3,385,667 United States Patent 0 "ice Patented May 28, 1968

1 2 3,385,667 given the designation ASTM 15-658. Calculated lattice MULYBDENUM DITELLURIDE AND HIGH parameters were (123.5182 A., and 0:13.9736 A., the TEMPERATURE, HIGH - PRESSURE SYN calculated density was 7.784 g./cc. THESES METHOD OF PREPARING SAME L. H. Brixner, Journal Inorganic Nuclear Chemistry, ieyer Shea Siiverman, Norristown, Pa., assigncr to Penn vol. 24, pp. 257—-65, (1962), prepared MoTe2 in single salt Chemicals Corporation, Philadelphia, Pa, a cor crystal form by means of transport reactions using bro poration of Pennsylvania mine as the transport agent. Crystallographic data was No Drawing. Filed Feb. 11, 1966, Ser. No. 526,702 taken on the basis of single crystal patterns using K a 4 Claims. (Cl. 23-204) radiation of Cu()t=1.5418 A). Lattice parameters were 10 (1:3.517 A. and 0:13.949 A., which constants were in ABSTRACT OF THE DISCLOSURE good agreement with those of Puotinen and Newnham, and Knop and MacDonald, supra. The MoTe2 exhibited A new form of molybdenum ditelluride having a rhom good semi-conducting properties having an electrical bohedral crystal structure is prepared by subjecting a mix resistivity measured at 25° C. and at —-196° C. of 8.5 ture of molybdenum and tellurium to a temperature of at and 1.34>< l03 ohm-cm, respectively. least about 1700° C. and a pressure of at least about 15 It is also reported by the US. Department of Com kilobars. merce, Ot?ce of Technical Service, Bulletin AD 265,121 (1961) that small single crystals of MoTe2 were obtained This invention relates to a new form of molybdenum by chemical transport reaction with bromine as the trans ditelluride and to a method of preparing molybdenum 20 port agent. The MoTez was sintered for 2 days at 600° telluride compounds. More particularly, this invention C. in vacuo and its density was then 6.85 g./ce. It was concerns a new compound of molybdenum and tellurium found to be thermally stable only up to 600° C. having a rhombohedral crystal structure and the empiri In accordance with the present invention, it has now cal formula M oTe2. The invention also concerns a unique, been discovered that molybdenum ditelluride having a high-temperature, higlrpressure, synthesis technique for rhombohedral crystal structure as described herein is pre preparing molybdenum telluride compounds. pared by subjecting, in the preferred embodiment, a mix Molybdenum ditelluride having a hexagonal crystal ture of elemental molybdenum and elemental tellurium structure and the appearance of grey platelets or powder to extreme temperatures, while simultaneously applying is known. André Morette, Compt. Rend, vol. 215, No. 3, high pressures thereto to avoid vaporization of the ele pp. 86-8, (1942), sealed the pure elements Mo and Te ments. In alternative embodiments of the process, the in vacuo and applied heat. Combination of the elements starting materials used can be compounds which are pre took place with incandesence at 450° C. with MoTeg be cursors of the new molybdenum ditelluride produced by ing formed at temperatures below 815° C. and Mo2Te3 the application of heat and pressure, i.e. those com formed above 815° C. The MoTez was‘ in the form of pounds which form the MoTe2 in situ, for example pulverulent, grey, hexagonal platelets, density d415=7.6 MoO3+Te, Mo-l-TeOz, MozTea-t-Te, and the above-de grams/cc. The compound oxidized when heated in air, scribed hexagonal erystal form of MoTe2. The rhornbo coloring the flame blue and emitting white clouds of hedral crystal form of MoTez prepared in accordance with telluriurn anhydride. Water wet it with diiliculty and had the ‘method of this invention is a charcoal-grey, highly re no effect thereon at ordinary temperatures. Hydrochloric ?ective, soft, polycrystalline material. acid and sulfuric acid, both cold and boiling, did not re 40 As previously stated, the practice of this invention re act with the MoTez, although it was attacked by dilute quires the application of an elevated temperature to con nitric acid and oxidized to TeO2 and M003. W. A. vert the raw materials described above into the new Morette, Ann. Chim. vol. 19, pp. 130-143, (1944), also rhombohedral crystalline form of molybdenum ditelluride. heated Mo with Te in an evacuated sealed tube, and then A temperature of at last about 1700° C. ‘is necessary, at removed the excess tellurium by distillation. The product least about 2000° C. being preferred. The practical upper Mo'l‘eg, d=7.6 grams/co, appeared as lamellar hexagonal temperature limit is about 2500° C. since no advantage is crystals and was stable between 580 and 815° C. achieved by going higher. The preferred range of operat D. Puotinen and R. E. Newnham, Acta Crystallo ing temperatures is from about 2000° C. to about 2200° graphica, vol. 14, pp. 6i9l_2, (196i), sealed stoichiomet C. The product MoTe2 will contain some hexagonal crys ric quantities of elemental Te and M0 in vacuo in car talline MoTe2 in addition to the desired rhombohedral bonized quartz ampules and reacted the materials at 1100° crystal form when the synthesis is conducted in the lower C. for several hours. The product MoTe2, further puri range of temperatures, with an increase in temperature ?ed by heat treatment in hydrogen at 850° C., had a favoring the yield of the rhombohedral form. It has been measured density of 7.81“ 0.02 g./cc. The crystal struc found that at temperatures above about 2100° C., the ture of the MoTez was determined from its X-ray powder product is essentially the rhombohedral crystal form. diffraction pattern using ?ltered CuK a radiation and The pressure used to maintain the feed materials (i.e., found to consist of a hexagonal unit cell with the lattice the mixture of molybdenum and tellurium, or mixture of constants: 11:3.519 A. and c=l3.964 A., with a calcu molybdenum and a precursor of tellurium, or mixture of lated speci?c gravity of 7.78 g./cc. telluriurn and a precursor of molybdenum, or mixture of O. Knop and R. D. MacDonald, Canadian Journal of 60 precursors that yield molybdenum ditelluride in situ) in Chemistry, vol. 39, pp. 897-904, (1961), sealed a stoi‘ a non-vapor state during the high temperature reaction is chiometric mixture of Mo and Te powders in an evacu at least about 15 kilobars (one kilobar equals 986.92 at ated quartz tube which was then held for 16 hours at mospheres), preferably at least about 25 kilobars and 460° C. The powder was removed, crushed, mixed thor more preferably at least about 45 kilobars. Pressures in oughly, rescaled and annealed at 600° C. for 48 hours. 65 the range of about 60 kilobars have been employed with The product MoTe2 was a loose, dark-grey powder of good results, however, no apparent advantage is gained by uniform appearance consisting of very ?ne needles and using pressures in excess of about 80 kilobars. platelets, density d425=7.681 g./ cc. X-ray powder diffrac The relative amounts of the aforesaid raw materials in tion patterns were taken with ?ltered CuK a radiation; all the mixture subjected to the process of this invention are lines could be indexed on a hexagonal unit cell. (This pat 70 such as to provide an atomic ratio of molybdenum to ' tern, which will be reproduced in part hereinafter, has tellurium in the charge generally within the range of been adopted as a standard for hexagonal Mo'l‘ez and about 1:2 to about 1:4. Although an excess of tellurium 3,385,667 3 4 greater than the 4 moles Te to one mole M0 is possible, the appearance of a ?ne, charcoal-grey powder. The no advantage is gained, and if a puri?ed product is de product is elementally analyzed with the results: Mo, sired more unreacted tellurium must be removed. The 27.7%, Te, 72.3%, atomic ratio of Te/M0=1.96. The preferred atomic ratio of Mo to Te is within the range of product is thus essentially of the empirical formula MoTeZ from about 1:2 to about 1:3, and most preferable is a which theoretically consists of Mo, 27.5%, Te, 72.5%, ratio of 1:21 to 1:2.5. atomic ratio of Te/Mo=2.0. The reaction period, which is not critical, may vary The X-ray powder diffraction pattern of the essentially from about 1 minute to 24 hours, although the reaction rhombohedral crystalline molybdenum ditelluride pre generally is completed in no more than about 5 minutes. pared above is determined using ?ltered CuKa radiation; Yields of from about 75% to 100% of the rhombohedral 10 the pattern is compared to the X-ray powder diffraction crystal form of MoTe2 are obtained without dif?culty pattern of molybdenum ditelluride having the hexagonal in the practice of this process with reaction periods of crystal structure as reported by Knop and MacDonald, from about 2 to 5 minutes. Unreacted tellurium, which supra. The comparison is set forth in Table I, below, is relatively volatile, may be removed from the product, where I values are the relative intensities of the individual if so desired, by vacuum sublimation. lines, relative to the strongest line in the pattern which The apparatus used in the illustrative examples that is given the value of 100 and “d” in angstrom units is follow is similar to that developed at the National Bureau the interplanar distance between lattice planes according of Standards and described in “Compact Multianvil Wedge to the “Bragg Equation.” The lines of the pattern of the Type High Pressure Apparatus,” E. C. Lloyd, U. 0. Hut new form of MoTez are indexed on a rhombohedral unit ton and D. P. Johnson, Journal of Research of the Na cell. tional Bureau of Standards, vol. 63C, No. 1, July-Septem TABLE I.—X-RAY POWDER DIFFRACTION PATTERNS OF ber 1959, pp. 59-64. In place of the V16" tetrahedral MOLYBDENUM DITELLURIDES sample holders used in the above reference, %" holders Hexagonal Form, Rhombohedral Form of with 1/2” anvil faces were employed in the examples ASTM 154658 This Invention which follow, and alternatively, 5716" holders were used with %" anvil faces. A polyester ?lm (“Mylar,” manu d I d I factured by DuPont Company) was used between the 6. 92 20 6. 0 75 (strong). 3. 47 14 3.02 25 (weak). anvil assemblies and the polytetra?uoroethylene sheet. 3. 04 4O 2. 83 100 (very strong). Additionally, a 0.003" wall boron nitride sleeve was 2. 547 100 2. 77 D0. 2. 056 35 2. 57 50 (medium) . used between the sample and the graphite heaters as 30 1. 840 30 l. 80 DO. electrical insulation. Force was applied to the tetrahedral 1. 757 40 1.74 D0. 1. 737 40 1. 555 25 (weak). anvil system by a Watson-Stillman 100-ton hydraulic 1. 445 16 1.235 20 laboratory press. Pressure calibration was done by mea 1. 116 14 suring the electrical resistance change of bismuth samples. Pressure was measured as a function of ram force and 35 Lattice Constants (in A.) the three discontinuities were considered to occur at Hexagonal MoTe; Rhombo‘uedral Mo’l‘ez 25.4, 27.0 and 82 kilobars. In all of the preparations, a a=3. 5182 {1:13. 70 thin sleeve of spectroscopic grade graphite was used as 0:13. 9736 c=20. 71 the heating element around the sample, and end plugs of the same material isolated the sample from the platinum It is well known that X-ray spectographs uniquely or silvered tabs that carried the current from the anvils characterize crystalline structure, and therefore, the to the heating sleeve. Temperature calibrations were done above X-ray diffraction pattern for the new rhombohedral by measuring the electrical power input required to ob MoTez is a de?nitive description of the compound. tain reaction temperatures which were indicated by a The electrical resistivity of the rhombohedral crystal Chromel-Alumel thermocouple, the tip of which was in line MoTe2, which is measured at ambient temperature good contact with the center of the graphite heating (about 25° C.) with a standard volt-ohmeter across two sleeve. The temperatures reported here are thus the highest separate axes of the cylindrical pellet as recovered from to which any part of the sample was subjected, and it the high pressure synthesis apparatus, is less than the elec should be recognized that the ends of the sample in each trical resistivity of the hexagonal crystalline form. The di case were somewhat cooler. Experience in repeated cali mension of said MoTe2 pellet are: radius=2 mm., brations indicates that the temperature values are uncer length: 1.5 mm. The electrical resistivity of the new com tain by approximately -l_-50° C., but the relative differ pound is an anisotropic value, i.e., 2>