Local Tetragonal Symmetry Research Articles

Two‐Photon Luminescence Excitation Spectra of Eu3+ in Yttrium Aluminium Garnet

AbstractThe results are reported of the first study of the 5D2 manifold of Eu3+ in the noninversion and inversion centres of YAG using the method of two‐photon luminescence excitation. Based on the number of Stark components and the two‐photon polarization dependences it is shown that the set of Stark components of 5D multiplet corresponds to tetragonal local symmetry. The components of 5D2 manifold are identified and the crystal field parameters are found for the non‐inversion centre. The europium ion distribution over the centres is determined showing that at least 60% of the ions are in the inversion centre.

V 4+ in SrTiO 3: A Jahn-Teller impurity

EPR results of V 4+, with S = 1 2 , in SrTiO 3 are reported. The tetragonal local symmetry of the impurity ion is related to strong T 2 g × ϵ g coupling as evidenced by intensity variations in the presence of stress. At 4.2 K the V 4+ EPR behaviour is related to the intrinsic local strain in SrTiO 3.

EPR study of central peak dynamics around the AsO44− center in KDA–KDP and ADA–ADP mixed crystals

Using the lifetime effects in the domain splitting of the ESP spectra as a detector of the polarization fluctuations around an AsO4−4 center, we have studied the dependence of the temperature T—where the local tetragonal symmetry of the high temperature paraelectric phase is broken on the time scale of the ESP experiment—on Tc in mixed KH2AsO4–KH2PO4 (KDA–KDP) and NH4H2AsO4–NH4H2PO4 (ADA–ADP) crystals. T decreases with Tc in KDA–KDP and increases with Tc in ADA–ADP crystals though the domain splitting is nearly constant. This can be understood only if the central peak width depends on the host lattice and thus varies with Tc. The width of the central peak in the polarization fluctuation spectrum, which changes in KDA from 280 MHz at 45 °C to 6 MHz at −90 °C does not depend on the concentration of the paramagnetic defect centers within the range studied. The results can be explained by a model where in the high temperature phase the mobile clusters of the low temperature phase are trapped by the defect centers.

Slow Ferroelectric Cluster Dynamics in KH2AsO4and KH2PO4fromCr5+Paramagnetic Resonance

We succeeded in substituting the paramagnetic ${\mathrm{Cr}}^{5+}$ ($S=\frac{1}{2}$) ion with the same size as ${\mathrm{As}}^{5+}$ in K${\mathrm{H}}_{2}$As${\mathrm{O}}_{4}$, for ${\mathrm{P}}^{5+}$ in K${\mathrm{H}}_{2}$P${\mathrm{O}}_{4}$, and in their deuterated isomorphs. In the crystals the high-temperature resonance spectrum showing a local tetragonal symmetry is broken on the time scale of the EPR experiment below ${T}^{*}>{T}_{C}$, so that orthorhombic spectra appear in the paraelectric tetragonal phase. ${T}^{*}$ shifts proportionally to the Curie temperature ${T}_{C}$ upon deuteration. We deduce the existence of ferroelectric clusters with lifetimes $\ensuremath{\tau}$ longer than 1.5\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}8}$ sec below ${T}^{*}$.

A simple theory of inhomogeneity linewidth broadening in MnxFe3−x,O4 ferrites

A simple theory of linewidth broadening due to thermal treatment is given. The theory is based on the assumption that the manganese-rich regions — clusters — with local tetragonal symmetry appear in the sample due to the alignment of tetragonally distorted Mn3 + ions. This is a source of local anisotropy fields, leading to two-magnon scattering. The size of clusters and consequently their contribution to the linewidth depends on the thermal treatment. The function giving the dependence of the linewidth broadening on the cluster size is calculated using Schlomann's autocorrelation function. The recent experimental results are qualitatively discussed in terms of this theory. For the temperature dependence of the effect an effective compositionx = 1·8 inside the cluster is assumed, which gives a qualitative agreement with experiment. The discussion of the dependence of linewidth broadening on the composition has only an illustrative character and shows that the theory is compatible with experiment. A qualitative disagreement found for the angular dependence of δ(ΔH) is believed to be a consequence of the high field approximation used.