The tetragonal MBe12structure of silver, palladium, platinum and gold

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Acta Cryst. (1958). 11, 122

T h e t e t r a g o n a l :~//Be~2 s t r u c t u r e of s i l v e r , p a l l a d i u m , p l a t i n u m a n d g o l d . By F. W. YON BAT- Cm~LDER and 1~. F. RAEUCHLE,* U. S. Naval Research Laboratory, Washington 25, D.C. , U . S . A .

(Received 17

The structure investigation repo~ted here is a continua- t ion of the work of yon Batchelder & l~aeuchle (1957). :Each compound was obtained by melting, several times, a precious-metal-beryl l ium mixture of approximately 1 : 15 atomic ratio in a tungsten arc furnace with a hel ium atmosphere.

Minute needle-like single crystals were obtained from these melts by electrolytic etch techniques and were examined by the precession method. The lattice constants and space group for AgBel~, PdB%2 and PtBel~ were determined from single-crystal precession diagrams taken around the a 0 and c o axes. These crystals are tetragonal, and since only the reflection absences characteristics of body centering occurred, the space group is ei ther I4 /mmm-D~, or a subgroup. The crystals are isostructural with MoBe~ (Raeuchle & yon Batchelder, 1955). The lattice parameters, using 2(Mo Ka) = 0.7107 A, and the calculated densities are reported in Table 1.

Table 1. Lattice constants for the M B e ~ compounds

Compound a 0 (A) c o (A) c/a Dx (g.cm. -3)

AgB%2 7.2844-0.005 4.2464-0.005 0.583 3.19 PdB%2 7.2714-0.005 4.2514-0.005 0.585 3.18 PtB%2 7-2374-0.005 4.2524-0.005 0.588 4.53

Each mel t was examined spectrochemically and no impuri ty of any consequence was found. Debye-Scherrer powder pat terns were also made from representat ive portions of each melt. Only the si lver-beryll ium melt was essentially a single phase which could be indexed as te t ragonal AgBex2. Oesterheld (1916) suggested tha t an intermetall ic compmmd might exist in the beryl l ium- silver system at 92-5 atomic % beryllium with the formula AgBel~.

Powder pat terns of the remaining melts show a strong face-centered cubic phase in addit ion to varying amounts of the tetragonal MBe12 phase. In the diffraction pat tern of the pal ladium-beryll i t~n melt, the strong face-centered cubic phase has the lattice constant a 0 ---- 5.969 A and can be identified wlth the lPdB% phase reported by Misch (1935). The P d B e ~ phase was moderate ly strong.

The powder pa t tern for the p la t inum-bery l l ium melt was indexed as a strong face-centered cubic phase with a 0 = 5.978 A, which is probably the compound PtB% (by analogy with PdB%) and a moderate ly strong PtBe12 phase. During the single-crystal investigation of this melt, a single crystal of the face-centered cubic phase was ob- ta ined and the lattice constant from the precession diagram was fotmd to be a 0 ----- 6.004 A. The discrepancy

* Present address: Olin Mathieson Chemical Corporation, New Haven, Conn., U.S.A.

October 1957)

between these two values for PtB% is probably due, as suggested by Misch (1935) in the case of PdB%, to the fact tha t the powder pa t te rn value may have been affected by an excess of beryllium which could lower the lattice constant from the value represented by the true stoichiometric ration.

The I)ebye-Scherrer pa t te rn of the gold-beryl l ium mel t was indexed as a very strong face-centered cubic phase wi th a 0 = 6.100 /~, which can be identified with AuBe 5 as reported by Misch (1935), and a very weak secondary phase of te t ragonal MB%~. The spectrochemical exam- ination indicated the presence of tungsten. A minu t e f ragment was obtained of essentially a single crystal of the tetragonal phase and examined by the precession camera around a 0 and c 0. This crystal was found to be isostructural with the tetragonal MBe12 compolmds reported, and had lattice constants

a 0 = 7.2434-0-005 ~, c o -~ 4-2523=0-005 A, c/a ---- 0.587 and Dx ~- 4.55 g.em. -3.

The composition of this was studied by the electron probe microanalyzer (Birks & Brooks, 1957) and gold plus varying larger amounts of tungsten were found to be present. This would indicate tha t the f ragment examined was not homogeneous in chemical composition. From the X-ray and electron probe evidence, gold and t tmgsten may joint ly form from the mel t small percen- tages of an MBele- type of compound. The compotmd formed could have the possible formula AuzWyB%2 (where x-~ y ---- 1).

A second gold-beryl l ium melt was made which showed negligible tungsten present. :In this the secondary phase was even weaker than in the first mel t and no information could be obtained.

We wish to thank Mr D. P. Fickle, of the National Bureau of Standards, for making our melts ; Mr S. Cress for spectrographic analysis of our melts ; and Messrs L. S. Birks and E. J. Brooks ~or microanalyzlng our small f ragment from the Au-Be melt .

R e f e r e n c e s

BATCtIELDER, F . W . void" ~: I~AEUCHLE, R . F . (1957). Acta Cryst. 10, 648.

BIRKS, L. S. & BROOKS, E . J . (1957). Rev. Sci. Instrum. To be published.

Misc , , L. (1935). Metallwirtschaft, 14, 897. OESTERHELD, G. (1916). Z. Anorg. Chem. 97, 6. RAEUCHLE, ]~. F. (~ BATCHELDER, F . W . voN (1955).

Acta Cryst. 8, 691.