Space Group Ama2 Research Articles

Rb2CdBr2I2: A New IR Nonlinear Optical Material with a Large Laser Damage Threshold

This paper describes the synthesis, crystal structure, and photophysical properties of a new compound Rb2CdBr2I2. It crystallizes in the noncentrosymmetric space group Ama2. In the crystal, all the distorted tetrahedron [CdBr2I2](2-) groups are arranged in a way such that all the Cd-I bonds are located in the same side of the Cd atoms resulting in a net polarization. Rb2CdBr2I2 showed a powder second harmonic generation (SHG) response 4 times that of KH2PO4 (KDP). The preliminary measurement indicated that it exhibits a large laser-induced damage threshold (LDT) of 190 MW/cm(2) which is 6 times that of AgGaS2. It also exhibits a wide transparent region (0.37-14 μm) with a relatively high (up to 490 °C) thermal stability. All these indicate that Rb2CdBr2I2 is a new promising candidate for NLO materials in the IR region.

Sodium–rare earth carbonates with shorite structure and large second harmonic generation response

Two non-linear optical crystalline carbonates (Na3Y(CO3)3 and Na3Gd(CO3)3) were synthesized under subcritical hydrothermal conditions, and both of them crystallized in the non-centrosymmetric space group Ama2 (40). Their crystal structures, analogous to shorite, can be described as alternate stackings of [Na(CO3)2]∞ and [Na2RE(CO3)2]∞ (RE = Y, Gd) layers, which are parallel to the bc plane. Second harmonic generation (SHG) studies indicated that Na3Y(CO3)3 and Na3Gd(CO3)3 have large SHG responses of approximately 3.57 and 3.54 times that of KH2PO4 (KDP), respectively, and the SHG responses were phase-matchable in the visible region. Furthermore, they exhibited wide transparent regions from UV to near IR with short UV cut-off edges at about 220 nm, suggesting that these crystals are promising UV non-linear optical (NLO) materials.

Hydrothermal preparation, structure and characterizations of noncentrosymmetric LiZnO(IO3)

A new lithium zinc iodate, LiZnO(IO3), has been prepared by hydrothermal reactions at 200 °C. Single-crystal X-ray diffraction experiment revealed that the compound crystallizes in a noncentrosymmetric orthorhombic space group Ama2 with cell parameters: a = 6.3386(6) Å, b = 11.2613(11) Å, c = 4.6840(5) Å, Z = 4. In the structure, the basic building units are the distorted ZnO6 octahedron, LiO4 tetrahedron and the IO3 group. These groups are linked together to form a three-dimensional structure by sharing the oxygen atoms. The DSC measurement demonstrates that the compound is stable up to 490 °C. Powder second-harmonic generation (SHG) test shows that the SHG effect is about 2 × KH2PO4.

Pressure-induced isostructural phase transition in Bi2Sr2CaCu2O8+δ

The high-pressure structures of an underdoped cuprate superconductor Bi2Sr2CaCu2O8+δ have been studied by synchrotron X-ray diffraction at pressures up to 36.5 GPa. We find that this superconductor retains its orthogonal structure with the space group Amaa in the pressure range studied. Upon compression, both the a and b axes first shrink monotonically up to 17.4 GPa from their ambient pressure values and keep these behaviors with positive compressibilities up to 36.5 GPa after experiencing expansion with negative compressibilities in the pressure regime between 17.4 and 23.7 GPa. However, the c axis decreases continuously with increasing pressure with a slow change at about 23.7 GPa. The results indicate an isostructural phase transition starting at 17.4 GPa and a structural collapse at around 23.7 GPa.

Single-crystal Structure Determination and Spectroscopic Characterization of KSr4(BO3)3

During attempts to synthesize a high-pressure phase of KSr4(BO3)3 using pressures of up to 12 GPa, single crystals of the normal-pressure phase KSr4(BO3)3 with sufficient quality for singlecrystal diffraction experiments could be obtained at 3 GPa and 1300 °C in a Walker-type multianvil apparatus. The single-crystal structure determination verified the published powder diffraction data of KSr4(BO3)3 revealing the non-centrosymmetric space group Ama2 with a = 1104:8(2), b=1199:1(2), c=688:8(2) pm and Z =4. Additionally, IR- and Raman-spectroscopic investigations were performed on single crystals of the compound.

KCa4(BO3)3:Ln3+ (Ln = Dy, Eu, Tb) phosphors for near UV excited white–light–emitting diodes

A series of doped KCa4(BO3)3:Ln3+ (Ln: Dy, Eu and Tb) compositions were synthesized by solid–state reaction method and their photoluminescent properties were systematically investigated to ascertain their suitability for application in white light emitting diodes. The X–ray diffraction (XRD) and nuclear magnetic resonance (MAS–NMR) data indicates that Ln3+–ions are successfully occupied the non–centrosymmetric Ca2+ sites, in the orthorhombic crystalline phase of KCa4(BO3)3 having space group Ama2, without affecting the boron chemical environment. The present phosphor systems could be efficiently excitable at the broad UV wavelength region, from 250 to 350 nm, compatible to the most commonly available UV light–emitting diode (LED) chips. Photoluminescence studies revealed optimal near white–light emission for KCa4(BO3)3 with 5 wt.% Dy3+ doping, while warm white–light (CIE; X = 0.353, Y = 0.369) is obtained at 1wt.% Dy3+ ion concentration. The principle of energy transfer between Eu3+ and Tb3+ also demonstrates the potential white–light from KCa4(BO3)3:Eu3+,Tb3+ phosphor. Whereas, single Tb3+ and Eu3+–doped systems showed bright green (Tb3+) and red (Eu3+) emissions, respectively. Having structural flexibility along with remarkable chemical/thermal stability and suitable quantum efficiency these phosphors can be promising candidates as white–light–emitter for near UV LEDs.

Redetermination of junitoite, CaZn2Si2O7·H2O

The crystal structure of the mineral junitoite, ideally CaZn2Si2O7·H2O (calcium dizinc disilicate monohydrate), was first determined by Hamilton & Finney [Mineral. Mag. (1985), 49, 91–95] based on the space group Ama2, yielding a reliability factor R of 0.10, with isotropic displacement parameters for all non-H atoms. The present study reports a structure redetermination of junitoite using single-crystal X-ray diffraction data from a natural sample, demonstrating that the space group of this mineral is actually Aea2, which can be attained simply by shifting the origin. Topologically, the structure models in the space groups Aea2 and Ama2 are analogous, consisting of chains of corner-sharing ZnO4 tetra­hedra parallel to the b axis, cross-linked by Si2O7 tetra­hedral dimers (the site symmetry of the bridging O atom is ..2) along a and c, forming a three-dimensional framework. The Ca2+ cations (site symmetry ..2) are situated in cavities of the framework and are bonded to five O atoms and one H2O mol­ecule (site symmetry ..2) in a distorted octa­hedral coordination environment. However, some bond lengths, especially for the SiO4 tetra­hedron, are noticeably different between the two structure models. Hydrogen bonding in junitoite is found between the water mol­ecule and a framework O atom.

Structure and optical properties of a noncentrosymmetric borate RbSr4(BO3)3

A new noncentrosymmetric borate, RbSr4(BO3)3 (abbreviated as RSBO), has been grown from Rb2OB2O3RbF flux and its crystal structure was determined by single crystal x-ray diffraction. It crystallizes in space group Ama2 with cell parameters of a=11.128(10)Å, b=12.155(15)Å, c=6.952(7)Å, Z=4. The basic structural units are isolated planar BO3 groups. Second harmonic generation (SHG) test of the title compound by the Kurtz–Perry method shows that RSBO can be phase matchable with an effective SHG coefficient about two-thirds as large as that of KH2PO4 (KDP). Finally, based on the anionic group approximation, the optical properties of the title compound are compared with those of the structure-related apatite-like compounds with the formula “A5(TOn)3X”.

Trabzonite, Ca4[Si3O9(OH)]OH: crystal structure, revised formula, new occurrence and relation to killalaite

AbstractThe crystal structure of the rare skarn mineral trabzonite, Ca4[Si3O9(OH)]OH, from the type locality near Ikizdere, Turkey and from the Upper Chegem caldera, Northern Caucasus, Kabardino-Balkaria, Russia has been solved and refined using single-crystal X-ray data. This shows that the chemical formula should be modified from Ca4(Si3O10)˙2H2O, reported in the original trabzonite description, to an OH-bearing composition. The crystal structure, which contains Si3O10 trimers embedded in a framework of CaO6–8 polyhedra, has orthorhombic symmetry, space group Ama2, a = 20.6, b = 9.1, c = 10.3 Å. The orthorhombic A-centred cell is easily obtained by transformation from the original monoclinic cell of P21/m symmetry. The revised formula means that trabzonite and foshagite are polymorphs; foshagite has wollastonite-like silicate chains and in contrast to trabzonite it does not contain silanol groups. The structure and composition of killalaite from both localities was also studied. Single-crystal X-ray structure refinement of killalaite from the Northern Caucasus confirmed it to be non-stoichiometric with a composition between Ca6[Si2O6(OH)]2(OH)2 and Ca7[Si2O7]2(OH)2(Z = 2). Trabzonite, killalaite and dellaite form a series of modular structures which differ mainly in the degree of condensation of the SiO4 units.

Two New Noncentrosymmetric (NCS) Polar Oxides: Syntheses, Characterization, and Structure–Property Relationships in BaMTe2O7 (M = Mg2+ or Zn2+)

Two new noncentrosymmetric (NCS) polar oxides, BaMgTe(2)O(7) and BaZnTe(2)O(7), have been synthesized and characterized, with their crystal structures determined by single crystal X-ray diffraction. The iso-structural materials exhibit structures consisting of layers of corner-shared MgO(5) or ZnO(5), Te(6+)O(6), and Te(4+)O(4) polyhedra that are separated by Ba(2+) cations. The Te(4+) cation is found in a highly asymmetric and polar coordination environment attributable to its stereoactive lone-pair. The alignment of the individual TeO(4) polar polyhedra results in macroscopic polarity for BaMgTe(2)O(7) and BaZnTe(2)O(7). Powder second-harmonic generation (SHG) measurements revealed a moderate SHG efficiency of approximately 5 × KDP (or 200 × α-SiO(2)) for both materials. Piezoelectric charge constants of 70 and 57 pm/V, and pyroelectric coefficients of -18 and -10 μC·m(-2)·K(-1) were obtained for BaMgTe(2)O(7) and BaZnTe(2)O(7), respectively. Although the materials are polar, frequency dependent polarization measurements indicated that the materials are not ferroelectric, that is, the observed macroscopic polarization cannot be reversed. Infrared, UV-vis diffuse spectroscopy, and thermal properties were also measured. Crystal data: BaMgTe(2)O(7), orthorhombic, space group Ama2 (No. 40), a = 5.558(2) Å, b = 15.215(6) Å, c = 7.307(3) Å, V = 617.9(4) Å(3), and Z = 4; BaZnTe(2)O(7), orthorhombic, space group Ama2 (No. 40), a = 5.5498(4) Å, b = 15.3161(11) Å, c = 7.3098(5) Å, V = 621.34(8) Å(3), and Z = 4.

Structure, growth and properties of a novel polar material, KSr4B3O9

A novel polar material, KSr4B3O9, has been obtained by the high temperature solution method for the first time. It crystallizes in the polar space group Ama2. The material exhibits a three-dimensional structure consisting of interconnecting KO10 groups, SrOx (x=8, 9) units and isolated BO3 triangles. DSC/TG curve shows that KSr4B3O9 melts incongruently. The calculated band structures and the density of states of KSr4B3O9 suggest that its direct gap is 3.897eV. The absorption spectrum indicates the absorption edge of KSr4B3O9 is about 302nm. It produces SHG intensity about as large as that of KH2PO4 (KDP) and is phase matchable.

Microtwinning hypothesis for a more ordered vaterite model

An orthorhombic fully ordered structural model is proposed for vaterite [space group Ama2, a=8.4721(5) Å, b=7.1575(7) Å, c=4.1265(4) Å, Z=4, and V=250.23(4) Å3]. It is based on a microtwinning hypothesis, with three domains rotated by 120° along the orthorhombic a axis, regenerating a pseudohexagonal habit. The solution came from direct space ab initio calculations applied to the powder diffraction data. However, five weak superstructure reflections seen in single-crystal and powder diffraction experiments, leading to a six times larger unit cell, are still unexplained.

Isothermal section of the Ag 2S–PbS–GeS 2 system at 300 K and the crystal structure of Ag 2PbGeS 4

The isothermal section of the Ag 2S–PbS–GeS 2 system at room temperature was investigated by XRD. The existence of two quaternary compounds, Ag 2PbGeS 4 and Ag 0.5Pb 1.75GeS 4, was confirmed, and the phase equilibria between the binary system components and the ternary and the quaternary compounds were determined. The crystal structure of Ag 2PbGeS 4 was studied using the single crystal X-ray diffraction. It was established that Ag 2PbGeS 4 crystallizes in an own structural type in non-centrosymmetric space group Ama2 with the lattice parameters a = 1.02390(4) nm, b = 1.02587(5) nm, c = 0.67701(3) nm.

Improved Synthesis of Bis(borano)hypophosphite Salts

A synthesis of the bis(borano)hypophosphite anion with various counterions has been developed to make use of more benign and commercially available reagents. This method avoids the use of potentially dangerous reagents used by previous methods and gives the final products in good yield. Details of the crystal structure determination of the sodium salt in space group Ama2 are given using a novel computational technique combined with Rietveld refinement.

ChemInform Abstract: LiEuPSe4 and KEuPSe4: Novel Selenophosphates with the Tetrahedral [PSe4]3- Building Block.

LiEuPSe4, the first quaternary lithium-containing selenophosphate, was synthesized as red polyhedra by reacting Eu with a molten mixture of Li2Se/P2Se5/Se at 750 degrees C. Similarly, the reaction of Eu with a molten mixture of K2Se/P2Se5/Se at 495 degrees C produced red polyhedral crystals of KEuPSe4. Both compounds are unstable in moist air. In addition, both compounds were plagued with crystal twinning. Acceptable crystal structure refinements could only be obtained by identifying the type of twinning and taking it into account in the final refinement. LiEuPSe4 crystallizes in the noncentrosymmetric space group Ama2 (no. 40) with a = 10.5592 (9) A, b = 10.415 (1) A, c = 6.4924(7) A, and Z = 4. The structure is three-dimensional and composed of EuSe8 distorted square antiprisms and PSe4 tetrahedral building blocks that create tunnels, running down the a axis, in which the Li ions reside. The Li ions are in a highly distorted tetrahedral coordination. KEuPSe4 crystallizes in the space group P2(1)/m (no. 11) with a = 6.8469(6) A, b = 6.9521(6) A, c = 9.0436(8) A, beta = 107.677(2) degrees, and Z = 2. The structure has two-dimensional character with layers composed of EuSe6 trigonal prisms and PSe4 tetrahedral units. Between the [EuPSe4]nn- layers the K ions reside in a bicapped trigonal prism of Se atoms. The structure of the [EuPSe4]nn- framework is similar to that found in CsPbPSe4. Both compounds are semiconductors with band gaps of 2.00 and 1.88 eV, respectively. Differential thermal analysis and infrared spectroscopic characterization are also reported.

Structural characterization of a new acentric Ruddlesden–Popper layered perovskite compound: LiHSrTa2O7

A new n = 2 member acentric Ruddlesden-Popper layered perovskite LiHSrTa(2)O(7) (LiDSrTa(2)O(7)) has been synthesized and structurally characterized from Rietveld treatment of its powder X-ray and high-resolution neutron diffraction data. It can be synthesized by a partial Li(+)/H(+) exchange from the mother phase Li(2)SrTa(2)O(7) either in solid state by NH(4)Cl or in dilute HNO(3) by controlling the amount of H(+). This compound crystallizes in the orthorhombic acentric space group Ama2 (no. 40) with lattice constants a = radical2a(p) approximately 5.5522(1) A, b = radical2a(p) approximately 5.5248(1) A and c approximately 18.7745(4) A. Classically, Ta(5+) ions occupy the octahedral sites of the kinked perovskite blocks and Sr(2+) ions completely fill the perovskite cages while Li(+) and D(+) ions are found in the interlayer spacing. Efficient positive second harmonic generation response, performed at room temperature on a polycrystalline sample, shows unambiguously the acentric character of this new phase. Interestingly, the choice of the acentric Ama2 space group to describe the structure is revealed only by high-resolution neutron diffraction data: in the interlayer spacing, Li(+) and D(+) cations are unequally distributed on different sites (two 4a sites for Li(+) and two 4b sites for D(+) ions).

Structural and Raman Spectroscopic Investigations of K4BaSi3O9 and K4CaSi3O9

AbstractThe crystal structures of K4BaSi3O9 and K4CaSi3O9 have been characterized by X‐ray diffraction techniques as well as Raman spectroscopy. The structure of K4CaSi3O9 has been refined from powder diffraction data via the Rietveld method using polycrystalline material prepared from solid state reactions. The compound is isostructural with form I of K4SrSi3O9. It crystallizes with 16 formula units in a cubic primitive cell (a = 15.94014(3) Å, V = 4050.20(1) Å3) and adopts space group $Pa{\bar 3}$. K4CaSi3O9 belongs to the group of cyclosilicates and contains highly puckered twelve‐membered [Si12O36]‐rings centered on the ${\bar 3}\rm -axes$. Five of the seven crystallographically independent alkaline and alkaline earth cations are surrounded by six oxygen ligands in the form of distorted octahedra, which share opposite triangular faces and form non‐intersecting columns parallel to the body diagonals of the cubic unit cell. This arrangement corresponds to one of the cubic cylinder or rod packings. The two remaining sites have more irregular coordination environments with eight next oxygen neighbors. High temperature X‐ray powder diffraction data have been collected to determine the thermal expansion of this material: between room temperature and 700 °C the coefficient of thermal expansion has a value of α = 12.9(2) × 10−6 [°C−1]. Single crystals of K4BaSi3O9 have been obtained from the devitrification of a glass with the same composition. The structure was determined from a single crystal diffraction data set collected at −100 °C and refined to a final R index of 0.0298 for 1288 observed reflections (I > 2σ(>I)). The compound is isostructural with modification II of K4SrSi3O9. Basic crystallographic data are as follows: space group Ama2, a = 11.0695(15) Å, b = 8.0708(10) Å, c = 11.905(2) Å, V = 1063.6(3) Å3, Z = 4. With respect to the silicate anions the material can be classified as a sechser single chain silicate. The crankshaft‐like chains run parallel to [100] and are linked by K and Ba cations, which are distributed among five crystallographically independent sites. The coordination polyhedra of two of the non‐tetrahedral cations can be described by distorted octahedra sharing opposite triangular faces. They build non‐intersecting columns parallel to [011] and [0‐11], respectively. The other sites exhibit more irregular coordination spheres with 7‐9 neighbours.

Preparation and crystal structures of two salts with the 5‐nitrotetrazolate anion

AbstractTetraphenylphosphonium 5‐nitrotetrazolate (2) was prepared by metathesis of sodium 5‐nitrotetrazolate dihydrate (1; NaNT) with tetraphenylphosphonium chloride in acetone. The new compound was fully characterized by vibrational (IR, Raman) and NMR (1H, 13C, and 14N) spectroscopies, elemental analysis, and mass spectrometry. Attempted synthesis of 2‐methyl‐5‐nitrotetrazole (2‐MeNT) by methylation of 1 with dimethylsulfate at reflux from acetonitrile failed, and crystals of an explosive compound with the formula (NaNT)2(H2O)2CH3CN (3), NT = 5‐nitrotetrazolate, formed. X‐ray diffraction techniques were used to determine the crystal structure of 2 and 3. Compound 2 crystallizes in the orthorhombic space group P212121 with four molecules in the unit cell and unit cell parameters a = 7.7413(4) Å, b = 13.624(1) Å, c = 21.252(1) Å, and V = 2241.5(2) Å3, whereas 3 crystallizes in the orthorhombic space group Ama2 with four formula unit in the unit cell and unit cell parameters a = 14.805(6) Å, b = 9.908(4) Å, c = 8.940(3) Å, and V = 1311.4(1) Å3. © 2009 Wiley Periodicals, Inc. Heteroatom Chem 20:35–44, 2009; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/hc.20509

Synthesis, structure and physicochemical characterization of a noncentrosymmetric, quaternary thiostannate: EuCu 2SnS 4

EuCu 2SnS 4 was prepared by a stoichiometric combination of the elements heated to 700 °C for 125 h. The structure was determined by single crystal X-ray diffraction methods. The compound crystallizes in the noncentrosymmetric, orthorhombic space group Ama2 with a=10.4793(1) Å, b=10.3610(2) Å, c=6.4015(1) Å, Z=4, R1=0.99% and w R2=2.37%. The structure type is that of SrCu 2GeSe 4. The structure can be described as a three-dimensional network built from near perfect SnS 4 and distorted CuS 4 tetrahedra together with EuS 8 square antiprisms. The dark red compound is a semiconductor with an optical bandgap of 1.85 eV.

Stability at high-pressure, elastic behaviour and pressure-induced structural evolution of CsAlSi5O12, a potential host for nuclear waste

The elastic and structural behaviour of the synthetic zeolite CsAlSi5O12 (a = 16.753(4), b = 13.797(3) and c = 5.0235(17) Å, space group Ama2, Z = 2) were investigated up to 8.5 GPa by in situ single-crystal X-ray diffraction with a diamond anvil cell under hydrostatic conditions. No phase-transition occurs within the P-range investigated. Fitting the volume data with a third-order Birch–Murnaghan equation-of-state gives: V 0 = 1,155(4) Å3, K T0 = 20(1) GPa and K′ = 6.5(7). The “axial moduli” were calculated with a third-order “linearized” BM-EoS, substituting the cube of the individual lattice parameter (a 3, b 3, c 3) for the volume. The refined axial-EoS parameters are: a 0 = 16.701(44) Å, K T0a = 14(2) GPa (βa = 0.024(3) GPa−1), K′ a = 6.2(8) for the a-axis; b 0 = 13.778(20) Å, K T0b = 21(3) GPa (βb = 0.016(2) GPa−1), K′ b = 10(2) for the b-axis; c 0 = 5.018(7) Å, K T0c = 33(3) GPa (βc = 0.010(1) GPa−1), K′ c = 3.2(8) for the c-axis (K T0a:K T0b:K T0c = 1:1.50:2.36). The HP-crystal structure evolution was studied on the basis of several structural refinements at different pressures: 0.0001 GPa (with crystal in DAC without any pressure medium), 1.58(3), 1.75(4), 1.94(6), 3.25(4), 4.69(5), 7.36(6), 8.45(5) and 0.0001 GPa (after decompression). The main deformation mechanisms at high-pressure are basically driven by tetrahedral tilting, the tetrahedra behaving as rigid-units. A change in the compressional mechanisms was observed at P ≤ 2 GPa. The P-induced structural rearrangement up to 8.5 GPa is completely reversible. The high thermo-elastic stability of CsAlSi5O12, the immobility of Cs at HT/HP-conditions, the preservation of crystallinity at least up to 8.5 GPa and 1,000°C in elastic regime and the extremely low leaching rate of Cs from CsAlSi5O12 allow to consider this open-framework silicate as functional material potentially usable for fixation and deposition of Cs radioisotopes.