Synthesis, structure and properties of K2(1-x)Rb2xAl2B2O7 and Cs1.39Tl0.61Al2B2O7 borates as the basis for preparing new oxide materials

1Baikal Institute of Nature Management, Siberian Branch, RAS, 6 Sakhyanova St., Ulan-Ude, 670047, Russia 2Lomonosov Moscow State University, GSP-1, 1 / 3 Leninskie Gory, Moscow, 119991, Russia 3Kirensky Institute of Physics, Federal Research Center KSC, Siberian Branch, RAS, 50 / 38 Akademgorodok, Krasnoyarsk, 660036, Russia 4Siberian Federal University, 82 Svobodniy Av., Krasnoyarsk, 660041, Russia


Introduction
Borates have attracted considerable attention because they have important practical applications as materials for nonlinear optical (NLO) effects, in particular optical second harmonic generation (SHG) and linear electro-optics effects (EOE) [1,2]. For example, β-BaB 2 O 4 , LiB 3 O 5 , CsB 3 O 5 and YCa 4 (BO 3 ) 3 O are all well-known NLO materials [3]. In 1998, K 2 Al 2 B 2 O 7 (KABO) as a new nonlinear optical crystal was reported by Hu et al. [4,5]. The material crystallizes in space group P321 (Z = 3) with the unit cell parameters a = b = 8.55800(2) Å , c = 8.45576(3) Å [6]. In the structure of KABO, the nearly planar (Al 3 B 3 O 6 ) networks perpendicular to the c-axis are connected to one another by bridging oxygen atoms, and the alkaline cations (K + ) are located between the layers. In the (Al 3 B 3 O 6 ) unit, each Al 3+ cation is linked with three terminal O atoms of the triangle (BO 3 ) 3− groups and a bridging oxygen ion to form a tetrahedral (AlO 4 ) 5− group.
Compared to β-BaB 2 O 4 and CsLiB 6 O 10 , the KABO crystal is free of moisture and possesses smaller walk-off and larger acceptance angles than BBO crystals, stable chemical and physical properties, and good mechanical properties. All of these merits indicate that KABO may be a competitive candidate for the fourth harmonic generation of Nd 3+ doped lasers, such as Nd:YAG, Nd:YLF and Nd:YVO 4 , etc. However, the KABO crystal has a problem concerning abnormal absorptions in the 200 -300 nm regions that greatly reduces the conversion efficiency of the fourth harmonic generation of Nd-based laser [7].
In order to improve the properties of KABO crystals, the structure and properties of doped KABO crystals were studied. The structure of Na-doped KABO was studied by Meng et al. [8 -11], the growth and properties of Fe-doped KABO crystals were studied by Wang et al. [12,13]. K 2 (Al 0.71 Ga 0.29 ) 2 B 2 O 7 (KAGBO) has a slightly smaller SHG coefficient and larger refractive indices [14].
Recently, a solid solution K 2(1-x) Rb 2x Al 2 B 2 O 7 with as high upper limit at room temperature as x ~ 0.8 has been discovered [15]. The vibrational properties and the electronic structure of borate KRbAl 2 B 2 O 7 (KRABO) are investigated in [16]. In [17], the ultraviolet nonlinear optical crystal (NLO) K 0.67 Rb 1.33 Al 2 B 2 O 7 (KRABO) was first obtained by the seed growth method. In this article, we continue to study the properties of KRbAl 2 B 2 O 7 . Also in the present work, a new borate Cs 1.39 Tl 0.61 Al 2 B 2 O 7 , isostructural to Cs 2 Al 2 B 2 O 7 [18] has been obtained.

Experimental
Polycrystalline samples of K 2(1-x) Rb 2x Al 2 B 2 O 7 (x = 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7 and 0.8) and CsTlAl 2 B 2 O 7 were prepared by solid-state synthesis using a mixture of Al(NO 3 ) 3 • 9H 2 O (pure), H 3 BO 3 (chemically pure grade), K 2 CO 3 (chemically pure grade), Rb 2 CO 3 (chemically pure grade), CsNO 3 (pure for analysis), TlNO 3 (pure for analysis) in stoichiometric ratio as starting materials. Initially, to minimize the content of water captured from the environment, the carbonates and nitrates were annealed at 300°C in dry air flow. The mixture of K 2(1-x) Rb 2x Al 2 B 2 O 7 (x = 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7 and 0.8) was ground in an agate mortar and first heated at 300°C, then slowly heated to 800°C for several days [15]. CsTlAl 2 B 2 O 7 was synthesized according to the following annealing scheme. The charges were first fired at 120°C for 1 hour, at 150°C for 1 hour, at 220°C for 3 hours, at 300°C for 5 hours, at 350°C for 15 hours, then chaffed and heated the next time at 400°C for 7 hours, at 450°C for 40 hours, at 520°C for 40 hours, at 600°C for 48 hours, at 700°C for 20 hours, at 800°C for 45 hours, and at 850°C for 25 hours. Repeated grindings are performed between sintering processes to improve the homogeneity of mixing. All products are obtained after natural cooling to room temperature. X-ray powder diffraction was used to identify the product.
The powder X-ray diffraction was recorded on a Bruker D8 ADVANCE X-ray diffractometer (Bruker, Berlin, Germany) with Cu-K α radiation (λ =1.5418 Å ) at room temperature. The scanning range is between 8 and 100° with a scanning width of 0.02 and a rate of 0.1° s −1 .
The thermal property was investigated by the differential scanning calorimetric (DSC) analysis and the thermogravimetric analysis (TGA) using a NETZSCH STA 449C TG / DSC / DTA thermal analyzer (NETZSCH, Berlin, Germany). A 15 mg sample of powder was placed in a Pt crucible and heated from room temperature at a rate of 10°C • min −1 in an Ar atmosphere.
The measurement of second harmonic generation (SHG) was carried out on polycrystalline samples. The minilite-I Nd: YAG laser operating in Q-switched mode at a repetition rate of 10 Hz produced the frequency doubling from λ ω =1064 nm to the second harmonics, λ 2ω = 532 nm in the sample. The green light of the second harmonics was collected with the lens and then directed to the photomultiplier tube. To detect the light only at 532 nm, a narrow band-pass interference filter was attached to the photomultiplier. The measured signal intensity (I 2ω ) from the sample was calibrated in relation to the α-quartz powder standard with a grain size of 3 µm. Q = I 2ω / I 2ω (SiO 2 ) quantitatively presents the SHG activity of the sample.
The dielectric loss tangent (tgα) was measured on a Novocontrol Beta-N impedance-analyzer in a ProboStat measuring cell using the double-contact method [19] in the frequency range of 0.3 Hz −1 MHz on heating at 20 -850°С with 2°С • min −1 ; ceramic pellets were 10 mm in diameter and 2 mm in height.
For all samples, a mass loss and one endothermic effect, which corresponds to the temperature of the phase transition, was observed in the temperature range of 900 -1000°C (Fig. 1, 2). The mass loss of the sample is due to the decomposition of K 2(1-x) Rb 2x Al 2 B 2 O 7 (x = 0.1-0.8).
The dependence of the temperature of phase transitions of solid solutions K 2(1-x) Rb 2x Al 2 B 2 O 7 on the parameter x is shown in Fig. 2.
An increase in the content of rubidium leads to a decrease in the temperature of the phase transition. It is known that there was a KABO-type structure detected for a powder sample with a nominal composition x = 0.9 and the solubility limit in K 2(1-x) Rb 2x Al 2 B 2 O 7 crystals can be estimated as x ~ 0.83 -0.9 at ambient conditions. In this connection, the temperature of the phase transition only for the composition x = 0.8 was determined.
A decrease in the temperature of the phase transition agrees with the fact that an increase in the rubidium content results in a quasilinear increase of the unit cell parameters and the cell volume [15].
A significant effect of SHG was found at room temperature for K 0.6 Rb 1.4 Al 2 B 2 O 7 Q = 70 (Fig. 3). The signals confirm the absence of an inversion center. Then the effect of SHG increases to 85 at a temperature of 645°C and remains constant with a further increase in temperature. When cooled, the effect of SHG slightly decreases, but it does not drop below I 2ω / I 2ω (SiO 2 ) = 65. Fig. 4 shows the temperature dependences of the dielectric loss tangent for K 0.6 Rb 1.4 Al 2 B 2 O 7 . The maxima of dielectric losses characteristic of the relaxational type, which with increasing frequency shift to higher temperatures, are revealed. Anomalies in dielectric loss tangent were revealed in the vicinity of 645°C, independent of the measurement frequency.
The diffraction data for the Rietveld analysis were collected at room temperature (27°C) with a Bruker D8 ADVANCE powder diffractometer in the Bragg-Brentano geometry and a Vantec linear detector. The operating parameters were:    Cu-K α radiation, step size 0.02°. Data were collected in the range of 2θ from 8 to 100°. Peak positions were determined with the EVA program, available in the PC DIFFRAC-PLUS software package supplied by Bruker. X-ray patterns of the title compound were indexed using the ITO program [20]. Replacement of cesium atoms by thallium ones in the compound Cs 2 Al 2 B 2 O 7 facilitates the formation of the phase Cs 1.39 Tl 0.61 Al 2 B 2 O 7 . For the finer structure analysis of Cs 1.39 Tl 0.61 Al 2 B 2 O 7 the single crystal data on the phase Cs 2 Al 2 B 2 O 7 , which crystallizes in the space group P2 1 / c , were used [18]. The refinement of atomic positions and occupancy of atoms leads to the formula: Cs 1.39 Tl 0.61 Al 2 B 2 O 7 .
The experimental and theoretical X-ray diffraction patterns obtained for Cs 1.39 Tl 0.61 Al 2 B 2 O 7 are shown in Fig. 5.
A good relation between experimental and theoretical curves is evident.
The parameters of the refinement and atomic coordinates of Cs 1.39 Tl 0.61 Al 2 B 2 O 7 are shown in Tables 1 and 2, respectively. The main lengths of interatomic bonds are given in Table S1 (Supplementary Material).
The structure of Cs 1.39 Tl 0.61 Al 2 B 2 O 7 is shown in Fig. 6. This is a three-dimensional framework built from cornersharing AlO 4 tetrahedra and BO 3 triangles with channels occupied by the Cs + and Tl + cations. The structure can be considered to be built up from the nearly planar [Al 2 B 2 O 10 ] rings, which are composed of two AlO 4 tetrahedra and two BO 3 triangles, connected alternately to each other by cornersharing.
Rb The DSC (Differential Scanning Calorimetry) curve exhibits one endothermic peak on the heating curve at 900°C, accompanied by the apparent weight loss observed on the TG (Thermogravimetric Analysis) curve (Fig. 7). The XRD pattern indicates that the molten residues were mainly raw materials instead of the original compound, which clearly demonstrates that 900°C is the decomposition temperature. Hence, Cs 1.39 Tl 0.61 Al 2 B 2 O 7 melts incongruently.    7 was synthesized and its structure in space group P2 1 / c was determined through Rietveld X-ray powder diffraction analysis.