Stillwellite Structure Research Articles

Thermal evolution of stillwellite, CeBSiO5, a natural prototype for a family of NLO-active materials

Stillwellite, ideally CeBSiO5, is a rare mineral, however, its discovery stimulated the intense studies of a new class of stillwellite-like materials with remarkable electrical and optical properties. With the use of the modern non–ambient single crystal X-ray diffraction, this work sheds some light on the evolution of stillwellite crystal structure during the process of high-temperature transformations noted for the most of these practically important compounds. Here a sample of natural stillwellite [stillwellite-(Ce)] from the Darai-Pioz alkaline massif (Tadjikistan) has been studied by means of electron microscopy, IR spectroscopy, single crystal and powder X-ray diffraction (XRD) in a wide temperature range from −180 up to 1200 ​°C. The dielectric properties are also measured in the range from −180 up to 500 ​°C. The low (LT) and the high temperature (HT) structure deformation and transformation are described from single XRD data including the detail analysis of deformation of cationic polyhedra on heating. The disorder-to-order phase transition occurs in the stillwellite structure at about 450–500 ​°C. Both LT (disordered) and HT (ordered) modifications are characterized as relatively low expansion materials: αvolume ​= ​15 and 19 ​× ​10-6 °C-1 for LT and HT phases, respectively. The high-temperature transformation of stillwellite involves some negative thermal expansion (αvolume ​= ​−11 ​× ​10-6 ​°C-1) caused by order-disorder process and presumably by the partial oxidation of Ce3+ to Ce4+. The dielectric properties show a maximum in the region of transformation as well, but the study of dielectric hysteresis loops did not confirm the ferroelectric order of stillwellite structure at room temperature. The high-temperature behavior and the decomposition process of stillwellite are dictated by the partial oxidation of cerium.

Theoretical investigation of dielectric properties of rare earth stillwellite compounds

Ab initio density functional calculations are performed to investigate the dielectric properties of LnBSiO 5 (Ln = Ce, Pr, Nd) with the stillwellite structure. The calculated structural parameters are found to agree well with existing experimental results. The three compounds possess insulating electronic structure with nearly isotropic high frequency dielectric permittivity tensors. On the other hand, the static dielectric permittivity tensors are found to be less isotropic. The anisotropy of static dielectric tensors are found to increase as the atomic number of the lanthanide increases.

Structural, electronic, vibrational, and dielectric properties of LaBGeO5 from first principles

Structural, electronic, vibrational, and dielectric properties of LaBGeO5 with the stillwellite structure are determined based on ab initio density functional theory. The theoretically relaxed structure is found to agree well with the existing experimental data with a deviation of less than 0.2%. Both the density of states and the electronic band structure are calculated, showing five distinct groups of valence bands. Furthermore, the Born effective charge, the dielectric permittivity tensors, and the vibrational frequencies at the center of the Brillouin zone are all obtained. Compared to existing model calculations, the vibrational frequencies are found in much better agreement with the published experimental infrared and Raman data, with absolute and relative rms values of 6.04 cm−1, and 1.81%, respectively. Consequently, numerical values for both the parallel and perpendicular components of the permittivity tensor are established as 3.55 and 3.71 (10.34 and 12.28), respectively, for the high-(low-)frequency limit.

Electrodynamic characteristics of the LaBGeO5 and LaBSiO5 glasses in the terahertz and infrared ranges

The methods of calculating the dielectric response functions for transmission (in the submillimeter range) and reflection (in the infrared range) of disordered materials have been compared using the LaBGeO5 and LaBSiO5 functional glasses with the stillwellite structure as an example. In the four-parameter factorized dispersion model, the dielectric parameters of the LaBGeO5 and LaBSiO5 glasses have been determined and the assignment of the spectral bands to fragments of the structure has been carried out.

Luminescence of rare earth borosilicates with the stillwellite and related structures

By hydrothermal reactions it was possible to prepare well-crystallized borosilicates of composition La 1 − x − y Ln x Ln′ y BSiO 5 with the trigonal stillwellite structure. Ln = Eu 3+ and Tb 3+ produce compounds with only weak luminescence. Compounds with Ln = Ce 3+ are strong UV phosphors with peak intensity at x = 0.08 and abrupt concentration quenching at x = 0.10. Compounds with Ln = Ce 3+ and Ln′ = Tb 3+ exhibit effective Ce 3+ → Tb 3+ coactivation and strong green luminescence. This family of borosilicates appears to be useful phosphor hosts, aside from the necessity of non-traditional synthesis.

Comprehensive study of crystallization and phase formation in (La,Gd)BGeO5 glass

The glasses with (25-x)La2O3·xGd2O3·25B2O3·50GeO2 composition corresponding to the (La,Gd)BGeO5 were prepared, and the substitution effect of gadolinium ions (Gd3+) on crystallization behavior and phase formation was examined by DTA, XRD and Raman techniques in detail. Ferroelectric (La,Gd)BGeO5 phase with hexagonal stillwellite structure were crystallized in the composition range of x = 0-15. Although no significant change in physical properties was observed in the whole composition range, crystallization behavior was drastically changed as the progressive substitution of Gd3+ was carried out (i.e., crystallization of monoclinic phase of (La,Gd)BGeO5 and high-pressure phase of Gd2Ge2O7 with a triclinic structure). Structure of the glasses and crystallized phases, and the phase formation mechanism were also discussed.

Systematic prediction of new ferroelectrics in space groups P3(1) and P3(2).

Release 2002/2 of the Inorganic Crystal Structure Database (FIZ Karlsruhe, Germany, and Institute Laue-Langevin, Grenoble, France) contains 62 entries in space group P3(1) and ten entries in space group P3(2) for 49 different materials including eight families with two or more isostructural members. The structural criteria for ferroelectricity are satisfied for 16 new structure types at a confidence level that depends on the reliability of each determination. LaBGeO(5), a mineral with stillwellite structure, was previously reported as ferroelectric and forms an additional family with seven other members or related structures that satisfy the criteria. Ten structures reported in space group P3(1) or P3(2) are dubious or incorrect, with atomic coordinates that satisfy supergroup symmetry. One material is probably pyroelectric but is unlikely to become ferroelectric, and three others are either incompletely solved or incompletely refined. Among the predicted new ferroelectrics are Cu(2)BaGeS(4,) Fe(3)(Fe,Si)O(4)(OH)(5), Se(4)S(5), K(2)HCr(2)AsO(10), IV-RbNO(3), Rb(2)Sc(NO(3))(5), Na(3)ReO(5), Nd(14)(GeO(4))(2)(BO(3))(6)O(8), CsHgCl(3,) Ba(2)Cu(2)AlF(11), KYF(4), SrS(2)O(6).4H(2)O, Cu(3)PbTeO(6)(OH)(2), ReH(CO)(4), Ni(2)(NH(3))(9)Mo(CN)(8).2H(2)O and the 6T polytype of Ca(1.89)Ta(1.80)Sm(0.16)Ti(0.10)O(7), in addition to beta-LaBSiO(5), PbBAsO(5) and BaBAsO(5) in the stillwellite family.

High-frequency EPR investigation of X-ray storage SrBPO5:Eu phosphor

High-frequency EPR (HFEPR) spectra of the SrBPO5:Eu with stillwellite structure, which was prepared using EuCl3.6H2O as a dopant, have been examined in order to clarify the Eu sites in it. They showed an intense doublet at 3.3T corresponding to g=2.0 as is expected for the 8S state ions. Also, weak satellites were observed on both sides of the central doublet signal symmetrically. This means that microwave energy at 90GHz is sufficiently larger than the zero-field splitting. Taking into account the crystal structure of stillwellite and the earlier PL investigation, a single Eu2+ site was assumed for the HFEPR simulation. As a result, it was found that HFEPR spectrum simulated using g=1.98161, ∣D∣=0.130T and λ=∣E/D∣=0.06 reasonably agrees well with the experimental one, supporting that the Eu2+ occupies the Sr site. The large ∣D∣ value small λ value obtained implies that the crystal field surrounding Eu2+ has a strong axial field with weak rhombic distortion.

Eu 2+ luminescence in stillwellite-type SrBPO 5− a new potential X-ray storage phosphor

Eu 2+-doped SrBPO 5 with stillwellite structure has been identified as a new luminescent system which shows intense photostimulated luminescence for UV and X-ray radiations. Various spectral features observed for the new system are very much similar to BaFBr : Eu 2+, the extensively applied phosphor system in digital radiography. Apart from its potential for application, the unusually low wavelength Eu 2+ emission observed has been explained by the absence of preferred orientation of d-orbital of Eu 2+ as dictated by crystal chemistry of the system.

Structures and phase transitions of in the temperature range

High-resolution neutron powder diffractometry has been used to characterize the structural phase transitions which occur in at 550 and . The change in lattice parameters of the trigonal unit cell as a function of temperature, observed in the range from ambient to , shows that the transition is associated with an abrupt contraction of the unit cell volume. The crystal structure stable in the temperature interval between 550 and is isomorphous with the stillwellite structure of in its polar variant stable up to . The phase transition at to a non-polar variant of the structure mimics the transition in . The structural parameters of these two phases have been refined from data taken at 650, 770 and . The structure of the phase stable at ambient temperature gives rise to extra reflections not given by the stillwellite structure. These can be indexed on a triple cell with and the structure has been determined and refined from a combination of neutron powder data and an ambient temperature single crystal x-ray diffraction study.

Zur Synthese und Kristallstruktur von Bariumboroarsenat Ba(BAsO5) mit einem Beitrag über Bariumorthoarsenat Ba3(AsO4)2 / Synthesis and Crystal Structure of Barium Boroarsenate Ba(BAsO5) with a Contribution on Barium Orthoarsenate Ba3(AsO4)2

Single crystals of Ba(BAsO5) (I) and Ba3(AsO4)2 (II) were obtained by melting B2O3, arsenic pentoxide and BaO at 1100°C. Both compounds crystallize trigonally, space groups (I): C3 2 - P31 (Nr. 144), (II): D3d 5 - R3̄m (Nr. 166), with the lattice parameters (I): a = 726.7(1), c = 710.6(1) pm, Z = 3 and (II): a = 576.7(1), c = 2116.3(2) pm, Z = 3. Ba(BAsO5) is isotypic to Pb(BAsO5) (Stillwellite structure) and Ba3(AsO4)2 to Ba3(PO4)2. The structure of Ba(BAsO5) is characterized by helical chains of rings of two borate tetrahedra and one arsenate tetrahedron. Barium is tenfold coordinated by oxygen. A rod-like sequence of coordination polyhedra, AsO4 - Ba(2)O10-Ba(1)O12-Ba(2)O10-AsO4 is characteristic in the crystal structure of Ba3(AsO4)2.

Pb6(AsO4)[B(AsO4)4] - ein neuartiger Kristallstrukturtyp im System PbO/B2O3/As2O5 mit einem Beitrag über Pb(BAsO5)/Pb6(AsO4)[B(AsO4)4] - a New Crystal Structure Type in the System PbO/B2O3/As2O5 with Remarks about Pb(BAsO5)

Abstract Single crystals of Pb6(AsO4)[B(AsO4)4] (I) and Pb(BAsO5) (II) were obtained by melting B2O3, As2O5 and PbO at 800 °C. X-ray investigation led to the space group (I): S4 1 -P4̄ (Nr. 81) with a = 715.4(1), c = 976.2(3) pm, Z = 1 and (II): C3 2 -P31 (Nr. 144) with a = 712.2(1), c = 693.5(2) pm, Z = 3. While (II) is isotypic to the Stillwellite structure, Pb6(AsO4)[B(AsO4)4] represents a completely novel type of crystal structure. Boron and arsenic are tetrahedrally coordinated by oxygen. These polyhedra form two kinds of isolated anions: AsO4 3- and B(AsO4)4 9-. Lead is eightfold coordinated by oxygen.

Stillwellite glass-ceramics with ferroelectric properties

The process of crystallization of LaBGeO 5 and PrBGeO 5 phases with stillwellite structure from the glassy state is studied. Exotherms on differential thermal analysis curves for lanthanum borogermanate glasses are interpretated by using the X-ray diffraction and the second harmonic generation method data. It is shown that acentric nuclei of the stillwellite phase are formed at the early stages of crystallization. There is not any metastable phase, prior to stillwellite precipitation. Glass-ceramic samples with high stillwellite content were synthesized, and their properties were measured and compared with the properties of a single crystal of LaBGeO 5.

Lanthanide borogermanates LnBGeO 5: Synthesis and structural study by X-ray diffractometry and vibrational spectroscopy

LnBGeO 5 borogermanates have been synthesized by solid-state reaction and investigated by powder X-ray diffractometry, infrared, and Raman spectroscopy. A hexagonal, stillwellite-like phase is obtained for Ln = La, Pr, and Nd (low-temperature phase). Smaller cations (Nd to Er and Y) lead to a monoclinic phase, Z = 4, with a = 9.80–10.03, b = 7.42–7.60, c = 4.79–4.96 Å, and β = 91°14′–91°54′, depending on the ionic radius of the rare earth. For the smallest lanthanides (Tm, Yb, Lu), no monoclinic phase is obtained: Ln 2Ge 2O 7 is eventually formed, with a total loss of B 2O 3. The vibrational spectrum of the monoclinic phase points to some analogies with the stillwellite structure, namely a tetrahedral coordination of boron, and an ordered distribution of the BO 4 and GeO 4 tetrahedra.