Hyperfine structure of EPR spectra of Cr5+ ions in irradiated sodium dichromate crystals

EPR spectra are studied of X-irradiated sodium dichromate crystals grown from an aqueous solution by evaporation at 31°C. Doublet lines of EPR-absorption are attributed to the Cr⁵⁺ ions in the CrO₄³⁻ and CrO₃⁻ radicals resulting from radiation decomposition of Cr₂O₇²⁻ and being in the lattice in two unequivalent positions. Hyperfine structure caused by interaction of an unpaired electron with Cr⁵³ nucleus were observed both at liquid nitrogen and room temperatures. For the line, caused by CrO₄³⁻ radical, g_y and A_y directions coincide and angles both between A_x, g_x and A_z, g_z make up ∼ 25°. The spectrum is described by usual spin Hamiltonian for S = 1/2 with following parameters (T = 77 K): for CrO: g_z = 1.984, g_y = 1.970, g_x = 1.961, |A_z| = 8.2 · 10⁻⁴ cm⁻¹, |A_y| = 13.7 × 10⁻⁴ cm⁻¹, |A_x| = 36.1 · 10⁻⁴ cm⁻¹; for CrO₃⁻: g_z = 1.915, g_y = 1.975, g_x = 1.985, |A_z| = 32.2 · 10⁻⁴ cm⁻¹.     

Hyperfine structure of Cr5+ EPR spectra in X-irradiated potassium halide crystals doped with CrO and Ca2+

The hyperfine structure has been investigated in the EPR spectra of X-irradiated KCl: CrO, Ca²⁺ and KBr: CrO, Ca²⁺ crystals. Preliminarily spectra were simplified by means of heating the crystals up to 400 K for KCl and 440 K for KBr, that destroys the less stable CrO · V centers. It is ascertained, that principal directions of g- and A-tensors do not coincide, and principal A-values are determined. Calculations, making use of the experimental meanings of g- and A-tensor components, showed, that the degree of distortion of oxygen tetrahedra in CrO · Ca²⁺ · V centers is almost the same in both KCl and KBr crystals; besides, these calculations suggest the existence of a strong covalent bonding between the central Cr⁵⁺ ion and four oxygen ligands in CrO ions.     

Growth and some properties of Ce3+-doped LiYbF4 single crystals

LiYbF₄ single crystals, nominally pure and doped with Ce³⁺ ions, of optical quality and up to 60 mm in diameter, have been grown by vertical directed crystallization. The optical and mechanical properties of the crystals have been studied. The refractive index dispersion for LiYbF₄ in the range of 0.4–0.6 μm can be described by the dependence n ²(λ) − 1 = Aλ²/(λ² − λ ₀²), where A = 1.14 and 1.21 and λ₀ = 0.074 and 0.080 μm for n _o and n _e, respectively. The sample microhardness exceeds 2.6 GPa. LiYbF₄ crystals are transparent in the range of 0.17–9 μm and have an absorption band in the range of 0.9–1.2 μm. It is shown that LiYbF₄ crystals doped with Ce³⁺ ions can be used as optical cut-off UV filters in the operating range λ = 0.25−0.28 μm.     

Spectroscopy and crystal-field analysis of Cr4+-doped transparent silicate glass-ceramics

We report transparent Cr⁴⁺-doped SiO₂–Al₂O₃–ZnO–Li₂O–K₂O glass-ceramics with broadband infrared luminescence. After heart-treatment, Li₂ZnSiO₄ crystallite was precipitated in the glasses, and its average size increased with increasing heat-treatment temperature. Racah parameters of Cr⁴⁺–Li₂ZnSiO₄ glass-ceramics have been calculated, and it was confirmed from absorption spectra that the energy levels of Cr⁴⁺-doped glass-ceramics are close to the cross point ¹E and ³T states. No infrared emission was detected in the as-made glass samples, but the broadband infrared emission centered at 1210 nm with the full width at half maximum (FWHM) of more than 250 nm was observed by exciting the glass-ceramics with excitation of an 808 nm laser diode. In order to analyze the located crystal field of Cr⁴⁺ ions, the emission spectra are fitted by multi-peak Gauss fitting. It is seen that the fluorescence spectra are fitted into two Gaussian bands at around 1195 and 1263 nm with band widths of 208 and 278 nm, respectively. The two Gaussian bands at around 1195 and 1263 nm have about the same decay rate, and hence they would probably originate from the same luminescent centers. The observed infrared emission could be attributed to Cr⁴⁺ ions at low-field sites in Li₂ZnSiO₄ glass-ceramics.     

Colour centre in YAG: Cr3+ crystals

YAG crystals doped with 0.01 wt% Cr were grown by Czochralski method using 98% Ar + 2% H₂ protective atmosphere. Four colour varieties of the crystals were prepared in the dependence of the Al₂O₃ :Y₂O₃ ratio in the melt and the sign of electrical potential above the melt level. New absorption bands were attributed to the O⁻ centre (400 to 480 nm), F centre (357, 500, and 833 nm) or Cr³⁺ at another site than octahedral (455, and 612 nm).     

LiTaO3:Cr3+晶体中Cr3+离子占位及其EPR参量的理论研究

利用Racah不可约张量算符法和Winger定理,建立了d3(C*3v)电子组态的能量矩阵及其全组态EPR理论;借助Newman的叠加晶场模型和自旋哈密顿理论,研究了EPR参量与LiTaO3:Cr3+晶体结构参数之间的定量关系.在此基础上,研究了LiTaO3:Cr3+的EPR参量及其电子光谱,理论与观测一致.定量研究表明:LiTaO3:Cr3+晶体中,Cr3+离子取代了Ta5+离子而不是Li+离子.     

EPR spectra of irradiated Na2Cr2O7 · 2 H2O crystals

The EPR spectra of X-irradiated Na₂Cr₂O₇ · 2 H₂O crystals grown from the aqueous solution at room temperature are studied. Three groups of lines are detected, which marked conventionally as C₁, C₂ and C₃. Each of the C₁ and C₂ line groups is a superposition of two doublets. The lines are interpreted as due to the Cr⁵⁺ ions in the distorted (stretched) oxygenous tetrahedron. The doublet structure is due to hyperfine interaction with nuclei of 1/2 spin which are protons of water. It is shown that the C₁ and C₂ line groups are due to the centers of the same type with different orientations in the lattice. The principal g-tensor values for these centres were as follows: g_z = 1.916, g_y = 1.978, g_x = 1.986.     

Cr3+ luminescence quenching in stoichiometric lithium niobate crystals

Cr³⁺-doped stoichiometric LiNbO₃ crystals were grown and detailed spectroscopic investigations were performed. The samples are characterized by extremely low frequency factor of the luminescence thermal quenching. Its numerical value 1.7 × 10⁸ s⁻¹ is about 10⁴ times lower than in crystals of lithium-aluminum fluorides. Under such conditions, radiative transitions of Cr³⁺ ions compete successfully with non-radiative ones, resulting in a relatively high quantum yield of the broadband luminescence at room temperature. The quenching of luminescence is counteracted by the effect of the dynamic removal of the radiative transition exclusion, and as a result, Cr³⁺ radiative lifetime in LiNbO₃ decreases from 8 to 4 μs when temperature grows up from 77 to 300 K. Therefore quantum yield of the broadband luminescence at room temperature is not 5–10%, as reported previously, but about 30%. The results obtained in the present study show the stoichiometric lithium niobate doped with Cr³⁺ ions to be a potential active media for tunable lasers.     

High-order picosecond SRS and self-SRS generation in Nd3+-doped CaMoO4, SrMoO4, and SrWO4 laser crystals

The results of a stimulated Raman scattering (SRS) spectroscopy study of Nd³⁺-doped CaMoO₄, SrMoO₄, and SrWO₄ crystals are presented. All the wavelengths of the Stokes and anti-Stokes generation observed in these crystals are identified. For the first time, the self-SRS conversion of the emission frequency of activator Nd³⁺-ions is attained in these lasing molybdates and tungstate with scheelite-type structures.     

Photoluminescence of annealed LLGG:Cr3+ crystals under high pressure

High pressure photoluminescence has been used for analysis of lattice disorder in La_2.7Lu_2.29Cr_0.01Ga₃O₁₂ and La_2.32Lu_2.59Cr_0.02Ga_3.07O₁₂ crystals. Photoluminescence of samples before annealing and those annealed for 5 h in oxygen and hydrogen atmosphere at 923 K has been measured. The pressure dependence of Cr³⁺ luminescence has been used to obtain the crystal field distribution (lattice disorder) and its dependence on the thermal treatment. The distribution energy of the ⁴T₂ state has been considered as a measure of disorder. Annealing in oxygen has been found to reduce the energies of ⁴T₂ states for all sites, and annealing in hydrogen – to increase these energies.     

CdSe晶体及掺Cr2+激光器的研究进展

以CdSe晶体光参量振荡器为代表的中红外激光器在生物、医疗和军事等诸多领域有着广泛的应用前景.本文总结了生长CdSe单晶的工艺方法包括熔体法、熔剂法和气相法等,其中常用的为气相法,近年来高压垂直梯度冷凝(HPVGF)技术也逐渐被采用.国内外将本征CdSe晶体用于光参量振荡器件(OPO),适用于多种激光器泵浦源,且输出功率不断提高.而Cr2+的掺杂能有效提高泵浦效率,实现波长的连续可调.     

EPR and Optical Absorption Spectra of Cr3+ Ions in Lithium Sulphate Single Crystals

Optical absorption and EPR spectra of Li₂SO₄ · H₂O crystals doped with Cr³⁺ are studied at liquid nitrogen temperature. The bands are found in absorption spectra with maxima about 17000, 23 800 and 37 200 cm⁻¹, assigned to the ⁴A₂ → ⁴T₂, ⁴A₂ → ⁴T₁ and ⁴A₂ → ⁴T₁ (⁴P) transitions, respectively. The crystal field theory parameters were determined and appeared to be as follows: Dq = 1700 cm⁻¹, B = 667 cm⁻¹, C = 3002 cm⁻¹. The lines resulting from Cr³⁺ ions are found in EPR spectra. All lines are doublets, which is indicative of presence of two magnetically unequivalent centre positions, and have the hyperfine structure resulting from interaction of the unpaired electron spin with Cr⁵³ isotope nucleus. Centres are oriented in such a way, that z-axes, corresponding to two centre positions, are situated at both sides of a-axis at an angle of about 3°. Spin Hamiltonian parameters were found as follows: g_x = 1.985, g_y = 1.984, g_z = 1.988, D = 0.130 cm⁻¹, E = 0.016 cm⁻¹, |A| = 17.8 · 10⁻⁴ cm⁻¹.     

Low temperature EPR spectra of Fe3+ centers in ternary layered TlGaS2 crystal

The results of low temperature electron paramagnetic resonance (EPR) study of Fe doped TlGaS₂ single crystal in the temperature range of 5‐300 K are presented. Iron was added to the growth mixture in amounts corresponding to a molar ratio Fe/Ga of about 1%. The EPR signal due to Fe³⁺ centers located at the centers of GaS₄ tetrahedrons has been observed. A transformation of EPR spectra has been observed on decreasing the temperature, which is attributed to lowering of the number of resonance lines. The simulation of rotational patterns of the resonance fields exhibits a decrease of the number of centers at low temperatures as a contrary to those of isostructural compounds of TlGaS₂. On comparing the EPR spectra of Fe³⁺ centers in TlGaSe₂, it has been revealed that the Tl atoms lose their zigzag alignment observed previously at room temperature and rearranged to equal structural positions on Tl chains on lowering the temperature. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Cr3+‐doped LiNbO3 crystals grown by the Bridgman method

The growth of LiNbO₃ crystals doped with Cr³⁺ ions in 0.1, 0.2, and 0.5 mol % concentrations by Bridgman method were reported. The Cr³⁺ ion concentrations in crystals were measured by inductively coupled plasma spectrometry. Electron paramagnetic resonance had been used to investigate the sites occupied by the Cr³⁺ ions. Two Cr³⁺ ion centers located at Li⁺ and Nb⁵⁺ sites (Cr_Li³⁺ and Cr_Nb³⁺ centers, respectively) were observed. Optical absorption and temperature‐dependence emission spectra of the Cr³⁺ ions were reported. The crystal‐field parameters and Racah parameters of the Cr³⁺ ion defect sites were reported and compared with those grown by Czochralski technique. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

GdAl3（BO3）4及其Yb^3＋激活激光晶体的研究

采用顶部籽晶法从K2Mo3O10-B2O3助熔剂体系中生长出尺寸达到20 mm的GdAl3(BO3)4(简称GAB)和Yb3 激活的晶体.测试了GAB晶体的<100>和(001)轴向热膨胀系数,确定了透光波长范围,计算了有效倍频系数和相位匹配角随波长的变化,结果表明GAB晶体在整个透光范围内均可实现相位匹配.测定了Yb3 :GAB晶体在室温下的偏振吸收和荧光光谱,进行了光谱计算,测试了晶体的激光性能,在1046 nm处实现了2.1 W激光输出,斜率效率达到58%.     

Er3+掺杂钨酸铅晶体的发光和光谱特征

本试验测试了Er3 掺杂钨酸铅晶体(PbWO4:Er3 )的吸收光谱,依据J-O理论,首次计算报道了光谱项特征:J-O强度参数、量子荧光效率、荧光分支比等,Ω2=3.75×10-20cm2,Ω4=0.67×10-20cm2,Ω6=0.41×10-20cm2。计算证实,PWO:Er3 中几乎有80%的激发能量非辐射跃迁转移致4I13/2能级,4I13/2的计算寿命是5200μs,J-O计算显示,在PWO中产生4I13/2→4I15/2和发射1.53μm.有高的几率。讨论了不同浓度Er3 掺杂对于吸收系数和PbWO4晶体光学吸收边的影响,讨论了Er3 掺杂PbWO4晶体的光致发光和X射线激发发光光谱,PbWO4:Er3 晶体中存在着从PbWO4基质到Er3 离子的能量传递,发光光谱的分析表明,这种能量传递是共振能量传递。     

Studies of EPR g factors for tetragonal and rhombic Cr5+ centers in ferroelectric PbTiO3

The anisotropic g factors (g_x, g_y, g_z) at low temperature and the motionally averaged isotropic g factor at high temperature for two Cr⁵⁺ centers, the tetragonal t‐Cr⁵⁺ center and the rhombic O‐Cr⁵⁺ center, in ferroelectric PbTiO₃ are calculated from the high‐order perturbation formulas based on a two‐mechanism model. In the model, the contributions to g factors from both the crystal‐field (CF) mechanism concerning the CF excited states and the charge‐transfer (CT) mechanism (which is neglected in CF theory) concerning the CT excited states are contained. The calculated results are in reasonable agreement with the experimental values. From the calculations, the defect models of t‐Cr⁵⁺ center (which is attributed to Cr⁵⁺ in the tetragonally‐compressed octahedron caused by Jahn‐Teller effect rather than in the tetragonally‐elongated octahedron in the host PbTiO₃) and O‐Cr⁵⁺ center (which is due to the t‐Cr⁵⁺ center perturbated by the electrical polarization perpendicular to the C₄ axis) are confirmed, and the defect structure of t‐Cr⁵⁺ center is obtained. It is found that in the precise calculations of g factors for the high valence state d¹ ions in crystals, both the contributions due to CF and CT mechanisms should be taken into account. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Peculiarities of the growth of PbWO4:Nd3+ and PbMoO4:Nd3+ single crystals

Crystallography Reports - The dependence of the Nd segregation coefficient on Nd concentration in PbWO4 and PbMoO4 melts and on the method of its introduction is analyzed. In a study of the...