Synthesis and crystal growth of SrLiH3 and EuLiH3; Ternary hydrides with the perovskite structure

Journal of Crystal Growth 6 (1970) 119—124 © North-Holland Publishing Co., Amsterdam

SYNTHESIS AND CRYSTAL GROWTH OF SrLiH3 AND EuLiH3 TERNARY HYDRIDES WITH THE PEROVSKITE STRUCTURE*t

J. E. GREEDAN** Department of Chemistry, Tufts University, Medford, Massachusetts 02155, U.S.A.

Received 29 July 1969 2~and Eu2~ The ternary metal hydrides, MLiU3, with M Sr which crystallize in the cubic perovskite structure were synthesized by direct reaction between lithium hydride and the appropriate metal dihydride. The reactants were obtained by direct reaction of the vacuum distilled (l0-~ Torr) parent metals with ultrapure hydrogen. Large single crystals of both SrLiH3 and EuLiH3 were grown from the melt using a modification of the Bridgman— Stockbarger technique. Contrary to a previous report these materials melt incongruently rather than congruently, and it was necessary to utilize an initial charge composition of 40 °/~ MH2— 60~ Lil-I to prevent the precipitation of MH2 upon melting. The peritectic melting point ofEuLiH3 was determined as 780 +

5 °Cusing DTA. Crystals of SrLiH

1. Introduction Studies of the electronic structure of solid metal hydrides are not abundant. It has generally been found that in order to gain useful information regarding the electronic properties of solids, one must carry out a wide battery of measurements such as electrical resistivity, optical absorption, Hall effect, and photoconductivity on well-defined single crystalline samples. To our knowledge only two published studies of the

The group of ternary hydrides, MLIH3, where 2+, Sr2~,and Eu2~seemed to represent an M = Ba attractive system for study. These materials were discovered by Messer and co-workers5’6) using X-ray crystallographic techniques and were found to crystal!ize in the ideal, cubic perovskite structure. Maeland and Andresen7) have verified the correctness of this assignment by neutron diffraction. Furthermore, Messer and Levy8) had reported that at least one member of the series, SrLiH 3, appeared to melt congruently at 745 °C.Thus, it appeared possible that single some crystals of thesemelt-growth perovskites technique could be obtained using standard and that studies similar to those previously cited could be done on these materials as well. In addition the high

properties of metal hydrides have been carried out on single crystals: the the seriesphysical of papers by Pretzel and 1’2) on properties, mostly co-workers optical, of lithium hydride and the more recent work of Libowitz and Pack3’4) on the electrical transport properties of the CeH 3_~system. In both of these cases the emphasis has been placed on electronic properties due largely to defects and impurities and not on the intrinsic electronic structure of the hydrides themselves,

3 were orange-brown and those of EuLiH3 were deep-red in color. The absorption edge at room temperature, determined by optical transmission measurements, was about 4.0 eV for SrLiH3 and 1.8 eV for EuLiH3. The room temperature resistivity of both materials was about l0~ ohm-cm. The crystals were quite brittle, quite sensitive to thermal shock, and tended to fracture conchoidally when cleavage was attempted by ordinary means. Chemical analysis suggested that the major impurities in both hydrides were oxygen and nitrogen and that these hydrides are not grossly non-stoichiometric with respect to hydrogen content.

symmetry of the cubic perovskite structure coupled with the obvious electronic simplicity of the Li~and H ions should provide an attractive system for theoretical study.

Taken in part from a thesis submitted by J. E. G. to the faculty of Tufts University, in partial fulfillment of the requirements for the degree of Doctor of Philosophy.

2. Experimental

t

2. 1.

*

Presented at the 156th national meeting of the American Chemical Society, Atlantic City, N.J. September 1968. ** Present address: Chemistry Department, Alumni Hall, University of Pittsburgh, Pittsburgh, Pennsylvania 15213, U.S.A.

119

MATERIALS

Lithium metal (99.96%) and lithium hydride (98.0 %) were obtained from Foote Mineral Co.

120

J.E.GREEDAN

Strontium metal (98%) was obtained from A. D. MacKay. Europium metal (99.9%, vacuum distilled) was obtained from Research Chemicals. Ultra-pure hydrogen gas, total impurities less than 10 ppm, was supplied by the Matheson Co.

2.4. PREPARATION OF LITHIUM I-IYDRIDE Commercial lithium hydride was recrystallized twice using the crystal growing apparatus and the procedures described in the following sections. This procedure was suggested and used with success by Pretzel et al. 1) and

2.2.

our

VACUUM SUBLIMATION

Strontium and europium metals were sublimed under — high vacuum (10 Torr) prior to reaction with hydrogen. The sublimation apparatus was an all metal

2

~

work seemed to confirm FREE ARATION 01 liii

their

observations.

I EROVSKII ES

Polycrystalline samples of SrLiH~ and

EuLiH~

assembly consisting of a ~3l6 stainless steel tube connected by a high vacuum flange to a high vacuum

could be prepared by direct reaction between equimolar amounts of LiH and MH

valve. An inner molybdenum cylinder, closed on one end, was used to contain the charge. A perforated cap of molybdenum sheet was placed over the open end of the tube. The closed end of the tube was heated using a small resistance furnace and the sublimed material collected on the molybdenvm cap and the upper 3 or 4 cm of the cylinder which extended out of the furnace. Complete sublimation of both metals occurred over a period of several hoLirs when the temperature of the charge end was maintained at 800 to 900 C. In both cases the sublimed metal was silvery in appearance. Following sublimation the high vacuum valve was closed and the 9) assembly was transferred to the atmosphere of which

out in the glove box and transferred to a capped. cylindrical, tLingsten carbide mortar and pestle belonging to a heavy-duty ~6 Wig-L-Bug. The mortar and pestle was sealed using electrician’s tape and was transferred to the Wig-L-Bug and vibrated for periods of I or 2 mm. Upon transfer back to the glove box the very finely powdered and intimately mixed charge was placed in a molybdenum boat which was in turn transferred to a quartz tube identical to that used in the preparation of the parent hydrides. The powdered charge was then heated to 700 C for hr under about one atmosphere of hydrogen. The phase purity of the products monitored using powder data and under thewas above conditions theX-ray reactions were always

an argon-filled glove box using Na—K liquid alloy was continuously gettered adsorbed on drierite.

found to be complete. The method described above differs from the original method of Messer et al. 56) who first alloyed the parent metals and hydrided this alloy. We believe that our technique has certain advantages over the alloy

2. 3.

PREPARATION OF STRONTIUM AND EUROPIUM HY-

DRIDES

2. The hydrides were weighed

The molybdenum tube and cap containing the

method. First of all, the hydrides are more tractable

sublimed metals was removed from the sublimation vessel in the glove box and transferred to a quartz tube equipped with a high vacuum stopcock. The quartz tube was transferred to a high vacuum hydrogen synthesis line and pumped to a vacuum of 10 6 Torr. A measured amount of ultra-pure hydrogen was introduced and the temperature was raised to about 300 to 400 ~C at which point the reaction commenced.

than the parent metals: they can be pulverized and intimately mixed in powdered form thus assuring optimum conditions for completion of the solid-state reaction and making unnecessary the tedioLis heat treatment cycles found essential in the earlier work. Secondly, it has been our observation that the hydrides are less reactive with respect to the glove box atmosphere than the parent metals. Thus, large amounts of

The reaction of hydrogen and sublimed metal went much more rapidly and at a lower temperature than a typical run using unsublimed metal. SrH 2 prepared in this manner was snow—white and EuH, ‘,sas grey. Following hydriding the quartz tube was removed to the dry box, the molybdenum tube removed, and (lie hydrides were scraped from the walls using a spatula.

the parent hydrides can be prepared initially and stored for relatively long periods of time (on the order of months) without contamination. For preparation of single-crystal samples the grinding and niixing steps were generally omitted and the perovskites were prepared in situ as a part of the regular crystal growth cycle.

SYNTHESIS AND CRYSTAL GROWTH OF

2.6.

SrLiH3 AND EuLiH3

121

blanket. The hollow support rod is fitted with a

CRYSTAL GROWTH—APPARATUS

The method of choice was the Bridgman—Stockbarger technique modified slightly to permit the use of a controlled atmosphere and a sealed crucible. A schematic of the apparatus used is shown in fig. i. ~

chromel—alumel thermocouple (J) which monitors temperature at the crucible tip. Temperature control to about ~ I °Cwas obtained by means of a Bristol proportionating thermal controller operated by another chromel—alumel thermocouple (K) placed in a conveniently located well. Vent (L) provides an access

K

~ B A

whereby the bomb can be either evacuated or filled

with hydrogen up to at least four atmospheres pressure. Water-cooled copper coils (M) protect the 0-ring gasket and the Teflon packing.

I

TABLE1

0

D

c

Optimum conditions for crystal growth

0

o o

//

0

0 0

I

Starting temperature Thermal gradients Cooling rate Hydrogen pressure*

/

§

M

Fig. I.

G H

Apparatus used in the growth of SrLiH3 and EuLiI-I3.

SrLiH3 —770 C; EuLiH3 —800 C 15 to 25 C/cm 2 to 4 C/hr 2 to 4 atm

Charge composition 60~cLiH— 40~MH2 * This is the pressure external to the welded crucible. At these temperatures the thin iron crucible walls are permeahle to hydrogen and thus the partial pressure of hydrogen is very likely ihe same inside the crucible as well. The total pressure inside the crucible was probably greater than hydrogen partial pressure because some argon froni the glove box was trapped during the welding operation. The outward buckling of the crucible cap observed in some runs is taken as e”idence of argon entrapment.

2.7 CHRYSTAL GROWTH—PROCEDURE

The

procedure

for a typical growth run will now be

this arrangement the welded iron crucible (A) and

described in some detail. The pure iron crucibles

tapered cap (B) containing the polycrystalline charge

(99.9% 0.013 in. wall) were pre-cleaned according to

is positioned on a hollow supporting rod (C) which

a procedure suggested by Gibb and Messer’°)and used

can be moved vertically with respect to the stationary furnace (D) at various rates down to 1.5 mm/hr. The crucible is held upright and in thermal contact with the tip of the support rod by means of an asbestos collar (E). The crucible and support rod are enclosed

successfully by Pretzel et al. 1) One or two crucibles were filled with commercial LiH and heated to 750— 800 °Cunder 20 cm Hg of hydrogen gas for 2 hr in a small stainless steel bomb. After cooling to room

in a z1r316 stainless steel bomb (F) complete with standard flange (G) and 0-ring gasket. This bomb is evacuable to at least l0~ Torr and can hold a pressure of at least 4 atm of hydrogen. A Teflon packing gland

temperature the hydride was removed by many washings with methanol. The crucibles together with the snug-fitting iron caps were transferred to a high vacuum line where they were heated to 500 °C for several hours under I atm of ultrapure hydrogen. The

(H) mounted in the bottom half of the flange effectively

furnace was then shut off and the crucibles were

seals the support rod during its traverse through the

degassed at less than 10 ~ Torr while cooling to room

furnace. The furnace itself consists of three separately

temperature. The quartz tube containing the crucibles

powered Kanthal wire heating units (I) wound on the same Alundum core. A constant voltage transformer was used to regulate the line voltage. Insulation for the furnace consists of firebrick and Fiberfrax ceramic

was then transferred to the glove box and stored until use. A clean crucible was selected and loaded with the appropriate polycrystalline charge in the glove box.

122

j.

e.

GREEDAN

The bottom few centimeters of the crucible were placed in a brass welding block and the open top was

analyzed for either strontium or europiLim and hydrogen. Lithium content was computed by difference.

connected to a simple apparatus which permitted evacuation of the crucible and subsequent filling with about 0.5 atm of ultra-pure hydrogen. After filling with hydrogen the crucible cap was forced into place, the vacuum assembly quickly removed, and the cap was

Variants of the standard oxalate method were used for the strontium and europium analyses. The europium analysis was carried out by Ledoux and Company of Teaneck, New Jersey. The hydrogen content of the crystals was determined by a gas evolution technique

spark-welded to the crLicible. The welding operation seldom required more than a few seconds,

utilizing the decomposition reaction of the hydrides with cold, dilute (I N) HCI. The HCI was purged of

The crLicible and its contents were then transferred

atmospheric gases by bubbling hydrogen through a

to the crystal growth apparatus. Following several cycles consisting of alternate evacuation of the bomb and flushing with ultra-pure hydrogen, one atmosphere

sintered glass disc for thirty minutes before use. A measured volume of this specially prepared HCI was allowed to react with the perovskite crystals in a sealed vessel held close to 0 C. The evolved gas was collected

of hydrogen gas was introduced and the furnace was turned on. About 6 hr were required to reach operating temperatures and during this time the hydrogen pressure was allowed to slowly increase to 30 or 40 psi. The starting temperatures were selected by adjusting the height of the crucible in the bomb and the values quoted are probably good to -~ 10 ~C. When the thermal cycle determined by the on—off controller was less than or equal to -E- I ~C the descent motors were turned on and the growth run was commenced. The crucible was then lowered through the temperature gradient provided by the furnace at the rate of 1.5 mm/ hr until the temperature measured at the crucible tip was was 150 to 200 ~C below the melting point of the material in question. This process usually required from 36 to 48 hr. At this point cooling was begun more rapidly by turning down the variacs each hour or so. Cooling to room temperature by this process reqtnred about 16 hr. The crucible was transferred unopened to the glove box where the contents were removed using a tubing cutter and long—nosed pliers to strip the thin walls away from the crystal which generally occupied the bottom one-third of the crucible. The crystal was not observed

in an air-jacketed gas burette and the measured volume was used to compute the hydrogen content of the crystals assLiming an ideal gas behavior. CaIc. for SrLiH 3: Sr, 89.9; H, 3.03; Li, 7.11. FoLind: Sr. 89.6; H, 2.95; Li, 7.48. CaIc. for ELiLiH3: Eu, 93.8; H, 1.84; Li, 4.29. Found: Eu, 93.0; H, 1.86; Li, 4.59, The molar ratios, H /M + + are for Sr. 3.00 .01 ; for Eu, 3.01 -I- .01. These results are thought to lie within the limits of experimental error for both the hydrogen and divalent metal analysis. 2.9.

CHEMICAL ANALYSIS

Metallic impurities were semi-quantitatively deter-

TABLE 2

Minor and trace impurities in SrL1 1-13 and EuLiIl3 In molar ppm

SrLiH, Ca

same apparatus used for crystal growth and in essentially the same manner except that the temperature of the charge was held stationary at about 100 ~C below the melting point for periods in excess of 100 hr.

Mg j Fe

EuLiH3

1

Ba

—

-

300

50

Fe Yb

50

10 ea.

Al

It)

Cu

0

Al Cu

)

300 ea.

Si CHEMICAL ANALYSIS

MINOR AND TRACE IMPURI-

mined using a combination of spectrographic and atomic absorption techniques. Nitrogen was determined using the micro-Kjeldahl method by Ledoux and

to stick to the iron walls. Annealing of the crystals was accomplished in the

2.8.

—

TIES

100

MAJOR COMPONENTS

Approximately 0.001 mole samples consisting of small single crystals of both perovskite hydrides were

500 ea.

500 Ca.

SYNTHESIS AND CRYSTAL GROWTH OF

Co. Oxygen was estimated to be present in roughly the same concentration as nitrogen. The results are shown in table 2. 3. Results and discussion Our observations doSrLiH not confirm the report All by 8) that melts congruently. Messer and Levy growth experiments using an initial charge of cornposition 50% LiH—50°/ SrH 2 resulted in the precipitation of SrH2 which was trapped in the conical tip of the crucible and remained there after cooling the crucible contents to room temperature. This phase was readily identified by X-ray powder data. In addition to this SrH2 phase the crucible also contained a largely single crystal phase of SrLiH3 which was located just above the SrH2 phase in the crucible and a top “phase” of a polycrystalline nature in which the only component detectable by X-ray analysis was SrLiH3. Upon steadily increasing the concentration of LiH in the initial charge the amount of precipitated phase was diminished and finally eliminated at about 60% LiH. We then interpret our results on the basis that SrLiH3 melts incongruently to solid SrH2 and a melt containing about 60% LiH from which SrLiH3 gradually freezes out as the temperature is lowered to the eutectic temperature where the entire melt becomes solid. Chemical analysis of the polycrystalline top “phase” indicated the presence of lithium and hydrogen in concentrations exceeding that expected for SrLiH3 thus giving support to our hypothesis. The incongruent melting point of EuLiH3 which had not been determined prior to this work was found to be 780 ±5 °Cby DTA. Using the conditions crystals of both SrLiHset forth in table I large single 3 and EuLiH3 were grown. These crystals had the configuration of the crucible and were 13 mm in diam. and from 13 to 25 mm in length. The crystals were identified as single by transmission Laue photographs. Crystals of both materials were highly colored, SrLiH3 being orange-brown 1), andunannealed EuLiH3 being deepcrystals of red. Unlike the colored case of blue, LiH these colors could not be which are often bleached by prolonged annealing under a hydrogen atmosphere. For example annealing periods of twenty days at one atmosphere hydrogen and for six days at four atmospheres hydrogen pressure produced no

SrLiH3

AND

EuLiH3

123

change in color. The annealing temperatures were 650 °Cfor Sr[.iH3 and 700 °Cfor EuLiH3. Also unlike LiH, these materials could not be cleaved using a razor blade and a small hammer and tended instead to fracture conchoidally. Furthermore, the samples were quite brittle and sensitive to heat. Forthe example, to solder to crystals attempts always resulted in indium fracture,metal directly X-ray powder data of samples prepared from single crystals indicated the absence of any other phase at this level of detection. Direct microscopic examination showed the absence of impurity inclusions. Observations with a polarizing microscope provided qualitative evidence that the crystals were mechanically strained, The room-temperature resistivities of both samples were about l0~ohm-cm and showed little change upon annealing. Absorption coefficients in the spectral regions below the absorption edges (1.8 eV for EuLiH3 and 4.0 eV for SrLiH3) are rather high, on the order of 60 cm ‘ in the case of EuLiH3 and 100 cm’ for SrLiH3, indicating the presence of optically active impurities or of scattering centers. The results of the chemical analysis suggest that with respect to hydrogen content the perovskites are essentially stoichiometric materials, at least to within the limits of analytical error. The failure of the crystals to bleach when exposed to a large excess of hydrogen supports this hypothesis as does the high resistivity of these materials which also fails to change significantly after the annealing operation. In this respect the ternary perovskites behave more like the more saline hydrides which tend to be essentially stoichiometric rather than the transition-metal or rare-earth metal1 binary i~i2) hydrides Also, as which are grossly non-stoichiometric might have been expected given the high chemical reactivity of hydrides toward common atmospheric gases, the major impurities detectable in both materials were oxygen and nitrogen followed by the other alkaline-earth metals. In all the total impurity content of both crystals is about 2000 ppm with no single impurity detectable at greater than 500 ppm.toFinally, no attempt was made by chemical analysis detect the possible presence of Eu~3 which is sometimes found to be a gross impurity in nominally divalent europium compounds’3). However, the observed value of the molar Curie constant, CM, taken from susceptibility data on EuLiH 3 is in agreement to within

124

J. F. GREEDAN

experimental error (I %) of the theoretical value. The details of the electrical, magnetic, and optical studies will be published elsewhere,

Hartunian for performing the DTA meastirement. This work was supported by the U.S.A.E.C.

4. Summary

References

It has been shown that large single-crystals of the perovskite hydrides can be grown using the Bridgman— Stockbarger technique in spite of the fact that these materials melt incongruently. To our knowledge this report makes the first time that any ternary hydride has been obtained in the form of large single-crystals.

2) F. E. Pretzel and C. C. Rushing, J. Phys. (Them. Solids 17 (l96l) 232.

The crystals have been shown to be essentially stoichiometric with respect to hydrogen content but to be rather impure by electronic standards, subject to

5) C. E. Messer, J. C. Eastnian, R. G. Mers and A. J. Macland,

mechanical strain, and to be of only moderate optical quality.

Acknowledgments The atithor wishes to thank Drs. T. R. P. Gibh, Jr. and C.E. Messer and Mr. W. A. Norder for many useful suggestions contributed during the course of this work. He is also indebted to Dr. G. G. Libowitz and the late A. Levesque for the design and partial construction of the crystal growth apparatus and to Mr. Byron

I) F. E. Pretzel, G. N. Rupert, C. L. Mader, F. K. Storms, G. V. Gritton and C. C. Rushing, J. Chem. Phys. Solids 16

(1960) 10.

3) G. G. Libowitz and J. G. Pack, iii:

Crystal Growth, Ed. H. S. Peiser (Pergamon Press, Oxford, 1967) p. 129. 4) G. G. Libowitz and J. C. Pack, J. Chem. Phys. 50 (1969)

lnorg. Chem. 3 (1964) 776. 6) C. E. Messer and K. Hardcastle, lnorg. Chcni. 3 (1964) 327. 7) A. J. Macland and A. F. Andresen, J. Cheni. Phys. 48(19(38) 4660.

8) C. E. Messer and I. S. Levy, lnorg. Cheni. 543 (1965). 9) T. R. P. Gibb, Jr., Anal. Cheni. 29 (1957) 584. 10) T. R. P. Gibb, Jr. and C. E. Messer, A Survey Report on Lithium Hydride, Tufts Univ. Report NYO-3957 (May 2, 1954); Supplement 1954—1957, Tufts Univ. Report NYO8022 (Aug. 31, 1957). II) G. G. Libowitz, The Solid-State Chemistry of Binary Metal Hydrides (Benjamin, New York, 1965). 12) T. R. P. Gibb, Jr., Primary Solid Hydrides, in: l’rogress in lnorqanic Chemistry )Interscience, New York, 1962). 13) M. W. Shafer, J. AppI. Phys. 36(1965)1145.