X-Ray studies in the ternary system Ti-Cu-Hg revealed the
existence of a new phase of the composition TiCuHg2. The cubic
unit cell has a lattice parameter a = 6.155 A and conta ins appr. 3
formula units. The crystal structure was determined from x-ray
powder diffraction data and found to belong to the ordered B 32
structure type. The density measurements indicate that the structure
is only 710/o filled

M. Pušelj

Z. Ban

English

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CCA-541 CR 0 AT IC A CHEM I CA ACT A 41 (1969) 79 548.7 :546.821.56.49 Original Scientific Paper The Crystal Structure of TiCuHg2 M. Puselj and Z. Ban Institute for Inorganic and Analytical Chemistry, The University, Zagreb, Croatia, Yugoslavia Received January 24, 1969 X-Ray studies in the ternary system Ti-Cu-Hg revealed the existence of a new phase of the composition TiCuHg2. The cubic unit cell has a lattice parameter a = 6.155 A and contains appr. 3 formula units. The crystal structure was determined from x-ray powder diffraction data and found to belong to the ordered B 32 structure type. The density measurements indicate that the struct-ure is only 710/o filled. INTRODUCTION In the intermetallic systems Ti-Cu, Ti-Hg and Cu-Hg, at the equiatomic ratio, three binary phases exist. All of them are structurally related to th e B 2 (CsCl) structure type: CuHg(y-brass), TiHg (CsCl) and 6-TiCu(AuCu) . We felt the study of the system TiHg-CuHg could give some interesting results mainly because of the well known potential sequence of crystal structures, found whenever a basic structural unit is a body centered cube, as for example in the system Fe-Al.1 The idea proved to be fruitful and the results of this investigations are presented in this paper. EXPERIMENTAL Materials and Methods Titanium powder (Johnson Matthey) 2 N, copper fillings (electrolytic copper) and high purity mercury (TOS, Ljubljana) were weighed and enclosed in Vycor tubes filled with purified argon under reduced pressure. The tubes were heated for 1 hour up to 700 °C and then slowly cooled down to room temperature. If components did not react completely, the same treatmeni was repeated. X-ray patterns were taken on General Electric XRD-6 diffractometer using nickel filtered CuK radiation. DTA data were obtained on a G. Netsch differental thermal analyzer. The density was determined picnometrically with decalin and methylen iodide. RESULTS AND DISCUSSION X-ray powder patterns of both terminal binary phases on the TiHg-CuHg tie line agree very well with literature data2•3• If a mutual solid solubility exists it is less than 50/o on both sides, because the strongest reflection belonging to the new phase appears to be visible beyond this value. The exact solubility limit is very difficult to determine because of the relatively poor quality of the X-ray diffraction patterns and due to the fact that in this case metallo-80 IVL PUSELJ AND Z. BAN graphy can not be used. A pure phase is obtained within the composition interval ranging from Ti0 .8 Cu1 . 2 Hg2 to TiCuHg2 • The phase decomposes above 200° C, and forms TiHg and CuHg. Above 300° C mercury starts to evaporate, leaving Ti and Cu as a residue (Fig. 1). We ascribe the exothermic effect at Temperature t•c l 360 200 Time Fig. 1. A typical DTA curve for TiCuHg ., (shortened form). 180° C to the stabilization (final ordering of atoms in the structure) or to the stress relief, because it varies in shape and area. No significant changes of the x-ray diffraction patern were found below and above this temperature. The indexing of the reflections showed that the unit cell is cubic with the lattice parameter a = 6.155 A. Only those reflections were present where h, k and 1 indices are all odd or even, which determines a face centered arrangement of heavy atoms. At first, chemical composition, together with the value of the lattice para-meter and the type of extinct reflections suggested that we were dealing with a Heussler type alloy4 • Trial calculations of some intensities immediatelly eliminated this assumption. If this were the case the ratio of the intensities for reflections 111 : 220 would be 1 : 400 which does not fit the experimental data. Later on we postulated three other possibilities with the atomic positions shown on Table I. The intensity calculations for all three possibilities are given in Table II. As it is easily noticeable from the data presented in Table II, the as-sumption No. II, where titanium and copper atoms are statistically distributed over the set of positions, is unacceptable, simply because the reflection 200 would in that case have zero intensity. Between the other two possibilities, we chose the one where the Ti-atoms occupy the positions within the unit cell and the copper atoms those on cube edges. In this way we obtain an ordered ternary structure belonging to B 32 structure type5• (Fig. 2). Some earlier papers mention the existence of ternary phases crystalizing in this structure type. The first of them6 is not conclusive but an excellent article, recently published by Pauly et al.,8 shows that a great number of intermetallic compounds tends to assume the same structure. These compounds are of the general composition Li2EX (E = Cu, Ag, Au; X = Al, Ga, CRYSTAL STRUCTURE OF TiCuHg2 TABLE I Three Alternative Possibilities for the Atomic Arrangement in the Structure of TiCuHg2 Ti 000 1/2, 1/2, 1/2 1/2, 0 ' 1/2 0 ' 1/2, 1/2 1/4, 1/4, 1/4 3/4, 3/4, 1/4 1/4, 3/4, 3/4 3/4, 1/4, 3/4 I II Cu 1/4, 3/4, 1/4 3/4, 1/4, 1/4 1/4, 1/4, 3/4 3/4, 3/4, 3/4 1/2, 0 ' 0 0 ' 1/2, 0 0 '0 ' 1/2 1/2, 1/2, 1/2 Hgc and Hgn the same position as in I; Ti and Cu statistically. III Hgc and Hgu the same position as in I; Cu and Ti change their positions . • Hg ~ T1 Q Cu Fig. 2. The illustra tion of the atomic a!Tangement in t.he crystal structure of TiCuHg;. hkl 111 200 220 113 222 004 224 333 602 TABLE II Diffractometer Data for TiCuHg2 (Cu K n radiation) 3.559 3.076 2.179 1.856 1.776 1.536 1.257 1.187 I 0.970 d e Io I 3.559 25 23 3.079 2 0.05 2.179 100 100 1.844 15 13 1.777 5 0.5 1.540 13 16 1.257 37 35 1.187 5 2 0.971 15 16 I,..,F2·p·LP where F = structure amplitude p = multiplicity factor II 24 0 100 16 0 17 35 1 16 and LP = Lorentz-polarization factor III 31 0.05 100 18 0.5 16 35 4 16 81 82 M. PUSELJ AND Z. BAN In, Tl, Si, Ge, Sn, Pb, Sb, Bi). Therefore the crystal structure of TiCuHg" is the first defined and ordered ternary compound containing mercury having the structure of the B 32-type. The most unusual result was obtained by measuring the density of the compound. The calculated x-ray density amounts 14.90 g/cm3. This would correspond to a structure denser than mercury itself. However, the table of the interatomic distances (Table III) for the postulated TABLE III Interatomic Distances in TiCuHg2 Atom Neighbours Distance (A) Hgr 6Cu 3.08 4 Ti 2.67 4 Hgn 2.67 Hgu 4Cu 2.67 2 Ti 3.08 Cu 4 Ti 2.67 crystal structure does not show unusually short contacts, except perhaps the Hg1-Hg11 distance (2.67 A) instead of the expected value 3.0 A. For this result, we offer the following explanation. It is a matter of com-mon knowledge that in the structures of NaTl (B 32) type, a very pronounced bond contraction is present7, suggesting an appreciable degree of ionicity. A typical value for the aforementioned contraction is 130/u. In our case, the unusually short Hg1-Hgrr distance is 13.80/o shorter than the expected one (the metallic radius of mercury was not corrected for the coordination 8; R Hg for C.Nl2 = 1.512). Therefore, we assume a partially unfilled structure which, although not very common among intermetallic compounds, seems to be the only reasonable explanation for the apparent disagreement between the observed and the calculated densities. The average experimental value for the density is 10.61 g/cm3. In other words the unit cell is only 71 O/o filled with all atoms comprising the structure and also in a statistical manner. REFERENCES 1. A. Tay 1 or, X-Ray Metallography, John Wiley and Sons, Inc., New York 1961, p. 452. 2. F. Sh o s s b erg er, Z. phys. Chem. 29 (1935) 65. 3. I. Pietrok ow sky, Trans. AIME 200 (1954) 219. 4. W. B. Pe arson, A Handbook of Lattice Spacings and Structures of Metals and Alloys, Pergamon Press, Oxford, 1958, p. 120. 5. ibid p. 94. 6. D. W. Levinson, D. J. Mc Pherson, Trans. Amer. Soc. Met. 48 (1956) 689. 7. A. F. We 11 s, Structural Inorganic Chemistry, Clarendon Press, Oxford 1962, p. 999. 8. H. Pa u 1 y, A. Weiss, and H. Witte, Z. Metallk. 59 (1968) 47. CRYSTAL STRUCTURE OF TiCuHg2 IZVOD Kristalna struktura TiCuHg2 M. Puselj i Z. Ban 83 Rendgenografskim istrazivanjem presjeka TiHg-CuHg u sistemu Ti-Cu-Hg ustanovljeno je postojanje ternarne faze TiCuHg2• Struktura je kubicna s parametrom elementarne celije a = 6,155 A. Od tri moguca rasporeda atoma u strukturi, na osnovi rezultata racunanja intenziteta, odabran je onaj koji predstavlja strukturu tipa NaTl (B-32) sa sredenim rasporedom svih atomskih vrsta. Mjerenjem gustoce ustanovljeno je da se radi o djelomieno popunjenoj kristalnoj strukturi, jer elemen-tarna celija saddi priblifoo tri formulske jedinice. INSTITUT ZA ANORGANSKU I ANALITICKU KEMIJU SVEUCILISTA, ZAGREB Primljeno 24. sljecnja 1969. 

The Crystal Structure of TiCuHg2

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