CdF2 Crystals Research Articles

A complete study on the temperature dependences of zero-field splitting b4 0 for Eu2+ and Gd3+ ions in CdF2 crystal

ABSTRACT This paper gives for the first time a complete equation to analyze the temperature dependence (or thermal variation) of spin-Hamiltonian parameter from low to high temperatures for transition metal and rare-earth ions in crystals. This equation includes not only the dynamic effect (only considered in previous papers) caused by electron–phonon interaction but also the static effect owing to lattice thermal expansion. Based on the complete equation, the thermal variations of zero-field splitting b4 0 from low to high temperatures for cubic Eu2+ and Gd3+ centers in CdF2 crystal are studied. The contributions due to static effect are estimated from the pressure dependence of zero-field splitting b4 0. The static parameters B (charactering the static effect) and the electron–phonon coupling parameters K (charactering the dynamic effect) for both systems are determined and the coefficients (B + K) in the complete equation are obtained. The results are discussed.

Optical Properties of Alkalyne Earth Fluoride and CdF2 Crystals Activated by Chromium Ions

Optical Properties of Alkalyne Earth Fluoride and CdF2 Crystals Activated by Chromium Ions

Determination of charge-compensated C 3υ (II) centers for Er3+ ions in CdF2 and CaF2 crystals* *Project supported by the National Natural Science Foundation of China (Grant No. 1170513), the Natural Science Foundation of Shaanxi Province, China (Grant No. Z20200051), the Foundation of the Education Department of Shaanxi Provincial Government, China (Grant No. 16JK1461), and the Scientific Research Foundation of Xi’an

A unified theoretical method is established to determine the charge-compensated C 3v (II) centers of Er3+ ions in CdF2 and CaF2 crystals by simulating the electron paramagnetic resonance (EPR) parameters and Stark energy levels. The potential (Er3+–F−–) and (Er3+––O2–) structures for the C 3v (II) centers of Er3+ ions in CdF2 and CaF2 crystals are checked by diagonalizing 364 × 364 complete energy matrices in the scheme of superposition model. Our studies indicate that the C 3v (II) centers of Er3+ ions in CdF2 and CaF2 may be ascribed to the local (Er3+–F−–) structure, where the upper ligand ion F− undergoes an off-center displacement by ΔZ ≈ 0.3 Å for CdF2 and ΔZ ≈ 0.29 Å for the CaF2 along the C 3 axis. Meanwhile, a local compressed distortion of the cluster is expected to be ΔR ≈ 0.07 Å for CdF2:Er3+ and ΔR ≈ 0.079 Å for CaF2:Er3+. The considerable g-factor anisotropy for Er3+ ions in each of both crystals is explained reasonably by the obtained local parameters. Furthermore, our studies show that a stronger covalent effect exists in the C 3v (II) center for Er3+ in CaF2 or CaF2, which may be due to the stronger electrostatic interaction and closer distance between the central Er3+ ion and ligand O2– with the (Er3+–F−–) structure.

Correlation between the Enthalpy of Melting and the Enthalpy of Anti-Frenkel Defect Formation in Homologous MF2 Fluorite Crystals

The homologous series of MF2 (M = Ca2+, Sr2+, Ba2+, Cd2+, Pb2+, Sm2+, Eu2+, Yb2+, Ra2+, and Hg2+) crystals having the fluorite structure (CaF2, space group $$Fm\bar {3}m,$$ Z = 4) holds an important place in fluoride materials science. A correlation between the enthalpy of anti-Frenkel defect formation (Hf) and the enthalpy of melting (Hm) was elucidated for CaF2, SrF2, BaF2, and PbF2 based on the most reliable experimental data set: Hf (kJ/mol) = 8.6Hm (kJ/mol) or Hf (eV) = 0.089Hm (kJ/mol). Experimentally determined Hm values were used to calculate the enthalpies of defect formation Hf = 2.0 and 2.6 eV for CdF2 and EuF2 crystals, respectively. The correlation is prognostic for the whole series of homologous MF2 fluorite crystals and for SrCl2.

Ab initio analysis of the optical spectra and EPR parameters of Ni2+ ions in CaF2 and CdF2 crystals

Crystals with fluorite type structure doped with transition metals (TM) in different oxidation states provide the possibility of investigating optical spectra, magnetic properties and ligand field (LF) parameters of TM in cubic eightfold coordination. If the doped ion in the host crystal matrix has a degenerate ground state then, due to the strong T⊗τ Jahn-Teller (JT) effect, the cubic coordination geometry of the impurity ion will be distorted to octahedral, with the change in its properties. In this paper we present the results of the ab initio investigation of absorption optical spectra, JT stabilization energy in the ground state, spin-Hamiltonian and ligand field parameters of divalent nickel ion doped in cubic CaF2 and CdF2 crystals. The analysis of new cluster [NiF6]4- with trigonal symmetry (D3d) at the Ni2+site was made based on the ab initio multireference methods (MR). All calculations are applied to the [NiF6]4- cluster embedded in an extended point charge field of host matrix ions built according to the Gellé-Lepetit procedure. After density functional theory (DFT) optimization of the doped crystals, the ab initio energy calculation of the electronic states and corresponding wave functions of Ni2+ ions is performed from the complete active space self-consistent field (CASSCF). The improved energy states obtained by using different approaches such as N-electron valence second order perturbation theory (NEVPT2), second order dynamic correlation dressed complete active space (DCD-CAS2), difference dedicate configuration interaction with three degrees of freedom (MRDDCI3) and spectroscopy-oriented configuration interactions (SORCI), were analyzed. In addition, the ab initio ligand field theory (AILFT) procedure allows extracting all LF parameters and spin-orbit coupling constant. The obtained results are discussed and the comparison with the available in the literature corresponding experimental values literature yields a reasonably agreement between theory and experiment, which justifies this new route of investigation.The structural mechanism around Ni2+ center through JT effect, combined with the above mentioned methodology, allows for more consistent and reliable calculations of optical spectra and spin-Hamiltonian parameters of any impurity ions doped in eightfold cubic coordination.

Synchrotron spectroscopy of confined carriers in CdF2-CaF2 superlattices

Luminescence spectroscopic and temporal dynamic properties of high energy elementary excitations in CdF2-CaF2 superlattices have been studied utilising excitation with vacuum ultraviolet and X-ray synchrotron radiation while comparing the results with those obtained for CdF2 and CaF2 bulk crystals. It is shown that the optical properties of the superlattice structures are determined by exciton emission in the CdF2 monolayers. The experimental manifestations of exciton confinement phenomena are discussed.

Broadening of the lines due to the 4f n -4f n − 15d transitions of Ce3+, Pr3+, and Tb3+ ions in the absorption spectra of CdF2 crystals

The structure of the absorption spectra of Ce3+, Pr3+, and Tb3+ ions in the vicinity of 4f-5d transitions has been investigated. At low temperatures the absorption spectra exhibit a weakly pronounced fine structure, in contrast to narrow-line spectra in crystals of Ca, Sr, and Ba fluorides. The spectra of Ce3+, Pr3+, and Tb3+ ions in CdF2 can be considered as the absorption spectra of these ions in alkali-earth fluorides, broadened by 60–75 cm−1. The broadening is related to the autoionization of electron from the local 5d(eg) level to the energy-degenerate states of the conduction band of CdF2 crystal.

Synthesis and study of spectroscopic properties of CdF2 crystals codoped with luminescent rare earth ions (Ho3+/Yb3+)

In this paper, we report the optical analysis of Ho3+ ions doped CdF2 single crystals. The pulled crystals were prepared by use of the Bridgman technique from a vacuum furnace in fluoride atmosphere after purification of the starting materials. Absorption, excitation, emission and fluorescence decay spectra were recorded at room temperature. The Judd–Ofelt (JO) analysis was applied to obtain the three phenomenological intensity parameters (Ωt, t=2, 4, 6) and the transition strengths. The JO intensity parameters Ω2, Ω4 and Ω6 for 4f–4f transitions of Ho3+ ions were computed from the optical absorption spectra using UV, visible and near infrared transitions. These parameters were then used to calculate the radiative transition probabilities (AJJ′), branching ratios (βJJ′) and radiative lifetimes (τrad) of the main laser emitting levels of Ho3+ ions. The obtained spectroscopic properties are compared to those of Ho3+ transitions in other fluoride and oxide hosts. The excitation spectrum in the UV–Visible spectral range is very close to the absorption spectrum indicating that all observed absorption levels can excite the holmium green emission corresponding to 5F4(5S2)→5I8 transition. The emission spectrum is mainly dominated by the green emission alongside the blue and red emissions. For the main transitions, there is a good agreement between the emission spectrum and the spontaneous emission probabilities given by the JO analysis. Using the Füchtbauer–Ladenburg method, the emission cross-section of the three main visible emission were determined in addition to other important laser parameters such radiative quantum efficiency and optical gain.We discuss the mechanisms of energy transfer in a forthcoming publication.

Growth and defect structure of CdF2 crystal and nonstoichiometric Cd1−x R x F2+x phases (R are rare earth elements and in): 6. Growth and ionic conductivity of Cd0.904In0.096F2.096 single crystal

Growth and defect structure of CdF2 crystal and nonstoichiometric Cd1−x R x F2+x phases (R are rare earth elements and in): 6. Growth and ionic conductivity of Cd0.904In0.096F2.096 single crystal

Additive colouring of CaF2:Yb crystals: determination of Yb2+ concentration in CaF2:Yb crystals and ceramics

When growing CaF2 crystal doped with rare-earth ions, most of these ions are present in a trivalent state. However, due to contact with graphite crucible, a small proportion of a number of ions (Eu, Sm, Yb and Tm) are reduced to a bivalent state. A similar situation takes place during fabrication of CaF2 ceramics doped with rare-earth metals. This fact is of particular importance for laser CaF2:Yb crystals (ceramics), a promising material for short-pulse, high-power, high-energy diode-pumped solid state lasers since the presence of bivalent Yb ions can be a source of thermal losses. To date, there has been no technique to determine Yb2+ concentration in as-grown crystals. The proposed technique is based on a total reduction of Yb3+ ions via the heating of as-grown CaF2 crystals with known concentration of Yb in the reducing atmosphere of metal vapour and determining the cross section of absorption bands of Yb2+ ions. The knowledge of these parameters allows estimation of the Yb2+ content in CaF2:Yb crystals or ceramics by analysing their absorption spectra. Examples of using this technique are given. The technology of CdF2 crystals reduction (an “additive colouring”) and features of colouring of crystals doped with rare-earth ions are considered.

The de Haas-van Alphen effect in nanostructures of cadmium fluoride

Measurements of the field and temperature dependences of static magnetic susceptibility demonstrate de Haas-van Alphen oscillations at high temperatures and low magnetic fields in sandwich nanostructures, which are represented by an ultranarrow p-type CdF2 quantum well confined by δ barriers heavily doped with boron on the surface of an n-type CdF2 crystal. The temperature dependences of the de Haasvan Alphen oscillation amplitudes indicate a small value of the effective mass of two-dimensional holes, as a result of which, the strong field assumption, μB ≫ 1, is fulfilled at high temperatures. It is for the first time that a periodic variation in the de Haas-van Alphen oscillation frequency is detected and is accompanied by a diamagnetic response as temperature is increased. This phenomenon manifests itself as synchronous temperature oscillations of the density and effective mass of two-dimensional holes as a result of the mesoscopic properties of δ barriers.

Dynamics of the transformation of bistable centers and lattice in CdF2 crystals under femtosecond pulsed photoexcitation

The kinetics of photoinduced absorption spectra of CdF2 crystals with bistable indium and gallium centers under femtosecond pulsed excitation has been experimentally investigated. Based on the example of indium ions, it is shown that the transmission band in the absorption spectrum of deep centers is formed for 0.8–1 ps, which significantly exceeds the photon absorption time. This process is interpreted as a result of displacement of indium ion from the initial interstitial position to a site of neighboring unit cell; the displacement velocity is estimated to be 200–250 m/s, a value close to the thermal velocity of this ion at room temperature. The times characteristic of the formation of free polarons as a result of the displacement of neighboring lattice ions have been experimentally estimated for the first time at a level of 0.8–1.2 ps. The capture times of free polarons by trivalent gallium and indium ions are estimated (5 and 10 ps, respectively), as well as the corresponding cross sections (2 × 10−16 and 8 × 10−16 cm2).

10.1007/s11449-008-3006-5

The microwave-cavity-based technique is used to study the processes of photoionization of electrons from donor levels to the conduction band in semiconductor CdF2 crystals doped with Y, In, or Ga. The samples were excited by periodic pulses of Nd-laser (λ = 1.06 μm, pulse width ∼10 ns) in the temperature range 6–77 K. The transient processes were detected in the absorption and dispersion modes related to variation of the imaginary and real parts of the complex permittivity ɛ1 − iɛ2 induced by the light pulses. The observed signals consisted of short peak at t ∼ 0, approximately 40–70 ns in length, and a long tail with a duration of ∼100 ms. The short peak is likely to be related to the stay of the photoexcited carriers in the conduction band, while the long tail is associated with the processes of excitation relaxation after the electrons coming back to the donor levels of the impurity band. The weak temperature dependence of the width of the peak at t ∼ 0 is explained by the tunneling mechanism of relaxation of electrons through the energy (or, probably, spatial) barrier separating the bound and free states of the carriers in the semiconductor CdF2.

Location of the energy levels of the rare-earth ions in BaF2 and CdF2

The location of the energy levels of rare-earth (RE) elements in the energy band diagram of BaF2 and CdF2 crystals is determined. The role of RE 3+ and RE 2+ ions in the capture of charge carriers, luminescence, and the formation of radiation defects is evaluated. It is shown that the substantial difference in the luminescence properties of BaF2: RE and CdF2: RE is associated with the location of the excited energy levels in the band diagram of the crystals.

Optical spectra of (CdF2)0.9(InF3)0.1 semiconductor solid solutions

The differences in the optical spectra of CdF2:In semiconductors with bistable DX centers (concentrated (CdF2)0.9(InF3)0.1 solid solutions) and “standard” samples with a lower impurity concentration used to record holograms are discussed. In contrast to the standard samples, in which complete decay of two-electron DX states and transfer of electrons to shallow donor levels may occur at low temperatures, long-term irradiation of a (CdF2)0.9(InF3)0.1 solid solution by UV or visible light leads to decay of no more than 20% deep centers. The experimental data and estimates of the statistical distribution of electrons over energy levels in this crystal give the total electron concentration, neutral donor concentration, and concentration of deep two-electron centers to be ∼5 × 1018 cm−3, ∼9 × 1017 cm−3, and more than 1 × 1020 cm−3, respectively. These estimates show that the majority of impurity ions are located in clusters and can form only deep two-electron states in CdF2 crystals with a high indium content. In this case, In3+ ions in a limited concentration (In3+ (∼9 × 1017 cm−3) are statistically distributed in the “unperturbed” CdF2 lattice and, as in low-concentrated samples, form DX centers, which possess both shallow hydrogen-like and deep two-electron states.

Relaxation processes upon photoexcitation of semiconductor CdF2 by laser pulses

The microwave-cavity-based technique is used to study the processes of photoionization of electrons from donor levels to the conduction band in semiconductor CdF2 crystals doped with Y, In, or Ga. The samples were excited by periodic pulses of Nd-laser (λ = 1.06 μm, pulse width ∼10 ns) in the temperature range 6–77 K. The transient processes were detected in the absorption and dispersion modes related to variation of the imaginary and real parts of the complex permittivity ɛ1 − iɛ2 induced by the light pulses. The observed signals consisted of short peak at t ∼ 0, approximately 40–70 ns in length, and a long tail with a duration of ∼100 ms. The short peak is likely to be related to the stay of the photoexcited carriers in the conduction band, while the long tail is associated with the processes of excitation relaxation after the electrons coming back to the donor levels of the impurity band. The weak temperature dependence of the width of the peak at t ∼ 0 is explained by the tunneling mechanism of relaxation of electrons through the energy (or, probably, spatial) barrier separating the bound and free states of the carriers in the semiconductor CdF2.

Dynamic wavefront-conjugating mirror based on CdF2 crystals with bistable in centers

The wavefront reversal upon degenerate four-wave interaction in a CdF2 crystal with bistable In centers is experimentally investigated using a pulsed ruby laser as a pump source. The reflectance and operating speed of the wavefront-conjugating mirror are measured and the quality of the reflected wave, as well as of the compensation of model phase distortions, is estimated. An operating speed of about 15 ns is obtained for such a mirror with a reflectance of up to 2% at room temperature. Compensation of model large-scale distortions yields a gain in the beam divergence of 20 and a quality of compensation of 1.05.

A dynamic phase-conjugate mirror based on CdF2 crystals with bistable centers

A four-wave phase-conjugate mirror on reflection gratings recorded in a CdF2 crystal with bistable centers has been studied experimentally. The mirror reflectivity and speed have been measured, and the coefficient of third-order nonlinearity of this medium has been estimated. The quality of the wave reflected from the phase-conjugate mirror has been investigated in model experiments.

The conductivity and Hall effect in CdF2:In and CdF2:Y

Temperature dependences of the static conductivity σ and Hall coefficient RH in CdF2 crystals doped with bistable indium and donor yttrium impurities have been measured. It is shown that this material contains different types of free carriers, i.e., electrons and polarons. A comparison of the calculated temperature dependences of σ and RH with the experimental data also shows that the impurity-band conductivity makes a significant contribution (due to hops of bound polarons or holes) to σ and RH.

Recording dynamic holograms in a semiconductor crystal CdF2:In

The kinetics of decay of a phase hologram in a semiconductor CdF2 crystal with bistable In centers is studied. Kinetic constants of the hologram decay are found, and the potential relief of the bistable center is plotted. The resolving power of the crystal is evaluated and recording of a transparency is demonstrated.