English

• 1. INTRODUCTION

  Nonlinear optical materials (NLO) play an irreplaceable role in laser science and technology^[1]. Especially, among them, short-wave ultraviolet (UV) (λ < 300 nm) NLO materials which have significant applications in modern precision manufacturing such as semiconductor photolithography and micro machining, attract researchers' most attention. However, up to date, there are only few NLO materials that can efficiently produce coherent light in the short-wave UV region via the direct second harmonic generation (SHG)^[2-4]. Therefore, it is necessary to explore short-wave UV NLO materials.

  For developing short-wave UV NLO materials, researchers previously have focused on π-conjugated systems that are based on planar triangle anionic groups with π-conjugated electrons, such as [BO₃]^3-, [CO₃]^2- and [NO₃]^-. The following noticeable examples include many borates, such as KBe₂BO₃F₂^[5], Sr₂Be₂B₂O₇^[6], Na₂Be₄B₄O₁₁^[7], Li₄Sr(BO₃)₂^[8], β-Rb₂Al₂B₂O₇^[9], NH₄B₄O₆F^[10], CsB₄O₆F^[11], and M₂B₁₀O₁₄F₆ (M = Ca, Sr)^{[12, 13]}, some carbonates like ABCO₃F (A = K, Rb, Cs; B = Ca, Sr)^[14], RbMgCO₃F^[15], and Y₂(CO₃)₃∙2H₂O^[16], and a number of nitrates like RbNa₂(NO₃)₃^[17] and Y(OH)₂NO₃^[18]. Beyond π-conjugated systems, recently, non-π-conjugated systems on account of [PO₄]^3- NLO anionic groups have drawn researchers' special interest. A lot of short-wave UV NLO phosphates were discovered, such as Ba₂NaClP₂O₇^[19], M₄Mg₄(P₂O₇)₃ (M = K, Rb)^[20], CsLi₂PO₄^{[21, 22]}, Ba₅P₆O₂₀^[23], RbBa₂(PO₃)₅^[24], Rb₂Ba₃(P₂O₇)₂^[24] and Ba₃P₃O₁₀X (X = Cl, Br)^[25]. In these phosphates, researchers proposed some distinct philosophies to design NLO materials. For example, our group proposed that the tailored synthesis strategy based on flexible fundamental building unit could help develop NLO materials with short absorption edges. In addition, in M₄Mg₄(P₂O₇)₃ (M = K, Rb), Halasyamani and co-authors suggest that low-polymerized phosphates with appropriate structures may have large SHG responses.

  Besides phosphates, sulfates composed of [SO₄]^2- groups also belong to non-π-conjugated systems. However, sulfates suffer releasing poisonous SO₃ under high temperature environment. This obstacle makes researchers have neglected sulfates as NLO materials for a long time, compared to phosphates, borates, carbonates and nitrates. Until fairly recently, our group and some other groups have utilized the water solution method and solvothermal method to synthesize some sulfate NLO materials, such as (NH₄)SbCl₂(SO₄)^[26], Li₈NaRb₃(SO₄)₆·2H₂O^[27], (NH₄)₂Na₃Li₉(SO₄)₇^[28], NH₄NaLi₂(SO₄)₂^[28], and CsSbF₂SO₄^[29]. Among these sulfates, some have good optical properties. For example, NH₄NaLi₂(SO₄)₂ has a short absorption edge below 186 nm, and exhibits the potential as deep-UV NLO materials. In addition, CsSbF₂SO₄, which is derived from the famous commercial UV NLO material KTiOPO₄, has a strong SHG response. Despite their good optical properties, all these sulfates have the same problem of low thermal stability. To overcome the problem, we chose alkaline metal and alkaline earth metal as cations and introduced them into sulfate systems by the flux method on account of the following considerations. First of all, the flux method is commonly used in other decomposing systems. In this method, alkaline metal sulfates and alkaline earth metal sulfates themselves can be as flux to decrease the melting points. Secondly, alkaline metal and alkaline earth metal are good for transmission in the UV region. As a result, we synthesized a new short-wave UV NLO P2₁2₁2₁ Cs₄Mg₆(SO₄)₈ (I) with high thermal stability up to 781 ℃ by the flux method in this work.

  2. EXPERIMENTAL

  2.1 Reagents

  Cs₂SO₄ (99.5%) and MgSO₄ (99.5%) were purchased from Aladdin and used as received.

  2.2 Synthesis of I

  Powder samples of I were prepared using the high-temperature solid-state method. A stoichiometric mixture of these raw materials was ground thoroughly, heated to 450 ℃ at the rate of 25 ℃/h, and then kept at this temperature for about 48 h. The products were ground thoroughly once more, heated to 620 ℃ at a rate of 25 ℃/h, and kept at this temperature for about 120 h. The phase purity of Ι was confirmed by the powder X-ray diffraction (XRD) analysis, which was carried out at room temperature on a Miniflex600 diffractometer equipped with Cu-Kα radiation. The scanning step width of 0.02° and scanning rate of 0.05 °·s^-1 were applied to record the patterns in the 2θ range of 10~70°. As shown in Fig. 1, the powder X-ray diffraction (XRD) patterns of prepared samples agree with the XRD patterns calculated from the single-crystal XRD data. Especially, there are some extra weak reflections at around 27.7°, which could be assigned to Cs₂Mg(SO₄)₂∙6H₂O.

  Figure 1

  

  Figure 1.  Calculated and experimental XRD patterns of Ι

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  2.3 Crystal growth of I

  I crystals were grown using the flux method. A mixture of Cs₂SO₄ (36.187 g, 0.1 mol) and MgSO₄ (36.111 g, 0.3 mol) was ground thoroughly using an agate mortar. The mixture was then placed into a Pt crucible, rapidly heated to 850 ℃ in a temperatureprogrammable electric furnace, and then held for 24 h to ensure that the melt was homogenized. The melt was allowed to cool to 700 ℃ at a rate of 4 ℃/h, and then cooled down to room temperature with the power off. I was gotten as colorless block crystals.

  2.4 Structure determination

  Colorless Ι crystals (0.13mm × 0.12mm × 0.10mm) were selected using an optical microscope for single-crystal XRD analysis. The diffraction data of I were performed on a Bruker APEX Ⅱ CCD diffractometer with graphite-monochromatized Mo-Kα radiation (λ = 0.71073 Å) at 200(2) K. The data collection, cell refinement, and data reduction of I were carried out with the program APEX3^[30]. A total of 7184 reflections were collected in the range of 3.25 < θ < 26.37° (–11≤h≤11, –9≤k≤12, –12≤l≤20), of which 2965 were independent (R_int = 0.0236) and 2917 were observed with I > 2σ(I). All the structures were solved by direct methods with program SHELXS and refined with the leastsquares program SHELXL^[31]. Final refinements include anisotropic displacement parameters. All the structures were verified using the ADDSYM algorithm from the program PLATON^[32], and no higher symmetries were found. The selected bond distances and angles are presented in Table 1.

  Table 1

  

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

  DownLoad: CSV

  Bond	Dis.		Bond	Dis.		Angle	(°)
  Cs(1)–O(2)	3.060(3)		Mg(2)–O(1)#7	2.357(3)		O(4)–S(1)–O(2)	107.98(18)
  Cs(1)–O(3)#1	3.069(3)		Mg(1)–O(5)	2.050(3)		O(4)–S(1)–O(3)	112.86(17)
  Cs(1)–O(6)#2	3.166(3)		Mg(1)–O(15)#9	2.065(4)		O(2)–S(1)–O(3)	112.0(2)
  Cs(1)–O(14)#3	3.246(3)		Mg(1)–O(2)	2.086(4)		O(4)–S(1)–O(1)	111.0(2)
  Cs(1)–O(4)	3.341(3)		Mg(1)–O(11)#9	2.117(3)		O(2)–S(1)–O(1)	108.72(17)
  Cs(1)–O(5)	3.401(4)		Mg(1)–O(4)#13	2.151(3)		O(3)–S(1)–O(1)	104.16(17)
  Cs(1)–O(16)#4	3.401(3)		Mg(1)–O(9)#4	2.153(3)		O(5)–S(2)–O(7)	110.2(2)
  Cs(1)–O(7)	3.411(4)		Mg(3)–O(10)	1.998(4)		O(5)–S(2)–O(6)	109.6(2)
  Cs(1)–O(9)#4	3.426(4)		Mg(3)–O(13)	1.998(3)		O(7)–S(2)–O(6)	107.9(2)
  Cs(1)–O(8)	3.530(4)		Mg(3)–O(14)#12	2.020(3)		O(5)–S(2)–O(8)	109.5(2)
  Cs(1)–O(7)#2	3.638(4)		Mg(3)–O(8)#10	2.021(3)		O(7)–S(2)–O(8)	108.8(2)
  Cs(2)–O(15)	3.087(3)		Mg(3)–O(1)#11	2.098(4)		O(6)–S(2)–O(8)	110.88(19)
  Cs(2)–O(11)#5	3.095(3)		S(1)–O(4)	1.476(3)		O(9)–S(3)–O(12)	108.7(2)
  Cs(2)–O(14)	3.205(3)		S(1)–O(2)	1.476(3)		O(9)–S(3)–O(10)	108.1(2)
  Cs(2)–O(8)#6	3.211(3)		S(1)–O(3)	1.485(3)		O(12)–S(3)–O(10)	108.3(2)
  Cs(2)–O(10)#7	3.227(3)		S(1)–O(1)	1.514(3)		O(9)–S(3)–O(11)	111.77(19)
  Cs(2)–O(6)#7	3.298(3)		S(2)–O(5)	1.454(3)		O(12)–S(3)–O(11)	110.3(2)
  Cs(2)–O(9)#7	3.323(3)		S(2)–O(7)	1.463(3)		O(10)–S(3)–O(11)	109.56(19)
  Cs(2)–O(12)#7	3.342(4)		S(2)–O(6)	1.465(4)		O(15)–S(4)–O(13)	111.31(19)
  Cs(2)–O(5)#7	3.388(4)		S(2)–O(8)	1.481(3)		O(13)–S(4)–O(14)	106.5(2)
  Cs(2)–O(13)#8	3.454(4)		S(3)–O(9)	1.471(3)		O(16)–S(4)–O(15)	110.9(2)
  Mg(2)–O(7)	2.007(4)		S(3)–O(12)	1.474(3)		O(16)–S(4)–O(13)	110.29(18)
  Mg(2)–O(12)	2.013(4)		S(3)–O(10)	1.475(4)		O(15)–S(4)–O(13)	111.31(19)
  Mg(2)–O(6)#2	2.042(4)		S(3)–O(11)	1.481(3)		O(16)–S(4)–O(14)	109.79(17)
  Mg(2)–O(16)#4	2.062(3)		S(4)–O(16)	1.477(3)
  Mg(2)–O(3)#7	2.166(3)		S(4)–O(15)	1.478(3)
  S(4)–O(13)	1.480(3)		S(4)–O(14)	1.497(3)
  Symmetry codes: #1: x–1/2, –y+5/2, –z+1; #2: x–1/2, –y+3/2, –z+1; #3: x–1, y+1, z; #4: –x+1, y+1/2, –z+3/2; #5: –x+1, y–1/2, –z+3/2; #6: x+1/2, –y+1/2, –z+1; #7: x, y–1, z; #8: –x+2, y–1/2, –z+3/2; #9: x, y+1, z; #10: x+1/2, –y+3/2, –z+1; #11: x+1, y–1, z; #12 –x+2, y+1/2, –z+3/2; #13: x+1/2, –y+5/2, –z+1

  2.5 Thermal stability

  The thermal stability was investigated by the differential scanning calorimetric (DSC) analysis and thermogravimetric (TG) analyses on a NETZSCH STA 449C simultaneous thermal analyzer instrument. About 20 mg of Ι was placed in Al₂O₃ crucibles, and heated at a rate of 10 ℃/min from room temperature to 850 ℃. The measurements were carried out in an atmosphere of flowing N₂.

  2.6 UV-Vis-NIR diffuse reflectance spectroscopy

  The UV-Visible (Vis)-Near infrared (NIR) diffuse reflection data were recorded at room temperature using a powdered BaSO₄ sample as a standard (100% reflectance) on a PerkinElmer Lamda-950 UV/Vis/NIR spectrophotometer. The scanning wavelength range is from 200 to 800 nm. Absorption (K/S) data were calculated from the following Kubelka-Munk function: F(R) = (1 – R)²/(2R) = K/S^[33], where R is the reflectance, K is the absorption, and S is the scattering.

  2.7 SHG measurements

  Powder SHG measurements were carried out with a Q-switched Nd: YAG laser at a wavelength of 1064 nm. Polycrystalline Ι samples were ground and sieved into the size range of 125~180 μm. The samples were pressed between glass slides and secured with tape in 1-mm thick aluminum holders containing an 8-mm diameter hole. They were then placed into a light-tight box and irradiated with the laser of λ = 1064 nm. The intensity of the frequency-doubled output emitted from the samples was collected by a photomultiplier tube. Crystalline KDP was also ground and sieved into the same particle size range and used as the references.

  2.8 Computational methods

  The CASTEP package was employed to perform the first principles calculations on I by the planewave psuedopotential methods^[34]. The norm-conserving pseudopotentials (Cs 5s²5p⁶6s¹, Mg 2s²2p⁶3s², S 3s²3p⁴, O 2s²2p⁴) were used to simulate ion-electron interactions for all constituent elements^[35]. A kinetic energy cutoff of 900 eV was chosen with Monkhorst-Pack k-point meshes.

  3. RESULTS AND DISCUSSION

  3.1 Single-crystal structure

  Ι is noncentrosymmetric (NCS) with an orthorhombic space group of P2₁2₁2₁ (No. 19). In the single-crystal structure of I, magnesium atoms take up three crystallographically independent sites to form Mg1O₆ octahedra, Mg2O₆ octahedra, and Mg3O₅ polyhedra. In addition, there are four crystallographically unique sites for sulfur atoms, which are tetrahedrally linked with four oxygen atoms to form distorted SO₄ tetrahedra. As shown in Fig. 2a, the single crystal structure of I can be described as a three-dimensional (3D) framework, which is constructed by Mg1O₆ octahedra, Mg2O₆ octahedra and SO₄ tetrahedra, with one-dimensional (1D) tunnels along the b-axis. In the 3D framework, Mg1O₆ octahedra are connected with six SO₄ tetrahedra via sharing with corners. For Mg2O₆ octahedra, they are corner-shared with four SO₄ tetrahedra, and edge-shared with one SO₄ tetrahedra. As illustrated in Fig. 2b and 2c, Mg3 atoms locate in 1D tunnels to help reinforce the 3D framework and make the single crystal structure stable. According to the reported results, all of the bond lengths and bond angles of S–O groups are reasonable^[26-29]. As shown in Fig. 2d, cesium atoms are surrounded by ten and eleven oxygen atoms with the Cs–O bond lengths between 3.087(3) and 3.638(4) Å. According to the bond valence calculations (Cs is +1.03 and +1.07; Mg is +1.99, +2.05, and +2.04; S is +5.79, +6.14, +5.98, and +5.86), these elements in the compound are all in their normal oxidation states

  Figure 2

  

  Figure 2.  (a) 3D framework of the single-crystal structure of I formed by SO₄, Mg1-O and Mg2-O groups. (b) Coordination environment of Mg3. (c) Single-crystal structure of I. (d) Coordination environments for Cs atoms. We omit Cs atoms from the single-crystal structure of I to make it more clear. The blue tetrahedra represent SO₄ groups, and yellow octahedra represent Mg1O₆ and Mg2O₆ groups. 1D tunnels are indicated with dashed red rings

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  3.2 Thermal stability

  In order to investigate the thermal stability, we carried out thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) for I. As illustrated in Fig. 3a, in the DSC curves, there is a sharp endothermal peak at 812 ℃ in the heating process and a sharp exothermal peak at 781 ℃ in the cooling process. Further combined with the results of TGA, we can know that I has high thermal stability and melts congruently. Significantly, as shown in Fig. 3b, the thermal stability of I far exceeds that of the reported sulfates NLO materials.

  Figure 3

  

  Figure 3.  (a) TGA and DSC curves of Ι. (b) Comparison of thermal stability among known sulfate NLO materials

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  3.3 Optical properties

  Since I is NCS, it should have SHG response. We carried out powder SHG measurements for I using a solid-state laser at the wavelength of 1064 nm. In the measurement, KH₂PO₄ (KDP) samples were chosen as the reference. As illustrated in Fig. 4a, the SHG intensity of Ι is about 0.2 × KDP in the same particle size of 125~180 μm. To test the absorption edge of I, we recorded its UV-Visible (Vis)-near infrared (NIR) diffuse reflectance spectra. From Fig. 4b, we can see there are no obvious absorption peaks in the wavelength range of 200 and 800 nm, which means that I has the ability of short-wave UV transparency.

  Figure 4

  

  Figure 4.  (a) SHG signals for I. KDP samples in the same particle sizes were chosen as the reference. (b) UV-vis-NIR spectra of I from 200 to 800 nm

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  3.4 The first-principles calculations

  In order to have insight into the micro-origin of the SHG response of I, the first-principles calculations were carried out based upon the CASTEP package^[34]. The energy band structure (Fig. 5a) suggests that I is an indirect material with the bandgap values of about 5.38 eV. The calculated energy bandgap values basically correspond with the experimental results (> 6.2 eV). The density of states (DOS) and partial DOS patterns of I are shown in Fig. 5b. We can see that the upper valence band regions are mainly composed of S 3p and O 2p orbitals, whereas the contribution from Cs⁺ and Mg²⁺ cations are relatively small. According to this result, I is similar to some other recently reported sulfate NLO materials^{[27, 28]} in which the S–O groups are NLO active groups.

  Figure 5

  

  Figure 5.  (a) Energy band structure of I. (b) DOS and partial DOS curves of I

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  4. CONCLUSION

  In summary, we have synthesized a sulfate shortwave UV NLO material, namely Cs₄Mg₆(SO₄)₈ (I), of high thermostability by the flux method. I crystallizes in a NCS space group P2₁2₁2₁ with a 3D framework. UV/vis/NIR diffuse reflectance spectra demonstrate that I has short-wave UV transparency. SHG measurements indicate that I has a SHG intensity of about 0.2 times that of KDP. The first-principles calculations reveal that S-O groups play a key role in optical properties of I. Notably, I is congruently melting with a melting point of 781 ℃ according to the TGA and DSC. The thermostability of I is much higher than the reported sulfate NLO materials. In the future, we will devote our efforts to growing big crystals of I, and developing some novel sulfate materials with high thermostability and good NLO properties using the flux method.

  
  
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  Figure 1  Calculated and experimental XRD patterns of Ι

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  Figure 2  (a) 3D framework of the single-crystal structure of I formed by SO₄, Mg1-O and Mg2-O groups. (b) Coordination environment of Mg3. (c) Single-crystal structure of I. (d) Coordination environments for Cs atoms. We omit Cs atoms from the single-crystal structure of I to make it more clear. The blue tetrahedra represent SO₄ groups, and yellow octahedra represent Mg1O₆ and Mg2O₆ groups. 1D tunnels are indicated with dashed red rings

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  Figure 3  (a) TGA and DSC curves of Ι. (b) Comparison of thermal stability among known sulfate NLO materials

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  Figure 4  (a) SHG signals for I. KDP samples in the same particle sizes were chosen as the reference. (b) UV-vis-NIR spectra of I from 200 to 800 nm

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  Figure 5  (a) Energy band structure of I. (b) DOS and partial DOS curves of I

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

  Bond	Dis.		Bond	Dis.		Angle	(°)
  Cs(1)–O(2)	3.060(3)		Mg(2)–O(1)#7	2.357(3)		O(4)–S(1)–O(2)	107.98(18)
  Cs(1)–O(3)#1	3.069(3)		Mg(1)–O(5)	2.050(3)		O(4)–S(1)–O(3)	112.86(17)
  Cs(1)–O(6)#2	3.166(3)		Mg(1)–O(15)#9	2.065(4)		O(2)–S(1)–O(3)	112.0(2)
  Cs(1)–O(14)#3	3.246(3)		Mg(1)–O(2)	2.086(4)		O(4)–S(1)–O(1)	111.0(2)
  Cs(1)–O(4)	3.341(3)		Mg(1)–O(11)#9	2.117(3)		O(2)–S(1)–O(1)	108.72(17)
  Cs(1)–O(5)	3.401(4)		Mg(1)–O(4)#13	2.151(3)		O(3)–S(1)–O(1)	104.16(17)
  Cs(1)–O(16)#4	3.401(3)		Mg(1)–O(9)#4	2.153(3)		O(5)–S(2)–O(7)	110.2(2)
  Cs(1)–O(7)	3.411(4)		Mg(3)–O(10)	1.998(4)		O(5)–S(2)–O(6)	109.6(2)
  Cs(1)–O(9)#4	3.426(4)		Mg(3)–O(13)	1.998(3)		O(7)–S(2)–O(6)	107.9(2)
  Cs(1)–O(8)	3.530(4)		Mg(3)–O(14)#12	2.020(3)		O(5)–S(2)–O(8)	109.5(2)
  Cs(1)–O(7)#2	3.638(4)		Mg(3)–O(8)#10	2.021(3)		O(7)–S(2)–O(8)	108.8(2)
  Cs(2)–O(15)	3.087(3)		Mg(3)–O(1)#11	2.098(4)		O(6)–S(2)–O(8)	110.88(19)
  Cs(2)–O(11)#5	3.095(3)		S(1)–O(4)	1.476(3)		O(9)–S(3)–O(12)	108.7(2)
  Cs(2)–O(14)	3.205(3)		S(1)–O(2)	1.476(3)		O(9)–S(3)–O(10)	108.1(2)
  Cs(2)–O(8)#6	3.211(3)		S(1)–O(3)	1.485(3)		O(12)–S(3)–O(10)	108.3(2)
  Cs(2)–O(10)#7	3.227(3)		S(1)–O(1)	1.514(3)		O(9)–S(3)–O(11)	111.77(19)
  Cs(2)–O(6)#7	3.298(3)		S(2)–O(5)	1.454(3)		O(12)–S(3)–O(11)	110.3(2)
  Cs(2)–O(9)#7	3.323(3)		S(2)–O(7)	1.463(3)		O(10)–S(3)–O(11)	109.56(19)
  Cs(2)–O(12)#7	3.342(4)		S(2)–O(6)	1.465(4)		O(15)–S(4)–O(13)	111.31(19)
  Cs(2)–O(5)#7	3.388(4)		S(2)–O(8)	1.481(3)		O(13)–S(4)–O(14)	106.5(2)
  Cs(2)–O(13)#8	3.454(4)		S(3)–O(9)	1.471(3)		O(16)–S(4)–O(15)	110.9(2)
  Mg(2)–O(7)	2.007(4)		S(3)–O(12)	1.474(3)		O(16)–S(4)–O(13)	110.29(18)
  Mg(2)–O(12)	2.013(4)		S(3)–O(10)	1.475(4)		O(15)–S(4)–O(13)	111.31(19)
  Mg(2)–O(6)#2	2.042(4)		S(3)–O(11)	1.481(3)		O(16)–S(4)–O(14)	109.79(17)
  Mg(2)–O(16)#4	2.062(3)		S(4)–O(16)	1.477(3)
  Mg(2)–O(3)#7	2.166(3)		S(4)–O(15)	1.478(3)
  S(4)–O(13)	1.480(3)		S(4)–O(14)	1.497(3)
  Symmetry codes: #1: x–1/2, –y+5/2, –z+1; #2: x–1/2, –y+3/2, –z+1; #3: x–1, y+1, z; #4: –x+1, y+1/2, –z+3/2; #5: –x+1, y–1/2, –z+3/2; #6: x+1/2, –y+1/2, –z+1; #7: x, y–1, z; #8: –x+2, y–1/2, –z+3/2; #9: x, y+1, z; #10: x+1/2, –y+3/2, –z+1; #11: x+1, y–1, z; #12 –x+2, y+1/2, –z+3/2; #13: x+1/2, –y+5/2, –z+1

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