Growth, Structure and Optical Characterization of Rb3Ti3P5O20 Single Crystal

Phosphate crystals attract much attention on account of their rich crystal structures and excellent physical and chemical properties. Herein, Rb3Ti3P5O20 single crystals were grown by the high temperature solution method using Rb2CO3 and NH4H2PO4 as the fluxes. This crystal, with non-centrosymmetric Pca21 space group, presents a three-dimensional framework structure composed of [TiO6] octahedron, [PO4] tetrahedra, and [P2O7] dimers. The electronic structure was measured via X-ray photoelectron spectroscopy. The measurements found that Rb3Ti3P5O20 has stronger Ti–O ionic bonding properties and weaker P–O covalent bonding properties compared to RbTiOPO4. Optical measurements indicated that Rb3Ti3P5O20 has a 3.54 eV band gap and a wide transmission range (0.33–4.5 μm). Theoretical calculations showed that Rb3Ti3P5O20 crystals have a moderate birefringence of 0.079 at 1064 nm. In addition, the relationship of the structure–property was studied using first-principles method. The results demonstrated that TiO6 octahedron played a significant role for the optical properties.


Introduction
Inorganic phosphate materials have received considerable attention due to their rich structures and excellent physicochemical properties, as well as possessing broad applications in the fields of nonlinear optics, ferroelectricity, luminescence, energy technologies, and catalysts [1][2][3][4][5]. For instance, KTiOPO 4 (KTP) and KH 2 PO 4 (KDP) crystals have been used for solid-state lasers as important nonlinear optical materials, and olivine-type LiFePO 4 is an outstanding cathode material for rechargeable Lithium batteries with high capacity [6][7][8]. In the structure of phosphate materials, one P atom is generally coordinated with four O atoms to shape into the [PO 4 ] tetrahedra. These [PO 4 ] tetrahedra units are also linked into different phosphorus oxygen groups by sharing oxygen atoms such as bisphosphonates, polyphosphate, and (PO 3 ) ∞ chains. These P-O groups can bond with other ionic polyhedrons to build three-dimensional framework structures [2,4,9].
Among the phosphates, rubidium titanyl phosphate (RTP) crystal has significant technological applications in electro-optic shutters, Q-switches, and pulse selectors based on its large dielectric constant, high laser damage threshold, and stable physicochemical properties [10,11]. The three-dimensional crystal structure of RTP is made up of an alternating connection of [TiO 6 ] octahedral groups and [PO 4 ] tetrahedral groups, and the resulting cavities are occupied by the alkali metal Rb atoms [12,13]. Research indicates that the large-distortion of [TiO 6 ] octahedron in the structure plays a major role in the optical properties of titanium-containing phosphates [14]. As well as RTP, RbTiP 2 O 7 and RbTi 2 (PO 4 ) 3 materials, containing Rb, Ti, P, and O elements, were previously reported because of the potential technological interest [15,16].
As new kinds of phosphates, Me 3 Ti 3 P 5 O 20 (Me stands for alkaline-earth metals) materials are non-centrosymmetric and contain [TiO 6 ] and [PO 4 ] functional units in the structure. K 3 Ti 3 P 5 O 20 crystal was first reported by Nagornyi et al. in 1993 and showed the potential application as a valuable NLO optical material [17]. Rb 3 Ti 3 P 5 O 20 crystal is the second material reported for Me 3 Ti 3 P 5 O 20 compounds, after K 3 Ti 3 P 5 O 20 [18]. However, there are few studies on Rb 3 Ti 3 P 5 O 20 crystals. The growth of bulk crystals and the property characterization were not carried out. In this work, Rb 3 Ti 3 P 5 O 20 single crystals were grown by the high temperature solution growth (HTSG) method and measured via powder X-ray diffraction (XRD) and Infrared (IR) spectroscopy. The electronic structure was studied by X-ray photoelectron spectroscopy (XPS). Linear optical properties and nonlinear optical property (frequency-doubled effect) were investigated. With the aim to better explain the structure-property relation, the density functional theory (DFT) method was applied to compute the density of states (DOS), energy band structure, and linear refractive indices (n).  PO 4 , and TiO 2 were thoroughly mixed and adequately ground in a molar ratio of 9:20:3 and put into a Pt pot [18]. First, the Pt pot with the batch was heated to 700 • C in the furnace and held for 12 h until all the decomposed gases (NH 3 , CO 2 ) were gone. Second, the mixture was put into a programmable temperature vertical resistance wire heating furnace and heated to 950 • C for 48 h. A platinum agitator blade was put into the solution for mechanical stirring for at least 36 h in order to increase the solution homogeneity. Third, the temperature of the solution was slowly decreased to 800 • C over two weeks and finally dropped to room temperature for seven days. Transparent colorless crystals were obtained from the excess flux by washing the cake in water.

Characterization
The XRD pattern was recorded by employing a Rigaku SmartLab 9KW X-ray diffractometer (Tokyo, Japan) set with the use of a Copper target X-ray source (λ = 0.15418 nm) in the diffraction angle range (2θ) of 10-60 • . The measurement had a scanning step size of 0.02 • and a scanning step time of 0.4 s at room temperature. A bulk transparent Rb 3 Ti 3 P 5 O 20 single crystal (0.12 × 0.1 × 0.1 mm 3 ) was chosen for the analysis of single crystal structure. The single crystal diffraction data collection was recorded via a Bruker D8 VENTURE PHOTON 100 diffractometer (Karlsruhe, Germany) with the use of a monochromatic Molybdenum target X-ray source (λ = 0.71073 Å) at 296.15 K. The crystal structure was resolved with the SHELXT structure solution program via Intrinsic Phasing and polished up with the SHELXL refinement package by making use of least square minimization [19]. The precise crystallographic data and detailed experimental conditions of Rb 3 Ti 3 P 5 O 20 crystal are listed in Table 1. Electron Probe Microanalysis (EPMA) was measured by a Shimadzu EPMA-1720H (Kyoto, Japan) for compositional analysis of the Rb 3 Ti 3 P 5 O 20 crystal. A well-polished wafer was chose for measurement. XPS spectra were surveyed by an X-ray photoelectron spectrometer (Thermo Fisher ESCALAB 250, Waltham, MA, USA) in an ultra-high vacuum (<10 −7 Pa) with the use of a monochromatic aluminum target X-ray source. The excess charging on the sample surface during the measurement was neutralized by the neutralization gun [20]. IR spectra were measured with a Nicolet NEXUS 670 infrared spectrometer (Thermo Nicolet Corporation, Madison, WI, USA). Fully ground crystalline powders were used for testing. Ultraviolet-Visible (UV-Vis) diffuse reflectance data for Rb 3 Ti 3 P 5 O 20 crystals were obtained by using a Hitachi UH4150 spectrophotometer (200-800 nm, Tokyo, Japan). The optical transmission spectra were measured on a Hitachi UH4150 UV-Vis-IR spectrometer (200-2000 nm) and a Nicolet NEXUS 670 FTIR spectrometer (2000-5000 nm) using a wellpolished crystal wafer of Rb 3 Ti 3 P 5 O 20 . The frequency-doubling effect of Rb 3 Ti 3 P 5 O 20 crystal was tested on a Q-switched Nd:YAG solid-state laser under a 1064 nm wavelength by the Kurtz-Perry technique, and a KDP was used as a reference [21]. The test conditions were not changed after the test start to ensure the accuracy of the results.

Computational Details
The electronic structure of Rb 3 Ti 3 P 5 O 20 was calculated by the ultra-soft pseudopotential method using DFT in the CASTEP module including density of states (DOS), band structure (Eg) and refractive indices (n) [22]. The 3D crystallographic structure file from single crystal structure determination of Rb 3 Ti 3 P 5 O 20 crystals was used for optimization and calculation. During the numerical calculation, generalized gradient approximation and Perdew-Burke-Ernzerhof functions were used to optimize the total energy of the system to a minimum. Thereby, the Rb 4p 6 5s 1 , Ti 3p 6 4s 2 3d 2 , P 3s 2 3p 3, and O 2s 2 2p 4 states were considered as the valence electrons. The K point mesh was set as 2 × 6 × 3 in the Brillouin region and the cut-off limit of kinetic energy was set at 350 eV. The refractive indices and birefringence of the Rb 3 Ti 3 P 5 O 20 crystal were computed based on the obtained band structure and DOS.

Crystal Growth
Due to the high thermal stability of the Rb 3 Ti 3 P 5 O 20 crystal, it can be grown below the melting point using the high temperature solution method [18]. To obtain high quality crystals, it is extremely vital to choose an appropriate flux. Previously, for decreasing the viscosity of the melt and the melting point, Rb 2 CO 3 and NH 4 H 2 PO 4 fluxes were utilized in our laboratory to grow high quality single crystals of pure RTP and doped-RTP [13,23,24]. Therefore, we have chosen Rb 2 CO 3 and NH 4 H 2 PO 4 to serve as fluxes for growing Rb 3 Ti 3 P 5 O 20 single crystals. As a result, the millimeter-sized transparent colorless single crystals were successfully obtained. Figure 1a shows the photograph of as-grown Rb 3 Ti 3 P 5 O 20 crystals with dimensions of 7 × 2 × 1 mm 3 and 3 × 3 × 1 mm 3 . The experimental powder XRD of Rb 3 Ti 3 P 5 O 20 crystal is plotted in Figure 1b. The diffraction peak positions of as-grown crystals correspond well to a standard Rb 3 Ti 3 P 5 O 20 pattern (PDF No. 82-1169) [18], suggesting that the Rb 3 Ti 3 P 5 O 20 crystals possess not only high purity but also good crystallinity. Meanwhile, the Rietveld refinement of XRD was carried out by GSAS 2 software and the final plot is shown in Figure 2. The compositional analysis of Rb 3 Ti 3 P 5 O 20 crystal shows that elemental ratios of Rb, Ti, P, and O are 3.55:3.25:6.12:20.42. birefringence of the Rb3Ti3P5O20 crystal were computed based on the obtained band structure and DOS.

Crystal Growth
Due to the high thermal stability of the Rb3Ti3P5O20 crystal, it can be grown below the melting point using the high temperature solution method [18]. To obtain high quality crystals, it is extremely vital to choose an appropriate flux. Previously, for decreasing the viscosity of the melt and the melting point, Rb2CO3 and NH4H2PO4 fluxes were utilized in our laboratory to grow high quality single crystals of pure RTP and doped-RTP [13,23,24]. Therefore, we have chosen Rb2CO3 and NH4H2PO4 to serve as fluxes for growing Rb3Ti3P5O20 single crystals. As a result, the millimeter-sized transparent colorless single crystals were successfully obtained. Figure 1a shows the photograph of as-grown Rb3Ti3P5O20 crystals with dimensions of 7 × 2 × 1 mm 3 and 3 × 3 × 1 mm 3 . The experimental powder XRD of Rb3Ti3P5O20 crystal is plotted in Figure 1b. The diffraction peak positions of as-grown crystals correspond well to a standard Rb3Ti3P5O20 pattern (PDF No. 82-1169) [18], suggesting that the Rb3Ti3P5O20 crystals possess not only high purity but also good crystallinity. Meanwhile, the Rietveld refinement of XRD was carried out by GSAS 2 software and the final plot is shown in Figure 2. The compositional analysis of Rb3Ti3P5O20 crystal shows that elemental ratios of Rb, Ti, P, and O are 3.55:3.25:6.12:20.42.  birefringence of the Rb3Ti3P5O20 crystal were computed based on the obtained band structure and DOS.

Crystal Growth
Due to the high thermal stability of the Rb3Ti3P5O20 crystal, it can be grown below the melting point using the high temperature solution method [18]. To obtain high quality crystals, it is extremely vital to choose an appropriate flux. Previously, for decreasing the viscosity of the melt and the melting point, Rb2CO3 and NH4H2PO4 fluxes were utilized in our laboratory to grow high quality single crystals of pure RTP and doped-RTP [13,23,24]. Therefore, we have chosen Rb2CO3 and NH4H2PO4 to serve as fluxes for growing Rb3Ti3P5O20 single crystals. As a result, the millimeter-sized transparent colorless single crystals were successfully obtained. Figure 1a shows the photograph of as-grown Rb3Ti3P5O20 crystals with dimensions of 7 × 2 × 1 mm 3 and 3 × 3 × 1 mm 3 . The experimental powder XRD of Rb3Ti3P5O20 crystal is plotted in Figure 1b. The diffraction peak positions of as-grown crystals correspond well to a standard Rb3Ti3P5O20 pattern (PDF No. 82-1169) [18], suggesting that the Rb3Ti3P5O20 crystals possess not only high purity but also good crystallinity. Meanwhile, the Rietveld refinement of XRD was carried out by GSAS 2 software and the final plot is shown in Figure 2. The compositional analysis of Rb3Ti3P5O20 crystal shows that elemental ratios of Rb, Ti, P, and O are 3.55:3.25:6.12:20.42.

Crystal Structure
The structure of the Rb 3 Ti 3 P 5 O 20 crystal was obtained by single crystal XRD analysis. It belongs to an orthogonal crystal system with a polar space group of Pca2 1 (No. 29) with the unit cell parameters a = 18.2967(17) Å, b = 6.3043(5) Å, c = 14.7942(15) Å, and Z = 4. The unit cell parameters are similar to those reported in a paper (a = 18.282(2) Å, b = 6.2932(7) Å, c = 14.773(2) Å) [18]. The structure diagram of the Rb 3 Ti 3 P 5 O 20 crystal along the b-axis is plotted in Figure 3a

Crystal Structure
The structure of the Rb3Ti3P5O20 crystal was obtained by single crystal XRD analysis. It belongs to an orthogonal crystal system with a polar space group of Pca21 (No. 29) with the unit cell parameters a = 18.2967(17) Å, b = 6.3043(5) Å, c = 14.7942(15) Å, and Z = 4. The unit cell parameters are similar to those reported in a paper (a = 18.282(2) Å, b = 6.2932 (7) Å, c = 14.773(2) Å) [18]. The structure diagram of the Rb3Ti3P5O20 crystal along the b-axis is plotted in Figure 3a  For the purpose of further verifying the types of phosphorus oxygen groups in Rb3Ti3P5O20 structure, IR spectra ( Figure 3b) were measured for the 600-3000 cm −1 wavenumbers. The peaks of 1232, 1206, and 1166 cm −1 are attributable to the asymmetric stretching vibrations of P-O bonds. The IR spectra, ranging from 800 to 1100 cm -1, are assigned to the symmetric stretching vibrations and the asymmetric stretching vibrations of P-O-P. The peak of 712 cm −1 is caused by the symmetric stretching vibrations of P-O-P. Hence, the IR spectra specify the existence of [PO4] tetrahedron and [P2O7] dimer, which coincide with the results obtained from the single crystal structure analysis of related phosphates [25,26].

Electronic Structure
The electronic structure of the Rb3Ti3P5O20 crystal was first measured by XPS and analyzed. The survey spectrum recorded for the Rb3Ti3P5O20 single crystal is shown in Figure  4a. The characteristic peaks of all constituent elements (Rubidium, Titanium, Phosphorus, and Oxygen) were found in the survey spectrum. The C 1s peak (284.6 eV) was attributed to hydrocarbonate contamination, and the line was used as a reference for the binding energy scale calibration. The calcium element was also tested as surface contamination. For the purpose of further verifying the types of phosphorus oxygen groups in Rb 3 Ti 3 P 5 O 20 structure, IR spectra ( Figure 3b) were measured for the 600-3000 cm −1 wavenumbers. The peaks of 1232, 1206, and 1166 cm −1 are attributable to the asymmetric stretching vibrations of P-O bonds. The IR spectra, ranging from 800 to 1100 cm -1, are assigned to the symmetric stretching vibrations and the asymmetric stretching vibrations of P-O-P. The peak of 712 cm −1 is caused by the symmetric stretching vibrations of P-O-P. Hence, the IR spectra specify the existence of [PO 4 ] tetrahedron and [P 2 O 7 ] dimer, which coincide with the results obtained from the single crystal structure analysis of related phosphates [25,26].

Electronic Structure
The electronic structure of the Rb 3 Ti 3 P 5 O 20 crystal was first measured by XPS and analyzed. The survey spectrum recorded for the Rb 3 Ti 3 P 5 O 20 single crystal is shown in Figure 4a. The characteristic peaks of all constituent elements (Rubidium, Titanium, Phosphorus, and Oxygen) were found in the survey spectrum. The C 1s peak (284.6 eV) was attributed to hydrocarbonate contamination, and the line was used as a reference for the binding energy scale calibration. The calcium element was also tested as surface contamination. Materials 2022, 15, x FOR PEER REVIEW 6 of 11 The deconvoluted high-resolution XPS spectra [27,28], of major elements in Rb3Ti3P5O20 crystal are shown in Figure 4b-e. The binding energy (BE) values and the BE difference of the constituent elements of the title compound are presented in Table 2. RbTi-OPO4 (RTP) and KTiOPO4 (KTP) are given for comparison. The BE values of Rb 3d5/2, Ti 2p3/2, P 2p, and O 1s for Rb3Ti3P5O20 crystal are 109.3 eV, 458.6 eV,132.7 eV, and 530.1 eV, respectively. The main peak of O 1s at 530.1 eV corresponds to lattice oxygen. The weak peak at 531.8 eV is usually attributed to defects and contamination (such as H2O and CO2) of the material surface. The BE values are similar to those of the corresponding elements in RTP and KTP crystals, and this indicates that the atoms are in similar chemical environments [29,30]. The BE difference ΔBE (M-O) = BE (O 1s)-BE(M), where M is the element to be analyzed, is usually used to evaluate the chemical bonding of the elements in crystal lattice because the parameters are insensitive to the surface charging effects [31][32][33]. As for Rb3Ti3P5O20, the BE difference values were calculated to be as follows: ΔBE

Optical Properties Characterization
The UV-Vis diffuse reflectance spectrum of Rb 3 Ti 3 P 5 O 20 crystals is shown in Figure 5a. The absorption data were calculated based on reflection spectra by using the Kubelka-Munk transformation equation: where S is the scattering coefficient, K is the absorption coefficient, and R is the reflectance [34]. The optical band gap of Rb 3 Ti 3 P 5 O 20 crystals is estimated to be 3.54 eV, which is corresponding to the absorption edge of 339 nm. For further obtaining a precise value of the absorption edge, the transmission spectrum (200-5000 nm) of a well-polished wafer was recorded. Figure 5b demonstrates  The UV-Vis diffuse reflectance spectrum of Rb3Ti3P5O20 crystals is shown in Figure  5a. The absorption data were calculated based on reflection spectra by using the Kubelka-Munk transformation equation: where S is the scattering coefficient, K is the absorption coefficient, and R is the reflectance [34]. The optical band gap of Rb3Ti3P5O20 crystals is estimated to be 3.54 eV, which is corresponding to the absorption edge of 339 nm. For further obtaining a precise value of the absorption edge, the transmission spectrum (200-5000 nm) of a well-polished wafer was recorded. Figure 5b demonstrates that the transmission cutoff edge can reach down to 331 nm in the UV region, which is shorter compared with the RTP crystal (350 nm). The IR region cutoff edge of Rb3Ti3P5O20 crystal is located at 4.5 μm. This shows the similarity to the KTP and RTP crystals. The Rb3Ti3P5O20 crystal is of high transparency in the UV-Vis-NIR region and has a wide optical transparency range of 0.33-4.5 μm. The step near 800 nm is caused by a test instrument (light source change). The absorption band at 3000 nm may be related to the formation of hydrogen bonds in the crystal (stretching vibrations of the O-H bond). The SHG response was measured because Rb3Ti3P5O20 crystal is non-centrosymmetric. The result is plotted in Figure 6. The SHG intensity is about 0.4 × KDP for a particle size range of 70-90 μm. The magnitudes of distortions of the TiO6 octahedra were calculated using the method proposed by P. Shiv Halasyamani [35].  The SHG response was measured because Rb 3 Ti 3 P 5 O 20 crystal is non-centrosymmetric. The result is plotted in Figure 6. The SHG intensity is about 0.4 × KDP for a particle size range of 70-90 µm. The magnitudes of distortions of the TiO 6 octahedra were calculated using the method proposed by P. Shiv Halasyamani [35].

Theoretical Calculations
To explore the structure-property relationship of the Rb3Ti3P5O20 crystals, the band structure and DOS were calculated based on the DFT method. As shown in Figure 7a, Rb3Ti3P5O20 is an indirect band gap material with an energy band gap width of 2.79 eV, which is smaller than the test value (3.54 eV). It can be attributed to the underestimation of the band gap by the DFT method [36]. The calculated total densities of states and partial densities of states (TDOS and PDOS) are plotted in Figure 7b. Optical properties of crystals are closely related to electronic transitions near the Fermi level (or the forbidden band). Therefore, it is vital to investigate the valence band top (VBT) and the conduction band bottom (CBB). From the spectra shown in Figure 7b, it can be inferred that: (1) the orbital effect of alkali metal cations (Rb + ) to electronic transitions near the forbidden band can be

Theoretical Calculations
To explore the structure-property relationship of the Rb 3 Ti 3 P 5 O 20 crystals, the band structure and DOS were calculated based on the DFT method. As shown in Figure 7a, Rb 3 Ti 3 P 5 O 20 is an indirect band gap material with an energy band gap width of 2.79 eV, which is smaller than the test value (3.54 eV). It can be attributed to the underestimation of the band gap by the DFT method [36]. The calculated total densities of states and partial densities of states (TDOS and PDOS) are plotted in Figure 7b. Optical properties of crystals are closely related to electronic transitions near the Fermi level (or the forbidden band). Therefore, it is vital to investigate the valence band top (VBT) and the conduction band bottom (CBB).

Theoretical Calculations
To explore the structure-property relationship of the Rb3Ti3P5O20 crystals, the band structure and DOS were calculated based on the DFT method. As shown in Figure 7a, Rb3Ti3P5O20 is an indirect band gap material with an energy band gap width of 2.79 eV, which is smaller than the test value (3.54 eV). It can be attributed to the underestimation of the band gap by the DFT method [36]. The calculated total densities of states and partial densities of states (TDOS and PDOS) are plotted in Figure 7b. Optical properties of crystals are closely related to electronic transitions near the Fermi level (or the forbidden band). Therefore, it is vital to investigate the valence band top (VBT) and the conduction band bottom (CBB). From the spectra shown in Figure 7b, it can be inferred that: (1) the orbital effect of alkali metal cations (Rb + ) to electronic transitions near the forbidden band can be From the spectra shown in Figure 7b, it can be inferred that: (1) the orbital effect of alkali metal cations (Rb + ) to electronic transitions near the forbidden band can be negligible.
(2) the electronic states of VBT between -5.0 eV and 0.0 eV are mainly constituted by O 2p orbitals and a spot of P 3p orbitals. (3) Figure 8, the refractive indices are characterized by a considerable anisotropy and n x > n z > n y , which indicates that the Rb 3 Ti 3 P 5 O 20 is a biaxial crystal. The values of the birefringence are 0.079 at 1064 nm and 0.104 at 532 nm. The relatively large birefringence at 1064 nm of Rb 3 Ti 3 P 5 O 20 is close to the value of KTP (experimental ∆n = 0.092 at 1064 nm).  Figure 8, the refractive indices are characterized by a considerable anisotropy and nx > nz > ny, which indicates that the Rb3Ti3P5O20 is a biaxial crystal. The values of the birefringence are 0.079 at 1064 nm and 0.104 at 532 nm. The relatively large birefringence at 1064 nm of Rb3Ti3P5O20 is close to the value of KTP (experimental Δn = 0.092 at 1064 nm).

Conclusions
In summary, large Rb3Ti3P5O20 single crystals with dimensions of 7 × 2 × 1 mm 3 and 3 × 3 × 1 mm 3 were successfully grown via the high temperature solution growth method in the Rb2O-TiO2-P2O5 system. The structural analysis indicates that there are two anionic groups [PO4] and [P2O7] in the structure of Rb3Ti3P5O20 crystal. The optical properties of the crystal, including UV-Vis diffuse reflectance spectra and transmission spectra, were studied for the first time. The results demonstrated that Rb3Ti3P5O20 displays a wide transparent range of 0.33-4.5 μm and has a relatively large band gap of 3.54 eV. The second harmonic generation measurement showed that the SHG intensity is about 0.4 × KDP. The weak distortions of [TiO6] octahedra might result in the weak SHG response. The band structure, the density of states, and the dispersive refractive indices were calculated by first principles calculations. The DOS spectra of Rb3Ti3P5O20 show that the valence band top is formed mainly by O 2p orbitals and the conduction band bottom is contributed to by Ti 3d orbitals. Therefore, distorted [TiO6] octahedra contributes considerably to the optical properties. Furthermore, Rb3Ti3P5O20 has a moderate birefringence of 0.079 at 1064nm. This work may be used as a reference for feasible optical application prospects of Rb3Ti3P5O20 crystals.

Conclusions
In summary, large Rb 3 Ti 3 P 5 O 20 single crystals with dimensions of 7 × 2 × 1 mm 3 and 3 × 3 × 1 mm 3 were successfully grown via the high temperature solution growth method in the Rb 2 O-TiO 2 -P 2 O 5 system. The structural analysis indicates that there are two anionic groups [PO 4 ] and [P 2 O 7 ] in the structure of Rb 3 Ti 3 P 5 O 20 crystal. The optical properties of the crystal, including UV-Vis diffuse reflectance spectra and transmission spectra, were studied for the first time. The results demonstrated that Rb 3 Ti 3 P 5 O 20 displays a wide transparent range of 0.33-4.5 µm and has a relatively large band gap of 3.54 eV. The second harmonic generation measurement showed that the SHG intensity is about 0.4 × KDP. The weak distortions of [TiO 6 ] octahedra might result in the weak SHG response. The band structure, the density of states, and the dispersive refractive indices were calculated by first principles calculations. The DOS spectra of Rb 3 Ti 3 P 5 O 20 show that the valence band top is formed mainly by O 2p orbitals and the conduction band bottom is contributed to by Ti 3d orbitals. Therefore, distorted [TiO 6 ] octahedra contributes considerably to the optical properties. Furthermore, Rb 3 Ti 3 P 5 O 20 has a moderate birefringence of 0.079 at 1064 nm. This work may be used as a reference for feasible optical application prospects of Rb 3 Ti 3 P 5 O 20 crystals.