X-ray Diffraction Study of the Structure and Defect System of Nominally Pure and Er<Superscript>3+</Superscript>- and Ce<Superscript>3+</Superscript>-Activated (Na<Subscript>0.5</Subscript>La<Subscript>0.5</Subscript>)MoO<Subscript>4</Subscript> Crystals

Nominally pure and activated (Er³⁺ and Er³⁺ + Ce³⁺) sodium lanthanum molybdate single crystals grown by the Czochralski technique from the (Na_0.5La_0.5)MoO₄ melt in different environments and then oxidized in air at 100°C are characterized in detail by x-ray diffraction (lattice parameters and structure refinement by the Rietveld method and single-crystal techniques). The results confirm that (Na_0.5La_0.5)MoO₄ crystallizes in a tetragonal scheelite structure (sp. gr. I₁/a). The crystals (especially unannealed crystals grown in neutral atmosphere) are shown to contain oxygen vacancies. In addition, some of the samples contain Mo vacancies. The Er³⁺ distribution over some of the activated crystals is highly inhomogeneous. As a result, the crystals contain Er-enriched zones with a distorted scheelite structure.     

Unusual structural properties of (Na0.5La0.5)MoO4:Er,Ce crystals

The structure of single crystals grown by the Czochralski technique from (Na_0.500La_{0.495 ? x} Ce_xEr_0.005)MoO₄ (x = 0.10, 0.125, 0.15, 0.175, 0.20) melts in a weakly oxidizing atmosphere (99–99.5% N₂ + 0.5–1% O₂) (NLM-0) was studied by x-ray diffraction. Some of the crystals received short-term annealing at 1000°C (NLM-2) and/or long-term annealing at 700°C (NLM-1). The oxygen in the structure of NLM:Er,Ce-0 and NLM:Er,Ce-1 with x = 0.20 was found to occupy two crystallographic sites at random. In the structure of the NLM:Er,Ce-2 crystal with x = 0.20, the oxygen atoms are fully ordered into one site, as in the scheelite structure. It cannot be ruled out that these crystals contain both Ce³⁺ and Ce⁴⁺ ions.     

Stoichiometry and purity of HgI2

Techniques given by ACS Reagent Specifications have been refined to allow the determination of deviations from stoichiometry in HgI_2+x at a level of x = 10^?4. Selected trace metals have also been determined with sensitivities of 0.5 – 10 mole ppm by atomic absorption spectrophotometry. HgI₂ in the form of commercial reagents, materials purified from these reagents by vacuum sublimation, and single crystals grown under a variety of conditions were investigated. The largest deviation from stoichiometry found corresponds to x = ?3×10^?3. No iodine excess has been observed.     

Magneto-optical properties of Tb0.81Ca0.19F2.81 and Tb0.76Sr0.24F2.76 single crystals

High optical quality terbium calcium fluoride (Tb_0.81Ca_0.19F_2.81) and terbium strontium fluoride (Tb_0.76Sr_0.24F_2.76) single crystals have been grown by the Czochralski method under controlled atmosphere. While both compounds present similar optical and magneto-optical properties (determined by the high concentration of Tb³⁺ ions), the lower absorption losses and high Faraday rotation exhibited by terbium calcium fluoride, compared to those of terbium strontium fluoride, lead to a notable figure of merit for Faraday isolators operating in the visible wavelength region. In particular at short wavelengths, below 500 nm, the figure of merit of terbium calcium fluoride is remarkably higher than that of the industrial standard terbium gallium garnet.     

Growth and spectral properties of Er:NaGd(WO4)2 crystal

A high optical quality Er³⁺-doped NaGd(WO₄)₂ single crystal with dimensions of ∅18 × 50 mm³ has been grown using the Czochralski method. The structure of the grown crystal was proved by X-ray powder diffraction. The accurate concentration of Er³⁺ ion in the crystal was measured. The absorption spectra, fluorescence spectra and fluorescence lifetime of the crystal were measured at room temperature. Green up-conversion luminescence has been observed when the crystal is excited at 965 nm.     

Growth and piezoelectric properties of Al-substituted langasite-type La3Nb0.5Ga5.5O14 crystals

The influences of aluminum substitution for gallium in the langasite-type La₃Nb_0.5Ga_5.5O₁₄ (LNG) crystals on their growth and electric properties were investigated. Al-substituted LNG (La₃Nb_0.5Ga_5.5−xAl_xO₁₄; LNGAx) single crystals up to the solubility limit x = 0.2 have been grown by the conventional Czochralski technique. The electric properties of the LNGAx crystals were investigated and compared to those of LNG. With Al substitution, the piezoelectric constants, d₁₁ and d₁₄, were slightly higher. The LNGAx crystals showed a temperature dependence of d₁₁ similar to that of the LNG crystal.     

Influence of Ce on the luminescence properties of Er/Yb:YVO4 crystals

YVO₄ single crystals doped with Ce³⁺, Er³⁺ and Yb³⁺ ions were grown by the Czochralsski technology. The luminescence properties of Er³⁺/Yb³⁺:YVO₄ single crystals with different concentration of Ce³⁺ were studied, and the energy transfer mechanism between Er³⁺, Yb³⁺ and Ce³⁺ was discussed based on their energy level properties. The branching ratios of the ⁴I_11/2 → ⁴I_13/2 transition in different samples were calculated. The results indicate that codopants of Ce³⁺ greatly enhance the population rate of the ⁴I_13/2 level due to the fast resonant energy transfer between Er³⁺ and Ce³⁺, i.e., ⁴I_11/2(Er³⁺) + ²F_7/2(Ce³⁺) → ⁴I_13/2(Er³⁺) + ²F_5/2(Ce³⁺).     

Crystal growth and scintillation properties of Er-doped Lu3Al5O12 single crystals

Er-doped Lu₃Al₅O₁₂ (Er:LuAG) single crystalline scintillators with different Er concentrations of 0.1, 0.5, 1, and 3% were grown by the micro-pulling-down (μ-PD) method. The grown crystals were composed of single-phase material, as demonstrated by powder X-ray diffraction (XRD). The radioluminescence spectra measured under ²⁴¹Am α-ray excitation indicated host emission at approximately 350 nm and Er³⁺ 4f-4f emissions. According to the pulse height spectra recorded under γ-ray irradiation, the 0.5% Er:LuAG exhibited the highest peak channel among the samples. The γ-ray excited decay time profiles were well fitted by the two-component exponential approximation (0.8 μs and 6-10 μs).     

Czochralski growth of Ba6Ti2Nb8O30

Ba₆Ti₂Nb₈O₃₀ (BTN) single crystals have been grown by the Czochralski method under a reducing atmosphere. Crystals grown in an oxidizing atmosphere were opaque polycrystals. It was found that BTN decomposes into several phases at temperatures above 1330°C. No congruent melt composition was found around stoichiometric composition in an oxygen or air atmosphere.     

Crystal growth of tetragonal GeO2 and Na4Ge9O20 and a comparison of their optical properties with glasses

Tetragonal GeO₂ has been prepared by the top-seed solution technique with different orientations of the seed. Large single crystals of Na₄Ge₉O₂₀ have been grown by the Czochralski method from a stoichiometric melt. Refractive indices of both crystals and of vitreous samples have been measured between 2 and 4.5 eV. Both crystals display an unusual dispersion of birefringence which is compared to stress induced birefringence of vitreous samples.     

Polarized spectral properties of Tm:K5Bi(MoO4)4 crystal

Trivalent thulium-doped K₅Bi(MoO₄)₄ single crystals were grown by the Czochralski method. Its polarized absorption and fluorescence spectra and fluorescence decay curves were recorded at room temperature. On the basis of the Judd-Ofelt theory, the spectral parameters of the Tm³⁺:K₅Bi(MoO₄)₄ crystal were calculated. The cross relaxations between Tm³⁺ ions were analyzed. The emission cross sections of the ³F₄ → ³H₆ transition were obtained by the Fuchtbauer-Ladenburg formula and then the gain cross sections around 1.9 μm were calculated. The peak emission cross section and width of emission band around 1.9 μm are comparable to those for Tm³⁺:YAG and the tunable range is about 280 nm for the potential ∼1.9 μm laser operation via the ³F₄ → ³H₆ transition.     

Luminescence properties of Ca0.65La0.35F2.35:Yb, Er with enhanced red emission via upconversion

Yb³⁺/Er³⁺ codoped Ca_0.65La_0.35F_2.35 materials with intense red emission via upconversion were prepared by a high temperature solid-state method. Based on the upconversion luminescence properties investigations, it was found that, under 980 nm excitation, Ca_0.65La_0.35F_2.35:20 mol.%Yb³⁺, xEr³⁺ showed intense red upconversion luminescence, which was ascribed to ⁴F_9/2 → ⁴I_15/2 transition of Er³⁺, although both green and red emissions could be detected. It was also found that the green and red emissions originated the two photon processes, and the ground-state absorption (GSA), excited-state absorption (ESA) and energy transfer (ET) processes between Er³⁺/Yb³⁺ ions and Er³⁺/Er³ ions were involved in the enhanced red emission mechanism.     

Czochralski single crystal growth of Sr.61Ba.39Nb2O6 for surface acoustic wave applications

Ferroelectric Sr_.61Ba_.39Nb₂O₆ (SBN) single crystals approximately 2 cm. in diameter and 5–6 cm. long have successfully been grown by the Czochralski technique. All the crystals were grown in the [001] direction in argon or oxygen, and it was found that the degree of difficulty of growth increased as the diameter of the crystals increased. Temperature dependence measurements on the poled Z-cut SBN crystals showed that this composition has temperature compensated orientations which make this material useful for surface acoustic wave devices.     

Synthesis and upconversion luminescence properties of Yb/Er codoped BaGd2(MoO4)4 powder

Synthesis and upconversion luminescence properties of the new BaGd₂(MoO₄)₄:Yb³⁺,Er³⁺ phosphor were reported in this paper. The phosphor powder was obtained by the traditional high temperature solid-state method, and its phase structure was characterized by the XRD pattern. Based on the upconversion luminescence properties studies, it is found that, under 980 nm semiconductor laser excitation, BaGd₂(MoO₄)₄:Yb³⁺,Er³⁺ phosphor exhibits intense green upconversion luminescence, which is ascribed to ²H_11/2 → ⁴I_15/2 and ⁴S_3/2 → ⁴I_15/2 transition of Er³⁺. While the observed much weaker red emission is due to the non-radiative relaxation process of ⁴S_3/2 → ⁴F_9/2 and ⁴F_9/2 → ⁴I_15/2 transition originating from the same Er³⁺. The concentration quenching effects for both Yb³⁺ and Er³⁺ were found, and the optimum doping concentrations of 0.5 mol% Yb³⁺ and 0.08 mol% Er³⁺ in the new BaGd₂(MoO₄)₄ Gd³⁺ host were established.     

Homogeneous Si0.5Ge0.5 bulk crystal growth as substrates for strained Ge thin films by the traveling liquidus-zone method

Compositionally uniform Si_0.5Ge_0.5 bulk crystals were grown by the traveling liquidus-zone method which we developed for alloy crystal growth. Grown crystals were characterized as substrates for compressive-strained Ge thin films for high mobility p-channels of complementary metal oxide semiconductor transistors. Compositional uniformity was excellent and crystallinity was also excellent for 10 mm diameter crystals. However, crystallinity was degraded for 30 mm diameter crystals although compositional uniformity was excellent. Transmission electron microscope (TEM) observation showed high dislocation density at the interface between a Si seed and a grown crystal due to lattice mismatch. However, the dislocation density decreased as crystal growth proceeded. High quality 30 mm diameter crystals will be grown when the single crystal length is extended judging from TEM results. In this paper, we report on the growth and characterization of Si_0.5Ge_0.5 crystals as substrates for strained Ge thin films.     

Crystal growth and scintillation characteristics of the Nd doped LiLuF4 single crystals

Sixty millimeter diameter single crystal of Nd³⁺ doped LiLuF₄ was successfully grown by the Czochralski technique. No remarkable absorption due to unfavorable impurities was observed from optical absorption measurements in the vacuum ultra-violet spectral region. The high crystallinity and homogeneous luminescence characteristics were found from X-ray rocking curve and cathode-ray luminescence respectively. X-ray excited luminescence spectrum was measured and the significant 4f²5d-4f³ luminescence at 182 nm was observed in the grown crystal. The pulse height spectrum was taken upon γ-ray irradiation. As a result, the grown crystals demonstrated sufficient response to the γ-ray showing the light yield of 420 ± 30 photons/MeV. The decay curve under α-ray irradiation was also investigated and described by two component decay kinetics which consists of the decay constants of 34 and 450 ns.     

Phase separation during the melting of oxide borates LnCa4O(BO3)3 (Ln=Y, Gd)

GdCa₄O(BO₃)₃ (GdCOB) and YCa₄O(BO₃)₃ (YCOB) single crystals have been grown from the melt by the Czochralski method. Subsequent DTA analysis of the single crystals showed different behavior for both substances. During the first heating both crystals showed a single sharp melting peak at 1490°C (GdCOB) or at 1504°C (YCOB), respectively. In subsequent heating/cooling runs only GdCOB shows one single peak, whereas YCOB shows five peaks. Moreover, phase separation of the melt can be observed by optical observation and by EDX measurements. This behavior can be explained by a miscibility gap in the YCOB melt. The YCOB crystal is formed from the stoichiometric melt by a monotectic reaction.     

X-ray diffraction study of La3Ga5.5Ta0.5O14 and La3Ga5.5Nb0.5O14 langasite-type single crystals

The structure of langatate (as-grown and vacuum-annealed: LGT-I and LGT-II, respectively) and langanite (seed- and tail-end portions: LGN-III and LGN-IV, respectively) single crystals grown by the Czochralski technique from charges of nominal composition La₃Ga_5.5M_0.5O₁₄ = La₃(Ga_0.5M_0.5)(1)Ga₃(2)Ga₂(3)O₁₄ (M=Ta⁵⁺, Nb⁵⁺) is studied by x-ray diffraction. The LGT-I and LGT-II crystals are shown to differ in the Ga and Ta distributions over crystallographic sites: La₃(Ga_0.52)Ta_0.48(2) ⁵⁺Ga₃(Ga_0.94Ta_0.06(1) ³⁺)₂O₁₄ in LGT-I (Ta⁵⁺ in the octahedral site Ga(1) and Ta³⁺ in the trigonal-pyramidal site Ga(3)) and La₃(Ga_0.55Ta_0.45(2) ⁵⁺)Ga₃Ga₂O_13.93(2)0.07 in LGT-II (Ta⁵⁺ in the octahedral site Ga(1) and oxygen vacancies in O(1)). The increased Ta content is responsible for the lower structural perfection of LGT-I. LGN-III and LGN-IV have essentially identical compositions, La₃(Ga_0.47Nb_0.53(1) ⁵⁺)Ga₅O₁₄ and La₃(Ga_0.48Nb_0.52(1) ⁵⁺)Ga₅O₁₄, respectively, but differ in polarity.__________Translated from Neorganicheskie Materialy, Vol. 41, No. 4, 2005, pp. 485–492.Original Russian Text Copyright © 2005 by Kuzmicheva, Tyunina, Domoroshchina, Rybakov, Dubovskii.     

Effect of starting melt composition on growth of La3Ta0.5Ga5.5O14 crystal

The phase diagram of the La₂O₃-Ta₂ O₅-Ga₂ O₃ system around La₃ Ta_0.5 Ga_5.5 O₁₄ (LTG) was investigated by a differential thermal analysis and the quenching method. La₃ Ta_0.5 Ga_5.5 O₁₄ single crystals were subsequently grown from different starting melt compositions using the Czochralski technique. The effect of the starting melt composition on crystal quality was examined by measuring the variation in the chemical composition and lattice parameters along the growth axis. On the basis of these experimental results, we have grown LTG single crystals over 2 inches in diameter.     

Polarized spectral properties and 1.5–1.6 μm laser operation of Er:Sr3Yb2(BO3)4 crystal

Undoped and Er³⁺-doped Sr₃Yb₂(BO₃)₄ crystals were grown by the Czochralski method. Room temperature polarized spectral properties of the Er:Sr₃Yb₂(BO₃)₄ crystal were investigated. The efficiency of the energy transfer from Yb³⁺ to Er³⁺ ions in this crystal was calculated to be about 95%. End-pumped by a diode laser at 970 nm in a hemispherical cavity, a 0.75 W quasi-CW laser at 1.5–1.6 μm with a slope efficiency of 7% and an absorbed pump threshold of 3.8 W was achieved in a 0.5-mm-thick Z-cut crystal glued on a 5-mm-thick pure YAG crystal with UV-curable adhesive.