US3428438A - Potassium tantalate niobate crystal growth from a melt - Google Patents

Description

CRYSTAh This improvement relates to a discovery of an improved method for producing improved large, mixed K'RN (KTa Nb O potassium tantalate niobate single crystals wherein l X 0, and the product obtained thereby. More particularly, the improvement concerns an improved method of synthesis of large, homogeneous single KTN crystals of improved electrical parameters from selected compositions of K(Ta,Nb)O preferably within the range of KTa Nb O and having controlled properties relating to composition, size, Curie temperature, dielectric constant, and optical clarity, and the products thereof.

In general, it is known that KTN is a mixed crystal of two perovskites, KTaO and KnbO which may be combined in almost any proportion by using known reducing temperature crystal forming technique, to grow crystals of small size and varying composition, dependent upon the changing solution concentration.

As illustrated in Triebwasser Patent No. 2,954,300, a phase equilibrium diagram for the system KNbO KTaQ' is also known to the art and used with reference to the general knowledge of crystal growth from cooling and slow evaporation of a saturated solution applying the Kyropoulos technique. Yet the surprising fact remains that the art is dependent upon non-homogeneous KTN crystals of small size and having a large measure of optical interference. Thus, reducing their value in electrooptical devices.

Accordingly, it is an object of this disclosure to provide an improved method of synthesizing crystals of the KTN type (KTa Nb O and On the order of KTa Nb O or X is on the order of .30.70, having a crystal size on the order of 30 x 20 x mm. to 40 x 40 x 30 mm. and weighing on the order of 34 grams to 142 grams.

Another object of this disclosure is to provide the art with the discovery of an improved and steady state method of synthesizing a greater and improved and more consistent yield of useful optical quality homogeneous single large crystals of KTN(KNbO KTaO and particularly KTa Nb O per size of melt, having a Curie temperature from 52 C. to +16 C., i3 C., a crystal size in excess of 30 x 20 x 5 mm., a crystal weight up to and about 142 grams of improved optical quality of reduced veiling and controlled electrical properties, and the product obtained thereby.

Further objects and advantages will be apparent from the following description in relationship to the accompanying drawing wherein:

FIG. 1 is illustrative of using the linear function of Curie temperature relative to crystal composition as herein contemplated; and

FIG. 2 is illustrative of selective melt compositions used to demonstrate optimum conditions to obtain optimum results as herein provided.

Additional objects and advantages will be recognized from the following description wherein the examples are given for purposes of illustration. To the, accomplishment of the foregoing and related ends, this invention then comprises the features hereinafter more fully described and inherent therein, and as particularly pointed out in the claims. Such illustrative embodiments are indicative of ted States meat the various ways in which the principle of our discovery, invention or improvements may be employed.

The method herein and herewith provided can be used for more consistently growing large homogeneous single KTN crystals of a size up to and about 142' grams of the mixed composition type of KTa Nb O w,here 1 X 0 and X is preferably on the order of .60 to .70. In general, the method provides for more uniform fKTN crystal growth at a constant temperature utilizing a steady state condition to yield large homogeneous crystals of improved high optical quality useful in electro-optical modulation of electromagnetic radiation. .1

With reference to FIG. 1, the composition applicable to the desired Curie temperature is determined by using the linear function of Curie temperaturen versus molar composition of KTa Nb O To select the composition of the melt 1 from which to produce the crystal, reference ismade to the phase equilibrium diagram of FIG. 2. For example, a selected composition being KTa Nb O as evidenced by the composition line dc, extends upwardly until it intersects the solidus curve 0.; An isotherm line is drawn from c which intersects the liquidus curve at a. This represents the composition of the liquid in equilibrium with the solid of composition e. The choice of this melt composition lies along the line ac. Thereby, it will be evident that two factors are used to determine the optimum composition of the melt: (1), the ratio of solid in equilibrium with liquid represented on the diagram by the length ratio of abzbc must be l to obtain the largest yield from the method; (2) since the melt will contain a solid phase in equilibrium with a liquid phase, the final composition must be selected after experimental trial to realize how much solid in the melt will interfere physically with ionic movement and with the growing crystal.

Illustrative of the method herein provided, a powdered composition of KTa Nb O in stoichiometric proportion, was mixed and added to a platinum crucible of 200 cc. capacity with excess K CO to the extent that amount of K 0 excess was 15 mol percent compared to stoichiometry. Whereas K CO is shown to work alone, it will be recognized that a small amount of additional material as SnO or the like, may be added to aid in stabilizing the pentavalent state of niobium, if desired. Further, the rate of crystal growth appears to be controlled by the amount of excess K 0 in the melt and where economically feasible, permitting slower growth by longer soak time, a reduced amount of such agent is permissible with even less lamellae and minimum optical interference. Initially, the mixture is raised to a melt temperature T above the liquidus curve, forming a complete liquid phase to ensure complete mixing of the materials and enable the addition of more crystal forming material mixed in powder form, in proportions as described previously, to raise the level of the melt and filling the crucible, or filling the crucible to a predetermined level. The mixture is then placed (or the mixture affected) in a furnace zone of constant temperature and maintained at T or in liquid form, preferably with a flow of oxygen maintained past the melt, about the crucible, or from the base and through the furnace during crystal growth. The temperature is then lowered to T (growth temperature) or melt and a small crystal seed of KTN composition, strung on a platinum wire, is immersed approximately 5 mm. below the surface of the melt maintained at a uniform melt tem perature, at T above 1200 C., in this instance. The seed is rotated slowly at a rate of about 8 rpm. with the melt temperature held constant or in a steady state within 1 C. of the melt temperature. Rotation of the crucible 1 Melt as used herein throughout the application refers to an equilibrium mixture of liquid and solid for a given charge composition at a given temperature.

may be affected, if desired. However, under the constant state condition, as herein prmided, the usually undesirable vertical thermal gradient is of advantage as normal convection currents circulate material from the bottom of the melt to the top where crystal growth occurs. The transfer of material occurs, not only through thermal gradient phenomena but homogeneity at the growth stage is maintainedQby ionic movement caused by concentration gradients in the melt, making rotation of the melt container unnecessary.

square in cross section and had clearl designated simple cube faces.

By considering the phase equilibrium diagram in relationship to the Curie temperature and growing crystals, by the above process, large crystals were grown in composition range on the order of from KTa Nb O to KTa Nb O the Curie temperature varied within :3? C. within a range of +16 C. to -52 C., and havingother advantageous properties, evidenced in the following table testing 5 mm. cubes cut therefrom,

TABLE I Crystal Dimensions, Weight Resistivity Avg. Retardation, composition mm. (g.) m-crn. Tei3 C. voltage KTaMNbMO; 30 x 30 x 5 34 5X10 -19 V:=2,810 V41=3,970 Ve=4,870 30 x 30 x 5 34 5X10" 45 Vh=3,500 30 x 26 X 7. 43 -52 35 x 35 x 101 9X10 +10 Vaf=700 KT8 so Nb.an7O3 40 X 40 X 142 8X10 9 +15 V =700 KTfijflNbjHOQ 30 x x 12 46 5X10 11 1 V;,=1,400 KTa wNb,4oO3 45 x 45 x 10 129 2X10 5 +16 Vn=700 In lowering from T to T the T temperature is passed at which solid c' is in equilibrium with liquid b. Crystals may precipitate out in a range of composition following the solidus curve from c" to c. However, this is not an equilibrium condition and the crystals react with the melt providing new material for equilibrium of seed crystal growth. Crystal growth continues until either of two con ditions exist: 1) equilibrium is reached and growth stops or (2) the level of the liquid in the crucible falls too low. Before the level falls too low and/or just about when the growth stops, as in conditions (1) and (2), the grown crystal is withdrawn to a position slightly above the melt and cooled down, as along the line cd of FIG. 2, at the rate of approximately 15 to 20 C. per hours, to room temperature.

In the above process, no cooling of the melt is per mitted during crystal growth for the production of high quality, homogeneous crystals of the type KTa Nb O as herein described. As will be recognized, the proper size and selection of an optimum melt composition is empirical relative to the crystal desired per melt batch. However, by proper selection of a melt composition in the range indicated, a large homogeneous single crystal can be grown under the steady state condition, as described.

As an alternative use of the above process, the growth of KTN crystals can be affected by use of Nb O Ta O and K CO mixed in stoichiometric proportion with 15 mol percent excess K 0 (using K CO The mixture, in powder form was used to fill a 200 cm. platinum crucible and melted down to a liquid state. While maintained in the melt state, as described, a seed crystal of KTaO (KTN) was suspended on a platinum wire strung through an aperture therein and the KTN crystal grown as in the above process. The desired growth temperature was determined by calibrating with the Curie temperature. That is, the growth temperature is maintained substantially constant to :1 C. in the range of over and above 1100 C., or at melt temperature, with the seed remaining in the melt mix for a period of from about four to five days. When growth' stops or the melt mix falls too low so as to change crystal composition, the grown crystal is lifted out of the melt and cooled to room temperature. By this method crystals with dimensions up to 40 X 40 x 30 mm. and weighing up to 142 grams have been grown in a period of four days time.

By the above method of crystal growth, homogeneous crystals of high optical quality with dimensions of from 30 x 30 x 5 mm. to 40 x 40 x 30 mm. and weighing as much as 142 grams were grown in periods of four to five days. Using an (001) seed, the crystals grew generally By comparison with known standard crystal material, the size differential provided by the herein described progess is discovered to afford a greater yield of optically usable material per run with the advantage of improved optical quality, thus providing a greater selectivity of more useful optical crystals per melt. That is, it has been discovered that the method provided herein consistently yields larger crystals of more homogeneity of composition with improved optical quality, electrical parameters and wide band gap.

Having described the present embodiments of our improvement is in the art in accordance with the patent statutes, it will be apparent that some modifications and variations may be made without departing from the spirit and scope thereof. The specific embodiments described are given by way of examples illustrative of our discovery. invention or improvement which is to be limited only by the terms of the appended claims.

What is claimed is:

1. The method of growing a homogeneous KTN crystal weighing onthe order of from 34 grams to 142 grams of a size ranging on the order of from 30 x 30 x 5 to 40 x 40 x 30 mm., having a Curie temperature within a range of +16 C. to 52 C., :3" C., and high optical quality comprising the steps:

(1). preparing a crystal forming liquid mixture of KTa Nb O and 1 X 0 in the presence of K 0 at crystal melting temperature;

(2 reducing the temperature of said mixture and maintaining the said mixture at a substantially constant temperature of equilibrium mixture of liquid and solid;

(3) suspending a seed crystal of KTN composition about 5 mm. below the surface of said mixture;

(4) maintaining the said mixture at a substantially constant temperature throughout crystal growth and with the seed crystal suspended therein; 1

(5) effecting the defined crystal growth for a period of up to about four to five days;

(6) with drawing the grown crystal from the said mixture and cooling said crystal to room temperature.

2. The method of claim 1 wherein the selected composition of the solid portion of the melt KTa Nb O is on the order of KTa Nb O and the K 0 is in stoichiometric excess up to about 15 3. The method of claim 1 wherein the selected composition of KTa Nb O of the melt mix is on the order of KTa Nb O and the K 0 is in stoichiometric excess up to about 15%.

4. The method of claim 1 wherein the selected com- 5 position of KTa Nh O of the melt mix is on the order of KTa Nb O and the K 0 is in stoichiometric ex cess up to about 15 5. The method of claim 1 wherein the selected com position of KTa Nb O of the melt mix is onthe order Of KTa.3o -1o and Nb.30 "7oo3 and the K30 is StoiChiometric excess up to about 15%,

6. The process of claim 1 including the step of maintaining a flow of oxygen past said mixture.

7. The method of claim 1 including the step of slowly rotating the seed crystal'in said liquid mix during the formation of said crystal.

8. The method of claim 1 includingthe step of providing a flow of oxygen past said liquid mix during crystal growth,

References Cited UNITED STATES PATENTS 2,758,008 8/1956 Reisman et a1. um- 23 21 3,065,046 11/1962 FOOS 23 51 5 3,092,448 6/1963 Kennedy 23 -15 3,129,061 4/1964 Dermatis et a1. 23-501 3,244,488 4/1966 Linares et a1. 23-301 3,346,344 10/1967 Levinstein et a1. 23- 301 10 3,366,510 1/1968 Daendliker M2 23-s1 NORMAN YUDKOFF, Primary Examiner,

U.S.C1.X.R.

Claims (1)

1. THE METHOD OF GROWING A HOMOGENEOUS KTN CRYSTAL WEIGHING ON THE ORDER OF FROM 34 GRAMS TO 142 GRAMS OF A SIZE RANGING ON THE ORDER OF FROM 30 X 30 X 5 MM. TO 40 X 40 X 30 MM., HAVING A CURIE TEMPERATURE WITHIN A RANGE OF +16* C. TO -52* C., $3*C., AND HIGH OPTICAL QUALITY COMPRISING THE STEPS: (1) PREPARING A CRYSTAL FORMING LIQUID MIXTURE OF KTAXNB1-XO3 AND 1>X>0 IN THE PRESENCE OF K2O AT CRYSTAL MELTING TEMPERATURE; (2) REDUCING THE TEMPERATURE OF SAID MIXTURE AND MAINTAINING THE SAID MIXTURE AT A SUBSTANTIALLY CONSTANT TEMPERATURE OF EQUILIBRIUM MIXTURE OF LIQUID AND SOLID; (3) SUSPENDING A SEED CRYSTAL OF KTN COMPOSITION ABOUT 5 MM. BELOW THE SURFACE OF SAID MIXTURE; (4) MAINTAINING THE SAID MIXTURE AT A SUBSTANTIALLY CONSTANT TEMPERATURE THROUGHOUT CRYSTAL GROWTH AND WITH THE SEED CRYSTAL SUSPENDED THEREIN; (5) EFFECTING THE DEFINED CRYSTAL GROWTH FOR A PERIOD OF ABOUT FOUR TO FIVE DAYS; (6) WITH DRAWING THE GROWN CRYSTAL FROM THE SAID MIXTURE AND COOLING SAID CRYSTAL TO ROOM TEMPERATURE.

Images