A.S. Chaves

Unusual sequence of commensurate-incommensurate phase transitions observed in KLiSO4

Abstract An unusual structural phase transition sequence is observed in KLiSO4 by EPR: high temperature prototype phase -incommensurately modulated phase - commensurately modulated phase - incommensurately modulated phase - low temperature prototype phase (reentrant phase). The low temperature phase has the same symmetry C66 of the high temperature prototype phase. The main feature of the modulation are rotations of the sulphate tetrahedra, which were precisely determined in the commensurately modulated phase.

EPR lineshape study of the incommensurate phase in γ-irradiated K2SeO4

Abstract The temperature dependence of the SeO4-4 EPR frequencies and the asymmetric broadening of the EPR lines in the incommensurate phase of K2SeO4 can be explained by an incommensurate spatial modulation of the g tensors which corresponds to the “broad” phase soliton limit. A comparison between the experimental and calculated lineshape shows a ≈1% volume fraction of commensurate regions in the middle of the incommensurate phase at 110 K.

Crystal optical studies of the phase transitions in KLiSO4

Abstract The sequence of phases and transition temperatures of KLiSO4 - hexagonal I (948 K) orthorhombic II (711 K) hexagonal III (205 K) trigonal IV (185 K) monoclinic V (83 K) VI (65 K) VII (38 K) VIII - was investigated using microscopically resolved measurements of linear birefringence (LB), refractive indices and optical activity (OA). Principal results are: (i) orthorhombic stripe domains intersecting under ±120° in the c plane characterize phase II; (ii) vanishing LB and residual OA hint at merohedrally twinned 3m symmetry in phase IV; (iii) none of the phases V thru VIII is axial; (iv) observation of three pairs of twin domains refute orthorhombicity of phase V.

EPR study of the ferroelastic phase transition in CsLiSO4

Abstract EPR spectra of SO-3 ion-radical in X-ray irradiated CsLiSO4 single crystals were used for the study of the ferroelastic phase transition at Tc = 203.0 K. The splitting (ΔH) of the SO3 line in the low-temperature ferroelastic phase has been interpreted as proportional to the square of the order parameter. The splitting shows the temperature dependence ΔH ∞ (Tc − T) 1.01± 0.01

Broadening of the Si doping layer in planar-doped GaAs in the limit of high concentrations

Abstract A series of samples of GaAs planar doped with Si, grown by MBE at low substrate temperatures and with different doping concentrations, is investigated. A comparison of Shubnikov-de Haas measurements and self-consistent numerical calculations shows that a broadening of the doped region occurs in spite of the low growth temperature. The broadening occurs via segregation of the Si impurities with the growth surface when the solid solubility limit of Si in GaAs is exceeded. For the growth conditions used this limit is determined to be (2.1 ± 0.2) × 1019 cm−3. At high doping densities an intrinsic compensation mechanism becomes active, limiting the concentration of conduction electrons.

EPR study of central peak fluctuations in an ADA-ADP mixed crystal☆

Abstract The spontaneous dynamic symmetry breaking in the high temperature phase EPR spectra of AsO 4- 4 centers in antiferroelectric NH 4 H 2 AsO 4 - - NH 4 H 2 PO 4 mixed crystals was studied as a function of the concentration of paramagnetic defects. The results are not compatible with a Halperin-Varma type “relaxing” defect model. The width of the central peak seen by EPR strongly decreases at lower temperatures.

The Order Parameter for the Incommensurate Transition in K2SeO4: an EPR Lineshape Study

The EPR lineshape of the SeO 4 - center was investigated in the normal-incommensurate transition of K 2 SeO 4 . The value β=0.33 ±0.03 was obtained for the critical exponent of the order parameter. A summary of the measured values of the critical exponent β found in the literature for structurally incommensurate systems is presented. The average value of those measurements, β=0.345 ±0.004, is in excellent agreement with the calculated value of β for the 3d XY model. The lineshape analysis seems also to indicate that phase solitons appear in a large region of the incommensurate phase.

EPR study of the T2+ center in the paraelectric and antiferroelectric phases of NH4H2PO4

Abstract The EPR T 2+ center in NH4H2PO4 was studied in the range of temperature 85K – 175K which includes the antiferroelectric transition. The center has the symmetry of NH+4 site, where it goes substitutionally, in both the paraelectric and antiferroelectric phases. In the antiferr o electric phase there is a charge compensations mechanism involving the repulsion of one of the neighbor protons in the hydrogen bond. A measurement of the splitting of the resonance lines for the field along the cristallographic direction a is presented as a function of temperature. This splitting is proportional to the order parameter.

Calculation of the susceptibility of an eight-site order-disorder model for ferroelectrics

Abstract The contribution of the diffusion of the titanium ion to the dielectric tensor, is estimated for all phases of BaTiO3 in thermodynamical model; the values obtained for the tetragonal phase are in good agreement with the difference between the phonon contribution and the capacitance measurements, for both directions parallel and perpendicular to the ferroelectric axis.

The anomalous lineshapes of the EPR spectra of the AsO4−4 center in CsH2AsO4

Abstract In order to fulfill the symmetry requeriments the EPR lines of the AsO4−4 center in CsDA crystals must be fitted with an anomalous lineshape like “two inverted gaussians”, for any orientation of the magnetic field. Superhyperfine interaction with one Cs+ ion can explain the line anomaly better than the “continous disorder” model previously proposed.