J. Petersson

Interacting one dimensional Ising spin chains as a model of the layered antiferroelectric squaric acid

Starting from the structure of the layered antiferroelectric squaric acid and assuming an ice rule type interaction mechanism of the protons it is shown that aboveTcthe molecular layers may be considered as a system of parallel and orthogonal nearly independent linear Ising spin chains with strong intrachain and comparatively weak interchain interactions. Whereas the former are treated exactly as nearest neighbour interactions the interchain interactions are taken into account either by the molecular field approximation or, in computer simulations, by nearest neighbour interactions. The parameters of the model are related to the unusually small transition entropy, to the high and nearly temperature independent dielectric constant aboveTc,and to neutron scattering data.

Specific heat of the two dimensional antiferroelectric squaric acid

The heat capacity of a single crystal of the two dimensional antiferroelectric squaric acid was measured particularly near the phase transition. In a wide temperature range belowTc a strong temperature dependence of the anomaleous specific heat is found which can be described by a critical exponentα′=0.5. The behaviour close toTc is determined by a latent heat thus indicating the phase transition to be of the first order. Near the phase transition the thermal behaviour can be described by a Landau type theory. The smallness of the transition entropy detected may be ascribed possibly to proton tunneling.

Observation of87Rb NMR satellite transitions in the commensurate and incommensurate phases of Rb2ZnCl4

It is shown that the first order quadrupole split NMR satellite transition frequencies of the87Rb nucleus can be detected in the paraelectric, incommensurate and ferroelectric phases of Rb2ZnCl4. From rotation patterns the electric field gradient tensor at the Rb sites is determined for the paraelectric phase. The data demonstrate a considerable influence of the structural changes in the incommensurate and ferroelectric phases on the observed NMR transition frequencies. For some crystal orientations the satellite transitions are followed through the incommensurate into the ferroelectric phase. Whereas in the former typical quasi continuous spectra are observed in the latter several sharp lines appear. The results are discussed in relation to the structural changes at the phase transitions.

Study of commensurate and incommensurate phases in NaNO2 by 23Na NMR

Abstract The temperature dependence was determined of the 23 Na NMR spectrum of NaNO 2 with special emphasis on the small incommensurate phase and the surroundings of the phase transition points. Whereas far away from the transitions satellite lines of Gaussian shape with the natural line widths are observed near the transitions broad line distributions are detected showing distinct edge singularities. The results are related to the sinusoidally modulated and homogeneous order parameters of the incommensurate and ferroelectric phases.

Brillouin-scattering at the structural phase transition of the two dimensional antiferroelectric squaric acid

Abstract Hypersonic elastic constants were determined by multipass Brillouin-spectroscopy between - 80°C and 130°C, particularly close to Tc. The spectra observed reflect the dynamics of the order parameter. Particularly a remarkable central peak could be detected.

Phase transition mechanism of the two dimensional antiferroelectric squaric acid

Abstract The results are presented and discussed of recent experiments concerning the mechanism of the structural phase transition in squaric acid. Various static material quantities, particularly the optical birefringence and the specific heat, behave in a manner which is consistent with a first order phase transition to be described by a Landau type equation of state. The order of magnitude of the order parameter frequency as derived from several experiments is given. Other experiments justify the two dimensional character of the interactions of the protons in this substance.

CRITICAL DYNAMICS ABOVE THE INCOMMENSURATE PHASE TRANSITION OF RB2ZNCL4 STUDIED BY NMR

Order parameter (OP) fluctuations are studied in the normal (N) phase of the incommensurately modulated (IC) system Rb2ZnCl4 by measuring the 87Rb NMR spin-lattice relaxation time T1. The results are analysed on the basis of dynamical-scaling laws. Since both the temperature and the frequency dependence of T1 reveal the slowing-down of the OP dynamics below the Larmor frequency near the N-IC phase transition, the critical dynamics could be studied quantitatively. As an upper bound for the phason frequency gap a value of 100 MHz is estimated.

Static dielectric properties of ammonium tetrachlorozincate

Abstract Single crystals of (NH4)2ZnCl4 (AZC) were grown from aqueous solution of molar composition 1:1 of the components NH4Cl and ZnCl2. The static dielectric constants ea, eb, ec were measured in the normal, the incommensurate and the various commensurate phases of AZC. Whereas ec is not both ea and eb are generally influenced differently by the various phase transitions. Special emphasis is laid upon the circumstances which cause a peculiar anomaly of ea to appear at about 46 [ddot]C. The temperature dependences of the spontaneous respectively induced polarization in the 3C, (7/2)C and 4C phases are determined. The domain pattern of the 4C phase observed by polarization optical methods for virgin samples differs considerably from that of samples which have been heated up to the IC phase.

Influence of hydrostatic pressure on the polarization dynamics of order-disorder ferroelectrics: AgNa(NO2)2

The characteristic ferroelectric dispersion of the complex dielectric constant of the ferroelectric AgNa(NO2)2 is measured as function of temperature and hydrostatic pressure. For the first time the temperature and pressure dependences of the static order parameter susceptibility and the order parameter relaxation time could be reproducibly determined. The results are interpreted in terms of a kinetic Ising model, whose parameters show a characteristic pressure dependence. The results are discussed on the basis of thermodynamic relations and related to the dynamical behaviour of other order-disorder ferroelectrics.

Dynamics of incommensurate crystals studied by NMR

Abstract The order parameter (OP) dynamics above and below the incommensurate (IC) phase transition of Rb2ZnCl4 and BCCD is investigated by means of nuclear spin-lattice relaxation measurements. The results are interpreted on the basis of a general formalism describing the spin-lattice relaxation rate in terms of the OP normal modes. The consequences for the interpretation of spin-lattice relaxation data in the IC phase both in the plane-wave limit and the soliton regime are pointed out. In particular, at the IC phase transition of Rb2ZnCl4 the critical slowing down to frequencies below 100 MHz is observed.