Local Jahn-Teller Distortion Research Articles

Phase transitions in CuZrF 6 and CrZrF 6: A Mössbauer and EPR study of local and cooperative Jahn-Teller distortions

CuZrF 6 and CrZrF 6 undergo phase transitions between 100 and 450K, which are induced by crystal packing effects and changes from dynamical to static Jahn-Teller distortions of the Cu(Cr)F 6 octahedra. We analyzed in particular the transitions of the Jahn-Teller type using 57Fe 2+, doped into the Cu 2+(Cr 2+) sites, as a Mössbauer probe. The quadrupole splitting is large in the region of static distortions and essentially reflects the distortion symmetry of the host Cu(Cr)F 6 polyhedra, while it vanishes in the case of a dynamical Jahn-Teller effect. Ligand field, EPR, and magnetic data are given in addition and are discussed with respect to the structures of the host compounds and the cooperative Jahn-Teller order of the tetragonally elongated Cu(Cr)F 6 polyhedra in the low-temperature phases.

Structural study of Pr 1− xCa xMnO 3 and Y 1− xCa xMnO 3 perovskites

The structural properties of Prx 1− x Ca x MnO 3 and Y 1− x Ca x MnO 3 perovskite systems were studied by X-ray and magnetic susceptibility measurements. The differences in the structural properties of both systems are attributed to the differences of tolerance factors, i.e. of ionic radii of the ions present and the results are interpreted as being due to simultaneous action of steric effect, orbit-lattice interactions of Mn 3+ ions and Coulomb interactions among 3 d electrons. Their individual contributions depending on the compositions are analyzed. Besides the usual cooperative Jahn-Teller effect existing in PrMnO 3 a new type of transformation was found which is connected, under condition of Mn 3+ ⇇ Mn 4+ valency migration, with the formation of local Jahn-Teller distortions. It is characterized by the equality of v hop and v phon at a critical temperature T crit; it has been found that T crit depends only very weakly on the composition. A correpsonding O'→O change in the yttrium system is, however, rather continuous which is explained by a strong influence of the cooperative buckling effect. An analysis of the magnetic data points to the presence of double exchange in the praseodymium system at high temperatures, while at low temperatures this mechanism applies only in the region, where cooperative Jahn-Teller effect exists, i.e. at x0.4.

Neutron diffraction experiments on CsCrI 3 at 300, 77, and 1.2 K

CsCrI 3 has been investigated by neutron powder diffraction at room temperature and 77 and 1.2 K. It undergoes a phase transition at 150 K due to the cooperative Jahn-Teller effect. The high-temperature form, α-CsCrI 3 (hexagonal, space group P6 3 mmc , a = 8.127(1) Å, c = 6.944(1)Å, Z = 2), adopts the BaNiO 3 structure with a local Jahn-Teller distortion. The low-temperature form, β-CsCrI 3 (orthorhombic, space group Pbcn, a = 8.102(1)Å, b = 13.792(1)Å, c = 6.900(1)Å, Z = 4), has a structure not yet been reported for a Jahn-Teller distorted BaNiO 3 structure. It is shown that the low-temperature form can be derived from the BaNiO 3 structure by means of a canting of triangles, formed by the three common I − ions of two adjacent CrI 6 4− octahedra. The magnetic structure of β-CsCrI 3 at 1.2 K is found to consist of an antiparallel sequence of ferromagnetic (0 0 1) planes with a magnetic moment in the ∥1 0 0∥ direction of 3.16 μ B.

Incommensurate Jahn-Teller transition in K 2PbCu (NO 2) 6

We report the results of a structural investigation of the intermediate phase of K 2PbCu(NO 2) 6 by neutron scattering technique. We have observed satellite reflections at ( H±0.416, K±0.430, 0) around each fundamental reflections. This clearly indicates that the crystal has the “incommensurate” structure in this phase. Our tentative model structure in this phase is that the sinusoidally modulated local Jahn-Teller distortion propagates along [110] with the wave vector | k| ≈ 0.42. In terms of pseudospin description, this phase is characterized as a “fan pseudospin” structure.

Mössbauer resonance of Fe 57 in oxidic spinels containing Cu and Fe

The Mössbauer spectra of Fe 57 in tetragonal and cubic CuFe 2O 4, CU 0.5Zn 0.5Fe 2O 4, and Cu 0.3Cd 0.3Fe 2O 4 have been investigated between 4.2–1000°K and in applied external magnetic fields. The spectra consist of a superposition of two apparent six-line patterns which are attributed predominantly to iron at the B sites (pattern I) and A sites (pattern II) of the crystal structure on the basis of their relative isomer shifts, hyperfine fields, quadrupole interactions, and thermal shifts. However, the intensity ratio of the Lorentzian fits of the two patterns does not correspond to the ratio of the Fe site occupancy factors as determined by diffraction methods. It is found that the patterns, especially I, are of a complex structure and exhibit anomalous line shapes due to various distributions of the neighboring cations and local Jahn-Teller type distortions of crystallographically equivalent sites. Both of these give rise to fluctuations in the A− B type super exchange interactions (resulting in varying hyperfine fields) and, to a lesser extent, to fluctuations in the quadrupole perturbation of the fields at the iron sites. The observed slight differences in the magnitude of the H eff( B) and the differential linewidths of II are interpreted in terms of a spin transfer from the A site cationic 3 d spin moments to the 4 S orbital of the B site Fe 3+. By use of applied magnetic fields the separation of the patterns is increased so that the Lorentzian fits yield approximately correct intensity ratios. In cubic CuFe 2O 4 and in the spinels containing Zn and Cd, the site distortions are such that patterns I and II cannot be resolved in the zero applied field spectra whereas this is possible if external fields are employed.

A new parameter for the description of the cubic manganese ferrite spinel lattice

By the measurement of specially chosen intensities of X-ray reflections on single crystals (doublets and triplets of indices), it was proved that in the case of manganese ferrite (Mm 1.66Fe 1.33O 4) the ϵ parameter ( u = 0.375 + ϵ) is not sufficient for the description of the spinel lattice. Another parameter δ was introduced which removed differences in observed X-ray intensities. This δ parameter was assumed to have a close connection with the local Jahn-Teller distortion of Mn 3+O 6 2− octahedra. On the basis of a rough model, we were able to estimate the number and the distortion, c/a value, of local distorted octahedra.

Comparison of lattice vibrations in nickel- and manganese-ferrite single crystals

X-ray measurements of the mean square amplitude of lattice vibrations previously made on polycrystalline samples were now repeated on single crystals of composition Mn1·66Fe1·33O4 and NiFe2O4. The greater value of the mean square amplitude for the Mn-ferrite is consistent with results on polycrystalline samples and is a further confirmation of the existence of local Jahn-Teller distortion in manganese ferrite.

Theoretical Model for Tetragonal-to-Cubic Phase Transformations in Transition Metal Spinels

The origin of the large tetragonal distortions which occur in a number of transition metal oxides having the spinel structure has been examined recently by Dunitz and Orgel in terms of the crystal field theory. According to these authors the macroscopic distortions arise as a consequence of a Jahn-Teller type distortion in the immediate environment of certain transition metal ions. Thus, all the observed large distortions in spinels have been correlated with the results of this crystal field treatment on the basis of the spatial ordering of the local distortions.In this communication we investigate the detailed properties of the transformations from tetragonal to cubic phases which are observed at elevated temperatures. An approximate model has been constructed which explicitly takes into account the interactions between local Jahn-Teller distortions about neighboring octahedral site cations. The configurational energy of the model has been derived in a completely general form in terms of occupation variables, and has been used to deduce the structure of the stable low-temperature phase. By the use of the usual methods of statistical mechanics it has proved possible to derive the thermodynamic behavior of the model, and hence to contribute to an understanding of the cooperative nature of these phase transformations. The temperature and composition dependence of the long-range order parameter, the thermodynamic functions, and the lattice parameters are calculated explicitly. The principal result of importance is the demonstration that the transformations from tetragonal to cubic spinel phases are thermodynamic transitions of the first order type. That is, a latent heat, a volume discontinuity, lattice parameter discontinuities, and a lambda anomaly in the heat capacity are to be observed at the transformation temperature. The available experimental evidence supports the conclusions drawn from the theoretical model. The agreement between theory and experiment is found to be semiquantitative in most of the cases considered.

Theoretical Model for Cubic-to-Tetragonal Phase Transformations in Transition Metal Spinels

According to Dunitz and Orgel, the large tetragonal distortions which occur in a number of transition metal oxides having the spinel structure are presumed to arise as a consequence of a Jahn-Teller type distortion in the immediate environment of certain transition metal ions. All the observed large distortions can be correlated with the results of this crystal field treatment on the basis of the spatial ordering of the local distortions. In this communication we investigate the detailed properties of the transformations from tetragonal to cubic phases which are observed at elevated temperatures. An approximate model has been constructed which explicitly takes into account the interactions between local Jahn-Teller distortions about neighboring B-site transition metal cations. By the use of the usual method of statistical mechanics it has proved possible to derive the thermodynamic behavior of this model, and hence to contribute to an understanding of the cooperative nature of these transformations. The principal result of importance is the demonstration that the transformations from tetragonal to cubic spinel phases are thermodynamically of the first order. That is, a latent heat, a volume discontinuity, lattice parameter discontinuities, and a lambda anomaly in the heat capacity are to be observed at the transformation temperature. The available evidence seems to support the conclusions drawn from the model.