in a temperature range from 298−723 K. No effects, caused by hydrolysis of initial samples, were observed in case of the samples loaded in an argon atmosphere. In contrast, DSC curves, recorded for the samples loaded in air, revealed endothermic effects at 560−580 K, caused by irreversible reactions between the samples and components of air. Obtained data confirmed that there were no traces of water in synthesized samples and indicated necessity of an inert atmosphere (Ar) for all manipulations with samples. 3. Investigation of Composition of Gaseous Phase by Knudsen Effusion Mass Spectrometry. a. Methods and Apparatus. Composition of the saturated vapor was studied by high-temperature mass-spectrometry that is a combination of the Knudsen effusion method with mass-spectrometric analysis of evaporated species (KEMS).18 The CeF4(s) samples were loaded into effusion cell in the glovebox in an argon atmosphere. An effusion orifice was sealed with a piece of an adhesive tape. Then, the effusion cell was installed in an evaporator of mass-spectrometer. Just before connection of the evaporator to an ion source of mass-spectrometer, the piece of an adhesive tape was removed. It was supposed, that during a short time period being on air (1−2 min) the inert argon atmosphere remained in the effusion cell. Increase of the temperature was not accompanied by appearance of significant amounts of HF or O2 molecules in the gaseous phase that revealed an absence of any hydrolysis processes in the system. The effusion cell was heated to adjusted temperature, which was controlled by a controller TERMODAT−16E3; temperature was measured by a Pt−Pt/Rh(10%) thermocouple. Deviations of actual temperature from the adjusted did not exceed ±1 K. Molecules of CeF4 were leaving the effusion cell through an effusion orifice and were entering an ionization chamber, where they were ionized by an electron bombardment (Eion = 70 eV). Formed positive ions were accelerated (Eacc = 3 keV) and analyzed in magnetic mass-analyzer MI−1201 (90°, r = 200 mm, resolution 500) according to the mass to charge ratio m/e. Currents of ions with defined m/e parameter were detected by a Faraday cup, and their intensities were measured by an electrometer (R = 1 × 1012 Ω). Low-intensity ion currents were measured by an electron multiplier. The part of the signal of ion current, originated by ionization of a molecular beam leaving the effusion cell, was separated from the background by a shutter, situated between the effusion cell and the ionization chamber. Measured ion currents of CeF3+ ions, formed during ionization of CeF4 molecules, I(CeF3+), are correlated with CeF4 pressure p(CeF4) by an equation:

Table 1. Calculated Enthalpies of CeF4 Sublimation material of effusion cell

temperature range, K

ΔsH°T(CeF4), kJ/mol

reference

Pt LaF3 Pt

886−1054 803−943 763−973

278.4 ± 12.4 209.4 252 ± 5

6 7 15

is accompanied by its decomposition with formation of CeF3(s), but reasons of thermolysis have not been clarified. The present research is focused on the processes taking place during evaporation of cerium tetrafluoride, that is, its sublimation and thermal decomposition (thermolysis).

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EXPERIMENTAL SECTION 1. Synthesis of Cerium Tetrafluoride. Samples of cerium tetrafluoride, used in this work, were synthesized by two different methods: high-temperature fluorination of cerium trifluoride CeF3 by molecular fluorine F2 at 603 K16 and room temperature fluorination of CeF3 by F2 in a medium of anhydrous HF applying UV irradiation:17 CeF3(s) + F2(g) → CeF4(s)

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All manipulations were performed under anhydrous conditions. Volatile compounds, such as anhydrous HF and F2 were handled on a nickel-Teflon vacuum line, while nonvolatile cerium trifluoride was handled in a glovebox (M. Braun) in an argon atmosphere. Reactions at room temperature were performed in reaction vessels made from tetrafluoroethylene−hexafluoropropylene (FEP; Polytetra GmbH, Germany) tubes (height 250−300 mm with i.d. 16 mm and o.d. 19 mm), heat-sealed on one end, and equipped with a Teflon valve on another end. Before use, reaction vessels were passivated by elemental fluorine at 1.1 bar for 2 h. Anhydrous HF (Linde, 99.995%) was treated with K2NiF6 (Advance Research Chemicals, Inc.) for several hours prior to use. Products of synthesis were characterized by X-ray powder diffraction and

p(CeF4 ) = (k /σ ) × I(CeF3+) × T

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where k is the constant of the instrument sensitivity, σ is the ionization cross section of CeF4 molecules with formation of CeF3+ ions, and T is the temperature. B

DOI: 10.1021/acs.jpca.5b04105 J. Phys. Chem. A XXXX, XXX, XXX−XXX

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Enthalpy of cerium tetrafluoride sublimation ΔsH°(CeF4) was determined from temperature dependence of saturated vapor pressure. Both samples, synthesized at high and room temperatures, were studied. Typical mass of the loaded sample was 15−25 mg. Ion currents I(CeF3+) were measured in temperature range of 750−920 K with stepping increase of temperature by 20 K (at each temperature the signal was measured three times). The lowest temperature was determined as temperature of appearance of well-measurable signal of CeF3+ ion currents. The highest temperature was temperature of appearance of PtFn+ (n = 0−4) ion currents in mass spectrum. Linear dependences ln(I(CeF3+) × T) − T−1 were analyzed by least-squares method resulting in enthalpy of CeF4 sublimation. After all experiments a solid residue was formed in the effusion cell. According to X-ray powder diffraction analysis the composition of the formed phase corresponded to cerium trifluoride. Before next experiment the cell was emptied, while CeF3 layers formed on inner surface were not removed. 4. Investigation of Knudsen Cell Inner Surface by Scanning Electron Microscopy. The inner surface of the effusion cell was analyzed by a field emission scanning electron microscope SUPRA 50 VP (LEO, Germany), equipped with secondary electron detector, at p = 40 Pa and U = 20 kV. The elemental composition of observed particles was studied by the energy-dispersive X-ray detector INCA X-MAX (Oxford Instruments, Great Britain) installed on the microscope. 5. Determination of Sample Masses. Mass of the empty cell prior to the sample loading, as well as mass of the cell with the residue, were measured by microbalance GR−200 (A&D, Japan) with an error of ±0.0001 g. Mass of the sample was measured by microbalance VL−210 (error ±0.0001 g), which was installed in a glovebox.

b. Nickel and Platinum Knudsen Effusion Cells. The samples were evaporated from effusion cells made from nickel or platinum. The nickel cell was passivated by molecular fluorine at T = 740 K and p(F2) ≈ 5 bar for ∼10 h. After fluorination, NiF2 protective layer was formed on the inner surface of the cell. It was found out that during isothermal evaporation of CeF4(s) from the preliminary fluorinated nickel cell at 873 K, sample sublimation is accompanied by significant thermal decomposition. Pressure of cerium tetrafluoride decreases by 25% in first 90 min of evaporation, and only 5% of the initial sample sublimes. The rest is reduced to CeF3(s). Obtained data clearly demonstrate that determination of saturated vapor pressure or enthalpy of sublimation by evaporation of CeF4(s) from fluorinated nickel cell is unacceptable. In a platinum effusion cell losses of the sample caused by its decomposition significantly decrease. The platinum effusion cell, used within present research, consisted of a body and a lid. The body was manufactured from platinum plate with thickness of 1.5 mm. The lid was made from platinum foil (thickness 0.1 mm). The lid was hermetically sealed with the body in a capsule made of stainless steel. Special attention was paid to the area of lid−body connection. Before each experiment, the corresponding edge of the body was polished, while the lid was treated with concentrated HNO3, water, ethanol, and acetone. After 3−4 uses the lid was changed to a new one. Prior to loading the sample the effusion cell was heated in vacuum at 500 K for ∼5 h. Formation of volatile fluorinated derivatives of platinum was observed only at 920 K. At this temperature signals of ions PtFn+ (n = 1−4) were detected in mass spectrum, and their intensities did not exceed 0.3% of the signal I(CeF3+). Thus, the platinum effusion cell was used for investigation of cerium tetrafluoride sublimation. c. Determination of p°(CeF4) and ΔsH°(CeF4). The product of a room-temperature synthesis was not used for determination of saturated vapor pressure of CeF4(s) because of possible change of initial mass of the sample, caused by evaporation of HF, which could remain in the product after synthesis. Typical initial mass of the sample synthesized at high temperature was 6−7 mg. It was isothermally evaporated at T = 873 K, which is 50 K lower than the temperature at which ions of PtFn+ (n = 0−4) were observed in mass spectrum. Value of the ion current I(CeF3+) was constant. The time of total vaporization of the sample t was determined by decrease of the signal I(CeF3+). After that temperature was decreased, the cell was removed from the evaporator and weighed. Increase of the mass was caused by formation of CeF3(s) during decomposition of CeF4(s). Calculated mass of decomposed sample was subtracted from initial mass of the sample, and finally, mass of evaporated sample was obtained. Saturated vapor pressure p°(CeF4) was calculated according to the Herz−Knudsen equation:

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RESULTS AND DISCUSSION 1. Sublimation of Cerium Tetrafluoride. During evaporation of CeF4(s), the mass spectrum of CeF4 {CeF4+ (3 × 10−5), CeF3+ (1), CeF2+ (0.19), CeF+ (0.21), Ce+ (0.10)} was constant in the temperature range of 750−920 K. The natural isotopic abundance ratio 140Ce/142Ce ≈ 8 was confirmed by comparison of corresponding ion currents of CeFn+ (n = 0−3). According to X-ray powder diffraction analysis the residue in the cell, formed during evaporation of CeF4(s), was CeF3(s). Among other trifluorides of rare earth metals, cerium trifluoride is a nonvolatile compoundits vapor pressure at 1370 K is p°(CeF3) = 8.4 × 10−2 Pa.19 Thermodynamic activities of CeF4 and CeF3 in their mixture at temperatures lower than 1081 K are equal to 1,20 so the pressure of cerium tetrafluoride, achieved in the effusion cell, is equal to its saturated vapor pressure, in spite of its partial decomposition with formation and accumulation of CeF3(s). Typical dependence of ion current I(CeF3+) upon time during isothermal evaporation of CeF4(s) at 873 K is presented in Figure 1. Results of experiments are given in Table 2. The average value of five measurements of the cerium tetrafluoride saturated vapor pressure at 873 K is p°(CeF4) = 0.28 ± 0.05 Pa. Calculated enthalpies of sublimation of CeF4(s), obtained by processing dependences ln(I(CeF3+) × T) − T−1, are presented in Table 3. The average of eight calculated values of ΔsH°(CeF4) is 257 ± 5 kJ/mol (confidence interval 0.95). This value is considered as enthalpy of sublimation of cerium tetrafluoride at 840 K.

p°(CeF4 ) = m v (CeF4 ) × (Seff × L)−1 × t −1 × (2πMRT )0.5 (4)

where mv(CeF4) is the mass of vaporized sample, Seff is the area of the effusion orifice, L is the Clauzing coefficient (calculated from geometrical parameters of an effusion orifice: radius r and length of a canal l; two different cells were used: r1 = 0.15 mm, l1 = 0.1 mm and r2 = 0.125 mm, l2 = 0.04 mm), t is the time of vaporization, M is the molar mass of CeF4, R is the gas constant, and T is the temperature of vaporization. C

DOI: 10.1021/acs.jpca.5b04105 J. Phys. Chem. A XXXX, XXX, XXX−XXX

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Table 4. Calculation of Enthalpies of CeF4 Sublimation (T = 298 K) basic value p°873(CeF4) = 0.28 ± 0.05 Pa ΔsH°840(CeF4) = 257 ± 5 kJ/mol

ΔsH°298(CeF4), kJ/mol (ZrF4)a

ΔsH°298(CeF4), kJ/mol (UF4)b

ΔsH°298(CeF4), kJ/mol (average)

277

271

274 ± 12

269

267

268 ± 14

a Calculations applying thermodynamic functions for ZrF4(s) and ZrF4(g).22 bCalculations applying thermodynamic functions for UF4(s) and UF4(g).22

equation for calculation the CeF4 saturated vapor pressure at the temperature range of 750−920 K was derived: log p°(CeF4) = −(13 400 ± 300)/T + (14.8 ± 0.4) Figure 1. Dependence of ion current I(CeF3+) upon time during isothermal evaporation of CeF4(s) at 873 K; Δt, h indicates time of evaporation.

Determined values of p°(CeF4) and ΔsH°(CeF4) are in good agreement with recently published data15 and dramatically contradict those previously reported.6,7 The first value of enthalpy of sublimation, depicted in Table 1, was obtained as average of eight values, calculated by the second law of thermodynamics.6 Experimental data were collected at temperatures of 886−1054 K. Such high temperatures could lead to solid-state fluorination of platinum effusion cell with formation of PtF4 and PtF2 in gaseous phase. Their further heterogeneous dissociation with formation and accumulation of platinum film on a surface of CeF4(s) was also possible. So, beside partial reduction of cerium tetrafluoride its interaction with platinum could lead to decrease of the effective area of evaporation and, finally, to overvalued enthalpy of sublimation of CeF4. The second value of ΔsH°(CeF4)7 was calculated by processing the only dependence ln(I(CeF3+) × T) − T−1. Moreover, it is stated that the effusion cell, used in experiments, had a leak that grew with increase of temperature. Finally, the temperature of the cell was measured with a large error (±15 K). All mentioned factors could have led to wrongly calculated value of ΔsH°(CeF4). 2. Thermal Decomposition of Cerium Tetrafluoride. After all experiments with cerium tetrafluoride evaporation a solid residue had been formed in the effusion cell. According to the X-ray powder diffraction analysis (XRPD analysis) it was the CeF3(s) phase. During one experiment, evaporation of CeF4(s) at 873 K was interrupted after 2 h. A sample in the effusion cell consisted of the phase CeF4(s) only, while CeF3(s) was not detected. These facts together with DSC data exclude supposition about possible partial hydrolysis of CeF4(s) as a main source of CeF3(s) and allow considering thermal decomposition of cerium tetrafluoride as an accompanying process of its sublimation. Any chemical reactions of solid-state fluorination in the system were also excluded, so formation of CeF3(s) had been caused only by release of stoichiometric quantity of fluorine from CeF4(s). The solid product of thermolysis was well-detectable by XRPD analysis, while the gaseous productfluorinewas not observed by mass spectrometer either during isothermal evaporation at 873 K or at higher temperatures up to 920 K. According to the equilibrium constant of a process of molecular fluorine dissociation:22

Table 2. Experimental Values of CeF4 Saturated Vapor Pressure experiment

m0a, mg

mva, mg

ta, h

p°(CeF4), Pa

1 2 3 4 5

7.1 7.0 6.0 7.1 7.0

2.9 2.7 1.5 2.7 3.7

21.3 22.2 13.7 21.8 53.5

0.33 0.29 0.26 0.30 0.21

m0initial mass of CeF4(s); mvmass of evaporated CeF4(s); t time of vaporization.

a

Table 3. Experimental Values of Enthalpy of CeF4 Sublimation experiment

temperature range, K

average temperature, K

ΔsH°(CeF4), kJ/mol

1 2 3 4 5 6 7 8

923−763 913−793 903−763 883−763 753−873 873−813 793−913 793−873

862 865 831 821 811 843 851 832

255.9 263.9 248.5 252.3 254.1 259.9 256.8 266.1

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To confirm consistency of these two independently determined thermodynamic values of cerium tetrafluoride sublimationp°(CeF4) and ΔsH°(CeF4)both were recalculated to the enthalpy of CeF4 sublimation at 298 K. Thermodynamic functions for CeF4(s) and CeF4(g) are not defined; nevertheless, it is possible to apply changes of function (HT − H298) and function (−G T + H298)/T in similar reactions of ZrF4(s) and UF4(s) sublimation. 21 Results of calculations are presented in Table 4. As it is shown, enthalpies of cerium tetrafluoride sublimation at 298 K, calculated using the value of its saturated vapor pressure, ΔsH°298(CeF4) = 274 ± 12 kJ/mol, and using the value of its enthalpy of sublimation at 840 K, ΔsH°298(CeF4) = 268 ± 14 kJ/mol, are in good agreement. It confirms that independently calculated values of p°(CeF4) and ΔsH°(CeF4) are consistent. On the basis of those data, the entropy of CeF4 sublimation was also calculated, ΔsS°(CeF4) = 188 ± 7 kJ/mol·K, and the

F2 ↔ 2F

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fluorine must be present in the system mostly as atomic species under described conditions. The equilibrium pressure of fluorine in the system CeF4(s)−CeF3(s) was estimated D

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The Journal of Physical Chemistry A applying enthalpies of formation of cerium fluorides (Δ f H° 298 (CeF 3 (s)) = −1732.6 ± 4.2 kJ/mol 10 and ΔfH°298(CeF4(s)) = −1935.2 ± 4.1 kJ/mol;8 change of the function (−GT + H298)/T was calculated using data for uranium fluorides22). According to calculations the equilibrium pressure of fluorine in a temperature range of 873−920 K is in the 1 × 10−5 ≤ peq(F) ≤ 6.6 × 10−5 Pa interval and does not exceed sensitivity threshold of the used equipment p ≈ 1 × 10−3 Pa. This does not contradict experimental data; however, amount of CeF3(s) that might be formed had to be much smaller than that observed experimentally. For example, at 873 K and peq(F) = 1 × 10−5 Pa, after ∼22 h of evaporation only 2.9 × 10−4 mg of CeF3(s) had formed. In reality, only less than the half of the initial quantity of CeF4(s) vaporizes, while the rest of it decomposes (Table 2). In this case partial pressure of fluorine, forming according to the reaction CeF4(s) → CeF3(s) + F

Figure 2. SEM image of the compact dense layer of CeF3(s) on the surface of the lid of the effusion cell.

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must reach observable value of p(F) ≈ 1.4 × 10 Pa at 873 K. In some experiments the temperature of the effusion cell was increased to 960 K; however, neither fluorine atoms F nor platinum fluorides were observed in the gas phase. This means that fluorine or products of fluorination were leaving the effusion cell in different ways. a. Cerium Tetrafluoride Thermal Decomposition in the Platinum Effusion Cell, Equipped with a Platinum Plate. In all experiments the major part of the formed CeF3(s) was distributed on the inner surface of the body and the lid of the effusion cell. This means that almost all initial sample of cerium tetrafluoride enters the gas phase, and then the processes of sorption and desorption of CeF4 molecules lead to formation of CeF4 layer equally distributed on the inner surfaces of the cell, which finally becomes a source of CeF4 molecules entering the gas phase. It is supposed that in the case when a surface of an evaporating sample Sv exceeds a surface of an effusion orifice Seff at 100 times or more, the pressure of the sample achieved in the cell is equal to saturated vapor pressure of the sample, p°(CeF4). Cerium tetrafluoride sublimation is accompanied by formation of CeF 3(s) particles. If the sample of CeF4 decomposed according to Reaction 7 with total elimination of fluorine to gas phase, then CeF3(s) would be distributed on surface of the cell in the same way as CeF4. However, scanning electron microscope (SEM) study of CeF3(s) distribution on the inner surface of the Knudsen effusion cell after complete evaporation of CeF4(s) samples revealed that CeF3(s) particles were distributed unequally on the inner surface of the cell; that is, thermal decomposition of CeF4 occurs only on specific zones inside the effusion cell according to reaction CeF4(s) → CeF3(s) + Fs

Figure 3. SEM image of the CeF3(s) crystals on the edge of the effusion orifice.

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Figure 4. SEM image of agglomerates of CeF3(s) particles on the surface of the lid along the zone of lid−body connection.

where Fs are fluorine atoms that leave CeF4(s) and migrate inside surface layer to drain zones (vacation mechanism of migration). Two areas with maximal CeF3(s) concentration could be distinguished: areas around the effusion orifice together with its edge (Figures 2 and 3) and zone of lid−body connection (Figure 4). These areas are transparent for Fs atoms due to their migration along Pt surface to a reducing agent situated outside of the cell (the capsule made of stainless steel). Concentration of CeF3(s) particles decreases with increasing distance from these zones.

Such unequal distribution of cerium trifluoride particles on the inner surface of the effusion cell characterizes thermolysis of cerium tetrafluoride as kinetic phenomenon initiated and proceeds due to irreversible drain of Fs atoms from the platinum effusion cell. Thus, fluorine atoms are found to be movable inside the adsorption layer on the inner surface of the platinum cell, which consists of CeF4(s) and CeF3(s). To confirm the assumption about atomic fluorine Fs migration in the system CeF4(s)−CeF3(s)−Pt the effusion E

DOI: 10.1021/acs.jpca.5b04105 J. Phys. Chem. A XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry A cell was equipped with special platinum triangular plate (Figure 5). The size of the plate fit the inner diameter of the effusion

Figure 5. Platinum triangular plate installed into the effusion cell.

Figure 6. SEM image of the corner of the Pt plate after CeF4(s) evaporation.

cell and was mounted in the way that the plate was in contact with the inner surface of the cell only by its vertexes, and the contact with the initial sample of cerium tetrafluoride was excluded. A hole with diameter equal to diameter of the effusion orifice, ⌀ = 0.3 mm, was made in the middle of the plate. During the first evaporation of CeF4(s) from the platinum effusion cell equipped with Pt triangular plate (experiment 1 in Table 5) the dependence of I(CeF3+) upon time had the same profile as in experiments without additional parts in the cell (Figure 1), and the calculated saturated vapor pressure of cerium tetrafluoride p°(CeF4) = 0.23 Pa corresponded to the recommended value at 873 K. Mass of the Pt plate had not changed, while masses of the body and the lid of the effusion cell had increased due to formation of CeF3 particles on their surfaces (experiment 1 in Table 5). Compact dense layers and detached particle agglomerates of CeF3(s) on the inner surfaces of the lid and the body of the cell were same as in experiments performed by applying the effusion cell without Pt plate (Figures 2−4). CeF3(s) powder on the bottom of the effusion cell was not observed. Although mass of the platinum triangular plate had not noticeably changed, CeF3(s) particles were also formed on its surface, so zones with different concentration of particles can be distinguished. There were few single CeF3 particles on the central part of the plate around the hole and on edges. In contrast, concentration of CeF3 particles on the vertexes of the plate was considerably higher, and larger agglomerates had been formed (Figure 6). Unequal distribution of CeF3(s) particles on the Pt plate also confirms that CeF3(s) formation is controlled by drain of fluorine atoms from the surface layer of CeF4. Since Fs atoms absorbed on the Pt plate diffuse to drain zones via three point

contacts (vertexes of triangle), the density of fluorine flow and rate of CeF3(s) accumulation are maximal at these three points. b. Cerium Tetrafluoride Thermal Decomposition in the Platinum Effusion Cell, Equipped with Ni Parts (Ring and Tube). As demonstrated by experiments described above, the main pathway of fluorine to the drain zones is migration of Fs atoms in adsorption layer. Obviously, presence of a reductant in the system would lead to formation of the “inner” drain zones of fluorine due to reaction between Fs and reductant. Moreover, taking into consideration mechanism of Fs diffusion, thermal decomposition of cerium tetrafluoride would also take place without direct contact of the sample with a reductant. To demonstrate the reliability of this supposition several experiments with the effusion cell equipped with nickel parts as a reductant were performed. In the first experiment it was planned to determine the accumulation of CeF3(s) phase on the platinum surface, caused by fluorine atoms drain to the reductant. For this purpose the sample was evaporated from the platinum effusion cell, equipped with platinum plate identical to one used in the previous experiment. A specially designed ring made from nickel foil (o.d. = 3 mm, i.d. = 2 mm, d = 0.2 mm) was maintained from the one side of the Pt plate, so that center of the ring matched the center of the plate (Figure 7). The ring was fixed by Pt holders. The evaporation of CeF4(s) lasted 14 h, and after that the surfaces of the Pt plate and Ni ring were analyzed by SEM. It was shown that the layer of cerium trifluoride had formed both on the nickel ring and on area of the platinum plate neighboring Ni ring (inside as well as outside the ring; Figure 8). The surface of the platinum plate distant from the nickel ring was free from CeF3(s) layer, while single CeF3(s) particles were detected.

Table 5. Experimental Data on CeF4(s) Evaporation exp 1 2 3

m0, mg 11 10 10.2

mv, mg 6.9 1.7 0.2

Δm(body), mg

Δm(lid), mg

Δm(plate), mga

t, h

p°(CeF4), Pa

1.9 0.1 0.7

0 7.1 6.0

71 14.5 16

0.23 0.29

b

1.8 1.0 (0.4)c 3.0 (0.9)

m0initial mass of CeF4(s) sample; mvmass of evaporated CeF4(s); Δmincrements of masses of body, lid and plate after evaporation; ttime of evaporation; p°(CeF4)calculated pressure of CeF4. arefers to: Pt-plate in experiment 1; Ni-plate in experiment 2; Ni-tube in experiment 3. b mass of the CeF3(s) layer formed on the internal surface of the cell’s body. cfor experiments 2 and 3: mass of formed CeF3(s), including mass of CeF3(s) layer, given in brackets. F

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Figure 9. SEM image of the CeF3(s) layer on the Pt plate (1) and on Ni ring (2).

In further experiments an attempt was made to quantitatively estimate the influence of a reductant on the process of CeF4(s) evaporation. Experimental data are presented in Table 5. In the experiment 2 amount of the evaporated sample was approximately the same as in the first experiment (Table 5), but a nickel triangular plate of a same size (S = 46 mm2) was installed in the effusion cell instead of the platinum one. The value of the ion current I(CeF3+) and its dependence upon time (Figure 10) were similar to value, observed in experiments, performed using the platinum effusion cell without additional Pt or Ni parts (Figure 1).

Figure 7. Platinum triangular plate and nickel ring installed into the effusion cell.

Figure 8. SEM image of the CeF3(s) layer on surfaces of the Pt plate (left and right sight) and the Ni ring (in the center).

Morphologies of the cerium trifluoride layers, formed on Pt and on Ni surfaces, were different. Formation of the CeF3(s) layer on the Pt surface could be caused only by drain of fluorine atoms Fs to the reductant. Thereby, formation of the layer is initiated on the areas neighboring to the nickel ring with further expansion in direction away from the ring. This mechanism is confirmed by orientation of CeF3(s) particles that matches the direction of Fs atoms drain (Figure 9). CeF3(s) particles with similar orientation and morphology form layers near the effusion orifice and the body−lid connection of the cell (Figure 3). In contrast, the CeF3(s) layer on the nickel ring had a developed relief without any particles orientation. The different morphology of the layer on the Ni ring is caused by a different mechanism of its formation. In contrast to uniform drain of Fs atoms on the Pt surface, the CeF3(s) layer on the Ni surface is being formed due to a reaction of solid-state fluorination 2CeF4(s) + Ni(s) = 2CeF3(s) + NiF2(s)

Figure 10. Dependence of ion current I(CeF3+) upon time (experiment 2, Table 5); Δt, h indicates time of evaporation.

The vapor pressure of CeF4, calculated after the second experiment was equal to p(CeF4) = 0.29 Pa, is in good agreement with the recommended value of p°(CeF4). It means that the presence of a reductant does not affect the value of the saturated vapor pressure of cerium tetrafluoride. After evaporation it was found that the whole surface of the Ni plate was covered by CeF3(s) layer, which was formed after reaction between Ni(s) and CeF4(s), condensed on the plate. According to the SEM analysis, the CeF3(s) layer was composed of particles of size 2−12 μm, forming elevations on the surface. The same relief was observed on the surface of the nickel ring used in the previous experiment. Weighing of Ni parts of the system revealed that almost 70% of the initial mass of the sample had been decomposed, forming

(9)

which runs irregularly and is initiated in reaction centers. This mechanism leads to the observed relief of the uneven layer with expressed elevations. G

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Figure 11. Dependence of I(CeF3+) upon time during CeF4(s) evaporation from the cell with installed nickel tube; Δt, h indicates time of evaporation.

With very close initial masses of the sample and surface areas of the reductant in experiments 2 and 3 (Table 5), the evaporation processes differ dramatically. It could be explained by significant differences in contact areas between the reductant and the body of the effusion cell: in experiment 2 there are three point contacts, while in experiment 3the whole outer surface of the tube. The mass-balance equation for cerium tetrafluoride evaporation in the effusion cell is

the CeF3(s) layer on the Ni surface. This resulted in decrease of evaporation time by 5 times, compared to the first experiment. Consequently, the amount of CeF3(s), formed on the lid of the cell due to Fs atoms draining through the effusion orifice, was also decreased. According to SEM images the surface area of the CeF3(s) layer around the effusion orifice vastly decreased as well as number of CeF3(s) particles on the area of the body−lid connection. In experiment 3 the nickel tube made of nickel ribbon (width: 2 mm, thickness: 0.1 mm, surface area: 40 mm2) was installed in the body of the effusion cell so that the outer diameter of the tube matched the inner diameter of the cell, resulting in much larger contact area between the reductant (Ni) and the inner surface of the effusion cell in comparison with experiment 2; contact between Ni tube and CeF4(s) sample was also excluded. Under these conditions the evaporation process dramatically changed (experiment 3, Table 5). Weighing of the effusion cell after evaporation revealed that only ∼2% of the initial sample had been evaporated, while the main quantity of CeF 3 (s) was accumulated on the inner surface of the Ni tube. CeF3(s) powder on the bottom of the cell, formed directly from the CeF4(s) sample, was also detected. Dynamics of the CeF4 vapor pressure changing is illustrated by the dependence of I(CeF3+) upon time (Figure 11) and has a completely different profile from those determined in previous experiments (Figure 10). In the beginning of evaporation, after increasing the temperature to 873 K, pressure of cerium tetrafluoride p(CeF4) became measurable only after 5 min and in further 3 h rose to a value equal to 0.3 p°(CeF4). After that a slow decrease of the p(CeF4) value took place. The same kinetic effectslow increase and further decrease of p(CeF4)was observed after resumption of evaporation after its interruption. Moreover, after resumption of evaporation the pressure p(CeF4) reached almost the same value that had been reached in the system before the evaporation was interrupted (Figure 11). Described effects were not observed during CeF4(s) evaporation in the absence of a reductant.

Q ↓ (CeF4 ) = Q ↑ (CeF4 ) + Q dec

(10)

where Q↓(CeF4) and Q↑(CeF4) are sorption and desorption flows of CeF4 molecules to and from the inner surface of the cell, Qdec is a rate of CeF4(s) decomposition to nonvolatile CeF3(s), which is equal to flow of Fs fluorine atoms directed to drain zones Q(Fs). In ideal case, when Qdec = 0 and Q↓(CeF4) = Q↑(CeF4), an achieved flow of sorption/desorption of CeF4 molecules is maximal and corresponds to saturated vapor pressure of CeF4. Under Knudsen conditions when a mean free path of molecules is much larger than diameter of effusion orifice, the effusion beam is directly proportional to the desorption flow of CeF4 molecules from the inner surface of the cell. Mass spectrometric measurements clearly demonstrate time dependence of the desorption flow of CeF4 molecules from the cell surface, affected by sample’s thermal decomposition, caused by surface drain of Fs atoms to a reductant. In experiment 2 the reductant (Ni triangular plate) had three point contacts with the cell’s body, and its influence on the desorption flow of CeF4 molecules from the inner surface of the cell was not significant due to low value of flow Q(Fs) directed to the reductant; sorption Q↓(CeF4) and desorption Q↑(CeF4) flows were almost equal, and vapor pressure, achieved in the cell, was close to the saturated vapor pressure of CeF4. In experiment 3 the reductant (Ni tube) had a maximal contact area with the cell’s body. Therefore, flow Q(Fs) increased leading to decrease of desorption flow Q↑(CeF4), which was a reason for observed decrease of p(CeF4) during isothermal evaporation of CeF4(s). H

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(6) Badtiev, E. B.; Chilingarov, N. S.; Korobov, M. V.; Sidorov, L. N.; Sorokin, I. D. Enthalpy of Formation and Electron Affinity of Cerium Tetrafluoride. High Temp. Sci. 1982, 15, 93−104. (7) Gibson, J. K.; Haire, R. G. Thermal Stabilities of Americium and Cerium Tetrafluorides. J. Less-Common Met. 1988, 144, 123−131. (8) Khanaev, E. I.; Storozhenko, T. P.; Afanas’ev, Yu.A. K Termokhimii Tetraftorida Ceriya. Deposited Doc. 1981, SPSTL 614 Khp-D81, Available in SPSTL, Russia. (9) Smith, D. W. A Simple Empirical Analysis of the Enthalpies of Formation of Lanthanide Halides and Oxides. J. Chem. Educ. 1986, 63, 228−231. (10) Cotton, S. Lanthanide and Actinide Chemistry; John Wiley & Sons Ltd: Chichester, England, 2006. (11) Goryunkov, A. A.; Mazej, Z.; Ž emva, B.; Strauss, S. H.; Boltalina, O. V. C60 Fluorination With Rare Earth Metal Tetrafluorides: an Extreme PrF4 Case. Mendeleev Commun. 2006, 16, 159−161. (12) Rau, J. V.; Cesaro, S. N.; Chilingarov, N. S.; Leskiv, M. S.; Balducci, G.; Sidorov, L. N. Mass Spectrometric and FTIR Spectroscopic Identification of FeF4 Molecules in Gaseous Phase. Inorg. Chem. Commun. 2003, 6, 643−645. (13) Kim, J. H.; Yonezawa, S.; Takashima, M. Reaction Between Cerium Trifluoride and Elemental Fluorine. J. Fluorine Chem. 2003, 120, 111−116. (14) Tressaud, A.; Demourgues, A.; Durand, E.; Belhomme, C.; Jourdan, A.; Morel, B. Tetrafluorure de Cerium, ses Procedes de Synthese et ses Utilisations, patent FR2898886 (A1), 2007, http:// www.patfr.com/200709/FR2898886.html. (15) Chilingarov, N. S.; Shlyapnikov, I. M.; Mazej, Z.; Moiseev, A. E.; Shlyapnikov, Ya.M.; Ishtubaev, I. V. The Study of Cerium Tetrafluoride Vaporization by Knudsen Effusion Mass Spectrometry. ECS Trans. 2013, 46, 191−195. (16) Wartenberg, H. Ü ber Einlge Höhere Fluoride (PbF4, CeF4, BiF5). Z. Anorg. Allg. Chem. 1940, 244, 337−347. (17) Mazej, Z. Room Temperature Synthesis of Lanthanoid Tetrafluorides (LnF4, Ln = Ce, Pr, Tb). J. Fluorine Chem. 2002, 118, 127−129. (18) Inghram, M. G.; Drowart, J. Mass Spectrometry Applied to High Temperature Chemistry. High Temperature Technology; McGraw-Hill: NewYork, 1960. (19) McCreary, J. R.; Thorn, R. J. Entropies and Enthalpies of Sublimation of Calcium and Cerium Fluorides; Correlation of Entropy and Enthalpy in Errors. High Temp. Sci. 1973, 5, 365−382. (20) Asker, W. J.; Wylie, A. W. Cerium Tetrafluoride. II. The System Cerium Tetrafluoride − Cerium Trifluoride. Aust. J. Chem. 1965, 18, 969−975. (21) Kireev, V. A. Metody Prakticheskih Raschetov v Termodinamike Himicheskih Reakcij; Khimiya: Moscow, Russia, 1970. (22) IVTANTHERMO, A Thermodynamic Database and Software System for the Personal Computer. User’s Guide; CRC Press: Boca Raton, 1993. (23) Chilingarov, N. S.; Skokan, E. V.; Rau, D. V.; Sidorov, L. N. A Mass-Spectrometric Study of Iron Trifluoride Decomposition. Russ. J. Phys. Chem. 1994, 68, 1068−1073. (24) Chilingarov, N. S.; Rau, D. V.; Nikitin, A. V.; Sidorov, L. N. A Mass-Spectrometric Study of Fluorination in the CoF2(s)-F and CoF2(s)-Pt(met)-F Systems. Russ. J. Phys. Chem. 1997, 71, 1455−1459. (25) Chilingarov, N. S.; Nikitin, A. V.; Rau, J. V.; Golyshevsky, I. V.; Kepman, A. V.; Spiridonov, F. M.; Sidorov, L. N. Selective Formation of C60F18(g) and C60F36(g) by Reaction of [60]Fullerene with Molecular Fluorine. J. Fluorine Chem. 2002, 113, 219−226.

CONCLUSIONS Within the present research, saturated vapor pressure p° and enthalpy of sublimation ΔsH° of cerium tetrafluoride CeF4 were determined by means of KEMS in the range of 750−920 K. Analysis of data, available in literature, and comparison of previously calculated enthalpies of CeF4 sublimation with one determined in present work were performed. It was discovered that cerium tetrafluoride evaporation is accompanied by thermal decomposition of the sample with formation of CeF3(s). Analysis of obtained KEMS and SEM data allows the statement that atomic fluorine, eliminated during CeF4(s) thermolysis, does not enter the gas phase. Formation of CeF3(s) is initiated and proceeds due to irreversible drain of Fs atoms from the system along the edge of the effusion orifice and body−lid connection of the cell. Although drain of Fs atoms from the system results in decomposition of almost a half of the sample, equilibrium of the sublimation process remains. When a reductant (Ni(s)), which does not have direct contacts with a sample, is present in the system, thermolysis of CeF4(s) is controlled by diffusion flows of Fs atoms, directed from the sample to a reductant. The paths of Fs migration are clearly revealed by CeF3(s) distribution on the inner surface of the cell. Suppositions about the existence of diffusion fluxes of fluorine atoms on the surface have been previously used for explanation of iron trifluoride thermolysis,23 fluorination in the system CoF2(s)−Pt(met)−F,24 and different mechanisms of fullerene C60(s) fluorination processes.25

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AUTHOR INFORMATION

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Authors acknowledge Russian Foundation for Basic Research (Grant No. 15−03−04987) and Slovenian Research Agency (Research Program P1−0045 Inorganic Chemistry and Technology) for financial support of the present work. Authors are also grateful to the master of the highest qualification A. S. Dunaev for the high quality of mechanical work during preparation of the experimental apparatus, Ya. M. Shlyapnikov and A. E. Moiseev for loading samples in an inert atmosphere, and Asst. Prof. Dr. E. I. Ardashnikova and Dr. I. B. Kucenok for reading the draft of the article and their constructive comments. The present work was performed using research instruments, acquired with the support of the program of Moscow State Univ. development.

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REFERENCES

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