The high-temperature lattice parameters of HoZn2

Journal of the Less-Common Metals Elsevier Sequoia S.A., Lausanne - Printed in the Netherlands

367

Short Communications The high-temperature

lattice parameters of HoZnz*

Much work has appeared in the literature on AB2 intermetallic compounds formed by the rare-earth metals with zinc. All these REZnz compounds are known to have the KHg2 (CeCuz)-type crystal structure but complete structural data are known for only a few of theml-4. However, no data describing the thermal behavior of these compounds are currently available, except the melting points of the compounds5 and several RE-Zn phase diagramsc-9. The objective of thisinvestigation was to study the thermal behavior of HoZnz by high-temperature X-ray powder diffractometry. HoZn2 was chosen because complete structural data are available for it, and because it is representative of the remainder of the REZn2 compounds. Experimental procedure

The compound used in this study was prepared in a manner described previous1~5. Metallographic examination of the alloy revealed that only a single phase was present. A portion of the alloy was reduced to -325 mesh powder by grinding in an agate mortar and subsequently encapsulated in IO mil beryllium foil according to a previously described procedurelo. The beryllium foil was pre-annealed at 850°C in vacuum. Before encapsulation, the alloy powder was cold-pressed into a rectangular wafer at N 8000 p.s.i. The encapsulated sample was then placed in a Tern-Pres Research, Inc., X-ray diffractometer furnace and alignedso that the surface of the sample powder and not the beryllium was coincident with the diffractometer rotation axis. CuKa(;l= 1.54178A) radiation was used and data were recorded between 20~ and 75”ze for temperatures between room temperature (-25°C) and 800°C. Two separate runs, each with a different sample prepared from the same alloy, were made. Lattice parameters at each temperature were calculated along with their standard deviations from sixteen reflections by Cohen’s least-squares method with the Nelson-Riley extrapolation function. The high-temperature diffractometer system was checked by making a run with a sample of high-purity Ag which was also encapsulated in Be foil. The lattice parameters for this sample were measured at the temperatures for which HU~UEROTHERY AND REYNOLDS~~report values. They were calculated in the manner above except that both CuKoll (A= 1.5405IA) and CuKol~(l= 1.54433A) data were employed for almost all reflections. The agreement was well within standard deviations in all cases. Results

The change * This investigation 3560-g.

in each lattice was supported

parameter

with temperature

is shown in Fig. I.

by the U.S. Atomic Energy Commission;

J. Less-Common

Report No. NYO-

Metals, 14 (1968)

SHORT COMMUNICATIONS

368

The minimum and maximum standard deviations for the calculated values were 0.007A and o.o28A, respectively. The results are interpreted as indicating a continuously increasing expansion of the Ho&s structure along each principal direction. Since the structure is built up of close-packed layers of atoms perpendicular to the a axis, the expansion can be expected to be anisotropic and it is greatest along the c direction. As can be seen in Fig. I, the behavior of each lattice parameter is approximately linear over the entire temperature range. The straight line shown for each parameter was obtainedfromaleast-squares calculation. The thermal expansion coefficients are then OI~=1.66 x IO-s/‘C, 01b=I.59 x IO-s/‘C, 01e =2.10 x IO-~/%.

I

-

400

ROOM

TEMPERATURE

600

000

(“cl

Fig. I. Lattice parameters vs. temperature for HoZnz. Shading denotes limits of error.

Two other REZna compounds, YZn# and SmZnzs, have beenreportedtoundergo a martensitic-type phase transformation at elevated temperatures. Although we have found no evidence for such behavior in HoZns, we are unable to account for certain deviations of the thermal expansion from linearity. It should be noted, however, that the deviations shown were definitely reproducible. Department of Materials Science, Metallurgy Section, The Pennsylvania State University, University Park, Pa. 16802 (U.S.A.) J. Less-Common

Metals,

14 (1968)

D. J. MICHEL E. RYBA

SHORT

I 2 3 4 5 6 7 8 g

IO II

369

COMMUNICATIONS

AND E. RYBA, Ada Cryst., 19 (1965) 687. AND E. RYBA, A&Z Cryst.,zr (1966)818. D. J.MICHEL AND E. RYBA, unpublished results. A. IANDELLI AND A. PALENZONA, Atti Accad. Nazi. Lincei, Rend. Classe Sci. Fis. Mat. Xat., 37 (1964) 165. I). J. MICHEL, E. RYBA AND P. K. KEJRIWAL,J. Less-Common Metals, II (1966) 67. 1’. CHIOTTI, J.T. MASON AND K.J.GILL, Trans. AIME, zz7 (Ig63)gIo. P. CHIOTTI AND J. T. MASON, Trans. AIME, 223 (1965) 786. 1’. CHIOTTI AND J. T. MASON, Trans. AIME, 239 (1967) 547. D. J. MICHEL AND E. RYBA, to be submitted to J. Less-Common Metals. D. J. MICHEL AND E. RYBA, Norelco Reporter, 14 (1967) 25. W. HUME-ROTHERY AND P. W. REYNOLDS, Proc. Roy. Sot. (London), A167 (1938)

D. J. MICHEL D. J. MICHEL

Received

September

3oth,

1967 J. Less-Common

Metals, 14 (1968)