Spin dynamics in the manganese tetramer compound α-MnMoO(4).

α-MnMoO(4) is a tetrameric magnetic cluster system which undergoes a transition to three-dimensional antiferromagnetic order at T(N) = 10.7 K. In the ordered state the Mn(2+) spins (s(ν) = 5/2) are ferromagnetically aligned within the tetramer, resulting in a total cluster spin S = 10. The magnetic excitation spectrum consists of eight excitation modes of tetramer origin propagating in the whole reciprocal space. From single-crystal inelastic neutron scattering investigations the three-dimensional coupling scheme is rationalized in the framework of a tetramer-based dispersion model. The tetramer states and the energy dispersion of all the magnetic excitations are described by Heisenberg-like intra- and intercluster exchange interactions, respectively, thus α-MnMoO(4) is a suitable tetrameric model system to study the interplay of these interactions.

Scintillation properties and anomalous Ce(3+) emission of Cs(2)NaREBr(6):Ce(3+) (RE = La,Y,Lu).

We report the optical and scintillation properties of the Ce(3+)-doped bromoelpasolites Cs(2)NaREBr(6) (RE = La,Y,Lu). The γ-ray scintillation light yield of these materials varies from 6000 to 17 000 photons per MeV absorbed γ-ray energy. At room temperature (RT), the γ-ray scintillation decay curves for all compounds show a fast component of 61 ns, whereas the intrinsic Ce(3+) decay time is 30 ns. The scintillation mechanism in elpasolites is addressed. In Cs(2)NaLuBr(6):Ce(3+) and Cs(2)NaYBr(6):Ce(3+), we observe for the first time the so-called Ce(3+) anomalous emission in bromide compounds. This emission previously observed for chloride compounds is an ultrafast Ce(3+) emission with a selective excitation mechanism. The decay time of the anomalous emission at 10 K in bromide compounds (∼7.80 ns) is faster than that in chloride compounds (∼9.90 ns). Two bands of the anomalous emission are resolved for the first time. The mechanism behind this emission is discussed.

Charge-Transfer Spectra and Bonding in Tetrahedral Mn(VI), Cr(V), and V(IV) and Mn(VII), Cr(VI), and V(V) Oxo Anions.

Density functional theory (DFT) calculations on the tetrahedral Mn(VI), Cr(V), and V(IV)(d(1)) oxo anions in their ground and lowest excited d-d and O --> M charge transfer (CT) states are reported and used to assign the electronic absorption spectra by reference to the spectra of the isoelectronic Mn(VII), Cr(VI), and V(V) (d(0)) and the Mn(V) and Cr(IV) (d(2)) anions. Calculated geometrical shifts along the totally symmetric metal-ligand vibration (alpha(1)) for electronic excitations are in agreement with data deduced from experimental vibronic fine structures, supporting the proposed assignments. Using a CT model including (as different from DFT) configuration interaction (CICT), it is shown that the CT excited states of MnO(4)(2)(-) at 17 000, 23 300, and 28 200 cm(-)(1) are due to d(2) (3)A(2)(2e(2)), (1)E(2e(2)), and (3)A(2)(2e(2)) final states combining with a single hole (L) on the ligand 1t(1) and 4t(2) orbitals, respectively. The higher 10Dq and smaller B values for the d(2)L(d(1)) states compared to those of the d(2) systems correlate with the shortening of the metal-ligand bond accompanying the removal of electrons from the antibonding d orbitals, leading to an increase in covalency and a change in the ordering of CT states for Cr(V) with (3)T(2)(2e(1)5t(2)(1))L (10Dq) at a higher energy than (1)E(2e(2))L (8B + 2C) as compared to Cr(IV) with nearly degenerate (3)T(2)(2e(1)5t(2)(1)) and (1)E(2e(2)) terms. This allows one to estimate the energy of the (3)A(2)(2e(2))L --> (1)E(2e(2))L transition from the CT (d(2)L) spectrum of Cr(V)(d(1)), which could not be observed for Cr(IV). From a comparison of calculated and experimental oscillator strengths and Huang-Rhys factors (S) for the lowest CT band in the V(V), Cr(VI), and Mn(VII) (d(0)) and the V(IV), Cr(V), and Mn(VI) (d(1)) oxo anions, it is shown that the increase in covalency from left to right in this series is accompanied by a reduction in band intensity and S for the progression in the alpha(1) vibration. An explanation of this result in terms of ionic contributions to the metal-ligand bond increasing from Mn(VI) to Cr(V) and V(IV) is proposed. Intensities of "d-d" transitions display the opposite trend; increasing covalency leads to stronger mixing between d --> d and CT excited states and thus an increase in intensity.

Quantum phase interference and Néel-vector tunneling in antiferromagnetic molecular wheels.

The antiferromagnetic molecular wheel Fe18 of 18 exchange-coupled Fe;{III} ions has been studied by magnetic torque, magnetization, and inelastic neutron scattering. The combined data show that the low-temperature magnetism of Fe18 is very accurately described by the Néel-vector tunneling (NVT) scenario, as unfolded by semiclassical theory. In addition, the magnetic torque as a function of applied field exhibits oscillations that reflect the oscillations in the NVT splitting with field due to quantum phase interference.

Direct observation of magnon fractionalization in the quantum spin ladder.

We measure by inelastic neutron scattering the spin excitation spectra as a function of applied magnetic field in the quantum spin-ladder material (C5H12N)2CuBr4. Discrete magnon modes at low fields in the quantum disordered phase and at high fields in the saturated phase contrast sharply with a spinon continuum at intermediate fields characteristic of the Luttinger-liquid phase. By tuning the magnetic field, we drive the fractionalization of magnons into spinons and, in this deconfined regime, observe both commensurate and incommensurate continua.

Quantum magnets under pressure: controlling elementary excitations in TlCuCl3.

We follow the evolution of the elementary excitations of the quantum antiferromagnet TlCuCl3 through the pressure-induced quantum critical point, which separates a dimer-based quantum disordered phase from a phase of long-ranged magnetic order. We demonstrate by neutron spectroscopy the continuous emergence in the weakly ordered state of a low-lying but massive excitation corresponding to longitudinal fluctuations of the magnetic moment. This mode is not present in a classical description of ordered magnets, but is a direct consequence of the quantum critical point.

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.

Huge transverse magnetic polarization in the field-induced phase of the antiferromagnetic molecular wheel CsFe(8).

The 1H NMR spectrum and nuclear relaxation rate T(1)(-1) in the antiferromagnetic wheel CsFe8 were measured to characterize the previously observed magnetic field-induced low-temperature phase around the level crossing at 8 T. The data show that the phase is characterized by a huge staggered transverse polarization of the electronic Fe spins, and the opening of a gap, providing microscopic evidence for the interpretation of the phase as a field-induced magnetoelastic instability.

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.

Néel-vector tunneling in antiferromagnetic molecular clusters.

From inelastic neutron scattering experiments, we demonstrate quantum tunneling of the Néel vector in the antiferromagnetic molecular ferric wheel CsFe8. Analysis of the linewidth of the tunneling transition evidences coherent tunneling.

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.

Quantum statistics of interacting dimer spin systems.

The compound TlCuCl(3) represents a model system of dimerized quantum spins with strong interdimer interactions. We investigate the triplet dispersion as a function of temperature by inelastic neutron scattering experiments on single crystals. By comparison with a number of theoretical approaches we demonstrate that the description of Troyer, Tsunetsugu, and Würtz [Phys. Rev. B 50, 13 515 (1994)10.1103/Phys. Rev. B 50, 13515] provides an appropriate quantum statistical model for dimer spin systems at finite temperatures, where many-body correlations become particularly important.

Design of luminescent inorganic materials: new photophysical processes studied by optical spectroscopy.

Recent spectroscopic results in the emerging area of transition-metal NIR-to-visible upconversion are related. The examples of Ti(2+)-, Re(4+)-, and Os(4+)-doped materials showing upconversion illustrate GSA/ESA, GSA/ETU, and photon avalanche multiphoton excitation mechanisms, respectively. Strategies for manipulation of such upconversion processes using the spectroscopic or magnetic properties of the host material are described. High-resolution low-temperature continuous-wave absorption and emission and time-resolved emission experiments combine to yield information about energy splittings, intensities, and excited-state dynamics, and assist in the design and development of luminescent materials showing novel multiphoton excitation properties.

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.

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.

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.

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.

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.

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.

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.