[en] The crystal structures of Rb₂ZnCl₄ at 300, 235, 100 and 60 K were investigated from neutron single-crystal diffraction data. Refinements in space groups Pnma (Z=4, average structure of the incommensurate phase at 300 and 235 K) and Pna2₁ (Z=12, 100 and 60 K) converged to R values of 0.084, 0.085, 0.046 and 0.025, respectively. The strong anisotropy of the thermal parameters of chlorine atoms at 300 and 235 K suggests the presence of a static modulated distortion in the incommensurate phase. However, this displacive model is not sufficient to account for the successive phase transitions. Evidence is given that the ZnCl₄ group undergoes a large libration motion. The orientation of each group derives from its position in the paraelectric phase (T > 302 K) by a rotation around the pseudo-hexagonal axis a which is the direction of tripling of the ferroelectric-phase unit cell (T < 189 K). The structure of this ferroelectric phase is given. At 60 K a space group is proposed for the low-temperature phase (T < 74 K) based both on an analysis of the experimental data and on group-theory considerations. The present structural results are compared with those obtained by NMR and NQR spectroscopy. (Auth.)

[en] At CERN there are currently 200 ventilation air handling units in production, used in many different applications, including building ventilation, pressurization of safe rooms, smoke extraction, pulsion/extraction of experimental areas (tunnel, cavern, etc.), and the ventilation of the computing centre. The PLC applications which operate these installations are currently being revamped to a new framework (UNICOS CPC). This work began 3 years ago, and we are now in a position to standardize the development of these HVAC applications, in order to reduce the cost of initial development (including specification and coding), testing, and long-term maintenance of the code. In this paper we will discuss the various improvements to the process, and show examples, which can thus help the community develop HVAC applications. Improvements include templates for the ^Functional Analysis specification document, standardized HVAC devices and templates for the PLC control logic, and automatically generated test documentation, to help during the Factory Acceptance Test (FAT) and Site Acceptance Test (SAT) processes. (author)

[en] Recent progress concerning lattice dynamics in quasicrystals, both theoretical and experimental, is discussed. The theory deals with the general description, which differs from that for ordinary crystals, and with model calculations and rigorous results for one-, two-, and three-dimensional systems. Experimental results are restricted mainly to quasicrystals with icosahedral symmetry. copyright 1997 The American Physical Society

[en] The low frequency dynamics related to the second order transition which takes place in ammonium deuterated oxalate has been investigated by coherent inelastic neutron scattering. This transition leads to a ferroelastic phase at zero pressure and to an incommensurate phase at 5kbar. At both pressures, our experiments show that the coupling between a reorientation motion of one family of ND₄⁺ ions and the TA phonons propagating along c^* and polarized along a drives the transition, the ND₄⁺ being coupled through a pressure dependent ANNNI model. Weak changes in the parameter explain the changes with pressure of the static and dynamical properties of the transition. 19 refs., 11 figs., 1 tab

[en] Inelastic neutron scattering experiments in K₂SeO₄, Cs₂SeO₄ and K₂CrO₄ are reported. The lowest Σ₃ - Σ₂ phonon branches along the a^* -axis in their Pnam phase have been characterized for the three compounds. The results are compared with recent lattice dynamical simulations using rigid-ion models. Potassium chromate, that simulations suggested to be incommensurate at low temperatures, evidences no structural phase transition down to 12 K, but its lowest Σ₂ phonon branch exhibits a significant softening and decreases more than 20% as the temperature is lowered from 300 K to 36 K. The softening mechanism in this compound and in potassium selenate is related to an increase of the effective interaction between an acoustic and an optical branch. (Author)

[en] The pretransitional fluctuations in the normal paraelectric phase have been studied by quasi-elastic neutron scattering. They are located at the satellite positions of the incommensurate phase. Correlation lengths σ_i are given and the dynamics is described by a relaxational process. We studied the width of the Σ2 phonon (soft in K₂SeO₄) as a function of q and T. We report a structural neutron analysis of the incommensurate phase, which is described by the P_Tss^Pnma (α00) superspace group. Structural results are connected with dynamic ones to explain the mechanism of the normal-incommensurate phase transition. (author)

[en] Heat capacity measurements of protonated lithium acetate dihydrate show a structural phase transition at T = 12 K. This finding is in contrast to earlier work, where it was thought that only the deuterated compound undergoes a low temperature structural phase transition. This finding is confirmed by low temperature ultrasound spectroscopy, where the structural phase transition is associated with a velocity decrease of the ultrasonic waves, i.e. with an elastic softening. We compare the thermodynamic properties of the protonated and deuterated compounds and discuss two alternatives for the mechanism of the phase transition based on the thermal expansion measurements.