Experimental studies of structurally incommensurate crystal phases

Perovskite-related oxynitrides – Recent developments in synthesis, characterisation and investigations of physical properties

Synthesis of perovskite-related layered AnBnO3n+2 = ABOX type niobates and titanates and study of their structural, electric and magnetic properties

The database on structural phase transitions is presented in the simple and easy accessible form of table. The symmetry changes and temperature for 3446 phase transitions and the references for each of 2242 registered crystalline materials with well defined stoichoimetry are the main data in this table. The rules for constructing and reading the database table are also presented.

Structural phase transitions in crystals. I. Database

Spherical?(S) and tesseral?(T) tensor operators?(TOs) have been extensively used in, for example, EMR and optical spectroscopy of transition ions. To enable a systematic review of the published tables of the operators and their matrix elements?(MEs) we have generated the relevant tables by computer, using Mathematica programs. Our review reveals several misprints/errors in the major sources of TTOs?the conventional Stevens operators (CSOs?components ) and the extended ones (ESOs?all q) for rank k = 2,4, and?6?as well as of some STOs with . The implications of using incorrect operators and/or MEs for the reliability of EMR-related programs and interpretation of experimental data are discussed. Studies of high-spin complexes like Mn12 (S = 10) and Fe19 (S = 33/2) require operator and ME listings up to k = 2S, which are not presently available. Using the algorithms developed recently by Ryabov, the generalized ESOs and their MEs for arbitrary rank k and spin S are generated by computer, using Mathematica. The extended tables enable simulation of the energy levels for arbitrary spin systems and symmetry cases. Tables are provided for the ESOs not available in the literature, with odd k = 3,5, and?7 for completeness; however, for the newly generalized ESOs with the most useful even rank k = 8,10, and?12 only, in view of the large listings sizes. General source codes for the generation of the ESO listings and their ME tables are available from the authors.

https://iopscience.iop.org/article/10.1088/0953-8984/16/32/018/pdf

The generalization of the extended Stevens operators to higher ranks and spins, and a systematic review of the tables of the tensor operators and their matrix elements

A new phase transition in Sr 2 Nb 2 O 7 was found at 215°C in addition to the already known phase transitions. The elastic compliance coefficients, the electromechanical coupling factor and the dielectric constant of an Sr 2 Nb 2 O 7 single crystal show anomaly around 215°C.

Phase Transition in Sr 2 Nb 2 O 7 at 215°C

The ferroelectric phase transition of mixed crystal systems of Rb 2 SO 4 –(NH 4 ) 2 SO 4 and CS 2 SO 4 –(NH 4 ) 2 SO 4 was studied by measuring the lattice constants, the dielectric constant, the spontaneous polarization and the proton spin-lattice relaxation time with functions of temperature and concentration of Rb + ion and Cs + ion. With the increase in the concentration of the ions in both mixed crystal systems, the following changes occur. (1) The ferroelectric phase transition temperature increases a few degrees until it reaches a certain concentration and decreases sharply. (2) The Curie-Weiss constant increases. (3) The phase transition changes from the first order to the second order at the certain concentration. (4) The maximum value of the spontaneous polarization increases about 10% until it reaches the certain concentration and decreases gradually. The results are discussed with a two-nonequivalent sublattice model.

Ferroelectric Phase Transition in Rb2SO4–(NH4)2SO4 and Cs2SO4–(NH4)2SO4 Mixed Crystals

The optical absorption and the interband Faraday rotation in LiTaO3 and LiNbO3 were measured at room temperature. The optical absorption in both crystals follows Urbach's equation and the effective temperatures defined by this equation are considerably higher than those of the cubic perovskites. The interband Faraday rotations are negative and about one order of magnitude smaller than those of the perovskites. The results are well described by a combination of indirect transition and direct transition which is estimated from the absorption data.

Optical absorption and interband faraday rotation in litao3 and linbo3

The decay of the 2.2 eV emission band in KTaO 3 single crystal was measured using an N 2 laser. The decay of the 2.2 eV emission band at low temperature is in good agreement with that of the 2.6 eV emission band. However, the decay of the 2.2 eV emission band differs from that of the 2.6 eV emission band above 70 K. The decay kinetics of the 2.2 eV emission band is explained by the theory of the recombination of holes trapped on two classes of acceptors and electrons trapped on one class of donors.

Kikuo Ohi

Papers

Phase Transition in Sr 2 Nb 2 O 7 at 215°C

Ferroelectric Phase Transition in Rb2SO4–(NH4)2SO4 and Cs2SO4–(NH4)2SO4 Mixed Crystals

Optical absorption and interband faraday rotation in litao3 and linbo3

The Photoluminescence Decay of the 2.2 eV-Emission Band in KTaO3 Single Crystal

Optical mode softening in the incommensurate phase of Sr2Nb2O7