The crystal structures of monoclinic ZrO2 [P21/c, a=5.1505 (1) [Fm3m, a=5.0858(1)./k, V= 131.55(1),/k 3, Z=4, Rwp= 0.083] have been refined by Rietveld analysis of 1.377 A neutron powder diffraction data collected at 295 K. In both tetragonal and cubic ZrO2, the stabilizer atoms randomly occupy the Zr site and charge balance is achieved by an appropriate number of vacancies on the O site. In cubic ZrO 2, the anions are displaced from their ideal fluorite positions by 0.025a in the [111] direction and there is evidence for the presence of either a small quantity of a tetragonal impurity phase, or a slight tetragonal distortion. IntroductionPure zirconia, ZrO2, is monoclinic at room temperature, tetragonal between ~ 1440 and ~2640 K, and cubic up to the melting point at ~2950K. The monoclinic phase is a distortion of the fluorite (CaF 2) structure with the Zr atom in seven coordination. In both high-temperature phases, the Zr atom assumes eight coordination, as in fluorite, but in the tetragonal form the O atom is substantially displaced from its ideal fluorite 1 1 ~ position. The tetragonal and cubic phases ~,~,~ of pure zirconia can be stabilized at room temperature by the addition of suitable oxides, namely MgO, CaO, 0108-7681/88/020116-05503.00Sc203, Y203 and certain rare-earth oxides. An orthorhombic form has also been prepared by quenching from high pressure and temperature (Suyama, Ashida & Kume, 1985) but this phase will not be considered further here.The crystal structures of, and mechanisms of the transformations between, the monoclinic, tetragonal and cubic phases are of considerable technical interest since they can be manipulated to provide optimized physical and chemical properties of the materials fabricated from the stabilized zirconia (Garvie, Hannink & Pascoe, 1975;Roth, 1975;Claussen, Ruhle & Heuer, 1984;Fisher, 1986). The so-called partially stabilized zirconias (PSZ), which are typically twophase cubic and tetragonal or single-phase tetragonal, are of importance for mechanical and structural applications. The fully stabilized zirconias (FSZ), which are normally single-phase cubic, are of interest for heating elements, oxygen sensors and fuel-cell applications.Crystal structure determinations have been performed on tetragonal ZrO 2 using X-ray powder diffraction intensities collected at 1470 to 2230 K by Teufer (1962). Monoclinic ZrO 2 (baddeleyite) has been studied at room temperature using X-ray single-crystal methods by McCullough & Trueblood (1959) and Smith & Newkirk (1965). Cubic Zr(Ca,Y)O2_ x solid solutions have been analyzed at various temperatures from both X-ray and neutron data by Carter & Roth (1968), Steele & Fender (1974), Faber, Mueller & Cooper (1978, Morinaga, Cohen & Faber (1979) and Horiuchi, Schultz, Leung & Williams (1984).© 1988 International Union of Crystallography C. J. HOWARD, R. J. HILL AND B. E. REICHERT 117The present study gives new refinements of the crystal structures and compositions of the three ZrO 2 polymorphs from high-resolution neutron pow...

13C N.m.r. spectra of the compounds [Os,(CO),,-,(PEt,),](n = 1 or 2) indicate that carbonyl exchange occurs selectively along one edge of the Os, triangle at low temperatures. The process involves only six of the 12n carbonyl groups in [Os,(CO),,-,(PEt3),](n = 1 or 2). and a mechanism involving a double carbonyl bridge is proposed.Laboratory, Lensfield Road, Cambridge CB2 1 EW A CARBON-^^ n.m.r. study of isotopically enriched [Os,(CO),,] from ca. 70-180 "C showed that rapid intramolecular scrambling of the equatorial and axial carbonyl groups 0ccurs.l At least three mechanisms may be proposed for this process, two involving the formation of CO bridges and the third a polytopal rearrangement about individual osmium atoms. It is not possible to differentiate between the mechanisms on the evidence a t present available. We have now undertaken a variabletemperature 13C n.m.r. study of some phosphine-substituted trinuclear osmium carbonyls in order to obtain further information about the mechanisms of carbonyl exchange in cluster compounds, although it must be borne in mind that the introduction of the phosphine may give rise to a different mechanism from that occurring in the parent carbonyls. There is evidence that the introduction of tertiary phosphines into polynuclear carbonyl clusters induces the formation of CO bridges2 I t therefore seemed reasonable to suppose that phosphinesubstituted derivatives [0s,(C0)1~-~(PEt,),] would more readily undergo CO scrambling by a CO migration process.However, in addition, and more interestingly, we find that for the clusters [Os3(CO)1z-,(PEt,),] (n = 1 or 2) two distinct fluxional processes occur at different temperatures. This has been found to be the case. RESULTS AND DISCUSSIONAt -60 "C the 13C n.m.r. spectrum of the trisubstituted species [OS,(CO),(PE~,),] (in toluene) showed two 13C carbonyl peaks at 6 199.4 and 186.4 p.p.m. (downfield from SiMe,) with an intensity ratio of ca. 2 : 1. The lower-field peak was split, but this is not clearly resolved.

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The X-ray structure determination of [Ru,(CO),,(C,Ph,)] confirms the previously proposed c/oso-Ru4C, unit with the alkyne capping a butterfly arrangement of the four ruthenium atoms. Crystals are monoclinic, space group PZ,/c, Z = 4, a = 9. 69(1), b = 16.88(2), c = 17.13(2) A, (3 = 90.67(5)"; R 0.056 for 2 929 independent reflections. Variable-temperature 13C n.m.r. spectroscopy has enabled the activation energy and entropy t o be measured for the mutual exchange of three carbonyl groups attached to one ruthenium, and also indicates that no exchange of carbonyls between different rutheniums occurs a t <373 K. REACTION of diphenylacetylene with [ R u ~( C O ) ~~]leads to formation of [Ru,(CO),,(C,Ph,)] .l The solid-state structure of this compound has now been determined by X-ray diffraction and this confirms the presence of a TABLE 1 Atomic co-ordinates ( x lo4) and isotropic temperature %la 3 103(1)

A single crystal X-ray diffraction study on [Os,(CO) 161 reveals a trigonal bipyramidal (D3J metal unit which is slightly distorted towards a capped butterfly arrangement (C,,). THE pink-red pentanuclear cluster [Os,(CO) 161 was first discovered by us as a product of the pyrolysis of [Os,(CO),,] in small amounts (ca. 7%) using extensive t.1.c.l The molecular formula [0s5(c0),,], like those of [os,(c0)1,] and 7 Reprints not available.[OS,(CO),,],~~~ was primarily deduced on the basis of mass spectrometry. We report here an X-ray structure determination of [Os,(CO),,] which supports our initial view that the carbonyl groups are iE an unusual arrangement around a distorted trigonal bipyramid of metal atoms.Crystal data: C,,O,,Os,, M 1399.2, trigonal, space group P3,21, a = b = 9-204(2), G = 24-818(4) A, 2 = 3, D, = 3.83 g ~m -~. Data were collected on an automated Stoe two-circle diffractometer using graphite monochromated