Full text of DOI:10.1016/0022-0248(71)90262-4

Journal of Crystal Growth 8 (1971) 3 37—340 North-Holland Publishing Co.
GROWTH OF EuO, EuS, EuSe AND EuTe SINGLE CRYSTALS*
T. B. REED and R. E. FAHEY
Lincoln Laboratory, Massachusetts Institute of Technology, Lexington, Massachusetts 02173, U.S.A.
Received 26 March 1970; revised manuscript received 20 August 1970
Single crystals of the europium monochalcogenides weighing up
to obtain crystals containing total impurities of about 200 ppm
primarily C and Ca. The crystals were also used subsequently
for growing crystals doped with Gd or La.
The other compounds were synthesized in sealed quartz am-
poules by the reaction of Eu metal at about 600 ~C with vapor
of the other element supplied by a reservoir at a lower temper-
ature. Single crystals similar in size and purity to those of the
oxide were then grown by the gradient cooling technique. The
effect of deviations from stoichiometry on optical and electrical
properties are discussed.
to 60 g have been grown from Eu-rich solutions and from stoi-
chiometric melts by using a gradient cooling technique. Since all
four compounds have high Eu partial pressures near their melting
points, the crystals were grown in weld-sea]ed tungsten crucibles
1.25 or 2.5 cm in diameter,
The oxide was synthesized and grown in the same run. Dried
Eu203 was heated with an excess of Eu metal to about 2200 ~C,
and the solution obtained was directionally frozen from the
bottom by cooling to room temperature at 5—50 ~C/hr. The re-
suIting boules were single crystals with total impurities ranging
from 250 to 5000 ppm. These crystals were then remelted, either
with or without additional Eu metal, and directionally frozen
The melting point of EuO, measured with an optical pyrometer
and a W—(W, 26% Re) thermocouple, was 1980±20 ~C. EuS,
EuSe and EuTe all melt between 2250 and 2500 C.
1
. Introduction
side by slowly cooling either metal-rich solutions or
stoichiometric melts sealed in tungsten crucibles. The
The divalent europium chalcogenides, which have synthesis and growth of EuO will be described in detail,
the rock saltcubic structure, are ofconsiderable current and variations in technique used for EuS, EuSe and
interest as magnetic semiconductors which are trans- EuTe will then be noted.
parent in the infrared1). Some of their characteristic
physical psoperties are summarized in table 12).
2
. EuO crystals
TABLE I
At the melting point of EuO, the partial pressure of
Physical properties of the divalent europium chalcogenides
Eu vapor overthe compound is probably about 1 atm.
This estimate4) isbased on an extrapolation of partial
[
Melting Lattice
Room
Critical
Corn-
pound
Color in
reflection
point*
param- temperature tempera-
pressure data for EuO—Eu mixtures5) and EuO—~
Eu304 mixtures6).] Therefore synthesis and crystal
growth must be performed in a sealedsystem. Tungsten
eter
abs. edge
ture
-
~
( C)
(A)
-
-
(eV)
(-K) - -
EuO
~
Magenta
~
2016
8
5.143
1.122
T,
=
69
crucibles 3 in. long and either 0.5 in. in diameter
~
T0
x 0.020 in. wall (total charge about 25 g) or I in. in
EuTe Dark green 2320—2450 6.598
1.959
9.6 diameter x 0.060 in. wall (total charge about 85 g)
.
. - - - : - -
-
were used for growing most of the crystals. They are
*
Determined in this study.
.
.
.
.
.
supplied with “insertion” lids, which are welded in an
argon atmosphere by means of a laboratory atc
The growth of EuO crystals from metal-rich solution melter7) at a current of about 100 A. During welding
was first described by Guerci and Shafer3). We have the crucible is held in a water-cooled cylindrical graph-
grown crystals of all four compounds up to 2 cm on a ite anode, as shown in fig. I, which restricts melting
*
This work was sponsored by the Department of the Air Force, to a narrow zone and keeps the charge cool. A few
337

338
T. It. REEl) AND R. Ii. FAHEY
INSERTION CAP
twenty atmospheres: the tungsten lids and bottoms
-
.
TUNGSTEN CATHODE ®
of the crucibles frequently bulge hut have never
broken.) The furnace is then cooled at about 10 C/hr
to l200 C and at 20 ~C/hr to room temperature. The
ARC
-
.
WELD
-
~
INSERT -Cu ANODE
temperature gradient is such that the solution freezes
from the bottom to the top.
r
After cooling the crucible is very brittle and can be
removed by gently cracking and peeling it from the
SLEEVE
wATER-COOLED
. ~
TUNGSTEN CRUCIBLE
CHARGE
ingot to which it adheres because of the excess metal.
The lower two-thirds of the ingot is generally a single
GRAPHITE INSERT
crystal with marked cubic cleavage perpendicular to
the <100> axis. Although it is difficult to remove the
Fig. I. Apparatus for s~eIding in~crtion caps to tungsten
crucibles.
crystal from the crucible without some cracking, cubes
up to 2cm on an edge and weighing over 30 g have been
recovered. Above the crystal there is a two-phase
legion of leathery texture, and above this is a layer of
excess Eu metal.
crystals were also grown in molybdenum crucibles
with similar results.
Synthesis and growth from solution are accom-
plished by sealing Eu203 (dried at 1000
C
~
in vacuum
The EuO crystals are moderately stable in air hut
and pressed into pellets) together with an excess of Eu tarnish noticeably in a few weeks. A typical mass
metal into a crucible in the ratio of one mole of Eu203 spectrographic analysis of the first portion of the
to two moles of Eu (l0 0 ~ excess Eu). One or two crystal to freeze shows C
=
100, K
8, As
all others less than I atomic p.p.m. (crystal 95).
=
50, Ca
=
35,
2,
crucibles are mounted in a cylindrical molybdenum
block (with a small sighting hole drilled through one
wall) inside a resistance furnace with tantalum or
tungsten heating elements8) as shown in fig. 2. With
argon at atmospheric pressure flowing through the
N
=
20, Sr
=
12, Te
=
12, Na
=
=
4. Mg
=
Doped or undoped crystals have been grown from
the synthesized compound by adding b’;, excess Eu
metal (by weight) and a dopant if required, and re -
furnace at 30 cm3/min, the temperature is slowly peating the above procedure. The concentrations of
raised to about 2300 ~C.(It is estimated that the euro- impurities in the first to freeze and last to freeze por-
pium pressure at this temperature reaches at least tions of crystal 48, grown from a solution doped with
8000 p.p.m. Gd are shown in fig. 3.
15cm
The sealed crucible method has also been employed
TANTALUM
HEATER
_________________________
TANTALUM
to grow single crystals of EuO from nominally stoi-
WATER-COOLED
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _
HEAT SHIELDS
chiometric melts, by using the synthesized compound
JACKET
TUNGSTEN
CRUCIBLES
in this manner, which do not adhere to the crucible,
do not contain a second phase of either Eu metal or
CRYSTAL
BLOCK
higher oxides. The formation of’ single phase material
SIGHT PORT
~
~~~MOLY
Twi n hi te ht homi us et l mta i dna dgn in peg or i en xst ch ewo s was ss Ed tueh t mae rt em t aEi lnu. eOdT h bme yecl mrt sy es ltcta iolnsn g go rabu t esa nii nnt lgeylde.
crystal in a sealed molybdenum crucible and measuring
the temperature of the thermal arrest observed on
GAS
—
GAS
ce( Wot eo r, l. i2nT6g h.’ e~ CRa ove n)e srt aih sget er mn vt oa crl eou sue up lolt esb t aawni ned re ewd iot fhbo t raa ni n5 7oe pd tc i ocwoa ill ti hnp gya r roWu mn—s -
CURRENT
on the same sample was 1980±20 ~C.
An attempt was made to grow crystals by slow
Fig. 2. Apparatus for growth of Eu chalcogenide crystals from
solution and melt,
cooling of melts containing excess oxygen. In all cases

GROW TH OF EuO, EuS, EuSe A N D EuTe SINGLE CRYSTALS
339
the crystals contained small amounts of Eu3O4 as a
second phase.
CRYSTAL 82-0
BOTTOM
CRYSTAL 82-0
TOP
1
000-
The results of mass spectrographic analysis of a
single crystal grown from a nominally stoichiometric
melt doped with 0.8 a t ~ Gd are shown in fig. 4. It
can be seen that the degree of purification due to
segregation of impurities is much less than that ob-
served in growth from Eu-rich solution (fig. 3).
In general undoped crystals grown from Eu-rich
solution are highly transparent in the infrajed from
a-
a.
4
Gd*
~ ~oo
N
0
~
4
a-
2
10
w
o
Sr
2
0
*
DOPANT 5000 PPM ADDED
2—10 ~m, with absorption coefficients in this region
of less than 1 cm ~, while crystals grown from a stoi- Fig. 3. Impurity segregation in EuO crystal grown from Eu-
rich solution, doped with 0.5 at% Gd. (Diagonal lines connect
chiometric melt have much larger absorption coeffi-
cients’). The room temperature electrical resistivity of
the last-to-freeze portions of doped and undoped
crystals grown from solution isas low as 10_I ohm-cm,
points for the same element and do not imply a linear distribu-
tion.)
while that of crystals grown from the melt is about 50 5 x l0 ~ ohm-cm at room temperature and increases
ohm-cm. The first-to-freeze portions of both solution slowly and smoothly with decreasing temperature to
and melt-grown crystals have resistivities above 106 about 5 x b0~ohm-cm at 50 °K , passing smoothly
ohm-cm, the limit of our measurement.
through the Curie point. The tesistivity of the Eu-
The homogeneity range of EuO is quite narrow and saturated sample isabout 102 ohm-cm at room temper-
difficult to measure. Weight change on combustion of ature, rises to a maximum of about l0 ~ ohm-cm at
EuO~to Eu703 gives values of x
=
1.000±0.005for 80 °K and then drops to about l 0 ~ ohm-cm at 50 °K .
typical single crystal samples, but there is sufficient This behavior, which is associated with the ferromag-
scatter in the results so that it has not yet been possible netic transition in EuO, has also been observed in some
to correlate them with other properties. The variation as-grown samples taken from the last-to-freeze por-
of the unit cell across the homogeneity range is also tions of solution-grown ingots2). These striking dif-
small and difficult to measure reliably. Shafer9) reports ferences between the Eu-saturated, as-grown, and 0-
a lattice parameter of 5.142 A for a number of annealed saturated crystals show that deviations from stoichiom-
samples containing excess metal and 5.143 A for etry are of great importance in determining the prop-
samples equilibrated with Eu3O4. Other values in the erties of EuO.
literature range from 5.142—5.145 A, as do all those
measured for EuO crystals in this laboratory, the
higher values being associated with excess oxygen.
The optical and electrical properties of EuO are
CRYSTAL-45
CRYSTAL-44
CRYSTAL-45
TOP
BOTTOM
Gd
Ca
extremely sensitive to the small variations in compo-
sition occurring during crystal growth or in subsequent
annealing. The effects of composition were demon-
strated by an annealing experiment on a high-resistivity
crystal of EuO. One part of the crystal was annealed at
~
1000
a.
a-
Yb
~
~
o
100
N
~
_ _ _ _ _ _ _ _ _ _ _ _ _ _
_ _ _ _ _ _ _ _ _ _ _ _ _ _
~‘—‘-—..~——~ .,_—
_
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _
Sr
a.
I—
a.
Sr
1400 ° Cfor 60 hr in a sealed crucible in the vapor over
Z
10
1
a mixture of Eu and EuO and another part was simi-
larly annealed in the vapor over a mixture of EuO and
Eu304. After annealing, these samples had lattice para-
meters of 5.1422 and 5.1438±0.005A respectively,
compared with the initial value of 5.1432 A.
0
0
Al
*DOPANT NOT PRESENT IN STARTING MATERIAL
000 PPM ADDED
8
Fig. 4. Impurity segregation in EuO crystal grown from nom i-
nally stoichiometric melt doped with 0.8 at% Gd.
The resistivity of the 0-saturated sample is about

340
T. B. REED A N I) R. L. FAHEY
3
. EuS, EuSe, and EuTe crystals
niufli vapor also increased the resistivity. The weight
changes accompanying the addition of chalcogenide
The other three Eu chalcogenides were synthesized or evaporation of europium suggest that the homo-
by reacting Eu metal with chalcogen vapor, using the
method of M iller’°) and Holtzbergi i), Chunks of Eu atom percent wide.
metal were placed at one end of a fused quartz tube In cooling experiments on the thiee compounds un-
0 cm long, and the stoichiometric amount of the melted material was observed in crucibles heated to
2250 C, measured in a blackbody hole with an optical
geneity ranges of the sulfide and selenide are several
2
chalcogen at the other end. After being evacuated and
-
sealed, the tube was placed in a horizontal mLlffle fur-
pyrometer. Crystals of all three compounds have been
grown from the melt by cooling from 2500 C, mdi-
nace 50 cm long with the metal kept below 600
C
and the chalcogen in a cooler region near one end of cating that their melting points all lie between 2250
the furnace, in order to prevent too rapid reaction,
After the reaction was essentially complete, usually in
and 2500 C.
-
a (lay or two, the temperature was raised to 900 C Acknowledgments
for 16 hr. There was no evidence of reaction with the
quartz.
We would like to acknowledge the assistance of
Mr. E. B. Owens in analysing the crystals and Mrs. M.
J. Button for measuring the lattice parameters.
Crystals of all three compounds were grown from
solution by adding 20 wt 0 < excess of Eu metal to the
synthesized material, heating to 2300 -C, and cooling References
in sealed tungsten crucibles at the same rate used for
I) J. 0. Dininiock, IBM J. Res. Develop. 14 (1970) 301.
EuO. As in the case of EuO, no special precautions
were necessary to obtain single crystals. The as-grown
crystals are almost opaque in the infrared, and their
electrical resistivity at room temperature is of the order
of I ohm-cm. These properties suggest the presence of
excess Eu metal. Therefore EuS and EuSe crystals
have been subjected to two annealing procedures de-
signed to increase the ratio of chalcogenide to metal.
After being annealed in a dynamic vacuum (10—2 torr)
at 1600 ~‘Cfor 48 hr. the crystals became insulating
2) M. R. Oliver, Conductivity Studies in EuO, Thesis, Electrical
Eng. Dept., M.I.T., Cambridge, Mass., 1970.
3
)
C. F. Guerci and M. W. Shafer, J. AppI. Phys. 37 (1966)
1406.
4) Solid State Research Report, Lincoln Lab.. M.I.T., Lexing-
ton, Mass., (1970:2) DDC AD7I I 074.
5) R. Hultgren, R. L. Orr, P. D. Anderson and K. K. Kelley,
Supplement to Selected Values of Thermodynamic Proper-
ties of Metals and Alloys (received from R. Hultgren).
6) J. M. Haschke and H. A. Eick, J. Phys. Chem. 73 (1969) 374.
7) T. B. Reed, Arc Techniques for Material Preparation and
Czocharalski Crystal Growth, in: High Tempei’ature Tech’
nologr (Butterworth~,London, 1969).
(
resistivity gleater than 106 ohm-cm) and quite trans-
8) T. B. Reed and R. E. Fahey, Rev. Sci. lnstr. 37 (1966) 59.
9
) M. W. Shafer, J. AppI. Phys. 36(1966)1145.
parent in the infrared and red, Annealing as-grown
crystals at 1000 C for 64 hr in I atm of sulfur or sele-
10) J. F. Miller, J. Electrochern. Soc. 106 (1959) 1043.
II) F. H oltzberg, personal com m unication.