Norman N. Greenwood

Ruthenium-99 Mössbauer studies of the magnetic properties of ternary and quaternary ruthenium(IV) oxides

Ruthenium-99 Mossbauer spectra have been obtained for the ternary oxides SrRuO3, CaRuO3, BaRuO3, and Y2Ru2O7 and for the quaternary oxides Sr(Ru1–xIrx)O3(x= 0·1 and 0·2) and Sr(Ru0·7Mn0·3)O3. The spectrum of SrRuO3 reveals the first example of a hyperfine magnetic field in a ruthenium compound; the internal field of 352 kG at 4·2 K is compatible with the ferromagnetic moment derived from neutron-diffraction data, and confirms the collective-electron magnetism model for this perovskite. A more precise value for the E2/M1 mixing ratio of 2·72 ± 0·17 has been obtained for the 99Ru Mossbauer γ transition. The oxide CaRuO3 shows only a single line and hence no magnetic ordering even at 4·2 K; it is not antiferromagnetic as previously reported but is Curie-Weiss paramagnetic. The oxide BaRuO3 also gives a singlet; there is no long-range antiferromagnetic ordering, but within each cluster of three face-sharing RuO6 units localised pairing of the 4d electrons takes place. The oxide Y2Ru2O7 is magnetically ordered at 4·2 K, with a hyperfine field of 126 kG. Substitution of ruthenium by iridium in the oxide SrRuO3 reduces the field at ruthenium by approximately 60 kG for each iridium nearest neighbour, whereas in the case of manganese substitution the reduction for each manganese neighbour is only 22 kG.

Polyhedral metallacarbaborane chemistry: preparation, molecular structure, and nuclear magnetic resonance investigation of [3-(η5-C5Me5)-closo-3,1,2-MC2B9H11] (M = Rh or Ir)

Reaction between Cs[nido-7,8-C2B9H11] and [{M(η5-C5Me5)Cl2}2] (M = Rh or Ir) yielded orange-yellow, air-stable crystals of [3-(η5-C5Me5)-closo-3,1,2-RhC2B9H11] [compound (2), 34%] or [3-(η5-C5Me5)-closo-3,1,2-IrC2B9H11] [compound (3), 96%] both of which were characterized by their assigned 11B and 1H n.m.r. spectra and by single-crystal X-ray diffraction analyses. Crystals of (2) were orthorhombic, space group P212121, with a = 1 081.0(2), b = 1 278.2(1), c = 1 278.4(2) pm, and Z = 4; R = 0.0197, R′ = 0.0204 for 1 756 observed reflections [I > 2.0σ(I)]. Crystals of (3) were also orthorhombic, space group P212121, with a = 1 076.3(1), b = 1 282.9(1), c = 1 292.8(2) pm, and Z = 4; R = 0.0286, R′ = 0.0307, for 1 712 observed reflections [I > 2.0σ(I)]. The n.m.r. properties of the C2B9H11 fragments of (2) and (3) are compared with the 11B and hitherto unreported 1H n.m.r. characteristics of the corresponding fragments of [3-(η5-C5H5)-closo-3,1,2-CoC2B9H11] (4), closo-1,2-C2B10H12, nido-7,8-C2B9H13, and [nido-7,8-C2B9H12]−, in order to assess any n.m.r. shielding patterns that might reveal bonding trends. The unique endo/bridging open-face hydrogen atom in [nido-7,8-C2B9H12]− is discussed in the light of its n.m.r. properties.

Kinetics and mechanism of the thermolysis and photolysis of binary boranes

The mechanisms by which gaseous boron hydrides so readily interconvert and build up into larger clusters has excited considerable academic and industrial interest for several decades. This paper describes recent progress that has been made in unravelling this complex series of interconversion reactions. Initial reaction rates have been studied mass spectrometrically to obtain rate equations, orders of reaction and energies of activation. Detailed and continuous product analysis for H2 and all the volatile boranes formed, coupled with a study of cothermolysis reactions of selected pairs of boranes gives further insight into the processes occurring. Crucial aspects of the thermolysis of B2H6, B4H1O, B5H11, and B6H1O are discussed, as are the effects of added H2 and the cothermolysis of B6H1O with alkenes. The final section presents data on the UV absorption spectra and photolytic stability of eight volatile boranes and the reaction kinetics of B6H1O photolysis.

A study of the new perovskite solid solution series SrFexRu1−xO3−y by ruthenium-99 and iron-57 Mössbauer spectroscopy

Abstract The magnetic and structural properties of the solid solution SrFexRu1−xO3−y (0 ⩽ x ⩽ 0.5) have been studied using 57Fe and 99Ru Mossbauer spectroscopy and other techniques. These phases, which are here reported for the first time, have a distorted perovskite structure. The iron substitutes exclusively as Fe3+ and thereby causes oxygen deficiency, but has little effect on the magnetic behaviour of the Ru4+ until x > 0.2, whereupon the metallic band system begins to revert to a localized electron structure. The properties of a sample with x = 0.3 are complex and intermediate in character. For x > 0.3 the oxygen deficiency is reduced by substantial oxidation to Ru5+ until at x = 0.5 the system corresponds to Sr2Fe3+Ru5+O6.

The first osmaboranes and a new iridatetraborane

Abstract The reactions of [Os(CO)ClH(PPh 3 ) 3 ] under mild conditions with the anions arachno -[B 3 H 8 ] − and nido -[B 5 H 8 − yield the first air-stable polyhedral osmaborane species arachno -[(HOsB 3 H 8 )(CO)(PPh 3 ) 2 ] (65%) and nido -[(OsB 5 H 9 )(CO)(PPh 3 ) 2 ] (80%) respectively. The 11 B and 1 H NMR properties of these osmaboranes are similar to those of their iridium analogues arachno -[(IrB 5 H 8 )(CO)(PPh 3 ) 2 ]. Mild thermolysis of nido -[(OsB 5 H 9 (CO)(PPh 3 ) 2 ] yields nido -[(OsB 4 H 8 )(CO)(PPh 3 ) 2 ] (40%) for which there is, as yet, no iridium analogue.

A study of the new cubic, ordered perovskites BaLaMRuO6 (M = Mg, Fe, Co, Ni, or Zn) and the related phases La2MRuO6 (M = Mg, Co, Ni, or Zn) by 99Ru Mössbauer spectroscopy and other techniques

Abstract 57 Fe and 99 Ru Mossbauer spectroscopy, coupled with magnetic susceptibility measurements down to 4.2 K, have been used to study the electronic and magnetic properties of the new cubic-ordered perovskites BaLa M RuO 6 ( M = Mg, Fe, Co, Ni, or Zn). The ruthenium is present in the +5 oxidation state in all the compounds except BaLaFeRuO 6 which contains iron(III) and ruthenium(IV). All the compounds exhibit long-range antiferromagnetic order, with Neel temperatures in the range 20–40 K. Mossbauer spectra for the new compound La 2 CoRuO 6 and the isostructural cubic perovskites La 2 M RuO 6 ( M = Mg, Ni, or Zn) confirm the presence of ruthenium(IV) in these phases and indicate that they are not ordered magnetically at 4.2 K.

A study of the ruthenium(V) perovskites M2LnRuO6 (M = Ca, Ln = Y, La, or Eu; M = Sr, Ln = Y; M = Ba, Ln = La or Eu) by 99Ru Mössbauer spectroscopy and other techniques

Abstract Ruthenium-99 Mossbauer spectroscopy has been used in conjunction with other techniques to examine the new perovskite phases Ca 2 Ln RuO 6 ( Ln = Y, La, or Eu) and the recently reported analogous strontium and barium compounds M 2 Ln RuO 6 ( M = Sr, Ln = Y; M = Ba, Ln = La or Eu). At 4.2°K the spectra show widely split magnetic hyperfine patterns ( B = 56–60 T) having chemical isomer shifts in the range 0.13-0.18 mm sec −1 relative to ruthenium metal, thereby confirming the presence of magnetically ordered ruthenium(V)ions in these phases. The computed linewidths for the yttrium compounds are close to the natural value, indicative of unique ruthenium environments and the existence of crystallographic order between Ru V and Y III ions on the B -sites. This is further corroborated by the observation of superlattice reflections in the X-ray powder diffraction pattern of Ca 2 YRuO 6 . Superlattice reflections are also observed for Ca 2 LaRuO 6 , Ba 2 LaRuO 6 , and Ba 2 EuRuO 6 . The Mossbauer spectra for the barium compounds feature sharp paramagnetic components in addition to the magnetically split ruthenium(V) resonances, and possible explanations for these unusual results are discussed. Magnetic susceptibility measurements above 80°K show that all the compounds follow a Curie-Weiss law with negative paramagnetic Curie temperatures, indicative of antiferromagnetism. Measurements down to 4.2°K for the calcium compounds reveal Neel temperatures in the region of 15°K when Ln = Y or La, whereas Ca 2 EuRuO 6 appears to be ferrimagnetic. Electrical resistivity measurements on compacted powders indicate that the materials are not metallic conductors. Mossbauer and powder X-ray diffraction measurements for the phase reported as Sr 2 LaRuO 6 indicate that it is in fact a mixture of Sr 2 RuO 4 and a solid-solution phase of the type Sr 2 La x Ru 1− x O 4 .

A study of the perovskite solid solution series LaxSr1−xRuO3 and LaxCa1−xRuO3 by ruthenium-99 Mössbauer spectroscopy

Abstract The magnetic, electronic, and structural properties of the solid solutions La x Sr 1− x RuO 3 and La x Ca 1− x RuO 3 have been studied by 99 Ru Mossbauer spectroscopy and other techniques. The La x Ca 1− x RuO 3 phases are reported for the first time and have been shown by powder X-ray diffraction measurements to be orthorhombically distorted perovskites. Electrical resistivity measurements on compacted powders show that all the phases are metallic with p ∼ 10 −3 , ohm-cm. Progressive substitution of Sr 2+ by La 3+ in ferromagnetic SrRuO 3 leads to a rapid collapse of the magnetic hyper-fine splitting at 4.2°K. For x = 0.25 some ruthenium ions still experience a magnetic field but for 0.4 ⩽ x ⩽ 0.75 only single, narrow resonance lines are observed, consistent both with the complete removal of the ferromagnetism and with the presence of an averaged ruthenium oxidation state in each phase, i.e., La x 3+ Sr 1− x 2+ Ru (4− x )+ O 3 rather than La x 3+ Sr 1− x 2+ Ru x 3+ Ru 1− x 4+ O 3 . LaRuO 3 and CaRuO 3 both give essentially single-line spectra at 4.2°K, indicating that the ruthenium ions in these oxides are not involved in long-range antiferromagnetic order but are paramagnetic. The solid solutions La x Ca 1− x RuO 3 (0 x ⩽ 0.6) give sharp symmetrical singlets with chemical isomer shifts (relative to the Ru metal) which move progressively from the value characteristic of Ru 4+ (−0.303 mm sec −1 ) toward the value for Ru 3+ (−0.557 mm sec −1 ), consistent with the presence of intermediate ruthenium oxidation states in these phases also.

Etude par effet Mössbauer des composés de type “bronzes fluorés” MxFeF3 (M = K, Rb, Cs, NH4)

Abstract The hexagonal, tetragonal and pyrochlore-type nonstoichiometric iron fluorides M x FeF 3 (M = K, Rb, Cs, NH 4 ) have been studied by Mossbauer spectroscopy over the temperature range 4.2 to 295 K. The magnetic transition temperatures have been determined. The ferrous and ferric ions remain in discrete oxidation states indicating the absence of charge hopping. The broadened lines of the spectra of the hexagonal and tetragonal phases are consistent with the disordering of Fe 2+ and Fe 3+ in the structure. By contrast, the narrow linewidths of the spectra of the pyrochlore-type phases characterize a structural ordering between the ferrous and ferric ions.

A study of the magnetic superexchange interactions in the solid-solution series CaxSr1−xRuO3 by ruthenium-99 Mo¨ssbauer spectroscopy

Abstract Ruthenium-99 Mo¨ssbauer spectroscopy has been used to examine magnetic superexchange interactions in the distorted perovskite solid-solutions CaxSr1−xRuO3 (x = 0.1, 0.2, 0.3, 0.4, and 0.5). The end members of this series also have a slightly distorted perovskite structure but CaRuO3 is Curie-Weiss paramagnetic, with only a single-line Mo¨ssbauer spectrum, whereas SrRuO3 is ferromagnetic and shows a broad well-resolved hyperfine pattern. For x ⩽ 0.2 a substantial proportion of the ruthenium atoms experience a magnetic flux density (hyperfine magnetic field) close to 35T, but inward collapse of the spectrum suggests that an increasing proportion of ruthenium atoms experience smaller flux densities. For samples with x ⩽ 0.3 there is an intense central “paramagnetic” component which increases rapidly with increasing x. The observed behaviour is incompatible with a conventional localized electron structure but can be interpreted satisfactorily on a collective electron model in which the average spin moment and hence the magnetic flux density at any given ruthenium atom is proportional to the strength of the exchange interactions with the six nearest-neighbour ruthenium atoms. The results imply that the greater electron-pair acceptor strength (Lewis acidity) of Ca2+ compared to Sr2+ results in a more effective competition with ruthenium for the oxygen anion orbitals involved in the superexchange interaction. It appears that, for a ruthenium to have a coupled spin-moment, it must have at least two exchange interactions through cube faces containing at least three strontium atoms. Possible origins of the reduced magnetic moment of SrRuO3 are discussed and it is suggested that the latter probably stems from spin-canting rather than from partial overlap of spin-up and spin-down bands.