Crystal Structure and Thermal Properties of SrC4H4O6·4H2O Single Crystals

Single crystals of strontium tartrate tetrahydrate, SrC4H4O6·4H2O, were grown at 308 K by a gel method using silica gel as the medium of growth. Differential scanning calorimetry, thermogravimetric-differential thermal analysis, and X-ray diffraction measurements were performed on the single crystals. The space group symmetry (orthorhombic P212121) and the structural parameters were determined at room temperature. The O–H–O hydrogen-bonded network formed between adjacent C4H4O6 molecules was found to extend along the a-axis. Weight losses due to thermal decomposition of SrC4H4O6·4H2O occurred in the temperature range of 350– 1200 K. The weight losses during decomposition were suggested to be caused by the evaporation of bound water molecules and the evolution of H2CO, 1/2O2, and 2CO gases. The chalky white substance remaining in the open vessel after decomposition was strontium oxide, SrO.

Strontium tartrate (SrC 4 H 4 O 6 ) is known to have two different crystal structures. One is monoclinic with space group P2 1 , and contains three independent water molecules in the structure. The positional and thermal parameters of the atoms in the crystal structure of SrC 4 H 4 O 6 ·3H 2 O have been reported by Ambady [10]. The other is orthorhombic with space group P2 1 2 1 2 1 , and contains four independent water molecules in the structure. The lattice parameters of SrC 4 H 4 O 6 ·4H 2 O obtained from powder X-ray diffraction are close to those of calcium tartrate tetrahydrate, CaC 4 H 4 O 6 ·4H 2 O [9,11]. However, the crystal structure of the SrC 4 H 4 O 6 ·4H 2 O crystal has not yet been determined. The crystal structure of the strontium salt is expected to be close to that of the calcium salt. Moreover, a detailed description of the thermal properties of SrC 4 H 4 O 6 ·4H 2 O has not yet been reported.
The purpose of this study is to determine the crystal structure of SrC 4 H 4 O 6 ·4H 2 O at room temperature using X-ray diffraction measurements, and to obtain the thermal properties of the strontium salt using differential scanning calorimetry (DSC) and thermogravimetric-differential thermal analysis (TG-DTA) measurements.

Crystal Growth
Single crystals of SrC 4 H 4 O 6 ·4H 2 O were grown in silica gel medium at 308 K using the single test tube diffusion method. The gels were prepared in test tubes (length of 200 mm, and diameter of 30 mm) using solutions of 2M Na 2 SiO 3 (20 ml), 1M C 4 H 6 O 6 (20 ml), and 2M CH 3 COOH (20 ml), and aged for one day. A solution (20 ml) of 0.5 M SrCl 2 ·6H 2 O was then gently poured on top of the gel. The crystals used were harvested after three months.

X-ray Crystal Structure Determination
The X-ray diffraction measurements were carried out using a Rigaku Saturn CCD X-ray diffractometer with graphite-monochromated Mo K α radiation (λ = 0.71073 Å). The diffraction data were collected at 299 K using an ω scan mode with a crystal-to-detector distance of 40 mm, and processed using the CrystalClear software package. The sample used was spherical in shape with a diameter of 0.30 mm. The intensity data were corrected for Lorentz polarization and absorption effects. The structure was solved with direct methods using the SIR2011 program and refined on F 2 by full-matrix least-squares methods using the SHELXL-2013 program in the WinGX package [18][19][20].

Thermal Measurements
DSC and TG-DTA measurements were respectively carried out in the temperature ranges of 105-450 K and 300-1250 K, using DSC7020 and TG-DTA7300 systems from Seiko Instruments Inc. Aluminium (for DSC) and alumina (for TG-DTA) open pans with no pan cover were used as the measuring vessels and reference pans. Fine powder samples prepared from crushed single crystals were used for the measurements. The sample amount varied between 2.88 and 3.94 mg, and the heating rates were 5 or 10 K min -1 under a dry nitrogen gas flow.

Crystal Structure
The crystal structure of SrC 4 H 4 O 6 ·4H 2 O was determined at room temperature by X-ray diffraction. The lattice parameters calculated from all observed reflections showed that the crystal belongs to an orthorhombic system. The intensity statistics and systematic extinctions in the observed reflections strongly indicated that the crystal belongs to an acentric point group, and the possible space group is P2 1 2 1 2 1 . Thus, the space group of SrC 4 H 4 O 6 ·4H 2 O was determined to be orthorhombic P2 1 2 1 2 1 with a = 9.4694(4) Å, b = 9.5130(3) Å, and c = 10.9703(3) Å. The atomic coordinates and thermal parameters for SrC 4 H 4 O 6 ·4H 2 O, including the positions of all hydrogen atoms, were determined at room temperature. A final R-factor of 4.01% was calculated for 5076 unique observed reflections. The relevant crystal data, as well as a summary of the intensity data collection and structure refinement parameters, are given in Table 1. Fig. 1 shows a perspective view of the crystal structure of SrC 4 H 4 O 6 ·4H 2 O. The positional parameters in fractions of a unit cell, and the thermal parameters are listed in Table 2. Selected bond lengths (in Å) and angles (in degrees) are given in Table 3. The hydrogenbond geometry (in Å and degrees) is presented in Table 4.
The observed lattice parameters along the a-and c-axes are slightly larger than those of CaC 4 H 4 O 6 ·4H 2 O, whereas the parameter along the b-axis is slightly larger than that determined by the powder X-ray diffraction method for SrC 4 H 4 O 6 ·4H 2 O [7-9,11]. The observed structure of the strontium salt contains four crystallographically independent H 2 O molecules, and is very close to that of the calcium salt [7,8]. The fundamental features of the obtained structure are as follows. Each Sr atom in the unit cell is surrounded by eight O atoms at distances from 2.527(2) to 2.676(2) Å to form a SrO 8 dodecahedron, as shown in Table 3. The average Sr-O distance is 2.594 Å. The lengths of the six O-C bonds in a C 4 H 4 O 6 molecule are in the range of 1.241(4)-1.428(4) Å, as shown in Table 3. The variation in O-C distances is probably related to differences in bond type. The    Table 3. The angle between the two least-squares planes  (3) 158(1) 224 (1) 184 (3) 180 (11) 231(12) 183(12) 11(9) 1(9) 3(9) C(2) 0.2821 (3) 0.1645 (3) 0.2378 (3) 143 (10) 216(12) 178(11) -10(9) 23(8) (3) 146 (11) 196(11) 183(10) -11(8) -4(9) 11(9) C(4) 0.4615 (3) 0.2397 (3) 0.3976 (3) 154 (11) 217(12) 216(12)      it is expected that these peaks can be attributed to the evaporation of water molecules. A very small endothermic peak in the DSC curve is also seen at 277.7 K. When the sample used for the above DSC measurement in the temperature range of 105-450 K was remeasured, the small and two large endothermic peaks were not observed in the DSC curve. Thus, the disappearance of the small and large peaks indicates that these peaks are mostly the result of evaporation of water from the sample. In general, there are many defects in the structure of a crystal, such as pores and cracks, and the aqueous solution used for crystal growth can infiltrate the defects. The very small peak at 277.7 K, which is close to 273 K, may be produced by the state change of the solution from ice (solid) to liquid in the defects of the SrC 4 H 4 O 6 ·4H 2 O crystal.

Fig. 2. DSC curve for the SrC 4 H 4 O 6 •4H 2 O
crystal on heating. The sample weight (powder) was 3.32 mg, and the heating rate was 5 K min -1 under a dry N 2 flux of 40 ml min -1 Fig. 3 shows the TG, differential TG (DTG), and DTA curves for SrC 4 H 4 O 6 ·4H 2 O in the temperature range of 300-1250 K. The weight of the sample (powder) used for the measurement was 3.46 mg, and the heating rate was 10 K min -1 under a dry nitrogen gas flow of 300 ml min -1 . The DTA curve exhibits three apparent endothermic peaks at 360.0, 399.3, and 1172.9 K, and three small exothermic peaks at 559.2, 740.1, and 956.7 K. The small peaks are indicated by arrows in Fig. 3. An enlarged view of the curve around 550 K is also shown (Fig. 3,  inset). In this enlarged view, a very small endothermic peak can be observed on the curve at 546.1 K, which is close to the exothermic peak at 559.2 K. The observed DTA peaks at 360.0 and 399.3 K correspond to the above-mentioned DSC peaks at 355.7 and 394.2 K, respectively. The slight differences of about 5 K between the peak temperatures are probably caused by the different heating rates (5 or 10 K min -1 ) and sample pans (aluminium or alumina) used for these measurements.   The DTG curve in Fig. 3 has peaks at 358.3, 395.9, 564.3, 741.9, 956.6, and 1169.1 K. The peak temperature of 956.6 K was determined from the numerical data for the DTG curve, as no obvious peak was detected around the temperature in the curve. These DTG peaks correspond to the respective DTA peaks. As the DTG curve is the first derivative of the TG curve, the DTA peaks are associated with the rate of weight loss on the TG curve. respectively. These values are close to the respective experimental weight losses of 9.5% and 3.1% around 740.1 and 956.7 K determined from the TG curve over the temperature ranges of 650-900 K and 900-1050 K, respectively. An endothermic peak was observed at 1172.9 K in the DTA curve, indicating that an evaporation reaction due to thermal decomposition occurs around this temperature. The theoretical weight loss caused by the evolution of 2CO gas is calculated to be 18.2% [=2×28.01/307.75]. This value is close to the experimental weight loss of 15.5% around 1172.9 K determined from the TG curve over the temperature range of 1050-1200 K. The experimental and theoretical weight losses, the molecules eliminated by evaporation, and the evolved gases at the various temperature ranges are summarized in Table 6.
Finally, the total theoretical weight loss from SrC 4 H 4 O 6 ·4H 2 O is calculated to be 66.3%. This

CONCLUSION
Single crystals of strontium tartrate tetrahydrate, SrC 4 H 4 O 6 ·4H 2 O, were grown in silica gel medium at 308 K by a diffusion method. The thermal properties and crystal structure of the single crystals were studied by DSC, TG-DTA, and Xray diffraction measurements. The crystal structure at room temperature was determined to be orthorhombic with space group P2 1 2 1 2 1 by means of single-crystal X-ray diffraction. It was confirmed that the structure consists of SrO 8 dodecahedra and a hydrogen-bonding network along the a-axis between adjacent