JOURNAL OF RESEARCH of the Notiona l Bureau of Standards-A. Ph ysics and Chemistry Vol. 73A, No.5, September- October 1969 Measured Enthalpy and Derived Thermodynamic Properties of Solid and Liquid Lithium Tetrafluoroberyllate, from 273 to 900 K 1 Thomas B. Douglas and William H. Payne 2 Institute for Basic Standards, National Bureau of Standards, Washington, D.C. 20234 (May 20, 1969) The enthalpy of a sampl e of lithium tetraAu oroberyllate, Li,BeF4 , of 98.6 percent purity was ?,easu. red re laLJ ve to 273 K a t eleven te mpe ratures from 323 to 873 K. Corrections we re appli ed fo r the Im purI li es and fo r ex t e n ~ lv e premelting below the m e lti~ g po int (745 K ). The e nthalpy and heat capacity, a nd the e ntropy a nd GIbbs free-energy functIOn rela LJ ve to the undetermined value of 5,°98 15 ' we re computed from empiri cal functIO ns of tem peratu re derived from the data and are tabuhied from 273 to 900 K. ' Key words: Drop calorimetry; enthalpy data; lithium beryllium Au oride; lithium te traAu orobe ryllate; premeltmg; th e rmodynamic properties. 1. Introduction The temperature-composition phase diagram of the condensed phases of the LiF-BeF 2 system has been As part of a long-term research program at the investigated in a number of laboratories. The version National Bureau of Standards on the thermodynamic in a fairly recent compilation of phase diagrams [3] is properties of the simpler li ght-element compounds, based on th e results of two groups of workers [4, 5]. the measure ments of hi gh-temperature enthalpy have Another composite di agram [6] is based on the results included several well-defin ed substances whi ch may of the above workers, as well as on those from two be regarded as double oxides or double fluorides of other publications [7 , 8] and additional work unpub­ lished at that time. A more recent version of the phase I two metals. S uch results have been published in two diagram, which is in essential agreement with the two papers for BeO · Ah03 (BeAh04) and BeO· 3Al2 0 3 already mentioned, is reproduced here as fi gure 1; it is (BeAlsO IO ) [1] ,3 are being obtained for 3LiF· AIF3 based largely on a comprehensive study at th e Oak (LiaAIF6 ), and are presente d in this paper for 2LiF · BeF2 Ridge National Laboratory [9]. One group of workers r (Li2 BeF4). The standard heats of formation and heat capacities of so me " double" compounds of this type [5] reported a compound LiF . 2BeF 2 not found in other are very nearly the same as the values calculated laboratories. There is general agreement, however, on additively from the parent binary compounds (in the the exi stence of the two solid compounds in the LiF­ same state), but in other cases there are considerable BeF 2 syste m indicated in fi gure 1. It will be noted that differences. Li2 BeF 4 , the compound of interest in this paper, is not The United States molten salt reactor program has quite congruent-melting, showing a peritectic point. , in recent years provided several notable demonstra­ The liquid fi eld is continuous, but when it has the tions of feasible examples. In nearly any foreseeable stoichiometric composition of LhBeF4 it may be thermal reactor of this type, the solvent for the fi ssile regarded as the liquid phase of this compound. Crystalline LhBeF4 exhibits an unusually small and/or fertile material is an LiF-BeF2 mixture of volume change when it melts (no more than 2 or 3 composition near that of Li2 BeF4 [2]. Accordingly, the percent [10]). This is attributed to the existence of thermodynamic properties of this compound are of two continuous void channels penetrating the unit cell great practical importance in this developing of the crystal, an unusual feature revealed by a recent y technology. quantitative structure determination [11]. A conse­ I Research sponsored by th e Advanced Researc h P rojects Agency, U.S. De partment of quence of this feature is that the crys tal density of Defense. un der ARPA Order No . 20 , and by the Ai r Force O ffi ce of Scie ntific Researc h _, Office of Aerospace Researc h, U.S. Air Force, un der AFOSR Contract No. ISSA 68-0004~ LhBeF4 is about 15 percent less than that computed 2 Present address: Science In fo rmat ion Ex change. 1730 M St. N. W., Washington, additively from the two component binary flu orides. D.C. 20036. 3 Figures in bracke ts indicate the li teratu re references at the end of thi s paper. An investi gation of the thermodynamic activities of 479 900 848 8 00 ~ I 700 ~ I 1 LiF + LlQUID u i ~ 600 \ -- \ 555 "" ---" 500 1\ ~ 458 V BeF2 (HIGH CUART Z TYPE ) .......... ' + :"" I QU ID 4 00 L -- ......... J -i--r- 360 L i F+ Li 2BeF4 "" I Li28 eF4 + BeF2 ( HIG H QUARTZ TYPE) --r--~. 300 --I 280 ,;:" I 1 m~ Li2~e F4 I L IBe F3 + BeF2 ( HIG H QUARTZ TYPE) ':;" .J L, Se F3 + 8eF2 ( LOW QUARTZ TY PE) ", 220 LiBe F3 CD I .J I 200 " L iF 10 20 30 4 0 50 60 70 80 90 oeF2 (mole %) FIGURE L The temperature-composition phase diagram of the LiF-BeF2 system (from ref [9])_ the components of the liquid LiF-BeF2 system has at about 400°C, in order to remove any volatile im­ recently been published [12]. purities that might have been produced by the acci­ Besides the above solid compounds in the LiF - BeF2 dental entrance of moisture during handling. The system, two gaseous compounds are known: from silver container was then sealed gas-tight by flame­ mass-spectrometric work the heats of formation at welding, and further enclosed within a container of 900 K of LiBeF3 (g) [13] and LizBeF4(g) [14] have been 80 Ni-20 Cr as a further precaution against escape of reported. The heat of formation of crystalline Li2BeF4 the toxic sample by volatilization or leakage during the at 298 K also has been reported [15]. subsequent enthalpy measurements. The measurements of enthalpy of Li2BeF4 reported After the completion of the enthalpy measurements, in this paper cover the temperature range 0 to 600°C. described in section 3, the sample was analyzed chemi­ Because the sampl~ was only 98.6 percent pure, there cally and spectrochemically in the Analytical Chemis­ was extensive "premelting" in the region 400-470 °C, try Division of the Bureau. The results of the chemical and the large corrections which this necessitated are analyses are given in table 1. The qualitative spectro­ sufficiently uncertain as to leave the derived heat of chemical analysis indicated the presence of the follow­ fusion much more uncertain than if the sample had ing additional elements in the ranges of abundance been highly pure. In addition, no low-temperature stated as weight percentages: 0.001-0.01 percent each 1 heat-capacity measurements on Li2BeF4 are available of Ag, AI, Cu, Fe, K, Mn, Na, Si, Ti, and Zr; 0.0001- to enable the calculation of values of absolute entropy 0.001 percent each of Ba, Ni, and Pb. (Thirty-five other from the data. elements were sought but not detected.) The deduction of the chemical composition of the 2. Sample sample was based entirely on table 1. It was assumed r that the metals were presen·t in their highest oxidation . The sample of lithium tetrafluoroberyllate was pre­ states, and the deficiency of fluorine in relation to the pared by the Oak Ridge National Laboratory, of Oak total equivalents of metal was assumed to be accounted Ridge, Tenn., by heating together in calculated propor­ for by oxygen, which was not analyzed for. Using the tions LiF and BeF2 to form a single liquid phase, mean analyses given in the last column of table 1, this followed by distillation of NHJIF2 from the material procedure accounted for 99.8 percent by weight of the in order to replace any oxygen present by fluorine. sample. However, the percentage of fluorine is obvi­ According to subsequent petrographic and x-ray exam­ ously the least precisely determined, so the percentage inations at ORNL, the colorless crystalline sample of this element was then taken to be 76.34 so that the consisted of a single phase within the detection limits percentages would add to exactly 100 percent. The of these methods. corresponding elemental composition of the sample After the sample was received, a portion of it (about then becomes the same as that of table 2, which was 3.3 g) was encased in a container of pure silver and used for correcting the thermal data to the basis of heated electrically in an efficient dry box for an hour pure Li2BeF4 in the manner described in section 4. 480 TABLE 1. Chemical analysis of the sample masses of parts of the container system, but not for impurities or premelting. For each furnace temperature the values are given in chronological order, although Element Individual analyses Mean the different furnace temperatures were run in some­ what random order. As always with this method, the Weight % Weight % times in the furnace adequate for reaching thermal Li a 13.87, a 13.85 b 13.85 equilibrium were predetermined from preliminary Be 9.36, 9.33 9.34 F 76.42,75.90, 75.56, 75.91 75.95 measurements at one temperature with deliberately Ca < 0.01 0.005 inadequate times.