Search

Low temperature specific heat measurements have been performed on Mn3B7O13Br and Fe3B7O13I boracites. As expected, we observe mag-netic ordering at low temperatures. The Neel temperature of the Fe3B7O13I (Fe-I) boracite (31.4 K) differs significantly from the Neel temperature of Fe3B7O13Cl (Fe-Cl) boracite (11.8 K, Schnelle et al., 1999). The same is true for the Mn3B7O13Br (Mn-Br) boracite, where we observe a transition at 25.9 K, while Schnelle et al. (1999) observed the transition in Mn3B7O13Cl (Mn-Cl) boracite at 11 K.

The static domain structure of ferroic Mn3B7O13Br, abbreviated in what follows as Mn-Br, synthetic boracite crystals have been analysed by polarized-light microscopy (PLM) and by field emission scanning electron microscopy (FE-SEM) at room temperature. As most boracites, Mn-Br possess dielectric, elastic, magnetic and optical properties of unusual interest. The stable phase of Mn-Br at room temperature has orthorhombic symmetry (point group mm2) which transforms to a cubic phase (space group ) when heating the crystal above 540 K. Optical examination of all samples grown by chemical transport reactions showed a bulky domain configuration which was resolved in a fine lamellar configuration under the electron microscope.

Zn3B7O13Cl and Zn3B7O13Br ferroic boracites were synthesized and their static domain structure and spontaneous birefringence were studied by polarized light microscopy as a function of temperature. The results confirm that the optical indicatrix of the orthorhombic phase in both compositions is biaxial positive type A.

Ferroelectric thin films of (Pb0.93La0.07)(Zr0.53Ti0.47)O3 (PLZT) were grown on Si single crystal substrates, by a high-oxygen pressure RF sputtering technique. Layers of Pt and La0.5Sr0.5CoO3 (LSC...

Thermal properties of Zn3B7O13X (X = Cl, Br, I) boracites have been investigated at the vicinity of the ferroic phase transitions underwent by this triad of crystals. The cubic --> orthorhombic phase transitions in these compositions appear to be first order.

The ferroic domain structure of Co 3 B 7 O 13 Cl boracite single crystals has been investigated by transmission electron microscopy (TEM) in conjunction with polarised-light microscopy (PLM). TEM studies revealed the existence of microtwins lamellae with (001) as twin plane.

The domain structures of Zn(3)B(7)O(13)Cl, Zn(3)B(7)O(13)Br and Zn(3)B(7)O(13)I boracite single crystals were studied by means of polarized light in conjunction with electron microscopy. Single crystals of the three compositions were grown by chemical transport reactions in closed quartz ampoules, at a temperature of 900 degreesC and were examined by polarizing optical microscopy (PLM), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). For both PLM and SEM, the same as-grown samples were used without having to resort to metallization of the crystal faces. For TEM the single crystals were crushed and mounted on holey carbon films. Comparative electron microscope images were useful for revealing the domain structure of these ferroelectric/ferroelastic materials previously observed between the crossed polars of an optical microscope. X-ray diffraction analysis of the pulverized crystals was performed for this triad of halogen boracites containing zinc as a common metal.

Since the early demonstration of the ferroelectric effect in boracites, the physical properties of several of these materials have extensively been studied. The purpose of this contribution is to summarize recent results on structural and thermal properties in some members of this large family of ferroics.

Results of our investigations of the fully ferroelectric-fully ferroelastic domain structure in Co3B7O13Cl single crystals by means of polarized-light and electron microscopy are reported. The isobaric heat capacity as well as the dielectric permittivity of this boracite have also been measured in the temperature range from 300 K up to 650 K.

The isobaric molar heat capacity of Ni3B7O13Cl boracite at the ferroelectric 43m-mm2 phase transition is reported. Spontaneous birefringence measurements on single crystals of this composition from room temperature up to the transition temperature are also reported. It is shown by detailed analysis that antiphase domains are not to be expected in the cubic phase.

The domain structure of Ni3B7O13Cl single crystals has been studied in the orthorhombic phase of this ferroic boracite by means of transmission electron microscopy. Calorimetric measurements on this composition are also reported.

Crystals of halogen manganese boracite, the mineral Mg3B7O13Cl, which is currently found in bedded sedimentary deposits of anhydrite, gypsum and halite, have been grown by chemical transport reactions and were examined by polarizing light microscopy and scanning electron microscopy. For both methods the same as-grown samples were used without having to metallize the crystal faces. Comparative electron microscope images were useful not only for observing the charging mechanism of an insulating sample bombarded by an electron beam but also for revealing the domain structure of these ferroelectric/ferroelastic materials previously observed between the crossed polars of a light microscope. EDS qualitative analysis of the crystal faces was performed for the three compositions under study, i.e. Mn3B7O13Cl, Mn3B7O13Br and Mn3B7O13I.

The static ferroelectric/ferroelastic domain structure in uncoated samples of Ni3B7O13Cl single crystals has been observed in the orthorhombic phase of this ferroic boracite by means of scanning electron microscopy and polarised light microscopy. Analytical and optical measurements on the crystal faces are also reported.

PZT films were successfully grown on SrTiO3 and Sr(Nb)TiO3 single crystal substrates by a high-oxygen pressure (about 1 Torr) rf sputtering technique. The structural properties of the films were evaluated by θ/2θ, ω and ϕ XRD-scans, and by RBS spectroscopy. XRD data showed epitaxial growth of the films, with the orientation relationships PZT [001] parallel to [001] of the substrate, and PZT [100] parallel to [100] of the substrate. RBS analyses showed a sharp film–substrate interface, with minimal lead diffusion. The ferroelectric hysteresis loops measured in the Pt/PZT/SNTO heterostructures show good ferroelectric behavior. The I–V characteristics measured in the Pt/PZT/LSCO/Si heterostructure showed that the conducting behavior is in agreement with the spatial charge limited current (SCLC) theory.

The principal dielectric constant, e33, and the spontaneous polarization, Ps of the trigonal, monoclinic and orthorhombic phases of Cobalt-Chlorine boracite has been measured between 300 to 623 K. At the onset of the 43m-mm2 transition at about 623 K, upon heating the permittivity undergoes a drastic jump downwards. Specific heat measurements in the vicinity of the cubic-orthorhombic phase transition are also reported.

Fully ferroelectric/fully ferroelastic crystals of Manganese-iodine boracite (Mn3B7O13I) were grown by chemical reactions in the vapour phase. Results of observations of the domain structure in uncoated samples of this composition, both by polarized light microscopy and by scanning electron microscopy are presented.

The phase transition sequence 3m − mm2 in Mn3B7O13Br (Mn-Br) boracite was investigated by means of spontaneous birefringence, dielectric constant, pyroelectric and thermal measurements. Results of calorimetric measurements give evidence of a new phase transition in this compound at about 440 K, however, this is not reconfirmed by dielectric and optical studies.

Single crystals of Mn 3 B 7 O 13 Br (MnBr) boracite were studied between 300 and 550 K by means of thermal, dielectric, pyroelectric and ferroelectric characterization, under simultaneous visual control of the ferroelectric/ferroelastic domain state of the samples. We observe an unusual change of sign of the pyroelectric coefficient in MnBr at about 440 K which can be an indication of the existence of a new phase transition in this compound. A preliminary DSC study seems to confirm the presence of such a transition.

Using polarizing microscopy in reflected light mode, we were able to observe domain structure in the BiSrCaCuO single crystals (BSCCO) at room temperature. Formation of domains structure is a phenomenon ocurring in ferroic phase transitions. In this paper we present experimental results of optical studies of the domain structure observed in BSCCO single crystals. We observed {110} domains walls only. No phase transition was observed at high temperature when the crystal was heated up to 700°C.

In a combined x-ray diffraction and density functional theory based study the bonding properties of several halogenide boracites have been investigated. We show that DFT calculations reliably reproduce the small spontaneous deformations of non-cubic boracites and that they therefore can be used to predict pressure-induced structural changes. Specifically, we predict the compressibility and the pressure-induced deformation of Mg-Cl boracite. Earlier findings concerning an unusual bond length in Co-Cl boracite are challenged. The arrangement of the halogenide ions and metal cations along chains running through the structure is rationalized in terms of electronegativity differences. We also show that the substitution of the halogenide or metal ions has no significant effect on the B-O bonds.