ZHONG Xiao-Yu WU En-Qiang YANG Sheng-Dong WANG Hao-Hang LIN Xiao-Xin SHEN Yao-Guo

(College of Physics & Electronic Information Engineering, Minjiang University, Fuzhou 350108, China)

ABSTRACT In this paper, a non-centrosymmetric compound of Rb2Ca2(SO4)3 has been synthesized by a high temperature solid-state reaction and high temperature melting method. Single-crystal X-ray diffraction analysis shows that Rb2Ca2(SO4)3 crystallizes in the cubic space group of P213, and its cell parameters are a = b = c = 10.5569(6) Å, Z= 4 and V = 1176.55(12) Å3, respectively. In the crystal structure, SO4 tetrahedra and CaO6 octahedra are connected with each other by a corner-sharing mode to construct the three-dimensional framework of Rb2Ca2(SO4)3. Optical measurements show that the title compound has a short ultraviolet absorption edge and a moderate second-harmonic generation response. The optical origin is illustrated by the electron band structure calculation. In addition, thermal stability is also studied by virtue of differential thermal/thermogravimetric analysis and powder XRD technique.

Keywords: crystal structure, nonlinear optics, solid-state reaction, sulfate;

1 INTRODUCTION

Nonlinear optical (NLO) crystals are important photoelectrical information functional materials in information optics, photolithography, medical treatment and other fields. A commercially available short-wave NLO material needs to meet some conditions, such as a non-centrosymmetric structure, short cut-off edge, large second-harmonic generation(SHG) response, good thermal stability, and crystal easy to grow. In addition, the use of toxic elements should be avoided in the process of crystal preparation[1]. According to the inorganic crystal structure database, most reported compounds have a central symmetry structure, and the probability of obtaining non-centrosymmetric compounds is relatively low[2].Consequently, it is still a challenge to design and construct a new NLO material which can work in the short-wave spectral region[3].

In the past few decades, the search for short-wave NLO materials has been typically limited to borates, because NLO-active triangle BO3groups in borates have comprehensive performance[4]. Many excellent NLO borates have been synthesized like KBe2BO3F2[5], Li6Zn3(BO3)4[6]and Li4Sr(BO3)2[7]. Due the short ultraviolet cut-off edge of PO4groups, phosphates have also received the attention of researchers, and some short-wave NLO phosphates with good performance are reported recently, such as Ba3P3O10X (X =Cl, Br)[8], LiCs2PO4[9], LiRb2PO4[10]and Na3Cd3B(PO4)4[11].Because SO4groups have the same tetrahedral configuration as PO4groups, sulfates are expected to have similar NLO properties as phosphates. Introducing ammonium ions and alkali metal elements into sulfates, Luo group reported two NLO sulfates of NH4NaLi(SO4)2and (NH4)2Na3Li9(SO4)7in 2019[8]. Both compounds can be used in deep ultraviolet region, and their SHG responses reach 1.1 and 0.5 times that of KH2PO4, respectively, indicating that sulfates are expected to be good candidates for deep ultraviolet NLO materials.

After investigation and research, it is reported that alkali metal and alkali-earth metal elements can improve the thermal stability of compounds[3]. Therefore, we screened the alkali metal and alkali-earth metal sulfate systems in the inorganic crystal structure database and selected Rb2Ca2(SO4)3[12]as the subject of study. In this context, the thermal stability, SHG effect and diffuse reflectance spectrum of Rb2Ca2(SO4)3will be studied. In addition, its optical origin is analyzed by virtue of structure and theoretical calculation.

2 EXPERIMENTAL

2. 1 Materials and methods

Rb2SO4(99.0%) and CaSO4(99.0%) were purchased from Aladdin and used without further purification. Differential thermal/Thermogravimetric analysis (DTA/TGA) was tested on a NETZSCH STA 449F3 simultaneous analyzer with bubbling nitrogen as the purge gas. The powder X-ray diffraction (XRD) data were collected on a Rigaku MiniFlex II diffractometer (CuKα radiation). The sampling range,interval and scanning rate are 2θ = 10º～70º, 0.02º and 0.4 º∙min-1, respectively. UV/Vis/NIR diffuse reflectance spectrum was collected using a PerkinElmer lamda-950 UV/Vis/NIR dispersive photometer at ambient temperature.

2. 2 Synthesis of Rb2Ca2(SO4)3

Pure polycrystalline Rb2Ca2(SO4)3was synthesized by a convenient high temperature solid-state reaction[12]. First, the above-mentioned chemical reagents were weighed at the molar ratio of Rb2SO4/CaSO4= 1/2, and then ground evenly with an agate mortar. Secondly, the mixture after grinding was placed in a platinum crucible followed by sintering at 973 K for 5000 min. In the calcining process, the mixture was ground twice. The purity of the prepared product was monitored by powder XRD analysis.

The technique of spontaneous crystallization is used to prepare Rb2Ca2(SO4)3single crystal. Firstly, Rb2SO4and CaSO4were mixed in the molar ratio of 1/3, and roasted at 1073 K in the electric stove for 2000 min. The nominal polycrystalline Rb2Ca3(SO4)4was obtained after cooling to room temperature. Secondly, the target compound of Rb2Ca2(SO4)3was synthesized by melting the mixture of Rb2Ca3(SO4)4and Rb2SO4in the molar ratio of Rb2SO4/Rb2Ca2(SO4)3= 1/3. Transparent Rb2Ca2(SO4)3single crystal can be selected to determine the single-crystal structure.

2. 3 X-ray structure determination

A colorless block crystal was selected and stuck on a glass fiber for data collection performed on a Bruker D8 diffractometer at 200(2) K using graphite-monochromatic MoKα radiation (λ = 0.71073 Å). Data simplification and absorption correction were performed on APEX3 program.In SHELXS program, the structure was solved by direct methods and then refined by full-matrix least-squares program SHELXL[13]including anisotropic displacement parameters. Finally, the rationality of the structure was tested by PLATON[14]. Crystal data for Rb2Ca2(SO4)3(Mr= 539.28 g/mol): cubic system, space group P213, a = 10.5569(6), b =10.5569(6), c = 10.5569(6) Å, V = 1176.55(12) Å3, Z = 4, T= 200(2) K, μ(MoKα) = 9.780 mm-1, Dc= 3.044 g/cm3, 7615 reflections measured (3.34º≤θ≤27.41º) and 904 unique(Rint= 0.0399, Rsigma= 0.0379) which were used in all calculations. The final R = 0.0174 (I > 2σ(I)) and wR =0.0391 (all data).

2. 4 Second-harmonic generation measurement

Using the innovative Kurtz and Perry method[15], the SHG signal of Rb2Ca2(SO4)3was recorded under the 1064 nm beam generated by the Q-switched Nd:YAG laser. The polycrystalline material was placed between the glass sheets and pressed into a circular box with a diameter of 8 mm. The samples were irradiated with 1064 nm laser and the output SHG signal was recorded by photomultiplier tube. In the same environment and operation steps, KH2PO4was also tested as a comparison.

2. 5 Methods of calculation

The density of states and electronic energy band were calculated by using the CASTEP program of density functional theory in the software package of material studio[16,17]. The function of generalized gradient approximation (GGA) model proposed by Perdew Burke emzerhoff (PBE)[18,19]was selected to record the performance of the exchange correlation. Most atoms use soft pseudopotential[20]. The energy cutoff momentum was set to 300 eV, and 2 × 2 × 2 k-point grid in Brillouin region was selected for the title compound.

3 RESULTS AND DISCUSSION

3. 1 Synthesis and characterization

Polycrystalline Rb2Ca2(SO4)3was synthesized by a convenient high temperature solid state reaction. Its purity was confirmed by powder XRD analysis in the range of 2θ =10º～70º (Fig.1). The experimental curve agrees well with the calculated one, which verifies the pure of polycrystalline Rb2Ca2(SO4)3. In the heating run of DTA curve (Fig.2),there is one endothermic peak in the temperature range of 1200～1300 K. After DTA experiment, the residue clumps together. Moreover, the TGA curve is almost a straight line,which confirms that most of the mass is preserved in the whole experiment. The DTA and TGA suggest that Rb2Ca2(SO4)3did not break into other products. To confirm this speculation, Rb2Ca2(SO4)3polycrystalline were melted at 1323 K in a muffle furnace for 11 hours and then cool slowly to outdoor temperature. The material was ground for powder XRD analysis and its XRD curve fits perfectly with the experimental one. All these results bear out the high thermal stability of the title compound.

Fig.1. Calculated, experimental and sintered XRD patterns for Rb2Ca2(SO4)3

Fig.2. DTA/TGA curve of Rb2Ca2(SO4)3

3. 2 X-ray crystal structure

In the single crystal structure of Rb2Ca2(SO4)3, the crystallographic positions of Rb, Ca, S, and O are 2, 2, 1 and 4, respectively. The 3D framework is composed of SO4tetrahedra and CaO6octahedra which are connected with each other in a corner-sharing mode (Fig.3). Rb+cations fill in the gap of the 3D framework to maintain the electrical neutrality of the compound. The bond distances of S-O,Ca-O and Rb-O lie in the ranges of 1.453(2)～1.462(2) Å,2.298(2)～2.330(2) Å and 3.017(2)～3.455(3) Å,respectively. The bond angles of O-S-O vary from 108.35(15)º to 110.69(15)º, indicating the SO4functional group is basically a regular tetrahedron. These data are generally reasonable according to the reported sulfates[8,12].Detailed bond parameters are listed in Table 1.

Fig.3. A ball-and-stick representation of Rb2Ca2(SO4)3. (a) SO4 tetrahedron and CaO6 octahedra; (b) 3D framework

Table 1. Selected Bond Lengths (Å) and Bond Angles (°)

The valences for the cations in Rb2Ca2(SO4)3, which calculated by the bond-valence-sum method[21,22], are consistent with their normal valence states (Rb, 1+; Ca, 2+;S, 6+).

3. 3 UV/Vis/NIR diffuse reflectance spectroscopy

The diffuse reflectance spectrum of the title compound was measured by a UV/Vis/NIR spectrophotometer in ambient temperature. The incident light wavelength was set in the range of 200～800 nm. Firstly, BaSO4powder was tested, and its reflectivity was set as the reference; then, a few samples were laid on the surface of BaSO4powder and tested under the same conditions. The result is shown in Fig.4. In the whole visible light region, the reflectance is as high as 80%. When the incident light wavelength is as low as 200 nm, the reflectivity is still above 80%, indicating that the UV cut-off edge of Rb2Ca2(SO4)3is lower than 200 nm.The fine UV transparence can also be visually observed from the transformed absorption curve in the energy range of 0～6.2 eV.

Fig.4. UV/Vis/NIR diffuse-reflectance spectrum of Rb2Ca2(SO4)3. The inset represents the absorption curve

3. 4 Nonlinear optical properties

The main requirement of NLO response is that the inorganic compound should crystallize in a non-centrosymmetric structure. Therefore, it is an effective way to test the structure using the SHG measurement. The SHG measurement of Rb2Ca2(SO4)3powder was tested at ambient temperature. Commercial KH2PO4samples were used for comparison. In the particle range of 0～63 μm, the magnitude of SHG signal is about 0.3 × KH2PO4(Fig.5),which proves that Rb2Ca2(SO4)3belongs to a non-centrosymmetric structure.

Fig.5. SHG signal of Rb2Ca2(SO4)3 and KH2PO4 under a 1064 nm laser radiation

3. 5 Electronic structure

In order to reveal the structure property relationship of Rb2Ca2(SO4)3, theoretical calculation was carried out. Fig.6a shows band structure diagram in which Rb2Ca2(SO4)3has a theoretical energy gap of 5.63 eV, which is basically consistent with the diffuse reflectance spectrum. Fig.6b shows the density of states (DOS) and partial DOS. It is well known that the electronic energy level transition near Fermi energy level has an important influence on the optical properties of a compound. At the top of the valance band ranging from -10 to 0 eV, the DOS are predominately occupied by O 2p and S 3p orbitals. Near the band gap ranging from 5 to 10 eV, the conduction band is mainly originated from Rb 5s and S 3s. Notably, the Ca2+cations have negligible contribution to DOS[23]. In other words,RbOnand SO4groups have important contributions to DOS near the forbidden zone. As a result, they play a decisive role in optical properties.

Fig.6. (a) Electronic band structure, (b) DOS and partial DOS plots

4 CONCLUSION

In summary, pure polycrystalline Rb2Ca2(SO4)3was synthesized by a high temperature solid-state reaction.Single-crystal diffraction measurement results indicate Rb2Ca2(SO4)3belongs to P213 space group, and it features a 3D framework consisting of SO4tetrahedra and CaO6octahedra. The Rb ions fill the holes in the framework. The crystal is stable below 1200 K. The UV cut-off edge of Rb2Ca2(SO4)3is lower than 200 nm. In addition, the SHG response of this crystal is about 0.3 times that of KH2PO4.The theoretical calculation about electron band structure and density of states indicates that RbOnand SO4groups have the most important contribution to the optical properties. The above results show that alkali metal and alkaline-earth metal sulfates may have potential applications as short-wave NLO materials.