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Graphite-like BC2 has yet to be synthesized; however, it stably exists in the Sc2B1.1C3.2 compound, where the boron atoms are arranged as far apart from each other as possible. Recently, a theoretical study on monolayer BC2 reported that in the most stable structure, B atoms are positioned adjacent to each other. We anticipated that graphite-like BC2 might take a different structure based on the electric charge. Therefore, we carried out first principles calculations to investigate whether this is true or not. The most stable structure among the six possible structures changed with the increase in the negative electric charge, which well explained both the previous results without contradiction. The most stable structure was also dependent on the pressure. The Li intercalation potential for BC2 was calculated to investigate its applicability as an anode for lithium-ion batteries. Our results revealed that Li atoms can be intercalated into BC2 to yield Li1.5BC2, whose gravimetric capacity is approximately 3.1 times higher than that of LiC6. However, the most stable structure with intercalated Li atoms became unstable when all the Li atoms were extracted. This feature may hinder the repetitive charge–discharge cycle of the anode and hence needs to be carefully considered.

The ternary borocarbide Sc2B1.1C3.2 is known to have a layered structure in which graphite-like BC2 layers and NaCl-like Sc2C layers are alternately stacked. Due to the complexity of the structure, X-ray analysis is unable to determine the precise structure of the BC2 layer, namely, how the boron atoms are arranged in the layer. Furthermore, the stacking manner of the two layers remains unclear. In this study, we have investigated the entire structure of Sc2B1.1C3.2 through first-principles calculations. The calculations reveal that the material is the most stable when B atoms are arranged to form 3 × 3 –R30° superlattices in the graphite-like layer. The interlayer bonding is mostly van der Waals-like, and the total energy is almost independent of the stacking method. The BC2 layer with the 3 × 3 –R30° structure can be regarded as an intermediate between similar graphite-like materials, BC and BC3. The electronic density of states of Sc2B1.1C3.2 exhibits metallic features, and electrons are transferred from Sc to B and C atoms, which compensates the lack of electrons in the BC2 layer. The origin of the undulation in the BC2 layer is not the inner electronic structure but the interaction between the BC2 and Sc2C layers. The weak localization found in the in-plane resistivity in experiments is explained by the domain boundaries of the 3 × 3 –R30° structure rather than the randomness of the arrangement of B atoms.

We have been investigating the high-temperature thermoelectric properties of some novel rare earth borides with a structure containing B12 icosahedra. Doping effects on the TE properties in such systems were investigated for the first time. A series of Nb-doped YB66 and C-doped YB66 single crystals were grown by the floating zone method. The Nb-doped compounds have approximate chemical formulas ranging from YNb0.30B66 to YNb0.33B66 while the C-doped compound has a formula of YB66C0.6. The effect of Nb-doping on the thermoelectric properties was not monotonic and appears to be complex. As a result of Nb-doping, the room temperature resistivity and the characteristic temperature T0 were considerably reduced. At room temperature the power factor of the Nb-doped YB66 sample with 89% site occupancy was three times greater than that of non-doped YB66. However, in the important high-temperature region, the non-doped sample actually exhibited the highest power factor for T > 550 K . Furthermore, owing to a structural feature of YB66, thermal conductivity actually increases with doping of transition metals. Taking into account all the thermoelectric properties, transition metal doping of YB66 is therefore not suitable for our purposes. On the other hand, doping of carbon, which is assumed not to go into the same sites as the transition metals, yielded a lowering of the thermal conductivity. Furthermore, contrary to Nb-doping, carbon doping did not result in a reversal of the relative magnitude of resistivity at extremely high temperatures and therefore, an increase in the figure of merit of factor 2 was realized at 1000 K.

The effect of transition metal doping in YB66 has been studied with respect to increase its thermal conductivity. Single crystals of YB66 doped with 4th, 5th and 6th group transition metals were grown by floating zone method. A detailed structure analysis suggested that an existing special B—B pair at the (1/4, 1/4, 1/4) site of YB66 could be attributed to a phonon scattering center that causes the amorphous-like low thermal conductivity. It was found that 5th and 6th group transition metals of V, Cr, Nb and Mo substitute the B—B pair. By contrast 4th group transition metals such as Ti and Zr were found to enter another (x, x, x) site with x ∼ 0.135. Thermal conductivity of both Nb-doped and undoped YB66 was measured in the range 4 K ≤ T ≤ 150 K. Nb doping increased thermal conductivity of YB66 with a factor of about 2.

Nb-doped YB 66 single crystals were grown by the floating-zone method using a 4 ellipsoidal mirror-type xenon lamp image furnace. Nb-doping is expected to improve the performance of YB 66 soft X-ray monochromators. Maximum site occupancy of Nb achieved was 95%. Full-width at half-maximum of the rocking curve measured for CuK α 10 0 0 reflection achieved 40–50 arcsec. Thermal conductivity of the crystal with the maximum site occupancy increased about two times that of undoped YB 66 crystals.

Crystal growth of rare-earth boron-rich solids in RE-B-C(-N) and RE-B-Si systems from Cu and Si solutions has been studied. The grown crystals are RE x B 22 C 3−δ N δ ( x =0.6–0.8, 0≤δ≤1), RE x B 28.5 C 4 (RE=Y, Ho, Er, Tm, Lu), REB 25 C 5 (RE=Y, Tb) and RE x B 12 Si ∼3 ( x =0.65–0.7, RE=Y,Gd–Lu). The first two phases are built with interconnected boron icosahedra and octahedra, which are regarded as homologous phases of B 4 C. REB 25 C 5 is a superstructure of YAlB 14 -type YB 25 C and RE x B 12 Si ∼3 is a new structure type. The crystal growth conditions and crystallography are described in this paper.

Crystals of Sc 2 AlB 6 were grown using scandium oxide and elemental boron as starting materials in a self-component aluminum solution under an argon atmosphere. The growth conditions for obtaining single crystals of relatively large size were investigated. Sc 2 AlB 6 single crystals were obtained in the form of prisms extending in the b-axis direction. The largest Sc 2 AlB 6 crystals prepared had maximum dimensions of about 0.4 x 0.4 x 4.2 mm 3 . The values of the Vickers microhardness and the electrical resistivity of Sc 2 AlB 6 crystals are 12.7 ± 0.8 GPa and 43 ± 8 μΩcm, respectively. The oxidation of Sc 2 AlB 6 crystals starts at about 773°C, and the weight gain after TG determination is 12.9 mass% at 1200°C. The oxidation products of Sc 2 AlB 6 crystals could not determined.

A novel quaternary scandium borocarbosilicide Sc3.67−xB41.4−y−zC0.67+zSi0.33−w was found. Single crystallites were obtained as an intergrowth phase in the float-zoned single crystal of Sc0.83−xB10.0−yC0.17+ySi0.083−z that has a face-centered cubic crystal structure. Single crystal structure analysis revealed that the compound has a hexagonal structure with lattice constants a = b = 1.43055(8) nm and c = 2.37477(13) nm and space group P 6 m2 (No. 187). The crystal composition calculated from the structure analysis for the crystal with x = 0.52, y = 1.42, z = 1.17, and w = 0.02 was ScB12.3C0.58Si0.10 and that agreed rather well with the composition of ScB11.5C0.61Si0.04 measured by EPMA. In the crystal structure that is a new structure type of boron-rich borides, there are 79 structurally independent atomic sites, 69 boron and/or carbon sites, two silicon sites and eight scandium sites. Boron and carbon form seven structurally independent B12 icosahedra, one B9 polyhedron, one B10 polyhedron, one irregularly shaped B16 polyhedron in which only 10.7 boron atoms are available because of partial occupancies and 10 bridging sites. All polyhedron units and bridging site atoms interconnect each other forming a three-dimensional boron framework structure. Sc atoms reside in the open spaces in the boron framework structure.

A novel ternary boron-rich scandium borocarbide Sc 4.5− x B 57− y + z C 3.5− z ( x =0.27, y =1.1, z =0.2) was found. Single crystals were obtained by the floating zone method by adding a small amount of Si. Single-crystal structure analysis revealed that the compound has an orthorhombic structure with lattice constants of a =1.73040(6), b =1.60738(6) and c =1.44829(6) nm and space group Pbam (No. 55). The crystal composition ScB 13.3 C 0.78 Si 0.008 calculated from the structure analysis agreed with the measured composition of ScB 12.9 C 0.72 Si 0.004 . The orthorhombic crystal structure is a new structure type of boron-rich borides and there are six structurally independent B 12 icosahedra I1—I6, one B 8 /B 9 polyhedron and nine bridging sites all which interconnect each other to form a three-dimensional boron framework. The main structural feature of the boron framework structure can be understood as a layer structure where two kinds of boron icosahedron network layer L1 and L2 stack each other along the c -axis. There are seven structurally independent Sc sites in the open spaces between the boron icosahedron network layers.

Single crystals of REMn 2 Si 2 (RE=Er, Tm, Yb, Lu) were grown from a high-temperature lead metal flux in an Ar atmosphere. The REMn 2 Si 2 crystals were plates enclosed by large {0 0 1} faces, and small {1 0 0} and {1 0 1} faces; maximum crystal size was 2.5×2.5×0.02 mm 3 . Chemical analyses of the crystals show the chemical composition with atomic ratios RE (RE=Er, Tm, Yb, Lu):Mn:Si=1:2:2. The values of the micro-Vickers hardness for the {0 0 1} faces of ErMn 2 Si 2 , TmMn 2 Si 2 , YbMn 2 Si 2 and LuMn 2 Si 2 are 5.0±0.4, 6.3±0.3, 7.9±0.3 and 6.1±0.4 GPa, respectively. According to thermogravimetric and differential thermal analyses, the oxidation of ErMn 2 Si 2 , TmMn 2 Si 2 , YbMn 2 Si 2 and LuMn 2 Si 2 crystals began to proceed at about 778, 978, 987, and 974 K, respectively. The results of magnetic susceptibility measurements down to low temperatures of the compounds are discussed.

Single crystals of a novel boron-rich scandium borocarbosilicide Sc0.83−xB10.0−yC0.17+ySi0.083−z were grown by the floating zone method using a four-xenon lamp mirror-type image furnace. Structure analysis using single-crystal X-ray diffraction data showed that Sc0.83−xB10.0−yC0.17+ySi0.083−z has a face-centered cubic with lattice constant a=2.03085 nm and space group of F 4 3m. The crystal composition calculated from the structure analysis was ScB12.0C0.65Si0.071 which agreed with the measured composition of ScB12.7C0.62Si0.08. In the crystal structure which is a new structure type of boron-rich borides, there are three structurally independent B12 icosahedra I(1), I(2) and I(3) and one B10 polyhedron which interconnect each other to form a three-dimensional boron framework. There are four fully occupied bridging sites, one is the C site and the other three sites are occupied statistically by B or C. The Sc atoms reside in three interstitial sites of the boron framework. Two Si sites are tetrahedrally surrounded by the bridging carbon sites and by the bridging boron/carbon mixed occupation sites, respectively. The boron framework structure can be understood more easily by introducing larger structure units, they are a super tetrahedra T(1) made up by four icosahedra I(1), a super tetrahedra T(2) made up by four icosahedra I(2) and a super octahedra O(1) made up by six icosahedra I(3).

Single crystals of a novel ScB19 family compound ScB19+xSiy were grown by the floating zone method using a four-lamp mirror-type image furnace. A small amount of silicon addition to ScB19 which decomposes at elevated temperatures without melting allowed it to coexist with the liquid phase and as a resultant made the floating zone crystal growth possible. Powder X-ray diffraction analysis confirmed the grown crystals of ScB19+xSiy to be isostructural to ScB19. It was found that the crystal structure of ScB19+xSiy solved based on single-crystal X-ray data is tetragonal with lattice constants of a, b=1.03081(2) nm, c=1.42589(3) nm, space group P41212 or P43212 and is basically isotypic with α-AlB12 structure type. In the crystal structure boron atoms form a three-dimensional framework based on interconnected B12 icosahedra and B22 polyhedra. The Sc atoms reside in three of five Al sites in the α-AlB12 structure and Si resides in a bridge site bonding two B22 units.

A new series of boron-rich solids ReB{sub 22}C{sub 2}N (Re: Y, Ho, Er, Tm, Lu) was synthesized by traditional solid-state reaction. The crystal structure of the representative compound YB{sub 22}C{sub 2}N was solved by direct method from powder X-ray diffraction (XRD) data and transmission electron microscope (TEM) analysis. The unit cell of the new structure is rhombohedral with space group R-3m (No. 166), lattice constant a = b = 5.623(0) {angstrom} and c = 44.785(3) {angstrom} with six formula units in one unit cell. The atoms of boron in the solids, like most of the boron-rich solids, exist with icosahedral and octahedral clusters, and the whole crystal shows a layered structure. The interconnected nine layers of icosahedron and three layers of octahedron in a unit cell build the whole framework of the new phase and rare earth metal atoms reside in voids of the octahedron layers. The neighboring icosahedral layers link through C-B-C chains besides the direct bonding of B-B. Both experimental and structural analysis indicated that the nitrogen atoms in the new phase can be replaced with carbon.

Single crystals of a new ternary boride Sc 2 AlB 6 were obtained from the Sc-Al-B system using high-temperature Al melts under an argon atmosphere. The growth conditions for large crystals were established. Sc 2 AlB 6 crystals were obtained in the form of needle-like rectangles extending along the direction of the b axis or in the form of thick platelets with large c planes. The crystals were examined by powder X-ray diffraction and chemical analyses, and the crystal structure of Sc 2 AlB 6 was investigated by single-crystal X-ray diffractometry. The crystal structure of Sc 2 AlB 6 is of the Y 2 ReB 6 type with the space group Pbam and the unit cell parameters are a = 0.8937(3) nm, b = 1.1226(3) nm, c = 0.3433(1) nm, V = 344.4(1) × 10 -3 nm 3 , Z = 4. The structure refinement converged at an R(F 2 ) value of 0.046 for 1545 reflections. The structural characteristics of Sc 2 AlB 6 are discussed.

In auxiliary metal fluxes (Si, Sn) at temperatures around 1650°C the crystal growth of “ScB 17 C 0.25 ” was successfully performed. While in the Si flux agglomerates of whisker-like crystals are formed, the change to Sn significantly improved the quality of the columnar crystals with respect to size and surface smoothness. Based on single-crystal X-ray data collected on a four-circle diffractometer using Mo Kα radiation and high-resolution transmission electron microscopy (HRTEM) the structure was solved by direct methods and refined to an reliability value R 1 of 0.055 for 661 F o >4 σ and 64 variables. The novel crystal structure belongs to the hexagonal system (space group P 6/ mmm ), with lattice constants a , b =14.5501(15) A and c =8.4543(16) A and is formed by a framework based on B 12 icosahedra which are radially connected to unusual “tubular” boron-based structural units. The Sc atoms reside on interstitial sites. In addition to the boron based clusters interstitial B atoms are found in the structure. The main structural features, which are governed by the arrangement of the boron clusters and the Sc atoms in the unit cell, could be unambiguously resolved by HRTEM, and simulated images match observed ones perfectly.

The 3 d (from Ti to Cu) and 4 d (from Zr to Mo) transition metals were doped to YB 66 . It was observed that the powder X-ray diffraction (XRD) intensities of their low-angle reflections like 200, 220, and 222 drastically changed from those of undoped YB 66 without changing face-centered cubic symmetry, except for Co- and Ni-doped YB 66 . On the other hand, the powder XRD patterns of Co- and Ni-doped YB 66 showed new peaks assigned to the 210, 300, 410, and 421 reflections and so on, indicating that the crystal lattice of Co- and Ni-doped YB 66 should be primitive. Single-crystal structure analysis of Cu-doped YB 62 showed that basic boron icosahedral framework is unchanged from that of undoped YB 66 . A new yttrium site at (0.25, 0.25, 0.25) and an interstitial Cu site at (0.144, 0.25, 0.25) were observed in addition to the original yttrium site at (0.0524, 0.25, 0.25). Single crystals of Cu-, Ni-, and Mo-doped YB 66 were grown by the floating zone method using a xenon lamp image furnace. Specific heats of Cu- and Ni-doped YB 66 were measured. A T linear contribution was observed in addition to the phonon T 3 term, suggesting that the amorphous-like phonon-related properties observed in undoped YB 66 were retained in the transition metal-doped YB 66 .

A detailed measurement of the specific heat of the B 12 icosahedral compound TbB 41 Si 1.2 was made. TbB 41 Si 1.2 crystals were grown by the floating zone method. Magnetic susceptibility measurements indicated that an antiferromagnetic-like transition occurs at 18 K in these crystals. TbB 50 and isostructural TbB 41 Si 1.2 are the first higher borides in which a magnetic transition was observed to occur. A hump in the specific heat is observed below 20 K, supporting the conclusion that a magnetic transition occurs in this system. An interesting magnetic field dependence was observed. Near the metamagnetic critical field of 31 kG, determined from magnetization measurements, the hump shifts to lower temperatures approaching 0 K. At 40 kG no hump is apparent. However, on further increasing the magnetic field to 55 and 70 kG, a new hump in the specific heat appears, indicating the possibility of the existence of an additional transition at high fields. Magnetization measurements of RE B 50 ( RE =Tb, Dy, Ho, Er) up to 55 kG showed that the compounds saturated universally at around only half the saturation magnetization value of free rare-earth ions, which also indicated the likelihood of a further transition. The structure of the RE B 50 -type compounds has a one-dimensional character with the rare-earth atoms forming an alternating bond length chain along the c -axis. These results are quite interesting in the context with the recent high activity in Haldane gap related compounds in which fundamental quantum phenomena are observed.

Single crystals of the ternary borides (Cr 1-x TM x ) 3 B 4 (TM = Ti, V, Nb, Ta, Mo, W) were grown by the flux method using molten aluminum as a solvent. The substances were subjected to chemical analyses and measurements of the unit cell parameters. Crystals of (Cr 1-x TM x ) 3 B 4 were grown in the form of thick platelets with large {010} planes. The largest crystals obtained show maximum dimensions of about 6x0.4x0.4 mm 3 . The solid solubilities of Mo and W in Cr 3 B 4 are discussed. In addition, measurements of microhardness and electrical resistivity were carried out, and the oxidation resistance at high temperature in air was studied.

A Sc-based boron carbide Sc2B1.1C3.2 with a two-dimensional graphite-like framework has been synthesized by a solid state chemical reaction route. Crystals have been grown successfully and the crystal structure could be assigned to represent a new structure type of rare-earth boron carbides. The trigonal crystal structure (a=b=23.710(9) A, c=6.703(2) A, P 3 m1 (No. 164)) is composed of alternate –[B1/3C2/3]∞–Sc–C–Sc–[B1/3C2/3]∞– layers. The boron and carbon atoms –[B1/3C2/3]∞– form a significantly puckered graphite-like layer in a rather disordered way. The Sc–C–Sc units appear to be rather loosely accommodated between the [B1/3C2/3]∞ units. Sc–Sc distances show a very close metal–metal contact. In the Sc–C–Sc units carbon atoms are surrounded by six Sc atoms in the form of a slightly distorted octahedron.

YB 66 crystals grown with an indirect heating floating zone method were extensively characterized using a variety of X-ray diffraction techniques such as synchrotron white beam transmission and reflection topography, section topography and conventional double-crystal rocking curve measurements using Cu K α and Mg K α sources. The observed crystal defects and deformations were correlated with changes in the crystal growth process. The interplay beween systematic characterization and process improvements results in the growth of high quality single crystals of YB 66 that can be used as an efficient element for monochromatization of synchrotron soft X-ray in the 1–2 keV energy range.

The arrangement of yttrium atoms in YB56with YB66structure was determined by high-resolution electron microscopy (HREM) using a slow-scan CCD camera. Digital HREM images recorded along the [100], [110], and [111] directions of the YB56crystals showed “averaged” yttrium atom positions in the boron clusters, which were also confirmed by image simulation. Digital HREM images recorded from thin regions (

Single crystals of a new yttrium borosilicide of YB44Si1.0have been grown by the floating zone method using a xenon lamp image furnace. X-ray powder diffraction, precession photography, and chemical analyses showed that the crystal obtained is a new YB50family yttrium borosilicide which has a composition of YB44.4Si1.04in the middle part of the crystal and an orthorhombic crystal structure with space groupPbamorPba2.

The structure of the clean YB{sub 66} (100) surface was studied with X-ray photoelectron spectroscopy, low-energy electron diffraction (LEED), and scanning tunneling microscopy (STM). A clean, well-ordered surface can be prepared by Ar ion bombardment followed by annealing to 1200{degrees}C. The fact that heating does not lead to the preferential loss of boron or yttrium indicates that the material evaporates congruently in vacuum. Although the bulk structure of YB{sub 66} is quite complex with over 1600 atoms per unit cell, both LEED and STM reveal that the surface structure has long-range two-dimensional periodicity described by a simple square lattice. The surface lattice constant determined by LEED is 21 {+-} 3 {angstrom}, whereas the bulk value is 23.44 {angstrom}. Large scale images obtained with STM reveal that the surface consists of flat (100) terraces separated by steps 11.7 {angstrom} (half unit cell) in height. Smaller scale STM images reveal a 11.7 {angstrom} square grid of objects. These objects are identified as icosahedral clusters of 156 boron atoms. In its reactivity toward oxygen, the surface shows behavior similar to that reported for a {beta}-rhombohedral boron (111) surface.

The crystal structure of a new compound, YB 41 Si 1.2 , has been determined by single-crystal X-ray diffractometry. The crystal was grown by the floating-zone method. The intensity data were collected on a four-circle diffractometer using CuK α radiation. The structure was solved by direct methods and refined with the aid of a full-matrix least-squares program to the R(R w ) value of 0.034 (0.040); 2686 independent reflections were used for 232 variable parameters. The structure of YB 41 Si 1.2 belongs to the orthorhombic system (space group Pbam), with lattice constants a = 16.674(1) A, b = 17.667(1) A, and c = 9.5110(7) A, and is composed of B 12 icosahedra and a new B 12 Si 3 polyhedral unit. Three Si sites of the B 12 Si 3 unit are assumed to be fully occupied by both Si and B atoms, and the actual chemical composition of this unit is estimated to be B 12 Si 158 B 15 . The boron framework is thus proved to be of a new structure type which is made up of five crystallographically different five B 12 icosahedra and the B 12 Si 3 unit. The Y atom occupies a large hole outside the icosahedra and the B 12 Si 3 unit. Interstitial sites, one Si and seven B sites, are also found outside the polyhedral units, with various occupancies from 100% to 9%.

The crystal structure of YB 62 and YB 56 of the YB 66 -type structure (cubic; space group, Fm 3 c ) has been studied by single-crystal X-ray diffractometry. The crystals were grown by the indirect heating floating-zone method; YB 62 ( a =23.4364(6) A) was obtained by growth at a congruent composition and YB 56 ( a =23.4600(9) A) by growth at an incongruent composition. The structure was refined with the aid of a full-matrix least-squares program to the R ( R w ) values of 0.069 (0.062) for YB 62 and 0.075 (0.069) for YB 56 . The boron framework of the present crystals is almost identical to that of YB 66 (S. M. Richards and J. S. Kasper, 1969, Acta Crystallogr. 25, 237). However, as expected from the chemical compositions of YB 62 and YB 56 , obtained by chemical analysis, there is a significant increase in the yttrium site occupancy; 0.532(4) for YB 62 and 0.575(5) for YB 56 . A marked anisotropy of thermal parameters was observed for the Y site, suggesting that disordered accommodation of the Y atom in the “Y-hole” has occurred.

Monochromator grade crystals of YB66 have been grown by the indirect heating floating zone (IHFZ) method. For growth at the congruent composition of [B][Y] = 62, some high quality crystals have been obtained by keeping a convex growth interface through the crystal using a minimized heating power and downwards drive. Reproducible growth of monochromator grade crystals has been achieved by growing crystals under an incongruent condition where the compositions of the feed rod and the zone are about [B][Y] = 56 and 40, respectively.

YB 41 Si 1.2 was grown by a floating zone method with infrared heating. The crystal structure of YB 41 Si 1.2 belongs to the orthorhombic system. The electrical resistivity, the Hall coefficient, and the Seebeck coefficient have been investigated from liquid nitrogen temperature to room temperature. The temperature dependence of the electrical resistivity was expressed as exp[( T 0 / T ) 1/4 ] and indicates semiconductive nature. The Hall mobility was estimated to an order of 0.1 cm 2 /V s or lower. The combined data of the Hall mobility and the electrical resistivity indicates that electrical conduction is not of the band conduction type but of hopping type. The Seebeck coefficient is almost temperature independent between 110 and 290 K with a value of about 180 μV/K. For temperatures lower than 110 K, the Seebeck coefficient decreases rapidly with decreasing temperature. The figure of merit of the thermoelectric conversion of YB 41 Si 1.2 was estimated and discussed using present data.

Single crystals of (La 1− x Pr x )B 6 were prepared in the composition range of lower than x = 0.3 using the RF heated floating zon method. Grain-boundary-free crystals were obtained in the range of higher than x = 0.18, that is, when the lattice constant decreased by more than 0.1% due to adding PrB 6 to LaB 6 . This value, 0.1%, agreed also with the case of adding CeB 6 [S. Otani, T. Tanaka and Y. Ishizawa, J. Crystal Growth 108 (1991) 425].

Single crystals of ( La 1- x Ce x ) B 6 were prepared by the RF heating floating zone method. The density of the subgrain boundary strongly depended on the Ce content. The boundary-free crystals were obtained in the composition range between x =0.3 and 0.6.

Single crystals of refractory metals, Mo, Ta and W, were prepared by an automated RF heated floating zone technique. In this method, a computer detected a zone shape from a mutual impedance between the work coil and the molten zone, and controlled the heating power and the feed rate. The molten zone could be passed more smoothly, compared with manual processes. Crystals 6 cm long and 1 cm in diameter were prepared using a 3 h zone pass with automatic control. The automatic growth program was prepared so that it can be applied to the growth of other refractory crystals.

Single crystals of YB 66 have been grown by the indirect heating floating zone method. The effects of growth rate and feed rod composition on the crystal quality were examined. The optimum growth rate was 25 mm/h. The congruent composition was confirmed to be [B]/[Y] = 62. Preliminary X-ray rocking curve analysis showed that the full width half maximum of the rocking curve of the best crystal was less than 12 min.

The stability of lithium atoms in α-rhombohedral boron was studied by the density functional theory and Car–Parrinello molecular dynamics (MD) simulations. At a low Li concentration (1.03 at%), a Li atom at the center of the icosahedral B12 site (the I-site) was found to be metastable, and the potential barrier was estimated at 775±25 K (=67±25 meV). Over 800 K, Li atoms began to escape from the B12 cage and settled at the tetrahedral site (the T-site) or at the octahedral site (the O-site). Li at the T-site was also metastable below 1400 K, and Li at the O-site was energetically the most favorable. At a high Li concentration (7.69 at%), the I-site changed to an unstable saddle point. The T-site was still metastable, and the O-site was the most stable. Regardless of concentration, MD simulations showed that Li atoms at the O-site never jumped to other sites below 1400 K. The migration of Li would be very slow below this temperature.

The modulated photoconductivity was measured in single crystal YB{sub 66}, essentially consisting of B{sub 12} icosahedral clusters like many other boron-rich borides. A model, in which localized states participate both in photocarrier generation and trapping well explains the phase shift between excitation and photo-current. For the localized states involved, an ionization energy of 0.19 eV is estimated.

We have found a series of new rare earth boron-rich solids in REBC(N) (RE: Y, Ho, Er, Tm, and Lu), systems and their structures are solved from single crystal and/or powder X-ray diffraction data. Structure analysis results show that they are homologous with B(4)C and also show trigonal symmetry. As the two basic structural units, boron icosahedra and octahedra in the new phases form in layers and stack in different sequences which form different phases. With increasing number of icosahedral layers stacking between two neighboring octahedral layers, the c-axis of the unit cell is increased and the other two edges of the unit cell are only changed slightly. Three monophases of the series have been synthesized both in powders and as single crystals. The vibrational modes of the homologous phases are analyzed from the Raman spectra and compared with that of B(4)C.

A method for automatic crystal preparation was developed for the RF heated floating zone technique. In this method, the heating power was computer-controlled on the basis of the impedance changes due to small changes in molten zone shape. LaB6 single crystals 6 cm long and 0.8 cm in diameter were prepared at a rate of 1 cm/h.

The complex dielectric functions of β -rhombohedral boron, of boron carbide with compositions between B 4.23 C and B 10.37 C and YB 66 , were measured between about 2.5 and 9.5 eV. Similarities of the electronic structures probably due to the icosahedra are indicated by obvious qualitative similarities of the spectra. Numerous critical points have been determined and confirm the dense sequence of energy bands. For β -rhombohedral boron the critical points are compatible with the combined density of states maxima derived from cluster calculations by K. Shirai and H. Nakamatsu (1994, in “Proc. 11th Int. Symp. Boron, Borides and Related Comp, Tsukuba, 1993,” JJAP Series 10, p. 70). Otherwise no satisfactory correlation with theoretical band structure calculations is possible. In the case of boron carbide the critical points are the same within the whole homogeneity range, while the values of the dielectric function at higher energies indicate composition-dependent densities of states, which are correlated with the density of B 12 icosahedra.

The evaporation of vanadium carbide during floating zone systems was examined. The evaporation composition and rate were measured as a function of the melt composition. At a composition of C/V = 0.72, the dependence of evaporation rate on ambient gas pressure ( P ) was measured at the range of 3.5–40 atm of helium and argon. The evaporation rate was proportional to P -0.70 in helium and P -0.31 in argon. The same results were obtained with TiC. These pressure dependences are shown to be expected for laminar and turbulent free convective mass transport.

In order to prepare a single crystal of TiC with a high purity using the floating zone method, the feed rods were prepared by the self-combustion reaction of Ti metal and carbon. The control of the composition of the feed rod, the impurity refining, the density of the feed rod and the degree of the reaction were examined in the process of self-combustion. Using the feed rod prepared in this way, a high purity single crystal with controlled composition was prepared by a modified zone leveling method.

High purity LaB 6 single crystals have been grown by multi-float zone passage. Crystal growth was done under a pressurized atmosphere to prevent vaporization and dissociation of LaB 6 . No seed crystals for the selection of a preferred growth axis were used. The existence of an easy axis of growth was not found. The impurity concentrations were analysed by emission spectrography and residual resistance ratio measurement. The results show that the purity of the crystals after three passes were improved by about one order of magnitude compared with the crystals after only one zone pass. The grown crystals consist of sub-grains whose crystal axes were slightly misorientated with respect to each other. Electrical resistivity measurements for the purest crystal reveal the characteristic feature of conduction property of LaB 6 , i.e., polar phonon scattering of electrons plays an important role in the scattering mechanism of LaB 6 in addition to impurity scattering and acoustic phonon scattering.

In the preparation of TiC single crystal with high purity from a self-combustion rod by a floating zone technique, the dependence of the crystal quality on the growth conditions was examined. The quality of the prepared crystals was determined by measuring the etch pit density and X-ray diffraction of the grown crystals. The quality of the crystals was partially improved by increasing the growth rate and decreasing the crystal diameter.

TiCx single crystals with a maximum carbon content (x = 0.96–0.97) were prepared by controlling the compositions of the initial molten zone and the feed rod (a modified zone leveling method). The homogeneity of the composition was checked along the growth and radial directions. The lattice constants, densities and impurities in the crystal, starting material, feed rod and evaporation product, were measured and reported. All impurities except W, whose carbide has a low vapor pressure, were found to be refined by evaporation. The effect of the helium ambient gas pressure was examined. It was found that good quality crystals could be prepared under relatively low pressure (4–5 atm). The good quality of the crystals was confirmed by X-ray transmission topography.

Vanadium carbide has a wide range of nonstoichiometric compositions where the atomic ratio of carbon to vanadium C/V varies between 0.71 and 0.88. Within this range, V8C7 and V6C5 phases exist due to carbon vacancy ordering in addition to the compound VCx (x

Single crystals of TiC x with various compositions have been prepared using a floating zone technique in a He ambient gas pressure of up to 30 atm. It was found that the composition of TiC x crystal rods, grown by the usual floating zone process, changes continuously along the growth direction of the crystal. The longitudinal variations in composition have been confirmed by chemical analysis of carbon, measurements of the electrical resistivity and the lattice constants by the X-ray diffraction method. The results were interpreted in terms of the phase diagram. Finally, the zone leveling technique has successfully been used to prepare the single crystals of TiC x with uniform composition.

NbCx single crystals with compositions controlled over the nonstoichiometric range (0.75

ZrC x single crystals have been prepared over the entire composition region by a floating zone technique. It was found that the crystal rods prepared by a usual floating zone method have composition changes along the growth direction, as predicted by the phase diagram, and that, in addition, the composition of the molten zone changes due to evaporation. ZrC x crystals with constant desired compositions have been prepared by controlling the compositions of the initial molten zone and the feed rod (a modified zone leveling method). The crystals were characterized by carbon analysis, impurity analyses, back-Laue and etching methods.

The temperature distribution in a growing crystal rod was calculated using a simplified model, in which the heating power was supplied by RF heating. The temperature distribution is determined by the Biot number (Bi) and the heating parameter δ/a (ratio of the skin depth δ to the rod radius a). In the case of refractory materials, crystal rods with 0.5 cm radius and RF heating (200kHz), one obtains Bi = 0.02–0.2 and δ/a = 0.2–0.6. Therefore, the temperature distributions were calculated for the cases of Bi = 0.01, 0.1, 1.0 and δ/a = 0, 0.2, 0.7. The calculated results were compared with the experimental results of Mo and TiC, which have the lowest and highest Biot numbers, respectively. The results agree well with each other.

TiCx single crystals have been prepared in the nonstoichiometric composition range (C/Ti < 0.91) by a floating zone technique. The crystal compositions were controlled by a modified zone leveling method because the crystal composition deviates much from the feed rod composition due to evaporation. The crystal compositions were determined by carbon analysis. The composition homogeneity was checked. The oxygen and nitrogen impurities were analyzed. The relationship between the composition and the lattice constant was determined. In addition, the evaporation rate and the evaporation composition were measured as a function of the zone composition by making use of the fact that the zone composition was kept constant during a zone pass. On the basis of these experimental results, the composition of the grown crystal was calculated as a function of the feed rod composition and the growth rate. The composition change due to evaporation was discussed.