Cooperative Jahn-Teller Transition Research Articles

Jahn-Teller Phase Transitions in Non-Stoichiometric Oxides with Perovskite Structure: Effect of Mechanical Activation

Structure phase transitions in nonstoichiometry oxides with perovskite structure were considered. The microscopic description of cooperative Jahn-Teller transition was suggested and the effects caused by mechanical activation of oxide systems (their conversion into nanostructural state, acceleration of oxidation-renewal reactions and immiscibility) were analized. The correspondence of the theoretical results to the data of experiment on phase states of mechanically activated oxide NdMnO3 is shown.

Unusual structural evolution in KCuF3at high temperatures by neutron powder diffraction

High-resolution neutron powder diffraction has been performed to study the structural evolution of the perovskite KCuF${}_{3}$ at temperatures up to 900 K. Results of the Rietveld refinement reveal an unusual site distortion that increases as temperature increases. In contrast to the widely accepted assumption that a cooperative Jahn-Teller transition may occur at 800 K, no phase transition was observed up to 900 K. We have made a comparative study of the Jahn-Teller distortion in fluorides and oxides with variables such as temperature, pressure, and the dilution by non-Jahn-Teller active ions in these compounds.

Exotic transition in the three-dimensional spin-liquid candidate Tb2Ti2O7

The insulator pyrochlore Tb${}_{2}$Ti${}_{2}$O${}_{7}$ is a three-dimensional spin liquid candidate: the strength of the magnetic interaction would normally induce order at $\ensuremath{\approx}$20 K, but according to published muon-spin relaxation and neutron diffraction measurements, it is still in a correlated paramagnetic state down to a temperature of 0.05 K. Our detailed experimental investigation shows that an exotic transition takes place at ${T}_{\mathrm{t}}=0.15\phantom{\rule{0.16em}{0ex}}(2)$ K. It is characterized by an anomaly in the muon precession frequency shift but not in the specific heat, which nevertheless reveals the signature of degrees of freedom at very low energy. It might be associated with a cooperative Jahn-Teller transition. The robustness of the experimental results is discussed.

Symmetry rules and strain/order-parameter relationships for coupling between octahedral tilting and cooperative Jahn–Teller transitions in ABX 3 perovskites. I. Theory

Space groups, order-parameter and strain/order-parameter coupling relationships in ABX3 perovskite structures which combine cooperative Jahn-Teller distortions and octahedral tilting have been investigated from the perspective of group theory using the computer program ISOTROPY. Two common Jahn-Teller ordering schemes are associated with the irreducible representations M2+ and R3+ of the space group Pm3m. A third, less-common ordering scheme is associated with Gamma3+. These combine with tilting instabilities associated with M3+ and R4+ to generate a predicted suite of Jahn-Teller structure types that includes many of the known structures of manganites, vanadates, Cu and Cr halides. Order-parameter coupling and possible phase transitions are described using Landau free-energy expansions, and general expressions for the relationships between symmetry-adapted spontaneous strains and particular order-parameter components are presented. These provide a general formal framework for determining structural evolution across multi-component order-parameter space and for characterizing the influence of tilting instabilities on Jahn-Teller instabilities or of Jahn-Teller ordering on octahedral tilting.

Symmetry rules and strain/order-parameter relationships for coupling between octahedral tilting and cooperative Jahn–Teller transitions in ABX 3 perovskites. II. Application

The structural evolution of selected perovskites containing Jahn-Teller cations has been investigated in the light of a formal analysis of symmetry hierarchies for phase transitions driven by octahedral tilting and Jahn-Teller cooperative distortions. General expressions derived from the strain/order-parameter coupling relationships allowed by symmetry are combined with observed changes in lattice parameters to reveal details of order-parameter evolution and coupling. LuVO3, YbVO3, YVO3 and CeVO3 are representative of systems which develop Jahn-Teller ordering schemes associated with irreducible representations M2+ and R3+ of the space group Pm3m. Tilting of their octahedra is associated with M3+ and R4+. The Pnma (M3+ + R4+ tilting) <--> P2(1)/a (M3+ + R4+ tilting, R3+ Jahn-Teller order) transition below room temperature is close to second order in character. Shear strains which depend primarily on tilt angles show little variation, implying that there is only weak coupling between the tilting and Jahn-Teller order parameters. The subsequent P2(1)/a <--> Pnma (M3+ + R4+ tilting, M2+ Jahn-Teller order) is first order in character, and involves either a reduction in the R4+ tilt angle or a change in the strength of tilt/Jahn-Teller order-parameter coupling. In LaMnO3, the isosymmetric Pnma (M3+ + R4+ tilting) <--> Pnma (M3+ + R4+ tilting, M2+ Jahn-Teller order) transition can be described in terms of a classical first-order transition conforming to a 246 Landau expansion with negative fourth-order coefficients. Strain evolution in Ba-doped samples suggests that the transition becomes second order in character and reveals a new strain relaxation mechanism in LaMnO3 which might be understood in terms of local strain heterogeneities due to the disordering of distorted MnO6 octahedra. Transitions in PrAlO3 and La(0.5)Ba(0.5)CoO3 illustrate the transformation behaviour of systems in which the Jahn-Teller ordering scheme is associated with the irreducible representation Gamma3+. Overall, coupled tilting + Jahn-Teller phase transitions in perovskites conform to mean-field behaviour, consistent with the underlying role of strain in promoting long interaction lengths.

Effect of Mn-substitution on magnetic and structural properties in FeCr 2O 4

We studied the magnetism and crystal structure of Fe 1 - x Mn x Cr 2 O 4 solid solutions to explore the interplay between the orbital and spin degrees of freedom in the spinel oxide system. Cooperative Jahn–Teller transition temperature decreases with partial substitution of Mn 2 + for Fe 2 + , because a Mn 2 + ion in the high-spin state ( e 2 t 3 ) has no orbital degree of freedom. In the partially substituted compounds, the ferrimagnetic transition induces the second structural transition from tetragonal to orthorhombic system, which might be explained by a possible competition between the single-cluster Jahn–Teller distortion and spin–orbit coupling.

Muon spin relaxation inDyVO4andTbVO4

Muon spin relaxation experiments have been performed on $\mathrm{Dy}\mathrm{V}{\mathrm{O}}_{4}$ and $\mathrm{Tb}\mathrm{V}{\mathrm{O}}_{4}$, both of which exhibit a cooperative Jahn-Teller transition. The data demonstrate that the nature of the magnetic ion, whether Kramers or non-Kramers, has a decisive effect on the nature of the observed spin relaxation. We interpret this result in terms of the difference in the relaxation induced by the spin-phonon interaction in the two systems.

Decoupling of orbital and spin degrees of freedom in Li1−xNaxNiO2

In the Li{sub 1-x}Na{sub x}NiO{sub 2} solid solutions, three different single-phase regions exist: for x{>=}0.9, for x{approx_equal}0.7, and for x{<=}0.3. Although the intermediate compound does not show the cooperative Jahn-Teller transition of NaNiO{sub 2}, its magnetic properties remain very similar, particularly with respect to the low-temperature three-dimensional magnetic ordering. Therefore, the strong coupling between orbital and spin degrees of freedom, characteristic of other oxides like perovskites, and usually invoked to explain the absence of both long-range orbital and magnetic ordering in LiNiO{sub 2}, does not take place in these layered compounds with {approx}90 deg. superexchange bonds. We also discuss the relevance of the O crystal-field splitting induced by the trigonal distortion in generating antiferromagnetic in-plane Ni-Ni interactions.

Resonant X-ray scattering and cooperative Jahn–Teller effect in LaMnO3 thin films

Abstract Epitaxial thin films of LaMnO3 were grown on MgO and SrTiO3. The cooperative Jahn–Teller transition occurred at different temperatures for the two films. It is shown that the intensity of the resonant (1 0 0) peak at the Mn K-edge is proportional to the 3 2 power of the orthorhombic strain for both films at all temperatures below the transition. This provides an evidence in favor of the band effect in accounting for the resonant scattering in LaMnO3. A significant intensity difference in the resonant scattering in the two films is pointed out.

Phase Transitions and Spin Relaxation in La0.95Sr0.05MnO3

We present a model calculation of the paramagnetic-resonance linewidth in La 0.95 Sr 0.05 MnO 3. Both the temperature and the angular dependencies can be described by contributions from the crystal field due to the cooperative Jahn–Teller transition.

Cooperative Jahn-Teller transition and resonant x-ray scattering in thin filmLaMnO3

Epitaxial thin films of stoichiometric ${\rm LaMnO_3}$ were grown on ${\rm SrTiO_3(110)}$ substrates using the pulsed laser deposition technique. From the high resolution x-ray diffraction measurements, the lattice parameters were determined as a function of temperature and the cooperative Jahn-Teller transition was found to occur at $T_{JT}$=573.0 K. Also measured was resonant x-ray scattering intensity of the orthorhombic (100) peak of ${\rm LaMnO_3}$ near the Mn K edge from low temperatures to above $T_{JT}$. We demonstrate that the integrated intensity of the (100) peak is proportional to the 3/2 power of the orthorhombic strain at all temperatures, and thus provide an experimental evidence that the resonant scattering near the Mn K edge in ${\rm LaMnO_3}$ is largely due to the Jahn-Teller effect.

Pressure effects of susceptibility and specific heat in PrCu 2

The pressure effects on magnetic susceptibility, specific heat and the co-operative Jahn–Teller transition temperature, T JT, in the single crystalline sample are presented. At P=1.2 GPa susceptibility shows remarkable maximum at T= T max while specific heat shows no anomalous change around T max. However, at T⪡ T JT the upturn of nuclear specific heat is developed with pressure. Our findings imply that a magnetic ordering is induced by pressure.

First-order phase transition inUO2:235Uand17ONMR study

A nuclear magnetic resonance (NMR) study is reported for a $5f$-cooperative Jahn-Teller (JT) system with antiferromagnetic ordering, ${\mathrm{UO}}_{2},$ where we have performed magnetic actinide ${(}^{235}\mathrm{U})$ as well as nonmagnetic ligand ${(}^{17}\mathrm{O})$ NMR studies in a $5f$-electron system. The observed ${}^{235}\mathrm{U}$ hyperfine interaction is consistent with an axially symmetric $5f$-wave-function character for ${\mathrm{U}}^{4+}$ below a first-order transition temperature ${T}_{\mathrm{N}}=30.8\mathrm{K}.$ Thus, the orbital degeneracy in the cubic crystalline field is lifted by a magnetic ordering combined with a JT distortion. On the other hand, modulation phenomena in the ${}^{17}\mathrm{O}$ spin-echo decay provide evidence of a lattice distortion just below ${T}_{\mathrm{N}},$ which gives rise to an axially symmetric electric-field gradient at the oxygen sites. These results indicate that, in a cooperative JT transition which occurs as a result of coupling between $5f$ quadrupoles and lattice distortion, the behavior of the $5f$ quadrupoles can be investigated using uranium NMR, while the lattice distortion can be studied with the nonmagnetic ligand NMR. Among magnetic structures and JT distortions, proposed on the basis of neutron scattering experiments and theoretical work, the noncollinear $3\mathit{k}$-type structure is the most likely, because either the $1\mathit{k}$ or $2\mathit{k}$ would cause an orthorhombic distortion. With respect to low-energy spin-wave excitations, nuclear spin-lattice relaxation rates ${T}_{1}^{\ensuremath{-}1}$ at both sites show a ${T}^{7}$ behavior below ${T}_{\mathrm{N}},$ which suggests the presence of gapless excitations due to strong magnon-phonon coupling.

Pressure effect on the cooperative Jahn–Teller transition in PrCu 2

We have studied pressure dependence on the cooperative Jahn–Teller specific heat in the orthorhombic PrCu 2 using a clamp-type pressure-cell. The pressure in the clamp-type pressure-cell is determined from the superconducting transition temperatures of Sn (999.99%) at liquid-helium temperatures. The pressure is ineffective on the specific heat for PrCu 2 up to 0.8 GPa below 5 K. Above 5 K, obvious depression in the specific heat peak is observed as pressure increases. The Jahn–Teller transition temperature T C increases with increasing pressure.

Anti-Jahn-Teller polaron inLaMnO3

Distortions of the oxygen sublattice couple to $e_g$ orbitals of Mn$^{3+}$ and drive a cooperative Jahn-Teller (orbital ordering) transition in LaMnO$_3$. A simple model for this transition is studied. Without further adjustment, the model predicts the shape and stability of small (anti-Jahn-Teller) polarons which form when holes are doped into the material. This leads to a new description of the lightly doped insulator, the anti-ferromagnetic to ferromagnetic transition, and the metal-insulator transition.

An Ultra-Low-Temperature Diffractometer Based on an 3He–4He Dilution Refrigerator used for Synchrotron-Radiation X-ray Diffractometry and Topography

The direct observation of crystal structure at ultra-low temperatures helps better understanding of phase transformations and phase stability. Two sets of 3He-4He dilution refrigerators with a top-loading facility were installed at BL-3C2 and 6C1 of the Photon Factory. The first is used exclusively for topography of quantum crystals of solid 3He and 4He for studies of lattice defects. Typical examples of topographs that indicate an annealing effect of solid helium, understood in terms of a quantum tunneling, are presented. The disappearance of downward diffraction Laue spots (2θB = 90°) is discussed on the basis of the temperature and other factors in solid helium. The second refrigerator is used for X-ray diffraction studies of crystalline solids. A splitting of the Laue spots from Cs2NaHoCl6 below 150 mK, giving evidence of a cooperative Jahn–Teller transition of the first kind at the lowest transition temperature ever observed, is briefly explained as an example of diffractometry.

Interacting phase transitions in Fe1+xCr2−xO4 (0≤x≤0.4)

The system Fe1+xCr2−xO4(0≤x≤0.4), in which a cooperative Jahn–Teller transition and a magnetic transition take place in the same temperature range, has been investigated by x-ray, magnetization, Mössbauer effect, and specific heat measurements. When 0.00≤x≤0.25, the Jahn–Teller transition temperature TD is higher than that of the magnetic transition TN, two peaks being observed in the specific heat. When x increases TD decreases slightly, whereas TN increases steeply, so that they coincide at x=0.275 with TD=TN=130 K, exhibiting a very sharp lambda type heat anomaly. When x&amp;gt;0.30, TD is expected to be lower than TN, but no distinct heat anomaly was observed in the expected temperature range. This indicates that the two cooperative phase transitions can couple when the higher temperature transition introduces a crucial part of the entropy lowering which would be expected after the lower temperature transition.

Recent Applications of Heat Capacity Measurement in Physicochemical Investigations

Abstract This review discusses the recent experimental heat capacity measurements which have been very useful in physicochemical investigations. Areas reviewed include critical point phenomena in systems such as fluids, magnetic systems, liquid crystals, co-operative Jahn-Teller transitions, etc. The uses of Cp measurements in the study of discrete energy levels in solids, in glasses at very low temperatures, in thin films and at high pressures are discussed. Calorimetric investigations in A-15 and other superconducting materials and applications of Cp measurements for evaluation of thermodynamic parameters in several new classes of materials are then briefly described. Finally, examples of applications of calorimetry in areas of biophysics, biological sciences and clinical medicine are cited. Two hundred and seventy references are cited and 25 figures are used for illustration.

Low-frequency dynamics of a cooperative Jahn-Teller transition: TbVO4

The low-frequency response of TbVO4 near its cooperative Jahn-Teller transition, TD=32.6K, has been measured by light scattering. Analysis above TD, based upon existing theory, provides the first quantitative description of the observed spectral behaviour of any singular dynamic central peak on the basis of a microscopic theory. The spectra below TD call for renewed theoretical effort.

Low-frequency dynamics in cooperative Jahn-Teller systems

We present a detailed study of the low-frequency dynamics of systems undergoing strain-induced cooperative Jahn-Teller transitions. Exact expressions relating the phonon susceptibilities to the electronic susceptibilities are given. The phonon and the electron response functions are calculated explicitly when the mean-field static correlation functions are used. In the limit that the decay rate of electronic fluctuations is much slower than the phonon frequency, we find that $\frac{{{\ensuremath{\chi}}^{\ensuremath{'}\ensuremath{'}}}_{\mathrm{QQ}}(\ensuremath{\omega})}{\ensuremath{\omega}}$ and $\frac{{{\ensuremath{\chi}}^{\ensuremath{'}\ensuremath{'}}}_{\mathrm{OO}}(\ensuremath{\omega})}{\ensuremath{\omega}}$ have a central peak. As the temperature is lowered toward the structural-phase-transition temperature the width of the central peak approaches zero and the central-peak intensity diverges for $\stackrel{\ensuremath{\rightarrow}}{\mathrm{k}}=0$. In the opposite limit we obtained the mean-field expressions for the phonon dispersion relation, the sound velocity, the elastic constant, and the ultrasonic attenuation coefficient. We also found a relation connecting the width of the central peak to the attenuation coefficient. In the fast-relaxation regime an explicit calculation of the response functions is possible using the exact static correlation functions. From this calculation we obtain the phonon dispersion relation, the sound velocity, the elastic constant, and the attenuation coefficient in terms of the exact static electronic susceptibilities. Our expression for the elastic constant reduces in the mean-field approximation to the result derived before by several authors. A calculation of the linewidths of the crystal-field manifolds is also given. In addition we comment on the asymptotic behavior near the critical point and the connection between our findings and the results of recent studies of elastic phase transitions which were obtained using renormalization-group techniques.