Phase equilibria in the pseudo-binary systems LiF–CaAlF5 and LiF–SrAlF5

Journal of Crystal Growth 235 (2002) 596–602

Phase equilibria in the pseudo-binary systems LiF–CaAlF5 andLiF–SrAlF5

H.B. Chena,*, Shiji Fanb, Haiping Xiaa, Yiting Feib

a Institute of Optoelectronic Materials, Ningbo University, Ningbo 315211, People’s Republic of ChinabShanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050, People’s Republic of China

Received 26 September 2001; accepted 17 October 2001

Communicated by R.S. Feigelson

Abstract

The phase equilibria in pseudo-binary systems LiF–CaAlF5 and LiF–SrAlF5 have been studied by means of

differential thermal analysis and X-ray diffraction. The two analogous phase diagrams show that the compounds

LiCaAlF6 and LiSrAlF6 melt congruently with the composition of LiF:CaAlF5=1:1 and LiF:SrAlF5=1:1 at 8141C and

7651C, respectively. There are two eutectic relationships in the system LiF–CaAlF5, one between the compounds LiF

and LiCaAlF6 with the eutectic temperature of 7051C, the other between LiCaAlF6 and CaAlF5 with the eutectic

temperature of 7811C. The system LiF–SrAlF5 also has two eutectic relationships at the left or right side of the

compound LiSrAlF6 with the eutectic temperatures of 6731C and 7051C, respectively. Using the feed materials with the

stoichiometry compositions, large-size fluoride laser crystals Cr3+:LiCaAlF6 and Cr3+:LiSrAlF6 with high quality have

been grown successfully by the vertical Bridgman method. r 2002 Elsevier Science B.V. All rights reserved.

PACS: 81.30.�t; 81.30.dz

Keywords: A1. Phase equilibria; A1. Differential thermal analysis; A1. X-ray diffraction; A2. Single crystal growth

1. Introduction

The fluoride crystals LiCaAlF6 and LiSrAlF6

with colquiriite structure were first reported asnovel laser hosts by the Lawrence LivermoreNational Laboratory [1–2]. The Cr3+-doped cry-stals Cr3+:LiCaAlF6 and Cr3+:LiSrAlF6 have beenproved to be efficient tunable all-solid-state lasingmedia in the near infrared wavelength region be-cause of their favorable laser and thermal–mecha-

nical properties. The recent discovery of Ce3+-doped crystals stimulated several research groupsto study the new laser media in the UV region [3–4]. In the past decade, extensive investigationsabout the growth of the fluoride crystal familyhave been made by different techniques such aszone-melting [1], Czochralski [5], TSSG [6] andBridgman method [4,7]. However, the phasediagrams for the ternary systems LiF–CaF2–AlF3

and LiF–SrF2–AlF3 have not been reported yet.As the basis of crystal growth, it is necessary forthe improvement of the growth process toinvestigate the phase equilibria of the systems.

*Corresponding author. Fax: +86-574-7605140.

E-mail address: chen [email protected] (H.B. Chen).

0022-0248/02/$ - see front matter r 2002 Elsevier Science B.V. All rights reserved.

PII: S 0 0 2 2 - 0 2 4 8 ( 0 1 ) 0 1 8 1 4 - 0

This paper reports on the phase diagrams ofpseudo-binary systems LiF–CaAlF5 and LiF–SrAlF5, which are partial diagrams concerningthe complete phase diagrams of the ternarysystems LiF–CaF2–AlF3 and LiF–SrF2–AlF3,respectively. The phase equilibria of the pseudo-binary systems have been established by means ofdifferential thermal analysis (DTA) and X-raydiffraction (XRD). Based on the proposed phasediagrams, large-size fluoride laser crystalsCr3+:LiCaAlF6 and Cr3+:LiSrAlF6 with highquality have been grown successfully by thevertical Bridgman method from the stoichiometriccompositions.

2. Experimental procedure

2.1. Phase equilibrium

The fluorides LiF, CaF2, SrF2 and AlF3 wereused as the starting materials, which were treatedby fluoridation at elevated temperature prior touse. The fluoride powders were placed in aplatinum crucible and sintered in an electricfurnace with anhydrous HF at 750–8001C for 8–10 h to ensure that the moisture contained in thefluorides was removed completely. According tothe appropriate molar ratios of the components,the fluorides were weighed out to prepare a seriesof 3.000 g batches. The mixtures were ground in amortar for 1 h to ensure sufficient homogeneity.The samples were stored in a dried container toavoid absorbing the moisture outside.

To determine the phase transition temperaturesof the pseudo-binary systems, the samples wereanalyzed by DTA using an STA-429 thermalanalyzer calibrated by SrCO3. An 80 mg samplewas tested under a high-purity nitrogen atmo-sphere at the heating and cooling rates of 101C/min with an accuracy of 731C. DTA and TGcurves could also be obtained simultaneously fromthe analysis. Based on the results of DTA, some ofthe samples were analyzed by XRD for phaseidentification after they had been subjected tofurther heat treatment. These samples were pressedinto pellets and wrapped in small platinumcrucibles. The crucibles were sealed and suspended

in a vertical furnace. After the samples had beenheated at an appropriate temperature for 3 h untilthe thermal equilibrium was reached, they weresubjected to quenching or cooling naturally.Quenching was accomplished by dropping thecrucible rapidly into an ice–water bath. Thesamples were taken from the crucibles and groundinto powder. XRD analysis was carried out by adiffractometer, using monochromatic Cu Ka ra-diation with a working voltage of 40 kV andcurrent of 80 mA.

2.2. Crystal growth

The crystals Cr3+:LiCaAlF6 and Cr3+:LiS-rAlF6 were grown by the vertical Bridgmanmethod. Using the purified fluorides by fluorida-tion treatment, the feed material was prepared bythoroughly mixing the components. The feedmaterial of 200–300 g was charged into a platinumcrucible with a seed at its bottom. The assembledcrucible was sealed in order to prevent effectivelythe volatilization of melt during crystal growth.The furnace temperature was controlled by aDWT-702 fine temperature controller with anaccuracy of 70.51C during the experiments. Afterthe furnace was heated to 920–9401C and 860–8801C for Cr3+:LiCaAlF6 and Cr3+:LiSrAlF6,respectively, seeding was performed by adjustingthe crucible to such a position that only the top ofthe seed was melted. The temperature gradientacross the solid–liquid (s–l) interface was 20–301C/cm and the growth rate was controlled in the rangeof 0.4–0.7mm/h. As-grown crystals were obtainedby stripping the crucibles after the growth hadfinished.

3. Results and discussion

The high-purity materials are essential to pre-pare the samples for measuring the phase dia-grams. The moisture contained in fluorides cancause hydrolysis at elevated temperature, whichbrings about oxygen substitution for fluorine inthe starting materials. The purification process byfluoridation must be applied to drive off themoisture of fluorides. The treated materials were

H.B. Chen et al. / Journal of Crystal Growth 235 (2002) 596–602 597

proved to be pure enough for the phase diagrammeasurements based on the fact that they hadbeen used to grow transparent green crystalsof Cr3+:LiCaAlF6 and Cr3+:LiSrAlF6 withoutmacroscopic defects in the present work.

CaAlF5 and SrAlF5 have been known ascongruently melting compounds [8–9], which canbe synthesized by CaF2 or SrF2 and AlF3 prior toprepare the samples. The compositions of sampleswere chosen by varying LiF molar percentagefrom 0 to 100 with the constant molar ratios ofCaF2:AlF3=1:1 and SrF2:AlF3=1:1. Tables 1 and2 list the compositions of the samples for LiF-CaAlF5 and LiF-SrAlF5 systems, respectively.

The DTA results show that evident weight lossesoccur during the higher temperature period. It canbe extrapolated that the stoichiometry of thesamples would shift towards CaF2 due to thehigher volatilization of LiF and AlF3. Moreover,

there is always a supercooling tendency upon thecooling procedure. Taking into consideration thestoichiometry shift and the supercooling, alltemperatures for the solidus and liquidus shouldbe taken from the heating curves.

As examples, the DTA and TG curves of thecrystal LiCaAlF6 are shown in Fig. 1(a). Theunique endothermic peak at 8141C on the heatingcurve corresponds to the melting point of thecompound. The measured data is consistent withthe result in the previous literature [10]. However,upon cooling of the sample, two exothermic peaksappear on the cooling curve at temperatures lowerthan the melting point. This example proves thatthe phase transition temperatures of the systemscan be determined by the heating curves. Fig. 1(b)shows the DTA and TG curves of the sample with55.5 mol% LiF (No. 13 in Table 1). The tempera-ture of the exothermic peak is much lower thanthat of the corresponding endothermic peakbecause of the stoichiometric shift and the super-cooling. The two temperatures of the endothermic

Table 1

DTA data of the pseudo-binary system LiF–CaAlF5

Sample

No.

Composition

(mol%)

Phase-transition

temperature (1C)

LiF CaAlF5 E L

1 0 100 898 (la)

2 6 94 781 (sa) 878 (l)

3 12 88 782 (m) 863 (l)

4 18 82 780 (ma) 843 (m)

5 24 76 782 (l) 825 (s)

6 33 67 783 (l) 802 (sha)

7 37 63 782 (l)

8 41 59 779 (l) 795 (sh)

9 44 56 780 (s) 806 (l)

10 47 53 778 (s) 809 (l)

11 50 50 814 (l)

12 52 48 702 (s) 810 (l)

13 55.5 44.5 705 (m) 794 (l)

14 59 41 704 (m) 787 (m)

15 62 38 704 (l) 775 (m)

16 66 34 707 (l) 758 (s)

17 70 30 708 (l) 735 (sh)

18 74 26 708 (l)

19 80 20 704 (l) 742 (sh)

20 86 14 704 (l) 777 (s)

21 93 7 703 (l) 801 (m)

22 100 0 850 (l)

aThe symbols l, m, s and sh indicate the large, moderate,

small and should peaks of DTA curves.

Table 2

XRD data for the pseudo-binary system LiF–CaAlF5

No. CaAlF5

(mol%)

Temperature (1C) Phase present

1 88 830 CaAlF5+glass

Cooling naturally CaAlF5+LiCaAlF6

2 76 805 CaAlF5+glass


3 53 795 LiCaAlF6+glass


4 50 Cooling naturally LiCaAlF6


Cooling naturally LiF+LiCaAlF6



7 20 725 LiF+glass


8 7 750 LiF+glass


H.B. Chen et al. / Journal of Crystal Growth 235 (2002) 596–602598

peaks on the heating curve should be determinedas the phase transition temperatures.

DTA data for the LiF–CaAlF5 and LiF–SrAlF5

systems are summarized in Tables 1 and 3,

respectively. In the tables, all phase transitionsare classified as eutectics (E) and liquidus (L) withthe melting points underlined. Tables 2 and 4 listthe XRD results for the LiF–CaAlF5 and LiF–SrAlF5 systems, respectively. Phase diagrams ofthe pseudo-binary systems LiF–CaAlF5 and LiF–SrAlF5 are plotted in Figs. 2 and 3 by synthesizingthe results of DTA and XRD. The phase equilibriaexhibited by the diagrams are described below.

System LiF–CaAlF5: The congruent meltingcompound LiCaAlF6 with a melting point of

Fig. 1. DTA and TG curves for: (a) crystal LiCaAlF6; (b) sample with 55.5mol% LiF.

Table 3

DTA data of the pseudo-binary system LiF–SrAlF5

Sample

No.

Composition

(mol% )

Phase-transition

temperature (1C)

LiF SrAlF5 E L

1 0 100 880 (la)

2 6 94 705 (sa) 863 (l)

3 12 88 706 (s) 851 (l)

4 18 82 706 (ma) 836 (m)

5 23 77 703 (m) 822 (m)

6 30 70 706 (m) 802 (m)

7 37 63 708 (m) 70 (m)

8 40 60 704 (m) 753 (s)

9 44 56 708 (m) 735 (s)

10 47 53 703 (s) 759 (m)

11 50 50 765 (l)

12 52 48 675 (m) 762 (l)

13 55.5 44.5 672 (m) 754 (l)

14 59 41 672 (m) 743 (m)

15 62 38 672 (m) 734 (s)

16 65 35 672 (l) 723 (s)

17 69 31 671 (l) 705 (sha)

18 74 26 671 (l)

19 79 21 674 (l) 715 (sh)

20 86 14 675 (l) 775 (s)

21 93 7 672 (m) 820 (m)

22 100 0 850 (l)

aThe symbols l, m, s and sh indicate the large, moderate,

small and should peaks of DTA curves.

Table 4

XRD data for the pseudo-binary system LiF–SrAlF5

No. SrAlF5 (mol%) Temperature (1C) Phase present

1 88 780 SrAlF5+glass

Natural cooling SrAlF5+LiSrAlF6

2 70 750 SrAlF5+glass


3 53 735 LiSrAlF6+glass


4 50 Natural cooling LiSrAlF6

5 44.5 720 LiSrAlF6+glass

Natural cooling LiF+LiSrAlF6

6 35 700 LiSrAlF6+glass


7 14 730 LiF+glass



Fig. 2. Phase diagram of the pseudo-binary system LiF–CaAlF5.

Fig. 3. Phase diagram of the pseudo-binary system LiF–SrAlF5.


8141C occurs at the molar ratio composition ofLiF:CaAlF5=1:1. There are two eutectic relation-ships in the system, one between the compoundsLiF and LiCaAlF6 with the eutectic temperatureof 7051C, the other between LiCaAlF6 andCaAlF5 with the eutectic temperature of 7811C.The eutectic compositions correspond to 74 mol%LiF and 37 mol% LiF, respectively. The com-pounds LiF and CaAlF5 melt congruently at8501C and 8981C, respectively.

System LiF–SrAlF5: This phase diagram isanalogous to that of the system LiF–CaAlF5.There are two eutectic relationships in the system,where the compound LiSrAlF6 melts congruentlyat 7651C with the molar ratio composition ofLiF:SrAlF5=1:1. The measured melting point ofLiSrAlF6 is in accordance with the result in theprevious literature [11]. The eutectic reactionbetween LiF and LiSrAlF6 occurs at the composi-tion of 74 mol% LiF as the melt is cooled to6731C, while the other eutectic reaction between

LiSrAlF6 and SrAlF5 corresponds to the composi-tion of 42.5 mol% LiF at 7051C. Comparing thesystems LiF–SrAlF5 and LiF–CaAlF5, the meltingpoint of LiSrAlF6 is much lower than that ofLiCaAlF6 and the eutectic temperatures in thesystem LiF–SrAlF5 are somewhat lower thanthose corresponding to the system LiF–CaAlF5.

The single crystals Cr3+:LiCaAlF6 andCr3+:LiSrAlF6 were grown successfully by thedescribed process. Taking into considerationthe proposed phase diagrams, the molarratios of LiF:CaF2:(AlF3+CrF3)=1:1:1 andLiF:SrF2:(AlF3+CrF3)=1:1:1 were adopted asthe compositions of the feed materials for thecrystals Cr3+:LiCaAlF6 and Cr3+:LiSrAlF6, re-spectively, in which 1mol% CrF3 was dopedinto the materials by substitution for AlF3. Fig. 4shows two boules of the crystals of (a) Cr3+:Li-CaAlF6, 28 mm in diameter and 120 mm in lengthand (b) Cr3+:LiSrAlF6, 25mm in diameter and90 mm in length. The two crystals were alloptically transparent with a bright green colora-tion. The examination by a polarizing microscopeshowed the bulk of the crystals to be free fromcracks, inclusions and scatter centers. XRDanalysis verified their crystalline phases to beLiCaAlF6 and LiSrAlF6, respectively. No segrega-tion phases were found to freeze on the top of thegrown boules if the feed materials were preparedwith the accurate stoichiometric compositions.

4. Summary

As the partial diagrams of the ternary systems ofLiF–CaF2–AlF3 and LiF–SrF2–AlF3, the phasediagrams in pseudo-binary systems LiF–CaAlF5

and LiF–SrAlF5 have been established by meansof DTA and XRD. The phase-transition tem-peratures can be determined by the heating curvesof DTA. The quenched samples were analyzedby XRD to identify the phases in the systems.The two analogous phase diagrams exhibitsimilar eutectic relationships, where the com-pounds LiCaAlF6 and LiSrAlF6 melt congruentlyat 8141C and 7651C with the compositionsof LiF:CaAlF5=1:1 and LiF:SrAlF5=1:1,respectively. Using the feed materials with the

Fig. 4. Fluoride laser crystal of: (a) Cr3+:LiCaAlF6 and (b)

Cr3+:LiSrAlF6.


stoichiometry compositions, large-size fluoridelaser crystals Cr3+:LiCaAlF6 and Cr3+:LiSrAlF6

have been grown successfully by the verticalBridgman method.

Acknowledgements

This work is supported by the National ScienceFoundation of China under Grant No. 59672002.The authors would like to express their sincerethanks to Engineer Yuexian Chen of ShanghaiInstitute of Ceramics, Chinese Academy ofSciences, for her help in differential thermalanalysis.

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Hongbing Chen was born in Shaanxi Province, China, on

August 29, 1964. He received the Ph. D. degree from Shanghai

Institute of Optics & Fine Mechanics, Chinese Academy of

Sciences in 1997. He worked as a postdoctor in Shanghai

Institute of Ceramics, Chinese Academy of Sciences from 1997

to 1999. He is now working as an associate professor in Ningbo

University. His current research interest is optoelectronic

materials. So far, he has published more than 40 papers.


Documents

Phase equilibria in the pseudo-binary systems LiF–CaAlF5 and LiF–SrAlF5