ISSN 0020-1685, Inorganic Materials, 2009, Vol. 45, No. 7, pp. 754–757. © Pleiades Publishing, Ltd., 2009. Original Russian Text © S.S. Ordan’yan, D.D. Nesmelov, S.V. Vikhman, 2009, published in Neorganicheskie Materialy, 2009, Vol. 45, No. 7, pp. 816–819. 754 INTRODUCTION Rare-earth borides offer a valuable combination of properties (melting point on the order of 3000 K, hard- ness of 26 GPa, metallic behavior of conductivity, the ability to absorb thermal neutrons and others), of which the thermionic activity at a relatively low work function (e.g., ϕ = 2.7 eV for LaB 6 ) is of particular interest. The above performance parameters suggest that LaB 6 -based materials are potentially attractive for a variety of appli- cations. Given that there are natural limitations on the properties of single-phase materials, research effort should be concentrated primarily on composites, in par- ticular, those that can be fabricated by ceramic process- ing techniques. Analysis of all known systems containing LnB 6 in equilibrium with other refractory compounds of tech- nological interest has shown that only a limited number of compounds may coexist with LnB 6 . These include refractory M IV–VI B 2 borides, B 4 C, and SiC. Previous studies were concerned with the T–x phase diagrams of the systems LaB 6 –å d B 2 [1], B 4 C–å d B 2 [2], and SiC – å d B 2 [3] and also with the LaB 6 –B 4 C [4] and LaB 6 –SiC [5] joins in the corresponding ternary and quaternary systems. Given that there is also information about the structure of the eutectic system B 4 C–SiC, it can be seen that there are many systems, such as LaB 6 –B 4 C–SiC, LaB 6 –B 4 C–å d B 2 , LaB 6 –SiC–å d B 2 , and LaB 6 –B 4 C– SiC–å d B 2 , for designing ceramics with various combi- nations of controlled structure-sensitive properties. An important point is that all of these systems have eutectic phase relations, which offers not only a number of tech- nological advantages in producing ceramics (model eutectics) by sintering [6] but also special properties, in particular, a reduced work function, typical of eutectic systems [7]. In this paper, we report the T–x phase diagram of the LaB 6 –W 2 B 5 system. Tungsten boride is of interest as a component of structural ceramics because it is nonreac- tive (forms eutectics) with B 4 C [8] and SiC [9]. In addi- tion, as shown previously [10, 11] W 2 B 5 is nonreactive with å d B 2 , and the W 2 B 5 –å d B 2 systems have eutectic phase relations. EXPERIMENTAL AND RESULTS In our preparations, we used pure-grade LaB 6 (Donetsk Plant of Chemical Reagents). W 2 B 5 was syn- thesized from high-purity tungsten powder and amor- phous boron at 1850°ë in pure-grade argon and was identified by X-ray diffraction (XRD). Only those sam- ples were used in further syntheses which contained no impurities according to XRD results (Fig. 1) and whose unit-cell parameters were close to reference values (‡ = b = 0.2983 nm, Ò = 1.3872 nm). Examination of pol- ished sections of melted W 2 B 5 (after arc melting) revealed no elemental boron inclusions. The LaB 6 and W 2 B 5 powders were ground in a vibra- tory mill with titanium diboride balls. Owing to the much greater hardness of titanium diboride (34.5 GPa) compared to LaB 6 and W 2 B 5 powders, contamination of the powder with TiB 2 during milling was insignifi- cant. Mixtures of powders 2.5 to 7.0 μm in particle size were prepared by vibration milling over the entire com- position range (table). The mixtures were pressed into prismatic samples, which were then sintered at 1900°C for 2 h in argon and used to determine the t m of the alloys. XRD examination showed that all of the sintered samples were two-phase, with no variations in the lat- tice parameters of the constituent phases, attesting to insignificant mutual solubility of the end-members. The samples were melted either by Joule heating or in tubular graphite heaters. In the former instance, the samples had a hole, and the beginning of melting was accompanied by sharp changes in its radiance, which was used to determine the melting point of the material. In this procedure, all of the samples ruptured at their Phase Relations in the LaB 6 –W 2 B 5 System S. S. Ordan’yan, D. D. Nesmelov, and S. V. Vikhman St. Petersburg State Technological Institute (Technical University), Moskovskii pr. 26, St. Petersburg, 190013 Russia e-mail: vihser@yandex.ru Received May 19, 2008; in final form, November 10, 2008 Abstract—The LaB 6 –W 2 B 5 join in the ternary system La–B–W is shown to have a eutectic phase diagram with t e = 2220°C and a eutectic composition of 30 mol % LaB 6 + 70 mol % W 2 B 5 . Data are presented on LaB 6 -con- taining systems potentially attractive for designing mixed-phase ceramics. DOI: 10.1134/S0020168509070097