3,472,621 United States Patent Office Patented Oct. 14, 1969 2 in yield does not justify the extra effort required to attain 3,472,621 such higher pressures. MOLYBDENUM DISELENDE HAVING A RHOM. As previously stated, the practice of this invention re BOHEDRAL CRYSTAL STRUCTURE AND METH OD OF PREPARING SAME quires a combination of both pressure and elevated tem Meyer S. Silverman, Norristown, Pa., assignor to Pennsalt peratures to convert the raw materials described above Chemicals Corporation, Philadelphia, Pa., a corpora into the new rhombohedral crystalline form of molyb tion of Pennsylvania denum diselenide. A temperature of at least about 800' No Drawing. Filed July 9, 1965, Ser. No. 470,907 C. is necessary, at least about 1600° C. being preferred. The portion of the term of the patent subsequent The practical upper temperature limit is about 2500 C. to May 28, 1985, has been disclaimed 10 The preferred range of operating temperatures is from Int. Cl. C01b 19/00; C01g 39/00 about 1600 C. to about 2000 C. The product MoSea U.S. Ct. 23-204 11 Claims will contain some hexagonal crystal form of the com pound in addition to the desired rhombohedral crystal form when the synthesis is conducted in the lower range 15 of temperatures, with an increase in temperature favoring ABSTRACT OF THE DISCLOSURE the yield of the rhombohedral form. It has been found Molybdenum diselenide having a rhombohedral crystal that at temperatures above about 1600 C., the product is structure and useful as a specialty lubricant is prepared essentially the rhombohedral crystal form. by Subjecting a mixture of elemental molybdenum and The relative amounts of the aforesaid raw materials in Selenium to the simultaneous effects of high heat and 20 the mixture subjected to the process of this invention are high pressure, i.e. a temperature of at least about 800 such as to provide an atomic ratio of molybdenum to C. and a pressure of at least about 10 kilobars. selenium in the charge generally within the range of about 1:2 to about 1:4. Although an excess of selenium greater than the 4 moles Se to one mole Mo is possible, 25 no advantage is gained, and if a purified product is desired This invention relates to a new form of molybdenum more unreacted selenium must be removed. The preferred diselenide and also to a method for preparing molyb atomic ratio of Mo to Se is within the range of about denum diselenide. More particularly, this invention con 1:2 to about 1:3, and most preferable is a ratio of 1:2.2. cerns a new compound of molybdenum and selenium hav The reaction period, which is not critical, may vary ing a rhombohedral crystal structure and the empirical 30 from about 1 minute to 24 hours, although the reaction formula MoSea, or expressed another way MoSea, and generally is completed in no more than about 5 minutes. a unique, high temperature, synthesis technique for pre Yields of from 75 to 100% of the rhombohedral crystal paring molybdenum diselenide. form of MoSea are obtained without difficulty in the Molybdenum diselenide having an hexagonal crystal practice of this process with reaction periods of from structure and a charcoal grey appearance is known. In 35 about 2 to 5 minutes. Unreacted selenium, which is quite accordance with this invention, it has been discovered volatile, may be removed from the product, if so desired, that the molybdenum diselenide having the rhombohedral by vacuum sublimation. crystal structure as described herein is prepared by sub The apparatus used in the illustrative examples that jecting, in the preferred embodiment, a mixture of ele follow is similar to that developed at the National Bureau mental molybdenum and elemental selenium to the com 40 of Standards and described in “Compact Multianvil bined action of extreme temperatures and pressures. The Wedge Type High Pressure Apparatus,” E. C. Lloyd, U.O. rhombohedral crystalline mloybdenum diselenide is a Hutton and D. P. Johnson in the Journal of Research of black, reflective, very soft, polycrystalline material. It has the National Bureau of Standards, vol. 63C, No. 1, July been discovered that heating the new rhombohedral form September, 1959, pages 59-64. In place of the 9A6' of MoSea under reduced pressures, e.g., at from about 45 tetrahedral sample holders used in the above reference, 900 to about 1000 C., and under pressures of less than %' holders with 4' anvil faces were employed in the about 25 mm. of Hg abs., converts the product into the examples which follow, and alternatively, 546' holders known, aforesaid, hexagonal crystal form of MoSea. were used with 34' anvil faces. A polyester film ("Mylar' In alternative embodiments of the process, the starting manufactured by Du Pont Company) was used between materials used can be compounds which are precursors 50 the anvil assemblies and the polytetrafluoroethylene sheet of the new molybdenum diselenide produced by the ap ("Teflon,” manufactured by Du Pont Company). Addi plication of the heat and pressure, i.e. those compounds tionally, a 0.003' wall boron nitride sleeve was used be which form MoSea in situ, for example, MoO--Se, tween the sample and the graphite heaters as electrical Mo--SeO2, MoC3--SeO2, and the hexagonal crystal form 55 insulation. Force was applied to the tetrahedral anvil sys of MoSe2. tem by a Watson-Stillman 100-ton hydraulic laboratory The pressure used to produce the molybdenum disele press. Pressure calibration was done by measuring the nide according to this invention, in simultaneous combina electrical resistance change of bismuth Samples. Pressure tion with heat acting on a mixture of molybdenum and was measured as a function of ram force and the three selenium, or a mixture of molybdenum and a precursor discontinuities were considered to occur at 25.4, 27.0 and of selenium, or a mixture of selenium and a precursor of 60 82 kilobars. In all of the preparations, a thin sleeve of molybdenum, or a mixture of precursors that yield molyb spectroscopic grade graphite was used as the heating ele denum diselenide in situ, is at least about 10 kilobars (one ment around the sample, and end plugs of the same ma kilobar equals 986.92 atmospheres), preferably at least terial isolated the sample from the platinum or silvered about 20 kilobars and more preferably at least about 40 65 tabs that carried the current from the anvils to the heat kilobars. The use of pressures in excess of 80 kilobars ing sleeve. Temperature calibrations were done by meas serves no practical purpose since the small improvement uring the electrical power input required to obtain tem 3,472,621 3 4. peratures which were indicated by a Chromel-Alumel TABLE I-Continued thermocouple, the tip of which was in good contract with the center of the graphite heating sleeve. The tempera Hexagonal, d, A.: Form tures reported here are thus the highest to which any 6.“ ------------------------------------ 78 part of the sample was subjected, and it should be recog 3.225 ----------------- - - - - - - - - - - - - - - - - - - 6 nized that the ends of the sample in each case were some 2.839 ----------------------------------- 63 what cooler. Experience in repeated calibrations indicates 2,788 ----------------------------------- 6 that the temperature values are uncertain by approximate 2.67 ----------------------------------- 7 ly 50 C., but the relative differences among the tem 2.369 ----------------------------------- 100 perature levels of the experiment are believed to be quite 2,151 ----------------------------------- 10 reliable. O 1914----------------------------------- 39 In each preparation the sample was first compressed in 1.641 ----------------------------------- 49 the high pressure apparatus, then heated, and held at the 1.615 ----------------------------------- 13 1591 ----------------------------------- 10 desired conditions for a measured period of time. The 1.464 ----------------------------------- 2 high pressure was then maintained until the power was 5 turned off and the sample had cooled to nearly ambient 1.422 ----------------------------------- 8 temperature. Cooling was very rapid in all cases. The 1.352 ---------------------------------- 13 product was removed as a compressed cylindrical pellet. 1.306 ----------------------- - - - - - - - - - - - - 6 1,246 ----------------------------------- 8 EXAMPLE 1. 1152 ----------------------------------- 33 20 A mixture of powdered molybdenum (99.8% purity, 'I' values are the relative intensity of the individual lines, relative to the strongest line in the pattern which is given the Fisher Scientific Company) and powdered selenium (tech Walue of 100. nical grade, 99-- percent purity, 200 mesh, Harshaw 'd' in Angstrom units is the interplanar distance between Chemical Corporation), the ratio of these raw materials lattice planes according to the Bragg Equation. in said mixture being one gram-mole of molybdenum to 25 It is well known that X-ray spectographs uniquely char 2.2 gram-moles of selenium, is charged to the above acterize crystalline structures, and therefore, the above described tetrahedral anvil apparatus. The mixture is com X-ray diffraction pattern for the new rhombohedral pressed to a pressure of 45 kilobars and then rapidly MoSe2 is a definitive description of the compound. heated to 1760 C. where it is held for 5 minutes. The A portion of the rhombohedral MoSea prepared above electrical power is switched off to quickly quench the 30 is heated to 1000 C. in a vacuum (25 mm. Hg) and reaction. After a cooling period of about 5 minutes, the held under these