ELECTRON REQUIREMENTS OF BONDS IN METAL BORIDES

Thesis/Dissertation · Mon Jan 01 00:00:00 EST 1962

OSTI ID:4784292

Johnson, R W

This study involved the preparation of YB/sub 2/, YB/sub 4/, YB/sub 6/, YB/sub 12/, CaB/sub 6/, SrB/sub 6/, and BaB/sub 6/, measurement of the Hall coef ficients of the yttrium borides, and measurement of resistance vs temperature for the above alkaline earth hexaborides. Polycrystalline samples of CaB/sub 6/ and BaB/sub 6/ were obtained by reaction of powdered boron and metal vapor in a sealed Ta crucible at 1000 to 1500 deg C, followed by arc melting. Single crystals of SrB/sub 6/ were obtained by holding Sr and boron, contained in a sealed thick-walled Ta tube, in a temperature gratlient from 1650 to 1800 deg C for four hours. The resistance vs temperature was measured up to 1185 deg C, and the slope of the curve was negative above 725 deg C, showing that SrB/sub 6/ is a semi-conductor. The energy gap calculated from the linear portion of a lnR vs l/ T curve was epsilon = 0.19 ev. Measurements of polycrystalline CaB/sub 6/ and BaB/sub 6/ gave similar results. Energy gaps obtained as above are: for CaB/sub 6/, epsilon = 0.20 ev; for BaB/sub 6/, epsilon = 0.06 ev. A new method for high temperature floating zone melting was developed for preparation of Hall effect samples of the yttrium borides. The zone is heated by an induction coil coupled with an r-f step-down transformer called an eddy current concentrator, and temperatures near 2800 deg C were easily obtained. The compounds were synthesized by arc melting together yttrium and crystalline'' boron amd were fabricated into 1/4-in. diameter rods by melting into a groove in the plate of the arc melter. Zone melting was done with zone compositions different from that of the rest of the rod, so that the peritectics YB/sub 6/ and YB/sub 122/ could be obtained by Mason and Cook's method. Single crystal samples of the non-cubics YB/sub 2/ and YB/sub 4/, and polycrystalline samples of the cubics YB/sub 6/ and YB/sub 12/ were obtained. Sample cutting was done with a Servomet sparkcutter. The Hall coefficient of YB/sub 2/, measured with the magnetic field parallel to the c axis of the crystal, is R/sub H/ = -20.5 plus or minus 2.5 x 10/sup -11/ meter/sup 3//coulomb. For YB/sub 4/ in a similar orientation, R/sub H/ = -213 plus or minus 9 x 10/sup -11/. For YB/sub 6/, R/sub H/ =-44.7 plus or minus 2.2 x 10/sup -11/. The YB/sub 12/ sample contained a finely divided non- conducting second phase, and the Hall coefficient corrected for that condition is R/sub H/ = -89 plus or minus 15 x 10/sup -11/, The electrical resistivities in OMEGA -cm of the above samples are: CaB/sub 6/, approximately 0.1; SrB/sub 6/ approximately 0.2. YB/sub 2/, 39 plus or minus 3 x 10/sup YB/sub 4/, 28.4 plus or minus 1.3 x 10/sup -6/; YB/sub 6/, 40.5 plus or minus 1.8 x 10/sup -6/ ; YB/s ub 12/, 94.8 plus or minus 3 x 10/sup -6/ According to LCAO treatments of boron atoms in the boride types MB/sub 2/, MB/sub 4/, MB/sub 6/, and MB/sub 12/ by various authors, if M is a divalent metal, any of these borides should be a semiconductor, and if M is trivalent the compounds should have one metallic electron per metal atom. For YB/sub 2/, YB/sub 6/, and YB/sub 12/, Hall coefficients calculated on that basis from R/sub H/ = 1/ne agree with the measured valiues within experimental error. The equation cannot be applied to YB/ sub 4/ because a free electron fermi surface for one electron per Ytrrium atom falls into four Brillouin zones. Indirect arguments for YB/sub 4/ based on its structural similarity to YB/sub 2/ and YB/sub 6/ lead to the conclusion that there is probably one electron per yttrium atom, in agreement with theory. (Dissertation Abstr., 23: No. 3, 1982)

• Research Organization: Originating Research Org. not identified
• NSA Number: NSA-17-002131
• OSTI ID: 4784292
• Resource Relation: Other Information: Thesis. Orig. Receipt Date: 31-DEC-63
• Country of Publication: Country unknown/Code not available
• Language: English

Related Subjects

PHYSICS
ALKALINE EARTH METALS
ATOMS
BARIUM BORIDES
BONDING
BORIDES
BORON
BRILLOUIN ZONE
CALCIUM BORIDES
CHEMICAL REACTIONS
COILS
COUPLING
CRYSTALS
CURRENTS
ELECTRIC ARCS
ELECTRIC CONDUCTIVITY
ELECTRIC DISCHARGES
ELECTRONS
ENERGY
EQUATIONS
ERRORS
FERMI LEVEL
HALL EFFECT
HEATING
HIGH TEMPERATURE
INDUCTION
INTERACTIONS
LATTICES
MACHINING
MAGNETIC FIELDS
MEASURED VALUES
MELTING
METALS
MONOCRYSTALS
PARTICLE MODELS
PLATES
POWDERS
PREPARATION
RADIO WAVES
RODS
SEALS
SEMICONDUCTORS
SPARKS
STATISTICS
STRONTIUM
TEMPERATURE
THALLIUM
THICKNESS
TRANSFORMERS
TUBES
VAPORS
YTTRIUM BORIDES
ZONE MELTING
ZONES