Icosahedral boron-rich solids are refractory materials composed of twelve-atom boron-rich icosahedral units with strong intericosahedral linkages. These distinctive structures admit unusual electronic and thermal transport properties. Here the distinctive (three-center) bonding which underlies these materials is first described. Then it is shown how insulators, semiconductors and highly degenerate (metal-like) materials emerge from the same basic structure with appropriate substitutions.

The electronic transport of the boron carbides is then addressed. The boron carbides are degenerate p-type semiconductors in which the charge carriers are diamagnetically aligned pairs of electrons which hop between icosahedra. Uniquely, this thermally activated hopping conductivity increases with increasing hydrostatic pressure. However, the Seebeck coefficient (thermoelectric power) is uncharacteristic of a degenerate semiconductor. Namely, the Seebeck coefficient is typically both large and an increasing function of temperature. In addition, despite the hardness and refractory character of these materials, their thermal conductivities can be surprisingly low with a glass-like temperature dependence. These features are manifestations of the distinctive structure and bonding of these solids. In fact, this novel mix of properties makes the boron carbides exceptionally good very-high-temperature p-type thermoelectric materials.

Icosahedral boron-rich solids have additional potential as high temperature semiconductors. In particular, the wide-gap icosahedral boronrich pnictides, B12P2 and B12As2, may be doped to form wide-gap refractory semiconductors. For example, replacement of the group V element with either a group VI or a group IV element is expected to yield n-type and ptype material, respectively.

Boron-rich borides represent a class of unique semiconductors. They combine the physico-chemical properties of refractory crystals with the electrical, optical, thermal properties typical for amorphous semiconductors [ 1–3 ].

Boron carbides exhibit an anomalously large Seebeck coefficient with a temperature coefficient that is characteristic of polaronic hopping between inequivalent sites. The inequivalence in the sites is associated with disorder in the solid. The temperature dependence of the Seebeck coefficient for materials prepared by different techniques provides insight into the nature of the disorder.

Measurements of the electrical conductivity, Seebeck coefficient and Hall mobility from -300 K to -1300 K have been carried out on multiphase hotpressed samples of the nominal composition B6Si. In all samples the conductivity and the p-type Seebeck coefficient both increase smoothly with increasing temperature. By themselves, these facts suggest small-polaronic hopping between inequivalent sites. The measured Hall mobilities are always low, but vary in sign. A possible explanation is offered for this anomalous behavior.

Boron carbides, B1-xCx with 0.085 ≤ × ≤ 0.200, generally contain both B12 and B11C icosahedra. However, the electronic transport with 0.1 ≤ × ≤ 0.2 is believed to occur by means of bipolaron hopping between only B11 C icosahedra [i]. We have calculated the changes in energy, atomic positlions and charge distribution when a pair of electrons is added to the isoelectronic icosahedral clusters B12 and (B11C)+. We simulate an icosahedron in a neutral lattice by bonding the icosahedral atoms to hydrogenic atoms which we constrain to be neutral. The computations are performed with a self-consistent molecular-orbital method, PRDDO.

We find a total energy reduction of -3.7 eV for two electrons added to a B12 icosahedron. Of this, -2.7 eV arises from the electrons filling the icosahedron's bonding orbitals. The remaining -1.0 eV comes from the contraction of the icosahedron's radius by -0.09 Å. For two electrons added to a (B11C)+ icosahedron we find a total energy reduction of -18.2 eV. Of this, -16.5 eV arises from filling the icosahedron's bonding orbitals. The remainder arises frop a -0.09 Å contraction of the icosahedron's radius. Thus, we find (B11C) icosahedra to be strongly energetically favored over B12 icosahedra as bipolaron sites.

The positive charge associated with a (B11C)+ icosahedron is distributed over the eleven boron atoms. Concomitantly, we find the added two electrons of the bipolaron to be distributed over all twelve sites of the B11C icosahedron. We find the energy difference between an electron pair added to B12 and (B11C)+ icosahedra to arise principally from the increased Coulomic attraction provided by the extra positive charge of the (B11C) icosahedron.

I have calculated the electronic structure of cubic B12 and B12 P2 using the linear augmented plane wave method. These calculations are selfconsistent and include calculation of the total energy. Within the strict muffin tin approximation (spheres centered only on atomic positionsconstant potential otherwise) these systems are unbound with respect to the atomic constituents. Using interstitial spheres, up to 77% of the unit cell can easily be filled to yield much more reasonable total energies. Although the improvement in the total energy is significant upon including the interstitial spheres, the band structure and densities of states are not significantly changed. Similarly, the real atomic charge distributions (B, P) are little changed, while the interstitial distribution does change to reflect the additional freedom provided by the interstitial spheres. Results including the rhombohedral and icosohedral distortions in both systems are discussed. Gaps are found to open up in the alpha rhombohedral boron and in the B12 P2 systems as are observed experimentally.

We have considered a series of conformations of the neutral complex [Mg+B12H12] including those in which the Mg is internal to the icosahedral B12 cage. Self-consistent molecular-orbital calculations to determine the total cluster energy have been carried out as the Mg atom is moved from the center of the borane to a position 10Å from the center (outside the borane cage). Two energy minima are found as a function of position. One is outside the borane cage. at 3.0–3.5Å from the center; the second is at the center of the borane cage. The central minimum is the deeper of the two if Mg 3d orbitals are included in the basis set.

Many atoms have a low ionization energy and are, as ions, of appropriate “size” to fit inside a B12 cage. Incorporation of such ions into B12 cages in solids may offer significant flexibility in the development of refractory semiconductors and thermoelectrics.

We compare electron spin resonance (ESR) spectra of B4C samples made by three methods. The samples are 1) two ceramics made by hot-pressing boron and carbon powders, 2) a polycrystal made by carbothermic reduction, and 3) a single crystal grown from a palladium metal melt. All samples show remarkably similar spectra between 2 and 100 K. In particular, all samples show a single Lorentzian absorption, a linewidth that decreases with increasing temperature, and an inverse temperature dependence of the integrated intensity. The integrated intensity of the paramagnetic spin signal corresponds to a density of 2×1019/cm2 localized spins. Having a single crystal sample enables us to meaningfully measure the angular dependence of the ESR linewidth. This angular dependence is consistent with the paramagetic centers being unpaired electrons centered on the central carbon atoms of positively charged C-C-C intericosahedral chains. These chains appear to replace 0.5 % of the positively charged C-B-C intericosahedral chains which occur in B4C.

Amorphous powders of boron isotopes have been studied in the temperature range 100–450 K using EPR method. It was shown that paramagnetic centers with g=2.0029 ± 0.0001 are the holes captured on trapping levels. The centers represent volume defects. On the basis of temperature dependent measurements of the concentration of paramagnetic centers and their linewidth an energetical parameter of trapping level equal to 0.11eV over valence band was defined. An observed strong narrowing of the EPR line is connected with the increase of the frequency of delocalization of electrons with temperature. The structure of powders was studied by electron microscope method. Experimental results confirm the existence of icosahedral arrangement in amorphous boron.

The specific heats of β-B and of B4C have been reviewed, and measurements of a different sample of B4C have been carried out below 80 K. A specific heat anomaly observed previously in boron carbides has been shown to be extrinsic in origin. The thermal conductivity of B1−x Cx, for x < 0.20, between 0.2 and 2000 K has also been reviewed, and recent measurements have been added. The magnitude and temperature dependence of the conductivity are somewhat similar to what is expected for amorphous boron, except for the characteristic plateau which is not clearly discernible. A possible explanation for the strong phonon scattering is discussed.

The thermal conductivity, k, of boron carbides of various B/C ratios, two modes of preparation – hot pressed and carbothermic, and two isotopic variants of boron – 11B and normal boron 10.81B, was measured from 300 to 1023 K. The density and composition of the samples were reflected in the magnitude and temperature dependence of k, and were investigated further with scanning electron microscopy, Rutherford backscattering spectroscopy, and Raman spectroscopy. While lower than theoretical density in B4C reduces k, the characteristic monotonic decline of k with increasing temperature is retained. This k-vs.-T behavior distinguishes B4C from material with larger B/C ratios for which the temperature dependence is essentially nil.

Large grain polycrystalline boron carbides have a high-temperature thermal conductivity which changes from being characteristic of a crystal to being glass-like as the carbon content is reduced from its maximal value. We relate this phenomenon, to compositional changes within the three-atom intericosahedral chains. With a reduction of the carbon concentration from its maximal concentration (20 %), a carbon atom within some of the three-atom (CBC) intericosahedral chains is replaced by a boron atom, thereby producing CBB chains. We estimate that the CBB chains are significantly softer than the CBC chains. Thus, with this reduction of carbon content the intericosahedral chains are inhomogeneously softened. This suppresses the coherent transport of heat through the chains. The remaining thermal transport occurs incoherently through vibrationally inequivalent structural units, i.e. “phonon hopping.”

The thermal conductivity of boron carbides at high temperatures presents a fundamental challenge to theory. One of the striking features is that, while in B4C it displays “normal” behaviour in that it decreases with increasing temperature, in B9C it is nearly temperature-independent. We address this feature through a model calculation which assumes the heat current to be due to carrier phonons whose frequency is modulated by lower-frequency phonons which “dress” the carrier phonons through strong interaction. Preliminary calculations show satisfactory but partial agreement with experiment for reasonable parameters and delineate areas for further investigation of the vibrational properties of boron carbides.

Boron carbides contain carbon atoms as substituents for boron in icosahedra. The B-C interaction is different in icosahedra from that in other geometric forms. Para-carborane, P-C2B10H12, offers an excellent system for the study of structure and interactions in boron-rich, carbon-containing icosahedra; its structure is well-characterized experimentally, and the infrared and Raman spectra have been observed and are relatively simple. Here we present an analysis of p-carborane by classical force field methods supplemented by quantum mechanical calculations. Complexity in the model cluster is introduced step-by-step beginning with B12 (Ih symmetry). The principal interaction constants extracted through interpretation of p-C2B10H12 spectra are kCB −2.0 × 105 dyne/cm, kBB (intrapentagon) −1.3 × 105 dyne/cm, and kBB (interpentagon) −1.55 × 105 dyne/cm.

Boron-rich refractory solids based on the rhombohedral structure of α-B exhibit electrical properties that range from a hopping-type semiconductor (boron carbide) to wide bandgap room temperature insulators (the boron pnictides B6P and B6As). As such, they are of interest for a variety of high temperature semiconductor applications. Preparation techniques for these unusual materials are reviewed, and new results on the crystal growth of boron carbides and B6As and on thin film deposition of B6P are presented.

Boron carbides with low C contents were produced from BC13 and CCl4 by chemical vapor deposition at temperatures of 1020–1500°C. The deposits were found to be between B13C2 and B13 C (8.3 to 13.3 at %C) in composition by means of Auger electron spectroscopy. Raman spectroscopy demonstrates that the specimens contain very little free carbon. Further, disordering of C in the chains and icosahedra of the rhombohedral boron carbide structure for C contents of less than 20 at.% is indicated by the Raman results. Singlephase rhombohedral boron carbides with C contents as low as 12.5 at.% (≈B7C) have been produced. The most B-rich specimens, containing 8.3 at.% (B11 C) or less C, were phase separated. At least one of the phases in the phase separated specimens is tetragonal. The electrical conductivity and Hall mobility of the B11C specimen are reported.

LaB6 powders of a particle size less than 2 μm can readily ge sintered to nearly pore free microstructures, when boron and carbon are used as additives. The microstructure consists of LaB6 B4C and C as seperate, finely divided phases. The preparatton of starting powders, the sintering process, and the resulting microstructures are discussed.

Detailed analytical electron microscope (AEM) studies of yellow whiskers produced by chemical vapor deposition (CVD)1 show that two basic types of whiskers are produced at low temperatures (between 1200°C and 1400°C) and low boron to carbon gas ratios. Both whisker types show planar microstructures such as twin planes and stacking faults oriented parallel to, or at a rhombohedral angle to, the growth direction. For both whisker types, the presence of droplet-like terminations containing both Si and Ni indicate that the growth process during CVD is via a vapor-liquid-solid (VLS) mechanism.

High resolution TEM images of boron carbide (B13C2) have been recorded and compared with images calculated using the multislice method as implemented by M. A. O'Keefe in the SHRLI programs. Images calculated for the [010] zone, using machine parameters for the JEOL 2000FX AEM operating at 200 keV, indicate that for the structure model of Will et al., the optimum defocus image can be interpreted such that white spots correspond to B12 icosahedra for thin specimens and to low density channels through the structure adjacent to the direct inter-icosahedral bonds for specimens of intermediate thickness (-40 > t > -100 nm). With this information, and from the symmetry observed in the TEM images, it is likely that the (101) twin plane passes through the center of icosahedron located at the origin. This model was tested using the method of periodic continuation. Resulting images compare favorably with experimental images, thus supporting the structural model. The introduction of a (101) twin plane through the origin creates distortions to the icosahedral linkages as well as to the intra-icosahedral bonding. This increases the inequivalence of adjacent icosahedral sites along the twin plane, and thereby increases the likelihood of bipolaron hopping.

The nature of interaction of point and linear defects in semiconductor boron doped with zirconium (∼ 1.5% at.) has been studied using electron microscope and internal friction methods. It was shown that doping with zirconium promoted the multiplication of polysynthetic twins and stacking faults. A computer simulated analysis of diffraction patterns was performed in terms of trigonal presentation. At the frequency of free torsional vibrations of ∼ 1Hz a high level of internal friction with maxima at 250, 300–320 and 380–420°C was revealed. It is supposed that the maxima are respectively due to: the twin-boundary motion accompanied by breaking of intericosahedral bonds (a); the process of ordering-disordering of atoms in the impurity atmospheres under the continuous change of the temperature and elastic fields of moving twinning dislocations in the {100} system (b); the impurity controlled twin-boundary motion in the {511} system.