Received 30 October 2014; revised 29 November 2014; accepted 31 December 2014

ABSTRACT

The research is devoted to the multicomponent system CaTiSiO₅-YFeSnO₅. The synthesis of solid solutions Ca_1−xY_xTi_1−xSn_xSi_1−xFe_xO₅ (x = 0 - 1.0, Δx = 0.1) was conducted in low-temperature plasma of hydrogen-oxygen flame. It was found that ions Ca²⁺, Ti⁴⁺ and Si⁴⁺ in the molecule of titanit may be substituted with t ions Y³⁺, Fe³⁺ and Sn⁴⁺. In this case, the system produces two phases of variable composition with broad regions of homogeneity. There were defined the boundaries of formed phases, crystallographic and electrical parameters of the solid solutions. All solid solutions have a semiconductor conductivity type, whose value is linearly dependent on the temperature and com- position of the sample.

Keywords:

Solid Solutions, Titanite (Sphene), Pseudobrookite, Electrical Properties, Structure

1. Introduction

Metal oxides containing transition elements possess unique physical properties, they are subject to numerous scientific studies and are successfully used in modern electronics. Particularly complex oxides containing d- elements, including oxides having the structure of sphene (titanite CaTiSiO₅) are of great practical interest.

Synthesis and study of solid solutions with the structure of sphene containing transition elements is not only of theoretical but also of practical interest. Propensity compounds with sphene form solid solutions with wide homogeneity region containing transition elements, indicates the perspective of their application as radio materials.

Possibility of full or partial replacement of the atoms in the crystal lattice of sphene leads to a natural change in the physical properties, representing both theoretical and practical interest. It was shown possibilities to replace the titanium atoms in the sphene structure with the atoms of tin [1] [2] . Gradual replacement of titanium atoms of tin atoms leads to the formation of a continuous series of solid solutions and homogeneous to a natural reduction of conductivity samples [3] . A significant change in the crystal lattice was not observed also at full replacement of silicon germanium atoms, leading to the formation of the germanium analogue of titanite (CaTiGeO₅) [4] . However it has been found that simultaneous replacement of atoms calcium and silicon with two atoms of trivalent elements such as Cr, Mn, Fe, Eu, etc. leads to a significant change in the lattice structure: the monoclinic crystal lattice of sphene becoming to rhombic structure, with the formation of the psedobrukit [5] - [7] . As a result, of the complete replacement of Ca²⁺ ions and Si⁴⁺ in sphene by ions Fe³⁺ and Nd³⁺ there was obtained the single crystal composition NdFeTiO₅ [6] . The investigation of CaTiSiO₅-Fe₂TiO₅ [7] has shown that replacing Ca²⁺ + Si⁴⁺® 2Fe³⁺ leads to a significant increase in electrical conductivity of samples, but replacing more than 50% calcium and silicon by iron, result is the formation of solid solutions with the structure of pseudobrookite.

The present communication is devoted to the study of simultaneous heterovalent substitution of all three cations in sphene: Ca²⁺ + Ti⁴⁺ + Si⁴⁺® Y³⁺ + Sn⁴⁺ + Fe³⁺. Simultaneous substitution of one divalent and two tetravalent (Ca, Ti, Si) atoms in a lattice of CaTiSiO₅ by two trivalent and one tetravalent (Y, Fe, Sn) represents not only the theoretical but also practical interest. To solve this problem, it has been investigated previously undescribed pseudo-binary system CaTiSiO₅-YFeSnO₅. There was built the diagram of the system’s state and defined the crystallographic and electrical parameters of the samples of compositions (CaTiSi)_1−x(YFeSn)_xO₅.

2. Experimental Procedure

Synthesis of solid solutions Ca_1−xY_xTi_1−xSn_xSi_1−xFe_xO₅ (0 ≤ x ≤ 1, Δx = 0.1) was carried out in parallel in the low-temperature plasma of hydrogen-oxygen flame (LP), and ceramic technology (CT) [8] . As starting substances were used the ultrapure oxides of relevant elements: CaO, TiO₂, SiO₂, Y₂O₃, SnO₂ and Fe₂O₃. Samples were synthesized by methods LP and TM then were heated at 1170 K for 6 hours followed by rapid cooling on a copper substrate. The calculations were performed using the software package [9] . The density of the samples was determined by pycnometric.

Conductivity of the samples was measured by the compensation in the air, using four silver electrodes [10] . The temperature dependence of the electrical conductivity was determined in the temperature range 293 - 900 K. Samples for measurement were prepared from powder passed through a laboratory CO-200 sieve (63 microns) after compression to form pills (d = 20 mm, l = 2.1 mm) at a pressure of 400 MPa. The permittivity of the samples was measured using the flat capacitor (20 V, 800 Hz). Molar polarization of the sample is calculated by the equation of Clausius-Mossotti.

3. Results and Discussion

Comparison of x-ray diagrams of the same composition synthesized by (LP) to (CD) revealed their identity. Following is the data obtained for the samples synthesized by (NP).

Based on X-ray diagrams, it was found that in these conditions the system CaTiSiO₅-YSnFeO₅ form two phases (α and β) of variable composition with a wide homogeneous regions: α-phase 0 ≤ x ≤ 0.45 and β-phase 0.70 ≤ x ≤ 1.00. Samples of the composition 0.45 ≤ x ≤ 0.70 contain two phases (Figure 1).

α-phase. Solid solutions of compositions from CaTiSiO₅ to Ca_0.55Y_0.45Ti_0.55Sn_0.45Si_0.55Fe_0.45O₅ crystallize in the lattice sphene. Thus, replacement of up to 45% of the ions Ca²⁺, Ti⁴⁺ and Si⁴⁺ ions on the Y³⁺, Sn⁴⁺ and Fe³⁺ did not result in a significant rearrangement of the crystal lattice sphene (Table 1). As seen from the data intoduction of ions Y³⁺, Sn⁴⁺ and Fe³⁺ ions instead of Ca²⁺, Ti⁴⁺ and Si⁴⁺ in the α-phase leads to a significant increase in the unit cell parameters. However, as can been expected, the densities of obtained solid solutions are also growing (Figure 2).

¹⁾±0.005; ²⁾±0.02.

Input Y³⁺ ions occupy seven vertex polyhedral voids that were previously occupied by ions Ca²⁺. The radii of these ions differ little from each other: r(Y) = 0.116 nm, r(Ca) = 0.130 nm [11] . Such substitution also contributes to the similarity of the electron shells of these ions: 3s²3p⁶ (Ca²⁺) and 4s²4p⁶ (Y³⁺). A Fe³⁺ ions, thus, occupy tetrahedral interstices previously occupied by ions Si⁴⁺.

Despite the greater tendency of Fe³⁺ ions to occupy the octahedral interstices, There are numerous examples where the Fe³⁺ ions are in tetrahedral cavities oxygen environment such as in the crystal lattice of Fe₃O₄ where in the tetrahedral voids occupied 1/3 of the ions Fe³⁺ [12] . As a result, even replacement of 45% of calcium ions and silicon in the α-phase does not result in significant changes in the lattice of sphene. As the authors of [13] , belonging of the sphene to the lower-monoclinic system is a consequence of the presence of calcium ions in the lattice, occupying polyhedral (semivershinnye) voids formed by distorted octahedra TiO₆. As a result, even replacement of 45% of calcium ions and silicon in the α-phase does not result in significant changes in the lattice of sphene. Hence, we expect that the replacement of ions Ca²⁺ on the Y³⁺ ions will lead to the gradual streamlining lattice, turning seven deformed lattice to the octahedral polyhedral. In fact, samples containing more than 70 at % Yttrium forms a crystal lattice of higher symmetry.

β-phase. When administered 55 at % and more ions Y³⁺, Fe³⁺ and Sn⁴⁺, instead of Ca²⁺, Si⁴⁺ and Ti⁴⁺ solid solutions crystallize in the orthorhombic symmetry in lattice of the psevdobrookit. The border of the homogeneity of β-phase extends in the region x = 0.70 - 1.0, which corresponds to the composition of Ca_0.30Y_0.70Ti_0.30Sn_0.70Si_0.30Fe_0.70O₅-YFeTiO₅.

The value of the lattice parameters of solid solutions of β-phase are shown in Table 2. Like the α-phase values of the elementary cells of solid solutions, as well as their densities are in the substantially rectilinear function of the composition (Figure 3).

¹⁾± 0.005.

As in the α-phase the increase of the value of “x” in the β-phase leads to an increase in cell volume. However, the transition from the monoclinic to the rhombohedral structure observed abrupt decrease in cell volume (Figure 4). In contrast to the α-phase, increasingthe ion content Y³⁺, Fe³⁺ and Sn⁴⁺ in the β-phase leads to a noticeable increase in the lattice parameters.

The synthesized solid solutions both α- and β-phases, are dielektriks with character of semiconductor conductivity. The determination results of conductivity, dielectric constant, the band gap, the molar polarizability and polarization of the synthesized solid solutions are shown in Table 3.

As can be seen from the data presented in Table 3, the introduction of tin, yttrium, and iron (p-, d-, and d- elements) in substituting calcium, silicon, and titanium (s-, p-and d-elements) leads to narrowing of the band gap and improves the conductivity of the solid solutions. Dielectric constant and band gap samples thus reduced. Increased content of yttrium, tin and iron leads to an increase in the values of the molar polarization of samples in areas of both phases. Simultaneously, there is a sharp, abrupt decrease in these parameters during the transition phase (a¦b), which is due to a sharp decrease in cell volume and an increase in density of the samples in the transition phase. Concentration dependences of the all parameters are linear in the fields of both phases (Figure 5).

4. Conclusion

The multicomponent system CaTiSiO₅-YFeSnO₅ was the subject of the research. It was found that ions Ca²⁺, Ti⁴⁺ and Si⁴⁺ in the molecule of titanit can be substituted with ions Y³⁺, Fe³⁺ and Sn⁴⁺ which leads to the formation of two phases of compositions Ca_1−xY_xTi_1−xSn_xSi_1−xFe_xO₅ with broad regions of homogeneity. There were defined the boundaries of the uniform phases at 1170 K. The samples α-phase (x = 0.0 - 0.45) crystallize in the structure of titanite. Samples b-phase (x = 0.70 - 1.0) crystallize in the orthorhombic crystal system structure

pseudobrookite. The electrical properties of solid solutions formed were also investigated. All samples are insulators with a semiconductor character of electrical conductivity. Complete replacement of the ions Ca²⁺, Ti⁴⁺ and Si⁴⁺ increases the conductivity of the samples by a factor of two.

References