SrLaAlO4 Single Crystals Research Articles

EPR and Optical Spectra of Cobalt in SrLaAlO4

The paramagnetic resonance and optical spectra of Co2+ ions in SrLaAlO4 single crystals are reported. The EPR spectrum shows one electronic transition consisting of eight hyperfine lines with anisotropic g-factor and nearly isotropic hyperfine interaction constant. A tetragonal distortion of the octahedral crystal field acting on Co2+ ions has been found. The optical spectrum consists of intensive lines corresponding to transition ΔS = 0 and weak intercombination lines. In the weak-field coupling scheme satisfactory agreement is obtained between the observed and calculated spectrum with Dq = 1030 cm—1 and Racah parameter B= 810 cm—1. Both the optical and EPR data indicate a covalent bonding of Co2+ to the nearest oxygen atoms.

Analysis of Thermal Shock during Rapid Crystal Extraction from Melts

A time-dependent stress field in LiNbO3 and SrLaAlO4 single crystals immediately after their extraction from melts has been found analytically. Numerical calculations show that during rapid extraction from melts the crystals are subjected to thermal shock which can be high enough to produce considerable dislocation density if there exists a plasticity zone or to induce cracks. During LiNbO3 crystal growth by the Czochralski method in a low temperature gradient high thermal stresses occur near the interface at the center and at the crystal periphery. After rapid extraction from the melt the thermal shock results in an increase in thermal stress over an order of magnitude. This can lead to the generation of high dislocation density at the bottom part of LiNbO3 crystal causing residual stress distribution after its cooling to room temperature. Times for the relaxation of stresses caused by the thermal shock are about 30 minutes and 30 seconds for 55 mm diameter LiNbO3 crystal and for 20 mm diameter SrLaAlO4 crystal, respectively. The magnitude and relaxation time of the stress caused by the thermal shock depend mainly on the crystal diameter, its thermal conductivity and the axial temperature gradient in the growth system. From analytical investigations a suitable method of crystal extraction from the melt may be obtained.

Electron spin resonance study of isolated Cr3+ ions and Cr3+ ion pairs in SrLaGaO4 and SrLaAlO4 single crystals

The ESR spectrum of isolated Cr3+ ions and nearest-neighbour Cr3+ ion pairs in SrLaGaO4 and SrLaAlO4 single crystals have been studied at X-band. A nearly isotropic, single line of isolated Cr3+ ions was observed for g = 1.980(2) in both materials. The zero-field splitting was small, therefore all allowed Cr3+ transitions nearly overlapped one another. For the nearest-neighbour Cr3+ ion pairs the ESR spectrum could only be observed for the S = 2 state, because broadening mechanisms did not affect this level. The coefficients of the isotropic exchange interaction J were determined from temperature dependencies of line intensity. The angular dependencies of ESR lines allowed the determination of the anisotropic exchange interaction constants. The following values have been found: J = 5.4(2)cm−1, g = 1.980(2), |DE| = 137(2) × 10−4cm−1, |EE| = 22(2) × 10−4cm−1 and J = 5.4(2) cm−1, g = 1.980(2), |DE| = 196(2) × 10−4cm−1, |EE | = 32(1) × 10−4cm−1 for SrLaGaO4 and SrLaAlO4 crystals, respectively.

Optical spectra and luminescence dynamics of Pr3+ and Nd3+ in SrLaAlO4 single crystals

Single crystals of SrLaAlO4: Pr3+ and SrLaAlO4: Nd3+ have been grown by the Czochralski method and their optical properties have been studied for different activator concentrations. The absorption and emission spectra of the activators are inhomogeneously broadened because the Sr2+ and La3+ ions are distributed randomly on the sites ofC4υ symmetry, however, the symmetry selection rules are still obeyed. Strong self-quenching of the praseodymium luminescence occurs by cross relaxation which is supposed to be assisted by phonon emission in the case of the3P0 level. Self-quenching of Nd3+ luminescence disappears at low temperature indicating that the condition of resonance in the cross-relaxation process is fulfilled only for higher components of the ground state.