Bondareva, O. S.

Fedorov, P. P.

Medvedeva, L. V.

Sobolev, Boris P.

Revistes Catalanes amb Accés Obert

THE THERMAL STABILITY OF
LiNaRZFs (R = Ho-Lu, Y) COMPOUNDS
P.P. FEDOROV, L.V. MEDVEDEVA, O.S. BONDAREVA,
B.I. SOBOLEV
Institute of Crystallography, USSR Acad. Sci., Moscow, USSR
The series of monoclinic compounds with the general formula Li-
NaR2Fs (R = Ho-Lu, Y) has been described in [1-3]. These compounds
may be used as promising host materials for luminofors [4]. The crystal
structure of LiNaR7F8 has been studied in [1,3] for R=Y, Yb. The structure
may be described as a three-dimensional framework built of [RFH] bicapped
trigonal prisms connected by edges and corners. The Li and Na cations oc-
cupy their positions in channels.
According to [1], Y-compound decomposes at 695 ± 5 C". No other
data concerning thermal stability of these compounds are available. Since
possible practical application of these materials is determined by their stabi-
lity we studied the thermal stability of the whole series of LiNaR,FH com-
pounds.
EXPIiRIMI NTAI.
In this work, the LiNaR7F8 compounds were studied by differential
thermal analysis (DTA) and powder X-ray diffraction techniques. For
synthesis we used LiF optical grade single crystals, NaF ("highly pure" gra-
de) and rare earth trifluorides ("chemically pure" grade) as starting mate-
rials. Sodium and rare-earth fluorides were remelted in advance in a fluori-
nating atmosphere of teflon thermal decomposition products.
The DTA was carried out in thin-wall graphite crucibles in static he-
lium atmosphere by the method described elsewhere [5,6]. The specimens
were annealed in sealed nickel containers in fluorinating atmosphere (pro-
ducts of thermal decomposition of teflon and BaF2-HF). The containers
were quenched in water, at a cooling rate of about 100 Ks'. X-ray diffrac-
tion analysis of the specimens was carried out on AVF-202E and HZG-4
diffractometers with Ni-filtered CuK„ radiation.
[BudI.Soc.Cat .Ccn.],VoI.XIII, NUm.1,1992
242 P.P. 11, DOROt', L.V. A1ED1'E1)EI'A, O.S. BOADARLI.I, B.P. SO BOLE V
RESULTS
We found that the synthesis of LiNaR,FB compounds upon melting of
the components taken in stoichiometric amounts always leads to the forma-
tion of a three-component mixture: X-ray powder diffraction patterns co-
rrespond to the superposition of the reflections of LiRF4 compounds [7,8],
fluorite-type phases (F) which are formed in NaF-RF3 systems [9-14] and
LiNaR,Fs compounds. It evidences incongruent-type melting of LiNaR,FH
compounds and (or) their decomposition upon cooling. The DTA thermal
heating curves of LiNaR2F5 (R = Ho -Tm, Y) exhibit 4 thermal effects, figure
1. The effects at about 700" C evidently correspond to non-variant proces-
ses. The annealing of a previously melted specimen of LiNaY,F8 at 500 "C
for 473 h leads to its complete decomposition. Its subsequent annealing at
T10C
800
600
Ely HaVEr Tm Yb Lu
rR3^._-
l ig. 1. Thermal ,tahilitY of LiNaR1F, compounds.
Butll.Soc.Cat .Cien.],V"LXI I I, Num.1,1992
THLR.11A1. STABILITY OF LzNaR21-rCOti1POUNDS 243
660 °C for 215 h and at 600° C for 50 h leads to secondary resynthesis of the
compound. On the basis of these data the thermal effect at 580 °C can be in-
terpreted as the formation of LiNaY,Fs in the solid state.
Figure 2 shows a suggested scheme of phase equilibria using our experi-
mental data in the section LiYF4-NaYF4 of the system LiF-NaF-YF3. The
LiNaY7F8 compound is stable in a narrow temperature range: 580 ± 5 °C is
T °C
900
IiYF4+1
I•F
8001
100
6001
IiYF4+1 *F
IiYF4+S
I
IiVF4tNaYF4
S+I-F
LiYF4
20 40 60
mole
S+NaVF4
F.[ +
80
NaYF4
NaYF4
Fig. 2. Polythermal section LiYF4 - NaYF4 of the LiF-NaF-YF, ternary system. I.: melt, S
LiNaY,F8, F : fluorite-type phase Nac; Ro.5+x F, '+,'
[Butll.Soc. Cat . Cicn.],VoI.XIII,Num.1,1992
'44 PT, /I l )ORO \ , L\ III 1)1T I)/ \ .1, O. S. 130.\ 1).1R/ V 1, l/.l' SOBOI_I k
the temperature of the formation of this compound upon heating, and at
700 ± 5 "C it melts incongruently. NaYF4 is dimorphic: the low-tempera-
ture [3-NaYI 3 modification crystallizes in the NaNdF4 structural type; the
high-temperature modification is a disordered fluorite-type one, which can
dissolve additional amount of YF3 (phase F). The temperature of the trans-
formation is 790 ± 5 "C [9,111.
Figure 3 shows a scheme of stable sections of a ternary LiF-NaF-YF3 sys-
tem at two temperatures: below the thermal stability of the 1 :1 :2 compound
at 500 "C and at 660 °C, where the 1:1 :2 compound is stable. The pseudo-
trigonal compound NaY,FV [ 15,161 has been obtained in the Nat-YF, sys-
tem at 600 " and 660 °C, but not Na,Y,,F3,, as described in [9].
The thermal behaviour of others LiNaR,Fs is similar to the behaviour
of the yttrium compound. The interpretation of thermal effects (figure 1) was
YF3
LiF
YF
NaF
Fig. 3. Scheme of stable sections of the - Iid state of the LiF-NaF- Yh, ternary system: a) at X00
"C; b) at 660 "C.
[ButIL $oc.Cat.( icn.],Vol.XIII,Num.1,1992
lJIER . i/.91. SI: b /LI'L)' O1 Li\ iR,I COiII'O['.A US 245
made by analogy with the LiF-NaF-YF3 system. The temperatures of ther-
mal effects for LiNaLu2Fs do not correspond to the general tendency. Pro-
bably, the type of phase equilibria in the LiF-NaF-LuF3 system differs
from other RF3 systems.
We have been unable to prepare 1:1:2 compounds for Dy and larger
rare-earth cations. We also tried to prepare such type compounds for sma-
ller Sc3 and In31 cations and we have obtained Li3Na3R2F12 garnets (R =
In, Sc) [17], mixed with other phases.
The region of LiNaR,F8 stability, determined by us (figure 1) is in good
agreement with the recommendation [2] for the temperature of the synthe-
sis of these compounds, as well as the temperature of LiNaY,F5 decomposi-
tion agrees with the data given in [1]. X-ray characteristics of the com-
pounds are practically identical with the data presented in [1-3]. The forma-
tion of LiNaR7F5 upon cooling of the melt inside the DTA equipment starts
at 50-100°C below the equilibrium temperature.
DISCUSSION
Thus, the field of stability of LiNaR,F8 has an ellipse-like shape in the
system of coordinates "temperature-ionic radius of trivalent cations". The
narrow interval of ionic radii of R3+ cations, where LiNaR2Fs structure is
stable, can be connected with the crystal-chemical toleration of 8-coordina-
ted polyhedra of [RF5] for a limited number of rare earth elements.
The LiNaR2Fs compounds are not stable under normal conditions, so
the application of any materials on such a basis is problematic.
Our results can be interpreted in terms of morphotropy, i.e., change of
stability of the given structure type of the group of compounds with the gi-
ven stoichiometry with changes in the qualitative chemical composition.
The phenomenon of polymorphism (phase transitons in the coordinates
"temperature-pressure") and morphotropy (phase transitions in the coor-
dinates "temperature-ionic radii") have very much in common, see figure 4.
As the compressibilities of anions are greater than those of cations, an in-
crease in pressure yields similar crystallochemical effects as an increase in
the. cationic radius.
For polymorphysm, there is the Clausius-Clapeyron thermodynami-
cal relation, which connects the slope of the curve of phase equilibria in the
coordinates "temperature-pressure" with changes of the molecular volume
V and entropy S upon phase transformation. This equation is correct for
equilibrium of n+1 phases in n-component systems [18]. As for morpho-
tropy, no strict quantitative relations are known up to now. It is probable
that dT/dr = ± x at T = °K [19].
[BudLSoc.Cat.Cien.],VoI.XIII,Nbm.1,1992
24( P.P. /I 1)0R0\, L,\. 111 1)111)1V 1, O.S. BO.AL).IRI 1.1, B.1'. S0B0LE1
PHASE TRANSFORMATIONS
POLYMORPHISM
( T;P )
P t
l
I I II
P
THERMODYNAMICS
Claueius-Clapeyron
relation
dT/dP = AV/L S
(V - volume,S - entropy;
n components , n+1 phases)
dT/dP = 0, p V = 0
IT/dP ='-, aS = 0 (T = 0)
GENERAL CASE
Tammann, 1903
T A V-0
oV=U
i
P I
r
cat.i
T
MORPHOTROPY
( T;rcat. )
I
I
11
rcat
dT/dr = + oo at T = 0
T1.
Pit;. 4. Rrlationti between the phenomena of polymorphism and niorphotropy.
rR3+
[But II.Soc.Cat .Cicn.1,Vol. XIII, Num.I,1992
THERMAL STABILITY Of LiN'tR,E, COMPOUNDS 247
On the basis of formal thermodynamic theory Tanmann predicted at
the turn of the century the general case of polymorphic transition: closed
ellipse-like curve [20,21]. Such a case has not been realized experimentally.
The series of LiNaR2Fs compounds gives a similar example, but a morpho-
tropic one.
ABSTRACT
The processes of formation and decomposition of LiNaR,F5 (R = Ho-Lu, Y)
compounds have been studied using thermal and X-ray phase analysis techniques. The
thermal stability of this structural type passes through the maximum in the family of
rare earth elements, namely, erbium and thulium. All the compounds melt with de-
composition and dissipate in solid state at temperatures below 500 'C. The stability re-
gion of these compounds in the coordinates "temperature - R3+ ionic radius" has an
elliptical shape. Polymorphic and morphotropic structural changes are analyzed as si-
milar phenomena, considered in different coordinates.
[Budl.Soc.Cat.Cien.],Vol.XIII,N1 n.1,1992
24S P.P. FEDOROI', L.1. iILDIZDLl:1, O.S. B0\1).IR1 1.-1, BT SOBOLL1
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The Thermal stability of LiNaR2F8 (R = Ho-Lu, Y) compounds

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