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H U Güdel

Publications and source records attributed to H U Güdel.

At least 19 recordsLinked to original sources
PubMed2007

Upconversion Between 4f-5d Excited States in Tm2+-Doped CsCaCl3, CsCaBr3, and CsCaI3.

Near-infrared to visible upconversion luminescence in CsCaCl3:Tm2+, CsCaBr3:Tm2+ and CsCaI3:Tm2+ is presented and analysed. The upconversion process involves exclusively the 4f-5d excited states of Tm2+, which is a novelty among upconversion materials. The presence of more than one long-lived 4f-5d excited state is the prerequisite for this. Multiple emissions from Tm2+ are observed in the title compounds. This is made possible by the favourable energy structure within the 4f-5d states and the low phonon energies of the materials. The energy positions of the relevant 4f-5d states, and thus the photophysical and light emission properties, are affected by the chemical variation along the series. The upconversion efficiency increases from chloride to iodide and the mechanism is found to be a combination of absorption and energy-transfer steps.

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PubMed2006

Field-induced magnetoelastic instabilities in antiferromagnetic molecular wheels.

The magnetic torque of the antiferromagnetic molecular wheel CsFe8 was studied down to 50 mK and up to 28 T. Below about 0.5 K phase transitions were observed at the field-induced level crossings (LCs). Intermolecular magnetic interactions are very weak excluding field-induced magnetic ordering. A magnetoelastic coupling was considered. A generic model shows that the wheel structure is unconditionally unstable at the LCs, and the predicted torque curves explain the essential features of the data well.

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PubMed2005

Ground states, excited states, and metal-ligand bonding in rare earth hexachloro complexes: a DFT-based ligand field study.

Metal (4f)-ligand (Cl 3p) bonding in LnCl(6)(3-) (Ln = Ce to Yb) complexes has been studied on the basis of 4f-->4f and Cl,3p-->4f charge-transfer spectra and on the analysis of these spectra within the valence bond configuration interaction model to show that mixing of Cl 3p into the Ln 4f ligand field orbitals does not exceed 1%. Contrary to this, Kohn-Sham formalism of density functional theory using currently available approximations to the exchange-correlation functional tends to strongly overestimate 4f-3p covalency, yielding, for YbCl(6)(3-), a much larger mixing of Cl 3p-->4f charge transfer into the f(13) ionic ground-state wave function. Thus, ligand field density functional theory, which was recently developed and applied with success to complexes of 3d metals in our group, yields anomalously large ligand field splittings for Ln, the discrepancy with experiment increasing from left to the right of the Ln 4f series. It is shown that eliminating artificial ligand-to-metal charge transfer in Kohn-Sham calculations by a procedure described in this work leads to energies of 4f-4f transitions in good agreement with experiment. We recall an earlier concept of Ballhausen and Dahl which describes ligand field in terms of a pseudopotential and give a thorough analysis of the contributions to the ligand field from electrostatics (crystal field) and exchange (Pauli) repulsion. The close relation of the present results with those obtained using the first-principles based and electron density dependent effective embedding potential is pointed out along with implications for applications to other systems.

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PubMed2004

Origin of higher order magnetic exchange: evidence for local dimer exchange striction in CsMn0.28Mg0.72Br3 probed by inelastic neutron scattering.

The origin of higher-order exchange interactions in localized S-state systems has been the subject of intensive investigations in the past. In particular, it has been suggested that a biquadratic exchange term may arise from the magnetoelastic energy. Here we report on the pressure and temperature dependence of the excitation spectra of magnetic Mn2+ dimers in CsMn0.28Mg0.72Br3 probed by inelastic neutron scattering. Biquadratic exchange and a strong distance dependence of the bilinear exchange are observed. It is shown that the mechanism of local exchange striction may explain the occurrence of biquadratic exchange in accordance with the elastic properties of the compound.

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PubMed2001

Interference dips in the single-crystal absorption spectrum of the ti-mu-fluoro-bis(trifluorochromate(III)) complex.

The lowest energy spin-allowed crystal field band of (Et(4)N)(3)Cr(2)F(9) shows a distinct interference dip at 15,000 cm(-1) with an approximate width of 200 cm(-1). This spectroscopic feature is due to spin-orbit coupling between the (2)E and (4)T(2) excited states and is analyzed with a set of two coupled potential energy surfaces. The minimum of the (4)T(2) potential surface is displaced along at least two normal coordinates. The modes involved cannot be directly determined from the unresolved absorption spectrum, but are obtained from Raman spectra and from the well-resolved spin-forbidden crystal field transition to the (2)A(1) state. The first mode with a frequency of 415 cm(-1) has predominant Cr-F stretching character; the second mode has a frequency of 90 cm(-1) and involves the entire Cr(2)F(9)(3-) dimer.

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PubMed2001

Chemical tuning of the photon upconversion properties in Ti(2+)-doped chloride host lattices.

The photophysical properties of Ti(2+)-doped NaCl and MgCl(2) at 15 K are compared. At this temperature both materials emit luminescence from their respective lowest excited states and from the (3)T(1g)(t(2g)e(g)) higher excited state. In Ti(2+):MgCl(2) the ligand field is significantly stronger than in Ti(2+):NaCl, leading to (1)T(2g)(t(2g)(2)) and (3)T(2g)(t(2g)e(g)) lowest excited states, respectively, in these materials. The near-infrared to visible upconversion mechanisms of these two materials are identified. The upconversion efficiency calculated for Ti(2+):MgCl(2) is approximately 2 orders of magnitude higher than for Ti(2+):NaCl. This is mainly due to the more efficient energy storage in the intermediate upconversion level in Ti(2+):MgCl(2), and its higher (3)T(1g)(t(2g)e(g)) --> (3)T(1g)(t(2g)(2)) luminescence quantum yield relative to Ti(2+):NaCl.

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PubMed2001

Influence of Jahn-Teller coupling on the magnetic properties of transition metal complexes with orbital triplet ground terms: magnetization and electronic Raman studies of the titanium(III) hexa-aqua cation.

Magnetization and electronic Raman data are presented for salts of the type Cs[Ga:Ti](SO(4))(2) x 12H(2)O, which enable a very precise definition of the electronic structure of the [Ti(OH(2))(6)](3+) cation. The magnetization data exhibit a spectacular deviation from Brillouin behavior, with the magnetic moment highly dependent on the strength of the applied field at a given ratio of B/T. This arises from unprecedented higher-order contributions to the magnetization, and these measurements afford the determination of the ground-state Zeeman coefficients to third-order. The anomalous magnetic behavior is a manifestation of Jahn-Teller coupling, giving rise to low-lying vibronic states, which mix into the ground state through the magnetic field. Electronic Raman measurements of the 1%-titanium(III)-doped sample identify the first vibronic excitation at approximately 18 cm(-1), which betokens a substantial quenching of spin-orbit coupling by the vibronic interaction. The ground-state Zeeman coefficients are strongly dependent on the concentration of titanium(III) in the crystals, and this can be modeled as a function of one parameter, representing the degree of strain induced by the cooperative Jahn-Teller effect. This study clearly demonstrates the importance that the Jahn-Teller effect can have in governing the magnetic properties of transition metal complexes with orbital triplet ground terms.

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PubMed2001

Green and red light emission by upconversion from the near-IR in Yb(3+) doped CsMnBr3.

Direct near-IR excitation of Yb(3+) 2F(7/2)-->(2)F(5/2) levels at 10126, 10138, and 10596 cm(-1) in CsMnBr3:0.5%Yb(3+) leads to three types of luminescence at cryogenic temperatures: near-IR Yb(3+) emission and green and red upconverted luminescence. The green luminescence around 20 000 cm(-1) is identified as cooperative Yb(3+) pair upconversion. The broad red upconversion luminescence band centered at 14 700 cm(-1) is ascribed to the 4T(1g)-->6A(1g) transition of Mn(2+). Pulsed measurements indicate a sequence of ground-state absorption and excited-state absorption steps for the red upconversion process. One- and two-color excitation experiments support this, and we conclude that the red upconversion occurs by an exchange mechanism involving Yb(3+) and Mn(2+). The Yb(3+) 2F(5/2)-->(2)F(7/2) near-IR emission around 10 000 cm(-1) is also observed after Mn(2+) excitation at 21 838 cm(-1). This is indicative of a Mn(2+) 4T(1g)--> Yb(3+) 2F(5/2) relaxation process, which is a potential loss process for upconversion efficiency.

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PubMed2001

Ferromagnetic exchange in the ground and (4)A(1) excited states of (Et(4)N)(3)Fe(2)F(9). A magnetic and optical spectroscopic study.

The synthesis, crystal growth, magnetic susceptibility, and polarized optical absorption spectra in the visible and near UV of (Et(4)N)(3)Fe(2)F(9) are reported. From single-crystal magnetic susceptibility data and high-resolution absorption spectra in the region of the (6)A(1) --> (4)A(1) spin-flip transition, exchange splittings in the ground and excited states are derived. Ferromagnetic ground and excited state exchange parameters J(GS) = -1.55 cm(-1) and J(ES) = -0.53 cm(-1) are determined, respectively, and the relevant orbital contributions to the net exchange are derived from the spectra. The results are compared with those reported earlier for the structurally and electronically analogous [Mn(2)X(9)](5-) pairs in CsMgX(3):Mn(2+) (X = Cl(-), Br(-)), in which the splittings are antiferromagnetic. This major difference is found to be due to the increased metal charge of Fe(3+) compared to Mn(2+), leading to orbital contraction and thus to a strong decrease of the orbital overlaps and hence the antiferromagnetic interactions.

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PubMed2001

High-resolution luminescence and absorption spectroscopy of Cs2GeF6:Os4+.

The luminescence spectrum of the Os4+ dopant ion occupying O(h) sites of the Cs2GeF6 host shows three sets of resolved transitions in the near-infrared at approximately 12000, 9000, and 6700 cm(-1), corresponding to intraconfigurational transitions between the 1T2g lowest excited electronic state and the Gamma1, Gamma4, and Gamma5/Gamma3 spinor levels of the 3T1g ground state, respectively. The octahedral OsF6(2-) chromophore does not emit from higher excited states, in contrast to related chloride and bromide host lattices, where several excited states show luminescence. The highly resolved single-crystal luminescence and absorption spectra are rationalized with ligand field parameters 10 Dq = 24570 cm(-1), B = 500 cm(-1), C = 2380 cm(-1), zeta = 3000 cm(-1). Transition intensities reveal an intermediate coupling situation for OsF6(2-): Whereas they generally follow the selection rules derived in the L-S coupling scheme, additional information can be gained from the j-j coupling limit. The resolved vibronic structure allows the identification of the most efficient ungerade parity enabling modes (vibronic origins) and shows that progressions along the a1g, e(g), and t2g modes occur for some transitions.

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PubMed2001

Exchange pathways in tris-mu-thiolato-bridged [Cr(2)L(3)](ClO(4)2Cl*H2O*MeOH (L=2,6-bis(aminomethyl)-4-tert-butyl-thiophenolate).

From polarized optical absorption and emission spectra of the tris-mu-thiolato bridged [Cr(2)L(3)](ClO(4))(2)Cl.H(2)O.MeOH (L = 2,6-bis(aminomethyl)-4-tert-butyl-thiophenolate) 1 in the visible and near UV, the exchange splittings of the (4)A(2) ground and the (2)E and (2)T(1) excited states are determined. In view of the large Cr-Cr distance of 3.01 A, the antiferromagnetic ground state splitting with J = 78 cm(-1) (H = J(S(A).S(B))) is large compared to other triply bridged Cr(3+) dimers. This can be rationalized using a model based on a valence bond approach, in which the exchange splittings are derived from configuration interactions between the ground electron configuration and ligand-to-metal (LMCT) as well as metal-to-metal (MMCT) charge-transfer configurations. It allows us to distinguish the interactions via ligand orbitals from the direct interactions between the metal-centered orbitals. We compare our results with those obtained for the tri-mu-hydroxo-bridged [Cr(2)(OH)(3)(tmtame)(2)](NO(3))(3) (tmtame = N,N',N' '-trimethyl-1,1,1-tris(aminomethyl)ethane) 2. In the latter the interactions via the ligands are negligible, whereas they play an important role in 1, due to the softness of the sulfur ligand atoms.

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PubMed2001

Magnetic polyoxometalates: anisotropic antiferro- and ferromagnetic exchange interactions in the pentameric cobalt(II) cluster.

The ground-state properties of the pentameric Co(II) cluster [Co(3)W(D(2)O)(2)(CoW(9)O(34))(2)](12-) were investigated by combining magnetic susceptibility and low-temperature magnetization measurements with a detailed inelastic neutron scattering (INS) study on a fully deuterated polycrystalline sample of Na(12)[Co(3)W(D(2)O)(2)(CoW(9)O(34))(2)].46D(2)O. The encapsulated magnetic Co(5) unit consists of three octahedral and two tetrahedral oxo-coordinated Co(II) ions. Thus, two different types of exchange interactions are present within this cluster: a ferromagnetic interaction between the octahedral Co(II) ions and an antiferromagnetic interaction between the octahedral and the tetrahedral Co(II) ions. As a result of the single-ion anisotropy of the octahedral Co(II) ions, the appropriate exchange Hamiltonian to describe the ground-state properties of the Co(5) spin cluster is anisotropic and is expressed as H = -2 summation operator(i= x,y,z)J(1)(i)[S(1)(i)S(2)(i) + S(2)(i)S(3)(i)] + J(2)(i)[S(1)(i)S(5)(i) + S(2)(i)S(5)(i) + S(2)(i)S(6)(i) + S(3)(i)S(6)(i)], where J(1)(i) are the components of the exchange interaction between the octahedral Co(II) ions and J(2)(i) are the components of the exchange interaction between the octahedral and tetrahedral Co(II) ions (see Figure 1d). The study of the exchange interactions in the two structurally related polyoxoanions [Co(4)(H(2)O)(2)(PW(9)O(34))(2)](10)(-) and [Co(3)W(H(2)O)(2)(ZnW(9)O(34))(2)](12)(-) allowed an independent determination of the ferromagnetic exchange parameters J(1)(x) = 0.70 meV, J(1)(y) = 0.43 meV, and J(1)(z) = 1.51 meV (set a) and J(1)(x) = 1.16 meV, J(1)(y) = 1.16 meV and J(1)(z) = 1.73 meV (set b), respectively. Our analysis proved to be much more sensitive to the size and anisotropy of the antiferromagnetic exchange interaction J(2). We demonstrate that this exchange interaction exhibits a rhombic anisotropy with exchange parameters J(2)(x) = -1.24 meV, J(2)(y) = -0.53 meV, and J(2)(z) = -1.44 meV (set a) or J(1)(x) = -1.19 meV, J(1)(y) = -0.53 meV, and J(1)(z) = -1.44 meV (set b). The two parameter sets reproduce in a satisfactory manner the susceptibility, magnetization, and INS properties of the title compound.

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PubMed2001

Optical dimer excitations and exchange parameters in (Et4N)3Cr2F9: first observation of the 4A2 --> 2A1 transition.

The synthesis, crystal growth, and polarized optical absorption spectra in the visible and near-UV of (Et4N)3Cr2F9 are reported. In the energy range 25800-27700 cm(-1) the 4A2 --> 2A1 (O notation) ligand field transition can be resolved in detail for the first time in any Cr3+ compound. This allows the determination of the antiferromagnetic ground-state exchange splitting with great accuracy: J = 25.9 cm(-1) and j = 0.27 cm(-1) using the Hamiltonian H = J(S(A).S(B)) - j(S(A).S(B))2, where j leads to deviations from the regular Landé pattern. The temperature dependence of the magnetic susceptibility is nicely reproduced by these parameters. A comparison with Cs3Cr2Cl9 and Cs3Cr2Br9 reveals an exponential dependence of the ground-state splitting upon the Cr-Cr distance in the [Cr2X9]3- dimers. This is the result of a dominant sigma-type orbital exchange pathway along the Cr-Cr axis.

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PubMed2001

Photon upconversion properties of Ni2+ in magnetic and nonmagnetic chloride host lattices.

Near-infrared to visible upconversion luminescence in Ni2+:CsCdCl3, Ni2+:CsMnCl3, and Ni2+:RbMnCl3 is presented and analyzed. In all three materials upconversion occurs via a sequence of ground-state absorption/excited-state absorption processes, which are both formally spin-forbidden transitions. Consequently, in the diamagnetic Ni2+:CsCdCl3 they are weak, and the efficiency of the upconversion process is relatively low. This is in clear contrast to the isostructural Ni2+:RbMnCl3 where the spin selection rule relaxes because of Ni(2+)-Mn2+ exchange interactions, leading to an intensity enhancement of the spin-flip transitions involved in the Ni2+ upconversion mechanism. This results in an exchange-induced enhancement of the upconversion rate in Ni2+:RbMnCl3 relative to Ni2+:CsCdCl3 by 2 orders of magnitude after two-color excitation into the maxima of the ground-state and excited-state absorption bands. In Ni2+:CsMnCl3 the Ni(2+)-Mn2+ exchange interaction does not play a significant role. This is due to the different Ni(2+)-Cl(-)-Mn2+ bridging geometry relative to Ni2+:RbMnCl3. In contrast to Ni2+:CsCdCl3 and Ni2+:RbMnCl3 where the upconversion luminescence occurs from Ni2+, in Ni2+:CsMnCl3 the upconverted energy is emitted from Mn2+ in the visible spectral region. This leads to an enhanced visible upconversion luminescence in Ni2+:CsMnCl3, relative to the other two samples where Ni2+ near-infrared inter-excited-state emissions compete with the visible upconversion luminescence.

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PubMed2000

Crystal structure and optical spectroscopy of Er2[Pt(CN)4]3.21H2O and Er2[Pt(CN)4]2.SO4.11.5H2O.

We have synthesized two forms of erbium tetracyanoplatinates, Er2[Pt(CN)4]3.21H2O (red form) and Er2[Pt(CN)4]2.SO4.11.5H2O (yellow form), and determined their crystal structures by X-ray diffraction. While the red form crystallizes in the orthorhombic space group Pbcn, with a = 15.4848(3) A, b = 13.8186(2) A, c = 19.07820(10) A, alpha = beta = gamma = 90 degrees, and Z = 4, the yellow form precipitates in the tetragonal space group I4cm, with a = b = 14.321(2) A, c = 13.338(3) A, alpha = beta = gamma = 90 degrees, and Z = 4. Both forms show [Pt(CN)4]2- chains but differ markedly in color and morphology. This is due to the incorporation of sulfate ions in the latter modification, leading to an increased Pt-Pt distance. The observed optical absorption and emission behavior of the title compounds is correlated with the Pt-Pt distances.

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PubMed2000

Superexchange in magnetic insulators: an interpretation of the metal-metal charge transfer energy in the Anderson theory.

The superexchange interactions in four three-center model systems A-L-B, for A and B being paramagnetic centers and L a diamagnetic bridging ligand, are analyzed by valence bond configuration interaction models in combination with fourth-order perturbation theory. We analyze the four distinct cases where a bridging ligand orbital simultaneously interacts with half-filled orbitals localized on A and B (case i), a half-filled orbital localized on A and an empty orbital localized on B (case ii), a full orbital localized on A and a half-filled orbital localized on B (case iii), and finally a full orbital localized on A and an empty orbital localized on B (case iv). In all four cases we compare our new results using localized orbitals with the equivalent results obtained using the Anderson ansatz of delocalized (magnetic) orbitals. The effective metal-to-metal electron transfer energy Ueff in the old formalism with delocalized orbitals is expressed in terms of the metal-to-metal electron transfer energy U and the ligand-to-metal electron transfer energy delta using localized orbitals. We find that the old formalism containing only Ueff is in general not sufficient. For cases i and ii we show that Ueff can be regarded as an effective U strongly reduced with respect to the free ion as a result of hybridization effects, whereas the same reduction of U for the cases iii and iv is not possible. The relevance and applicability of our theoretical results is demonstrated on magnetochemical data from the literature.

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