Investigation of electro-optical properties of gradient potassium niobate

. The gradient perovskite material of potassium niobate is investigated in this work. As a result of the research, a gradient change in the properties was found: transformations of the IR absorption spectrum, the magnitude of the half-wave voltage depending on the composition of the grown potassium niobate crystal. The value of the half-wave voltage for plates cut from crystalline potassium niobate varied from 1500 to 1700 V with a change in the composition of potassium niobate in the range K x Nb 2-x O 5-2x at x=0.9..1.


Introduction
Interest in perovskite energy has emerged relatively recently. The values of the conversion coefficient of solar energy into electricity for perovskite single-layer solar cells are comparable to silicon (23%), they are superior to silicon materials in terms of cost and ease of production [1].
Potassium niobate KNbO3 also belongs to an extensive class of perovskites. Potassium niobate refers to materials with incongruent melting character, possessing good electrooptical properties, in the form of solid solutions with piezoelectric properties [2]. When crystal plates are grown, their composition will continuously change. Along with this, the width of the forbidden zone will also change. Additional alloying of potassium niobate with the formation of solid solutions of perovskites expands the range of converted solar radiation. Potassium niobate is also a nonlinear optical material that allows converting optical radiation by frequency [3].
A method for producing lithium niobate perovskites with a controlled gradient in composition has been tested relatively recently [4]. The detected gradient change in properties (absorption band edges, optical stability, etc.) prompted the application of this method to a new gradient perovskite material, potassium niobate. The study of the optical and physical characteristics of grown potassium niobate crystals, depending on the composition of crystal plates and on the coordinate, is an important stage in the construction of functional radiation conversion devices based on gradient crystals.

Experimental procedure
Potassium niobate crystals were grown by the modified Czochralski method with liquid feeding. Growth equipment -a system of platinum crucibles of various diameters. The system of crucibles was deposited with compositions KxNb2-xO5-2x, where x for the inner and outer crucible differed in accordance with the phase diagram of the K2O-Nb2O5 system.
The melt was heated by induction using a medium-frequency generator with a power of 60 kW with frequency adjustment to the resonance of the inductor-crucible system. The pulling speed of the crystal varied from the minimum at the stage of the cylindrical part to 0.5-1 mm/h at the stage of cone formation.
The grown crystals with a potassium concentration gradient along the length were oriented by X-ray methods and cut into plates for further studies. Cut plates of potassium niobate of various compositions were treated with polishing pastes to form plane-parallel parallelepipeds and plates for electro-optical and optical measurements. Figure 1 shows a typical scheme for determining the half-wave voltage in grown gradient crystals of potassium niobate. To study the dependence of the half-wave voltage on the composition of the crystal by the main components, a helium-neon laser LGN-208A was used, the voltage applied to the crystal was regulated by a power supply unit B5-50 ( Fig.1). The characteristic dependence of the photodetector signal on the applied voltage to the potassium niobate crystal is shown in Fig.2.
It can be seen from Fig.2 that the value of the half-wave voltage was 1500..1700 V in the case of potassium niobate of the composition KxNb2-xO5-2x at x=0.9..1. The orientation of the crystal along <111>, the geometry of the samples is 3mmx3mmx1mm.

Results and discussion
When constructing a model of functional devices for converting radiation into gradient crystals, it should be noted that the use of many physical methods for determining the composition is limited by a significant dependence of physical properties, including the degree of blurring of the maximum on the liquidus-solidus curve, on the degree of intrinsic structural perfection of the crystal and the presence of uncontrolled impurities [5]. In this case, these methods can give different results for crystals that differ in the defect state, with the same ratio of the main components in them. For example, the position of the optical absorption edge can largely be determined by the deficiency or presence of impurities that create optically active energy sublevels in the band gap [6]. One of the most accessible methods of indirect determination of the composition of grown crystals of lithium niobate and potassium niobate is the determination of the UV edge of the absorption band and studies of the IR spectra of OH-group oscillations in the mid-IR range [5,6]. The scheme of the experimental setup for measuring absorption spectra in the mid-IR range is shown in Fig.3   The analysis of vibrations of hydroxyl groups in the mid-IR range allows us to assess the defective structure in potassium niobate. The appearance of hydrogen ions in the crystal lattice may more often be caused as a method of charge compensation in potassium niobate during growth, although it may also be the result of crystal growth in a reducing atmosphere. The presence of a reducing atmosphere causes, among other things, the color in the visible range of potassium niobate single crystals.
When a hydroxyl group is formed as a defect, an intermediate proton combines with the nearest oxygen ion, this defect leads to the appearance of OH absorption bands in the IR region.
Another method for assessing the quality of the grown potassium niobate crystal for the presence of inclusions of foreign phases in it can be the form of the Raman spectra. A typical Raman spectrum of a sample of a single crystal of potassium niobate, in comparison with lithium niobate, contains a frequency shift in the Raman lines [4]. These crystals belong to different types of symmetry, respectively, and the shape of the Raman spectra will be very different, however, in the region of 580 cm -1 and 630 cm -1 , corresponding to the vibrations of oxygen octahedra in lithium niobate, there is a shift in frequencies up to 535 cm -1 and 602 cm -1 in potassium niobate [4].
Another characteristic feature in the Raman spectrum of potassium niobate crystals compared to lithium niobate crystals is the position of the line in the region of stretching bridge vibrations of oxygen atoms or impurity metal cations, whose positions in octahedra may differ [5].
In the vibrational spectrum of lithium niobate, the stretching bridge vibrations of oxygen atoms are located in the frequency range of 850-900 cm -1 [5]. The appearance of bands corresponding to them in the Raman spectrum of potassium niobate indicates a phase of grown potassium niobate single crystals with noncentrosymmetric oxygen octahedra, an orthorhombic phase [4].
In the studied samples of potassium niobate, only one line [4] located at 834 cm -1 was observed in the Raman spectrum, which indicates a high proportion of the structural perfection of the grown single crystals with an equivalent arrangement of the vast majority of the corresponding cations of the same name inside oxygen octahedra.
The IR absorption spectra behave similarly. The performed studies of the transformation of the IR absorption spectra of OH groups should correlate with the transformation of the composition of grown potassium niobate gradient crystals and with the transformation of the electro-optical properties (half-wave voltage). When comparing the IR spectra of hydroxyl groups in lithium niobate and potassium niobate, it can be noted that in the case of potassium niobate at room temperature, there is one band with a maximum in one σ-polarization at 3504 cm -1 and a half-width of 2.4 cm -1 and a band with a maximum at 3507 cm -1 in the other πpolarization and a half-width of 12 cm -1 [4]. According to the literature data, in the case of a polydomain crystal, both bands are observed at once [5].
When light propagates along the direction of the spontaneous polarization vector, a 3507 cm -1 line is observed in the crystal, the intensity of which decreases when propagating in other directions. This result can be explained by the formation of an OH bond along the O-O direction of oxygen octahedra coinciding with the vector of spontaneous polarization. The behavior of the band at 3504 cm -1 with a change in polarization is explained by fluctuations of dipoles along the direction of coupling [5].
It is known from the literature data that this oscillation band of OH groups corresponds to the orthorhombic phase of potassium niobate [5]. When the temperature drops below 260 K, when the crystal is in the rhombohedral phase, a line with a maximum of 3489 cm -1 appears in the IR spectrum of OH-group oscillations.
Such a small half-width of the line with a maximum of 3507 cm -1 in the oscillation spectrum of OH groups (Fig.4) indicates a high degree of structural ordering of the grown crystals and indicates that a monodomain crystal of potassium niobate has been grown.

Conclusions
As a result of the conducted studies, a gradient change in the properties was found: transformations of the IR absorption spectrum, the magnitude of the half-wave voltage depending on the composition of the grown potassium niobate crystal. The value of the halfwave voltage for plates cut from crystalline potassium niobate varied from 1500 to 1700 V with a change in the composition of potassium niobate in the range KxNb2-xO5-2x at x=0.9..1.
The type of IR absorption spectra of grown gradient crystals of potassium niobate is explained by the formation of an OH bond along the O-O direction of oxygen octahedra coinciding with the vector of spontaneous polarization. The behavior of the band at 3504 cm -1 with a change in polarization is explained by fluctuations of dipoles along the direction of coupling, which corresponds to the orthorhombic phase of the grown gradient potassium niobate KxNb2-xO5-2x at x=0.9..1.