Electrical and nonlinear optical properties of KTiOPO<Subscript>4</Subscript> single crystals doped with niobium,antimony, and tantalum

Single crystals of K_{1 ? x}Ti_{1 ? x}Nb_xOPO₄ (KTP: Nb), K_{1 ? x}Ti_{1 ? x}Sb_xOPO₄ (KTP: Sb), and K_{1 ? x}Ti_{1 ? x}Ta_xOPO₄ (KTP: Ta) solid solutions are grown and their dielectric, conducting, and nonlinear optical properties are investigated. The maximum contents x of niobium, antimony, and tantalum impurities in the crystals are equal to 0.11, 0.23, and 0.25, respectively. The doping of the KTiOPO₄ crystals with niobium, antimony, and tantalum brings about the formation of additional potassium vacancies and additional potassium positions and, as a consequence, an increase in the ionic conductivity σ₃₃. An increase in the doping level leads to a smearing of the ferroelectric phase transitions and a decrease in the phase transition temperatures. The permittivity of the doped crystals exhibits a broad relaxation peak in the temperature range 200–600°C.     

Refinement of the K0.96Ti0.96Nb0.04OPO4 structure

The structure of a K_0.96Ti_0.96Nb_0.04OPO₄ single crystal was established by the methods of the X-ray diffraction analysis. Additional positions of K atoms were established. In the superionic state, these atoms provide ionic transport in the crystals.     

Structure of the RbTi<Subscript>0.98</Subscript>Zr<Subscript>0.02</Subscript>OPO<Subscript>4</Subscript> single crystal at temperatures of 293 and 105 K

This paper reports on the results of precision X-ray structural investigations of a RbTi_0.98Zr_0.02OPO₄ single crystal at temperatures of 293 and 105 K. It is established that the observed decrease in the temperature of the ferroelectric phase transition in RbTiOPO₄ crystals doped with zirconium is associated with the increase in the Rb-O bond lengths. The structural factors responsible for the decrease in the electrical conductivity in these crystals are revealed. An analysis is made of the structure of the helical channels which in crystals of this family are considered to be a decisive factor for the manifestation of superionic conduction. It is shown that, in structures of the KTiOPO₄ (KTP) type, the migration of ions in channels is most hindered inside the cavities.     

Synthesis,physical properties,and atomic structure of niobium-doped RbTiOPO<Subscript>4</Subscript> crystals

Single crystals of solid solutions Rb_1?xTi_1?xNb_xOPO₄(RTP: Nb) were grown and the temperature dependences of their dielectric and nonlinear optical properties and electric conductivity were studied. The maximum possible niobium content in these crystals is close to x = 0.1. The niobium impurities decelerate growth of {100} faces, and crystals take a plate-like habit. With increasing doping level, ferroelectric phase transitions diffuse and their temperature decreases. A specific feature of the dielectric properties of RTP: Nb crystals is the appearance of a broad relaxation maximum ε₃₃ in the temperature range 200–600°C caused by the formation of vacancies in the rubidium cation sublattice. The intensity of second-harmonic generation under laser irradiation decreases with increasing niobium content. The atomic structure of a crystal with x = 0.01 is studied and it is established that niobium substitutes for titanium only in Ti(1) positions.     

Structural state of a radiation-modified Ti<Subscript>50</Subscript>Ni<Subscript>47</Subscript>Fe<Subscript>3</Subscript> single crystal

The structural state of a Ti₅₀Ni₄₇Fe₃ single crystal irradiated by fast neutrons (F = 2.5 × 10²⁰ cm⁻²) at 340 K was studied by thermal neutron diffraction at 78 and 295 K. The melt of this composition was chosen with the purpose of designing a radiation-resistant material exhibiting a shape-memory effect. It was found that the melt remains crystalline after irradiation, whereas the Ti₄₉Ni₅₁ crystal studied earlier becomes amorphous after an analogous irradiation. In spite of the fact that the main structural motif of the crystal remains unchanged after irradiation, martensitic transformations in the crystal do not occur and, consequently, the shape-memory effect is not retained. The radiation resistance of this class of crystals was estimated.     

Structural reasons for the nonlinear optical properties of KTi<Subscript>0.96</Subscript>Zr<Subscript>0.04</Subscript>OPO<Subscript>4</Subscript> single crystals

This paper reports on the results of the precision X-ray structural investigations of KTi_0.96Zr_0.04OPO₄ single crystals at room temperature. It is established that the incorporation of zirconium atoms into the structure of KTiOPO₄ (KTP) crystals does not lead to substantial changes in the framework structure and results only in an insignificant decrease in the scatter of the distances in the PO₄ tetrahedra and the formation of more symmetric (TiZr)O₆ octahedra as compared to the TiO₆ octahedra. However, the incorporation of zirconium atoms into the KTP structure is accompanied by the redistribution of the electron density in the crystal as a whole, so that the electron density increases in the region of the positions occupied by the potassium atoms. This changes the nonlinear optical properties of the given series of crystals, which are estimated from the intensity of the second harmonic generation signals.     

Structural study of new compound Bi<Subscript>2.53</Subscript>Li<Subscript>0.29</Subscript>Nb<Subscript>2</Subscript>O<Subscript>9</Subscript> by the powder neutron diffraction method

A new compound of composition Bi_2.53Li_0.29Nb₂O₉ was synthesized in the course of the search for new materials with high ionic conductivity. Its crystal structure was determined from the neutron diffraction data. The new compound Bi_2.53Li_0.29Nb₂O₉ is crystallized in the orthorhombic system, sp. gr. Cmc2₁, and unit-cell parameters a = 24.849(1) Å, b = 5.4536(3) Å, and c = 5.4619(2) Å at T = 290 K (a = 24.843(2) Å, b = 5.4456(5) Å, and c = 5.4546(5) Å at T = 10 K). Within the temperature range 10–870 K, no structural phase transitions were revealed. The atomic coordinates and the thermal factors in the isotropic approximation were refined by the Rietveld method at 290 and 10 K. The data obtained were analyzed based on the calculated local balance of bond strengths.     

Structure refinement of cadmium cerium(IV) phosphate Cd<Subscript>0.5</Subscript>Ce<Subscript>2</Subscript>(PO<Subscript>4</Subscript>)<Subscript>3</Subscript>

Cadmium cerium orthophosphate Cd_0.5Ce₂(PO₄)₃ is synthesized by precipitation from aqueous solutions. The structure refinement from powder X-ray diffraction data is preceded by the sample preparation and structure solution. The refinement is carried out by the Rietveld method (ADP-2 diffractometer, Cu _Kα radiation, Ni filter, 15° < 2θ < 120°, 2θ-scan step 0.02°, counting time 10 s per step). All calculations are carried out using the WYRIET program (version 3.3) within the sp. gr. P2₁/n. The structure is refined with anisotropic displacement parameters for cations and isotropic displacement parameters for oxygen atoms.     

Ti/Zr isomorphism in wadeite: The crystal structure of the titanium-dominant K<Subscript>2</Subscript>(Ti<Subscript>0.55</Subscript>Zr<Subscript>0.45</Subscript>)Si<Subscript>3</Subscript>O<Subscript>9</Subscript> member of the series

The crystal structure of the titanium-rich mineral wadeite K₂(Ti_0.55Zr_0.45)Si₃O₉ from rischorrites of the Khibiny Alkaline Massif (Kola Peninsula, Russia) is studied by X-ray diffraction (XCalibur-S diffractometer, R = 0.0459): a = 6.8611(6) Å and c = 10.0611(9) Å; space group P6₃/m, Z = 6, D _x = 3.03 g/cm³. It is shown that the unit-cell parameters and volume of the mineral of mixed (Ti/Zr) composition are naturally intermediate between those of the terminal members of the isomorphous wadeite-based K₂ZrSi₃O₉–K₂(Ti_0.55Zr_0.45)Si₃O₉–K₂TiSi₃O₉ series. The expected correlation is due to the ionic radii of Zr⁴⁺ and Ti⁴⁺ which determine the lengths of Zr/Ti–O bonds in octahedra. The data of field observations and microscopic studies show that the Ti-dominant wadeite is formed on the basis of primary zirconium mineral in the course of a late imposed process under unique geochemical conditions.     

Ionic conductivity of KMgCr(MoO<Subscript>4</Subscript>)<Subscript>3</Subscript> molybdate

The ionic conductivity σ of KMgCr(MoO₄)₃ crystal has been investigated in a temperature range of 575–932 K by impedance spectroscopy in the frequency range of (5–5) × 10⁵ Hz. Ternary molybdate was obtained from the initial MgMoO₄ and KCr(MoO₄)₂ reagents by solid-phase technique in air at 923–973 K for 200 h. The temperature dependence σ(T) of a ceramic sample exhibits a jump of σ by a factor of about 4 at 833 ± 5 K, which is caused by the first-order phase transition. The σ value above the phase-transition temperature reaches 6 × 10^–4 S/cm (932 K) at an ion-transport activation enthalpy of 0.84 ± 0.05 eV. The most likely carriers in KMgCr(MoO₄)₃ are K⁺ cations.     

X-Ray diffraction study of KTiOPO<Subscript>4</Subscript> single crystals doped with hafnium

Single crystals of KTi_{1 − x} Hf _x OPO₄ (x = 0.015(2), 0.035(1), and 0.128(1) are reinvestigated by precision X-ray diffraction at room temperature. It is found that the implantation of hafnium atoms in the crystal structure of KTiOPO₄ does not lead to significant changes in the framework and affects only the positions of the potassium atoms in the channel. Our studies reveal the displacements of the potassium atoms from their main and additional positions in the structure of pure KTP in all three structures studied. The largest displacements from the K1′ and K1″ additional positions are observed in the structure with x = 0.035. At this hafnium concentration, the occupancy of the main positions of potassium atoms decreases and the occupancy of the additional positions increases in relation to those in KTP. This redistribution of potassium atoms enhances the nonuniformity of distribution of the electron density in the vicinity of their positions, which is probably responsible for the increase in the nonlinear susceptibility of KTP crystals that contain 3.5% hafnium in relation to crystals of pure KTP.     

Diffusion Paths for Interstitial Impurities in Different Polymorphic Modifications of Niobium Silicide Nb<Subscript>5</Subscript>Si<Subscript>3</Subscript>

Structural models of the α, β, and γ modifications of Nb₅Si₃ silicides, which are used as a reinforcing phase in composites obtained in situ based on the Nb?Si system, have been constructed by computer simulation methods. A geometric analysis of unit cells is performed using the H-poisk program to estimate the voids existing in the structures. The results of measuring the number of voids and their sizes are reported. A conclusion about possible diffusion paths of interstitial boron, carbon, nitrogen and oxygen atoms is drawn based on the calculation results, and the solubility of these impurities in the structure of each Nb₅Si₃ modification is estimated.     

Synthesis and Crystal Structure of New Indium Iodate (K<Subscript>0.6</Subscript>Na<Subscript>0.4</Subscript>Ba)In[IO<Subscript>3</Subscript>]<Subscript>6</Subscript>

Crystals of new indium iodate (K_0.6Na_0.4Ba)In[IO₃]₆ were prepared by the hydrothermal synthesis. The unit-cell parameters are a = 11.3984(3) Å, с = 11.3817(3) Å, sp. gr. R3?. The chemical formula of the compound was derived from the structure determination and refinement with anisotropic displacement parameters to R = 0.0284. In the structure the InO₆ octahedra share vertices with six umbrella-like [IO₃]–groups typical of iodates and form isolated 3?-symmetric charged \(\rm{In[IO_{3}]_6^{3-}}\) clusters. Large Ba, K, and Na cations occupy a common site on a threefold axis due to the isomorphous substitution and compensate the charge of the clusters. The new structure extends the family of the recently discovered alkali-metal and barium iodates containing Ti and Zr atoms in octahedral sites. The iodate K₂Ge[IO₃]₆ containing Ge atoms in the centers of octahedra is the parent compound of this structural family.     

Crystal structure of KTi0.93Sn0.07OPO4

To reveal correlations between the atomic structure and properties of the KTi_1-x SnxOPO₄ solid solutions, single crystals of the KTi_0.93Sn_0.07OPO₄ composition were synthesized and their structure was determined. The crystals belong to the KTiOPO₄ family; orthorhombic, sp. gr. Pna2₁. The unit-cell parameters are a = 12.831, b = 6.410, and c = 10.584 Å. The partial substitution of Sn atoms for Ti atoms results in the formation of more symmetric coordination polyhedra in the structure framework and the change of the physical properties of the crystals.     

Structural transition from vacancy-ordered tetragonal perovskite to tetragonal potassium tungsten bronze by the example of Na<Subscript>4</Subscript>Bi<Subscript>2</Subscript>Nb<Subscript>10</Subscript>O<Subscript>30</Subscript>

The phenomenological transition from the perovskite structure type to the potassium tungsten bronze structure type is considered. The structural motif of vacancy-ordered tetragonal perovskite (VOTP) with the space group P4/m is constructed. The atomic coordinates in the unit cell of VOTP and the Madelung energies for the unit cells of VOTP and tetragonal potassium tungsten bronze are calculated. The transition is experimentally observed in Na₄Bi₂Nb₁₀O₃₀ prepared by the solid-state reaction method. The X-ray diffraction pattern measured after the first annealing showed good agreement with the theoretical X-ray diffraction pattern calculated for VOTP Na₄Bi₂Nb₁₀O₃₀.     

Crystal and magnetic structure of the Sm<Subscript>0.45</Subscript>Sr<Subscript>0.55</Subscript>MnO<Subscript>3</Subscript> manganite studied by neutron powder diffraction

Neutron diffraction investigation of the ¹⁵²Sm_0.45Sr_0.55MnO₃ manganite is performed. The diffraction data are compared with the magnetic and transport properties of this compound. The parameters of the crystal and magnetic structures are determined. Manganite belongs to the orthorhombic system (sp. gr. Pnma) and has a perovskite-like structure in the entire temperature range under study (1.5–260 K). The ground state of the ¹⁵²Sm_0.45Sr_0.55MnO₃ manganite at low temperatures is a single-phase A-type antiferromagnetic insulator with T _N ～ 180 K.     

Piezo-optic properties of incommensurately modulated K<Subscript>2</Subscript>ZnCl<Subscript>4</Subscript> crystals

The effect that uniaxial pressures along the principal crystallophysical directions have on the dispersion and temperature dependences of the birefringence in incommensurate K₂ZnCl₄ crystals has been investigated in a wide temperature range. This parameter has been established to be fairly sensitive to uniaxial pressures. The spectral and temperature dependences of combined piezo-optic coefficients are analyzed. Significant anomalies of these coefficients are found at the incommensurate-commensurate phase transition. They are due to the change in the induced birefringence that results from spontaneous polarization and to the significant effect of uniaxial stress on the soliton structure of the crystal. The contributions that the electrooptic effect, the elasto-optic effect, and the order parameter make to the spontaneous changes in the piezooptic coefficients are separated.     

Investigation of the structural conditionality for changes in physical properties of K<Subscript>3</Subscript>H(SO<Subscript>4</Subscript>)<Subscript>2</Subscript> crystals

With the aim of elucidating the nature of anomalies in the physical properties of K₃H(SO₄)₂ crystals that arise as the temperature grows, the dielectric and optical properties of the crystals are studied, an X-ray diffraction analysis of single-crystal and polycrystalline specimens are performed, and the morphology and chemical composition are studied by scanning electron microscopy and energy-dispersive X-ray spectroscopy. As a result of the studies performed, a phase transition from the phase with the monoclinic symmetry (space group C2/c) to the phase with the trigonal symmetry (space group R $ \bar 3 $ \bar 3 m) is found in a number of K₃H(SO₄)₂ specimens at a temperature of ≈457 K, the responsibility of the dynamically disordered hydrogen-bond system for the rise of high proton conductivity in the high-temperature phases of the crystals of this family is confirmed, and data on the solid-phase reactions proceeding at high temperatures are obtained.     

Domain Structure and Orientational Ordering of NO<Subscript>2</Subscript> Groups in K<Subscript>2</Subscript>Ba(NO<Subscript>2</Subscript>)<Subscript>4</Subscript> Crystals

Polarization-optical studies of the domain structure of K₂Ba(NO₂)₄ crystals and differentialscanning calorimetric measurements have been performed in the vicinity of the high-temperature phase transition. The orientational ordering of NO₂ atomic groups is analyzed and the temperature dependence of the birefringence coefficient is theoretically described.     

Structure of Gd<Subscript>0.95</Subscript>Bi<Subscript>0.05</Subscript>Fe<Subscript>3</Subscript>(BO<Subscript>3</Subscript>)<Subscript>4</Subscript> single crystals at 293 and 90 K

The structure of GdFe₃(BO₃)₄ single crystals has been studied by X-ray diffraction at 293 and 90 K. The crystals are grown from a flux in the Bi₂Mo₃O₁₂–B₂O₃–Li₂MoO₄–Gd₂O₃–Fe₂O₃ system. The results of chemical analysis and structural study show that these crystals contain bismuth as an impurity. It is found that bismuth atoms are located at gadolinium sites in the structure. A decrease in the temperature is accompanied by a lowering of the symmetry from sp. gr. R32 (at 293 K) to sp. gr. P3₁21 (at 90 K). The presence of two types of iron chains with different geometries at 90 K promotes a change in the magnetic properties of these crystals with a decrease in the temperature.