Wl?T * % master JZ31-/7/ * Electron Requirements of Bonds in Metal Borides ROBERT W. JOHKSONtAND A. H. DAANE Institute for Atomic Research and Department of Metallurgy Iowa State University, Ames, Iowa Abstract-—Electrical measurements have been made on CaB^, SrB^, BaB^, YB^, YB4, YB^» and YB [2 for the purpose of testing models of the electronic structure of the boron atoms in the four boride types represented. The compounds CaB^, SrBfc, and BaB^ were found to be semiconductors. The Hall coefficients of YB2, YB^, and YB 12 were measured and found to correspond to one free electron per yttrium atom in each compound. The Hall coefficient of YB^ was also measured, but could not be interpreted because the free electron theory does not provide a theoretical value for comparison with experi­ ment. The results are in agreement with the models proposed. INTRODUCTION The study of electron-deficient boron hydrides has benefited greatly from the application of the method of linear combination of atomic o. bitale (LCAO) to the study of the bonds within the molecules. The increase in understanding has enabled many strange varieties of struc­ ture to be viewed within a coherent framework of one theory, and allowed facsimile fries i Microfilm Mee I /< / ^ Avei table beet the Office ef Technical Services Wesh inftoa 23, D. C- 2 n#w compounds to be predicted, many of which were later found to exist. * Many of the known borides are electron-deficient in the same sense as are the boron hydrides, but are not as well understood as the latter. The structure types which are electron-deficient are MB^, MB( , MB[2, some reported borides of unknown structure such as YB^Q, 2 and the various forms of elemental boron. LCAO studies have been published for M B^, MB j 2*^ elemental boron. The recent discussion by Lipscomb and Britten^ covered all of the above boride types except fo r YB^q, and \n addition included MB^, for which a bonding scheme was suggested. The present work was initiated in order to provide experimental evidence with which to evaluate the models for the electronic structure of the boride ty#es MB^, MB^, MB^, and MB^* (The models themselves will be described later.) The methods for experimentally testing the LCAO mode s for the borides differ in principle from those which are useful for boron hydrides. All of the electrons in boron hydride molecules are considered to take part in the co-valent bonding, and a knowledge of the composition and structure usually allows a fair test of the theoretical model to be made. For the borides, metals of different valence often can form the same boride type, so that one cannot assume that all valence electrons are used for bonding in each case. The assumption that the boride formed from the metal of lowest valence is the one for which all of the metal »tom electrons are needed for B-B bonds, is also open to question unless reinforced with other information, because an iso- morphous bo ids of a metal of yet lower valence may exist but be r 3 undiscovered. In the present work the method used for judging the pro­ posed models is to find out whether the boron atoms have the requisite number of electrons suggested by the theory. It is implied in the theory that the electrons participating in B-fi bonds are bound, and unable to move under the influence of an electric field unless somehow excited (by thermal energy for instance), and any metallic properties are to be accounted for by the presence of more valence electrons than are required by the boron atoms. Experimentally it is possible to determine whether all of the valence electrons are bound, and if some are not bound it is often possible to determine the number of ’free' electrons per atom. The former condition corresponds to semiconducting behavior, whick is reflected in the electrical resistance-tem perature relationship. In the latter case the free electron concentration is often obtained directly from the Hall coefficient. Accordingly, these two types of electrical properties were chosen for measurement on suitable borides. The proposed models^ are such that any of the four boride types would be expected to be a semiconductor if made from an alkaline earth metal. Previous experience had shown that all but the alkaline earth hexaborides would be very difficult to prepare in a form necessary for measuring electrical properties, so only the alkaline earth hexaborides were investigated for semiconductivity. The yttrium borides were chosen for Hall coefficient measurement since the models for each indicated the presence of one free electron per yttrium atom. Yttrium was chosen because it is trivalent, forms the four boride types to be investigated, and because information on the yttrium - boron phase diagram^ was availa­ ble (se e F ig . 1). 4 WEIGHT P£N CENT BORON 20 30 40 50 50 <50 AiOMtC PER CENT BORON V F i* . 1. The y ttr iu m - bo’•on sy ste m lice fo o tn o te £), The preparation ol boride tem ples for Hall effect measurement Is difficult because of their very high melting points, extreme brittle­ ness, and ease with which they are contaminated at temperatures required N in their preparation. Single crystals were preferred for samples to be checked for semiconductivity, and were essential for Hall coefficient measurements on non-cubic borides. It was necessary to develop a new method for floating sone melting in order to obtain m aterial of suitable form and purity. The details of that method are being published sepa­ rately, and only the data obtained from the samples will be described h e re . EXPERIMENTAL DETAILS Sample Preparation The method chosen for the preparation of the yttrium borides was floating seme melting. Some of the advantages to be gained by its use were: a) No crucible is required; b) Slow rates of solidification could be achieved by controlling the traverse rate of the sone along the rod; c) Mason and Cook7 recently showed that single phase bulk samples of peritectic compounds could be prepared by a slight modification of the sons melting procedure. {YBfc and YB^ ^ *r« peritectic compounds.) Condition b) is likely to result in large grains from which single crystals might be cut. The only previously described heat source for floating tone melting at the neceseary temperatures (see Fig. 1) is electron bombardment, which was ruled out in this case by the fact that in the vacuum necessary for its operation the borides change composition 6 «t temperature* near their melting points. The method that was developed in order to attain the neceaeary temperatures in an argon atmosphere used an induction coil coupled with a special type of RF step-down transformer called an eddy current concentrator. The yttrium borides were synthesized from yttrium met a.’ (<»9.9%) and boron pieces (99. 0%) of average diameter 3 mm. The necessary quantities of these elements were melted together in an arc melter under srgon to form 'buttons" weighing about 25 g. The buttons were remelted nto a trough in the water-cooled copper plate to form rods about 1/4 in. in diameter and 4 in. long, of nearly circular cross section. For each run, Mason and Cook'*7 sone leveling technique was em­ ployed. This method differs from ordinary sone melting in that the latter has a molten sone of the comnosition of the compound required and at a temperature above its melting point, while the former uses a sone having a composition and temperature corresponding to a point on an adjacent liquidus line. In sone leveling, movement of the sone causes the compound in question to precipitate out of solution; in ordinary sone melting, solidification is by normal freesing. If the rate of sone travel is low enough, the compound will come out of solution on crystals already present instead of precipitating as separate crystals, and allow a single phase bulk sample of the compound to be grown. The sone will maintain its composition as it moves, since the material going into solution at one liquid-solid interface has the same composition as the compound coming out of solution at the other interface. The employment of sone leveling * brings several advantages: first, if the liquidus line in question is • 7 i relatively steep, it is much raster to maintain constant temperature and ■one length; second, peritectic compounds can be prepared by sone level­ ing but not by ordinary rone melting; third, a compound which exists over a range of composition will be produced with uniform c imposition by sone leveling, but not necessarily by ordinary sone melting. The desired sise and shape of a sample for Hall effect measurement was that of a flat rectangular slab of length, width, and thickness approxi­ mately 1. 1, 0. 3 and 0. 1 cm respectively. The necessary cutting was done very satisfactorily with a Servomet sparkcutter (Manufactured by Metals Research Ltd. , Cambridge, England). In the cases of YB^, and # YB^ crystals could be parted by cleavage along 00! planes, and the shaping finished by sparkcutting. Preparation of CaBfe, SrBfc and BaB^ If a material is to be checked for the semiconducting property by the usual method of measuring the change of electrical resistivity with tsmperaturs, the sample should be a single crystal. In that case the possibility of strange electrical effects due to grain boundaries need not be considered. The sample must be very pure, since most impurities drastically affect the electrical behavior of semiconductors. On the other hand, the sise and ahape of the crystal are not critical.