Fermi-level engineering of BaTiO3 by alkali codoping: increasing the near-infrared absorption by rhodium

Using electron paramagnetic resonance, optical absorption, and fast spectroscopy of light-induced absorption changes, it is shown that codoping BaTiO₃:Rh with Na_Ba acceptors raises the charge state of Rh³⁺ to Rh⁴⁺. Subsequent oxidation under high oxygen pressures can lower the Fermi level to Rh^4+/5+, leading to increased infrared absorption. The light-induced charge transfer in such specimens is characterised by “one center” behaviour. Received: 18 November 1998 / Revised version: 23 December 1998 / Published online: 7 April 1999     

Performance of photorefractive crystals as derived from EPR-based defect studies: application to BaTiO<Subscript>3</Subscript>:Rh and Ba<Subscript>0.77</Subscript>Ca<Subscript>0.23</Subscript>TiO<Subscript>3</Subscript>:Rh

A method is introduced which allows us to predict the performance of a photorefractive material quantitatively using electron paramagnetic resonance (EPR)-based defect studies. This includes the determination of the defect densities and the parameters governing their light-induced charge changes. On this basis the effective trap densities and the photorefractive response times are calculated. All quantities can be determined without theoretical simplifications such as employed in previous approaches to the problem. The method is applied to BaTiO₃ and congruently melting Ba_0.77Ca_0.23TiO₃, both doped with rhodium. The iron defects, present as background contaminations, are fully taken into account. Their influence on the intensity saturating the space charges is calculated on this basis. The complete energy dependences of the absorption cross sections of all optically active Rh and Fe defects are given. PACS 42.70.Nq; 76.30.Mi; 78.40.-q     

Photochromic behaviour of doped Bi4Ge3O12 single crystal scintillators

The changes induced by ultraviolet (UV) illumination on the optical absorption and electron paramagnetic resonance (EPR) spectra of Bi₄Ge₃O₁₂ single crystals, doped either with Fe (and Gd) or Mn, have been followed at room temperature (RT). In both crystals several overlapping optical absorption bands develop under UV illumination, covering from 0.7 eV up to the band edge of the matrix. The optical damage can be bleached by heating the samples above RT or by illumination with visible light. Although these optical changes temporarily correlate with the variation of the Gd³⁺ and Mn²⁺ concentrations, it has been concluded that other defects are present and partially responsible for the optical damage.     

ODMR and EPR investigations of Fe centers in KTaO3

The analysis of EPR spectra obtained from iron doped KTaO₃ crystals in the as-grown state revealed three dominant iron centers: Fe³⁺-O_I, axial Fe-centers with spinS = 3/2 and rhombic Fe³⁺. By comparison with data from literature possible assignments for the center withS = 3/2 are discussed. For the rhombic species the temperature dependence of the main parameters of the Spin- Hamiltonian was measured. The result makes it most plausible that only one rhombic iron center exists in KTaO₃, in contrast with literature. The understanding of the EPR spectra allows us to assign transitions, observed at very low magnetic fields by optically detected magnetic resonance (ODMR), to this rhombic Fe center. On this basis, the magnetic circular dichroism (MCD) of this defect could be identified using the method of tagged-MCD. This spectrum is compared to the tagged-MCD of Fe³⁺-O₁ and of axial Fe⁴⁺ centers, which may be generated metastably by optical charge transfer. Considerably different structures in the MCD spectra of both Fe³⁺ centers indicate different local surroundings and electronic states.Dedicated to O. F. Schirmer on the occasion of his 60th birthday     

LiNbO3 with the damage-resistant impurity indium

We report on a detailed investigation (EPR, SHG, optical absorption, luminescence and Raman scattering) on the new damage-resistant impurity indium in LiNbO₃, where the increased photoconductivity strongly reduces the photorefractive effect. EPR and optical absorption measurements point to a complete disappearance of the Nb antisite in LiNbO₃: In for all In concentrations. We believe that the very effective driving out of Nb antisites by In is due to the trivalent charge state of In and the possibility of charge self-compensation. Similarities in the properties of Mg-, Zn- or Indoped samples are discussed. Simultaneous doping with In and Zn leads to an addition of both contributions, in particular for optical frequency doubling and luminescence. Raman studies prove that In does not improve the stoichiometry of the crystals. Indium doping provides the possibility to control simultaneously phase-matching conditions and to reduce drastically photorefraction. Therefore, In co-doped LiNbO₃ compositions are promising materials for applications after solving contemporary growth problems.     

Observation of a donor exciton band in silicon

Far-infrared transmission measurements on P-doped Si at donor concentrations below the metal-insulator transition show absorption bands from donor pairs. We identify the lowest energy band as charge transfer excitations from a donor to its neigbhor, forming excitons (D⁺D^-) in the Mott-Hubbard gap. The results suggest that charge transfer states in complexes dominate the low-energy optical excitations as the insulator approaches the Mott density.     

YAG晶体色心的EPR谱

本文描述了8种YAG样品的室温和77K的EPR谱.从这些谱的参数与光谱的实验数据对照,认为晶体中有三种顺磁性的色心,其中g～2.00和g～1.98两条顺磁谱相应于光谱2×10~4cm~(-1)～3.4×10~4cm~(-1)的吸收带,为YAG基质晶体中的缺陷俘获一个电子构成的电子型缺陷中心;而 g～30.60的顺磁谱线与氧空位和掺杂的金属离子Mg~(2+) 、Cr~(3+)有关,推测为氧空位团或氧空位与金属离子构成的S>1/2的复合顺磁中心缺陷.YAG晶体随着在大气氛中高温退火,氧的进入和金属离子的掺入,造成电子和空间体积的不平衡,从而使晶体色心浓度加大.     

Phenomena associated with diffusion of trivalent iron in amethyst quartz

In natural amethyst crystals from different locations changes in the EPR, optical and infrared spectra were measured as a function of annealing temperature. After heating to 650°C for two hours the concentration of Fe(III) on substitutional sites with alkali ions as charge compensators is equal in all crystals irrespective of its initial concentration. Establishment of a heterogeneous equilibrium due to formation of iron oxide particles (presumably Fe₂O₃) previously characterized by their EPR and optical spectra is thus confirmed. Initially unequal distributions of Fe(III) among the three equivalent sites of the quartz structure also become equal under this treatment. From comparison with literature data for MnCl₂ precipitates in NaCl the average particle size can be estimated to be about 100 nm allowing an estimate of the diffusion coefficient of Fe(III) at 650°C. In one amethyst with a high concentration of hydrogen a large amount of the substitutional Fe(III) is charge compensated by hydrogen. Thermal destruction of these centers does not correlate with the intensities of infrared absorption bands in the OH stretching vibration region.     

Gap levels of Ti<Superscript>3+</Superscript> on Nb or Li sites in LiNbO<Subscript>3</Subscript>:(Mg):Ti crystals and their effect on charge transfer processes

The Ti dopant occupying Li or Nb sites and the charge transfer processes induced by thermochemical reduction and optical bleaching treatments have been investigated in LiNbO₃ systems using optical absorption and EPR. The Ti³⁺ centers, built preferentially at Nb sites in heavily Mg-codoped crystals, are shown to have absorption bands at 1.62±0.08 eV and 2.65±0.25 eV, which are similar or slightly redshifted compared to Ti³⁺ centers at Li sites in LiNbO₃:Ti crystals. The Ti_Nb^4+/3+ gap level plays an important role in the trapping of electron-polarons in LiNbO₃, double-doped with Mg and Ti; in particular, an enhanced optical detrapping sensitivity for pumping in the 1.3–2.8 eV range is observed, which may be relevant for applications in integrated optics. Evidence indicating the possible existence of bipolarons involving the Ti dopant is presented. PACS 77.84.Dy; 71.55.Ht; 71.38.-Mx; 78.40.Ha; 76.30.Fc     

EPR and optical absorption studies of VO doped l-alanine (C3H7NO2) single crystals

VO²⁺ doped l-alanine (C₃H₇NO₂) single crystals and powders are examined by electron paramagnetic resonance (EPR) and optical absorption spectroscopy. Three magnetically different sites are resolved from angular variations of l-alanine single crystal EPR spectra. In some specific orientations each VO²⁺ line splits into three superhyperfine lines with intensities of 1:2:1 and maximum splitting value of 2.23 mT. The local symmetries of VO²⁺ complex sites are nearly axial. The optical absorption spectra show three bands. Spin Hamiltonian parameters are measured and molecular orbital coefficients are calculated by correlating EPR and optical absorption data for the central vanadyl ion.     

Obtaining the Optical and EPR Parameters of Vanadyl-Doped Sodium Dihydrogen Phosphate Dihydrate Powders by Experimental and Theoretical Methods

In this study, the spin-Hamiltonian parameters (g and A), molecular orbital coefficients, and other spectroscopic properties of vanadyl-doped sodium dihydrogen phosphate dihydrate (NaH₂PO₄·2H₂O) powders have been investigated by experimental and theoretical methods, including electron paramagnetic resonance (EPR) and optical absorption spectroscopies. The results show axially symmetric crystalline field around VO²⁺ ion. The optical absorption spectra exhibit three characteristic bands of VO²⁺ ions in tetragonal symmetry. EPR and optical data were used in a complementary way to calculate spin-Hamiltonian parameters and molecular orbital coefficients. The octahedral and the tetragonal field parameters were theoretically calculated on the basis of crystal field theory. These parameters were used to determine various bonding parameters which characterize the nature of bonding in the complex. The theoretical results are supported by experimental results.     

Optical properties of group-V atom-vacancy pairs in silicon

Abstract

Two optical absorption bands are identified as arising from the negative charge state of each of the P-, As-, and Sb-vacancy pairs in silicon. The more prominent band for each is at 6150, 6000, and 5500 cm^?1, respectively. A second broader partially overlapping band at ～8500 cm^?1 is also present for each. Dichroism produced in the bands by uniaxial stress reveals a static Jahn-Teller distortion of opposite sign to that for the neutral state previously studied by EPR. The absorption bands can be identified as electron transitions from filled to empty orbitals of a simple one-electron molecular orbital model for the defect.     

Studies of impurity levels in Rh-doped and Ce-doped photorefractive BaTiO3

Impurity levels, light-induced and thermo-induced charge transfer processes in Rh-doped and Ce-doped photorefractive BaTiO₃ were studied. The thermal depths and optical transitions of these impurity levels in the crystals were determined by using such methods as light-induced absorption, thermo-induced absorption, and grating dark decay. The origins of these impurity levels were discussed. We demonstrated, for the first time to our knowledge, that thermo-induced absorption spectroscopy is a useful and complementary technique for investigating the impurity levels in photorefractive crystals. With this technique, the deep impurity level generated by Ce in BaTiO₃:Ce was revealed and identified. Received: 7 July 1999 / Revised version: 24 September 19999 / Published online: 2 February 2000     

EPR and Optical Absorption Study of VO2+ Ions Doped Potassium Hexaaquazinc (II) Sulfate Single Crystals

X-band single-crystal electron paramagnetic resonance (EPR) studies are done on VO²⁺ ions doped in potassium hexaaquazinc (II) sulfate, K₂[Zn (H₂O)₆] (SO₄)₂ (PHZS) at room temperature. The spin Hamiltonian parameters, i.e., g and A tensors and their direction cosines, are evaluated by the standard diagonalization procedure using angular variation of the EPR spectra in three planes (ab, bc* and c*a), with the help of a computer program. The EPR spectrum is simulated using the EasySpin program to verify the calculations. The detailed EPR analysis indicates the presence of two magnetically inequivalent VO²⁺ sites. Both the vanadyl complexes are found to take up the substitutional position in the host lattice. The optical absorption spectrum of VO²⁺ ions doped in PHZS single crystal at room temperature is also recorded and four main d–d transfer bands in the visible region are assigned. The theoretical band positions are obtained using energy expressions and a good agreement is found with the experimental values. With the help of assigned bands the crystal-field parameters (Dq, Ds and Dt) are evaluated. Finally, with the optical and EPR data, the nature of bonding in the complex is discussed.     

Electronic excitations and optical response of metal nanocomposites under heavy ion implantation

The optical transmission and ion-induced luminescence under implantation of copper ions into quartz glass (a-SiO₂) have been measured to study the processes of formation of copper nanoparticles. It is shown that in situ measurements are more informative in comparison with the ordinary approach—investigation of the properties of ion-implanted nanocomposites only after implantation. A series of experiments was performed to prove that the ion-induced luminescence band at 545–550 nm is due to Cu⁺ ions dissolved in a-SiO₂. The combined use of in situ optical techniques makes it possible to monitor the states of implanted copper (metal nanoparticles and dissolved atoms) by the change in the optical absorption near the surface plasmon resonance of nanoparticles and by the intensity of ion-induced luminescence of Cu⁺ states in solid solution. It is shown that the optical bands of defects, dissolved copper, and nanoparticles can be separated within a simple linear approximation. Near the surface plasmon resonance and defect bands, ion-induced transient optical absorption has been revealed. The transient optical absorption near the surface plasmon resonance is explained by the temperature effect. The relationship between the electronic excitation, radiation-induced optical response, and the kinetics of nanoparticle formation is analyzed. Several stages of nanoparticle formation have been established: accumulation of implanted copper in solid solution, nucleation of nanoparticles, coalescence, growth of nanoparticles, and saturation of nanocomposites.     

Study on the defect of sulfur substituting for phosphorus in potassium dihydrogen phosphate

The electronic structure and geometric distribution of neutral and charge states of the point defect sulfur substituting for phosphorus (S_p) in potassium dihydrogen phosphate (KH₂PO₄) have been investigated using a first-principles method. The energy gap is lowered to about 2.2 eV and 6.5 eV from the Density Function Theory (DFT) value of 7.3 eV by a -2 charge and neutral state, respectively. However, no defect states appear in the energy gap for the +1, -1, and +2 charge states of the S_p defect. The results show that the defects S_p with neutral and -2 charge states might contribute to the transient optical absorption under the high energy laser irradiation.     

晶体C_3H_7NO_2:VO~(2+)的自旋哈密顿参数及局域结构

在晶体场理论(CF机制)和电荷转移机制(CT机制)的基础上,结合双自旋-轨道耦合参量模型、自旋哈密顿量的高阶微扰公式及晶场能量公式计算得到C_3H_7NO_2:VO~(2+)晶体的自旋哈密顿量(EPR参量)和吸收光谱,及电荷转移跃迁能级.对比只采用CF机制和采用双重机制下的计算结果,发现高价钒离子(V~(4+))掺杂在C_3H_7NO_2晶体中的CT机制对于其EPR参量的影响不能忽略.     

EPR,optical and superposition model study of Mn2+ doped L+ glutamic acid

Electron paramagnetic resonance (EPR) study of Mn²⁺ doped L+ glutamic acid single crystal is done at room temperature. Four interstitial sites are observed and the spin Hamiltonian parameters are calculated with the help of large number of resonant lines for various angular positions of external magnetic field. The optical absorption study is also done at room temperature. The energy values for different orbital levels are calculated, and observed bands are assigned as transitions from ⁶A_1g(s) ground state to various excited states. With the help of these assigned bands, Racah inter–electronic repulsion parameters B = 869 cm^?1, C = 2080 cm^?1 and cubic crystal field splitting parameter Dq = 730 cm^?1 are calculated. Zero field splitting (ZFS) parameters D and E are calculated by the perturbation formulae and crystal field parameters obtained using superposition model. The calculated values of ZFS parameters are in good agreement with the experimental values obtained by EPR.     

Optical absorption of triplet molecular excitons in alkali cyanides

The optical absorption spectra of alkali cyanides in the UV region present a set of weak absorption bands which are identified as triplet a'³Σ⁺ molecular excitons. The nature of the molecular exciton transitions in the ionic-molecular crystals is discussed and the existence of an admixture between molecular exciton and charge transfer exciton states is suggested.     

Light-induced charge transport in BaTiO3 via three charge states of rhodium

By combined studies of electron spin resonance and optical absorption at low temperatures, the charge-transfer bands of Rh⁵⁺ and Rh⁴⁺ are identified to be peaked near 1.6 and 1.9 eV, respectively. On this basis, the light-induced charge-transfer processes in BaTiO₃:Rh are unraveled at room temperature. It is shown that three charge states of Rh are involved, leading to two levels: the shallow Rh^4+/5+ and the deeper Rh^3+/4+ level. The optical behaviour of these two levels corresponds to those expected from a two-center model. The present paper represents the first atomic-scale identification of three charge states of one element leading to optical two-level response.