Type Mixed Crystals Research Articles

Widely Tunable Angular Non-Critical Phase-Matching Wavelengths from 0.72 to 1.42 μm Based on RE1xRE21−xCOB Mixed Crystals

The angular non-critical phase-matching (A-NCPM) second-harmonic-generation (SHG) properties of RE1xRE21−xCOB (RE1, RE2 = Y, Gd, La, Tm, Sm, and Nd) type mixed crystals including NCPM wavelength and conversion efficiency were detailedly investigated. Theoretical calculations manifest that the A-NCPM SHG scope of these crystals is 0.72~1.42 µm, and in experiments, the A-NCPM SHG waveband of 0.72~1.25 µm has been realized, by changing the ratio of the rare-earth elements RE1 and RE2 in RE1xRE21−xCOB crystals. Comparing to the temperature-dependent A-NCPM SHG of 0.95~1.34 µm in LiB3O5 (LBO) crystal, the composition-dependent A-NCPM SHG of 0.72~0.95 µm in RE1xRE21−xCOB type crystals is unique and has special significance for the frequency conversion of Ti:Sapphire lasers. Relationships between the birefringence and radius of rare-earth ion RE3+ in RE1xRE21−xCOB mixed crystals were discussed. Aiming for the A-NCPM SHG of 0.72~1.42 µm, we supply a clear, completed, and optimized solution on how to select the compositions of RE1xRE21−xCOB mixed crystals. Under focusing light beam conditions, high efficient A-NCPM SHG for both OPO and Ti:sapphire lasers were realized experimentally by using long Y- and Z-cut RE1xRE21−xCOB crystal samples.

Generalized Pseudo-Unit-Cell model for long-wavelength optical phonons of multinary mixed crystals: Application to AxB_1−xCyD_1−y type mixed crystals

Long-wavelength optical phonons in multinary mixed crystals are studied based on the Pseudo-Unit-Cell model. A unitary matrix method is developed to calculate the eigenfrequencies of optical phonons in multinary mixed crystals. The analytical expressions of oscillator strengths and dielectric constants of the multinary mixed crystals are obtained as a function of the phonon frequencies. The results indicate that the composition dependence of oscillator strengths shows clearly the phonon-mode behaviors of the mixed crystals. The theory and calculation method can be applied to any type of multinary mixed crystals. It is found that there is a composition independent point for the dielectric constant of quaternary mixed crystals.

An aspect on relaxor behaviours and phase boundaries in ferroelectric mixed crystals

In order to search for Pb-free relaxor substances and to define relaxor behaviour, a theoretical aspect is presented from not only microscopic but also phenomenological viewpoint. In order–disorder type mixed crystals, the microscopic approach is attempted under conditions similar to those for relaxor perovskite-type oxides. Basing on the theory for the precursor to glass state, two kinds of technique of X-ray diffuse scattering (static properties) and quadrupole perturbed nuclear magnetic resonance (NMR-NQR) (dynamic properties) are discussed in ferroelectric (FE) -- antiferroelectric (AFE) mixed crystals of Rb1−x(NH4)xH2PO4. After investigating both boundary regions between FE and glass (G) phases, and between AFE and G phases, relaxor-like behaviours as a large dispersion in dielectric constants are found in the AFE-G phase boundary region where the gradual growth of wave number to make the energy minimum and a discontinuous broad NMR-NQR spectrum appear in quite different forms from those of the FE-G phase boundary region. On the other hand, in perovskite-oxides, the conditions for relaxor appearance are discussed within the phenomenological theory by comparing with the microscopic analysis in hydrogen-bonded systems.

Composition Dependency of Lattice Anisotropy of TlBX2-type Chain Mixed Crystals

Variation of the lattice parameters of TlTl 1-x In x Se 2 chain mixed crystals with composition have been studied by X-ray diffraction technique. The lattice anisotropy ( c/a ) of the TlBX 2 -type mixed crystals changes linearly with substitution of the atoms located both at the center and at the vertices of the BX 4 tetrahedra. A brief survey of the important features of the effect of isomorphic atom substitution on the lattice anisotropy of TlBX 2 -type mixed crystals with chain structure has been presented.

Effect of Isomorphic Atom Substitution on Lattice Parameters of TlBX_{2}-type Mixed Crystals

The variation of the lattice parameters of TlBX_{2}-type mixed crystals with composition have been investigated by X-ray diffraction technique. The lattice anisotropy (c/a) of the mixed crystals changes linearly depending on substitution of the atoms located both at the centre and at the vertices of the BX_{4} tetrahedra. For some of the mixed crystals studied, structural phase transition (monoclinic \rightleftharpoons tetragonal) is observed due to atom substitution when the sum of tetrahedral covalent radii of the B and X atoms in BX_{4} tetrahedra reaches a critical value of about 0.255 nm.

Phase relationships in mixed metal germanium sulfide systems (Mn 2GeS 4T 2GeS 4, where T = Co, Ni, Zn)

The influence of substitution of Mn 2+ by Co 2+, Ni 2+, and Zn 2+ in the olivine Mn 2GeS 4 has been investigated by x-ray powder methods on polycrystalline samples. Contrary to similar systems starting from spinel type compounds we could not obtain new layered materials in these systems. A series of olivine type mixed crystals are formed in the system Mn 2−xCo xGeS 4 for 1 ≥ x ≥ 0. In the corresponding Ni system the olivine type region reaches from x = 0 to at least x = 1.7. Zn 2+ cannot replace Mn 2+ in the olivine structure. Substitution of Zn 2+ in Zn 2GeS 4 by Mn 2+ changes the structure from a defect sphalerite to defect wurtzite type. Co 2GeS 4 and Ni 2GeS 4 could not be obtained.

Concentration dependent vibrational mode behaviour in the mixed crystal system SnS xSe 2- x

The laser Raman and far-infrared spectra of CdI 2 type mixed crystals SnS x Se 2- x where 0⩽ x⩽2 were measured in the region up to 500 cm −1. The spectra rule out any possibility of one mode behaviour of the optically active lattice vibrations and indicate a two mode behaviour in the series. A modified random element isodisplacement (MREI) model is applicable.

Vibrational properties of one- and two-mode behaviour in spinel type mixed systems Zn1− xCdxCr2S4

The infrared reflectivity spectra of spinel type mixed crystals Zn 1− x Cd x Cr 2S 4 have been measured from 40 to 900 cm −1. Four or five bands were observed at both end members in the region of intermediate composition. The three bands in higher energy show one-mode behaviour and the other two bands show two-mode behaviour. The concentration dependences of these mode frequencies are interpreted by a model proposed already, in which the 2 basic units and the 6 force constants are contained. By comparing the force constants associated with the Zn-site in several spinels, it is suggested that the two-mode behaviour arises from the large difference in the force constants between the two end members. The E g (260 cm −1) and T 2g (160 cm −1) Raman modes for ZnCr 2S 4 were also observed and used for the determination of the force constants.

Temperature dependence of the FIR reflection spectra of the skutterudites CoAs 3 and CoSb 3

The temperature dependence of the TO and LO phonon frequencies and of the coupled plasmon-LO phonon modes was determined from the FIR reflection spectra of the skutterudites CoAs 3 and CoSb 3 and of some skutterudite type mixed crystals in the frequency range 30–700 cm −1 from 80 to 700 K. A strongly temperature-dependent plasma frequency of free carriers is found in CoAs 3. No free-carrier contribution to the FIR reflectivitity, however, was observed in the case of CoSb 3. From the FIR spectra of cationic (Co 1− x Fe xSb 3, Co 1− x Ni x Sb 3, Co 1−2 x Fe x Ni x Sb 3) and anionic (CoAs 3− x P x ) substituted mixed crystals an anionic defect structure is proposed to be responsible for the free-carrier contributions in CoAs 3.

Nuclear quadrupole resonance and nuclear magnetic resonance studies of K2PtCl6 type mixed crystals

Phase transitions from cubic to lower symmetry phases in antifluorite (K2PtCl6) structures have been ascribed to lattice mode-softening. We study the effect of impurities on lattice vibrations and transition temperatures in such structures. For mixed rhenates we find a lowering of the frequency of the lattice vibrations with increasing impurity content and a corresponding increase in transition temperature. For mixed stannates the n.q.r. signal disappears at high temperature, perhaps due to reorientational motion of the stannate anion.

Pressure Dependence of Infrared Eigenfrequencies of KI, RbI, and Their Mixed Crystals

The pressure dependence of the long-wavelength transverse-optic-phonon frequency of KI and RbI was determined by infrared transmission measurements. The mode Grüneisen parameters thus obtained compared well with those calculated from a rigid ion model using Born-Mayer-type potentials. The phase transition to CsCl structure at appropriate pressures was noted for both crystals, accompanied by a lowering of the k lim →≃0 TO phonon frequency. Infrared transmission measurement and its pressure dependence confirms that the mixed-crystal system K1−xRbxI forms an intermediate class of mixed-crystal system ``half-way'' between the ``two-mode''- and ``one-mode''-type mixed crystals.