The Jahn-Teller effect and orientational order in (C_nH_2n+1NH₃)₂CuCl₄, n=1, 2, 3

Abstract

Crystal optical and x-ray diffraction studies show that the high-temperature phase I of the perovskite-type layer compounds (C,I.12,,1NH,),CuC1, with n = 1, 2, 3 is Jahn-Teller distorted orthorhombic with space group Bbcm (Dif). As a consequence, the order of the CnH2n+ INH3 group leads to two configurations, realised in a monoclinic phase I1 with space group P2,/c (C&) and an orthorhombic phase I11 with Pbca (D::). The transition sequences found above 300 K depend on n: I +-T,, = 348 K+ I1 for n = 1; I + T,, = 364 K-+ I1 +-T& = 356 K, Te2 = 338K+ I11 forn = 2; and I +-T,, = 434K+ I11 forn = 3. The domain phenomena at T,, and T,, point to a subtle balance between the tilt and order configurations of phases I1 and 111. Birefringence and lattice constants indicate a very loose inter-layer contact in phase I.

Investigations of High Temperature Phase Transitions in (NH3c2H5)2CuCl4Crystals

“…Discontinuous nature of both high temperature transitions is confirmed by their temperature hysteresis (ΔT1 = 1.5 K and ΔT2 = 2.5 K). Nevertheless, the obtained value of ΔT2 is much lower than those presented in [2]. Such a discrepancy would be connected with a different sample quality and preceding thermal history.…”

“…They consist of ionic layers of CuCl 6 octahedra sandwiched between the two sheets of (C2H 5 NH3)+ complexes [1,2]. The NH3 polar head of the alkylammonium ion points toward the layer of CuCl6 and forms hydrogen bonds with Cl-anions in CuCl6.…”

Low-frequency Raman study of the ferroelectric phase transition in a layeredCuCl4-based organic-inorganic hybrid

“…Most of the CuCl 4 -based compounds 19,21,22 undergo a similar phase transition in the range between 320 and 360 K. In analogy to the present example of PEACuCl, the apparent Cmca → P cba (D 18 2h → D 15 2h ) (Ref. 50) transitions reported in other CuCl 4 -type hybrids might actually be ferroelectric transitions to symmetries lower than P bca.…”

“…The variety of structural and physical phenomena found in this family of compounds can be ascribed to the large differences in interaction strengths and length scales associated with the chemical bonding: The organic molecules and inorganic layers are composed of strong covalent and ionic bonds, respectively; both ionic and hydrogen bondings are of importance at the organic-inorganic interface, and van der Waals forces largely govern the molecular arrangement in the organic layers. A large number of experiments have been performed in order to reveal the origin of the various physical phenomena in layered hybrids: x-ray diffraction, [17][18][19] differential scanning calorimetry, 17,20 birefringence and microscopy, [17][18][19][20][21][22] NMR and 35 Cl nuclear quadrupole resonance [23][24][25][26] (NQR), Brillouin scattering, 27 neutron diffraction, 28,29 ultrasound experiments, 30,31 and IR/Raman spectroscopy (e.g., Refs. [32][33][34][35].…”

Structure analysis of the high-temperature phases of [NH3(C3H7)]2CuCl4. II. The modulated phase

“…The three commensurate phases a,/3 and ~ have been discussed in the preceding article (Doudin & Chapuis, 1990). At nearly 380 K, satellite reflections appear at locations ha* + kb* +/c* + mq where q = 0.18a* (Holocher, 1986;Jahn, Holocher, Knorr & Ihringer, 1986;Jahn, 1988;Doudin & Chapuis, 1989;Holocher, Jahn, Knorr & Ihringer, 1989). Phase/3 reenters at the y-6 transition.…”