Hans Schmid

Some Properties of Ferromagnetoelectric Nickel‐Iodine Boracite, Ni3B7O13I

Ferroelectricity and weak ferromagnetism have been found to set on simultaneously in Ni3B7O13I at about 64°K. This is evidenced by dielectric hysteresis, spontaneous Faraday effect, quadratic magnetoelectric hysteresis, etc. The strong coupling between the mutually perpendicular spontaneous polarization—[001]—and spontaneous magnetization—[110]—is such that, when the former is reversed, the latter turns by 90°. The magnetic point group is most probably m′m2′. Dielectric constant, magnetic, and magnetoelectric susceptibilities and magnetic coercive field are shown as a function of temperature.

Die Synthese von Boraziten mit Hilfe Chemischer Transportreaktionen

Zusammenfassung Es wird ein in Quarzampullen arbeitendes Transportreaktionsverfahren beschrieben, mit dem eine grosse Anzahl von Boraziten (Me3B7O13X, Me = ein zweiwertiges Metall, X = Cl, Br oder J) hergestellt wurde; darunter waren die Cr-Br, Cr-J-, Cu-Cl-, und Cu-Br-Borazite bis jetzt nicht bekannt. Die Verbindungen wurden mieist in Einkristallform erhalten. MeO lasst sich am besten mit Hilfe der reversiblen Reaktion MeO(f) + 2 HX(g) = MeX2(g) + H2O(g) oder MeO(f) + X2(g) = MeX2(g) + 0.5 O2 und CuO uber die Kupfer(I)halogenide transportieren. B2O3 lasst man vorzugsweise als flussigen Film zum wachsenden Kristall diffundieren. Bei hohen HX-und H2O-Drucken bildet sich Borazit unter Beteiligung von B2O3-Transport uber die Gasphase. Es werden thermodynamische Abschatzungen der Lage einer Reihe von Reaktionsgleichgewichten fur den Transport von MeO und B2O3 vorgenommen. Die Ergebnisse werden in bezug auf die beobachteten Kristallisationseigenschaften der Borazite besprochen. Auf Grund von MeO- und B2O3-Transport uber die Gasphase wurden mehrere bekannte und einige neue Uebergangsmetallborate als Borazitnebenprodukt erhalten. In einem orientierenden Versuch konnte Mg-Cl-Borazit bei etwa 150 bis 180°C mittels Transportreaktionen in wassriger Losung dargestellt werden. Die Gitterkonstanten und α-β-Umwandlungstemperaturen der dargestellten Borazite wurden bestimmt. Co-J-Borazit zeigt einen Alexandriteffekt. Die Fe-Cl-, Fe-Br-, Ni-Cl-, Ni-Br-, Ni-J- und Cu-Br-Borazite zeigen Thermochroismus.

Dielectric properties of boracites and evidence for ferroelectricity

Abstract The dielectric constants of Ni-Cl, Ni-Br, Co-Br and Co-I boracites have each a maximum at a temperature which coincides with that of a phase transformation we have observed optically. Motion of ferroelectric domain walls under an applied electric field has been sought for in Ni-Cl boracite and found to occur at temperatures as far as 80° below that of the phase transition.

Magnetic susceptibilities of some 3d transition metal boracites

Abstract Effective magneton numbers and Curie-Weiss temperatures of eleven 3d-metal boracites have been determined from susceptibility versus temperature measurements. All investigated boracites display negative Curie-Weiss temperatures. Crue5f8Iue5f8 and Niue5f8Iue5f8boracites become anti-ferromagnetic at about 90° and 120°K, respectively. Niue5f8Iue5f8boracite shows an anomalous peak of susceptibility at about 60°K which coincides with the temperature of the simultaneous onset of ferro-electricity and weak ferromagnetism.

Proprietes magnetiques des boracites des metaux de transition (3d)

Resume Nous avons mesure les proprietes magnetiques de 13 boracites de metaux de transition, pour des temperatures comprises entre 1,8° et 300°K. Tous les composes sordonnent antiferromagnetiquement a basse temperature et possedent un moment ferromagnetique lie a lordre antiferromagnetique, a lexception des boracites de chrome.

Pyroelectricity and related properties in the fresnoite pseudobinary system Ba2TiGe2O8-Ba2TiSi2O8

Complete solid solubility is found in the pseudobinary system Ba2TiGe2O8-Ba2TiSi2O8 by studies on ceramics and single crystals (Czochralski technique). The spontaneous birefringence perpendicular to the polar axis of the orthorhombic low temperature phase has been measured versus temperature and silicon contents: the temperature of the phase transition of species 4mmFmm2 is of second order and decreases from 1103 K at 0 at % Si to about 400 K at 40 at % Si. The pyroelectric coefficient in the orthorhombic mm2 and the tetragonal 4 mm phase is of the order of 10−6 C m−2 K−1 in the entire system and has a positive temperature coefficient for all compositions studied except for Ba2TiGe2O8, in which the pyroelectric coefficient changes sign at about 308 K. The spacegroup of the orthorhombic phase is proposed to be C2v11-Cmm2.

Mössbauer effect and optical evidence for new phase transitions in Fe-Cl-, Fe-Br-, Fe-I-, Co-Cl- and Zn-Cl- boracite

Abstract Mossbauer measurements on Fe-Cl-, Fe-Br-, and Fe-I- boracite showed a new transition temperature which was recognized optically as a phase transition from orthorhombic to trigonal symmetry. The same type of transition was observed optically in Co-Cl- and Zn-Cl- boracite.

Gas phase synthesis of epitaxial layers of nickel-chlorine boracite on chromium-chlorine boracite

Abstract Layers of Ni-Cl boracite have been epitaxially grown on Cr-Cl boracite at 900°C. At the reaction temperature both substrate and layer had the cubic 4 3m symmetry whereas at room temperature the layer is ferroelectric (symmetry mm2) and the substrate is still cubic. The layers have been produced both by (i) CVT reactions in closed quartz ampoules and by (ii) two variants of a CVD process using either the reactants BCl3(g) + H2O(g), or B2O3(1) + H2O(g) for the formation of the intermediate gaseous (BOCl)3 and the B2O3 hydrates. The reactant NiCl2(g) has been obtained by the reaction Ni(s) + Cl2(g) = NiCl2(g) at about 1000°C inside the reactor. Layer thicknesses from 1 up to 100 μm have been achieved. The growth rate was of the order of 1 μm/min. The best substrates were found to be those with smooth, as-grown surfaces; these layers were characterized after cooldown by a pattern of ferroelectric 180° domains with the spontaneous polarization oriented perpendicularly to the surface, an average domain size between 1 and 10 μm and by good contrast (at λ = 546 nm) between antiparallel domains placed between crossed polarizers and a retarder. The ferroelectric poling fields were > 107 V m−1.

Review of ferroelectric materials usable for passive electro-optic alphanumeric display devices

Abstract Among optical field effect phenomena of known solid state materials, only three are suitable for alphanumeric displays: (i) electric field switching of the spontaneous birefringence, (ii) electric field induced birefringence and (iii) electric field switching of the spontaneous optical rotatory power (electrogyration). The birefringence effects are necessarily coupled with mechanical deformations, whereas electrogyration is not. Among the known ferroelectrics, only a few, Gd2(MoO4)3, boracites, Bi4Ti3O12, BaTiO3 and PLZT come close to meet the requirements for alphanumeric displays. For large scale production, the development of shear free or very low shear compositions-for single crystals, layers and ceramics-appears mandatory. Possibilities will be discussed. Some PLZT compositions are in an advanced development state.

Spontaneous polarization, dielectric constant, D.C. resistivity, and specific heat of orthorhombic boracite Fe3B7O13I

Abstract Several properties of ferroelectric orthorhombic Fe-I boracite have been measured; some individual values are: spontaneous polarization Ps (25°C) = 3.9 ± 0.1 × 10−2 cm−2, dielectric constant e33 (25°C) = 11, D.C. resistivity ρ (71°C, mm2) = 1.1 × 108 Ωm. ρ (71°C, 43m) = 3 × 106 Ωm, volume specific heat Cv (25°C) = 3.5 ± 0.3 × 106 Jm−3 K−1, latent heat ΔQ = 3.1 + 0.3 × 103 Jmole−1, transition enthalpy ΔS = 8.9 ± 0.5 Jmole−1 K−1.