Static Jahn-Teller Distortion Research Articles

Spin-Orbit-Lattice Entangled State in A_{2}MgReO_{6} (A=Ca, Sr, Ba) Revealed by Resonant Inelastic X-Ray Scattering.

The 5d^{1} ordered double perovskites present an exotic playground for studying novel multipolar physics due to large spin-orbit coupling. We present Re L_{3} edge resonant inelastic x-ray scattering (RIXS) results that reveal the presence of the dynamic Jahn-Teller effect in the A_{2}MgReO_{6} (A=Ca, Sr, Ba) family of 5d^{1} double perovskites. The spin-orbit excitations in these materials show a strongly asymmetric line shape and exhibit substantial temperature dependence, indicating that they are dressed with lattice vibrations. Our experimental results are explained quantitatively through a RIXS calculation based on a spin-orbit-lattice entangled electronic ground state with the dynamic Jahn-Teller effect taken into consideration. We find that the spin-orbit-lattice entangled state is robust against magnetic and structural phase transitions as well as against significant static Jahn-Teller distortions. Our results illustrate the importance of including vibronic coupling for a complete description of the ground state physics of 5d^{1} double perovskites.

Jahn-Teller Distortions and Phase Transitions in LiNiO2: Insights from Ab Initio Molecular Dynamics and Variable-Temperature X-ray Diffraction.

The atomistic structure of lithium nickelate (LiNiO2), the parent compound of Ni-rich layered oxide cathodes for Li-ion batteries, continues to elude a comprehensive understanding. The common consensus is that the material exhibits local Jahn-Teller distortions that dynamically reorient, resulting in a time-averaged undistorted R3̅m structure. Through a combination of ab initio molecular dynamics (AIMD) simulations and variable-temperature X-ray diffraction (VT-XRD), we explore Jahn-Teller distortions in LiNiO2 as a function of temperature. Static Jahn-Teller distortions are observed at low temperatures (T < 250 K) via AIMD simulations, followed by a broad phase transition that occurs between 250 and 350 K, leading to a highly dynamic, displacive phase at high temperatures (T > 350 K), which does not show the four short and two long bonds characteristic of local Jahn-Teller distortions. These transitions are followed in the AIMD simulations via abrupt changes in the calculated pair distribution function and the bond-length distortion index and in X-ray diffraction via the monoclinic lattice parameter ratio, amon/bmon, and δ angle, the fit quality of an R3̅m-based structural refinement, and a peak sharpening of the diffraction peaks on heating, consistent with the loss of distorted domains. Between 250 and 350 K, a mixed-phase regime is found via the AIMD simulations where distorted and undistorted domains coexist. The repeated change between the distorted and undistorted states in this mixed-phase regime allows the Jahn-Teller long axes to change direction. These pseudorotations of the Ni-O long axes are a side effect of the onset of the displacive phase transition. Antisite defects, involving Li ions in the Ni layer and Ni ions in the Li layer, are found to pin the undistorted domains at low temperatures, impeding cooperative ordering at a longer length scale.

Evidence for nanosized magnetic clusters of Ce-ions in the archetypal heavy fermion metal CeB6

Fine details of crystal structure of archetypal CeB6 hexaboride with heavy fermions are studied at temperatures 85 and 500 K by precise X-ray diffraction technique. Small static Jahn-Teller distortions of a simple cubic lattice are observed at these temperatures, leading to emergence of (i) dynamic charge stripes along selected directions <110>, <100>, and <111> in the crystals in combination with (ii) vibrationally coupled pairs of Ce ions. Instead of a Currie-Weiss type behavior, the temperature dependence of magnetization M∼(T − TCrand)−0.8 with TCrand∼TQ∼3.3 K was deduced in a wide temperature range 5–800 K for various directions of external magnetic field, which indicates the Griffiths phase formation with nanosized clusters of magnetic ions in CeB6. Moreover, in contrast to the scenario of a single-ion Kondo lattice, when revealing the power-law behavior of the magnetic contribution ρm(T)∼T−0.4 to the resistivity in the range of 8–90 K, we conclude in favor of the regime of weak localization of charge carriers in CeB6. Fourier maps of electron density confirm the conclusion about the nanosized magnetic clusters of Ce ions in this archetypal strongly correlated electron system with unusual magnetic ground state.

Low temperature singularities of electron density in a two-gap superconductor ZrB12

Fine details of crystal structure of ZrB12 two-gap superconductor are studied at low temperatures by precise x-ray diffraction technique. Small static Jahn-Teller distortions of a face centered cubic lattice are observed in the range 30–70 K, leading to emergence of two types singularities of electron density (ED). These are: (i) dynamic charge stripes along selected directions <110> and <112> in the crystal and (ii) triangular lattice of the ED antinodes located in the interstices of the boron lattice in the {111} planes. The second anomaly was detected for the first time, and it is very pronounced for ZrB12, being intensified at low temperatures. The anisotropy of the two-gap superconductivity in ZrB12 is confirmed by measurements of the low temperature heat capacity in a magnetic field directed along three principal axes in the crystal, H || [100], H || [110] and H || [111]. We conclude in favor of the magnetic field-induced anisotropy arising from the interaction of vortex lattice in this superconductor with fluctuating ED (charge stripes) of a two-type filamentary structure.

Effect of Static Jahn-Teller Distortion on the Li+ Transport in a Copper Hexacyanoferrate Framework.

Prussian blue (PB) and its analogues (PBAs) are potential cathode-active materials for rechargeable lithium-ion batteries. Although a body of research has assessed the performances of various PB/PBA cathodes with an eye to practical use, the underlying Li+-transport mechanism is still unclear. Focusing on copper hexacyanoferrate (CuHCF), a PBA that exhibits static Jahn-Teller (JT) distortion, we theoretically investigate how the framework's distortion affects the pathways and energetics of the Li+ transport. Density functional theory calculations of a local structure model of CuHCF reveal that the static JT distortion makes the favorable Li+-transport pathways quasi-two-dimensional, contrary to an intuitive picture of isotropic Li+ diffusion within the regular jungle-gym framework. The pathways are mutually interconnected, thereby creating an almost barrierless transport network. To better understand the distortion-induced transport anisotropy, we visually analyze the framework's electronic structure and noncovalent Li+-framework interactions. This study helps deepen the fundamental understanding of intrinsic Li+-transport properties of a distorted porous framework.

Transition-Metal Monofluorophosphate Ba2M2(PO3F)F6 (M = Mn, Co, and Ni): Varied One-Dimensional Transition-Metal Chains and Antiferromagnetism.

Four transition-metal monofluorophosphates, with a chemical formula of Ba2M2(PO3F)F6 (M = Mn, Co, Ni, and Cu), have been synthesized hydrothermally using phosphoric and fluorophosphoric acids. The structures of Ba2M2(PO3F)F6 were intensively investigated by single-crystal and powder X-ray diffraction. Their networks exhibited three-dimensional frameworks formed by cis-MO2F4 octahedra and PO3F tetrahedra. Ba2M2(PO3F)F6 (M = Mn, Co, and Cu) are isostructural, crystallizing in the monoclinic space group P21/c, while Ba2Ni2(PO3F)F6 adopted an unpredetermined structure, crystallizing in the monoclinic space group P21/n. The different arrangements of the same cis-MO2F4 moieties and PO3F groups within one-dimensional chains lead to different frameworks in this late-first-row transition-metal series. By a comparison of normalized bond lengths with their normal counterparts, the more elongated octahedral cis-MO2F4 units in Ba2Cu2(PO3F)F6 reveal the presence of static Jahn-Teller distortion, uncovering the puzzling reason for the exceptional structure of Ba2Ni2(PO3F)F6 in the series. Ba2M2(PO3F)F6 (M = Mn, Co, and Ni) forms spin chains of M2+ made of coupled dimers or tetramers, showing dominant antiferromagnetic behavior.

Zeeman interaction and Jahn-Teller effect in the Γ8 multiplet

We present a thorough analysis of the interplay of magnetic moment and the Jahn-Teller effect in the $\Gamma_8$ cubic multiplet. We find that in the presence of dynamical Jahn-Teller effect, the Zeeman interaction remains isotropic, whereas the $g$ and $G$ factors can change their signs. The static Jahn-Teller distortion also can change the sign of these $g$ factors as well as the nature of the magnetic anisotropy. Combining the theory with state-of-the-art {\it ab initio} calculations, we analyzed the magnetic properties of Np$^{4+}$ and Ir$^{4+}$ impurity ions in cubic environment. The calculated $g$ factors of Np$^{4+}$ impurity agree well with experimental data. The {\it ab initio} calculation predicts strong Jahn-Teller effect in Ir$^{4+}$ ion in cubic environment and the strong vibronic reduction of $g$ and $G$ factors.

FeCr2O4spinel to near megabar pressures: Orbital moment collapse and site-inversion facilitated spin crossover

The interplay between lattice, orbital, and spin degrees of freedom in iron chromite (spinel $[\mathrm{Fe}]{\mathrm{C}{\mathrm{r}}_{2}}{\mathrm{O}}_{4})$ has been investigated to near megabar pressures. The cubic-to-tetragonal transition, from static Jahn-Teller distortions at the Fe locality, rises from ${T}_{\mathrm{J}\text{\ensuremath{-}}\mathrm{T}}\ensuremath{\sim}135\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ at ambient pressure to 300 K by $\ensuremath{\sim}12\phantom{\rule{0.16em}{0ex}}\mathrm{GPa}$. The tetragonal distortion progressively increases and orbital moment quenching is triggered beyond $\ensuremath{\sim}24\phantom{\rule{0.16em}{0ex}}\mathrm{GPa}$, as monitored by the magnetic hyperfine field ${H}_{hf}$ at Fe sites. In the range 30--60 GPa, original $\mathrm{F}{\mathrm{e}}^{2+}$ tetrahedral sites with an unquenched orbital moment $({H}_{hf}\ensuremath{\sim}20\phantom{\rule{0.16em}{0ex}}\mathrm{T})$ coexist with newly evolved $\mathrm{F}{\mathrm{e}}^{2+}$ at tetrahedral sites having the orbital moment quenched and a resultant large ${H}_{hf}\ensuremath{\sim}35\phantom{\rule{0.16em}{0ex}}\mathrm{T}$. Additionally, new $\mathrm{F}{\mathrm{e}}^{2+}$ sites having distinguishable orbital moment quenching signatures $({H}_{hf}\ensuremath{\sim}42\phantom{\rule{0.16em}{0ex}}\mathrm{T})$ are discerned. Those sites also have other Fe nuclear hyperfine interaction parameter values typical of $\mathrm{F}{\mathrm{e}}^{2+}$ in octahedral coordination. There is a concurrent change to a steeper decrease of unit-cell volume as pressure rises above $\ensuremath{\sim}30\phantom{\rule{0.16em}{0ex}}\mathrm{GPa}$. These electronic and lattice responses are interpreted as signatures of progressive partial spinel inversion from high-spin $\mathrm{Fe}\ensuremath{\leftrightarrow}\mathrm{Cr}$ tetrahedral/octahedral site exchange, triggered near $\ensuremath{\sim}30\phantom{\rule{0.16em}{0ex}}\mathrm{GPa}$. Beyond 60 GPa a new diamagnetic low-spin M\ossbauer spectral component emerges. This is preceded by an inflection and discontinuity in the pressure dependence of the resistance and tetragonal unit-cell volume, respectively. By $\ensuremath{\sim}93\phantom{\rule{0.16em}{0ex}}\mathrm{GPa}$, half of the iron is low spin in octahedral sites from evolved tetragonal-spinel inversion processes. The remainder reside in tetrahedral high-spin sites with ${H}_{hf}\ensuremath{\sim}30\phantom{\rule{0.16em}{0ex}}\mathrm{T}$. The charge gap from electron correlations does not close, despite an anticipated appreciable band broadening from a $\ensuremath{\sim}30%$ unit-cell volume reduction upon pressurization to $\ensuremath{\sim}93\phantom{\rule{0.16em}{0ex}}\mathrm{GPa}$. This is attributable to an increase in effective on-site repulsion (Hubbard ${U}_{\mathrm{eff}}$) with increasing pressure or at spin crossover, specific to the $\mathrm{C}{\mathrm{r}}^{3+}({d}^{3})$ and $\mathrm{F}{\mathrm{e}}^{2+}({d}^{6})$ electronic configurations, respectively. Thus a $\ensuremath{\sim}200\phantom{\rule{0.16em}{0ex}}\mathrm{meV}$ ``Mott'' gap prevails in the mixed spin-state of the partially inverted tetragonal phase $[\mathrm{F}{\mathrm{e}}_{1\ensuremath{-}x}\mathrm{C}{\mathrm{r}}_{x}]{\mathrm{F}{\mathrm{e}}_{x}\mathrm{C}{\mathrm{r}}_{2\ensuremath{-}x}}{\mathrm{O}}_{4}\phantom{\rule{0.28em}{0ex}}(x\ensuremath{\sim}0.5)$ up to near a megabar (100 GPa) densification.

Conducting π Columns of Highly Symmetric Coronene, The Smallest Fragment of Graphene.

Coronene, which is the smallest D6h -symmetric polycyclic aromatic hydrocarbon, attracts particular attention as a basic component of electronic materials because it is the smallest fragment of graphene. However, carrier generation by physical methods, such as photo- or electric field-effect, has barely been studied, primarily because of the poor π-conduction pathway in pristine coronene solid. In this work we have developed unprecedented π-stacking columns of cationic coronene molecules by electrochemical hole-doping with polyoxometallate dianions. The face-to-face π-π interactions as well as the partially charged state lead to electrical conductivity at room temperature of up to 3 S cm(-1) , which is more than 10 orders of magnitude higher than that of pristine coronene solid. Additionally, the robust π-π interactions strongly suppress the in-plane rotation of the coronene molecules, which has allowed the first direct observation of the static Jahn-Teller distortion of cationic coronene molecules.

Nanoscale orbital excitations and the infrared spectrum of a molecular Mott insulator: A15-Cs3C60.

The quantum physics of ions and electrons behind low-energy spectra of strongly correlated molecular conductors, superconductors and Mott insulators is poorly known, yet fascinating especially in orbitally degenerate cases. The fulleride insulator Cs3C60 (A15), one such system, exhibits infrared (IR) spectra with low temperature peak features and splittings suggestive of static Jahn-Teller distortions with a breakdown of orbital symmetry in the molecular site. That is puzzling, since there is no detectable static distortion, and because the features and splittings disappear upon modest heating, which they should not. Taking advantage of the Mott-induced collapse of electronic wavefunctions from lattice-extended to nanoscale localized inside a caged molecular site, we show that the unbroken spin and orbital symmetry of the ion multiplets explains the IR spectrum without adjustable parameters. This demonstrates the importance of a fully quantum treatment of nuclear positions and orbital momenta in the Mott insulator sites, dynamically but not statically distorted. The observed demise of these features with temperature is explained by the thermal population of a multiplet term whose nuclear positions are essentially undistorted, but whose energy is very low-lying. That term is in fact a scaled-down orbital excitation analogous to that of other Mott insulators, with the same spin 1/2 as the ground state, but with a larger orbital momentum of two instead of one.

Absence of Jahn−Teller transition in the hexagonal Ba3CuSb2O9 single crystal

With decreasing temperature, liquids generally freeze into a solid state, losing entropy in the process. However, exceptions to this trend exist, such as quantum liquids, which may remain unfrozen down to absolute zero owing to strong quantum entanglement effects that stabilize a disordered state with zero entropy. Examples of such liquids include Bose-Einstein condensation of cold atoms, superconductivity, quantum Hall state of electron systems, and quantum spin liquid state in the frustrated magnets. Moreover, recent studies have clarified the possibility of another exotic quantum liquid state based on the spin-orbital entanglement in FeSc2S4. To confirm this exotic ground state, experiments based on single-crystalline samples are essential. However, no such single-crystal study has been reported to date. Here, we report, to our knowledge, the first single-crystal study on the spin-orbital liquid candidate, 6H-Ba3CuSb2O9, and we have confirmed the absence of an orbital frozen state. In strongly correlated electron systems, orbital ordering usually appears at high temperatures in a process accompanied by a lattice deformation, called a static Jahn-Teller distortion. By combining synchrotron X-ray diffraction, electron spin resonance, Raman spectroscopy, and ultrasound measurements, we find that the static Jahn-Teller distortion is absent in the present material, which indicates that orbital ordering is suppressed down to the lowest temperatures measured. We discuss how such an unusual feature is realized with the help of spin degree of freedom, leading to a spin-orbital entangled quantum liquid state.

Electronic structure ofBa3CuSb2O9: A candidate quantum spin liquid compound

Using density-functional methods, we study the electronic structure of ${\mathrm{Ba}}_{3}\mathrm{Cu}{\mathrm{Sb}}_{2}{\mathrm{O}}_{9}$, a candidate material for the quantum spin liquid behavior. We study both the triangular lattice as well as the recently proposed hexagonal lattice structures with flipped Cu-Sb dumbbells. The band structure near the Fermi energy is described very well by a tight-binding Hamiltonian involving the Cu (${e}_{g}$) orbitals, confirming their central role in the physics of the problem. A minimal tight-binding Hamiltonian for the triangular structure is presented. The Cu (${d}^{9}$) ions (a single ${e}_{g}$ hole in the band structure) present in the compound are expected to be Jahn-Teller centers, while the nature of the Jahn-Teller distortions in this material is still under debate. Solving a simple model by exact diagonalization, we show that electronic correlation effects in general enhance the tendency towards a Jahn-Teller distortion by reducing the kinetic energy due to correlation effects. Our density-functional calculations do indeed show a significant Jahn-Teller distortion of the $\mathrm{Cu}{\mathrm{O}}_{6}$ octahedra when we include the correlation effects within the Coulomb-corrected GGA+U method, so that the Jahn-Teller effect is correlation driven. We argue for the presence of a random static Jahn-Teller distortion in the hexagonal structure rather than a dynamical one because of the broken octahedral symmetry around the $\mathrm{Cu}{\mathrm{O}}_{6}$ octahedra and the potential fluctuations inherently present in the system caused by a significant disorder, which is believed to be present, in particular, due to the flipped Cu-Sb dumbbells.

Crystal structure, oxidation state and magnetism of SrxLa2−xCu0.5Ru0.5O4 (x=1, 1.5)

SrxLa2−xCu0.5Ru0.5O4 (x=1, 1.5) oxides with K2NiF4-type structure were prepared by solid state reaction and characterized by powder X-ray diffraction, X-ray photoelectron spectroscopy, magnetic and electrical resistivity measurements. The SrLaCu0.5Ru0.5O4 phase was obtained for the first time with a negligible amount of impurities. The octahedral Cu/RuO6 units are more elongated in SrLaCu0.5Ru0.5O4 than in Sr1.5La0.5Cu0.5Ru0.5O4 indicating a greater extent of static Jahn–Teller distortion. XPS suggests that mixed ion pairs Ru5+/Ru4+↔Cu+/Cu2+ are present in SrLaCu0.5Ru0.5O4, while Ru remains as Ru5+ and Cu as Cu2+ in Sr1.5La0.5Cu0.5Ru0.5O4. Both samples show spin-glass behavior, which can be explained by competition between ferromagnetic and antiferromagnetic superexchange interactions. The negative Weiss temperature estimated for SrLaCu0.5Ru0.5O4, −318K, is significantly lower than −11.5K deduced for Sr1.5La0.5Cu0.5Ru0.5O4 which may be related to the higher static Jahn–Teller distortion in the former oxide.

Temperature-dependent and anisotropic optical response of layered Pr0.5Ca1.5MnO4probed by spectroscopic ellipsometry

We study the temperature dependence as well as anisotropy of optical conductivity (${\ensuremath{\sigma}}_{1}$) in the pseudocubic single crystal Pr${}_{0.5}$Ca${}_{1.5}$MnO${}_{4}$ using spectrocopic ellipsometry. Three transition temperatures are observed and can be linked to charge-orbital (${T}_{\mathrm{CO}/\mathrm{OO}}\ensuremath{\sim}$320 K), two-dimensional-antiferromagnetic (2D-AFM) ($\ensuremath{\sim}$200 K), and three-dimensional AFM (${T}_{\mathrm{N}}\ensuremath{\sim}$125 K) orderings. Below ${T}_{\mathrm{CO}/\mathrm{OO}}$, ${\ensuremath{\sigma}}_{1}$ shows a charge-ordering peak ($\ensuremath{\sim}$0.8 eV) with a significant blue shift as the temperature decreases. Calculations based on a model that incorporates a static Jahn-Teller distortion and assumes the existence of a local charge imbalance between two different sublattices support this assignment and explain the blue shift. This view is further supported by the partial spectral weight analysis showing the onset of optical anisotropy at ${T}_{\mathrm{CO}/\mathrm{OO}}$ in the charge-ordering region (0.5--2.5 eV). Interestingly, in the charge-transfer region (2.5--4 eV), the spectral weight shows anomalies around the ${T}_{2\mathrm{D}\ensuremath{-}\mathrm{AFM}}$ that we attribute to the role of oxygen-$p$ orbitals in stabilizing the CE-type magnetic ordering. Our result shows the importance of spin, charge, orbital, and lattice degrees of freedom in this layered manganite.

The nature of Cr center in GaN: Magnetic anisotropy of GaN:Cr single crystals

Magnetization measurements of the strain-free bulk GaN:Cr single crystals of wurtzite structure are reported. Strong magnetic anisotropy at low temperatures (2–10 K) was observed. The data were analyzed assuming Cr2+(d4) configuration. The crystal field model taking into account cubic field of tetrahedral symmetry, trigonal field along the c-axis simulating hexagonal structure, tetragonal static Jahn-Teller distortion, and the spin-orbit interaction provide a good description of the experimental magnetization data.

Investigation of the g factors and defect structures for the tetragonal Ti3+ and Ti3+–Nb5+ centers in the low-temperature rhombohedral phase of BaTiO3

In this study, the electron paramagnetic resonance g factors g // and g ⊥ of two tetragonal Ti3+ defect centers, the isolated Ti3+ center and the Ti3+–Nb5+ center, in the low-temperature rhombohedral phase of BaTiO3 are calculated from the complete diagonalization (of energy matrix) method and the perturbation theory method. Both the methods are based on the two-spin–orbit (SO)-parameter model. In the model, the contributions to the g factors due to both the SO parameters of the central 3d1 ion and the ligand ion are included. The calculations are related to the defect models and defect structures of the two tetragonal Ti3+ centers. In the isolated Ti3+ center, the Ti3+ ion is assumed to occupy the approximately cubic octahedral Ti4+ site. The octahedron surrounding the Ti3+ undergoes a static Jahn–Teller distortion (characterized by the ligand displacement a) from approximately cubic to tetragonally compressed. In the Ti3+–Nb5+ center, the substitutional Ti3+ is charge compensated by an Nb5+ ion at the nearest neighboring Ti4+ site along the C 4-axis. The O2− ion intervening Ti3+ and Nb5+ has an additional displacement Δ z toward Nb5+ owing to the electrostatic interaction. From the calculations, the g factors of the two tetragonal Ti3+ centers are reasonably explained by either method. Also, the defect models of both the Ti3+ centers are confirmed and their defect structures (characterized by the ligand displacements a and Δ z) are obtained. The results are discussed.

Studies on the spin-Hamiltonian parameters of two Cu2+ centers and their defect structures due to Jahn–Teller effect for Cu2+-doped ZnGa2O4 spinel

A complete diagonalization (of energy matrix) method based on the cluster approach (where the covalence effect due to the admixture between the orbitals of the central d n ion and that of ligands is considered) was applied to calculate the spin-Hamiltonian parameters (g factors, g | and g ⊥, and hyperfine structure constants, A | and A ⊥) for two Cu2+ centers in ZnGa2O4 spinel at the temperature T ≈ 110 K. In both Cu2+ centers, Cu2+ ions occupy the trigonally-distorted octahedral sites 16d and the octahedrons undergo the static Jahn–Teller distortions from trigonal to tetragonally-elongated and tetragonally-compressed, respectively. The calculated spin-Hamiltonian parameters of the elongated Cu2+ center show good agreement with the experimental values and those of the compressed Cu2+ center are close to the observed values. The better agreement of spin-Hamiltonian parameter between calculation and experiment for the compressed Cu2+ center can be acquired by considering a small admixture of state to the ground state due to the vibrational motion of ligands. The defect structures of both Cu2+ centers (characterized by the ligand displacements a 1 and a 2, respectively) in ZnGa2O4 spinel were also determined from the calculation.

Spin-state transitions in PrCoO3investigated by neutron scattering

Pulsed neutron diffraction and inelastic neutron scattering were performed on the orthorhombic perovskite PrCoO${}_{3}$ to investigate possible spin-state transitions arising from Co${}^{3+}$ ions. Although the crystal symmetry is orthorhombic, no evidence for a Jahn-Teller distortion is observed in either the average or local crystal structures, suggesting that the intermediate-spin Co${}^{3+}$ state, if present, does not give rise to a static Jahn-Teller distortion. While previous measurements suggested that spin excitations may occur at about 200 K, we find from the inelastic neutron measurements that the crystal-field excitations from Pr${}^{3+}$ dominate the inelastic spectrum, and no magnetic excitation linked to Co${}^{3+}$ can be clearly discerned up to room temperature.

X-ray absorption and neutron diffraction studies of(Sr1 − xCex)MnO3: transition from coherent to incoherent static Jahn–Teller distortions

We have carried out Ce L- and Mn K-edge x-ray absorption near edge structure (XANES)measurements to experimentally determine the oxidation states of both Ce and Mn in(Sr1 − xCex)MnO3 (x = 0.1–0.4). It was found that although Ce is predominantly4 + at low dopinglevels (x = 0.1 and 0.15), the Ce valency decreases with increasing Ce doping (reaching a value of around3.5 + atx = 0.4). Theaverage Mn oxidation state decreases with the increase of Ce content, with the percentage of Jahn–Teller activeMn3 + ions increasingfrom 26% (x = 0.1) to 57% (x = 0.4).Precise structural parameters were also obtained from high resolution neutron diffraction studies for sampleswith x = 0.1–0.3. The crystal structure remains tetragonal inI4/mcm forx ≤ 0.3. The octahedral tilt angle increases with increasing Ce content, but the distortion of theMnO6 octahedra is reducedsignificantly at x ≥ 0.2 due to a transition from long-range ordered Jahn–Teller distortions to incoherent staticdistortions.

ChemInform Abstract: Structure of Trisodium Hexafluoromanganate(III).

Na3MnF6, Mr = 237.90, monoclinic, P21/n, a = 5.4709 (9), b = 5.6830 (6), c = 8.0734 (12) A,/3 = 88.964 (7) °, V= 250.97 (6) A 3, Z = 2, D x = 3.15 gcm -3, A(Mo Ka) = 0.7107 A., /x = 27.6 cm -1, F(000) = 224, T = 293 K, R = 0.0152, wR = 0.0144 for 550 unique reflections (Fo > 30.(Fo)). The structure is isostructural with that of the cryolite Na3A1F6 and shows the weakest static Jahn-Teller distortion known to date for (MnF6) 3- octahedra (d(Mn--F) axial 2.0176 (8), mean equatorial 1.880 (1) A). The ordering of the elongated octahedra is ferrodis- tortive. Of the Na ions, one third have octahedral and the remainder have bicapped trigonal prismatic eight coordination. Introduction. Single-crystal studies on sodiumhexa- fluorometallates(III) Na3MmF6 (M m = Cr, Fe) revealed cryolite structures closely related to that of NaaA1F6 (Naray-Szabo & Sasvari, 1938; Hawthorne & Ferguson, 1975), space group P21/n. The Cr com- pound crystallizes in the same space group (Brunton, 1969), while for the structure of the iron compound P2~ is assumed (Matvienko, Yakubovich, Simonov, Ivashchenko, Melnikov & Belov, 1981). For several other compounds Na3MF6 (M = Ga, Sc, Ti, V, Mn, Co, Ni, Cu) cryolite-type unit cells were deduced from powder diffraction data (cf. review of Babel & Tressaud, 1985). In the course of studies on the dependence of Jahn-Teller distortion on the struc- ture type, we were interested in the crystal structure of the Mn m compound. Na3MnF6 was mentioned for the first time by Siebert & Hoppe (1971). Lattice parameters have been proposed by Bucovec & Siftar (1975) (a=5.56, b=5.84, c=8.10A, /3=90.7 ° ) which were confirmed roughly by our single-crystal investigation reported here. Experimental. The title compound was prepared by heating a stoichiometric mixture of NaF and MnF3 * To whom correspondence should be addressed.