The growth and properties of strontium barium metaniobate, Sr1−xBaxNb2O6, a tungsten bronze ferroelectric

Journal of Crystal Growth 1 (1967) 311—314 © North-Holland Publishing Co., Amsterdam

THE GROWTH AND PROPERTIES OF STRONTIUM BARIUM METANIOBATE, Sr1 ~Ba~Nb2O6, A TUNGSTEN BRONZE FERROELECTRIC

A. A. BALLMAN and H. BROWN Bell Telephone Laboratories, Incorporated, Murray Hill, New Jersey, U.S.A. Received 29 May 1967 The ferroelectric phase Sri_~Ba5Nb,O6has been grown as large been theCzochralski composition range SrSingle crystals have single grown crystalsover by the technique. 0.7~Ba~.25Nb2O6to Sr,.25Ba0.75Nb2O,. The crystals are yellow-amber in color, transparent from 0.4 ym to 6.0 sum, show a room temperature tetragonal symmetry and appear to be in the point group 4mm. The optical character is

uniaxial negative 11e = 2.27.

1. Introduction

tamed in iridium crucibles and heated inductively.

Solid solutions and ionic substitutions have been studied in some detail by many investigators as a means of modifying the properties ferroelectric 2 + orofCa2 + ions formaterials, Ba2 + in The substitution of Pb barium titanate to alter the Curie temperature1) and the solid solution of potassium niobate and potassium tantalate (KTN)24) to achieve a particular dielectric effect are examples of these techniques. Further examples of the modification or of the production of ferroelectric properties occur in the group of materials generally described as nonstoichiometric compounds and of particular concern to the present work, are compounds of the type AB 206. Nonstoichiometric compounds in the BaNb2O6—PbNb2O6 5), BaNb2O6—BaZrO3 BaNb2O6—SrNb2O6 7) and other 8~0) have 6) been studied in some detail. Most systems of the compounds formed in these systems appear to possess the same structure commonly referred to as a tungsten bronze. The present study deals with the

Starting materials were strontium and barium carbonates 99.999 %‘~and niobium pentoxide 99.99 %~‘.Sampies ofthe desired composition were melting weighedtook and place. added directly to an iridium crucible while

SrNb2O6—BaNb2O6 system, the growth of large single crystals from the melt and certain of the properties exhibited by crystals containing different ratios of strontium to barium. 2. Experimental The single crystals described in this work were grown by the Czochralski technique. The charges were con-

and the indices of refraction are n0

=

2.31 and

A large dielectric anomally occurs at 70 °Cfor the composition Sr0.75Ba0.25Nb2O6 and extends to 220 °C for the barium rich member. Ferroelectric hysteresis loops have been observed at room temperature as well as exceptionally large linear electrooptic effects.

Care was excercised to avoid errors in composition produced by spattering as the carbonates decomposed. The charge was added in small increments as the molten level approached the upper region of the crucible in order to minimize spattering losses. Crystals were grown initially by causing the melt to spontaneously nucleate on a platinum wire. The polycrystal so produced was then grown to a small diameter (about 2 mm) to effect the growth of a single crystal. The crystal was then grown to larger dimensions which oriented crystal seeds could be cut forfrom subsequent growth single experiments. Growth rates of about 10—i 5 mm/hr were used with rotation rates of 40—60 rpm. Single crystals were grown over the composition range Sr 0 75Ba0 .2 5Nb2O6 to Sr02 5Ba075 Nb206. The melting point over this range measured by uncorrected optical pyrometer readings was 1500 °C±10 °C. The analytical determination for the barium, strontium and niobium content in the crystals and the melt * Johnson-Mathey Company, Ltd., London, England, via United Mineral and Chemical Company, New York City, New York, U.S.A.

311

312

A. A. BALLMAN AND H. BROWN

was done by X-ray fluorescence’ 1) and is reported to be accurate to within ±1.0 % of the amount present in the sample. The measurement ofthe capacitance change as a function of temperature was done by plotting continuously both the output of a thermocouple placed next to the crystal in the furnace and the capacitance of the crystal obtained from the output of a Type 130 Tektronix L.C. meter on an X—Y recorder. A detailed description ofthe circuit was given in an earlier paper1 2),

_______ ________

~1j~LJ~

____

3. Results and discussion Fig. I.

1_ Ba Nb,06 Czochralski growth

The ferroelectric structure of lnterest* first occurs when strontium is replaced with barium at the level Sr07 5Ba0 .2 5Nb2O6 and persists up to the composition Sr02 5Ba0 5Nb2O6. Melts were prepared and crystals were grown at Sr0 125Ba0 875Nb2O6 and Sr0875 Ba 0125Nb2O6 and in each case the end member structures of BaNb2O6 and SrNb2O6 were identified by X-ray powder diffraction methods. Table 1 shows the analytical data obtained on the grown crystals at several of the compositions studied. The analytical data shown in table 1 suggest that the melt composition is near to the crystal composition. A small but persistent excess cation to anion ratio is

(x 1.5).

evident, however, and would be consistent with the nonstoichiometric nature group. 7) reported a ofthe similartungsten—bronze finding in his study of Francombe the BaNb2O6—SrNb2O6 system. Single crystals shown in fig. 1 have been grown in air as large as 1.0 cm in diameter and 7.0 cm in length. The crystals are dark amber as grown and bleach to a yellow or pale amber after annealing in oxygen at 1400 °Covernight. Single crystals show a room ternperature tetragonal symmetry and appear to belong to the point group 4mm. The lattice parameters measured on the Sr0 ~ Ba0 25Nb2O6 composition gave values of: 5 a = 12.43024 ±2 x 10 ~and c = 3.941341 ±1 x i0—~

.~

* The detailed crystal structure of this tetragonal tungsten bronze is currently under study and will be reported separately~).

.

TABLE

2+

Melt composition Baa+ Nb

Sr —

0.125 0.25 0.50 0.50 0.50 0.60 0.70 0.70 0.75 0.75 0.75 0.75 0.75 0.75 0.75 0.75 0.75 0.875 1.0 *

1.0 0.875 0.75 0.50 0.50 0.50 0.40 0.30 0.30 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.125 —

1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0

Sr —

0.114 0.351 0.532 0.534 0.537 0.628 0.724 0.720 0.774 0.768 0.761 0.771 0.768 0.776 0.781 0.782 0.788 0.966 0.974

1

Ba

Analysis Nb

0

Cation to anion ratio

Structure

0.971 0.836 0.661 0.487 0.488 0.472 0.384 0.302 0.300 0.265 0.256 0.270 0.271 0.272 0.256 0.256 0.256 0.260 0.072

2.000 2.011 1.976 1.962 1.957 1.982 1.975 1.949 1.962 1.925 1.952 1.939 1.919 1.923 1.937 1.929 1.927 1.908 1.927 2.000

5.971 5.977 5.953 5.926 5.914 5.963 5.951 5.899 5.924 5.851 5.905 5.879 5.840 5.856 5.875 5.859 5.855 5.819 5.855 5.974

0.971 0.944 1.024 1.038 1.044 1.016 1.025 1.053 1.039 1.080 1.050 1.064 1.086 1.082 1.056 1.075 1.077 1.098 1.077 0.974

BaNb2O6 orthorhombic BaNb2O0 orthorhombic Tetragonal tungsten bronze Tetragonal tungsten bronze Tetragonal tungsten bronze Tetragonal tungsten bronze Tetragonal tungsten bronze Tetragonal tungsten bronze Tetragonal tungsten bronze Tetragonal tungsten bronze Tetragonal tungsten bronze Tetragonal tungsten bronze Tetragonal tungsten bronze Tetragonal tungsten bronze Tetragonal tungsten bronze Tetragonal tungsten bronze Tetragonal tungsten bronze Tetragonal tungsten bronze SrNb~Oo* SrNbiO6*

-

2O6~

Single crystals of Sr

—

Structure unknown, but different from BaNb2O6 and the tungsten bronze.

313

THE GROWTH AND PROPERTIES OF STRONTIUM BARIUM METANIOBATE 100

80

60

I

I

I

I

I

-

-

II-.

Z 40 Ui

-

u Ui

20

-

0 _______________________________________________________ 0 I 2 3 4 5 6 MICRONS

Fig. 2.

7

Optical transmission of Sr

0.75Ba0.25Nb2O0.

_____________________________________________ I I I 06

0.5

—

COMPOSITIONS

A Sr075 Ba0.25 Nb2 06 B Sr050 880.50 Nb2 06

-

Sr0~25880.75 Nb2 06 MPLES O026~ THICK

anomally occurs for the particular composition being studied indicating the change from a polar to a nonpolar state. Attempts at measuring the spontaneous polarization have been unsuccessful due to a lack of complete saturation of the specimen. Tentative results, however, indicate a polarization in excess of 30 pC-cm2 for the composition Sr 0 75Ba025Nb2O6. A plot of the dielectric change as a function oftemperature for several compositions of Sr1_~Ba~Nb2O6 is shown in fig. 3. No anomality of the dielectric constant was observed in samples of SrNb2O6 or BaNb2O6 from liquid nitro7), but disagrees gen to about 600 °C. This is in agreement with the results of Isupov’ 8) and 1Francombe 9) that BaNb with a report by Coates 2O6 is ferroelectric with a Curie temperature of approximately 70 °C.Coates does report rather special firing techniques to obtain the ceramic used in his were studynecessary and perhaps this could explain discs the difference in properties he obtains.

0

I

32

::

I

Fig, for 280- 4 shows the approximate • FRANCOMBE phase boundaries I

~

-

0 THIS WORK 0;

I

240-

__________________________________________

-

oI 0

______________________________________________________

LIQ.N2

Fig. 3.

26

60 130 CURIE TEMPERATURE (°c)

199

Capacitance as a function of temperature for Sr1..~Ba~Nb2Oe.

4). The optical at 25 °C using CuKIX at 1.540562 A’ character is uniaxial negative, and the indices of refraction at 6328 A are n 0 = 2.31byand ‘~e = 2.30. A density 3 was obtained pycnometric techniques, of 5.4 g-cm and the hardness on the mho scale is about 5.5. The optical transparency shown in fig. 2 extends from about 0.4 pm to 6.0 pm. Piezoelectricity has been detected by a positive response when the crystals are placed in a Giebe—Scheibe circuit’ 5), Electromechanical couplings in the order of 30% have been obtained by resonance techniques’6). Ferroelectric hysteresis loops have been observed by

o

._.200-

Z

~Uiw 0. ~

ieo

.

(J,

-

III I I

a. I

-

z Z 0

-

40

-

4 -J 0.

z z

•.•...•..• °

60

/ /‘

.

-

/ FERROELECTRIC TETRAGONAL TUNGSTEN

-

BRONZE PHASE

0

I 20

0

40

I

60

80

tOO

SrNb 2O6

using a Sawyer—Tower circuit’ 7), The hysteresis ioops disappear at the temperature where the large dielectric

z

-)

120

.0

NON-POLAR BIREFRINGENT PHASE

.0

~

0~_ CU

I

0

w ~

Fig. 4.

MOLE PERCENT

BaNb2O6

BaNb2O6

Phase boundary and Curie temperature versus composition for Sr1_~Ba~NbiOo.

314

A. A. BALLMAN AND H. BROWN

the system and the Curie temperature dependence upon composition. The data shown here agree reasonably well with Francombe’s7) earlier work, although the abrupt discontinuity in the Curie temperature versus composition slope is less noticeable in the present study. Francombe’s work shows an abrupt change in slope, and this change is supported by lattice parameter data which show a similar change over the same composition range. The measurement of the eiectro-optic coefficients are currently under study and will be reported in detail in a separate publication20). Tentative results show a particularly large linear electro-optic effect which ranges from about 40 V to 400 V for a 1:1 aspect ratio at 6328 A. The low voltage for half wave retardation (—~40 V) occurs at the high strontium concentration and increases as the barium concentration is increased. The similarities in the compound reported here (Sr, _~Ba~Nb 2O6) and many of those reported by previous investigators is quite apparent. It is of interest to note that the tetragonal tungsten bronze structure can be obtained by several methods an present AB206 study corn7) andinthe pound. substitution Francombe’sinwork employ the “A” site to arrive at a formula of (Ba24Sr24)> ,Nb 6) and Fang9) 206. site Goodman used substitution in the “B” to obtain (Ba2 ~) (Nb5 4Ti4~) 8) employed no 2O6. Ridgleytheand Wardof niobium ion to substitution but reduced valence obtain Sr> ,(Nb5 4Nb44) 206. The abovecompounds all show the same X-ray powder patterns typical of a tetragonal tungsten bronze. The data suggest that the primary requisite for the formation of the bronze structure is that the niobium ion be reduced. Ridgley and Ward’s work show this quite and Fang a plus four 4 ~)simply, ion to Goodman replace a part of theemploy plus five niobium (Ti (Nb5 ~). The current work and the work of Francombe indicate that the mixture of Sr2 + and Ba2 + causes a partial reduction of the niobium, and the formula probably is more accurately written as (Sr2 + Ba2 )>~ (Nb5~Nb4~) 2O6.

4. Conclusion The tungsten bronze structure* appears to be a particularly favorable host in which ionic substitutions and solid solutions may be used to alter ferroelectric properties. A wide range of Curie temperatures, indices of refraction, densities, and other physical properties, can be obtained by the introduction of specific ions. The availability of a wide variation in properties can be advantageously used to produce a crystal with a very specific requirement for nonlinear optic or elec~ ITo-optic use. References 1) G. M. Grotenhuis and A. G. Barkow, I. Am. Ceram. Soc.Durst, 33 (1950) 133. 2) A. Reisman and E. Banks, J. Am. Chem. Soc. 80(1958)1877. 3) A. Reisman, S. Treibwasser and F. Holtzberg, J. Am. Chem. Soc. 77 (1955) 4228. ~ ~ Treibwasser, Phys. Rev. 114 (1959) 63. 5) E. C. Subbarao, J. Am. Ceram. Soc. 42 (1959) 448. 6) G. Goodman, J. Am. Ceram. Soc. 43 (1960) 105. 7) M. H. Fang, Francombe, Acta. Cryst. 131. 9) P. H. W.R.S.Ward, Brower, R. 13 S. (1960) RothSoc. and Marzullo, 8) D. Ridgley and I. Am. Chem. 77S. (1955) 6232. Bull. Am. Phys. Soc. [2] 4 (1959) 64. 10) V. A. Isupov and V. I. Kosyakov, Zh. Tekh. Fiz. 28 (1958) 2175. 11) S. M. Vincent and J. E. Kessler, private communication. 12) A. A. Ballman, H. J. Levinstein, C. D. Capio and H. Brown, to J. Am.J. Ceram. Soc. (1967). 13) submitted S. C. Abrahams, L. Bernstein and P. Jamieson, to be published. 14) R. L. Barns, private communication. 15) and A. Scheibe, Z. Physik 33 (1925) 760. 16) E. A. Giebe Warner, private communication. 17) C. B. Sawyer and C. H. Tower, Phys. Rev. 35 (1930) 269. 18) V. A. Isupov, lzv. Akad. Nauk SSSR Ser. Fiz. 21 (1957) 402. 19) V. Lenzo, Coates and H.Spencer F. Kay, and Phil.A.Mag. 3 (1956) submitted 1449. 20) R. P. V. E. G. A. Ballman, to Appl. Phys. Letters. 21) F. Jona and G. Shirane, Ferroelectric Crystals (Pergamon London, and New York, 1962). 22) Press, L. Mandelcorn, Ed.,MacMillan, Non-Stoichiomerric Compounds (Academic Press, New York, 1964).

*

A good survey of materials of this type appears in refs. 21

and 22.