Search

In Sr9 – xMxTm(VO4)7 system, solid solutions with M = Ca (0 ≤ x ≤ 2) and M = Pb (0 ≤ x ≤ 1) have been prepared by solid-phase synthesis at 1373 K. At room temperature, the single-phase samples are isostructural to the Ca3(VO4)2 ferroelectric (space group R3c, Z = 6). The materials have been characterized by methods of the X-ray phase analysis (XPA), the second harmonic generation (SHG), and the dielectric spectroscopy. The intensity of the change in the SHG signal is changed within the range of 30–50 units with respect to a quartz standard in the dependence on cation–substitute and its concentration. A reversible ferroelectric phase transition is observed in the temperature range 850–960 K. The transition is accompanied by a sharp maximum in the curve of the dependence of the dielectric permittivity on temperature. The absence of the SHG signal at temperatures higher than the transition temperature indicates the centrosymmetricity of the paraelectric phase.

Sr9In(VO4)7 was prepared by a solid-state method at 1270 K in air. This vanadate has the β-Ca3(PO4)2-type structure and crystallizes in polar space group R3c. The structural parameters of Sr9In(VO4)7 were refined by the Rietveld method from laboratory powder X-ray diffraction data (XRD): the lattice parameters are a = 11.18016(9) A and c = 39.6170(3) A with Z = 6. In3+ cations occupy the octahedral M5 site, Sr2+ cations occupy the M1, M2, and M3 sites of the β-Ca3(PO4)2-type structure, and the M4 site remains vacant. Sr9In(VO4)7 was characterized by differential thermal analysis (DTA), optical second-harmonic generation (SHG), high-temperature XRD, and dielectric measurements. All these methods prove the existence of a ferroelectric–paraelectric phase transition at Tc = 974 K. This transition is compared with a similar transition in Ca9In(PO4)7 with lower Tc = 902 K. The polar-to-centrosymmetric phase transition in such compounds has a quite unique mechanism of the order–disorder type. The structural transition involves slight shifts of the M1, M2, M3 cations and the E2O4, E3O4 tetrahedra, while half of the E1O4 tetrahedra (E = P or V) statistically reverse their orientation along the three-fold axis, so that the centre of symmetry appears in the structure as a whole. To invert the E1O4 tetrahedron, one oxygen anion should pass a large neighbouring cation (Sr2+ or Ca2+) that is only possible when intense rotational vibrations of the tetrahedra are excited at high temperatures. The lower Curie temperature in Ca9In(PO4)7 corresponds to the smaller rotational vibration amplitude of the P1O4 tetrahedron required to reverse this tetrahedra at Tc in comparison with V1O4 in Sr9In(VO4)7.

Ca9–xBaxBi(VO4)7 (0 ≤ x ≤ 1.5) solid solutions with the β-Ca3(PO4)2-type structure were prepared by a solid-state method. Powder X-ray diffraction study of 0 ≤ x ≤ 0.7 showed that Bi3+ and Ba2+ cations were completely incorporated into the β-TCP-type host framework up to x = 0.7. Ca9-xBaxBi(VO4)7 (x = 0.25, 0.5) structures were refined by the Rietveld method using powder synchrotron X-ray diffraction data. DSC, SHG and dielectric properties measurements of Ca9-xBaxBi(VO4) (x = 0, 0.25, 0.5) revealed two reversible first-order phase transitions (PT1 and PT2). Increase of Ba2+ content in Ca9-xBaxBi(VO4)7 leads to lowering PT1 and PT2 phase transitions temperatures due to the increase of cell volume and the structure looseness. Nonlinear optical activity of Ca9–xBaxBi(VO4)7 reaches its maximum response at x = 0.5. The amount of (M1–(M3)–M2) dipoles in Ca9–xBaxBi(VO4)7 (0

Solid solution of Ca10.5–1.5xYbx(PO4)7 (0 ≤ x ≤ 1) was synthesized in the β-Са3(РО4)2-type structure by a standard solid-state method. Second-harmonic generation, differential scanning calorimetry, and dielectric measurements showed the presence of a reversible first-order phase transition of ferroelectric type between the room-temperature polar R3c structure to centrosymmetry. The temperature of the phase transition decreases from 1203 K for x = 0 to 788 K for x = 1, while the second-harmonic generation response decreases from 1.4 to 0.17 α-SiO2 powder standard units. The luminescence spectra of Ca10.5–1.5xYbx(PO4)7 point to two structural Yb3+ positions in full agreement with the structural data. The intensity of the luminescence emission line at ∼975 nm due to the 2F5/2 → 2F7/2 transition reaches the maximum at x = 0.667 in Ca10.5–1.5xYbx(PO4)7. The Yb3+ luminescence spectra studied as a function of the chemical composition 0

Crystal structure, and thermal, dielectric and optical second-harmonic activities were investigated for whitlockite-type Ca8.6Pb1.9(VO4)7 (1), Ca7Pb3.5(VO4)7 (2) and Ca5.6Pb4.9(VO4)7 (3) single crystals. The refinement of the structure revealed splitting of the M3 and M4 sites into two and three sub-positions, respectively. Splitting of the M3 and M4 sites strongly enhances nonlinear optical activity, measured on powders, with the effect vanishing at ferroelectric phase transitions at elevated temperatures. Single crystals (1) and (2) undergo two structural transformations: ferroelectric one between 750–1150 K and a subsequent phase transition at higher temperatures. The structural mechanism of the ferroelectric phase transition is analyzed with respect to the rearrangement of Ca2+ and Pb2+ over their positions in the ferroelectric and paraelectric phases. Large Pb2+ ion substitution for calcium results in unit cell expansion with consequent extending of ion migration channels, thus leading to enhanced ionic conductivity in the Pb-rich materials. It is concluded that in spite of steric hindrances for lead cations, they effectively move between positions M2 → M4 → M1 or M1 → M4 → M2 together with calcium cations across the crystal. The cation conduction pathways as well as the migration energies of Ca2+ and Pb2+ cations were calculated by the bond valence energy landscape (BVEL) method. Among the obtained single crystals, (3) demonstrated the highest ionic conductivity, maximum second harmonic generation (SHS) signal and the lowest temperature of ferroelectric phase transition.

Photoluminescence excitation and emission spectra of new phosphate Ca8ZnEu(PO4)7 have been investigated. The influence of the composition on the luminescence intensity has been shown. The substitution of Zn2+ for Ca2+ in Ca9Eu(PO4)7 with the formation of Ca8ZnEu(PO4)7 leads to the increased emission intensity 5D0 → 7F2 by about 1.5 times and to decreased number of bands from three to one in the range of 5D0 → 7F0 transition. Conditions of preparation do not influence the luminescent properties and distribution of Eu3+ cations in the structure. DSC, SHG data and dielectric spectroscopy reveal that Ca8ZnEu(PO4)7 belongs to antiferroelectrics (Tc = 836 K). The first-order antiferroelectric/paraelectric transformations are accompanied by a changing symmetry (R 3 ¯ с to R 3 ¯ m). The structure of Ca8ZnEu(PO4)7 (sp. gr. R 3 ¯ с) was refined by the Rietveld method. The Zn2+ cations occupy the M5 site. The Eu3+ and Ca2+ ions are located in the M1 and M3 sites of β-Ca3(PO4)2-type structure.

Single crystals of Ca9Y(VO4)7 (1), Ca9Y(VO4)7:Li+ (2) and Ca9Y(VO4)7:Mg2+ (3) were grown by the Czochralski method. Their chemical composition was analyzed by ICP spectroscopy and their crystal structure was examined by single crystal X-ray analysis. The crystals are characterized by trigonal symmetry, space group R3c. Hexagonal unit-cell parameters are as follows: a=10.8552(1) Å, c=38.0373(2) Å, V=3881.65(1) Å3 for 1; a=10.8570(1) Å, c=38.0161(3) Å, V=3880.77(4) Å3 for 2; a=10.8465(1) Å, c=38.0366(2) Å, V=3875.36(3) Å3 for 3. All crystals are characterized by β-Ca3(PO4)2-type structure with statistical distribution of Ca2+ and Y3+ over M1, M2 and M5 sites in different ratios and with completely empty M4-cationsite. The impurity of Mg2+cations in structure 2 has been detected in octahedral M5 site. Ferroelectric phase transitions are evidenced by DSC and SHG. At about 1220 and 1300 K, they demonstrate phase transitions. Upon heating the symmetry of the crystal structure changes according to the scheme R3c→R3̅c→R3̅m and is restored during consequent cooling. The first of them is of ferroelectric and the second of non-ferroelectric nature. Even a small amount of impurities in Ca9Y(VO4)7 structure is accompanied by a noticeable decrease in the temperature of the ferroelectric-paraelectric phase transition.

A series of new phosphates Ca9-xZnxLa(PO4)7:Ho3+ with the β-Ca3(PO4)2-type structure was synthesized by the solid state route. An intense near infra-red (NIR) emission according to intraconfigural 4f-4f transitions of Ho3+ ions 5I7 – 5I8 (~2 μm) and 5I6 – 5I8 (~1.156 μm) was observed. The obtained phases were studied by a combination of methods including synchrotron powder X-ray diffraction, dielectric spectroscopy, second harmonic generation, differential scanning calorimetry, luminescence spectroscopy. The structure of Ca8ZnLa(PO4)7 was refined by the Rietveld method in centrosymmetric space group R 3 ‾ c. The Ca2+ → Zn2+ substitution in the M5 site leads to a transformation from polar R3c space group (x = 0 – 0.5) to centrosymmetric R 3 ‾ c space group (x = 0.6–1) and to the increased integral intensity of luminescence with maxima at x = 1. It was concluded that the crystal site engineering in the Ho3+-containing β-Ca3(PO4)2-type hosts offers a promising way to obtain new NIR phosphors for use in the creation of biocompatible bone tissue fillers.

The purchase of trade establishments is an important and complex process, on which companies spend a lot of resources in order to get the maximum benefit. There are many well-established methods for making a forecast of the required number of goods, but none of them can guarantee an accurate result, because in the matter of forecasting, there cannot be a method that produces a result with 100% accuracy. The object of our research is the process of creating a purchase forecast for trade enterprises. The subject is the analysis of the quality of the purchase forecast for retail outlets. To determine the quality of the purchase forecast of the retail outlets, it is necessary to understand how the purchasing process takes place, as well as highlight the factors that affect the forecast quality.

The article is devoted to the analysis and construction of a multivariate regression model. The mode of operation of generalized heat points is considered. The positive and negative aspects of using heat points to maintain the temperature at the desired level have been identified. The analysis of the multivariate model is carried out. The input data for the generated model has been defined. The factors that affect the temperature in structurally isolated rooms have been identified. A multifactorial regression model of temperature control in heated rooms has been built, with the help of which it is possible to determine how much heat needs to be supplied to a particular structurally isolated room, which will allow more efficient use of resources to maintain the temperature at the required level.

Ca9.5–1.5xBixCd(VO4)7 (0 ≤ x ≤ 1) solid solutions with the whitlockite-type structure (SG R3c) were synthesized by a standard solid-state method in air. Structures of Ca9.5–1.5xBixCd(VO4)7 (x = 0.167, 0.5, 0.833) were refined by the Rietveld method from synchrotron powder X-ray diffraction data. Nonlinear optical properties of the whitlockite-type compounds can be designed and increased by an order of magnitude through appropriate isovalent and aliovalent substitutions for Ca2+ cations. Dielectric and temperature second harmonic generation investigations revealed the presence of a reversible ferroelectric phase transition in the range from 1331 K to 1055 K. The phase transition temperature monotonically decreases while nonlinear optical activity of Ca9.5–1.5xBixCd(VO4)7 strongly increases with increasing Bi3+ content. Keywords: Vanadates, Ferroelectric properties, Nonlinear optical properties, Crystal structure

The crystal structure, thermal, dielectric and second harmonic generation (SHG), and nonlinear optical activity data for whitlockite-type Ca9Yb(VO4)7 single crystals were obtained on one and the same sample produced by means of the Czochralski method. The crystal structure refinement has revealed that Yb3+ cations substitute for Ca2+ ions only in the M1, M2 and M5 positions of the whitlockite-type structure. Dielectric, differential thermal analysis and SHG data have shown that Ca9Yb(VO4)7 belongs to the family of high-temperature Ca3(VO4)2 ferroelectrics with Curie temperature Tc = 1221 K, where the symmetry changes from R3c to Rc. At higher temperatures a previously unknown complementary phase transition is discovered at T2 = 1276 K and is associated with the symmetry change during heating from Rc to Rm. Unlike other whitlockites, two phase transitions in Ca9Yb(VO4)7 are separated by a broad interval (ΔT = 55 K) which allows one to register two phase transitions by DSC and dielectric measurements. According to the thermal type both transitions are classified as first-order transformations and their structural mechanisms are considered. Inhomogeneity in the cation distribution is argued to have a crucial influence on the optical quality and ferroelectric domain structures of Ca9Yb(VO4)7 and other whitlockite-type laser crystals.

This paper describes a study of the nature and the mechanism of ion transfer in Ca6.25Pb4.25(VO4)7 with β-Ca3(PO4)2-type structure by the Tubandt method. The comparison of weight change to the quantity of electricity passed through the stack allowed us to state that migration of both Ca2+ and Pb2+ cations was present. The transference numbers were found to be 0.33 and 0.67 for Ca2+ and Pb2+ cations, respectively. The phase composition of samples after the Tubandt measurements was studied by powder X-ray diffraction and scanning electron microscopy. The quantitative phase analysis carried out by the Rietveld method using XRD diffraction data revealed the formation of five phases in anodic and cathode pellets other than Ca6.25Pb4.25(VO4)7. The quantity of phases in the samples depends on the location of pellets during the Tubandt measurements.

The article discusses the construction of the simulation-dynamic model of the manufacturing details process in the workshop. As part of the production process, the workshop prepares workpieces, performs processing operations and packaging of details, as well as controls defective workpieces during processing operations. The model is built, taking into account the percentage of defect probabilities during the operation. It also shows the work process of the enterprise workshop for 8 hours. A model management interface was created, which includes an input interface through which data is entered, as well as an output interface that allow seeing the results of the work done. On the grounds of the conducted experiments, conclusions were drawn regarding the available data, a comparison of the experimental results was carried out, as well as illustrating how the model visualizes changes in the manufacturing process of details in the workshop.

Ca 8- x Pb x M Bi(VO 4 ) 7 , M 2+ = Mg 2+ , Zn 2+ (0 ≤ x ≤ 1.5) and M 2+ = Ca 2+ , Cd 2+ (0 ≤ x ≤ 2) solid solutions with whitlockite-type structure have been prepared by solid state reactions in the form of powders and ceramics. Ferroelectric phase transitions (FPT) are revealed in the cause of dielectric, calorimetric and temperature second harmonic generation (SHG) investigations. It is found that ferroelectric Curie points in solid solutions decrease with x from 1000–1070 K to 730–800 K. The intensity of SHG firstly increases with x and then decreases, the highest SHG signal belongs to Ca 7 PbCdBi(VO 4 ) 7 and Ca 6.5 Pb 1.5 CdBi(VO 4 ) 7 . Variation of Pb 2+ concentration is shown to have a minor influence on the coherence length, therefore the SHG augmentation in solid solutions is attributed to optical nonlinear coefficient d >. The structure of Ca 6.5 Pb 1.5 MgBi(VO 4 ) 7 is resolved and crystal structure changes in solid solutions are analyzed. The results are interpreted in view of site occupancy by two- and trivalent cations and their influence on formation of spare space essential for Pb 2+ and Bi 3+ non-bonded electrons stereo-activity in whitlockite-type crystal structures. For the first time contribution to optical nonlinearity from VO 4 3− anionic groups in different positions is also discussed.

A series of Ca8MTb1–xEux(PO4)7 (M = Mg2+, Zn2+, Ca2+; 0 ≤ x ≤ 1) with β-Ca3(PO4)2-type structure have been synthesized via a standard solid-state method. X-ray diffraction, second harmonic generation, photoluminescence emission and excitation spectra, and decay time curves were used to characterize solid solutions. The color coordinates of as-prepared samples can be shifted from green to red regions by the increasing of Eu3+ concentration. Red-to-orange (R/O) and green-to-blue (G/B) factors indicate a higher local symmetry of emitting Eu3+ and Tb3+ ions environment in Ca8MTb1–xEux(PO4)7 (M = Mg, Zn) in comparison to Ca9Tb1–xEux(PO4)7. The study shows that Zn-incorporation in the β-Ca3(PO4)2 matrix increases the luminescence intensity by 15%. The presented results indicate that the synthesized solid solutions can be considered as new single-phase phosphors.

A solid state route was used for the preparation of whitlockite-type solid solutions Ca9Y(NbO4)x(VO4)7–x and Ca9+0.5xYb (GeO4)x(VO4)7–х in both powder and ceramic forms. The complex of methods, including synchrotron powder diffraction were applied for the characterization of the first-time obtained compounds. The limits of the anionic substitution were found up to х = 0.28 (Nb) and 0.35 (Ge). The synthesized solid solutions show ferroelectric properties. The Curie temperatures Tc were determined and gradually rise with x. The increase of Tc correlates with the enlargement of tetrahedral units upon Nb5+ or Ge4+ substitutions for V5+ cations. The intensity of optical second harmonic generation (SHG) induced by a Nd:YAG laser depends non-monotonically on the grain size in powder Ca9Y(NbO4)x(VO4)7–x samples, marking the length of the phase matching lc = 15 μm for x = 0.28 in comparison with lc = 9.14 μm in Ca9Y(VO4)7. Manifold increase in the SHG intensity in both solid solutions is partly related to the appearance of more polarizable Nb5+ and Ge4+ cations in the E1O4 tetrahedra on the 3-fold axis, resulting in an enlarged birefringence of the materials. The photoluminescence study of the Ca9+0.5xYb(GeO4)x(VO4)7–х reveals the same environment of the Yb3+ ions all alone the solid solution. The intensity of the luminescence rises with germanium substitution for vanadium about 2 times.

Ca8MgR(PO4)(7) = La, Pr, Nd, Sm-Lu, and Y) phosphates with a beta-Ca-3(PO4)(2) related structure were prepared by a standard solid-state method in air. Second-harmonic generation, differential scanning calorimetry, and dielectric measurements led to the conclusion that all Ca8MgR(PO4)(7) are centrosymmetric and go to another centrosymmetric phase in the course of a first-order antiferroelectric phase transition well above room temperature (RT). High-temperature electron diffraction showed that the symmetry changes from R (3) over barc to R (3) over barm during the phase transition. Structures of Ca8MgR(PO4)(7) at RT were refined by the Rietveld method in centrosymmetric space group R (3) over barc. Mg2+ cations occupy the M5 site; the occupancy of the M1 site by R3+ cations increases monotonically from 0.0389 for R = La to 0.1667 for R = Er-Lu, whereas the occupancy of the M3 site by R3+ cations decreases monotonically from 0.1278 for R = La to 0 for R = Er-Lu. In the case of R = Er-Lu, the M3 site is occupied only by Ca2+ cations. P1O(4) tetrahedra and cations at the M3 site are disordered in the R (3) over barc structure of Ca8MgEu(PO4)(7). Using synchrotron X-ray powder diffraction, we found that annealing conditions do not significantly affect the distribution of Ca2+ and Eu3+ cations between the structure positions of Ca8MgEu(PO4)(7). Luminescent properties of CasMgEu(PO4)(7) powder samples were investigated under near-ultraviolet (n-UV) light. Excitation spectra of CasMgEu(PO4)(7) show the strongest absorption at about 395 nm that matches with commercially available n-UV-emitting GaN-based LED chips. Emission spectra show an intense red emission due to the D-5(0) -> F-7(2) transition of Eu3+. (C) 2016 Elsevier B.V. All rights reserved.

We have prepared Ca9.5 − xPbxM(PO4)7 (M = Mg, Zn, Cd) and Ca10.5 − xPbx(PO4)7 solid solutions. A polar whitlockite-like (sp. gr. R3c) crystal structure exists in the range 0 ≤ x ≤ 1.5 for all of the M cations in Ca9.5 − xPbxM(PO4)7 and in the range 0 ≤ x ≤ 2.5 for Ca10.5 − xPbx(PO4)7. X-ray powder diffraction profile analysis results for Ca8.5PbCd(PO4)7 powder demonstrate that the small divalent M cations reside predominantly on the octahedral site M5 of the whitlockite structure, the calcium cation occupy the M1–M3 sites, and the lead cations are located primarily on the M4 site. Differential scanning calorimetry, second-harmonic generation, and dielectric permittivity data indicate that all of the synthesized phosphates are high-temperature ferroelectrics. The highest Curie temperatures are offered by the x = 0.5 materials, in which most of the lead resides in the spacious oxygen polyhedra M4 and only a small amount of lead is incorporated into the smaller polyhedra around M1–M3. The nonlinear optical activity has a maximum in the middle of the solid-solution series and is an order of magnitude higher than that of the parent, lead-free phases.

We have prepared solid solutions based on whitlockite-structure ferroelectrics, Ca9 − x Pb x R(PO4)7 (R = Sc, Cr, Fe, Ga, In), through Pb substitution for Ca. Single-phase Ca9 − x Pb x R(PO4)7 materials with a polar whitlockite-like structure (sp. gr. R3c) exist in the range 0 ≤ x ≤ 1.5 for all of the R metals studied. X-ray powder diffraction profile analysis results for R = In (x = 0.5, 0.8, and 1.0) demonstrate that the trivalent cations reside on the octahedral site M5 of the whitlockite structure, the calcium cation occupy the M1–M3 sites, and the lead cations are located predominantly on the M3 site. Differential scanning calorimetry, dielectric permittivity, and second-harmonic generation data attest to a ferroelectric phase transition, whose temperature is 580–610°C in Ca9R(PO4)7 and decreases monotonically to 480–520°C as x increases to 1.5.

High-temperature second-harmonic-generation, x-ray powder diffraction, dielectric, differential scanning calorimetry, and structural data are presented to show that ${\mathrm{Sr}}_{9}\mathrm{In}{(\mathrm{P}{\mathrm{O}}_{4})}_{7}$ exhibits an antiferroelectric (AFE) phase transition of the second order at $773\ifmmode\pm\else\textpm\fi{}3\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. ${\mathrm{Sr}}_{9}\mathrm{In}{(\mathrm{P}{\mathrm{O}}_{4})}_{7}$ complements a group of ${\mathrm{Ca}}_{3}{(\mathrm{V}{\mathrm{O}}_{4})}_{2}$-type ferroelectric phosphates and vanadates and is the first example of an AFE material in this family. Structural data obtained for the AFE and the paraelectric phase of ${\mathrm{Sr}}_{9}\mathrm{In}{(\mathrm{P}{\mathrm{O}}_{4})}_{7}$ show evidence that both displacive and order-disorder contributions are responsible for the physical properties of the ${\mathrm{Ca}}_{3}{(\mathrm{V}{\mathrm{O}}_{4})}_{2}$-type materials.