Differential scanning calorimetry studies on structural phase transitions and molecular motions in (CH₃NH₃)₂MBr₄and [(CH₃)₂NH₂]₂MBr₄(M = Zn, Cd, and Hg)

Abstract

Structural phase transitions and molecular motions in (CH3NH3)2MBr4 and in [(CH3)2NH2]2MBr4 (M = Zn, Cd, and Hg) were investigated by differential scanning calorimetry (DSC) between about 130 K and the melting points. All of the compounds showed at least one structural phase transition over the temperature region investigated. The values of the transition entropies obtained suggest that these transitions are of the order-disorder type. N-H···Br hydrogen bonds were found to play an important role in the stability of P21/c room-temperature phases of the title compounds. In the highest-temperature solid phases of all title compounds, orientational orders of cations seem to be completely lost.

Crystal Structures of Piperazinium Tetrahalogenometallates (II) [C4H12N2]MX4 (M = Zn, Hg; X = Br, I)

“…The low-temperature phases of these compounds have the space group P2 1 /c, which is also found the room-temperature phase of [CH 3 [18]. We found that the transition temperatures increase as the number of N-H···Br hydrogen bonds increases, suggesting that N-H···Br hydrogen bonds stabilize the crystal lattices.…”

Crystal structures, phase transitions, and nuclear magnetic resonance of organic–inorganic hybrid [NH2(CH3)2]2ZnBr4crystals

“…2). This result is consistent with the DSC data reported by Horiuchi et al 39 The transition enthalpies (Δ H ) obtained from the peak area for 401 and 436 K are 9.79 and 6.64 kJ mol −1 , respectively, whereas that for 459 K is 13.79 kJ mol −1 , which is determined using the same method. Furthermore, exothermic peaks attributed to cooling are observed at 313, 361, 386, and 421 K, and the resulting entropies are 0.74, 3.56, 2.21, and 13.07 kJ mol −1 , respectively.…”

Structural phase transition, thermal analysis, and spectroscopic studies in an organic–inorganic hybrid crystal: [(CH3)2NH2]2ZnBr4

“…was measured at 291 K. [ 15 ] Moreover, a recent study found that [(CH 3 ) 2 NH 2 ] 2 ZnBr 4 showed at least one structural phase transition between ≈130 K and the melting points. [ 24 ] This has prompted us to investigate the structure of DMA‐ZnBr 4 at low temperatures ( T = 150 K) in order to characterize the phase transition observed by DSC at 281 K.…”