Chemistry

G. V. SAMSONOV

PREPARATION AND PROPERTIES OF SCANDIUM DIBORIDE

(Presented by Academician I. I. Chernyaev on 7 IV 1960)

Numerous studies carried out in recent years have thoroughly investigated the compounds of the transition metals of the first period (titanium, vanadium, chromium, manganese, etc.) with boron, which are hard and refractory compounds finding ever broader use in various branches of technology ($^1$). The compound with boron of the first element of the first transition period—scandium—remains completely unstudied; its atom, with the electronic configuration $3s^2 3p^6 3d^1 4s^2$, has the least filled $3d$ shell, with one electron.

Fig. 1. Physical properties of Sc, Ti, V, Cr and their diborides. $\alpha_t \cdot 10^6$ ($\mathrm{MeB_2}$)—coefficient of thermal expansion of the diborides, $\frac{1}{\mathrm{deg}}$; $T_{\mathrm{m}}$ ($\mathrm{MeB_2}$)—melting temperature of the diborides, °C; $H_{\mathrm{m}}$ ($\mathrm{MeB_2}$)—microhardness of the diborides (under a load of 30 g), kg/mm$^2$; $\varepsilon_1(\mathrm{Me})$—first ionization potentials of the metal atoms, eV; $1/Nn$ (Me)—factor of the degree of incompleteness of the $d$ shell of the metal atom; $\dfrac{R_{\mathrm{MeB_2}}}{R_{\mathrm{Me}}}$—ratio of the values of the specific resistivities of the diborides and the corresponding metals.

Therefore it is of interest to compare the physical properties of scandium compounds with the compounds of other transition metals of the first period, in particular the physical properties of the compounds with boron.

Individual samples of scandium diboride $\mathrm{ScB_2}$ were obtained by us earlier; however, they were heavily contaminated with boron carbide, from which they were separated by specific gravity in heavy liquids. On the preparations obtained in this way, N. N. Zhuravlev and A. A. Stepanova ($^2$) determined the crystal structure of $\mathrm{ScB_2}$, which proved to be hexagonal (structural type $\mathrm{AlB_2}$), identical with the structures of the diborides of other transition metals of groups IV–VI of the periodic system, including the diborides of titanium, vanadium, and chromium. According to this work, the identity periods of $\mathrm{ScB_2}$ are: $a = 3.140 \pm 0.002$; $c = 3.510 \pm 0.002$ kX, $c/a = 1.118$, calculated from this ...

density \(\delta_x = 3.67\ \text{g/cm}^3\). In subsequent work, carried out by us jointly with B. M. Tsarev, G. A. Kudintseva, and V. S. Neshpor, an attempt was made to determine the principal parameters of thermionic emission of scandium diboride; however, on the basis of X-ray analysis data it was established that in the course of heating in vacuum \(\mathrm{ScB_2}\) loses part of the metal, transforming into scandium hexaboride, which has a cubic lattice of the \(\mathrm{CaB_6}\) type with period \(a = 4.355\ \text{kX}\). The electron work function for this compound was found to be 2.96 eV, and the constant \(A\) in the Richardson equation was \(4.6\ \text{A/cm}^2\cdot\text{deg}^2\); the secondary-emission coefficient was 0.58 and the radiation coefficient at \(1600^\circ\) was 0.6.

In the present work a systematic investigation was carried out of the conditions for obtaining scandium diboride by the reaction between \(\mathrm{Sc_2O_3}\) and boron in vacuum, with evolution of the volatile lower boron oxide of composition \(\mathrm{BO}\) or \(\mathrm{B_2O_2}\):

\[ \mathrm{Sc_2O_3 + 7B = 2ScB_2 + 3BO}. \]

It was thereby established that the maximum completeness of the reaction is attained at \(1800\)—\(1850^\circ\) and holding at this temperature for 1 h; the product thus obtained contains 32.6% boron, as compared with 32.5% boron in \(\mathrm{ScB_2}\) by calculation. The pycnometric density of the diboride powder, equal to \(3.65\ \text{g/cm}^3\), agrees well with the X-ray density.

To determine the physical properties of \(\mathrm{ScB_2}\), specimens were sintered from its powder by hot pressing in graphite molds without a specially created protective atmosphere at \(2000\)—\(2050^\circ\) for 7—10 min, under a load providing a pressure on the sintered powder of \(100\ \text{kg/cm}^2\). The microhardness of sintered \(\mathrm{ScB_2}\), measured under a load of 50 g, is \(1742 \pm 337\ \text{kg/mm}^2\); the average density of the hot-pressed specimens is \(3.56\ \text{g/cm}^3\); the specific electrical resistivity is \(7\)—\(15\ \mu\Omega\cdot\text{cm}\); the thermal coefficient of the thermo-e.m.f. is \(-7.7\ \mu\text{V/deg}\); the thermal expansion coefficient at \(20\)—\(800^\circ\) is \(9.48\cdot10^{-6}\); the radiation coefficient, measured in the range from 1035 to \(1770^\circ\) by the method of \((^3)\), proved to be practically unchanged and equal (at \(\lambda = 650\ \text{m}\mu\)) to 0.89. Finally, the melting point of \(\mathrm{ScB_2}\), determined by the method described in \((^4)\), is \(2250^\circ\). Table 1 gives a comparison of some physical properties of the diborides of scandium, titanium, vanadium, and chromium.

Table 1

These data show that the properties characterizing, to one degree or another, the strength of the crystal lattice pass through an extremum for titanium diboride, with a corresponding decrease in strength both toward scandium diboride and toward the diborides of vanadium and chromium. Since in the structures of diborides the boron atoms are bonded to one another by covalent bonds into flat nets, in which each boron atom is surrounded by three neighbors, the bond between the atomic complexes of boron and the metal atoms is effected chiefly through the latter, as is indicated by the clear correlation between the course of variation of the melting point,

microhardness, and coefficient of thermal expansion of diborides, on the one hand, and the first ionization potentials of the atoms of transition metals, on the other. This observation at the same time shows that the indicated properties are determined mainly by the state of the \(4s\)-electrons, while the \(d\)-electrons of the metal atoms take a much smaller part in bonding with the boron complexes, as is shown by the lack of correspondence between the variation of these properties and the variation of the incompleteness factor of the \(d\)-electron shells of transition metals. The latter, on the contrary, determine the degree of scattering of conduction electrons, as is shown by the agreement of the values \(1/Nn\), where \(n\) is the number of electrons in the \(d\)-shell and \(N\) is the principal quantum number of this shell [5], with values indicating an increase in electrical resistance upon the formation of borides relative to the metal. The value of the specific electrical resistivity of scandium is unknown; however, extrapolating to \(\mathrm{ScB_2}\) the ratio \(R_{\mathrm{MeB_2}} : R_{\mathrm{Me}}\) as a function of \(1/Nn\), we obtain \(R_{\mathrm{ScB_2}} : R_{\mathrm{Sc}} = 0.13\), whence the resistance of scandium should lie in the range from 55 to 115 \(\mu\Omega\cdot\text{cm}\). Thus, the variation of the resistance of diborides as a function of the scattering ability of the \(d\)-shells confirms the earlier views on this question set forth in [6]*.

Consequently, scandium diboride is a compound whose properties allow it to be placed in the series \(\mathrm{ScB_2—TiB_2—VB_2—CrB_2}\) and to be regarded as the end member of this series; moreover, the strength of the crystal lattice of \(\mathrm{ScB_2}\) is determined predominantly by the state of the \(s\)-electrons, while the electrical properties are determined by the state of the \(d\)-electrons, which also applies to the diborides of the other metals of the first transition period.

In conclusion, it should be noted that the melting temperature, hardness, and coefficients of thermal expansion of \(\mathrm{ScB_2}\) and \(\mathrm{CrB_2}\) are unusually close, which, together with the high electrical conductivity and, consequently, thermal conductivity of \(\mathrm{ScB_2}\), as well as its specific gravity being 35% lower than that of \(\mathrm{CrB_2}\), makes the use of \(\mathrm{ScB_2}\) promising in light heat-resistant alloys, in which chromium boride is currently used [7]. O. I. Shulishova took part in the experimental portion of the work.

Institute of Cermets and Special Alloys
Academy of Sciences of the Ukrainian SSR

Received
5 IV 1960

REFERENCES

1. G. V. Samsonov, Ya. S. Umanskii, Hard Compounds of Refractory Metals, Moscow, 1957.
2. N. N. Zhuravlev, A. A. Stepanova, Kristallografiya, 3, 83 (1958).
3. T. I. Serebryakova, Yu. B. Paderno, G. V. Samsonov, Optics and Spectroscopy, No. 3, 410 (1960).
4. G. V. Samsonov, E. V. Petrash, Metalloved. i obr. metallov, No. 4, 19 (1955).
5. G. V. Samsonov, DAN, 93, 689 (1953).
6. G. V. Samsonov, ZhTF, 26, 716 (1956).
7. K. I. Portnoi, G. V. Samsonov, Boride Alloys, Moscow, 1959.

* Using the data of this work, the specific electrical resistivity of scandium may be assumed to be \(\sim 90\ \mu\Omega\cdot\text{cm}\), whence the most probable value of the resistance of \(\mathrm{ScB_2}\) proves to be \(11.7\ \mu\Omega\cdot\text{cm}\).