Ferroelectric Metal–Organic Frameworks

Synthesis, Structure, and Properties of Organic-Inorganic Perovskites and Related Materials

The study of changes in the refractive indices (with consequent changes of birefringence) of a transparent magnetic crystal which accompany changes in the magnetic order is becoming more popular. The authors review why this is. The first reason is that birefringence can be measured very accurately: the different experimental arrangements are reviewed. The second reason is because a birefringence measurement is an integrational spectroscopic technique and therefore it is studied both experimentally and theoretically as a branch of magneto-optics and hence gives information on the detailed energy level structure of the solid. The third reason is that in a number of interesting systems the birefringence is proportional to the magnetic energy over a wide temperature range and it is often a more convenient method of obtaining the magnetic specific heat than direct specific heat measurements; this is particularly true in magnetic crystals which show low dimensional ordering. The last reason is that in all magnetic crystals the birefringence change should vary like one of the thermodynamic critical exponents near to the phase transition. They review in detail the reasons why birefringence studies have become so successful for measuring critical exponents in pure and particularly mixed crystals.

https://iopscience.iop.org/article/10.1088/0034-4885/47/5/002/pdf

Linear optical birefringence of magnetic crystals

We demonstrate that ethylammonium copper chloride, (C2H5NH3)2CuCl4, a member of the hybrid perovskite family is an electrically polar and magnetic compound with dielectric anomaly around the Curie point (247 K). We have found large spontaneous electric polarization below this point accompanied with a color change in the sample. The system is also ferroelectric, with large remnant polarization (37??C/cm2) that is comparable to classical ferroelectric compounds. The results are ascribed to hydrogen-bond ordering of the organic chains. The coexistence of ferroelectricity and dominant ferromagnetic interactions allows to relate the sample to a rare group of magnetic multiferroic compounds. In such hybrid perovskites the underlying hydrogen bonding of easily tunable organic building blocks in combination with the 3d transition-metal layers offers an emerging pathway to engineer multifuctional multiferroics.

/pdf/multiferroicity-and-hydrogen-bond-ordering-in-c-2-h-5-nh-3-2-2qwccfwdqg.pdf

Multiferroicity and hydrogen-bond ordering in ( C 2 H 5 NH 3 ) 2 CuCl 4 featuring dominant ferromagnetic interactions

CsCl type NH 4 Br crystal has been treated as an Ising spin-phonon coupled system to explain the phase transitions associated with the orientational order of NH 4 + ions. The appearance of two ordered phases is explained as essentially due to the fact that the direct interaction between NH 4 + ions stabilizes parallel ordering while the indirect interaction through phonons stabilizes the antiparallel ordering. Based on a microscopic hamiltonian describing the spin-phonon coupled system, several experimental results have been analized such as (i) anomalous lattice expansions (ii) orientational order parameters (iii) spontaneous displacement of Br - ions (iv) P - T phase diagram (v) X-ray critical diffuse scattering. Semiquantitative agreements are obtained. Especially the property of correlated fluctuations of displacement of Br - ions seems to give a support on the validity of the microscopic model used.

I.R. Jahn

Papers

Birefringence, X-ray and neutron diffraction measurements on the structural phase transitions of (CH3NH3)2 MnCl4 and (CH3NH3)2FeCl4

Birefringence and order parameter of ammonium bromide

Phase diagram of the solid solution system NH4Cl1-xBrx as determined by optical methods

Change of the optical birefringence associated with the antiferromagnetic ordering of MnF2, FeF2, CoF2, and NiF2

Birefringence in tetragonal antiferromagnets