Method of producing plate single-crystal of gadolinium molybdate
United States Patent 1191 Akiy ama et a1.  
 1451 Apr. 29, 1975 METHOD OF PRODUCING PLATE SINGLE-CRYSTAL OF GADOLINIUM MOLYBDATE [75] lnventors: Seikichi Akiyama, Kokubunji;  
 Hirotsugu Kozuka, Tokyo, both of Japan [73] Assignee: Hitachi, Ltd., Tokyo, Japan [22] Filed: July 26, 1972 211 App]. No.: 275,163  
 [58] Field of Search..... 23/301 R, 301 SP; 423/263, 423/21; 148/16; 252/462 [56] References Cited UNITED STATES PATENTS 3.146.137 8/1964 Williams l48/l.6 3,194,691 7/1965 Dikhoff 23/301 SP 3,437,432 4/1969 Borchardt.... 423/263 3.621213 11/1971 .lcn t 23/301 SP 3,775.066 11/1973 Yumoto 23/301 SP OTHER PUBLICATIONS Preparation of Single Crystals, Lawson Butterworth Scientific Pub. 1958, London, pp. 1013.  
 Primary ExaminerA. Louis Monacell Assistant ExaminerHiram H. Bernstein Attorney, Agent, or FirmCraig &amp; Antonelli [57] ABSTRACT A method of producing a thick plate single-crystal of gadolinium molybdate having smooth (001) faces by the Czochralski method in which a (110) face of a rotating seed crystal is brought into contact with a melt of crystal material and the crystal is pulled under the following temperature conditions:  
 The temperature gradient at the surface of the melt from the center thereof in the radius direction: +1 to +5C/mm The temperature gradient from the surface of the melt to 10 mm above the surface of the melt: 5 to lOC/mm The temperature gradient from 10 mm above the surface of the melt in an upward direction: 1 to -2C/mm The thickness of the plate crystal is determined by the rotational velocity, i.e., the number of revolutions per minute.  
 13 Claims, 1 Drawing Figure REVOLUTIONS PER MINUTE METHOD OF PRODUCING PLATE SINGLE-CRYSTAL OF GADOLINIUM MOLYBDATE BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method of producing a thickness controllable plate single-crystal of gadolinium molybdate. 4  
 2. Description of the Prior Art It is known that a single-crystal of gadolinium molybdate has a very excellent characteristics as an optical material because it is a ferroelectric ferroelastic and light-transmissive material. Thus, a gadolinium molybdate single-crystal has peculiar properties and hence is an excellent new material usable. for various appli ances. However, it has also a disadvantage that it is difficult to produce a good quality single-crystal. In particular, when a gadolinium molybdate single-crystal is employed as an electro-optic material, it is required that it has smooth crystal faces and, at the same time, a uniform thickness. However, to date satisfactory crystals which meet all of these requirements have not been produced by any known method.  
 SUMMARY OF THE INVENTION An object of the present invention is to provide a method of producing a thick plate single-crystal of gadolinium molybdate having smooth crystal faces and, at the same time, a uniform thickness.  
  Another object of the present invention is to provide a method of producing a plate single-crystal of gadolinium molybdate which can control the thickness of the plate crystal.  
  The present inventors have found that in a wellknown pulling or Czochralski method of growing single-crystals of gadolinium molybdate, the growth state of crystals depends to a great extent on the selected crystal plane of a seed crystal which is in contact with a melt, and that a preferable crystal plane for preparing good quality thick plate crystals is a l plane. However, smooth crystal faces cannot be provided by the usual pulling method even if a (l 10) crystal plane of a seed crystal is put into contact with a melt. Therefore, the present inventors have further found that the temperature distribution at the surface of the melt as well as the temperature distribution from the surface of the melt in the direction of crystal pulling to the pulled crystal plays an important role in preparing smooth crystal faces.  
  The present invention is based on these findings. The present invention provides a method of growing a single-crystal of gadolinium molybdate by pulling a rotating gadolinium molybdate seed crystal brought into contact with a melt consisting of 1 mol. of gadolinium oxide and 3 mol. of molybdenum oxide characterized in that the crystallographic plane of the seed crystal in contact with the melt is a l 10) plane, that the temperature distribution at the surface of the melt is such a one as having a positive temperature gradient of lto 5C/mm in the radial direction from the center of the melt toward the wall of the crucible in which the melt is contained, and that the temperature distribution from the surface of the melt upwards in the pulling direction perpendicularly to the surface of the melt is such a one as having a negative temperature gradient of 5 to l0/mm in the range from the surface of the melt to a height of 10 mm therefrom and a negative temperature gradient of 1 to 2 C/mm from 10 mm up.  
  By this inventive method a plate gadolinium molybdate single-crystal having a uniform thickness can be prepared. The growth rate of a gadolinium molybdate crystal is greater at its crystal plane than at its (001) crystal plane which is perpendicular to the I I0) plane, and hence the (001) face becomes a fairly smooth face. The present invention takes the advantage of this property to prepare a thick plate singlecrystal of a uniform thickness. The thickness of this crystal can be varied to a certain extent by varying the rotational velocity of the seed crystal at the time of the crystal pulling. For example, the thickness is about 21 mm at 60 rpm and about 10 mm at rpm. The reason why the crystallographic plane of the seed crystal in contact with the melt is selected to be (110) in the present invention is to develop the (001) face of the growing crystal. Further, the reason why the temperature distribution at the surface of the melt and that from the surface of the melt to the upward direction are determined as above is that these temperature conditions enable a crystal to grow keeping its thickness constant. In more detail, the reason why the temperature gradient at the surface of the melt from the center of the surface in the radial direction to the wall of the crucible is taken to be +l to +5C/mm is that when the temperature gradient is less than lC/mm, it is difficult to grow a smooth (001) face, and when the temperature gradient is greater than 5C/mm, the thickness of the plate crystal is insufficient.&#39;The reason why the temperature gradient from the surface of the melt up to 10 mm in the upward direction is selected to be 5 to 10C/mm (absolute value) is that when the temperature gradient is less than 5C/mm, the thickness of the plate crystal is insufficient, while when it is greater than 10C/mm. the development of the (001) face is suppressed to degrade the smoothness thereof. Further. the reason why the temperature gradient from 10 mm above the surface of the melt in the upward direction is selected to be 1 to 2C/mm is that outside this range the thickness of the plate crystal is stepped or graded in the growth direction and uniform thickness is not ensured.  
  In the present invention the pulling velocity of the rotating seed crystal is within a range of l to 50 mm/hr. and preferably and practically 6 to 10 mm/hr.  
  According to the present invention, once the temperature distribution is set in the initial stage of crystal pulling, no temperature variation is necessary during the crystal growth in contrast to the prior art method of crystal growth in which complicated temperature control was necessary to produce a crystal of a desired geometry.  
 BRIEF DESCRIPTION OF THE DRAWING The sole FIGURE is a characteristic of the thickness of a plate single-crystal of gadolinium molybdate produced versus the rotational velocity or the number of revolutions per minute of a seed crystal according to the method of the present invention.  
 DESCRIPTION OF THE PREFERRED EMBODIMENTS A powder mixture of 1 mol. of gadolinium oxide and 3 mol. of molybdenum oxide was formed, after being further shattered and mixed, into plate chips and then was fired at 700C for 4 hours in air. The fired chips were again shattered, mixed up, and formed into plate chips which were fired at 1.000C for about 4 hours in air. The thus obtained fired material was put in a platinum crucible and heated to 1,170C to melt in air. Then, a seed crystal of gadolinium molybdate having (1 10) orientation was cut in a wedge form and fixed to a platinum support. A (110) face of the seed crystal was then brought into contact with the melt at the center portion ofits surface, and gradually pulled (at about 8 mm/hr.) while being rotated at 60 r.p.m. to produce a thick plate single-crystal. The temperature gradient at the surface of the melt at 10 mm from the center of the surface in the radial direction was set at 3.5C/mm, the temperature gradient in the range from the surface of the melt to 10 mm above the surface in the vertical direction was 10C/mm, and that in the range from 10 mm above the surface in an upward direction was l.lC/mm. The produced flat plate single-crystal of gadolinium molybdate was 30 mm wide, 21 mm thick and 50 mm long.  
  According to this experiment, it was found that the relation between the thickness of the produced plate crystal and the rotational velocity of the seed crystal at the pulling operation was as shown in FIG. 1, from which it can be understood that the thickness of the plate crystal can be varied as desired by varying the rotational velocity. That is, when the rotational velocity of the seed crystal is varied between 50 and 180 r.p.m., the thickness of the plate crystal varies between 24 and 8 mm.  
  Though the thickness of the plate crystal increases with the reduction of the rotational velocity. it is difficult to produce a plate single-crystal when the rotational velocity is lower than 30 r.p.m. because the development of the (001) face is suppressed. Consequently. in order to produce a plate single-crystal it is necessary for the rotational velocity to be at least 30 r.p.m., and practically at least 50 r.p.m. On the other hand, the upper limit of the rotational velocity of the seed crystal in the method according to the present invention is 180 r.p.m. because at above 180 r.p.m. the temperature control at and around the surface of the melt is difficult.  
 The preferable length of the seed crystal was 3 to 15 By varying the temperature gradient at C/mm. 10 mms distance from the center of the surface of the melt in the crucible in the radial direction over a range of0.5 to 6C/mm, it was found that at a range of 1 to 5C/mm thick plate single-crystals having excellent smooth surfaces could be produced similarly to the case of the temperature gradient 3.5C/mm/ However, when the temperature gradient was both 0.5C/mm and 6C/mm. the thickness of the plate crystal was not uniform.  
  Also from experiments made by varying the temperature gradient over an extension of from the surface of the melt to 10 mm therefrom in an upward direction between 4 and l 1C/mm, it was found that substantially the same results as in the case of the abovementioned temperature gradient -lC/mm were provided at a range of to lOC/mm, but at both 4C/mm and l 1C/mm the thickness of the plate crystal was not uniform.  
  As a result of varying the temperature gradient at mm from the surface of the melt and above between O.5C/mm and 3C/mm, it was found that substantially the same results as in the case of the abovementioned temperature gradient l.lC/mm were obtained within the range of1 to 2C/mm, but at both O.5C/mm and 3C/mm the thickness of the plate crystal was not uniform.  
  Generally, it is necessary that the control of the temperature distribution in the upward direction is performed from the surface of the melt up to the distance at least 1.3 times the length of the grown crystal (the length of the crystal protruding from the surface of the melt), for example, when the length of the grown crystal is 15 mm, the range of the temperature control is up to 20 mm from the surface of the melt. The range of temperature control may be extended to a higher position, but temperature control up to the height of two times the length of the grown crystal is practical from the point of view of the installation.  
 We claim:  
  1. A method for producing a plate single-crystal of gadolinium molybdate comprising forming a melt mixture of l mol. of gadolinium oxide and 3 mol. of molybdenum oxide, heating the melt so as to establish a temperature gradient at the surface of the melt in a radial direction from the center of the surface of a positive 1 to 5C/mm, a temperature gradient from the surface of the melt in an upward direction up to 10 mm from the surface ofa negative 5 to 10C,/mm and a temperature gradient from 10 mm above the surface in an upward direction ofa negative 1 to 2C/mm, and pulling a seed crystal of gadolinium molybdate rotating at about 30 to 180 r.p.m. and whose (1 l0) face is in contact with the surface of the melt from the melt without changing the heating conditions of the melt.  
  2. A method of producing a plate single-crystal of gadolinium molybdate according to claim 1, in which the pulling velocity is 1 to 50 mm/hr.  
  3. A method of producing a plate single-crystal of gadolinium molybdate according to claim 1, in which the height of the zone above the melt surface in which the temperature gradient is controlled as specified in claim 1 is at least 1.3 times the length of the grown crystal.  
  4. A method of producing a plate single-crystal of gadolinium molybdate according to claim 1, in which the length of the seed crystal is 3 to 15 mm.  
  5. A method for producing a plate single-crystal of gadolinium molybdate according to claim 1, wherein said seed crystal is pulled from said melt in a direction perpendicular to the surface of said melt.  
  6. A method of producing a plate single-crystal of gadolinium molybdate according to claim 5, wherein the pulling velocity is about 1 to 50 mm/hr.  
  7. A method of producing a plate single-crystal of gadolinium molybdate according to claim 6, wherein the pull velocity is about 6 to 10 mm/hr.  
  8. A method of producing a plate single-crystal of gadolinium molybdate according to claim 1, wherein the pulling velocity is about 8 mm/hr, wherein the rotational velocity of the seed crystal is about 60 rpm, and wherein heating is accomplished so that the temperature gradient at the surface of the melt at about 10 mm from the center of the surface in the radial direction is about 3.5C/mm, the temperature gradient from the surface of the melt to 10 mm above the surface in a vertical direction is about lOC/mm, and the temperature gradient in the range from 10 mm above the surface of the melt in a vertical direction is about l.lC/mm.  
  9. A method of producing a plate single-crystal of gadolinium molybdate according to claim 1, wherein the temperature gradient above the surface of the melt is maintained up to a distance of at least 1.3 times the length of the grown crystal measured from the melt surface.  
  10. A method of producing a plate single-crystal of gadolinium molybdate according to claim 2, in which the rotational velocity of the seed crystal is about 50 to 180 rpm.  
  11. A method for producing a plate single-crystal of gadolinium molybdate comprising pulling a rotating seed crystal of gadolinium molybdate from a melt mixture of one mole of gadolinium oxide and three moles of molybdenum oxide, said seed crystal having its (110) face in contact with the melt, said seed crystal being pulled from the melt in the direction parallel to the 110 crystallographic direction of the seed crystal, said melt having been heated in such a way that the temperature gradient at the surface of the melt in a radial direction from the center of the surface is positive 1 to 5C/mm, the temperature gradient from the surface of the melt in an upward direction up to mm from the surface of the melt is negative 5 to 10C/mm and the temperature gradient from 10 mm above the surface of the melt in an upward direction is negative 1 to 2C/mm,  
 whereby a plate single-crystal is produced having a thickness of at least about 8 mm and a pair of smooth and uniformly spaced apart parallel faces.  
  12. A method of producing a plate single-crystal of gadolinium molybdate according to claim 11, in which the rotational velocity of the seed crystal is 30 to rpm, and the pulling velocity is l to 50 mm/hr.  
  13. A method for producing a plate single-crystal of gadolinium molybdate having a predetermined thickness between about 8 and 21 mm comprising forming a melt mixture of one mol. of gadolinium oxide and 3 mol. of molybdenum oxide, heating the melt so as to establish a temperature gradient at the surface of the melt in a radial direction from the center of the surface of a positive 1 to 5C/mm, a temperature gradient from the surface of the melt in an upward direction up to 10 mm from the surface of a negative 5 to 10C/mm and a temperature gradient from 10 mm above the surface in an upward direction of a negative 1 to 2C/mm, and pulling a seed crystal of gadolinium molybdate rotating at about 30 to 180 rpm and whose (1 l0) face is in contact with the surface of the melt from the melt without changing the heating conditions of the melt, the rotational speed of the seed crystal corresponding to the desired thickness of the gadolinium molybdate single crystal whereby a gadolinium molybdate single crystal having said predetermined thickness is formed.