ABSTRACT
Pressure has a profound effect on the paraelectric and ferroelectric properties of perovskite crystals. In this paper we theoretically investigate the effect of pressure on the cubic-to-tetragonal phase transition and on the soft mode dynamics of some classical perovskite crystals: BaTiO(3), PbTiO(3), and KNbO(3). We use a model consisting of three subsystems: electrons, phonons, and off-center displacements treated as spins. Experiments show that pressure has a large effect on the tunneling and hopping of the off-center displacements, that in turn strongly affect the pressure dependence of the transition temperature and the soft mode frequency. This model, with a very small number of adjustable parameters, accounts quantitatively for the experimentally measured nonlinear pressure dependence of the cubic-to-tetragonal phase transition temperature, up to the critical pressure where the transition temperature is zero. It also accounts quantitatively for the pressure dependence of the soft mode frequency, which is finite at the phase transition in spite of the fact that the phase transition at elevated pressures is second order, and for the pressure dependence of the electronic gap energy.
ABSTRACT
The complete atomic structure of a five-monolayer film of LaAlO3 on SrTiO3 has been determined for the first time by surface x-ray diffraction in conjunction with the coherent Bragg rod analysis phase-retrieval method and further structural refinement. Cationic mixing at the interface results in dilatory distortions and the formation of metallic La(1-x)SrxTiO3. By invoking electrostatic potential minimization, the ratio of Ti{4+}/Ti{3+} across the interface was determined, from which the lattice dilation could be quantitatively explained using ionic radii considerations. The correctness of this model is supported by density functional theory calculations. Thus, the formation of a quasi-two-dimensional electron gas in this system is explained, based on structural considerations.
ABSTRACT
On average 0.04% of the population is affected by craniostenosis. It is believed there are anatomo-clinical forms at moderate risk and others at high risk. The neurosurgical indications are very different according to various Authors. The forms at high risk which are caused by an isolated synostosis of the coronal system with brachicephalia are either associated to other synostosis (oxicephalia) or to facial synostosis should always be operated, with various techniques, within the first 6 months of life. Basing on the experience of 3 cases of craniostenosis, deliberately not operated on, followed and controlled for over 20 years from the decision of not to operate, all in excellent general condition both neurological and psychiatric, the Authors consider and discuss the criteria of surgical indications with particular regard to clinical objectivity (implicity disregarding a precise value of the craniometric data).
