Raman Spectroscopic Study of the Structural Phase Transitions in K₂SnCl₆

Abstract

Raman spectra of I<,SnC16 are investigated in the temperature range between 50 and 300 K. The observed low temperature spectra are compatible with the proposed monorlinic structure. Two bands that exhibit a soft behaviour are assigned to a zone contor and zone boundary rotational mode, respectively. It is supposed that these modes drive the structural phase transitions a t T , = 263 K and T , = 255 K.Die Raman-Spektren von K,SnC16 werden im Temperaturbereich von 50 bis 300 K untersucht.Das beobachtete Tieftemperatur-Spektrum steht in Einklang mit der vermuteten monoklinen Strulitur. Zwei Bandon, die ein Soft-Verhalten xeigen, werden Rotationsmoden zugeordnet, die zum Zentrum bzw. zum Rand der Brillouin-Zone gehoren. Diese Moden werden als Ursache fur die strukturellen Phasenubergange bei T, = 263 K und T z = 255 K angesehen.

Optical-Vibrational Properties of the Cs2SnX6(X = Cl, Br, I) Defect Perovskites and Hole-Transport Efficiency in Dye-Sensitized Solar Cells

“…Cs 2 SnCl 6 exhibits four Raman peaks according to the theoretical predictions (Figure ). Considering the Raman studies in several isostructural systems containing the [SnX 6 ] 2– anions − with the O h point group symmetry, we attribute the peaks as ν­(A 1g ) = 309 cm –1 , ν­(E g ) = 232 cm –1 and δ­(F 2g ) = 168 cm –1 , the strongest being the ν­(A 1g ) symmetric stretching. The low frequency band at about 51 cm –1 is attributed to the ν L (F 2g ) vibration involving the translational motion of the heavy Cs atoms.…”

Defect Perovskites under Pressure: Structural Evolution of Cs2SnX6 (X = Cl, Br, I)

“…The defect halide perovskites possess diverse crystallographic phases according to the size of the cations and the anions and a rich phase diagram in temperature and pressure space. Early studies have shown that, upon cooling, they undergo cooperative octahedral tilting and rotations in symmetry-lowering phase transitions. − In the past, some A 2 MCl 6 defect perovskites (A = Cs, Rb, K and M = Sn Te, Pt) were systematically examined, including K 2 SnCl 6 and [(CH 3 ) 4 N] 2 SnCl 6 , which showed no phase changes but small anomalies in the frequency versus pressure curves at low pressures. Very recently, a combined XRD and Raman high pressure study on Cs 2 SnBr 6 has found no phase transformation up to 20 GPa, in agreement with our findings, which we will discuss in detail below .…”

Development of inorganic and hybrid perovskite materials and their application in photovoltaic devices

“…doubly degenerate ν(Eg) due to the Sn-X asymmetric stretching vibration, and triply degenerate δ(F2g) as the X-Sn-X asymmetric bending deformation. The lowest frequency detected Raman band is due to the triply degenerate ν L (F2g) lattice mode, which involves vibrations of the organic moiety (CH3)3S + in the rigid [SnX6] lattice 153,163,164 . Frequencies of these vibrational modes are recorded in Table 5-5 and are very similar to those observed in Cs2SnX6 (X = Cl, Br, I) 153 .…”

“…Moreover, the band gap values of ((CH3)3S)2SnX6 (X = Cl, Br, I) are slightly larger than the analogous Cs2SnX6 compounds with 3.9, 2.7 and 1.26 eV for X = Cl, Br and I, respectively. 153 Similarly, to conventional A2BX6 perovskite materials, substitution of halogen atoms at the X-site will subsequently affect optical absorption properties, which are attributed to the smaller and more electronegative nature of the chloride and bromide anions; in both cases this would increase the magnitude of the band gap 164,165 . Further, the analogue compounds of MA2SnI6 and FA2SnI6 show a monotonic increase in optical band gap 1.35 eV and 1.37 eV, respectively 164 .…”

“…153 Similarly, to conventional A2BX6 perovskite materials, substitution of halogen atoms at the X-site will subsequently affect optical absorption properties, which are attributed to the smaller and more electronegative nature of the chloride and bromide anions; in both cases this would increase the magnitude of the band gap 164,165 . Further, the analogue compounds of MA2SnI6 and FA2SnI6 show a monotonic increase in optical band gap 1.35 eV and 1.37 eV, respectively 164 . Despite the absence of connectivity between the octahedra, all A2SnI6 (A = Cs, CH3NH2, H2NCHNH2, (CH3)3S) compounds exhibit a direct optical band gap around 1.3 eV.…”