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Z. Naturforsch. 2015; 70(5)a: 325–331

Elena V. Nikolaeva*, Irina D. Zakiryanova, Iraida V. Korzun, Andrey L. Bovet and Boris D. Antonov Interaction between and Barium Containing Chloride Melt

Abstract: Thermal analysis was applied to determine the technological processes and parameters of the products liquidus temperatures in the NaCl–KCl–BaCl2–BaO sys- fabricated using ionic solvents. tem, with BaO concentration varied from 0 to 6 mole%. The barium oxide in alkali chloride melt is The temperature dependence of the BaO solubility in known to be lower than that in contain- the NaCl–KCl–BaCl2 eutectic melt was investigated; the ing solvents [6–12]. Thus, barium oxide solubility in the thermodynamic parameters of BaO dissolution were cal- molten NaCl–KCl equimolar mixture is about 0.028–0.048 culated. The caloric effects of melting of the NaCl–KCl– mole% in the temperature range of 700–800 °C [11].

BaCl2 eutectic with barium oxide and barium oxychloride Whereas according to Neumann et al. [12], barium chlo- additions were studied. The type, morphology, and com- ride may dissolve 20 mole% BaO at 1000 °C. The large position of oxychloride ionic groupings in the melt were differences in the solubility of the oxide indicate differ- determined in situ using Raman spectroscopy. ent dissolution mechanisms in the alkali chloride and alkaline-earth chloride melts [6]. The BaO solubility in the

Keywords: Barium Oxychloride; Liquidus Temperature; molten NaCl–KCl–BaCl2 mixtures increases at 700 °C from

Raman Spectroscopy; Solubility; Thermal Analysis. 0.77 to 10.94 mole% with increasing of BaCl2 content from 5 to 55 mole% [9]. The similar tendency was observed by Boghosian et al. [6] in the NaCl–BaCl melts. DOI 10.1515/zna-2014-0370 2 Received December 24, 2014; accepted March 2, 2015; previously Solodkova et al. [9] proposed the following reac- published online April 14, 2015 tion to explain the enhanced BaO solubility observed

with increasing BaCl2 concentration in NaCl–KCl–BaCl2 (n–2)– – (m–2)– melt: BaO + [BaCln] + (m–n)Cl = [Ba2OClm] . Such a scheme of barium oxide dissolution agrees with 1 Introduction Boghosian et al. [6]: the solubility data show that each

BaO added needs one BaCl2 to dissolve. Nevertheless, the Molten salt mixtures of alkali and alkali-earth chlorides model calculations in combination with the experimental are widely used as electrolytes in high temperature tech- data [6] indicate that the complexes Ba4O2Cl4 and Ba3OCl4 nological processes [1, 2]. Thus, one of the methods of may be predominant in similar systems. Though the direct barium and its alloys production is electrolysis of molten high temperature spectral experiment reveals formation 4– mixtures of BaCl2 with alkali chlorides [2]. However, oxide of Me2OHal6 complex groupings (where Me = Mg, Ca, admixtures are always formed in molten salts due to Ba; Hal = I, Cl, F) in pregnant solutions of alkaline-earth high-temperature hydrolysis of melts or their interactions in halide melts [7]. with container materials, as well as active components We have previously studied the BaO solubility in of atmosphere [3–5]. These admixtures may form insolu- molten BaCl2 (27 mol%)–KCl (73 mol%) eutectic. The ble products or bind molten salt cations into oxychlo- oxide dissolution was found to proceed according to the ride complex groupings, which inevitably influence the chemical mechanism with formation of barium oxychlo-

ride Ba4OCl6 [13]. According to Neumann et al. [12] and

Frit et al. [14] different oxychlorides (Ba4OCl6, Ba3O2Cl2 and *Corresponding author: Elena V. Nikolaeva, Institute of High Ba4O3Cl2) may be formed in BaO–BaCl2 system. Temperature Electrochemistry, Yekaterinburg, Russian Federation; The objective of this work is to provide a detail anal- and Ural Federal University, Yekaterinburg, Russian Federation, ysis of BaO dissolution mechanism in the molten NaCl– E-mail: [email protected] Irina D. Zakiryanova, Iraida V. Korzun, Andrey L. Bovet and Boris KCl–BaCl2 eutectic. With this aim in view the initial D. Antonov: Institute of High Temperature Electrochemistry, crystallisation temperatures of the NaCl–KCl–BaCl2–BaO Yekaterinburg, Russian Federation system were investigated, and the BaO solubility in the 326 E.V. Nikolaeva et al.: Interaction between Barium Oxide and Barium Containing Chloride Melt

NaCl–KCl–BaCl2 eutectic melt was determined; the caloric effects of barium oxide and barium oxychloride interac- tions with the solvent-salt were studied; the direct high temperature spectral experiment was performed. Raman spectroscopy allows detection of the melt structure in situ. The method provides sufficient data to determine the composition of ionic complexes and the compositional changes due to BaO dissolution in the NaCl–KCl–BaCl2 melts. The XRD-analysis was also used to check the inter- action products in samples under investigation.

2 Experimental

2.1 Chemicals Figure 1: XRD data for frozen Ba4OCl6 sample.

Chemically pure grade NaCl (99.9 %) (Russian State Standard Speci- determinations were usually made for each composition. The total fication (RSSS) No 4233-77), KCl (99.8 %) (RSSS No 4234-77) and BaCl2 error of the liquidus temperature measurement did not exceed 5°. (99.5 %) (RSSS No 4108-72) salts obtained from Vekton (Moscow, ­Russia) were preliminary dehydrated under vacuum at stepwise tem- perature growth and melted in argon atmosphere. Then the salts were 2.3 Technique of Synchronous Thermal Analysis (STA) additionally purified by floating-zone refining. The eutectic BaCl2 (27 mol%)–KCl (37 mol%)–NaCl (036 mol%) mixture was prepared by The studies were carried out using the STA 449C Jupiter thermal ana- fusion of the weighted portions of the prepared salts. Barium oxide lyser produced by NETZSCH Company (Germany). This device allows was prepared before each experiment from (99.5 %) measuring by differential scanning calorimetry (DSC) and thermal (RSSS No 3777-76) obtained from Vekton (Moscow, Russia) using a gravimetry (TG) in a wide temperature range. well-known technique [15]. The prepared chemical reagents were All measurements were performed in an argon atmosphere. The tested by Fourier transform (FTIR) analysis. samples were placed into the alundum or Pt-Rh alloy crucibles with 2– – The vibration bands related to CO3 , OH and adsorbed water were vented covers and were heated with the rate of 10°/min. According not observed. All operations with the prepared chemical reagents to the technical parameters of the experimental set, the temperature and mixtures were carried out in a box in a atmosphere. detection error did not exceed 1°.

The Ba4OCl6 solid phase synthesis was described in literature [14], and XRD-analysis of barium oxychloride was performed (crys- tallographic group P63mc, Z = 2, wurtzite). We synthesised Ba4OCl6 2.4 The High Temperature Raman Spectra Analysis according to reaction: The Raman spectra were recorded using the optic fiber spectromet- BaO( solid) +→3BaCl(molten)BaOCl (molten). 246 ric Ava-Raman complex (Avantes, The Netherlands), which includes

The ground mixture of the BaO and BaCl2 weighted portions was the monochromatic laser (λ = 532 nm, radiation power of 50 mW). heated up to the temperatures of nearly 50 degrees above the Ba4OCl6 The optic diffraction scheme 180° was used to record the spectra. (980 ± 10 °C) [14] in argon and was held at these tem- A soldered quartz glass ampoule with the inner diameter of 4 mm peratures for several hours. The Ba4OCl6 phase formation is proved was used as an optic cell at chloride eutectics spectra recording. A by the XRD-analysis (Fig. 1). tray of 20 mm length and 5 mm height served as a container for chemically aggressive BaO containing melts. The experiments were performed in an inert argon atmosphere. The Raman spectra record- 2.2 Technique of Liquidus Temperatures Determination ing setup is presented in Figure 2.

The method of thermal analysis (analysis of the melt cooling curves) was used to define the liquidus temperatures of the BaCl (27 mol%)– 2 3 Results and Discussion KCl (37 mol%)–NaCl (36 mol%) + BaO system, where BaO concentra- tion varied from 0 to 6 mol%. The measurements were performed in an inert argon atmosphere. The composition under study was placed 3.1 Physico-Chemical Properties of Ba OCl into the alundum crucible. The hermetical quartz container with the 4 6 crucible inside was inserted into a massive metallic block to provide the required cooling rate (2–3°/min). Temperature was measured by The thermal gravimetric and spectral studies of Ba4OCl6 means of a Pt–PtRh thermocouple. Each thermocouple was checked have been first performed to obtain data on the oxychlo- periodically at the freezing point of aluminum or . Several ride thermal stability and vibration modes. E.V. Nikolaeva et al.: Interaction between Barium Oxide and Barium Containing Chloride Melt 327

Figure 4: Raman spectra of crystalline Ba4OCl6 at the room Figure 2: The Raman spectra recording setup: 1, block; 2, the melt temperature (a), temperature dependence of Ba4OCl6 vibration under study; 3, nickel cell; 4, optic quartz window; 5, focus object- frequencies (b). glass; 6, fiber optic Raman probe; and 7, laser beam and scattered light. frequencies at elevated temperatures were then used to The DSC curves demonstrate a single endothermic interpret spectra of barium-containing oxide-chloride peak at 991 ± 1 °C, which corresponds to the Ba4OCl6 melts. melting (according to literature data [14] tm = 980 ± 10 °C). Specially conducted long-term experiments on the

The barium oxychloride melting enthalpy calculated Ba4OCl6 sample in air did not demonstrate any interaction according to the DSC data is 231.3 ± 6.9 kJ/mol. The sample with water vapours and CO2: the sample spectra before mass remains constant in the temperature interval of and after dwelling in air are identical. There were no lines 2– – 20–1020 °C (Fig. 3) according to the TG data. related to CO3 , OH , and adsorbed water observed. Figure 4 demonstrates the Raman spectra and char- acteristic vibration modes of solid Ba4OCl6 at different temperatures. The phonon modes of Ba4OCl6 at the room 3.2 Liquidus Temperatures and BaO temperature are 158, 187, 324, 343, and 379 cm–1. The loca- Solubility tion of vibration modes at 600 °C correlates with the data [16]. The Raman spectra testified the thermal gravimetric The defined in present study melting points of the triple data on Ba4OCl6 thermal resistivity: the increasing tem- BaCl2 (27 mol%)–KCl (37 mol%)–NaCl (36 mol%) eutectic perature to a linear shift of vibration frequencies to (540 ± 5 °C) agrees with literature data (542 °C) [17] within the area of smaller values, a relative intensity of bands did the accuracy of measurement error. The measurements not change, formation of new bands was not observed. carried out by the DSC method showed that the eutectic

The obtained data on the values of Ba4OCl6 vibration melting temperature was 544 ± 1 °C.

Figure 3: DSC curve of the barium oxychloride sample. 328 E.V. Nikolaeva et al.: Interaction between Barium Oxide and Barium Containing Chloride Melt

The dependence of liquidus temperatures on the added barium oxide concentration (Ni) is presented in Figure 5. At first, the BaO additions decrease the tempera- ture of initial crystallization, and then they increase it. For the compositions with BaO exceeding 0.015 mole fraction, which denotes the minimum on the liq- uidus curve, the barium oxide concentration in the melt corresponds to the barium oxide solubility at liquidus temperature. The solubility values thus obtained agree well with published data [9] determined by isothermal saturation method (Fig. 6, curve 2). Figure 6 illustrates the tem- perature dependence of BaO solubility in the lnS –1/T Figure 6: Temperature dependence of barium oxide solubility: 1, in the NaCl–KCl–BaCl eutectic melt; 2, literature data [9]. coordinates, which may be approximated by (1) with a 2 confidence coefficient 0.95 in temperature range from Table 1: Thermodynamic characteristics of BaO solubility. 540 °C to 875 °C.

0 0 (4376.17± 2.8) t, °C S, m.f. ∆Gm , ∆Gsol , RTln(γ(BaO), γ(BaO) ln S =±(1.270.08)−±0.024. (1) (T, K) kJ/mol kJ/mol kJ/mol T 600 (873) 0.0237 43.57 27.15 –16.41 0.10 On the basis of these data, the thermodynamic dissolution 700 (973) 0.0397 41.15 26.10 –15.06 0.16 parameters may be defined. For the studied BaO concen- tration range (up to 6 mole%) the BaO activity coeffi- fusion after the experiment (Fig. 7) illustrate that Ba OCl cients are assumed to be unchangeable. The Gibbs energy 4 6 is present in the solid phase. This compound is assumed changes during the BaO dissolution can be expressed by to be formed during the interaction between BaO and the (2) with the overcooled liquid barium oxide being a stand- chloride melt. ard state for solution at given temperature

00 ∆∆GTsol()=−RT ln SG=+m()TRT ln γ(BaO). (2)

0 3.3 Caloric Effects Here, ∆GTm() is the change of the phase transition Gibbs energy at T temperature; γ(BaO) is the BaO activity The caloric effects were measured at heating of oxide- coefficient in the solution at T temperature. Table 1 pro- chloride compositions by the DSC method to verify the vides the thermodynamic characteristics of barium oxide data obtained by the analysis of the melt cooling curves. dissolution calculated by (2) using published data [18, 19] At first the DSC curves were recorded during heating and the experimental values of solubility. of the preliminary smelted chloride eutectic and barium The RTlnγ(BaO) value characterises the abundant changes in Gibbs energy of mixing of liquid overcooled oxide and a solvent melt. The data in Table 1 demon- strates that this value is slightly negative, which denotes the exothermic interaction between barium oxide and solvent particles at their mixing. The XRD data of frozen

Figure 5: Liquidus temperatures of the [NaCl–KCl–BaCl2 eutectic + Figure 7: XRD data for frozen [NaCl–KCl–BaCl2 eutectic + BaO

BaO (Ni)] system. (4 mole%)] melt. E.V. Nikolaeva et al.: Interaction between Barium Oxide and Barium Containing Chloride Melt 329 oxide composition. The DSC data demonstrated the When the solution becomes oversaturated, the solid phase dependence on different factors attributed to the peculi- of Ba4OCl6 is extracted: arities of samples preparation: the temperature, dwelling Ba OCl(solution)B<=> aOCl (solid). (4) time at this temperature, and the hardening rate (quench- 46 46 ing and slow cooling). The influence of these factors was In addition, the removal of a part of barium chloride from difficult to define and interpret. the molten eutectic leads to the formation of the solid Additional experiments were performed to simplify phase of the alkali metal chlorides. The new endothermic the DSC data interpretation: the compositions of chloride peak on the DSC curve in the area of 626 °C demonstrates eutectic and the significant BaO or Ba4OCl6 additions were the melting of the obtained mixture. studied (Fig. 8). The direct interactions between eutectic The endothermic peak of eutectic melting is registered and barium oxide (oxychloride) might be observed and on the DSC curve of the chloride eutectic with Ba4OCl6 were not influenced by any factors connected with the addition, the exothermic peak is not observed. preparation of the samples. Thus, the performed studies of caloric effects at The DSC curves of chloride eutectic with barium melting of chloride eutectic with barium oxide and barium oxide addition (Fig. 8, curve 1) demonstrates the endo- oxychloride additions testified our thermodynamic calcu- thermic peak in the area of 544 °C corresponding to the lations and proved the chemical mechanism of barium eutectic melting and the abrupt peak of exothermic effect. oxide dissolution in molten chloride eutectic. The appearance of exothermic peak testified the chemi- cal reaction between barium oxide and molten chloride eutectic. A new endothermic peak is observed at the fol- 3.4 The Structure of the Barium-Containing lowing heating at the temperature exceeding the eutectic melting point. The subsequent study (Fig. 8, curve 2) was Oxide–Chloride Melts performed. The sample was not removed from the device. The Raman spectrum of homogeneous NaCl–KCl–BaCl In this case, the endothermic peak of eutectic melting 2 eutectic melt is presented in Figure 9 (curve 1). In addition and the following exothermic peak were not observed. to the intense Rayleigh wing scattering the band of small However, the second endothermic peak remained, and its intensity in the area of 200 cm–1 is observed. This band we height increased. attributed to a symmetric valence vibration of the [BaCl ] The interaction of solid barium oxide and chloride 6 ionic groupings. melt, which is followed by exothermic effect (exothermic This conclusion was made according to empiric peak on the DSC curve right after the endothermic peak) dependence of the symmetric vibration frequency on can be described as follows: coordination number [20]: the increasing of coordi-

BaO( solid) +=3BaCl(24liquid)BaOCl6 (solution). (3) nation number resulted in the decreasing of valence

Figure 8: DSC curves: 1 and 2, subsequent measurements for the NaCl–KCl–BaCl2 eutectic with barium oxide (10 mole%) addition; 3, the

NaCl–KCl–BaCl2 eutectic with barium oxychloride (10 mole%) addition. 330 E.V. Nikolaeva et al.: Interaction between Barium Oxide and Barium Containing Chloride Melt

having a greater ionic potential in comparison with that of a one. 4– The Ba2OI6 groupings of D3d symmetry were previ- ously found when studying the structure of oxide–halide CsI–BaO melts by IR-spectroscopy [7]. In addition, the position of vibration band observed in the IR spectrum corresponds to that in our research. Therefore, the appearance of a vibration band close to the 300 cm–1 may be due to the chemical dissolution

of barium oxide and formation of [Ba2OCl6] oxychloride

groupings of D3d symmetry according to the reaction + 4− Figure 9: Raman spectra: 1, NaCl–KCl–BaCl eutectic melt, 660 °C; BaO( solid) [BaCl]6 (solution) 2 (5) ° 4− 2, NaCl–KCl–BaCl2 eutectic melt + BaO (6 mole%), 580 C; 3, NaCl– → [BaO26Cl ](solution).

KCl–BaCl2 eutectic + Ba4OCl6 (saturated melt), 600 °C. The increasing of dwelling time of the chloride melt in contact with BaO results in the formation of the larger vibration frequency. According to the data on X-ray amount of oxychloride groupings in the melt and, thus, structural analysis of the crystalline BaCl the chlorine 2 increasing intensity of corresponding vibration band. anions coordination number (CN) is equal to 9 [21] and In this regard, the enhancement of the barium oxide in molten BaCl it changes insignificantly (CN = 7.7) [22]. 2 solubility with increasing of barium chloride concentra- At 600 °C in the crystalline barium chloride spectrum tion in the melt is clarified: the larger content of [BaCl ] the band at 171 cm–1 area is observed [16]. Conversely, 6 groupings results in larger content of “construction mate- in BaCl –CsI molten mixture at 677 °C, the tetrahedron 2 rial” for [Ba OCl ] oxychloride groupings. BaCl I 2– ionic grouping with the CN = 4 and vibration 2 6 2 2 To confirm the existence of the [Ba OCl ] grouping in frequencies close to the 250 cm–1 was found [7]. The band 2 6 the oxide-chloride melt and to relate the observed vibra- of valence symmetric vibration of BaCl 2– complex anion 4 tion band at the 300 cm–1 area to the vibration of this par- in the organic composition [N(CH ) ] BaCl is situated 3 4 2 4 ticular grouping we have additionally registered Raman in the same frequency area [23]. On the basis of these spectrum of the molten NaCl–KCl–BaCl eutectic with data and empiric dependence of vibration frequency on 2 abundant addition of Ba OCl . Figure 9 (curve 3) demon- coordination number, the vibration band recorded in 4 6 strates the vibration band in the area of 200 cm–1, which the NaCl–KCl–BaCl eutectic melt spectrum (200 cm–1, 2 correlates with the valence symmetric vibration band 670 °C) is related to the vibration of the octahedron of the [BaCl ] grouping in the molten chloride eutectic BaCl 4– grouping. 6 6 (curve 1). Two vibration bands at 301 and 280 cm–1 areas The heating of NaCl–KCl–BaCl eutectic mixture with 2 are analogous to those found in the oxide-chloride melt additions of 6 mole% of BaO above the eutectic melting (Fig. 9, curve 2). In addition, the position of the high fre- point (544 °C) but not exceeding the corresponding liqui- quency band is shifted by 20 cm–1 as compared to the dus temperature, allowed to note the following peculiari- vibration band, corresponding to the Ba OCl solid phase ties in Raman spectra of heterophase melt: the intensity 4 6 at the same temperature. Such a low frequency shift is due of the vibration band at 200 cm–1, corresponding to the to the solid phase dissociation in chloride melt according [BaCl ] ionic grouping vibration, decreases as compared 6 to the reaction to that in the chloride melt spectra; and two new vibra- –1 tion bands appear at the 300 cm area and their intensi- Ba46OCl(solid) + 12Na(K)Cl( melt ) ties increase during the melt dwelling time (the maximum 4− → [BaO26Cl ](solution) (6) dwelling time was 1.5 h). 4−+ ++2[ BaCl6 ](solution)12Na( K) . The analysis of the obtained spectroscopy results directly demonstrates the chemical mechanism of BaO dis- The Ba4OCl6 dissociation as described by (6) is pro- solution in barium-containing chloride melts. Indeed, the moted by the crystalline Ba4OCl6 structure [14]: two types intensity of the vibration band at 200 cm–1 area decreases of Ba-based anion polyhedra share atom with each as barium oxide is added. That is directly connected to the other. decreasing of the chloride grouping amount. The [BaCl6] Thus, the newly obtained spectral data confirmed groupings destruct in the presence of oxygen anions, the chemical mechanism of barium oxide dissolution in E.V. Nikolaeva et al.: Interaction between Barium Oxide and Barium Containing Chloride Melt 331 chloride melts – identified the type, structure, and com- [4] M. V. Smirnov and O. Yu. Tkacheva, Electrochim. Acta 15, 2681 position of oxychloride groupings in the melt. Another (1992). [5] V. L. Cherginets, Oxoacidity: Reactions of Oxo-Compounds in type of oxychloride complexes like those proposed by Ionic Solvents, Elsevier Science, Amsterdam 2005. Boghosian et al. [6] may be formed in solvents with larger [6] S. Boghosian, A. Godo, H. Mediaas, W. Ravlo and T. Osvold, BaO solubility. Acta Chem. Scand. 45, 145 (1991). [7] A. A. Khokhryakov and A. M. Khokhlova, Melts 5, 408 (1989). [8] A. 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