M. Berkowski

Photoluminescence studies of Mn4+ ions in YAlO3 crystals at ambient and high pressure

Detailed investigations of the photoluminescence properties of Mn4+ (3d3) ions in YAlO3 have been performed in the temperature range 10–600 K. The luminescence of Mn4+ ions due to the transition consists of two zero-phonon lines (R lines) at 691.3 and 692.7 nm, which became visible only at low temperature, and their vibronic sidebands that cover the range of 660–740 nm. The thermal quenching of the luminescence intensity due to the non-radiative decay occurs at temperatures above 420 K. The temperature dependence of the luminescence decay time shows a quasi-linear decrease from τ = 4.9 to 1.6 ms in the temperature range from 90 to 420 K (with a temperature coefficient −0.01 ms K−1) that makes YAlO3:Mn a potentially good phosphor for a fibre optic fluorescence thermometer in this temperature range. The high-pressure low-temperature luminescence measurements in a diamond-anvil cell reveal similar pressure coefficients for Mn4+ and Cr3+ dopant ions in YAlO3, equal to 1.16 cm−1 kbar−1 and 1.08 cm−1 kbar−1, respectively.

Investigation of Eu3+ sites in SrLaGa3O7, SrLaGaO4 and SrLaAlO4 crystals

Abstract Trivalent europium has been used as a probe of symmetry of crystalline sites occupied by rare earth ions and of electron-phonon coupling in crystals belonging to two wide families of compounds with general chemical formulae ABCO 4 and ABC 3 O 7 ( A = Ca , Sr ; B = Y , La - Gd ; C = Al , Ga ). Random distribution of crystallochemically different ions over the lattice sites results in the spread of energy of Eu 3+ excited states in the three matrices, but the site symmetry is preserved in SrLaAlO 4 and SrLaGaO 4 . Strongly perturbed sites behave as one quasicentre with single decay time of luminescence. The multiphonon relaxation rates for the 5 D 2 level of Eu 3+ have been analysed with a point charge nonlinear relaxation mechanism proposed recently. Unusually low electron-phonon coupling in SrLaGaO 4 :Eu 3+ system has been found.

CaNdAlO4 perovskite substrate for microwave and far‐infrared applications of epitaxial high Tc superconducting thin films

Single‐crystal perovskite calcium‐neodymium aluminate CaNdAlO4 has been grown and tested as a substrate material for high Tc superconducting thin films. The material is chemically stable, can be fabricated into the form of large, twin‐free wafers, permits deposition of epitaxial Y‐Ba‐Cu‐O and Bi‐Sr‐Ca‐Cu‐O films, and exhibits low dielectric permittivity (e≂20) and very low losses (tan δ≂10−3 at 100 K) at sub‐THz frequencies. Its high potential in microwave applications of thin‐film high Tc superconductors is clearly demonstrated.

Optical spectra of a LaGaO3 crystal singly doped with chromium, vanadium and cobalt

Abstract Crystals of lanthanum orthogallate containing chromium, vanadium or cobalt were grown by the Czochralski method and their optical spectra were measured at room and 5K temperature. Recorded absorption and luminescence spectra of chromium ions which are convenient probes of number and type of available sites revealed that Cr3+ ions in LaGaO3 occupy identical sites with distorted octahedral symmetry and consistent with a strong crystal field case. Absorption spectra of vanadium doped LaGaO3 have been attributed to V4+ and V3+ ions coexistent in the matrix whereas a short lived and broad band luminescence recorded in the 12000–15000 cm−1 region has been attributed to the 2E–2T2 transition of V4+ ions. Absorption spectra of LaGaO3 doped with cobalt were found to be consistent with a mixed valency system containing Co2+ and Co3+. This activator–host combination did not show luminescence strong enough to be recorded with our detection systems.

Czochralski growth and structural investigations of La1−xNdxGaO3 solid solution single crystals

Abstract Growth of single crystals in the pseudobinary LaGaO 3 –NdGaO 3 system by the Czochralski method was investigated. It was found that solid solution crystals La 1− x Nd x GaO 3 exist in the whole concentration range. The structure of the crystals was investigated by X-ray diffraction method. It was found that the unit cell volume decreases linearly with increasing Nd concentration in agreement with Vegards law. With increasing Nd concentration x the lattice parameters a and c decrease, whereas b increases. At x =0.32 parameters a and b become equal and the crystal adopts the pseudo-tetragonal structure. The differential thermal analysis proved that the temperature of the first-order phase transition observed in LaGaO 3 at 150°C rises linearly with Nd concentration at the rate of 20.5°C/Nd mol% up to the melting temperature. The segregation coefficients of Nd in LaGaO 3 and La in NdGaO 3 have been found to be k eff (Nd)≈0.77 and k eff (La)≈1.3.

Anisotropy of optical properties of SrLaAlO4 and SrLaAlO4:Nd

Abstract Single crystals of SrLaAlO 4 and SrLaAlO 4 :Nd have been grown by the Czochralski method and their optical properties have been studied in polarized light. The crystals are positive uniaxial, and their birefringence is stable with respect to temperature in the 290 K–520 K temperature region. Undoped crystal is transparent from 240 nm to 7.6 μm. Intensities of absorption and spontaneous emission of Nd 3+ are slightly lower for light polarized perpendicular than those for light polarized parallel to the optical axis. Laser action in SrLaAlO 4 :Nd has been obtained for the first time using a diode laser as a pump. Stimulated emission occurs in the 1073 nm-1080 nm spectral region. Poor laser performance is attributed to the self-quenching of Nd emission.

Optical properties and lasing of BaLaGa3O7 single crystals doped with neodymium

Abstract Large single crystals of BaLaGa3O7 containing Nd were prepared by the Czochralski method and studied experimentally by absorption and emission spectroscopy. The dependence of the spectroscopic properties of the crystals on temperature, Nd concentration and light polarization was determined. Room temperature laser action associated with the 4 F 3 2 − 4 I 11 2 transition of Nd3+ was achieved.

SrLaGaO4SrLaAlO4 solid solutions: new promising substrate materials for HTSc

Abstract Different substrate materials with K2NiF4 structure for HTSc epitaxy have been compared with the best substrate materials with perovskite structure. The growth of SrLaGaO4 and SrLaAlO4 crystals, as well as their solid solutions on 〈100〉 and 〈001〉 oriented seeds, has been investigated. The optimal conditions for crystal growth occurred when the active afterheater was applied to decrease the vertical temperature gradient. The optimum starting compositions for crystal growth of SrLaGaO4SrLaAlO4 solid solutions have been found. The single crystals of SrLaAl1− xGaxO4, obtained up to now enabled preparation of substrates with lattice constant a in the range 0.3754–0.3774 nm, while the SrLaGa1−xAlxO4 crystals are good for substrates with lattice constant in the range 0.3843–0.3826 nm.

Conditions of maintenance of a flat crystal/melt interface during Czochralski growth of bismuth germanium oxide single crystals

Abstract Investigations of Czochralski growth of Bi 12 GeO 20 and Bi 4 Ge 3 O 12 single crystals in crucibles with different diameters and a constant crystal diameter/crucible diameter ratio d D = 0.42 have been carried out. The effect of the change in height of the melt layer in the crucible on the temperature distribution in the melt and on the critical crystal rotation rate at which a flat crystal/melt interface is formed, is determined. The temperature dependences of the viscosity and density of the melts of the two bismuth germanium oxides and the values of their volumetric expansion coefficients are established from separate measurements. After a suitable processing of the experimental results it is shown that they satisfy the general correlation proposed earlier on the basis of model studies for the critical crystal rotation rate and the geometrical and physical parameters of the system crucible-melt-crystal.

The Czochralski growth of BaLaGa3O7 single crystals

Abstract Single crystals of BaLaGa 3 O 7 have been grown by the Czochralski method. Most crystals have been pulled in the 〈001〉 direction. The crystals obtained contain more Ga 2 O 3 and BaO and less La 2 O 3 with respect to the stoichiometric composition. A distinct dependence of the chemical composition on the growth plane orientation has been observed. The composition of the crystals (cylindrical part) which grew on the (001) plane may be expressed by the formula Ba 1.05 La 0.925 Ga 3.025 O 6.975 .