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Preparation and dislocation density determination of large, pure RbMnF3 and KMnF3 single crystals
Mat. Res. Bull. Vol. 7, pp. 573-582, 1972. in the United States.
P e r g a m o n P r e s s , Inc.
Priated
PREPARATION AND DISLOCATION DENSITY DETERMINATIONOF LARGE,
PURE RbMnF3 AND ~nF 3 SINGLE CRYSTALS R. H. Plovnick and S. J. Camobreco Materials Science Center Cornell University Ithaca, New York 14850
(Received April 19, 1972; Communicated by R. L. Sproull)
ABSTRACT A detailed description is given of apparatus and methods, based on Czochralski growth, for the routine preparation of RbMnF3 and KMnF3 single crystals of high optical and chemical quality. A technique for etch pit formation to reveal dislocations in these crystals is also discussed. Introduction Because RbMnF3 and KMnF3 have the crystallographically simple perovskite structure and represent ideal examples of Heisenberg antiferromagnets, considerable attention has been devoted in recent years to their optical, magnet i c , thermal, and elastic properties (1). Although crystals have been produced by a variety of techniques including flux (2), Bridgman (3), zone melting (4), and Czochralski (5) methods, there has surprisingly been no detailed report in the literature to date concerning the preparation of large, unstrained, stoichiometric single crystals of high purity. Furthermore, despite the fact that dislocations and small-angle grain boundaries can markedly affect lattice dynamical properties in crystals, no attention seems to have been given to assessment of the number of lattice defects present in the RbMnF3 and KMnF3 Supported by the Advanced Research Projects Agency through the Materials Science Center at Cornel] University, MSC Report #1775.
573
574
RbMnF 3 AND KMnF 3 SINGLE CRYSTALS
Vol. 7, No. 6
crystals studied. As an i n i t i a l effort in a current program to produce more nearly perfect RbMnF3 and KMnF3 crystals for investigation of their optical and thermal properties, we have developed a method for the routine production of crystals of good optical quality, high chemical purity, and large size.
This method is
based on the Czochralski technique (6) of crystal pulling from the melt, which is generally capable of yielding crystals with significantly lower dislocation densities than crystals grown by gradient methods.
In this report, we des-
cribe in detail our apparatus and methods of crystal preparation, as well as a technique for etch pit formation to permit estimation of the dislocation densities in the RbMnF3 and KMnF3 crystals produced. Experimental A.
Preparation of Crystal Feed Material The preparation of suitable polycrystalline RbMnF3 or KMnF3 feed material
consists of intimately mixing appropriate binary Rb or K and Mn compounds and heating the mixture in anhydrous HF to remove the last traces of moisture and oxides.
We have obtained crystals of similar quality from either metal carbon-
ate or fluoride starting materials. I.
Preparation from Carbonates. The starting materials and their im-
purity levels are listed in Table I.
Impurity levels in crystals ultimately
produced from these starting materials are considerably lower, as will be noted later.
Preparation is carried out as follows, based on the method of
Henk et al. (7), in accordance with the reactions K2CO3 + 2MnCO3 + 6HF --~ Rb2CO3 + 2MnCO3 + 6HF --~
2KMn____F 3 + 3H20 + CO2 2RbMnF3 + 3H20 + CO2
(1) (2).
The desired weight of Rb2CO3 (K2CO3) is dissolved in the minimum amount of d i s t i l l e d water, filtered into a plastic beaker, and converted to RbF.xHF (KF.xHF) by the dropwise addition of 48% HF with constant stirring.
A I0%
excess of HF over that required by equation l or 2 is added. This RbF.xHF (KF.xHF) solution is then added dropwise to a stirred suspension of the equivalent amount of MnCO3 in a plastic beaker, using the minimum quantity of d i s t i l l e d water to suspend the MnCO3.
After being stirred overnight, the pro-
duct mixture is then filtered with suction through ashless paper on a plastic Buchner funnel, the precipitate transferred to a plastic vacuum desiccator, and drying completed over NaOHpellets with mechanical pumping over a two day period.
Vol. 7, No. 6
RbMnF 3 AND KMnF 3 SINGLE CRYSTALS
575
TABLE i rlaximum Major Impurity Levels in Carbonate StartinQ Materials (in ppm atomic) Impurity
Mallinckrodt 'AR' MnCO3
Mallinckrodt 'AR'K2CO3
A l k a l i s & earths
1500
(see individual
Ca/Mg
see ' A l k a l i s & earths
I00
<1
Me
"
200
80
Cs
"
---
400
K
"
---
370
Li
"
---
< I0
Ba
"
---
I00
C1
I00
30
Heavy metals, as Pb
20
5
<1
Fe
lO
5
5
Ni
lO
Zn
500
.
.
.
.
.
.
Sulfate
I00
.
.
.
.
.
.
Insoluble matter
lO0
I00
2.
Kawecki 'High P u r i t y ' Rb2CO3 elements)
---
---
<1
Preparation from Binary Fluorides.
I00
S t a r t i n g materials are RbF (or
KF) and MnF2, f o r which p u r i t i e s of commercially available materials vary from 'reagent grade' q u a l i t y to zone-refined 'Optran' grade (BDH Chemicals L t d . ) in which the major i m p u r i t i e s are a l k a l i s and earths at levels below I00 ppm. Stoichiometric q u a n t i t i e s of the respective binary f l u o r i d e s are weighed under conditions of low humidity (less than 30% R.H.) into a p l a s t i c container, which is then sealed and tumbled to i n t i m a t e l y mix the reactants.
Although RbF is
very hygroscopic, rapid weighings under conditions of low humidity minimize moisture pickup. The f l u o r i d e powder from e i t h e r of the above schemes is then vacuum-baked to remove residual moisture and entrapped gases, and f i n a l l y stream of anhydrous HF gas to remove the f i n a l
sintered in a
traces of moisture and oxides.
This procedure is carried out as f o l l o w s , using the apparatus shown in Figure I .
576
RbMnF 3 AND KMnF 3 SINGLE CRYSTALS
Vol. 7, No. 6
6
1/
"\ FIG. 1
Hydrofluorination apparatus. Numbered components are: (I) inconel tube with water-cooled enas, (2) nichrome resistance furnace, (3) platinum boat, (4) HF cylinder, (5) argon cylinder, (6) drying column, (7) flow meter, (8) and (9) one-way valves, (I0) trap, ( I I ) liquid N2 cold trap, (12) thermocouple gauge, (13) pressure-vacuum gauge. Approximately 80 g of fluoride powder is loaded into a 25 cm long platinum combustion boat, which is centered in the inconel tube of Fig. I.
The system
is evacuated overnight at 200°C, then backfilled with ultrahigh purity argon gas and heated to 500°C under a continuous argon stream.
The argon flow is
then stopped while a steady flow of anhydrous HF gas is established (HF cylinder is maintained at 40°C via a water bath not shown in Fig. I ) .
The
argon flow is then restarted, and the temperature raised to approximately 900°C for one hour with the argon/HF mixture flowing.
The furnace is next
cooled to 500°C in the gas mixture, and f i n a l l y to room temperature in argon alone. B.
Crystal Growth The polycrystalline feed material is transferred from the hydrofluorina-
tion apparatus to an 80 cm3 platinum crucible which is loaded into the stainless steel crystal pulling furnace detailed in Figure 2.
(The furnace and
empty crucible have previously been cleaned thoroughly and baked under vacuum at approximately 5 x 10-5 torr for several hours at 1200°C.)
The furnace
chamber is continuously evacuated and gradually heated to about 300°C over a period of 24 hours in order to outgas the chamber and feed material. Vacuum lines are then closed and the system is backfilled with s u f f i c i e n t ultrahigh purity argon gas to generate 0.I atm overpressure at crystal growth temperature
Vol.
7, No. 6
(about I000-I050°C).
RbMnF 3 AND KMnF 3 SINGLE CRYSTALS
577
The temperature is increased to melt the feed material,
and standard Czochralski crystal pulling then carried out.
In general, a
[I00] cleaved seed crystal is used, fastened with platinum wire to the stainless steel, water-cooled pulling rod (Fig. 2).
The crystal is pulled from the
melt at a speed of about 1 cm per hour with 3 RPM rotation. dimensions attained are 6 x 2 x 2 cm.
Typical boule
After growth to the desired length has
been completed, the boule is raised just above the melt and cooled to room temperature over a two hour period.
'%
~ToPump
FIG. 2 Crystal oulling furnace and argon ourification system. Numbered components are: (l} seed, (2) mel~ in Pt crucible, (3) graphite support, (4) graphite heating element, (5) graphite heat shield, (6) stainless steel, water-cooled chamber, (7) pulling rod (8) quartz window, (9) cold trap, (I0) liquid N2, ( I I ) heater at 400°C, (12) 87.5% Zr-12.5% Ti getter alloy, (13) ultrahigh purity argon. The single crystal boules thus produced are clear pink, free of l i g h t scattering, and show no strain pattern when observed through crossed polarizers. The boules are easily cleaved in directions perpendicular to [I00] with either a single-edged razor blade or preferably a highly sharpened steel wedge. Crystal pieces of other desired c~stallographic orientation are obtained by cutting the boules with a wire saw in conjunction with a SiC-isopropanolglycerine slurry. Figure 3 shows an as-grown RbMnF3 boule, a wired seed, and a cleaved, polished section. C.
Chemical Analysis of Pulled Crystals
Chemical analysis of crystals by means of flame emission (Rb,K), atomic absorption (Mn), and potentiometric (F) analysis verified the theoretical stoichiomet~ within the error limits of the analytical methods. Furthermore,
578
RbMnF 3 AND KMnF 3 SINGLE CRYSTALS
Vol. 7, No. 6
chemical analysis of RbMnF3 crystals pulled from deliberately nonstoichiometric melts containing either a I0% excess or deficiency of RbF showed the crystals to s t i l l have RbMnF3 stoichiometry within the error limits. A spark source mass spectrometric impurity survey was carried out for several crystals, and showed no impurities present at levels greater than lO-lO0 ppm atomic. Although this is expected for crystals produced from zonerefined starting materials, for those produced using much less pure starting materials i t FIG. 3 As-grown RbMnF: boule, wired seed, and polished section.
indicates that appreciable purification has occurred during crystal preparation.
For
example, in Table 2 , which summarizes spark source mass spectrometric data for a RbMnF3 crystal produced from carbonate starting materials, note the markedly decreased levels of the elements Zn, Ba, K, Cl, Na, and Cs as compared with their concentration in the starting materials (Table I). Since crystal pulling was evidently effective in reducing the level of certain impurities, i t seemed possible that repeated crystal pulling might achieve even further purification.
To test this possibility, two RbMnF3
boules were remelted and a 'double grown' boule pulled from the combined melt. Spark source mass spectrometry revealed no significant improvement, however, in the purity of this double grown RbMnF3 crystal. Although more repeated crystal pullings might eventually result in significant improvement, this approach was not further investigated. Zone refining RbMnF3 and KMnF3 prior to crystal pulling could also be a useful means of further purifying these materials. D.
Etch Pit Dislocation Densities of Pulled Cry.stals Crystal perfection assessment in terms of relative dislocation densities
Vol. 7, No. 6
RbMnF 3 AND KMnF 3 SINGLE CRYSTALS
579
TABLE 2 Spark Source Mass Spectrometric Impurity Survey of a RbMnF3 Crystal Produced from Rb2CO3 and MnCO3 Starting Materials
Elements Detected
Concentration (ppm atomic)
Co, N , 0
I0-I00
Ba, Ca, Ce, CI, Cr, Cs, Cu, Fe, Gd, Ge, K, Mg, Na, Ni, Ti, V
I-I0
As, In, Mo, Rh, Ru, S, Sn, Sr, Y, Zn
0.I-I
Pd, Se, Zr
<0.I
Possible interference on N and 0 determination; range given is highest possible concentration.
has been applied to certain metals, alloys, oxides, chalcogenides, pnictides, and halides (8).
Successful application of the technique requires a chemical
etchant which will reveal dislocation microstructure in the crystal of interest.
Becauseno prior attention seems to have been given to dislocation etch
pit formation in ternary fluorides such as RbMnF3 and KMnF3, we f i r s t had to determine a suitable etchant for these materials.
In view of chemical simi-
l a r i t i e s between the ternary fluorides and LiF, we decided to try etchants which had been reported to reveal dislocations in LiF.
Several trials estab-
lished that the acetic acid-HF-FeF3 etchant reported by Gilman et al. (9, lO) for LiF was most effective for RbMnF3 and K~nF3. The etchant was prepared by mixing 50 ml of 48% HF, 50 ml of glacial acetic acid, and O.5,ml of 48% HF saturated with FeF3.
Cleaved [lO0] crystal
pieces of RbMnF3 and ~nF 3 of approximately l cm2 surface area were continuously rotated in the etching solution for various times, then rinsed well in 95% ethanol, petroleum ether, and air-dried. The etched crystals were then viewed under 400X magnification on a Reichert microscope equipped with a Nomarski interference contrast attachment. Well-developed octahedral etch pits were seen on the (lO0) faces after three seconds etching time. The etch pit size increased linearly with etching time. Optimum etching time with freshly prepared, room-temperature etchant solution was from 3 to 5 seconds. Figure 4 shows a typical etched KMnF3 (lO0) crystal surface, and Figure 5a-c shows the effect of increased etching time on etch pit development on a (lO0) RbMnF3 surface.
Assuming that each etch pit marks the point of emergence
580
RbMnF 3 AND KMnF 3 SINGLE CRYSTALS
Vol. 7, No. 6
on the (I00) cleavage face of a dislocation in RbMnF3 and KMnF3, as has been shown to be the case for LiF (9), the dislocation density is easily computed by counting the number of etch pits per unit area on photographs such as those shown in Fig. 4 and 5a.
(This cannot be done for overetched surfaces,
as in Fig. 5c, where there is excessive etch pit overlap.)
In this manner, we
have obtained average dislocation densities for both RbMnF3 and KMnF3 of approximately 5 x IO5 to l x IO6/cm2, very similar to densities typically observed for seed-pulled LiF and KCl ( l l ) .
FIG. 4 Dislocation etch pits on (lO0) cleavage face of KMnF3. Magnification: 4OOX. In order to determine the sensitivity of the observed dislocation densities to thermal treatment, RbMnF3 crystal cuttings were reheated to 9OO°C in evacuated, Ptlined quartz capsules, rapidly quenched to room temperature, etched and studied.
The
dislocation density was doubled by this treatment.
The effect of deliberately
introduced impurities on the dislocation density was also studied, for the case of Li and Co in RbMnF3. Crystals were produced for which flame emission analysis showed respectively 12O ppm Li and 1424 ppm Co in cleaved boule sections.
Etched pieces
showed dislocation densities in the same range as undoped crystals.
Finally, the
FIG. 5 Dislocation etch pits on (lO0) cleavage face of RbMnF3 after etching time of (a) 3 sec, (b) 5 sec, (c) 7 sec. Magnification: 400X.
Vol. 7, No. 6
RbMnF 3 AND KMnF 3 SINGLE CRYSTALS
581
effect of plastic deformation was investigated by compressing a RbMnF3 crystal along a (lO0) direction using techniques similar to those reported by Sproull et al. (12) for LiF.
In order to reduce the length of a crystal measurably
without cracking, i t was necessary to heat the crystal to about 160°C during compression.
A RbMnF3 crystal reduced 0.7% in length, freshly cleaved, and
etched, showed a dislocation density approaching 107/cm2, an order of magnitude increase over that in as-grown crystals.
The dislocation density in
RbMnF3 and ~nF 3 is thus shown to be sensitive to thermal and mechanical strain in the same way as LiF (g, lO, 12). Summary We have described in detail a method for the routine production of RbMnF3 and ~nF 3 single crystals of adequate size, purity, optical quality, and perfection for use in lattice dynamical property studies.
The Czochralski techni-
que employed has the advantage of significantly purifying relatively impure crystal feed material of certain impurities, including Ba, Cl, Cs, K, Na, and Zn.
We have determined that an etching reagent consisting of 48% HF, glacial
acetic acid, and FeF3 is effective in producting etch pits which highlight dislocations in RbMnF3 and KMnF3, making i t possible to assess the dislocation density in these materials.
This technique provides a useful means for deter-
mining the relative perfection of RbMnF3 and KMnF3 crystals, and should aid future efforts to produce more nearly perfect crystals of these materials.
AcKnowledgements We thank Professor R. O. Pohl and Mrs. J. B. Hartmann for helpful advice and for t h e i r continued interest in this work, G. E. Schmidt for assistance in construction of the hydrofluorination apparatus and preparation of the Figures, and Dr. J. R. Roth and N. G. Nunez of the Cornell Analytical Chemistry F a c i l i t y for the chemical analyses. I.
References For a comprehensive bibliography through 1968, see J. %. Friebely and W. J. Ince, Lincoln Laboratory Library Report (Contract AF 19(628)-5167), AD-677285, August 8, 1968. "Properties of the Compounds RbMnF3, ~nF 3 and CsMnF3".
2.
S. Ogawa, J. Phys. Soc. Japan 14, I l l 5 (1959).
3.
W. W. Holloway, E. W. Prohofsky, and M. Kestigian, Phys. Rev. 139, 3A, A954 (1965).
4. 5.
O. Beckman, I. Olovsson, and K. Knox, Acta Cryst. L3, 506 (1960). K. Nassau, J. Appl. Phys. 32, 1820 (1961).
6.
J. Czochralski, Z. Phys. Chem. 92, 219 (1917); Z. Anorg. Chem. 144, 131 (1925).
582 7.
RbMnF 3 AND KMnF 3 SINGLE CRYSTALS
Vol. 7, No. 6
P. O. Henk, D. Gabbe, and K. Bangerskis, Tech. MemoNo. 3, Microwave and Quantum Magnetics Group, Dept. of Electrical Engineering and Center for Materials Science and Engineering, M.I.T., Sept. 1966 (unpublished). 8. See, for example, S. Amelinckx, The Direct Observation of Dislocations. Academic Press, New York (1964). 9. J. J. Gilman and W. G. Johnston, J. Appl. Phys. 2_]_7,lOl8 (1956). lO. J. J. Gilman, W. G. Johnston, and G. W. Sears, J. Appl. Phys. 29, 747 (1958). I I . R. W. Dreyfus, Alkali Halides, in The Art and Science of Growing Crystals, p. 414. J. J. Gilman, ed. John Wiley & Sons, Inc., New York (1963). 12. R. L. Sproull, M. Moss, and H. Weinstock, J. Appl. Phys. 30, 334 (1959).
P e r g a m o n P r e s s , Inc.
Priated
PREPARATION AND DISLOCATION DENSITY DETERMINATIONOF LARGE,
PURE RbMnF3 AND ~nF 3 SINGLE CRYSTALS R. H. Plovnick and S. J. Camobreco Materials Science Center Cornell University Ithaca, New York 14850
(Received April 19, 1972; Communicated by R. L. Sproull)
ABSTRACT A detailed description is given of apparatus and methods, based on Czochralski growth, for the routine preparation of RbMnF3 and KMnF3 single crystals of high optical and chemical quality. A technique for etch pit formation to reveal dislocations in these crystals is also discussed. Introduction Because RbMnF3 and KMnF3 have the crystallographically simple perovskite structure and represent ideal examples of Heisenberg antiferromagnets, considerable attention has been devoted in recent years to their optical, magnet i c , thermal, and elastic properties (1). Although crystals have been produced by a variety of techniques including flux (2), Bridgman (3), zone melting (4), and Czochralski (5) methods, there has surprisingly been no detailed report in the literature to date concerning the preparation of large, unstrained, stoichiometric single crystals of high purity. Furthermore, despite the fact that dislocations and small-angle grain boundaries can markedly affect lattice dynamical properties in crystals, no attention seems to have been given to assessment of the number of lattice defects present in the RbMnF3 and KMnF3 Supported by the Advanced Research Projects Agency through the Materials Science Center at Cornel] University, MSC Report #1775.
573
574
RbMnF 3 AND KMnF 3 SINGLE CRYSTALS
Vol. 7, No. 6
crystals studied. As an i n i t i a l effort in a current program to produce more nearly perfect RbMnF3 and KMnF3 crystals for investigation of their optical and thermal properties, we have developed a method for the routine production of crystals of good optical quality, high chemical purity, and large size.
This method is
based on the Czochralski technique (6) of crystal pulling from the melt, which is generally capable of yielding crystals with significantly lower dislocation densities than crystals grown by gradient methods.
In this report, we des-
cribe in detail our apparatus and methods of crystal preparation, as well as a technique for etch pit formation to permit estimation of the dislocation densities in the RbMnF3 and KMnF3 crystals produced. Experimental A.
Preparation of Crystal Feed Material The preparation of suitable polycrystalline RbMnF3 or KMnF3 feed material
consists of intimately mixing appropriate binary Rb or K and Mn compounds and heating the mixture in anhydrous HF to remove the last traces of moisture and oxides.
We have obtained crystals of similar quality from either metal carbon-
ate or fluoride starting materials. I.
Preparation from Carbonates. The starting materials and their im-
purity levels are listed in Table I.
Impurity levels in crystals ultimately
produced from these starting materials are considerably lower, as will be noted later.
Preparation is carried out as follows, based on the method of
Henk et al. (7), in accordance with the reactions K2CO3 + 2MnCO3 + 6HF --~ Rb2CO3 + 2MnCO3 + 6HF --~
2KMn____F 3 + 3H20 + CO2 2RbMnF3 + 3H20 + CO2
(1) (2).
The desired weight of Rb2CO3 (K2CO3) is dissolved in the minimum amount of d i s t i l l e d water, filtered into a plastic beaker, and converted to RbF.xHF (KF.xHF) by the dropwise addition of 48% HF with constant stirring.
A I0%
excess of HF over that required by equation l or 2 is added. This RbF.xHF (KF.xHF) solution is then added dropwise to a stirred suspension of the equivalent amount of MnCO3 in a plastic beaker, using the minimum quantity of d i s t i l l e d water to suspend the MnCO3.
After being stirred overnight, the pro-
duct mixture is then filtered with suction through ashless paper on a plastic Buchner funnel, the precipitate transferred to a plastic vacuum desiccator, and drying completed over NaOHpellets with mechanical pumping over a two day period.
Vol. 7, No. 6
RbMnF 3 AND KMnF 3 SINGLE CRYSTALS
575
TABLE i rlaximum Major Impurity Levels in Carbonate StartinQ Materials (in ppm atomic) Impurity
Mallinckrodt 'AR' MnCO3
Mallinckrodt 'AR'K2CO3
A l k a l i s & earths
1500
(see individual
Ca/Mg
see ' A l k a l i s & earths
I00
<1
Me
"
200
80
Cs
"
---
400
K
"
---
370
Li
"
---
< I0
Ba
"
---
I00
C1
I00
30
Heavy metals, as Pb
20
5
<1
Fe
lO
5
5
Ni
lO
Zn
500
.
.
.
.
.
.
Sulfate
I00
.
.
.
.
.
.
Insoluble matter
lO0
I00
2.
Kawecki 'High P u r i t y ' Rb2CO3 elements)
---
---
<1
Preparation from Binary Fluorides.
I00
S t a r t i n g materials are RbF (or
KF) and MnF2, f o r which p u r i t i e s of commercially available materials vary from 'reagent grade' q u a l i t y to zone-refined 'Optran' grade (BDH Chemicals L t d . ) in which the major i m p u r i t i e s are a l k a l i s and earths at levels below I00 ppm. Stoichiometric q u a n t i t i e s of the respective binary f l u o r i d e s are weighed under conditions of low humidity (less than 30% R.H.) into a p l a s t i c container, which is then sealed and tumbled to i n t i m a t e l y mix the reactants.
Although RbF is
very hygroscopic, rapid weighings under conditions of low humidity minimize moisture pickup. The f l u o r i d e powder from e i t h e r of the above schemes is then vacuum-baked to remove residual moisture and entrapped gases, and f i n a l l y stream of anhydrous HF gas to remove the f i n a l
sintered in a
traces of moisture and oxides.
This procedure is carried out as f o l l o w s , using the apparatus shown in Figure I .
576
RbMnF 3 AND KMnF 3 SINGLE CRYSTALS
Vol. 7, No. 6
6
1/
"\ FIG. 1
Hydrofluorination apparatus. Numbered components are: (I) inconel tube with water-cooled enas, (2) nichrome resistance furnace, (3) platinum boat, (4) HF cylinder, (5) argon cylinder, (6) drying column, (7) flow meter, (8) and (9) one-way valves, (I0) trap, ( I I ) liquid N2 cold trap, (12) thermocouple gauge, (13) pressure-vacuum gauge. Approximately 80 g of fluoride powder is loaded into a 25 cm long platinum combustion boat, which is centered in the inconel tube of Fig. I.
The system
is evacuated overnight at 200°C, then backfilled with ultrahigh purity argon gas and heated to 500°C under a continuous argon stream.
The argon flow is
then stopped while a steady flow of anhydrous HF gas is established (HF cylinder is maintained at 40°C via a water bath not shown in Fig. I ) .
The
argon flow is then restarted, and the temperature raised to approximately 900°C for one hour with the argon/HF mixture flowing.
The furnace is next
cooled to 500°C in the gas mixture, and f i n a l l y to room temperature in argon alone. B.
Crystal Growth The polycrystalline feed material is transferred from the hydrofluorina-
tion apparatus to an 80 cm3 platinum crucible which is loaded into the stainless steel crystal pulling furnace detailed in Figure 2.
(The furnace and
empty crucible have previously been cleaned thoroughly and baked under vacuum at approximately 5 x 10-5 torr for several hours at 1200°C.)
The furnace
chamber is continuously evacuated and gradually heated to about 300°C over a period of 24 hours in order to outgas the chamber and feed material. Vacuum lines are then closed and the system is backfilled with s u f f i c i e n t ultrahigh purity argon gas to generate 0.I atm overpressure at crystal growth temperature
Vol.
7, No. 6
(about I000-I050°C).
RbMnF 3 AND KMnF 3 SINGLE CRYSTALS
577
The temperature is increased to melt the feed material,
and standard Czochralski crystal pulling then carried out.
In general, a
[I00] cleaved seed crystal is used, fastened with platinum wire to the stainless steel, water-cooled pulling rod (Fig. 2).
The crystal is pulled from the
melt at a speed of about 1 cm per hour with 3 RPM rotation. dimensions attained are 6 x 2 x 2 cm.
Typical boule
After growth to the desired length has
been completed, the boule is raised just above the melt and cooled to room temperature over a two hour period.
'%
~ToPump
FIG. 2 Crystal oulling furnace and argon ourification system. Numbered components are: (l} seed, (2) mel~ in Pt crucible, (3) graphite support, (4) graphite heating element, (5) graphite heat shield, (6) stainless steel, water-cooled chamber, (7) pulling rod (8) quartz window, (9) cold trap, (I0) liquid N2, ( I I ) heater at 400°C, (12) 87.5% Zr-12.5% Ti getter alloy, (13) ultrahigh purity argon. The single crystal boules thus produced are clear pink, free of l i g h t scattering, and show no strain pattern when observed through crossed polarizers. The boules are easily cleaved in directions perpendicular to [I00] with either a single-edged razor blade or preferably a highly sharpened steel wedge. Crystal pieces of other desired c~stallographic orientation are obtained by cutting the boules with a wire saw in conjunction with a SiC-isopropanolglycerine slurry. Figure 3 shows an as-grown RbMnF3 boule, a wired seed, and a cleaved, polished section. C.
Chemical Analysis of Pulled Crystals
Chemical analysis of crystals by means of flame emission (Rb,K), atomic absorption (Mn), and potentiometric (F) analysis verified the theoretical stoichiomet~ within the error limits of the analytical methods. Furthermore,
578
RbMnF 3 AND KMnF 3 SINGLE CRYSTALS
Vol. 7, No. 6
chemical analysis of RbMnF3 crystals pulled from deliberately nonstoichiometric melts containing either a I0% excess or deficiency of RbF showed the crystals to s t i l l have RbMnF3 stoichiometry within the error limits. A spark source mass spectrometric impurity survey was carried out for several crystals, and showed no impurities present at levels greater than lO-lO0 ppm atomic. Although this is expected for crystals produced from zonerefined starting materials, for those produced using much less pure starting materials i t FIG. 3 As-grown RbMnF: boule, wired seed, and polished section.
indicates that appreciable purification has occurred during crystal preparation.
For
example, in Table 2 , which summarizes spark source mass spectrometric data for a RbMnF3 crystal produced from carbonate starting materials, note the markedly decreased levels of the elements Zn, Ba, K, Cl, Na, and Cs as compared with their concentration in the starting materials (Table I). Since crystal pulling was evidently effective in reducing the level of certain impurities, i t seemed possible that repeated crystal pulling might achieve even further purification.
To test this possibility, two RbMnF3
boules were remelted and a 'double grown' boule pulled from the combined melt. Spark source mass spectrometry revealed no significant improvement, however, in the purity of this double grown RbMnF3 crystal. Although more repeated crystal pullings might eventually result in significant improvement, this approach was not further investigated. Zone refining RbMnF3 and KMnF3 prior to crystal pulling could also be a useful means of further purifying these materials. D.
Etch Pit Dislocation Densities of Pulled Cry.stals Crystal perfection assessment in terms of relative dislocation densities
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RbMnF 3 AND KMnF 3 SINGLE CRYSTALS
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TABLE 2 Spark Source Mass Spectrometric Impurity Survey of a RbMnF3 Crystal Produced from Rb2CO3 and MnCO3 Starting Materials
Elements Detected
Concentration (ppm atomic)
Co, N , 0
I0-I00
Ba, Ca, Ce, CI, Cr, Cs, Cu, Fe, Gd, Ge, K, Mg, Na, Ni, Ti, V
I-I0
As, In, Mo, Rh, Ru, S, Sn, Sr, Y, Zn
0.I-I
Pd, Se, Zr
<0.I
Possible interference on N and 0 determination; range given is highest possible concentration.
has been applied to certain metals, alloys, oxides, chalcogenides, pnictides, and halides (8).
Successful application of the technique requires a chemical
etchant which will reveal dislocation microstructure in the crystal of interest.
Becauseno prior attention seems to have been given to dislocation etch
pit formation in ternary fluorides such as RbMnF3 and KMnF3, we f i r s t had to determine a suitable etchant for these materials.
In view of chemical simi-
l a r i t i e s between the ternary fluorides and LiF, we decided to try etchants which had been reported to reveal dislocations in LiF.
Several trials estab-
lished that the acetic acid-HF-FeF3 etchant reported by Gilman et al. (9, lO) for LiF was most effective for RbMnF3 and K~nF3. The etchant was prepared by mixing 50 ml of 48% HF, 50 ml of glacial acetic acid, and O.5,ml of 48% HF saturated with FeF3.
Cleaved [lO0] crystal
pieces of RbMnF3 and ~nF 3 of approximately l cm2 surface area were continuously rotated in the etching solution for various times, then rinsed well in 95% ethanol, petroleum ether, and air-dried. The etched crystals were then viewed under 400X magnification on a Reichert microscope equipped with a Nomarski interference contrast attachment. Well-developed octahedral etch pits were seen on the (lO0) faces after three seconds etching time. The etch pit size increased linearly with etching time. Optimum etching time with freshly prepared, room-temperature etchant solution was from 3 to 5 seconds. Figure 4 shows a typical etched KMnF3 (lO0) crystal surface, and Figure 5a-c shows the effect of increased etching time on etch pit development on a (lO0) RbMnF3 surface.
Assuming that each etch pit marks the point of emergence
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RbMnF 3 AND KMnF 3 SINGLE CRYSTALS
Vol. 7, No. 6
on the (I00) cleavage face of a dislocation in RbMnF3 and KMnF3, as has been shown to be the case for LiF (9), the dislocation density is easily computed by counting the number of etch pits per unit area on photographs such as those shown in Fig. 4 and 5a.
(This cannot be done for overetched surfaces,
as in Fig. 5c, where there is excessive etch pit overlap.)
In this manner, we
have obtained average dislocation densities for both RbMnF3 and KMnF3 of approximately 5 x IO5 to l x IO6/cm2, very similar to densities typically observed for seed-pulled LiF and KCl ( l l ) .
FIG. 4 Dislocation etch pits on (lO0) cleavage face of KMnF3. Magnification: 4OOX. In order to determine the sensitivity of the observed dislocation densities to thermal treatment, RbMnF3 crystal cuttings were reheated to 9OO°C in evacuated, Ptlined quartz capsules, rapidly quenched to room temperature, etched and studied.
The
dislocation density was doubled by this treatment.
The effect of deliberately
introduced impurities on the dislocation density was also studied, for the case of Li and Co in RbMnF3. Crystals were produced for which flame emission analysis showed respectively 12O ppm Li and 1424 ppm Co in cleaved boule sections.
Etched pieces
showed dislocation densities in the same range as undoped crystals.
Finally, the
FIG. 5 Dislocation etch pits on (lO0) cleavage face of RbMnF3 after etching time of (a) 3 sec, (b) 5 sec, (c) 7 sec. Magnification: 400X.
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RbMnF 3 AND KMnF 3 SINGLE CRYSTALS
581
effect of plastic deformation was investigated by compressing a RbMnF3 crystal along a (lO0) direction using techniques similar to those reported by Sproull et al. (12) for LiF.
In order to reduce the length of a crystal measurably
without cracking, i t was necessary to heat the crystal to about 160°C during compression.
A RbMnF3 crystal reduced 0.7% in length, freshly cleaved, and
etched, showed a dislocation density approaching 107/cm2, an order of magnitude increase over that in as-grown crystals.
The dislocation density in
RbMnF3 and ~nF 3 is thus shown to be sensitive to thermal and mechanical strain in the same way as LiF (g, lO, 12). Summary We have described in detail a method for the routine production of RbMnF3 and ~nF 3 single crystals of adequate size, purity, optical quality, and perfection for use in lattice dynamical property studies.
The Czochralski techni-
que employed has the advantage of significantly purifying relatively impure crystal feed material of certain impurities, including Ba, Cl, Cs, K, Na, and Zn.
We have determined that an etching reagent consisting of 48% HF, glacial
acetic acid, and FeF3 is effective in producting etch pits which highlight dislocations in RbMnF3 and KMnF3, making i t possible to assess the dislocation density in these materials.
This technique provides a useful means for deter-
mining the relative perfection of RbMnF3 and KMnF3 crystals, and should aid future efforts to produce more nearly perfect crystals of these materials.
AcKnowledgements We thank Professor R. O. Pohl and Mrs. J. B. Hartmann for helpful advice and for t h e i r continued interest in this work, G. E. Schmidt for assistance in construction of the hydrofluorination apparatus and preparation of the Figures, and Dr. J. R. Roth and N. G. Nunez of the Cornell Analytical Chemistry F a c i l i t y for the chemical analyses. I.
References For a comprehensive bibliography through 1968, see J. %. Friebely and W. J. Ince, Lincoln Laboratory Library Report (Contract AF 19(628)-5167), AD-677285, August 8, 1968. "Properties of the Compounds RbMnF3, ~nF 3 and CsMnF3".
2.
S. Ogawa, J. Phys. Soc. Japan 14, I l l 5 (1959).
3.
W. W. Holloway, E. W. Prohofsky, and M. Kestigian, Phys. Rev. 139, 3A, A954 (1965).
4. 5.
O. Beckman, I. Olovsson, and K. Knox, Acta Cryst. L3, 506 (1960). K. Nassau, J. Appl. Phys. 32, 1820 (1961).
6.
J. Czochralski, Z. Phys. Chem. 92, 219 (1917); Z. Anorg. Chem. 144, 131 (1925).
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P. O. Henk, D. Gabbe, and K. Bangerskis, Tech. MemoNo. 3, Microwave and Quantum Magnetics Group, Dept. of Electrical Engineering and Center for Materials Science and Engineering, M.I.T., Sept. 1966 (unpublished). 8. See, for example, S. Amelinckx, The Direct Observation of Dislocations. Academic Press, New York (1964). 9. J. J. Gilman and W. G. Johnston, J. Appl. Phys. 2_]_7,lOl8 (1956). lO. J. J. Gilman, W. G. Johnston, and G. W. Sears, J. Appl. Phys. 29, 747 (1958). I I . R. W. Dreyfus, Alkali Halides, in The Art and Science of Growing Crystals, p. 414. J. J. Gilman, ed. John Wiley & Sons, Inc., New York (1963). 12. R. L. Sproull, M. Moss, and H. Weinstock, J. Appl. Phys. 30, 334 (1959).