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Nonlinear optical crystals play important roles in modern laser science and technology. However, the design and growth of new nonlinear optical (NLO) materials is still a challenging issue for researchers. Due to the excellent performance of Mg3B7O13Cl crystal, we paid attention to the optimization of its structure, in order to find new NLO materials with favorable properties. Here, Zn3B7O13Cl crystals were obtained by a high-temperature solution method. Its structure was determined to be the trigonal symmetry with a polar space group of R3c, which is more highly symmetric than that of Mg3B7O13Cl (Pca21). The experimental and theoretical investigations demonstrated that the title compound exhibits a short absorption cutoff (band gap ∼6.53 eV), moderate SHG responses (2.2 times that of KDP at 1064 nm), and the improved birefringence, which results from the large distortion and anisotropy of borate groups and zinc polyhedrons. Therefore, the structural modification of Mg3B7O13Cl by zinc cations achieves a balance between the deep-ultraviolet transparency, the nonlinear optical effect, and the moderate birefringence, which is very significant for the design of practical NLO materials.

Domain walls in Cu–Cl boracite develop as a consequence of an improper ferroelastic, improper ferroelectric transition, and have attracted close interest because some are conductive and all can be mechanically written and repositioned by application of an electric field. The phase transition and its associated dynamical properties have been analysed here from the perspective of strain and elasticity. Determination of spontaneous strains from published lattice parameter data has allowed the equilibrium long-range order parameter for F 4 ̄ 3c → Pca21 to be modelled simply as being close to the order–disorder limit. High acoustic loss in the cubic phase, revealed by resonant ultrasound spectroscopy, is consistent with the presence of dynamical microdomains of the orthorhombic structure with relaxation times in the vicinity of ∼10−5–10−6 s. Low acoustic loss in the stability field of the orthorhombic structure signifies, on the other hand, that ferroelastic twin walls which develop as a consequence of the order–disorder process are immobile on this time scale. A Debye loss peak accompanied by ∼1% elastic stiffening at ∼40 K is indicative of some freezing of defects which couple with strain or of some more intrinsic freezing process. The activation energy of ⩾∼0.01–0.02 eV implies a mechanism which could involve strain relaxation clouds around local ferroelectric dipoles or freezing of polarons that determine the conductivity of twin walls.

During switching, the microstructure of a ferroelectric normally adapts to align internal dipoles with externally applied electric fields. Dipolar regions (domains), that are favourably oriented, grow at the expense of those in unfavourable orientations and this is manifested in a predictable field-induced motion of the walls that separate one domain from the next. Here, we report the discovery that specific charged 90o domain walls in copper-chlorine boracite move in the opposite direction to that expected, increasing the size of the domain in which polarisation is anti-aligned with the applied electric field. As a consequence, polarisation-field (P-E) hysteresis loops, inferred from optical imaging, show negative gradients and non-transient negative capacitance, throughout the P-E cycle. Switching currents (generated by the relative motion between domain walls and sensing electrodes) confirm this, insofar as their signs are opposite to those expected conventionally. For any given bias, the integrated switching charge due to this anomalous wall motion is found to be directly proportional to time, indicating that the magnitude of the negative capacitance component should be inversely related to frequency (for a given applied ac field). This passes Jonscher’s test for the misinterpretation of positive inductance (associated with an inverse square relationship) and gives confidence that field-induced motion of these specific 90o charged domain walls generates a measurable negative capacitance contribution to the overall dielectric response.

Domain walls in Cu–Cl boracite develop as a consequence of an improper ferroelastic, improper ferroelectric transition, and have attracted close interest because some are conductive and all can be mechanically written and repositioned by application of an electric field. The phase transition and its associated dynamical properties have been analysed here from the perspective of strain and elasticity. Determination of spontaneous strains from published lattice parameter data has allowed the equilibrium long-range order parameter for F 4̄ 3c → Pca21 to be modelled simply as being close to the order–disorder limit. High acoustic loss in the cubic phase, revealed by resonant ultrasound spectroscopy, is consistent with the presence of dynamical microdomains of the orthorhombic structure with relaxation times in the vicinity of ∼10−5–10−6 s. Low acoustic loss in the stability field of the orthorhombic structure signifies, on the other hand, that ferroelastic twin walls which develop as a consequence of the order–disorder process are immobile on this time scale. A Debye loss peak accompanied by ∼1% elastic stiffening at ∼40 K is indicative of some freezing of defects which couple with strain or of some more intrinsic freezing process. The activation energy of ⩾∼0.01–0.02 eV implies a mechanism which could involve strain relaxation clouds around local ferroelectric dipoles or freezing of polarons that determine the conductivity of twin walls.

In this study, solubility measurements regarding boracite [Mg3B7O13Cl(cr)] and aksaite [MgB6O7(OH)6·2H2O(cr)] from the direction of supersaturation were conducted at 22.5 ± 0.5 °C. The equilibrium constant (log10K0) for boracite in terms of the following reaction, Mg 3 B 7 O 13 Cl cr + 15 H 2 O l ⇌ 3Mg 2 + + 7B OH 4 − + Cl − + 2H + is determined as −29.49 ± 0.39 (2σ) in this study. The equilibrium constant for aksaite according to the following reaction, MgB 6 O 7 OH 6 · 2H 2 O cr + 9H 2 O l ⇌ Mg 2 + + 6B OH 4 − + 4H + is determined as −44.41 ± 0.41 (2σ) in this work. This work recommends a set of thermodynamic properties for aksaite at 25 °C and 1 bar as follows: ΔHf0 = −6063.70 ± 4.85 kJ·mol−1, ΔGf0 = −5492.55 ± 2.32 kJ·mol−1, and S0 = 344.62 ± 1.85 J·mol−1·K−1. Among them, ΔGf0 is derived from the equilibrium constant for aksaite determined by this study; ΔHf0 is from the literature, determined by calorimetry; and S0 is computed in the present work from ΔGf0 and ΔHf0. This investigation also recommends a set of thermodynamic properties for boracite at 25 °C and 1 bar as follows: ΔHf0 = −6575.02 ± 2.25 kJ·mol−1, ΔGf0 = −6178.35 ± 2.25 kJ·mol−1, and S0 = 253.6 ± 0.5 J·mol−1·K−1. Among them, ΔGf0 is derived from the equilibrium constant for boracite determined by this study; S0 is from the literature, determined by calorimetry; and ΔHf0 is computed in this work from ΔGf0 and S0. The thermodynamic properties determined in this study can find applications in many fields. For instance, in the field of material science, boracite has many useful properties including ferroelectric and ferroelastic properties. The equilibrium constant of boracite determined in this work will provide guidance for economic synthesis of boracite in an aqueous medium. Similarly, in the field of nuclear waste management, iodide boracite [Mg3B7O13I(cr)] is proposed as a waste form for radioactive 129I. Therefore, the solubility constant for chloride boracite [Mg3B7O13Cl(cr)] will provide the guidance for the performance of iodide boracite in geological repositories. Boracite/aksaite themselves in geological repositories in salt formations may be solubility-controlling phase(s) for borate. Consequently, solubility constants of boracite and aksaite will enable researchers to predict borate concentrations in equilibrium with boracite/aksaite in salt formations.

Charged domain walls form spontaneously in Cu-Cl boracite on cooling through the phase transition. These walls exhibit changed conductivity compared to the bulk and motion consistent with the existence of negative capacitance. Here, we present the dielectric permittivity and DC resistivity of bulk Cu-Cl boracite as a function of temperature (-140 {\deg}C to 150 {\deg}C) and frequency (1 mHz to 10 MHz). The thermal behaviour of the two observed dielectric relaxations and the DC resistivity is discussed. We propose that the relaxations can be explained by the existence of point defects, most likely local complexes created by a change of valence of Cu and accompanying oxygen vacancies. In addition, the sudden change in resistivity seen at the phase transition suggests that conductive domain walls contribute significantly to the conductivity in the ferroelectric phase., Comment: 9 pages, 4 figures

A cubic boracite with substituted boron sites, Li4B4M3O12Cl (M = Al, Ga), derived by fully replacing the tetrahedral BO4 units in the parent lithium chloroboracite, Li4B7O12Cl, with AlO4 or GaO4 units, has been discovered. These substituted compounds have the largest unit cell dimensions of known boracites and are formed as the stable primary phase in highly crystalline glass-ceramics derived from the Li2O–B2O3–M2O3–LiCl quarternary system. The conductivity of Li4B4M3O12Cl glass-ceramics at room temperature was ∼10−5 S cm−1, an order of magnitude larger than the highest conductivity recorded for Li4B7O12Cl glass-ceramics. The Li4B4Al3O12Cl glass-ceramic is stable in contact with Li metal and exhibits a wide electrochemical window between 0 and 6 V vs. Li/Li+ and a Li+ ion transport number of ∼1.

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.

Occurrences of borate minerals in the Zechstein salt-bearing deposits of the Klodawa salt dome (central Poland), and the manner of their development, suggest that the minerals were formed at high temperatures, that considerably exceed the temperature estimated from the thermal gradient (~180°C). Research on rock salt and potassium-magnesium salts containing congolite and boracite, respectively, are consistent with high-temperature processes of transformations affecting the salt rocks in certain sections of the salt dome. The chemical composition of, and daughter minerals occurring in, primary fluid inclusions in halite, originating from the congolite zone, indicated a very high proportion of potassium and magnesium in the brines from which the halite crystallised. The thermal transformations observed in inclusions indicate a halite crystallisation temperature exceeding 420°C. Anhydrite crystals, co-occurring with borate minerals, represent unique features as to the distribution and composition of solid and fluid inclusions. These features indicate crystallisation or recrystallisation in conditions that differed considerably from those typical of the salt dome, with the involvement of solutions of changing chemical compositions. The crystals contain multiple solid inclusions of transparent and non-transparent minerals, among which we have focused on carnallite. The relationships of carnallite with liquid and gaseous phases indicate, as in the congolite zone, migration of very highly concentrated brines with respect to potassium and magnesium, or even transport of carnallite in the form of melt (liquid). Measurements of fluid inclusion homogenization temperatures, within the range from 197.8 to 473.8°C, supported a high-temperature origin for these minerals in hydrothermal conditions.

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.

The evaporites on the Khorat Plateau comprise one of the largest potash deposits in the world, and their origin has long been a controversial problem. Based on boron isotope measurements from borate as a good indicator to distinguish marine and nonmarine evaporites, the borates of potash layers were identified and the results were used to indicate the source of evaporites in the study area as representative of the whole Khorat Plateau. The results show that the main borates include boracite and hilgardite. The boracites occur as crystals and ooids with minor amounts of hilgardite aggregates. The range of δ11B values is from +21.30‰ to +32.94‰ (averaging +29.74‰) and falls in the range of marine borates. The δ11B values in the potash layer are seemingly variable because of the influence of fluvial influx when salts were precipitating. Although the composition of seawater has possibly been modified, the evidence from boron isotopic composition implies that the evaporites in the study area or in the whole Khorat Plateau are marine deposits.

Pseudocubic crystals of (Fe,Mg,Mn) 3 B 7 O 13 Cl borate, ≤ 1.2 mm in size and yellowish through pale-violet to pale-violet-brown and brownish in color, were found in the Klodawa salt dome, located within the Mid-Polish Trough (central Poland), an axial part of the Polish basin belonging to a system of Permian-Mesozoic epicontinental basins of Western and Central Europe. The crystals occur in adjacent parts of the Underlying Halite and Youngest Halite units bordering the Anhydrite Pegmatite unit around the PZ–3/PZ–4 (Leine/Aller) boundary. The internal texture of the crystals reflects phase-transitions from rhombohedral congolite (space-group symmetry R 3 c ) through orthorhombic ericaite ( Pca 2 1 ) to a cubic, high-temperature, mainly Fe-dominant analogue of boracite ( F 43 c ), indicating formation of the phases under increasing temperature. Currently, the parts of the crystals showing orthorhombic and cubic morphology generally represent congolite and, much less frequently, trembathite paramorphs after higher-temperature structural varieties of (Fe,Mg,Mn) 3 B 7 O 13 Cl and (Mg,Fe,Mn) 3 B 7 O 13 Cl, which are unstable at room-temperature. The phase-transitions R 3 c → Pca 2 1 and Pca 2 1 → F 43 c recorded in the morphology of the growing crystals took place at temperatures of 230–250 °C and 310–315 °C, respectively, whereas the total range of (Fe,Mg,Mn) 3 B 7 O 13 Cl borate crystallization was determined to be ca . 100 °C to 350–360 °C. The primary congolite crystallized as a result of the metasomatic reaction of Fe and Mn carriers present within the host rock-salts of the Underlying Halite and Youngest Halite with (BO 3 ) 3− -bearing brines and fluids released from primary evaporate sediments (probably mainly from Upper Younger Halite with potash impurities of the PZ–3 cyclothem) in diagenetic and metamorphic processes induced by disturbances in halokinetic stability, intensive faulting of the pre-Zechstein basement and decompression, and, connected with movements of evaporates, their upward flow in the crust and the formation of a diapiric structure. The phase-transitions R 3 c → Pca 2 1 and Pca 2 1 → F 43 c have been forced by inflows of Mg-bearing fluids sourced from relatively thick loads of potash salts (Younger Potash) of the PZ–3 cyclothem, containing inclusions of carnallite, kainite, and kieserite. The brines and fluids have utilized cracks in Pegmatite Anhydrite as convenient pathways for migration in the dome, in contrast to the host rock-salts, which are too solid to be penetrated by the fluids for distances greater than a few meters.

Boracite is a magnesium borate mineral with formula: Mg(3)B(7)O(13)Cl and occurs as blue green, colorless, gray, yellow to white crystals in the orthorhombic - pyramidal crystal system. An intense Raman band at 1009 cm(-1) was assigned to the BO stretching vibration of the B(7)O(13) units. Raman bands at 1121, 1136, 1143 cm(-1) are attributed to the in-plane bending vibrations of trigonal boron. Four sharp Raman bands observed at 415, 494, 621 and 671 cm(-1) are simply defined as trigonal and tetrahedral borate bending modes. The Raman spectrum clearly shows intense Raman bands at 3405 and 3494 cm(-1), thus indicating that some Cl anions have been replaced with OH units. The molecular structure of a natural boracite has been assessed by using vibrational spectroscopy.

Historical flashlights are presented on the real life background of the discovery in 1964 of the first ferromagnetic/ferroelectric single crystalline material, Nickel Iodine Boracite. The subsequent stony trail until support for scientific follow-up, finally obtained in 1978, is summarised. Leaving out intermediate research milestones, the present day state-of-the-art is succinctly resumed, culminating in the insight that a variety of boracite phases are ferroelastic, ferroelectric, ferromagnetic and ferrotoroidic in the same phase, representing herewith useful model substances for studying a high degree of multiferroic complexity.

Solvothermal reactions of Cd(CH3COO)2�2H2O and NaOH with 1,3,5-benzenetricarboxylic acid (H3BTC) in a mixture of H2O/DMA (DMA = N,N?-dimethylacetamide) gave rise to two novel 3D porous metal?organic frameworks (MOFs), [Cd3Na6(BTC)4(H2O)12]�H2O (1) and [Cd3Na2(BTC)3(H2O)3]�[H2N(CH3)2]�H2O (2), which were characterized by single crystal and variable temperature powder X-ray diffraction (VTPXRD), IR spectroscopy, elemental analyses, inductively coupled plasma measurements, and coupled thermogravimetric?mass spectrometric analyses. Compound 1 crystallizes in the tetragonal space group P42/n and could be described as a 2-fold interpenetrated net having a boracite (bor) topology, bridged by {Na3O12} trimers. Compound 2 crystallizes in the monoclinic space group Cc with 1D channels distributed in an unusual honeycomb fashion along the b-axis. {Na2O10} dimers, (CH3)2NH2+ ions, and guest water are located in these channels. TG-MS and VTPXRD studies revealed that 1 had a high thermal stability up to 340 �C under air, and 2 was stable up to 310 �C. The gas-adsorption investigation disclosed that compound 1 exhibited H2 uptake of 1.11 wt % at 77 K and 1.0 bar, with a high adsorption heat (8.4 kJ mol?1).

New iron-zinc chlorine single crystals of Fe1.5Zn1.5B7O13Cl boracite were grown by chemical transport reactions in closed quartz ampoules, at a temperature of 1173 K. The crystal structure was characterized by X-ray powder diffraction (XRD) using the Rietveld refinement method and belongs to the trigonal/rombohedral system with space group R3c (No. 161). The cell parameters were a = 8.5726(1) angstroms, c = 21.0116(4) angstroms, V = 1337.26(3) angstroms3 and Z = 6. The refinement successfully proceeded and ended with sound merit figure values chi2 = 2.25, R(B) = 6.12%. Chemical analysis was performed with X-ray energy dispersive spectroscopy (EDS) and X-ray fluorescence (XRF). Ferroelectric nano and micro reoriented domains were found in this material using polarizing optical microscopy (PLM), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The examination by TEM showed that in the trigonal/rombohedral system of Fe1.5Zn1.5B7O13Cl nanodomain structures exist. Thin (50-100 nm) mostly planar domains parallel to (100) were frequently observed in Fe1.5Zn1.5B7O13Cl boracite.

A structural model of the cubic paraelectric phase of a Fe3B7O13Br crystal belonging to the boracite family has been developed using the data obtained by single-crystal X-ray diffraction with due regard for the results of extended X-ray absorption fine structure (EXAFS) spectroscopy. It has been shown that the best agreement between the data obtained by these two methods is achieved within a model assuming a disorder in the arrangement of both the Fe and Br atoms and a high degree of correlation of their displacements. It has been found that, during the phase transition from the rhombohedral ferroelectric phase to the cubic paraelectric phase, no significant transformation of the structure is observed on a local level. In this case, a change in the macroscopic symmetry occurs predominantly as a result of the variation in the set of possible spatial orientations of stable structural fragments, which is characteristic of order-disorder phase transitions.

The cubic iron hydroxy boracite Fe3B7O13OH・1.5H2O was synthesized from Fe2O3 and B2O3 under high-pressure/high-temperature conditions of 3 GPa and 960 °C in a modified Walker-type multianvil apparatus. The crystal structure was determined at room temperature by X-ray diffraction on single crystals. It crystallizes in the cubic space group F4̄3c (Z = 8) with the parameters a = 1222.4(2) pm, V = 1.826(4) nm3, R1 = 0.0362, and wR2 = 0.0726 (all data). The B-O network is similar to that of other cubic boracites.

A brief review of research performed on ferroelectromagnetic crystals over the 50years following the discovery of these materials by G. A. Smolenskiĭ and his colleagues is given. During the first decades single crystals and solid solutions of ferrloelectromagnetic substances were synthesized and the first evidence of an interaction between the ferroelectric and magnetic subsystems was obtained. The first two colossal magnetoelectric (ME) effects in nickel-iodine boracite were discovered. The theory explained qualitatively the observed and predicted new ME phenomena. Subsequently, the class of ferroelectromagnets grew, and compounds in which the electric polarization was induced by a spiral magnetic structure appeared. Measuring techniques improved. The discovery at the end of the 20th century of a new optical method of second-harmonic generation set the conditions for a new renaissance in ME studies. An entire series of new colossal ME effects attesting to the realization of cross ME control of the electr...

To investigate the paraelectric-ferroelectric phase transitions in Co3B7O13Cl and Co3B7O13Br boracites, correlations and scaling relations were measured from the domain wall structure at different temperatures using a multifractal analysis. We obtained high-resolution digital photographs and characterized the evolution of the complexity of the domain wall configuration by evaluating its fractal dimension. The change in the energetic state, the specific heat and the persistence of the domain structure was evaluated by measuring the changes of the spatial distribution of walls. We found two kinds of memory in the domain structure of the cobalt bromine boracite.

Solvothermal treatment of copper halides and 3-amino-5-mercapto-1,2,4-triazole (H2amtz) produced three isostructural complexes [Cu31X4(H1.55amtz)24](SO4)8·xDMF [X = Cl, Br and I, x ∼ 32], which show 3-D networks with bor topology constructed by Cu-centered CuCu4 tetrahedra and CuCu3 triangles with an acentric cubic superlarge cell.

The specific heat capacity of mostly single-crystalline samples of 21 boracite compounds ${M}_{3}{\mathrm{B}}_{7}{\mathrm{O}}_{13}X$ with $M$ a $3d$ transition metal (Cr, Mn, Fe, Co, Ni, Cu, Zn) or Mg and $X$ a halogen (Cl, Br, I) is determined. In combination with magnetic susceptibility data the magnetic ordering of the ${M}^{2+}$ ions at ${T}_{N}$ is investigated in detail. The fully ferroelectric/fully ferroelastic structural phase transitions at higher temperatures are measured by differential scanning calorimetry. In the Cr-Br, Cr-I, Cu-Cl, and Cu-Br compounds, previously unknown magnetic phases were found. Magnetic order in the boracites is characterized by the quantum and classical spin states of the ${M}^{2+}$ ions, a variable degree of structural distortion, orbital effects, and competing exchange interactions. The Cu-Cl, Cu-Br, and Ni-Cl boracites exhibit broad maxima of magnetic specific heat and of magnetic susceptibility above ${T}_{N}$ caused by low-dimensional or frustrated magnetic interactions. Co boracites display additional broad anomalies below ${T}_{N}$ originating from continuous spin reorientations and effective $S=1/2$ ground states. Indications for spin reorientations are also observed for Fe boracites. New phases appear in high magnetic fields for some Co and Fe boracites, which is not the case for the Mn compounds. Stronger magnetic frustration is deduced for the cubic Cr compounds. For the latter compounds and Ni-I boracite magnetostructural phase transitions are observed.

A new zinc brome boracite Zn3B7O13Br has been grown by a chemical transport reaction in closed quartz ampoules at 920 K. The crystal structure was characterized by Rietveld refinement. Ferroelectric nano and micro reorientable domains were found in this material using polarizing optical microscopy (PLM), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Chemical analysis was performed with x-ray energy dispersive spectroscopy (EDX). In the crystal, a new structure transition at 586 K from orthorhombic (Pca 21) to cubic cell () has been found. This transition was corroborated by differential scanning calorimetry (DSC).

The study investigated the domain structure of nickel bromine boracite single crystals, by means of polarised-light in conjunction with transmission electron microscopy. Single crystals of Ni3B7O13Br were grown by chemical transport reactions in closed quartz ampoules, in the temperature range of 1130 K and were examined by polarising optical microscopy (PLM), and transmission electron microscopy (TEM). PLM was also used in order to study the behaviour of birefringence as a function of temperature. 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 this fully ferroelectric/fully ferroelastic material previously observed between the crossed polars of an optical microscope. X-ray diffraction analysis of the crystal under study was performed at room temperature. (c) 2005 Published by Elsevier B.V.

Landau theory is used to describe the ferroelectric trigonal phase of Boracites. The temperature dependence of the principal reciprocal susceptibility is obtained. The calculated result shows that at the Curie temperature Tc, the spontaneous polarization jumps. The component of the principal reciprocal susceptibility perpendicular to polarization jumps but the component parallel to polarization does not. A possible experimental scheme for the measurement of the principal reciprocal susceptibility components is presented.

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.

A calculation of the magnetoelectric and dielectric susceptibilities is carried out for a layered structure in which thin toroidal layers alternate with dielectric layers not found in a state of spontaneous toroidal ordering. The toroic considered is nickel iodine boracite. It is shown that an anomalous increase of the magnetoelectric and dielectric susceptibilities can occur in the vicinity of the temperature of the induced toroidal phase transition.

The local structure around Br ion has been studied by Br K-edge XAFS in rhombohedral, orthorhombic and cubic phases of Fe 3 B 7 O 13 Br at temperatures from 300 to 660 K. It has been shown, that in the rhombohedral and orthorhombic phases Br-Fe distances were in a good agreement with X-ray diffraction data, however in the cubic phase the local environment of Br couldn't be described by the ideal cubic model. Br-Fe atomic distribution in the cubic phase appeared to be similar to that in the low-temperature ferroelectric phases. This shows that the cubic phase is essentially disordered.

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.

Solutions for the components of toroidal moment T 1 ( x 1 ), magnetization M 2 ( x 1 ) and polarization P 3 ( x 1 ), constituing a domain wall in a ferrotoroic phase of boracites, are obtained. The wall is assumed to be plane and perpendicular to the x 1 coordinate axis. All quadric in relativistic invariants terms are neglected in comparison with the exchange invariants.

Walkerite, ideally Ca 16 (Mg,Li,□) 2 [B 13 O 17 (OH) 12 ] 4 Cl 6 ·28H 2 O, is a new mineral species from the Potash Corporation of Saskatchewan (New Brunswick Division) mine at Penobsquis, near Sussex, Kings County, New Brunswick. It occurs in the Upper Halite Member of the Windsor Group evaporites, associated with halite, hydroboracite, hilgardite, volkovskite, boracite, szaibelyite, a mica-group mineral and anhydrite. The crystals of walkerite are colorless to white, fibrous to acicular, commonly forming bundles of fibers to 7 mm in length. The crystals are transparent to translucent, with a vitreous luster and a white streak. It is soft (Mohs hardness of 3), brittle with no apparent cleavage, but with a splintery fracture along [001]. Walkerite is biaxial positive, α 1.516 ± 0.002, β 1.532 ± 0.002 and γ 1.554 ± 0.002 (for λ = 590 nm); 2 V meas. = 82(3)°, 2 V calc. = 82°. There is no dispersion and no pleochroism. Optical orientation: X = a , Y = b and Z = c . It is orthorhombic, space group Pba 2, with unit-cell parameters refined from powder data: a 15.52(1), b 22.74(1), c 8.761(4) A, V 3091(2) A 3 and Z = 1. The strongest six powder-diffraction lines [ d in A( I )( hkl )] are: 12.820(100)(110), 7.785(80)(200), 6.319(40)(121), 5.649(30)(211), 3.176(30)(170) and 2.570(30)(461,550). Electron-microprobe analyses, a thermogravimetric analysis and induced coupled plasma – mass spectrometry results, supported by crystal-structure analysis, infrared-absorption spectrometry and secondary-ion mass spectrometry, yielded Li 2 O 0.12, Na 2 O 0.13, K 2 O 0.07, CaO 23.05, FeO 0.32, MgO 0.58, Cl 4.91, B 2 O 3 47.17 and H 2 O 25.48, O = Cl −1.11, total 100.72 wt.%. The empirical formula based on 150 anions, is (Ca 15.60 Na 0.16 K 0.06 ) ∑15.82 (Mg 0.55 Li 0.31 Fe 0.17 ) ∑1.03 B 51.43 O 68 (OH) 48 [Cl 5.26 (OH) 0.74 ] ∑6.00 H 2.53 ·28H 2 O; the idealized formula is presented above. The calculated density (from the empirical formula) is 2.05 g/cm 3 , and the measured density is 2.07 g/cm 3 . The structure has been refined to R = 0.040. The highly corrugated sheets of borate polyhedra are cross-linked by Ca and Mg polyhedra. The Cl atoms and H 2 O groups are located within the ovoid channels. The structure is compared to that of other complex borates, such as penobsquisite and pringleite.

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.

The physical phenomena related to manifestation of the second order magnetoelectric susceptibility in optical region are considered. In magnetoelectrics of the type EB 2 this susceptibility contributes to magneto-induced spatial dispersion optical phenomena such as a non-reciprocal birefringence or a non-reciprocal linear dichroism. The analysis of experimental data in boracite and rare-earth sesquisulphide single crystals shows that the magnetoelectric mechanism is the basic one in formation of these phenomena. In magnetic semiconductors type Cd 1 m x Mn x Te the magnetoelectric effect results in the frequency independent part of non-reciprocal birefringence. The results of the theoretical treatment of the second order magnetoelectric susceptibility in the optical region are presented.

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.

The microscopic theory of second-order magneto-electric susceptibility in optical region is presented for the case of optical transition 4 A 2 ( 4 T 1 ) → 4 E( 4 T 1 ) in the boracite Co 3 B 7 O 13 I. Both the splitting of Kramers doublets of the ground and excited states and the magnetic field dependence of electric and magnetic dipole transition matrix elements contribute to this susceptibility. The experimental data on dispersion, temperature dependence and absolute value of non-reciprocal birefringence in Co 3 B 7 O 13 I can be adequately described in considered model.

The crystal structure of the rhombohedral phase of Fe3B7O13Br was determined and compared with the structures of Fe3B7O13Cl and Fe3B7O13I. The influence of the ionic radii of the halogen atom on the structural features of the crystals of the boracite family is discussed.

The diapiric structures of the Polish Lowlands are tectonically deeply seated down to the autochthonous Zechstein strata at a depth of ca. 6 km. In the process of deep burial and halokinetic diapirism, the salt rocks were subjected to diagenetic and metamorphic transformations, with the temperature being an essential factor. Considering the thermal gradient, a temperature of up to ca. 200°C can be achieved in the salt dome within a depth range from several hundred metres to 6 km, which may lead to transformations of the majority of salt minerals. Phase transitions of two borate minerals – boracite and congolite from the Klodawa salt dome – provide evidence for higher temperatures in the salt dome rocks. The authigenic euhedral crystals of those borate minerals display their external habitus in ambient temperature in the form of regular symmetry (F43 c – pseudo-regular polyhedrons), whereas their internal structure is lower: orthorhombic (Pca2 1 ) for boracite and rhombohedral (R3C) for congolite. The heating and cooling of boracite and congolite crystals show reversible phase transition. At a temperature of ca. 270°C, boracite crystals change their symmetry: orthorhombic ↔ cubic. In the case of congolite three reversible phase transitions within a temperature range of 50–339°C can be observed: rhombohedral ↔ monoclinic ↔ orthorhombic ↔ cubic symmetry. Those phase transitions, confirmed experimentally in our study, clearly document at least local occurrences of temperatures exceeding 339°C in the Klodawa salt dome.

The Fe-Cl boracite is prepared by a procedure, consisting of solution mixing, heating, grinding and H-2 reduction. The high valence metal ion Fe3+ reacts with H-2 to form the low valence metal ion Fe3+ so that the corresponding boracite can be synthesized. The lattice parameters and the phase transition temperatures are determined by means of X-ray diffraction, transmission electron microscopy and differential scanning calorimetry. The hydrogen not only plays a role in reduction, but also prevents the oxidation of the mixture. The ferroelectric property at room temperature of the sample has been substantially demonstrated. (C) 2001 Elsevier Science BY All rights reserved.

A phenomenological theory of the sequence of two second-order phase transitions with close temperatures is considered; such transitions occur in the Ni-Br boracite. The thermodynamic potential is written as a function of polarization Pi, magnetization Mi, and toroidal moment Ti vectors and fields Ei and Hi; Ti is treated as an order parameter. It is assumed that only one coefficient of Ti2passes through zero as T decreases. The possibility of a sequence of two proper ferrotoroidal phase transitions along the T1 and T2 components is demonstrated. Spontaneous Ti, Pi, and Mi vector values and equations for susceptibility tensors (dielectric χij = dPi/dEj, magnetic kij = dMi/dHj, and magnetoelectric αij = dPi/dHj = dMj/dEi) were obtained for three phases. Some of these values have well-defined anomalies in the vicinity of transitions. All possible sequences of ferrotoroidal phase transitions in boracites are considered. Depending on two potential coefficient values, these sequences may consist of one, two, or three such transitions.

Linear nonreciprocal dichroism that is odd in magnetic field B was observed in the transverse geometry and studied for cubic (symmetry class T d ) phase of boracite Co3B7O13I. Nonreciprocal dichroism was observed in the range of absorption bands corresponding to the transitions of the Co2+ ion in the energy range ΔE=1.2–3.2 eV. The sign and magnitude of nonreciprocal dichroism depend on the mutual orientation of the magnetic field and crystallographic axes. Nonreciprocal dichroism refers to the phenomena of magnetically induced spatial dispersion, and its anisotropy is typical for the manifestation of the second-order magnetoelectric susceptibility in the optical range.