Crystal imperfections and structural phase transitions in perovskites

High‐resolution X-ray scattering measurements of the antiferrodistortive phase transitions in the perovskites SrTiO₃, RbCaF₃ and KMnF₃ have recently revealed the existence of two length scales in the critical scattering above T_e . Here we review these observations and discuss how they might be related to the well known existence of two time scales (the "central peak" problem) in the critical dynamic response function above T_e . We reach the tentative conclusion that within a few degrees of T_e both the time and length scales of the fluctuations are strongly influenced by defects and that the intrinsic critical behaviour is swamped by these effects in most crystals.     

Superconducting critical temperature, Tc, and phase equilibrium diagram of A3B β-W) type compounds

A correlation between the superconducting critical temperature, T_c, and solid solution range (SSR) appearing in the phase equilibrium diagram of A₃B (β-W) type compounds has been observed. Among the compounds of incongruent melting (peritectic or peritectoid) type, the T_c is directly proportional to the solid solution range. Further, in the low temperature range, the solid solution range invariably is found on the A element-rich side of the stoichiometric composition (75 at. % A: 25 at. % B), suggesting a fundamental importance in the “A-chain integrity,” viz., an A element can substitute for a B element which is not a part of the chain, but a B element cannot replace an A element which forms a part of the chain) which is believed to be a key to superconductivity.     

Possible preferential order-disorder in A15(β-W type) compounds

A possible existence of preferential disorder in the B sites in A₃B(β-W type) compounds is demonstrated. This appears to support the “A-chain integrity” recognized in these compounds. Therefore, the correlation between the order-disorder parameters and the superconducting critical temperatures, investigated in the past, should be reexamined.     

The Tc-SSR correlation in A3B(A15-type) compounds

The 28A₃B(A15-type) compounds, for which both the phase equilibrium diagram and the superconducting critical temperature, T_c, are known, can be divided into two groups. The first group consists of compounds whose SSR (solid solution range) is either zero or extends only to the A-rich side of the stoichiometric composition. The second group consists of the compounds whose SSR extends to both sides or only to the B-rich side. The first group of compounds generally has high T_c's and follow the T_c-SSR correlation proposed earlier.Both the grouping and the T_c-SSR correlation are directly relatable to “A-chain integrity” and thus lend further support to the thesis that “A-chain integrity” is of fundamental importance in the superconductivity of these compounds.     

Levelcrossing Experimente im Tm I-Spektrum

A semi phenomenological dynamical theory for the rotations of the BO₆ octahedra in ABO₃ perovskites is proposed, which accounts for the soft mode and the central peak forT≧T _c observed by neutron scattering in SrTiO₃. The neutron scattering cross section is discussed. The EPR line width is predicted to diverge as ?^?ν(1–2η) and as ?^?ν(2–2η) for three- and two dimensional correlations with ?=(T-T _c )/T _c . The theory is generalized to other structural transitions.     

Shell-model analysis of helion and triton inelastic scattering from90Zr including core polarization effects

A microscopic analysis of the inelastic scattering of helions and tritons from⁹⁰Zr is performed using the shell-model wavefunctions and phenomenological effective interactions. Effects of the core polarization are studied assuming a collective model for the core of closed shells. Core coupling parametersA _L , are obtained from the measured electromagnetic transition rate,B(EL). For those transitions for which no meauredB(EL) values are available,A _L values which fit the data have been extracted. The radial forms of both the Gaussian and Yukawa interactions were studied. Effects of the non-locality of the bound states on the predicted cross sections are found to be small. As in the case of proton inelastic scattering, it is found that the valence (direct) contributions are small and the transitions are dominated by core polarization. The strength of the effective interaction required to fit the data is reduced by a factor of two or more when the core polarization contributions are included.     

Electrical and thermal properties of Pr2/3(Ba1−xCsx)1/3MnO3 manganites

Electrical and thermal properties of Pr_2/3(Ba_1?x Cs _x )_1/3MnO₃ (0 ≤ x ≤ 0.25) manganite perovskites are reported here. Two insulator-metal (I-M) transitions (T _P1 &T _P2) are observed in the electrical resistivity (ρ) of the pristine Pr_2/3Ba_1/3MnO₃ (PBMO) sample, and they are systematically shifted to lower temperatures with increasing Cs substitution. An upturn in ρ is noticed below 50 K in these perovskites, presumably due to the combined effect of weak localization, electron-electron and electron-phonon scattering. It is found that the absolute value of room-temperature thermoelectric power (TEP) gradually decreases with increasing Cs content, implying the annihilation of the charge carriers with doping. An analysis of the electrical resistivity and thermoelectric power data indicates that the paramagnetic insulating state above T _P1 is governed by the small polaron hopping due to a non-adiabatic process. It is argued that the electron-magnon scattering processes are responsible for low temperature metallic behavior of TEP. A distinct specific heat peak below T _P1 is observed, attributed to the magnetic ordering, and its broadening with Cs-doping corresponds to the increase of magnetic inhomogeneity. Further, the temperature variation of thermal conductivity and the low temperature plateau in κ has been associated with the delocalization of Jahn-Teller polarons and transition from Umklapp scattering to a defect-limited scattering, respectively.     

Raman scattering study of pressure-induced phase transitions in AIIB2IIIC4VI defect chalcopyrites and spinels

A^IIB₂^IIIC₄^VI defect chalcopyrites (DC) and spinels were investigated by Raman scattering spectroscopy under hydrostatic pressure up to 20 GPa. All these compounds were found to undergo a phase transition to a Raman inactive defect NaCl-type structure. The phase transition is reversible for spinels and irreversible for DC. From the analysis of the pressure behavior of Raman-active modes, it was concluded that the phase transition from spinel to NaCl-type structure is direct in MnIn₂S₄ and CdIn₂S₄, while it occurs via an intermediate LiVO₂-type NaCl superstructure in MgIn₂S₄. The observed differences in the pressures and the paths of the pressure-induced phase transitions in A^IIB₂^IIIC₄^VI compounds are discussed.     

Magnetic properties of the rare-earth diborodicarbides (RB2C2)

The magnetic susceptibility of RB₂C₂ has been measured in the temperature range of 3–300 K. Curie-Weiss fits to the susceptibilities led to effective moments in agreement with those expected for R³⁺ ions. The RB₂C₂ (R = Ce, Nd, Sm, Gd, Tb, Er, and Tm) compounds are antiferromagnetic. Metamagnetic transitions at low fields were observed for CeB₂C₂ and TbB₂C₂. The compounds, DyB₂C₂ and HoB₂C₂, are ferromagnets with complex magnetic structures. Praseodymium borocarbide becomes a Van Vleck paramagnet at low temperature. The magnetic ordering temperatures of these compounds are discussed in terms of their crystal structure and the RKKY theory.     

Energy band structure and modulation spectra of A2B4C25 semiconductors

The results of the energy band structure calculations of A²B⁴C₂⁵ compounds are reviewed, and the differences in the energy spectra passing from A³B⁵ semiconductors to their closest ternary analogs are described. The origin of the lowest conduction band minima of A²B⁴C₂⁵ compounds was determined from the slopes of the fundamental absorption edge and the pressure coefficients of the energy gap. The investigations of the valence band structure from electroreflectance (ER), thermoreflectance (TR) and wavelength modulated absorption (WMA) spectra are reviewed. In the higher energy region the ER and TR spectra of A²B⁴C₂⁵ compounds were found to be more complicated in comparison with those of their binary analogs. A model of an assignment of the structures in optical spectra of A²B⁴C₂⁵ compounds is discussed.     

The magnetic properties of a finite superlattice with two disordered interfaces

Within the framework of the effective field theory, based on a probability distribution technique, we examine the critical and compensation behaviors of a ferrimagnetic alternating superlattice on a simple cubic structure. The superlattice consists of k unit cells each of which consists of L layers of spin-1/2 A atoms, L layers of spin-1 B atoms and a disordered interface with two layers in between that is characterized by a random arrangement of A and B atoms A_pB_1−pA_1−pB_p with a negative coupling A − B. Considering a finite and infinite superlattices, the effect of the thickness of the film and the surface exchange coupling on the magnetic properties are studied. The obtained results show a number of characteristic phenomena.     

Resonant Raman study of LO + acoustic phonon modes in CdSe

In studying resonant Raman scattering in the vicinity of the A and B excitons of CdSe, we have observed three new Raman peaks. Two of the peaks have been identified as two-phonon modes consisting of a longitudinal optical (LO) phonon plus respectively a transverse acoustic (TA) and a longitudinal acoustic (LA) phonon. A theory which involves the scattering of photoexcited B excitons to the A exciton by acoustic phonons via the piezoelectric exciton-phonon interaction was found to explain quantitatively the peak positions, lineshape and resonance enhancements of the observed peaks.     

Franck-Condon factors and absolute transition probabilities for the if (B3Π0+-X1Σ+) transition

Improved spectroscopic constants have been used to calculate Rydberg-Klein-Rees (RKR) potentials and Franck-Condon factors for the IF(B³Π₀₊-X¹Σ⁺) transition. The Franck-Condon factors are generally in good agreement with previously calculated values, but differ by as much as 30% for transitions from higher levels of the B-state. Several experimentally measured relative transition moment functions have been evaluated and the best scaled, so that the total transition probability calculated for each B-state vibrational level, A(v'), matched measured values. The scaled function was then used to calculate individual transition probabilities, A(v',v), for the vibronic transitions.     

The microwave spectrum of trans 1-nitropropene

Rotational transitions of 1-nitropropene arising from the ground vibrational state and from three excited states of the nitro torsional vibration have been assigned. The values of the rotational constants in MHz are: A₀=10 650B₀=2028.56C₀1722.16A₁10 615 B₁=2028.47 C₁=1725.11 A₂10 570 B₂ 2028.31 C₂= 1727.32 A₃= 10 512 B₃2028.11 C₃=1729.37The dipole moment components are μ_a = 4.52 D, μ_b = 0.42 D and μ_total = 4.54 D. From the lack of observable internal rotation splittings the barrier to internal rotation of the methyl group is shown to be greater than 2250 cal/mole.     

Formation time of F centers in alkali halides

It is argued that in most alkali halides an appreciable fraction of the self-trapped excitons may undergo non-radiative transitions from vibrationally excited states of B_3u to the A_1g state during the relaxation of the self-trapped excitons created by ionizing radiation. Numerical calculations show that the non-radiative transition probabilities, from vibrational levels of B_3u near or above the crossing point of the A_1g and B_3u potential curves to the A_1g state, are consistent with the observed formation times of F centers in various alkali halides. The exceptional case of KI is also discussed.     

Magnetic properties of the mixed spin-5/2 and spin-3/2 Blume-Capel Ising system on the two-fold Cayley tree

We use exact recursion relations to study the magnetic properties of the half-integer mixed spin-5/2 and spin-3/2 Blume-Capel Ising ferromagnetic system on the two-fold Cayley tree that consists of two sublattices A and B. Two positive crystal-field interactions Δ₁ and Δ₂ are considered for the sublattice with spin-5/2 and spin-3/2 respectively. For different coordination numbers q of the Cayley tree sites, the phase diagrams of the model are presented with a special emphasis on the case q = 3, since other values of q reproduce similar results. First, the T = 0 phase diagram is illustrated in the (D _A = Δ₁/J,D _B = Δ₂/J) plane of reduced crystal-field interactions. This diagram shows triple points and coexistence lines between thermodynamically stable phases. Secondly, the thermal variation of the magnetization belonging to each sublattice for some coordination numbers q are investigated as well as the Helmoltz free energy of the system. First-order and second-order phase transitions are found. The second-order phase transitions become sharper and sharper when D _A or D _B increases. The first-order transitions only exist for some appropriate non-zero values of D _A and/or D _B . The corresponding transition lines never connect to the second-order transition lines. Thus, the non-existence of tricritical points remains one of the key features of the present model. The magnetic exponent β ₀ of the model is estimated and found to be ¼ at small values of D _A = D _B = D and β ₀ = ½ at large values of D. At intermediate values of D, there is a crossover region where the magnetic exponent displays interesting behaviours.     

Continuum limit of A Z2 lattice gauge theory

Phase diagrams of lattice gauge theories have in several cases lines of first-order transitions ending at points at which continuous (second-order) transitions take place. In the vicinity of this critical point, a continuum field theory may be defined. We have analyzed here a Z₂ gauge plus matter model (which has no formal continuum limit) and identified the critical point with a usual Ø⁴, globally Z₂ invariant, field theory. The analysis relies on a mean field functional formalism and on a loop-wise expansion around it, which is reviewed.     

Disordered ferromagnetic alloys — Effective medium

The transition temperature and critical concentration of randomly disordered A_χB_1?χ alloys, where A is the magnetic constituent of the alloy, have been investigated. The effects of second neighbor A-A and of nearest neighbor A-B interactions, both ferromagnetic and antiferromagnetic, have been included. Molecular field, assumed to hold within small enough regions, together with percolation theory are used to formulate an effective magnetic medium.     

Microwave spectra of SiH2DF and SiHD2F: Harmonic force field and molecular structure

The ground vibrational state rotational spectrum of SiH₂DF and SiHD₂F has been studied in the frequency region 24–58 GHz. The observed transitions have been used to obtain the A, B, C rotational constants and three of the five quartic distortion constants present in the reduced Hamiltonian of Watson.The A, B, C rotational constants obtained in this work in combination with the B constants of the six symmetric isotopes were used to determine accurate substitution and average structures. An approximate equilibrium structure is also estimated.The general harmonic force field of silyl fluoride has been redetermined using all existing data.     

Light scattering by polariton modes of KNbO3 in orthorhombic phase

The polariton modes of A₁, B₁ and B₂ species in biaxial KNbO₃ are observed by near forward Raman scattering. The oscillator strengths and dielectric constants along the principal axes are determined from the polariton dispersion. The mode assignments of each species are also discussed.