Physicochemical Properties of the (LiF+ CaF2) eut+ LaF3 System

Article pubs.acs.org/jced

Physicochemical Properties of the (LiF + CaF2)eut + LaF3 System: Phase Equilibria, Volume Properties, Electrical Conductivity, and Surface Tension Blanka Kubíková,*,† Miroslav Boča,† Jarmila Mlynáriková,† Veronika Gurišová,† Michal Šimurda,† Zuzana Netriová,† and Michal Korenko†,‡ †

Institute of Inorganic Chemistry, Slovak Academy of Sciences, Dúbravská cesta 9, 845 36 Bratislava, Slovakia Engineering College, Valparaiso University, 1900 Chapel Drive, Valparaiso, Indiana 46383, United States

‡

S Supporting Information *

ABSTRACT: The physicochemical behavior of the molten system (LiF (1) + CaF2 (2))eut + LaF3 (3) has been studied and the phase equilibria, density and volume properties, electrical conductivity, and surface tension have been selected for investigation. Well-established methods for determination of these physicochemical properties of molten salts have been used, such as thermal analysis, the Archimedean method of hydrostatic weighing, and the phase shift and maximum bubble pressure methods. A significant change in all investigated properties was detected in the region of x3 = 0.04−0.06. In regard to phase equilibria, this region is close to and may contain the eutectic point; in volumetric properties, the initial volume contraction (on increasing mol %) starts to reverse. A sharp change of electrical conductivity reflects the changes in ionic composition, and surface tension also shows different behavior below and above this region.

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INTRODUCTION Molten lanthanide fluorides have been investigated for possible use in the nuclear fuel industry in the following two different roles: as models for the pyrochemical processing, reprocessing, and separation of the heavier (and usually more radioactive) actinide systems and as models or potential components in cooling/heating/heat transport systems. For these purposes, several candidate systems with two or more components have been selected and investigated including almost all lanthanides. However, this investigation has not been systematic even though much data has been gathered over a long period of time. When considering only systems with the most frequently investigated LaF3, such systems are, for example, LiF + LaF3, NaF + LaF3, KF + LaF3, RbF + LaF3, LiF + NaF + LaF3, as well as others.1−6 The majority of these research reports are focused on phase equilibria, and other properties are investigated only occasionally, such as NMR, extended X-ray absorption fine structure (EXAFS), or X-ray photoelectron spectroscopy.5,7,8 The investigation of basic physicochemical properties (like density, surface tension, viscosity, and electrical and thermal conductivity) has been more or less random even when these properties are important for potential applications. The system LiF (1) + CaF2 (2) + LaF3 (3) is numbered among those with the potential for practical use in nuclear applications. However, only one report dealing with the modeling and calculation of the phase diagram is accessible.3 The aim of this work is to provide a consistent set of data for the physicochemical properties of the title system, including © 2016 American Chemical Society

density (with subsequent calculation of volume properties), surface tension, viscosity, and electrical conductivity.

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EXPERIMENTAL SECTION The following chemicals have been used in the experimental section: LiF, CaF2, LaF3, NaCl, KCl, NaF, and KF. Some of the chemicals were dried for 2 h before use, that is, LiF at 723 K, CaF2, KCl, and NaF at 773 K, and KF at 473 K under vacuum. A glovebox with controlled nitrogen atmosphere was used for storage of the chemicals. Table 1 summarizes all information about used chemicals. Phase Equilibria. The thermal analysis method was used for determination of the temperatures of individual thermal effects. Ten grams of the homogenized mixture was weighed out into the platinum crucible and placed in the resistant furnace. A heating rate of 5 K min−1 and cooling rate of 1.5 K min−1 were used for recording of the heating and cooling curves. The primary crystallization (Tpc/K) and eutectic (Te/K) temperatures were evaluated from the cooling curves. The precision of the temperature determination was checked by comparison with the melting temperatures of pure salts (NaCl, KCl, NaF, and KF) and the uncertainty in the temperature measurement was determined to be u(T) = 2 K. A Pt−Pt10Rh thermocouple was used for measurements. Each experiment Received: June 26, 2015 Accepted: February 28, 2016 Published: March 7, 2016 1395

DOI: 10.1021/acs.jced.5b00536 J. Chem. Eng. Data 2016, 61, 1395−1402

Journal of Chemical & Engineering Data

Article

found to have a value u(r) = 0.5 × 10−3 m. The uncertainty in the immersion depth determination u(h) was u(h) = 0.5 × 10−3 m. The precision of the pressure measurement was expressed as an uncertainty u(p) = 0.5 Pa. The uncertainty in the temperature measurement u(T) was u(T) = 2 K. A detailed description of the measurement procedure and the experimental setup can be found in other published work.14,15

Table 1. Sample Description Table with Drying Temperature (Td/Ka) chemical name lithium fluoride calcium fluoride lanthanum fluoride sodium chloride potassium chloride sodium fluoride potassium fluoride

source

initial mole fraction purity

purification method

Td/K

SigmaAldrich Merck

0.99

drying

723

b

drying

773

ChemPur

0.999

none

Slavus

0.999

none

Merck

0.995

drying

773

Alfa Aesar

0.99

drying

773

Aldrich

0.99

drying under vacuum

473

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RESULTS AND DISCUSSION Phase Equilibria. The temperatures of primary crystallization, together with the eutectic temperatures of the investigated system (LiF (1) + CaF2 (2))eut + LaF3 (3) (xLiF = x1, xCaF2 = x2, xLaF3 = x3), are summarized in Table 2 and data are graphically shown on Figure 1. Table 2. Temperature of Primary Crystallization (Tpc/K) and Eutectic Temperature (Te/K) of the (LiF (1) + CaF2 (2))eut + LaF3 (3) System for Different Compositions, Together with Investigated Cooling Temperature Range T/ K Obtained at Pressure p = 0.1 MPaa

a

Standard uncertainties u is u (Td) = 0.5 K. bSupplier has indicated purity only as pure.

was performed under inert argon atmosphere. The LabView software environment was used for data collection. Further information about the procedure can be found in our previous publications.9,10 Density. The Archimedean method was used for determination of the density of investigated system. A platinum vessel suspended on a platinum wire with a diameter of d/m = 0.3 × 10−3 (u(d) = 0.05 × 10−3 m), attached to the bottom of an electronic balance unit, was used as the measuring body. The dependence of the vessel volume on temperature was determined by calibration using molten NaCl and KF. The temperature was measured using a Pt−Pt10Rh thermocouple. The uncertainty in the temperature measurement was u(T) = 2 K. A PC with the LabView software environment was used for controlling the measuring device and for evaluation of the experimental data. The experimental device and procedure have been described in more detail elsewhere.11,12 Electrical Conductivity. The electrical conductivity was investigated using a method based on the principle of phase shift measurements with a conductometric cell made of a pyrolytic BN capillary and platinum−rhodium electrodes. The dimensions of the pBN capillary were as follows: d = 4 mm (u(d) = 0.5 mm) and l = 100 m (u(l) = 0.5 mm). The Pt10Rh alloy for the electrode was placed in a fixed position inside the pBN capillary. A Pt−Rh crucible with 35 g of the investigated mixture (u(m) = 0.0005 g) was used as a counter-electrode. A Pt−Pt10Rh thermocouple was used for the temperature determination with an uncertainty of u(T) = 2 K. An impedance/gain phase analyzer (a National Instruments highperformance modular chassis controlled by Labview software) was used to measure the cell impedance. Further experimental details of the procedure can be found elsewhere.13 Surface Tension. The surface tension of the studied system was determined using the maximum bubble pressure method in a water-cooled resistance furnace. The surface tension of each sample was measured at five or six different temperatures in the range above the temperature of primary crystallization. The surface tension measurements were performed at four different depths of immersion (usually (2, 3, 4, and 5) × 10−3 m) yielding six surface tension values for each temperature. The precise machining of the capillary orifice is important for getting correct and exact results. The deviation of the orifice diameter we express as an uncertainty u(r), which we have

x1

x2

x3

Tpc/K

0.790 0.786

0.210 0.209

0.000 0.005

1036 1035

0.782

0.208

0.010

1033

983

1273−473

0.774

0.206

0.020

1030

987

1273−473

0.751

0.199

0.05

1019

987

1273−473

0.711

0.189

0.100

1001

989

1273−473

0.695

0.185

0.120

1010

990

1273−473

0.671

0.179

0.150

1018

988

1273−473

0.632

0.168

0.200

1034

990

1273−473

0.593

0.157

0.250

990

1273−473

0.553

0.147

0.300

990

1273−473

0.514

0.136

0.350

990

1373−473

1020b

Te/K

T/K 1073−973 1273−473

solidified phases LiF, CaF2, LaF3 LiF, CaF2, LaF3 LiF, CaF2, LaF3, LaOF LiF, CaF2, LaF3, LaOF LiF, CaF2, LaF3, LaOF LiF, CaF2, LaF3, LaOF LiF, CaF2, LaF3, LaOF LiF, CaF2, LaF3, LaOF LiF, CaF2, LaF3, LaOF LiF, CaF2, LaF3, LaOF

a

Standard uncertainties u are u(x1) = 0.001, u(x2) = 0.001, u(x3) = 0.001, u(Tpc) = 2 K, u(Te) = 2 K, u(T) = 2 K and u(p) = 10 kPa. bNot primary crystallization.

The eutectic temperatures of the quasi-binary system at ca. 989 K fits with the eutectic temperature of 985 K (at x1 = 0.684, x2 = 0.177, x3 = 0.139) previously reported for the ternary system,1 calculated by a quasi-chemical model from DSC data. On the basis of this model, one peritectic point was also reported at 1002 K and at x1 = 0.664, x2 = 0.188, x3 = 0.148. Both points are very close to each other in the ternary system and the cross section line (molar ratio LiF (1)/CaF2 (2) = 0.79/0.21) represented by (LiF (1) + CaF2 (2))eut + LaF3 (3) lies between the ternary eutectic (molar ratio LiF (1)/CaF2 (2) = 0.794/0.206) and ternary peritectic (molar ratio LiF (1)/ CaF2 (2) = 0.779/0.221) points. The temperatures of primary crystallization for the investigated system might differ slightly from those that could be calculated based on that previous work.1 The reason is that in this work the thermal analysis method was used and the temperatures of primary crystal1396



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from thermal analysis showed no other crystalline phase except the initial ones (LiF, CaF2 and LaF3). With increasing LaF3 content, a small amount of LaOF was also present as an impurity (several reasons are possible, for example, oxygen impurities in Ar or imperfectly sealed furnace). Moreover, with increasing LaF3 content the background contribution to the XRD signal increases as well, and the formation of some glassy phase cannot be excluded. This would help explain the observed undercooling effect at higher LaF3 concentration. Density and Volume Properties. With density and subsequent volume properties, some unusual behavior of molar volume was found for our investigated system (LiF (1) + CaF2 (2))eut + LaF3 (3). Table 2 summarizes the fitted values of the properties a/g·cm−3 and b/g·cm−3·K−1, together with their respective standard deviations σa/g·cm−3 and σb/g·cm−3· K−1 for 11 measured compositions (S1−11). The fitting was performed by linear regression analysis according to the following equation

Figure 1. Temperatures of primary crystallization, □, together with eutectic temperatures, ○, of the (LiF (1) + CaF2 (2))eut + LaF3 (3) system. Temperatures are expressed in Kelvin.

ρ = a − bT

(1)

The density increases monotonously with increasing LaF3 (3) content from a value of 2.040 g·cm−3 for (LiF (1) + CaF2 (2))eut up to 3.728 g·cm−3 at 1173 K. We find the density of the eutectic mixture (LiF (1) + CaF2 (2))eut at 1173 K to be 2.040 g·cm−3 for x1 = 0.79, which is close to the reference density of 2.020 g·cm−3 for x1 = 0.80 and temperature 1170 K.31,32 The experimental dependence of density, ρ, on temperature, T, for 11 samples of the investigated system, is shown on Figure 2. Table 4 summarizes the values of the density for three selected temperatures calculated according to eq 1, and Figure 3 shows the concentration dependency of density at these temperatures. The molar volumes (Vm,i/cm3·mol−1) at particular compositions and selected temperatures of the investigated system can be easily calculated according to the equation

lization were obtained from the cooling curves, while that work used the DSC method for the determination of the temperatures of primary crystallization.1 On the basis of the data of the present work, a eutectic point is expected around 6 mol % of LaF3. The left side of the phase diagram of the investigated system might indicate the formation of a simple eutectic system. However, there are several indications that the system is more complicated. First of all, it should be mentioned that the system LiF (1) + LaF3 (3) seems to be simple eutectic one1,16−20 (although in the systems with all other alkaline metals some other phases were reported3,16,17,21−24), while the system CaF2 (2) + LaF3 (3) is not a simple eutectic one;25−28 fields of solid solution are present, but the formation of any other phases in phase diagrams have never been reported although the formation of some ternary phases, like Ca0.65La0.35F2.35 and Ca0.9La0.1F2.1, have been reported separately.29,30 This means that the formation of ternary fluorides may be expected in the system (LiF (1) + CaF2 (2))eut + LaF3 (3). Another thermal effect, which cannot be considered as effect of primary crystallization at higher LaF3 concentrations, was recorded both in this work (at 1020 K and x1 = 0.553, x2 = 0.147, x3 = 0.300) as well as in in ref 1. XRD patterns of solidified samples

Vm, i =

∑i xiMi ρi

(2)

where xi is the molar fraction, Mi/g·mol−1 is the molar mass and ρi/g·cm−3 is the density at a particular composition. Table 5 summarizes the values of the molar volume for three selected temperatures calculated according to eq 2; calculated values for three different temperatures are shown on Figure 4.

Table 3. Regression Coefficients a/g·cm−3, b/g·cm−3·K−1, Corresponding Standard Deviations sda/g·cm−3, sdb/g·cm−3·K−1, and Temperature Range, T/K, of Investigated Melts in the System (LiF (1) + CaF2 (2))eut + LaF3 (3)a Obtained at Pressure p = 0.1 MPaa

a

sample no.

x1

x2

x3

a/g·cm−3

sda × 103/g·cm−3

b × 103/g·cm−3·K−1

sdb × 106/g·cm−3

T/K

S1 S2 S3 S4 S5 S6 S7 S8 S9 S10 S11

0.790 0.786 0.782 0.774 0.751 0.711 0.671 0.632 0.593 0.553 0.514

0.210 0.209 0.208 0.206 0.199 0.189 0.179 0.168 0.157 0.147 0.136

0.000 0.005 0.010 0.020 0.050 0.100 0.150 0.200 0.250 0.300 0.350

2.758 2.716 2.800 2.747 2.917 3.245 3.500 3.701 4.152 4.390 4.569

7.0 1.0 1.0 0.7 0.7 1.0 1.6 1.1 0.8 1.8 2.0

6.119 4.874 5.291 4.291 4.400 4.590 4.364 4.413 6.116 6.705 7.163

5.9 0.9 0.8 0.6 0.6 0.8 1.3 0.8 0.6 1.4 1.7

1103−1173 1123−1223 1123−1223 1173−1293 1183−1273 1153−1283 1203−1283 1273−1373 1223−1373 1273−1373 1323−1423

Standard uncertainties u are u(x1) = 0.001, u(x2) = 0.001, u(x3) = 0.001, u(T) = 2 K and u(p) = 10 kPa. 1397



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ple,9,11,33−39 but occasionally the presence of local minima can occur (in the cross sections of multicomponent systems the situation can be even more complicated).40 This is the case for the investigated systems in this work. The molar volume of (LiF (1) + CaF2 (2))eut + LaF3 (3) initially decreases on addition of LaF3 up to x3 = 0.01. However, with further addition of LaF3 (up to x3 = 0.35) the molar volume increases, and at ca. x3 = 0.04 the value of the molar volume is comparable with that of the pure solvent. Such behavior is not typical for majority of molten systems in which molar volume usually increase/decrease without local extremes. The molar volume of pure molten LaF3 was not possible to measure as its melting temperature is either (two values are given in the literature) 1773 K41 or 1766 K;42 it can merely be estimated that the molar volume of molten LaF3 could be higher (by ca. x3 = 0.15−0.25) than its room temperature value of 33.20 cm3· mol−1. From these reasons, excess molar volume cannot be calculated. Instead, partial molar volume of LaF3, V3̅ /cm 3·mol−1 can be calculated according to the equation

Figure 2. Experimental dependence of density, ρ, on temperature, T for 11 samples of the investigated system (LiF (1) + CaF2 (2))eut + LaF3 (3).

Table 4. Calculated Values of Density ρ/g·cm−3 of the Molten System (LiF (1) + CaF2 (2))eut + LaF3 (3) at Selected Temperaturesa x1

x2

x3

ρ1123K/g·cm−3

ρ1173K/g·cm−3

ρ1223K/g·cm−3

0.790 0.786 0.782 0.774 0.751 0.711 0.671 0.632 0.593 0.553 0.514

0.210 0.209 0.208 0.206 0.199 0.189 0.179 0.168 0.157 0.147 0.136

0.000 0.005 0.010 0.020 0.050 0.100 0.150 0.200 0.250 0.300 0.350

2.071 2.169 2.206 2.265 2.423 2.730 3.010 3.205 3.410 3.637 3.764

2.040 2.144 2.179 2.244 2.401 2.707 2.988 3.183 3.377 3.603 3.728

2.010 2.120 2.153 2.222 2.379 2.684 2.966 3.161 3.344 3.570 3.692

⎛ ∂V ⎞ Vi̅ = Vm + xj⎜ m ⎟ ⎝ ∂xi ⎠T , P , x

(3)

i≠j

−1

where Vm/cm ·mol is molar volume of mixture and xi, xj are molar fractions. Partial molar volume of LaF3 represents the change on volume of 1 mol LaF3 (3) when dissolved in a large amount of (LiF (1) + CaF2 (2))eut. For the calculation, the first four experimental points were used and concentration dependence of molar volume was expressed using Redlich− Kister type equation. It is not possible to use all experimental points of molar volume as no simple polynomial function describing properly the unusual molar volume concentration dependence can be found. 3

Vm1123K = 17.77(x1 + x 2) − 60.12(x1 + x 2)2 x3

a

Standard uncertainties u are u(x1) = 0.001, u(x2) = 0.001, u(x3) = 0.001, u(T) = 2 K and combined expanded uncertainty Uc is Uc(ρ) = 0.007 g·cm−3 (0.95 level of confidence).

+ 5669.00(x1 + x 2)x32

(4)

Then applying eq 3 partial molar volume of LaF3 (3) can expressed as V3̅ 1123K = 7398.12(x1 + x 2)2 − 7458.24(x1 + x 2)3

(5)

Consequently, when (x1 + x 2) → 1, V3̅ 1123K = −60.12 cm 3·mol‐1

This value indicates relatively extensive volume contraction that can be explained in the view of ionic composition changes as follow. It is expected that the very compact LaF3 solid phase in which each La atom is coordinated by nine fluorine atoms will decompose and its volume will increase while dissolving in (LiF (1) + CaF2 (2))eut. However, the formed components will have strong tendency to make coordination bonds with free F− anions from ionic (LiF (1) + CaF2 (2))eut thus resulting to the volume contraction. The final effect will then result in overall volume contraction as a result of the two described opposite processes. If this process would continue at any LaF3 (3) composition, the concentration of free F− would continuously decrease and logically it can be expected that the activity of free Li+ (or Ca2+) cations would increase. Consequently, the formation of a ternary compound (e.g., LiLaF4) that would be expected does not seem to be the case. As molar volume increases with further addition of LaF3 some other process

Figure 3. Density, ρ, of the molten system (LiF (1) + CaF2 (2))eut + LaF3 (3) at three selected temperatures: □, 1123 K; ○, 1173 K; and Δ, 1223 K.

The dependence of molar volume on composition in binary systems of molten salts is usually monotonic, for exam1398



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Table 5. Calculated Values of Molar Volume Vm/cm3·mol−1 and the Standard Deviations of Molar Volume sdVm/cm3·mol−1 of the Molten System (LiF (1) + CaF2 (2))eut + LaF3 (3) at Selected Temperaturesa

a

x1

x2

x3

V1123K /cm3·mol−1 m

sdV1123K × 102/cm3·mol−1 m


sdV1173K × 102/cm3·mol−1 m


sdV1223K × 102/cm3·mol−1 m

0.790 0.786 0.782 0.774 0.751 0.711 0.671 0.632 0.593 0.553 0.514

0.210 0.209 0.208 0.206 0.199 0.189 0.179 0.168 0.157 0.147 0.136

0.000 0.005 0.010 0.020 0.050 0.100 0.150 0.200 0.250 0.300 0.350

17.81 17.38 17.45 17.69 18.51 19.34 20.18 21.43 22.48 23.26 24.59

8.0 1.1 1.1 0.7 0.7 0.9 1.0 0.9 0.7 1.5 2.0

18.08 17.57 17.66 17.86 18.68 19.50 20.33 21.58 22.67 23.48 24.82

8.0 1.2 1.1 0.8 0.7 1.0 1.5 1.0 0.7 1.6 2.0

18.35 17.78 17.87 18.03 18.85 19.67 20.48 21.73 22.92 23.70 25.06

9.0 1.2 1. 2 0.8 0.8 1.0 1.6 1.0 0.8 1.6 2.0

Standard uncertainties u are u (x1) = 0.001, u (x2) = 0.001, u (x3) = 0.001, u (T) = 2 K.

Table 7. Calculated Values of Conductivity σ/S·cm−1 of the Molten System (LiF (1) + CaF2 (2))eut + LaF3 (3) at Selected Temperaturesa x1

x2

x3

σ1123K/S·cm−1

σ1173K/S·cm−1

σ1223K/S·cm−1

0.790 0.782 0.774 0.751 0.711 0.671

0.210 0.208 0.206 0.199 0.189 0.179

0.000 0.010 0.020 0.050 0.100 0.150

5.82 5.93 5.97 6.31 5.70 5.45

6.29 6.45 6.50 6.85 6.14 5.85

6.77 6.96 7.04 7.38 6.59 6.24

a

Standard uncertainties u are u(x1) = 0.001, u(x2) = 0.001, u(x3) = 0.001, u(T) = 2 K and combined expanded uncertainty Uc is Uc(σ) = 0.04 S·cm−1 (0.95 level of confidence).

temperature range can be measured but then linear temperature dependence of conductivity cannot be applied as was shown in refs 2 and 43. Figure 5 shows the recorded dependencies of conductivities for all measured mixtures. Conductivity of (LiF (1) + CaF2 (2))eut at 1173 K is 6.29 S· cm−1. However, this value is relatively far from ref 1, 7.20 S· cm−1 at 1170 K.44,45 We are not aware of any data for binary systems LiF (1) + LaF3 (3) or CaF2 (2) + LaF3 (3). In the investigated system, electrical conductivity initially increases with increasing LaF3 (3) content up to the value of 6.846 S· cm−1 at x3 = 0.05 (at T = 1173 K). With further increase of LaF3 (3) content, electrical conductivity decreases, as can be clearly seen in Figure 6. Measurements were possible only up to x3 = 0.15. The conductivity of the system is influenced by the ionic components present in the system. It is generally expected

Figure 4. Molar volume of the molten system (LiF (1) + CaF2 (2))eut + LaF3 (3) at three selected temperatures: □, 1123 K; ○, 1173 K; and Δ, 1223 K.

probably will dominate, but based on only volume properties they cannot be identified and some more sophisticated experiments would be necessary, for example, in situ EXAFS measurements or others. Electrical Conductivity. In order to get the concentration dependence of the electrical conductivity at selected temperatures, linear regression analysis has been applied to obtain regression coefficients from the experimental data of electrical conductivity at six measured compositions (S1−S7). Table 6 shows the calculated regression coefficients of the fitting and Table 7 summarizes the values of the electrical conductivity for three selected temperatures. It should be noted that even higher

Table 6. Regression Coefficients a/S·cm−1, b/S·cm−1·K−1, Corresponding Standard Deviations sda/S·cm−1, sdb/S·cm−1·K−1, and Investigated Temperature Range, T/K, for the Temperature Dependence of Conductivity of Molten System (LiF (1) + CaF2 (2))eut + LaF3 (3) Obtained at Pressure p = 0.1 MPaa

a

sample no

x1

x2

x3

a/S·cm−1

sda/S·cm−1

b/S·cm−1·K−1

S1 S3 S4 S5 S6 S7

0.790 0.782 0.774 0.751 0.711 0.671

0.210 0.208 0.206 0.199 0.189 0.179

0.000 0.010 0.020 0.050 0.100 0.150

2.268 2.859 3.109 2.820 1.884 1.246

0.079 0.065 0.035 0.075 0.162 0.176

0.010 0.010 0.011 0.011 0.009 0.008

sdb/S·cm−1·K−1 9.4 7.8 4.2 9.2 2.0 1.9

× × × × × ×

10−05 10−05 10−05 10−05 10−04 10−04

T/K 1064−1165 1041−1161 1041−1163 1035−1148 1034−1161 1132−1254




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that fluorine atoms will not dissociate into a free anionic state. The initial increase in conductivity on LaF3 (3) addition indicates the presence of a larger amount of some more mobile component than in (LiF (1) + CaF2 (2))eut. At x3 = 0.05, this trend reverses and the content of this more conductive species decreases. This situation is consistent with observations of phase equilibria and volume properties where again changes of the properties of the system are seen in the region between x3 = 0.04−0.06. Surface Tension. Surface tension was calculated from the experimental data according to the following equation γ=

r Pmax,1h2 − Pmax,2h1 2 h 2−h1

(6)

where r/m is the capillary radius, and Pmax,i/Pa is the maximum bubble pressure at immersion depth hi/m. The uncertainties of the parameters have been already given in the Experimental Section. Surface tension has been calculated at the same three selected temperatures as the other physicochemical properties studied in this work. The calculated coefficients from the linear regression analysis for evaluation of the concentration dependency of the surface tension are summarized in Table 8 and the values of the surface tension at selected temperatures are given in Table 9. The concentration dependence of surface tension of the system (LiF (1) + CaF2 (2))eut + LaF3 (3) is plotted in Figure 7. Surface tension increases with increasing amount of LaF3 (3) in the molten eutectic LiF (1) + CaF2 (2). However, the rate of increase in surface tension on addition of LaF3 is significantly higher for LaF3 content below x3 = 0.05.

Figure 5. Experimental dependence of conductivity, σ, on temperature, T, for six samples of the investigated system (LiF (1) + CaF2 (2))eut + LaF3 (3).

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CONCLUSION The phase equilibria and physicochemical properties of the molten (LiF (1) + CaF2 (2))eut + LaF3 (3) system have been investigated in restricted area. The eutectic point lies close to x3 = 0.06 and the eutectic temperature reaches the value ca. 989 K. However, the studied system seems not to be a simple eutectic one. At higher LaF3 concentration, such thermal effect was recorded that could not be attributed to the temperature of primary crystallization. No typical behavior has been observed for the dependence of molar volume on composition. Small additions of LaF3 to the eutectic composition of (LiF (1) + CaF2 (2)) have caused decreasing molar volume. At x3 = 0.01, molar volume increases up to x3 = 0.35. This behavior was very probably caused by significant ionic changes in the studied system at lower LaF3 content in (LiF (1) + CaF2 (2))eut. Relatively extensive volume contraction was assumed based on the value of partial molar volume of LaF3 in infinitesimally diluted solution. It was also observed that ionic changes in the

Figure 6. Conductivity of the molten system (LiF (1) + CaF2 (2))eut + LaF3 (3) at three selected temperatures: □, 1123 K; ○, 1173 K; and Δ, 1223 K.

that small free ions will make a bigger contribution to conductivity than larger, bulkier ions. In the solid state of LaF3 (3), each fluorine atom coordinates to three lanthanum atoms, which is in contrast to, for example, AlF3 where each fluorine atom makes bridges between two aluminum atoms. This means that in LaF3 each lanthanum atom is coordinated to nine fluorine atoms (in contrast to six fluorine atoms in, for example, AlF3). When dissolving such a compound in a (LiF (1) + CaF2 (2))eut melt one might expect

Table 8. Regression Coefficients a/mN·m−1 and b/mN·m−1·K−1, Corresponding Standard Deviations sda/mN·m−1, sdb/mN·m−1· K−1, and Temperature Range, T/K, for Surface Tension of Investigated Melts in the System (LiF (1) + CaF2 (2))eut + LaF3 (3) Obtained at Pressure p = 0.1 MPaa

a

x1

x2

x3

a/mN·m−1

sda/mN·m−1

b × 102/mN·m−1·K−1

sdb × 102/mN·m−1

T/K

0.790 0.751 0.711 0.671

0.210 0.199 0.189 0.179

0.000 0.050 0.100 0.150

323.7 368.7 382.6 586.5

17.9 5.4 1.7 46.2

9.3 10.8 11.7 28.3

1.6 0.4 0.1 4.0

1112−1188 1163−1240 1178−1255 1186−1263




Article

Table 9. Calculated Values of Surface Tension γ/mN·m−1 of the Molten System (LiF (1) + CaF2 (2))eut + LaF3 (3) at Selected Temperaturesa

a

x1

x2

x3

γ1123K/mN·m−1

sdγ1123K/mN·m−1

γ1173K/mN·m−1

sdγ1173K/mN·m−1

γ1223K/mN·m−1

sdγ1223K/mN·m−1

0.790 0.751 0.711 0.671

0.210 0.199 0.189 0.179

0.000 0.050 0.100 0.150

219.6 247.5 251.8 269.2

25.1 7.4 2.2 62.6

215.0 242.1 246.0 255.1

25.7 7.5 2.3 64.0

210.4 236.7 240.2 240.9

26.2 7.7 2.3 65.2

Standard uncertainties u are u(x1) = 0.001, u(x2) = 0.001, u(x3) = 0.001.

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ACKNOWLEDGMENTS The present work was financially supported by the Scientific Grant Agency of the Ministry of Education of the Slovak Republic and the Slovak Academy of Sciences under nos. 2/ 0095/12 and 2/0116/14.

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Figure 7. Surface tension of the molten system (LiF (1) + CaF2 (2))eut + LaF3 (3) at three selected temperatures: □, 1123 K; ○, 1173 K; and Δ, 1223 K.

investigated system have an influence on the electrical conductivity, which after initial increase suddenly decreases at the higher content of LaF3. The turning point was observed at x3 = 0.05. The influence of ionic changes has been clear to see also on the dependency of surface tension on composition, where the most significant increase of surface tension was observed at x3 = 0.05.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jced.5b00536. Temperature dependences (T) of density (ρ) for 11 samples of the molten system (LiF (1) + CaF2 (2))eut + LaF3 (3) refers to density experimental data LiF-CaF2LaF3 excel file. (ZIP) Temperature dependences (T) of conductivity (σ) for six samples of the molten system (LiF (1) + CaF2 (2))eut + LaF3 (3) refers to conductivity experimental data LiFCaF2-LaF3 excel file. (ZIP) Temperature dependences (T) of surface tension (γ) for four samples of the molten system (LiF (1) + CaF2 (2))eut + LaF3 (3) refers to surface tension experimental data LiF-CaF2-LaF3. (ZIP)

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*Tel: +421 2 59410 421, Fax: +421 2 59410 444, E-mail: [email protected]. Notes

The authors declare no competing financial interest. 1401



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Physicochemical Properties of the (LiF+ CaF2) eut+ LaF3 System

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