Monoclinic Site Symmetry Research Articles

Temperature dependence of local structural changes around transition metal centers Cr3+ and Mn2+ in RAl3(BO3)4 crystals studied by EMR

Theoretical investigations are carried out to determine the temperature dependence of the local structural parameters of Cr3+ and Mn2+ ions doped into RAl3(BO3)4 (RAB, R = Y, Eu, Tm) crystals. The zero-field splitting (ZFS) parameters (ZFSPs) obtained from the spin Hamiltonian (SH) analysis of EMR (EPR) spectra serve for fine-tuning the theoretically predicted ZFSPs obtained using the semi-empirical superposition model (SPM). The SPM analysis enables to determine the local structure changes around Cr3+ and Mn2+ centers in RAB crystals and explain the observed temperature dependence of the ZFSPs. The local monoclinic C2 site symmetry of all Al sites in YAB necessitates consideration of one non-zero monoclinic ZFSP (in the Stevens notation, b21) for Cr3+ ions. However, the experimental second-rank ZFSPs (D=b20, E=1/3b22) were expressed in a nominal principal axis system. To provide additional insight into low symmetry aspects, the distortions (ligand's distances ΔRi and angular distortions Δθi) have been varied while preserving monoclinic site symmetry, in such way as to obtain the calculated values (D, E) close to the experimental ones, while keeping b21 close to zero. This procedure yields good matching of the calculated ZFSPs and the experimental ones, and enables determination of the corresponding local distortions. The present results may be useful in future studies aimed at technological applications of the Huntite-type borates with the formula RM3(BO3)4. The model parameters determined here may be utilized for ZFSP calculations for Cr3+ and Mn2+ ions at octahedral sites in single-molecule magnets and single-chain magnets.

Electron Paramagnetic Resonance Characterization of Mn2+Ion in GaAs:Mn Crystal

The electron paramagnetic resonance (EPR) of a new Mn 2+ center in a GaAs:Mn single crystal has been investigated with an X-band spectrometer at 3.8 K. The spectroscopic splitting parameters g i j and the zero-field splitting parameters B k q were fully determined using the effective spin Hamiltonian. The local site symmetry of the Mn 2+ ion turns out to be monoclinic from the rotation patterns together with the spectroscopic splitting parameters and the zero field splitting parameters. The new Mn 2+ center with monoclinic site symmetry is believed to originate from interstitial Mn 2+ ions. The energy levels of the ground state for the Mn 2+ impurity ion embedded in a GaAs crystal, are calculated as a function of applied magnetic field.

A Variable-Temperature X-Band EPR Study of the Gd3+ Ion in a La2Si2O7 Crystal Characterized by Monoclinic Site Symmetry

Electron paramagnetic resonance (EPR) studies on a single crystal of diamagnetic compound La2Si2O7, potentially a phosphorescent/luminescent/laser material, with the Gd3+ ion substituting for the La3+ ion, were carried out at X-band (9.61 GHz) over the 4–295 K temperature range. The asymmetry exhibited by the Gd3+ EPR line positions for the orientations of the external magnetic field about the magnetic Z- and Y-axes in the ZY-plane was ascribed to the existence of monoclinic site symmetry at the site of the Gd3+ ion, as confirmed by the significant values of the spin Hamiltonian parameters g YZ , b 2 −1, b 4 −m (m = 1, 3), b 6 −m (m = 1, 3, 5), estimated by fitting all EPR line positions observed at room temperature for the orientation of the magnetic field in the magnetic ZX- and ZY-planes using a rigorous least-squares fitting procedure. At 8 K measurements were only carried out for orientation of B in the magnetic ZX-plane, due to difficulty in orientation of the crystal inside the cryostat, enabling estimation of all spin Hamiltonian parameters b n m except those characterized by negative m values and g YZ . The absolute sign of the zero-field splitting parameter b 2 0 was determined to be negative from the relative intensities of the lines at 8 K.

Variable temperatureX-band EPR ofGd3+inLaNbO4andPrNbO4crystals: Low-symmetry effect, influence of host and impurity paramagnetic ions on linewidth, and onset of antiferromagnetism

EPR studies on ${\mathrm{Gd}}^{3+}$-doped diamagnetic ${\mathrm{LaNbO}}_{4}$ and paramagnetic ${\mathrm{PrNbO}}_{4}$ crystals containing ${\mathrm{Nd}}^{3+}$ and ${\mathrm{Ce}}^{3+}$ impurity Kramers ions were carried out at the X band (9.61 GHz) in the 4--295 K temperature range. The observed asymmetry in the extrema of ${\mathrm{Gd}}^{3+}$ EPR line positions in the angular variations of the EPR lines in the two hosts for the various transitions observed for the orientations of the external magnetic field about the Z and Y magnetic axes in the ac crystallographic plane was ascribed to the existence of the low, monoclinic site symmetry at the ${\mathrm{Gd}}^{3+}$ ion. The ${\mathrm{Gd}}^{3+}$ spin-Hamiltonian parameters were estimated from EPR line positions in the two crystals at 8, 73, and 295 K. The variation of the ${\mathrm{Gd}}^{3+}$ EPR linewidth as a function of temperature in the paramagnetic ${\mathrm{PrNbO}}_{4}$ host between 100 and 295 K is explained to be due to the dynamical dipolar and exchange interactions between the impurity ${\mathrm{Gd}}^{3+}$ ions and the host paramagnetic ${\mathrm{Pr}}^{3+}$ ions. The increase in the linewidth with decreasing temperature below 30 K in the two crystals implies the onset of antiferromagnetic ordering, consistent with ${T}_{c}=1.6$ and 2.4 K in ${\mathrm{LaNbO}}_{4}$ and ${\mathrm{PrNbO}}_{4}$ crystals. The observed decrease in the integrated intensity of ${\mathrm{Gd}}^{3+}$ EPR signal with lowering temperature in the two crystals is ascribed to the coupling of ${\mathrm{Gd}}^{3+}$ ions to the impurity ${\mathrm{Nd}}^{3+}$ and ${\mathrm{Ce}}^{3+}$ Kramers ions present in the samples.

The magnetic states of in

We have investigated the compounds and with the aim of establishing the magnetic energy levels and wavefunctions of the and ions. We report neutron inelastic scattering, specific heat capacity and susceptibility measurements, and analyse the data in two stages to obtain a model for the crystal field acting on the and ions. First we find the best model for the crystal field assuming tetragonal symmetry, and then we refine this model to include terms describing the actual monoclinic site symmetry . By this process we obtain a good description of the available data. We find that the low-temperature magnetic properties of in are dominated by two singlets, separated in energy by 4.3 meV. The level scheme consists of five, roughly equally spaced doublets, the ground and first excited states separated by 7.1 meV. The doublets are split by an effective magnetic field from the ordered moments, and the size of the splitting of the ground-state doublet is 1.5 meV.

Monoclinic Spin Hamiltonian Analysis of EPR Spectra of Mn2+ in BiVO4 Single Crystals

AbstractX‐band EPR spectra of Mn2+ in ferroelastic BiVO4 single crystals at room temperature taken in three orthogonal planes are analyzed using a twestage least‐squares fitting procedure. The absence of site splitting from the spectra shows unequivocally that Mn2+ lies on a site with two‐fold rotation symmetry, whose two fold axis is parallel to the monoclinic crystal axis. The low symmetry effects arising from the monoclinic site symmetry are evidenced by the observed coincidence of the extrema for transitions between different Zeeman levels along the monoclinic axis, and by the 180° rotational symmetry in the monoclinic plane and a small noncoincidence of the turning points in this plane. In the first stage, the spectra are fitted using the monoclinic form of the electronic Zeeman and fine structure terms (of second and fourth degree) appropriate for the crystallographically determined orientation of the monoclinic axis. In the second stage, fitting of the nuclear hyperfine tensor (A) and the nuclear quadrupole tensor (P) is carried out with fixed values of the electronic Zeeman tensor (g) and fine structure terms, i.e. the tensor (D) and the fourth‐degree terms, as obtained in the first stage. The principal axes of g, D, A, and P are found to be non‐coincident in the monoclinic plane, as would be generally expected for a monoclinic site.

Observability of forbidden hyperfine transitions along the principal axes in the EPR of low-symmetry complexes

It is shown that forbidden hyperfine transitions of non-zero intensity are not observable for the Zeeman field orientation along the two-fold symmetry axis, when it is parallel to a crystal-field axis, of paramagnetic complexes, characterized by monoclinic site symmetry; on the other hand, they are observable for all orientations of the Zeeman field for complexes characterized by triclinic symmetry. Under the assumptions of isotropic hyperfine tensor and vanishing nuclear quadrupole interaction expressions are given for the intensities of the forbidden hyperfine transitions (ΔM=±1, Δm=±1; M and m are the electronic and nuclear magnetic quantum numbers, respectively), relative to those of the allowed hyperfine transitions (ΔM=±1, Δm=0) for an arbitrary Zeeman-field orientation. The three cases, where the monoclinic symmetry axis is parallel to a crystal-field (magnetic) axis have been specifically considered.