## 3 Results and Discussion  
Figure 1a shows the XRD patterns of the resultant Cu\({}_{2}\)SnSe\({}_{3}\) samples after melting and annealing with different cooling rates. Samples 1 and 2 were cooled from 1273 K to room temperature in 24 h and 48 h, respectively. Sample 3 was directly quenched in salt water. Sample 4 was annealed at 923 K for 24 h after quenching in salt water. The XRD patterns indicate that Cu\({}_{2}\)SnSe\({}_{3}\) can be synthesized by melting at 1273 K for 12 h and annealing. The Cu\({}_{2}\)SnSe\({}_{3}\) matrix is a disordered zincblende structure with a lattice parameter of 5.684 A. Figure 1b shows the XRD patterns of different samples from 10\({}^{\circ}\) to 40\({}^{\circ}\). There was no significant difference between sample 1 and sample 2, while the sample 4 had fewer peaks, which may be indexed to the monoclinic structure [3]. The structure transformation in Cu\({}_{2}\)SnSe\({}_{3}\) from monoclinic to cubic can be attributed to the different cooling rates. [PERSON] reported that slow cooling could result in a monoclinic structure while water quenching led to a disordered cubic zincblende-like structure [17]. [PERSON] et al. reported that an order-disorder transformation in Cu\({}_{2}\)SnSe\({}_{3}\) can occur at 723 K and the modified phase was found to be sphalerite [18]. In order to comprehensively and thoroughly study these phases, [PERSON] et al. found that the phase in low diffraction position was likely to be SnSe\({}_{2}\) on the basis of differential thermal analysis (DTA) [19]. Therefore, the structure of Cu\({}_{2}\)SnSe\({}_{3}\) can be described as an adamantane compound which is derivative of the sphalerite structure. In this study, the Cu\({}_{2}\)SnSe\({}_{3}\) material prepared by the melting, quenching, and annealing process was selected as the matrix phase (sample 4) to carry out the following experiment. The Cu\({}_{2}\)SnSe\({}_{3}\) bulk ingot was artificially ground into powder, as shown in Figure 2a. It can be seen that the size distribution of Cu\({}_{2}\)SnSe\({}_{3}\) particles is not uniform. In order to obtain smaller uniform particles, the powder was subjected to mechanical ball milling, and the SEM image of Cu\({}_{2}\)SnSe\({}_{3}\) powder after ball milling is shown in Figure 2b. The XRD of Cu\({}_{2}\)SnSe\({}_{3}\) matrix was analyzed before  
Figure 4 shows the XRD patterns of \(x\) vol %TiO\({}_{2}\)/Cu\({}_{2}\)SnSe\({}_{3}\) (\(x\) = 0, 0.6, 1.0, 1.4, 1.8) composites after ball-milling. The XRD patterns of composites samples are similar to that of Cu\({}_{2}\)SnSe\({}_{3}\) matrix samples. The diffraction peaks of all the samples correspond to the Cu\({}_{2}\)SnSe\({}_{3}\) phase (PDF#65-4145). The peak of TiO\({}_{2}\) phase is hardly found in the XRD patterns, which may be attributed to its low content. In addition, no diffraction peaks corresponding to other phase was found, which indicates that there was no reaction between the introduced nano-TiO\({}_{2}\) particles and the Cu\({}_{2}\)SnSe\({}_{3}\) matrix in the process of ball-milling. Figure 5a shows the TEM image of the starting powders of TiO\({}_{2}\) nanoparticles. It is shown that the size of spherical TiO\({}_{2}\) particles is 10-20 nm. Figure 5b-f shows the SEM images of the fracture surface of TiO\({}_{2}\)/Cu\({}_{2}\)SnSe\({}_{3}\) composite samples. It can be observed in Figure 5b that the grain size of Cu\({}_{2}\)SnSe\({}_{3}\) matrix is about 3 \(\upmu\)m. Rapid sintering process with SPS technology can effectively inhibit the growth of grains, which would result in more grain boundaries and scatter more phonons. It also can be found in Figure 5c,d, and f that the nano-TiO\({}_{2}\) particles are mainly distributed on the grain boundary.