Search

An examination of manganese silicates and germanates revealed unusual structural motifs and extremely different chemistries, with identical hydrothermal reactions forming K2Mn2Si3O9 versus K11Mn21Ge32O86(OH)9(H2O). The germanate is exceptional in both its c-axis length (exceeding 76 A) and unit cell volume (nearly 18000 A3), the largest known polygermanate structure to our knowledge.

Two new alkali vanadate carbonates with divalent transition metals have been synthesized as large single crystals via a high-temperature (600 °C) hydrothermal technique. Compound I, Rb2Mn3(VO4)2CO3, crystallizes in the trigonal crystal system in the space group P1c, and compound II, K2Co3(VO4)2CO3, crystallizes in the hexagonal space group P63/m. Both structures contain honeycomb layers and triangular lattices made from edge-sharing MO6 octahedra and MO5 trigonal bipyramids, respectively. The honeycomb and triangular layers are connected along the c-axis through tetrahedral [VO4] groups. The MO5 units are connected with each other by carbonate groups in the ab-plane by forming a triangular magnetic lattice. The difference in space groups between I and II was also investigated with Density Functional Theory (DFT) calculations. Single crystal magnetic characterization of I indicates three magnetic transitions at 77 K, 2.3 K, and 1.5 K. The corresponding magnetic structures for each magnetic transition of I were determined using single crystal neutron diffraction. At 77 K the compound orders in the MnO6-honeycomb layer in a Neel-type antiferromagnetic orientation while the MnO5 triangular lattice ordered below 2.3 K in a colinear ‘up–up–down’ fashion, followed by a planar ‘Y’ type magnetic structure. K2Co3(VO4)2CO3 (II) exhibits a canted antiferromagnetic ordering below TN = 8 K. The Curie–Weiss fit (200–350 K) gives a Curie–Weiss temperature of −42 K suggesting a dominant antiferromagnetic coupling in the Co2+ magnetic sublattices.

A new member of the descloizite family, a cobalt vanadate, SrCo(VO4)(OH), has been synthesized as large single crystals using high-temperature and high-pressure hydrothermal methods. SrCo(VO4)(OH) crystallizes in the orthorhombic crystal system in space group P212121 with the following unit cell parameters: a = 6.0157(2) A, b = 7.645(2) A, c = 9.291(3) A, V = 427.29(2) A3, and Z = 4. It contains one-dimensional Co-O-Co chains of edge-sharing CoO6 octahedra along the a-axis connected to each other via VO4 tetrahedra along the b-axis forming a three-dimensional structure. The magnetic susceptibility of SrCo(VO4)(OH) indicates an antiferromagnetic transition at 10 K as well as unusually large spin orbit coupling. Single-crystal magnetic measurements in all three main crystallographic directions displayed a significant anisotropy in both temperature- and field-dependent data. Single-crystal neutron diffraction at 4 K was used to characterize the magnetically ordered state. The Co2+ magnetic spins are arranged in a staggered configuration along the chain direction, with a canting angle that follows the tipping of the CoO6 octahedra. The net magnetization along the chain direction, resulting in ferromagnetic coupling of the a-axis spin components in each chain, is compensated by an antiferromagnetic interaction between nearest neighbor chains. A metamagnetic transition appears in the isothermal magnetization data at 2 K along the chain direction, which seems to correspond to a co-alignment of the spin directions of the nearest neighbor chain. We propose a phenomenological spin Hamiltonian that describes the canted spin configuration of the ground state and the metamagnetic transition in SrCo(VO4)(OH).

The reactions of Ln2O3 (Ln = La Lu) with ReO2 were examined in high temperature hydrothermal water (650 °C). In all cases the rhenium oxide was added in a three-fold excess. No external mineralizer was needed and the rhenate itself acted as a sufficient mineralizer to form high quality single crystals of several crystalline products. These form in good yield with compositions varying as a function of the size of the rare earth ion. The largest sesquioxide, La2O3, forms the well-known La4Re6O19 phase, but when Ln = Nd or Pr, a new product, Ln2Re2O7(OH), is isolated. Reactions with Ln = Sm Tb form Ln4Re2O11, and those with Dy Lu form Ln2ReO5. The new Ln2Re2O7(OH) series was characterized as a new structure type by single crystal X-ray diffraction. This structure features a tetrameric Re4O16 cluster embedded in a lanthanide oxide framework. Additional detailed structural data is also provided for Tb4Re2O11 and Eu4Re2O11, which were not previously reported for the Ln4Re2O11 family. In Ln2Re2O7(OH), no direct Re Re bond (Re Re = 2.60 A) is observed while Ln4Re2O11 appears to possess a rhenium-rhenium double bond with a Re Re distance of 2.42 A. Single crystal Raman data supports both of these characterizations. Magnetic data is reported for Ln2Re2O7(OH) and Tb4Re2O11, and their behavior appears to be dominated by the f-element magnetic moments. At low temperatures Tb4Re2O11 displays possible canted antiferromagnetic coupling.

Single crystals of a new transition metal adelite-descloizite-type structure were synthesized using a high temperature (580 °C) high-pressure hydrothermal technique. Single crystal X-ray diffraction and energy dispersive X-ray analysis (EDX) were used to investigate the structure and elemental composition, respectively. SrNi(VO4)(OH) crystallizes in an acentric orthorhombic crystal system in the space group P212121 (no. 19); Z = 4, a = 5.9952(4) Å, b = 7.5844(4) Å, c = 9.2240(5) Å. The structure is comprised of a Ni–O–V framework where Sr2+ ions reside inside the channels. Single-crystal magnetic measurements display a significant anisotropy in both temperature- and field-dependent data. The temperature dependent magnetic measurement shows antiferromagnetic behavior at TN~8 K. Overall, the magnetic properties indicate the presence of competing antiferromagnetic and ferromagnetic interactions of SrNi(VO4)(OH).

The magnetic properties are reported for three members of the glaserite series of compounds, Na2BaM(VO4)2, M = Mn, Mn0.6Co0.4, and Co. Large single crystals are grown using a high-temperature hydrothermal synthesis method. This structure type exhibits a triangular magnetic lattice in which M2+O6 octahedra are interconnected with nonmagnetic (VO4)3- groups. All the structures crystallize at room temperature with rigid trigonal symmetry (space group P3 m1); however, at lower temperatures both Na2BaMn(VO4)2 and Na2BaMn0.6Co0.4(VO4)2 undergo a structural transition to lower symmetry (monoclinic, C2/ c). The bulk magnetic measurements indicate that Mn- and Co-structures are antiferromagnetic and ferromagnetic, respectively. Na2BaMn0.6Co0.4(VO4)2 does not show any long-range ordering down to 0.5 K, although a broad heat capacity anomaly near 1.2 K suggests short-range magnetic order or freezing into a spin-glass-like state related to the chemical disorder and resulting competing magnetic interactions. The magnetic structures of Na2BaMn(VO4)2 and Na2BaCo(VO4)2 were determined using neutron powder diffraction. At zero magnetic field, Na2BaMn(VO4)2 possesses an antiferromagnetic structure with the moments ordered in a Neel-type arrangement and aligned along the C4 axis of the octahedra. Under applied magnetic field at 0.3 K, the evolution of the magnetic structure toward a fully polarized state is observed. Na2BaCo(VO4)2 represents a ferromagnetic (FM) magnetic structure with Co moments aligned parallel to the c-axis direction. The relationships between these structures and magnetic properties are discussed.

Reactions are reported of early rare earth oxides, RE2O3 (RE = La, Pr, Nd) with Ta2O5 under hydrothermal conditions (650 °C, 1.5 kbar) in concentrated aqueous hydroxide (20-30 M KOH) as a mineralizer. Under various stoichiometries several members of two new structure types were isolated, Ln2TaO5(OH) (Ln = La, Pr) and Ln3Ta2O9(OH) (Ln = Pr, Nd). The analogous niobate La2NbO5(OH) was also obtained. Both structure types were characterized by single crystal X-ray diffraction and contain pentavalent tantalatum oxide octahedra and complex rare earth oxide frameworks. The Ln2TaO5(OH) structure type contains Ln-O8 and Ln-O9 building blocks and TaO6 octahedra in a 3-D framework. It contains a 3-D rare earth oxide framework formed by from zig-zag chains of rare earth oxides linking sheets of rare earth oxides. The tantalates form edge-shared Ta2O10 dimers occupying gaps in the rare earth oxide frameworks. The structure of Ln3Ta2O9(OH) contains two types of 2-D rare earth oxide slabs built of seven and eight coordinate rare earth metals. The tantalate units form 2-D slabs through a multiple corner-sharing scheme of TaO6 octahedra. The Ln3Ta2O9(OH) structure type has an interesting close structural relationship to the previously reported rare earth titanate La5Ti4O15(OH), which is discussed. The presence of hydroxide in the lattice is confirmed by IR spectroscopy and the H atom locations are assigned unambiguously using bond valence sums.

Large single crystals of Yb2Ge2O7 in the cubic Fd3m space group, are synthesized and characterized from a high-temperature hydrothermal method (650°C/200 MPa in 1 M KF). The cubic phase displays spin frustration and possibly nonclassical quantum-spin behavior at low temperature. This is the first report of single crystals of this important phase of size and quality suitable for single-crystal neutron diffraction.

The growth of large high quality single crystals of three lithium silicates and germanates Li 2 SiO 3 , Li 2 SiO 5 , and Li 2 GeO 3 is described. The crystals are all grown using hydrothermal fluids at 650 °C and 1 kbar using simple oxide feedstock and 3–6 M LiOH serving as both the lithium ion source and mineralizer. The crystals could be grown by spontaneous transport without any specific seeding, and grew between 2 and 10 mm/edge and without evidence of cracking, physical twinning or multiple domain structures. The crystals were characterized by single crystal diffraction, UV–vis spectroscopy and Raman spectroscopy. All the crystals are acentric and polar. Preliminary nonlinear optical properties were investigated using the Kurtz method and indicate that the crystals exhibit Type-1 phase matching ability, and Li 2 SiO 3 demonstrates a significant nonlinear optical conversion efficiency, making it of potential interest as a UV NLO crystal.

Reactions of rare-earth oxides with TiO2 were performed in high temperature (650–700 °C) hydrothermal fluids. Two different mineralizer fluids were examined, 20 M KOH and 30 M CsF, and their respective products analyzed. When concentrated KOH fluids were used, single crystals of a variety of new OH− containing species were isolated and structurally characterized: RE5Ti4O15(OH) (RE = La, Er) I, Sm3TiO5(OH)3II and RE5Ti2O11(OH) (RE = Tm–Lu) III. La5Ti4O15(OH) I crystallizes in the orthorhombic space group Pnnm with unit cell dimensions of a = 30.5152(12) A, b = 5.5832(2) A, c = 7.7590(3) A and V = 1321.92(9) A3, Z = 4. Sm3TiO5(OH)3II crystallizes in the monoclinic space group P21/m with unit cell parameters of a = 5.6066(2) A, b = 10.4622(4) A, c = 6.1258(2) A and β = 104.7390(10)°, V = 347.50(2) A3, Z = 2. Lu5Ti2O11(OH) III crystallizes in the monoclinic space group C2/m with unit cell dimensions of a = 12.1252(9) A, b = 5.8243(4) A, c = 7.0407(5) A, β = 106.939(3)° and V = 475.65(6) A3, Z = 2. When concentrated fluoride solutions are used, mostly RE2Ti2O7 type compounds were isolated in either cubic or monoclinic phases. In the case of cerium, Ce2Ti4O11IV was isolated that crystallizes in the monoclinic space group C2/c with unit cell parameters of a = 13.6875(7) A, b = 5.0955(3) A, c = 12.8592(7) A, β = 108.964(2)° and V = 848.18(8) A3, Z = 4. The synthesis, structural characterization, and supporting characterization are reported for all compounds. The work highlights the complementary nature of hydroxide and fluoride fluids in studying the reactivity of refractory oxides.

Two new halide-containing cesium manganese vanadates have been synthesized by a high-temperature (580 °C) hydrothermal synthetic method from aqueous brine solutions. One compound, Cs3Mn(VO3)4Cl, (1) was prepared using a mixed cesium hydroxide/chloride mineralizer, and crystallizes in the polar noncentrosymmetric space group Cmm2, with a = 16.7820(8) A, b = 8.4765(4) A, c = 5.7867(3) A. This structure is built from sinusoidal zig-zag (VO3)n chains that run along the b-axis and are coordinated to Mn2+ containing (MnO4Cl) square-pyramidal units that are linked together to form layers. The cesium cations reside between the layers, but also coordinate to the chloride ion, forming a cesium chloride chain that also propagates along the b-axis. The other compound, Cs2Mn(VO3)3F, (2) crystallizes in space group Pbca with a = 7.4286(2) A, b = 15.0175(5) A, c = 19.6957(7) A, and was prepared using a cesium fluoride mineralizer. The structure is comprised of corner sharing octahedral Mn2+ chains, with trans fluoride ligands acting as bridging units, whose ends are capped by (VO3)n vanadate chains to form slabs. The cesium atoms reside between the manganese vanadate layers, and also play an integral part in the structure, forming a cesium fluoride chain that runs along the b-axis. Both compounds were characterized by single-crystal X-ray diffraction, powder X-ray diffraction, and single-crystal Raman spectroscopy. Additionally, the magnetic properties of 2 were investigated. Above 50 K, it displays behavior typical of a low dimensional system with antiferromagnetic interactions, as to be expected for linear chains of manganese(ii) within the crystal structure.

The structural and magnetic properties of a glaserite-type Na2BaFe(VO4)2 compound, featuring a triangular magnetic lattice of Fe2+ (S = 2), are reported. Temperature dependent X-ray single crystal studies indicate that at room temperature the system adopts a trigonal P3m1 structure and undergoes a structural phase transition to a C2/c monoclinic phase slightly below room temperature (Ts = 288 K). This structural transition involves a tilting of Fe-O-V bond angles and strongly influences the magnetic correlation within the Fe triangular lattice. The magnetic susceptibility measurements reveal a ferromagnetic transition near 7 K. Single crystal neutron diffraction confirms the structural distortion and the ferromagnetic spin ordering in Na2BaFe(VO4)2. The magnetic structure of the ordered state is modeled in the magnetic space group C2'/c' that implies a ferromagnetic order of the a and c moment components and antiferromagnetic arrangement for the b components. Overall, the Fe magnetic moments form ferromagnetic layers that are stacked along the c-axis, where the spins point along one of the (111) facets of the FeO6 octahedron.

Three new strontium manganese vanadates, Sr 2 Mn 2 (V 3 O 10 )(VO 4 ) ( I ), Sr 3 Mn(V 2 O 7 ) 2 ( II ), and Sr 2 Mn(VO 4 ) 2 (OH) ( III ), were prepared using a high temperature (580 °C) hydrothermal method with various chloride salts as the mineralizer. Minor differences in the chloride stoichiometry led to significant differences in product. Compound I crystallizes in the monoclinic space group P 2 1 / c ( a = 6.8773(12) A, b = 15.061(3) A, c = 11.609(2) A, β = 96.745(8)°), and consists of edge-shared octahedral manganese(II) dimers coordinated by trimeric [V 3 O 10 ] and monomeric [VO 4 ] groups. Compound II crystallizes in the tetragonal crystal system, P 4 3 2 1 2 ( a = 6.9951(2) A, c = 25.4390(7) A), and is built from monomeric manganese(II) octahedra chelated by two pyrovanadate [V 2 O 7 ] groups and linked to each other by additional pyrovanadates to form layers. Compound III is a noncentrosymmetric variation on the brackebuschite structure type, crystallizing in the monoclinic space group P 2 1 ( a = 7.6316(3) A, b = 6.1204(3) A, c = 8.6893(3) A, β = 111.3940(10)°). The structure is composed of octahedral manganese(III) edge-sharing chains coordinated to corner-sharing monomeric [VO 4 ] groups, thereby forming a manganese vanadate chain. All compounds were characterized by single-crystal X-Ray diffraction, powder X-Ray diffraction, infrared spectroscopy and single-crystal Raman spectroscopy. Density functional theory calculations were employed to investigate the relative stability of compound III .

Structural variations across a series of barium rare earth (RE) tetrasilicates are studied. Two different formulas are observed, namely those of a new cyclo-silicate fluoride, BaRE2Si4O12F2 (RE = Er3+–Lu3+) and new compounds in the Ba2RE2Si4O13 (RE = La3+–Ho3+) family, covering the whole range of ionic radii for the rare earth ions. The Ba2RE2Si4O13 series is further subdivided into two polymorphs, also showing a dependence on rare earth ionic radius (space group P{\overline 1} for La3+–Nd3+, and space group C2/c for Sm3+–Ho3+). Two of the structure types identified are based on dinuclear rare earth units that differ in their crystal chemistries, particularly with respect to the role of fluorine as a structural director. The broad study of rare earth ions provides greater insight into understanding structural variations within silicate frameworks and the nature of f-block incorporation in oxyanion frameworks. The single crystals are grown from high-temperature (ca 953 K) hydrothermal fluids, demonstrating the versatility of the technique to access new phases containing recalcitrant rare earth oxides, enabling the study of structural trends.

Two terbium germanates have been synthesized via high-temperature and high-pressure hydrothermal synthesis with 20 M KOH as a mineralizer using Tb4O7 as a starting material. Tb13(GeO4)6O7(OH) crystallizes in trigonal space group R3, is built up of isolated GeO4 units, and contains a complex arrangement of terbium oxide polyhedra. K2TbGe2O7 is a terbium(4+) pyrogermanate that is isostructural with K2ZrGe2O7 and displays a rare stable Tb4+ oxidation state in the solid state.

Manganese vanadate fluorides were synthesized using high-temperature hydrothermal techniques with BaF2 as a mineralizer. Ba3Mn2(V2O7)2F2 crystallizes in space group C2/c and consists of dimers built from edge-sharing MnO4F2 trigonal prisms with linking V2O7 groups. Ba7Mn8O2(VO4)2F23 crystallizes in space group Cmmm, with a manganese oxyfluoride network built from edge- and corner-sharing Mn2+/3+(O,F)6 octahedra. These octahedra form alternating Mn2+ and Mn2+/3+ layers separated by VO4 tetrahedra. This latter compound exhibits a canted antiferromagnetic order below TN = 25 K.

A higher-temperature hydrothermal approach (650–700 °C) has been employed in the crystal growth of Lu 2 O 3 and its lanthanide-doped analogs. Carefully controlled thermal gradients of 30° or less were also used to minimize the number of nucleation sites. The resulting crystals exhibit improvements in size and optical clarity over those grown at 600–650 °C. These outcomes are likely also attributed to a greater stability of Lu 2 O 3 relative to LuO(OH) at the higher temperature conditions. The doping of Lu 2 O 3 single crystals has been extended to encompass all spectroscopically active trivalent rare earth ions. Absorption spectra have been obtained of a wide range of lanthanide-doped Lu 2 O 3 single crystals from 80 to 298 K and the spectra of Nd:Lu 2 O 3 are reported as a representative example herein.

Large single crystals of A2Mn2V2O7F2 (A=Rb, Cs) and Mn2VO4F were grown using a high-temperature (~600 °C) hydrothermal technique. Single crystal X-ray diffraction and powder X-ray diffraction were utilized to characterize the structures, which both possess MnO4F2 building blocks. The A2Mn2V2O7F2 series crystallizes as a new structure type in space group Pbcn (No. 60), Z=4 (Rb2Mn2V2O7F2: a=7.4389(17) A, b=11.574(3) A, c=10.914(2) A; Cs2Mn2V2O7F2: a=7.5615(15) A, b=11.745(2) A, c=11.127(2) A). The structure is composed of zigzag chains of edge-sharing MnO4F2 units running along the a-axis, and interconnected through V2O7 pyrovanadate groups. Temperature dependent magnetic susceptibility measurements on this interesting one-dimensional structural feature based on Mn2+ indicated that Cs2Mn2V2O7F2 is antiferromagnetic with a Neel temperature, TN=~3 K and a Weiss constant, θ, of −11.7(1) K. Raman and infrared spectra were also analyzed to identify the fundamental V–O vibrational modes in Cs2Mn2V2O7F2. Mn2(VO4)F crystalizes in the monoclinic space group of C2/c (no. 15), Z=8 with unit cell parameters of a=13.559(2) A, b=6.8036(7) A, c=10.1408(13) A and β=116.16(3)°. The structure is associated with those of triplite and wagnerite. Dynamic fluorine disorder gives rise to complex alternating chains of five-and six-coordinate Mn2+. These interpenetrating chains are additionally connected through isolated VO4 tetrahedra to form the condensed structure.

New complex manganese vanadate materials were synthesized as high-quality single crystals in multi-millimeter lengths using a high-temperature, high-pressure hydrothermal method. One compound, Mn5(VO4)2(OH)4, was grown from Mn2O3 and V2O5 in 3 M CsOH at 580 °C and 1.5 kbar. Changing the mineralizer to 1 M CsOH/3MCsCl leads to the formation of another product, Mn6O(VO4)2(OH). Both compounds were structurally characterized by single-crystal X-ray diffraction (Mn5(VO4)2(OH)4: C2/m, Z = 2, a = 9.6568(9) Å, b = 9.5627(9) Å, c = 5.4139(6) Å, β = 98.529(8)°; Mn6O(VO4)2(OH): P21/m, Z = 2, a = 8.9363(12) Å, b = 6.4678(8) Å, c = 10.4478(13) Å, β = 99.798(3)°), revealing interesting low-dimensional transition-metal features. Mn5(VO4)2(OH)4 possesses complex honeycomb-type Mn-O layers, built from edge-sharing [MnO6] octahedra in the bc plane, with bridging vanadate groups connecting these layers along the a-axis. Mn6O(VO4)2(OH) presents a more complicated structure with both octahedral [MnO6] and trigonal bipyramidal [MnO5] units. A different pattern of planar honeycomb sheets are formed by edge-shared [MnO6] octahedra, and these sublattices are connected through edge-shared dimers of [MnO5] trigonal bipyramids to form corrugated sheets. Vanadate groups again condense the sheets into a three-dimensional framework. Infrared and Raman spectroscopies indicated the presence of OH groups and displayed characteristic Raman scattering due to vanadate groups. Temperature-dependent magnetic studies indicated Curie-Weiss behavior above 100 K with significant anti-ferromagnetic coupling for both compounds, with further complex magnetic behavior at lower temperatures. The data indicate canted anti-ferromagnetic order below 57 K in Mn5(VO4)2(OH)4 and below 45 K in Mn6O(VO4)2(OH). Members of another class of compounds, K2M3(VO4)2(OH)2 (M = Mn, Co), also containing a honeycomb-type sublattice, were also synthesized to allow a comparison of the structural features across all three structure types and to demonstrate extension to other transition metals.

A series of transition metal vanadate crystals were prepared using a high temperature (580 °C) hydrothermal method. The compounds all had the general formula A 2 AEM (VO 4 ) 2 ( A =K, Na, Li; AE =Ba, Sr; M =Co, Fe, Mn). They are all variations of the glaserite structural type and range in symmetry from P- 3 m 1 to P -3 to P 2 1 / c . Most of the derivatives contain a planar three-fold rotation operation, making them possible spin frustration candidates. Single crystal structural analyses were performed on many of the derivatives to obtain a detailed understanding of the distortions of the tetrahedral building blocks that accommodate the symmetry distortions. A hydrothermal growth method was developed to grow high quality single crystals of sizes up to 2–3 mm/edge. This method can be generalized for large crystal growth to enable magnetic and neutron diffraction studies that require relatively large single crystals.

As intermediate products of the dehydration of hydroxides or the hydration of oxides, rare earth oxyhydroxides (REOOH) have important implications for synthesis and potential applications in luminescent devices. Using high temperature oxide melt solution calorimetry and thermal analysis, thermodynamic properties of REOOH (RE ​= ​Eu to Lu), including enthalpies of dehydration and formation, were determined. The exothermic enthalpies of formation from oxides and endothermic enthalpies of dehydration demonstrate that oxyhydroxides are not only synthetic but also thermodynamic intermediates in rare earth oxide - water systems. The linear relationship between enthalpies of formation from oxides and ionic radius of RE3+ confirms the key role of cation size in the stability of REOOH, and reveals that REOOH with lighter rare earth elements are easier to form and harder to dehydrate, both thermodynamically and kinetically.

A method for the synthesis of single crystals of BaSnO3 is reported. The growth employs a hydrothermal process using simple Ba(OH)2 and SnO2 powders in water at 650 °C and 140 MPa, with 5 M CsOH mineralizer in welded inert metal liners. Solubility studies indicate that the crystals have positive solubility coefficients that increase with temperature and mineralizer concentration in the growth regimes investigated. The tetravalent tin ions are somewhat aggressive toward the inert metal liners, especially silver and gold, requiring platinum liners for longer term growth runs. The (1 1 0) and (1 0 0) faces are commonly observed on the as-grown crystals. The crystals are thermally and chemically stable and can be readily polished as substrates for subsequent epitaxial growth. Crystals that are n-doped (La:BaSnO3) can be grown by the addition of the desired amount of La2O3 to the initial feedstock. The band gap for this hydrothermally-grown BaSnO3 was measured using the Tauc plot method to be 2.94 eV, with approximately 0.3 eV shift to higher energy upon n-doping.

The solid-state chemistry of novel metal silicate and borosilicate crystals containing Eu2+ and Eu3+ in high temperature hydrothermal fluids was investigated. It was found that europium ions could be readily incorporated into a number of crystals using a variety of simple glass-based feedstocks. The hydrothermal growth reactions were performed at nearly 600 °C in 1–5 M NaOH mineralizer fluids, and this was found to be a versatile route to both europium ion oxidation states with Eu acting as either a fundamental structural building block or a dopant ion. Several new single crystals were identified including two new apatites as well as a new europium borosilicate. The apatites crystallize in space group P63/m with a = 9.4619(13) A and c = 7.0054(14) A for Eu10(SiO4)6O2, and a = 9.4413(13) A and c = 6.9087(14) A for NaEu9(SiO4)6O2. The new borosilicate, Eu2SiB2O8, crystallizes in space group Pbcn with a = 13.310(3) A, b = 4.4247(9) A and c = 9.2394(18) A. The investigation of Eu-doped borosilicates demonstrated the strong blue emission generated from 370 nm excitation that was expected for the Eu2+ doped materials, as well as the less intense orange-red emission expected for the Eu3+ dopants. The hydrothermal reactions naturally facilitated a degree of europium reduction in Eu:Ba3Si2B6O16. It was found that the oxidation states of europium in the final product could be further controlled by using hydrazine as a reducing reagent.

The structural variations of several alkali metal rare earth fluoride single crystals are summarized. Two different stoichiometric formulations are considered, namely those of ARE2F7 and ARE3F10 (A = K, Rb, Cs; RE = Y, La–Lu), over a wide range of ionic radii of both the alkali and rare earth (RE) ions. Previously reported and several new single-crystal structures are considered. The new single crystals are grown using hydrothermal methods and the structures are compared with literature reports of structures grown from both melts and hydrothermal fluids. The data reported here are combined with the literature data to gain a greater understanding of structural subtleties surrounding these systems. The work underscores the importance of the size of the cations to the observed structure type and also introduces synthetic technique as a contributor to the same. New insights based on single-crystal structure analysis in the work introduce a new disordered structure type in the case of ARE2F7, and examine the trends and boundaries of the ARE3F10 stoichiometry. Such fundamental structural information is useful in understanding the potential applications of these compounds as optical materials.

A new series of transition metal vanadates, namely, Ba2M(VO4)2(OH) (M = V(3+), Mn(3+), and Fe(3+)), was synthesized as large single crystals hydrothermally in 5 M NaOH solution at 580 °C and 1 kbar. This new series of compounds is structurally reminiscent of the brackebuschite mineral type. The structure of Ba2V(VO4)2(OH) is monoclinic in space group P21/m, a = 7.8783(2) Å, b = 6.1369(1) Å, c = 9.1836(2) Å, β = 113.07(3)°, V = 408.51(2) Å(3). The other structures are similar and consist of one-dimensional trans edge-shared distorted octahedral chains running along the b-axis. The vanadate groups bridge across edges of their tetrahedra. Structural analysis of the Ba2Mn(VO4)2(OH) analogue yielded a new understanding of the Jahn-Teller effect in this structure type. Raman and infrared spectra were investigated to observe the fundamental vanadate and hydroxide vibrational modes. Single-crystal temperature-dependent magnetic studies on Ba2V(VO4)2(OH) reveal a broad feature over a wide temperature range with maximum at ∼100 K indicating that an energy gap could exist between the antiferromagnetic singlet ground state and excited triplet states, making it potentially of interest for quantum magnetism studies.

An investigation into the crystal chemistry of the alkali thorium silicates resulted in the formation of five novel materials by hydrothermal synthesis. Of these, Rb4Th2Si12O30, and K2ThSi4O10F2 are new structure types crystallizing in space groups P1 and Pnma, respectively. The structure of Rb4Th2Si12O30 reveals a silicate sheet structure with eight-, six-, five-, and four-membered silicate rings. Crystals of a related composition, Cs2ThSi6O15, were also synthesized. They crystallize in space group Cmc21 and have a silicate sheet containing eight-, six-, and four-membered silicate rings. The structure of K2ThSi4O10F2 consists of a tubular silicate column comprised of eight-, six-, and four-membered silicate rings and features chains of fluorine-bridged Th atoms. Two new wadeite analogues Cs2ThSi3O9 and Rb2ThSi3O9 were also discovered and constitute the first actinide representatives in this mineral family. All products were crystallized from reactions of ThO2 and silica with alkali fluorides or hydroxid...

The synthesis and upconversion properties of trigonal glaserite-type K 3 Y(VO 4 ) 2 co-doped with Er 3+ /Yb 3+ , Ho 3+ /Yb 3+ , or Tm 3+ /Yb 3+ were studied. Powder samples were synthesized by solid state reactions at 1000 °C for 48 h, while well-formed hexagonal single crystals of the same were grown hydrothermally using 10 M K 2 CO 3 at 560–650 °C. Infrared-to-visible upconversion by Er 3+ /Yb 3+ , Ho 3+ /Yb 3+ , or Tm 3+ /Yb 3+ codoped-K 3 Y(VO 4 ) 2 glaserite powder and single crystals was observed, and the upconversion spectral properties were studied as a function of different Er 3+ , Tm 3+ , Ho 3+ , and Yb 3+ ion concentrations. The process is observed under 980 nm laser diode excitation and leads to strong green (552 nm) and red (659 nm) emission for Er 3+ /Yb 3+ , green (549 nm) and red (664 nm) emission for Ho 3+ /Yb 3+ , and blue (475 nm) and red (647 nm) emission for Tm 3+ /Yb 3+ . The main mechanism that allows for up-conversion is attributed the energy transfer among Yb 3+ and the various Er 3+ /Ho 3+ /Tm 3+ ions in excited states. These results illustrate the large potential of co-doped alkali double vanadates for photonic applications involving optoelectronics devices.

A series of trivalent transition metal germanates with a general formula of A3M2Ge4O14 (A ​= ​Sr, Ba, M ​= ​Mn and Fe) was synthesized using a high-temperature (580 ​°C) hydrothermal technique. Compound I, Ba3Mn2Ge4O14, is a new structure type that is characterized by one-dimensional Mn–O–Mn chains made from edge-sharing highly distorted Mn3+O6 units, and exhibits antiferromagnetic behavior below 15 ​K with field induced metamagnetic transitions. The A3Fe2Ge4O14 (A ​= ​Sr and Ba, II and III, respectively) series belongs to the well-known langasite structure type, crystallizing in space group P321. In A3Fe2Ge4O14, there is one octahedral Fe3+ site and one mixed-occupancy Fe3+/Ge4+ site that form the one-dimensional substructure. The magnetic susceptibility data of Ba3Fe2Ge4O14 reveals a much simpler magnetic structure with an antiferromagnetic transition near 5.5 ​K. The Jahn-Teller distortion of Mn3+ appears to be influential in the phase formation, leading to significantly different structures for these Mn3+ and Fe3+ compounds with similar chemical formulas.

Several new borate compounds, Ba11B26O44(PO4)2(OH)6 (1), Li9BaB15O27(CO3) (2), and Ba3Si2B6O16 (3) were synthesized containing other hetero-oxyanion building blocks in addition to the borate frameworks. They were all prepared under hydrothermal conditions and characterized by single crystal and powder X-ray diffraction, and IR spectroscopy. Crystal data: For 1; space group P21/c, a=6.8909 (14) A, b=13.629 (3) A, c=25.851 (5) A, β=90.04 (3)°; For 2; space group P-31c, a=8.8599 (13) A, c=15.148 (3) A; For 3; space group P-1, a=5.0414 (10) A, b=7.5602 (15) A, c=8.5374 (17) A, α=77.15 (3)°, β=77.84 (3)°, γ=87.41 (3)° for 3. Compounds 1 and 2 contain isolated oxyanions [PO4]3− and [CO3]2− respectively, sitting in channels created by the borate framework, while structure 3 has the [SiO4]4− groups directly bonded to the borate groups creating a B–O–Si framework.

K2ThF6 and K7Th6F31 were prepared hydrothermally as single crystals. Their structures were investigated with single-crystal X-ray diffraction at ambient conditions and at high pressures using diamond anvil cells. The results of this study indicate that the two materials, with chains of face-sharing tricapped trigonal prisms ThF9 and with layers of square antiprisms ThF8 sharing their edges in Th2F14 doublets, respectively, are structurally stable at least to about 9 GPa at room temperature. This observation suggests that, in general, moderate high pressure has no effect on the coordination polyhedra around the Th atoms and the topology of the structure of complex thorium fluorides. The bulk compressibility of the thorium fluorides entirely depends on the alkali metal present in the structure.

Three new barium manganese vanadates, all containing hexagonal barium chloride layers interpenetrated by [V2O7]4– groups, were synthesized using a high-temperature (580 °C) hydrothermal method. Two of the compounds were prepared from a mixed BaCl2/Ba(OH)2 mineralizer, and the third compound was prepared from BaCl2 mineralizer. An interesting structural similarity exists between two of the compounds, Ba2Mn(V2O7)(OH)Cl and Ba4Mn2(V2O7)(VO4)2O(OH)Cl. These two compounds crystallize in the orthorhombic space group Pnma, Z = 4, and are structurally related by a nearly doubled a axis. The first structure, Ba2Mn(V2O7)(OH)Cl (I) (a = 15.097(3) A, b = 6.1087(12) A, c = 9.5599(19) A), consists of octahedral manganese(II) edge-sharing chains linked by pyrovanadate [V2O7] groups, generating a three-dimensional structure. Compound II, Ba4Mn2(V2O7)(VO4)2O(OH)Cl (a = 29.0814(11) A, b = 6.2089(2) A, c = 9.5219(4) A), is composed of manganese(III) edge-sharing chains that are coordinated to one another through pyrovanadat...

This paper describes the hydrothermal chemistry of alkali hafnium fluorides, including the synthesis and structural characterization of five new alkali hafnium fluorides. Two ternary alkali hafnium fluorides are described: Li(2)HfF(6) in space group P31m with a = 4.9748(7) Å and c = 4.6449(9) Å and Na(5)Hf(2)F(13) in space group C2/m with a = 11.627(2) Å, b = 5.5159(11) Å, and c = 8.4317(17) Å. Three new alkali hafnium oxyfluorides are also described: two fluoroelpasolites, K(3)HfOF(5) and (NH(4))(3)HfOF(5), in space group Fm3m with a = 8.9766(10) and 9.4144(11) Å, respectively, and K(2)Hf(3)OF(12) in space group R3m with a = 7.6486(11) Å and c = 28.802(6) Å. Infrared (IR) spectra were obtained for the title solids to confirm the structure solutions. Comparison of these materials was made based on their structures and synthesis conditions. The formation of these species in hydrothermal fluids appears to be dependent upon both the concentration of the alkali fluoride mineralizer solution and the reaction temperature. Both X-ray and visible fluorescence studies were conducted on compounds synthesized in this study and showed that fluorescence was affected by a variety of factors, such as alkali metal size, the presence/absence of oxygen in the compound, and the coordination environment of Hf(4+).

As new compositions and configurations of single crystals are specified for optical applications the crystal growth community must continue to optimize and explore new options for developing such materials. New crystal growth technology and a growing understanding of certain chemical and physical phenomena sometimes implore us to reconsider our perceptions even about the most studied systems. Thus, the hydrothermal growth of YAG was reexamined after nearly 40 years of inactivity and met with promising new results. While the hydrothermal growth of YAG at 500 °C was previously met with slow growth rates or poor crystal quality we have found that growth at 600–630 °C yields good quality single crystals at rates of about 0.5 mm/side/week on (100) seeds. In the current study, high quality epitaxial layers of transition metal and rare earth doped YAG have been grown from 0.1 to 2.75 mm in thickness. This has enabled several more advanced proof of concept designed growth experiments including the growth of co-doped Ca 2+ ,Cr 4+ ,Nd:YAG for self Q-switching, and the growth of Sm:YAG around Nd:YAG to potentially suppress amplified spontaneous emission. The interface quality of the substrate and the new growth was investigated and application of a very narrow thermal gradient at the early stages of growth has proven to greatly reduce optical scattering at the interface. The morphology of hydrothermally-grown YAG crystals is also discussed and several intermediate forms are occasionally observed.

A new family of mixed alkali–alkaline earth metal thorium fluorides based on the formulation A3Ba2Th3F19 (A = Na, K, Rb) has been synthesized and structurally characterized. The compounds crystallize in the cubic space group Im-3m with a = 12.1222 (14) and 12.3052 (14) A for the K and Rb end members, respectively. This is only the second thorium fluoride reported as a single crystal structure to incorporate alkali and alkaline earth metals into the same structure. Crystals were obtained from the equimolar reaction of Ba(OH)2·H2O, ThF4 and ABF4 in a 2 M AF solution under hydrothermal conditions (545 °C and 1 kbar). Barium atoms substitute at the alkali metal sites and preferential site substitution is supported by the crystallographic analysis. Additional substitution at the alkali metal site has also been shown for K2.2Na0.8Ba2Th3F19 and K1.85Rb1.15Ba2Th3F19 using dissimilar alkali tetrafluoroborate and fluoride mineralizers in the reaction. A new family of mixed alkali–alkaline earth metal thorium fluorides has been synthesized and structurally characterized by single crystal X-ray diffraction, greatly adding to the limited crystal chemistry reported about such compounds.

Three novel hydrated borates Ba2B5O9(OH) (1), Sr2B5O9(OH) (2) and Li2Sr8B22O41(OH)2 (3) have been synthesized hydrothermally and their structures determined. Compounds (1) and (2) are isostructural, crystallizing in space group P21/c and having lattice parameters of a=6.6330(13) A, b=8.6250(17) A, c=14.680(3) A, β=93.46(3)° and a=6.4970(13) A, b=8.4180(17) A, c=14.177(3) A, β=94.35(3)°, respectively. Compound (3) crystallizes in P-1 with lattice parameters of a=6.4684(13) A, b=8.4513(17) A, c=14.881(3) A, α=101.21(3)°, β=93.96(3)°, γ=90.67(3)°. While similar in their axis lengths, (3) differs greatly in structure and formulation from (1) and (2). The structure of (1) and (2) is contrasted to that of the well-known mineral hilgardite (Ca2B5O9Cl·H2O).

The synthesis and structure of the isostructural acentric compounds Sr(3)Be(2)B(5)O(12)(OH) (1) and Ba(3)Be(2)B(5)O(12)(OH) (2) are reported for the first time. These compounds crystallize in the space group R3m, and the unit cell parameters are a = 10.277(15) Å and c = 8.484(17) Å for 1 and a = 10.5615(15) Å and c = 8.8574(18) Å for 2. The structures consist of a network of [Be(2)B(4)O(12)(OH)] units interwoven with a network consisting of MO(9) polyhedra (M = Sr, Ba) and BO(3) triangles and exemplify how acentric building blocks such as [BO(3)](3-), [BO(4)](5-), and [BeO(4)](6-) can be especially suitable to build noncentrosymmetric long-range structures. Both networks are centered on the 3-fold rotation axis and present themselves in alternating fashion along [001]. Acentricity is imparted by the alignment of the polarities of BO(3) and BeO(4) environments. Infrared spectroscopy has been used to confirm the local geometries of B and Be, as well as the presence of hydroxide in the crystal structure. Another interesting feature of these compounds is the presence of disorder involving Be and B at the tetrahedral Be site. The degree of the disorder has been confirmed by observing a noticeable shortening of average Be-O bond distances.

Crystals of Ba{sub 2}TiSi{sub 2}O{sub 8}, Sr{sub 2}TiSi{sub 2}O{sub 8}, and Ba{sub 2}VSi{sub 2}O{sub 8}, all belonging to the fresnoite family were prepared by the hydrothermal method. Attempts to synthesize other members by substitution at the alkaline earth site, transition metal site, and tetrahedral site were not successful, but did lead to the formation of a variety of other mineral phases as well as the incommensurate structure, Ba{sub 2}TiGe{sub 2}O{sub 8}. Structural studies on this compound and analysis of the other known fresnoite phases were performed to elucidate the nature of the incommensurate structure. -- Graphical abstract: Materials crystallizing from the hydrothermal TiO{sub 2}+SiO{sub 2}+Ba(OH){sub 2} system. Display Omitted Research highlights: {yields} A variety of products crystallized from the Ba(OH){sub 2}-TiO{sub 2}-SiO{sub 2} hydrothermal system. {yields} Isolation of several fresnoite derivatives. {yields} Structural study on the incommensurate Ba{sub 2}TiGe{sub 2}O{sub 8} material.

The growth of fresnoite, Ba 2 TiSi 2 O 8 , by hydrothermal synthesis has led to spontaneous generation of large, (4–5 mm) optically clear crystals from 6 M KF mineralizer solutions. Growth was achieved at relatively low synthesis temperatures (575 °C) comparative to fresnoite synthesis by Czochralski or flux methods. Bulk crystal growth possibilities were explored by transport reactions performed in both fluoride and hydroxide mineralizers with 25–45 °C temperature gradients. Growth rates of 0.14×0.19×0.22 mm 3 /week were established in 6 M KOH, which is significantly slower than standard hydrothermal rates of 1 mm/week. Although relatively slow, the hydrothermal method has been demonstrated as a synthesis route to high quality single crystals of fresnoite.

Single crystals of KNbO{sub 3} (KN), KTaO{sub 3} (KT), and KTa{sub 1-x}Nb{sub x}O{sub 3} (x=0.44, KTN) have been prepared by hydrothermal synthesis in highly concentrated KOH mineralizer solutions. The traditional problems of inhomogeneity, non-stoichiometry, crystal striations and crystal cracking resulting from phase transitions associated with this family compounds are minimized by the hydrothermal crystal growth technique. Crystals of good optical quality with only minor amounts of metal ion reduction can be grown this way. Reactions were also designed to provide homogeneous distribution of tantalum and niobium metal centers throughout the KTN crystal lattice to maximize its electro-optic properties. Synthesis was performed at relatively low (500-660 {sup o}C) temperatures in comparison to the flux and Czochralski techniques. This work represents the largest crystals of this family of compounds grown by hydrothermal methods to date. -- Graphical Abstract

Recently developed glass-clad semiconductor core optical fibers offer potential advantages over present optical fiber materials, including greatly enhanced Raman cross-sections and extended infrared transparency. While fibers have been fabricated that exhibit a high degree of crystallinity there has not been any in-depth analysis of the nature of the crystallographic orientation of the core material relative to the fiber axes. This crystallographic analysis is of important scientific and technological value since optical fiber fabrication is a highly non-equilibrium process; consequently, achieving high degrees of crystallinity is counter-intuitive. In this work, the crystallographic orientation of germanium core optical fibers was analyzed using single crystal X-ray diffraction and electron backscatter diffraction techniques. Over nearly a 100 mm length of fiber the Ge cores were found to be polycrystalline with the 〈1 0 0〉 and 〈1 1 0〉 orientations dominant implying a dendritic growth mechanism. Single crystal regions were observed routinely in lengths greater than 8 mm with the longest being about 15 mm.

Single crystals of hafnia stabilized with yttrium, neodymium, holmium and erbium 0.25 mm in size were formed in hydrothermal solutions by spontaneous nucleation in 20 M KOH at 750 °C. Positive transport conditions were established at 650 °C using a single crystal of cubic zirconia as a substrate for growth of yttrium stabilized hafnia at a rate of 0.084 mm/week. During the study single crystals could only be formed with high concentrations of the stabilizer dopants by this method. This is the lowest synthesis temperature reported for any single crystal growth method of this refractory material.

In this paper we report the synthesis, solubility and growth of large YVO 4 and Nd:YVO 4 transparent single crystals in aqueous NaOH under hydrothermal conditions. YVO 4 crystals formed from hydrothermal spontaneous nucleation by (1) reaction of Y 2 O 3 and V 2 O 5 , (2) reaction of Y 2 O 3 and Na 3 VO 4 or (3) recrystallization of Czochralski-grown YVO 4 . Crystals synthesized by these hydrothermal methods were of good quality, exhibiting no signs of vanadium reduction. YVO 4 was found to have a positive solubility coefficient from 375 to 500 °C in 2–6 M NaOH, as well as a positive dependence on mineralizer concentration. Single crystals of YVO 4 and Nd:YVO 4 were grown on YVO 4 seeds using 2 M NaOH as a mineralizer and exhibited the expected tetragonal morphology. Growth rates between 0.097 and 0.13 mm/day were observed. Nd:YVO 4 growth using temperatures of 450 °C in the dissolution zone and 410 °C in the growth zone produced a crystal 15×15×7 mm 3 in size after a 4-week growth period.

Single crystals of Sc 2 O 3 hold great promise as potential hosts for high-energy lasers, but the extremely refractory nature of this oxide makes crystal growth difficult. Hydrothermal methods have been used to synthesize and grow single crystals of Sc 2 O 3 and Er:Sc 2 O 3 at a temperature over 1800 °C below their melting point. Scandia crystals up to 6 mm in size have been obtained by spontaneous nucleation. A preliminary hydrothermal growth study was performed and rates of up to 0.16 mm/day were observed for seeded growth of both Sc 2 O 3 and Er:Sc 2 O 3 . Hydrothermal scandia has also been doped with trivalent Ho, Tm and Yb, making this an attractive technique for the growth of refractory optical crystals.

Crystals of a family of compounds, ABe 2 BO 3 F 2 (ABBF, A=K, Rb, Cs, Tl), have been grown hydrothermally. Each of these materials was studied using the powder SHG technique and exhibited promising NLO behavior. Seeded crystal growth was demonstrated and the growth conditions were optimized by modifying the temperature, thermal gradient and mineralizer concentration. RbBe 2 BO 3 F 2 crystals possessed a particularly good combination of SHG intensity, favorable crystal habit and fast growth rates. High quality crystals suitable for advanced deep-UV NLO studies were grown at rates of 0.11 mm/day on (0 0 1) and 0.12 mm/day perpendicular to (0 0 1).

Large single crystals of the acentric, tetragonal lithium metaborate, γ-LiBO 2 , have been grown using the hydrothermal method. Favorable conditions for the synthesis of γ-LiBO 2 were found over the course of a phase stability study of lithium borates in supercritical aqueous base. Hydrothermal synthesis was achieved by: (1) conversion of the α phase, (2) direct crystallization from B 2 O 3 and LiOH or (3) recrystallization of previously crystallized γ-phase crystallites. Solubility of γ-LiBO 2 in dilute aqueous LiOH was found to be 2–5 wt% from 460 to 75 °C with a positive solubility coefficient. Crystal growth experiments were designed to take advantage of the favorable solubility conditions within this range. Seeded growth occurred at a maximum rate of 2.1 mm/day when the dissolution zone temperature was 535 °C and the growth zone temperature was 495 °C. This permitted the routine growth of centimeter-size crystals in less than 1 week. These crystals possess an interesting combination of optical properties, including a band gap of about 9 eV and high non-linear optical (NLO) susceptibility.

The use of high temperature hydrothermal reactions to prepare crystals having mineralogically-related structures is described. Complex naturally occurring minerals can have fascinating structures and exhibit important features like low dimensionality, noncentrosymmetry, or ion channels that can provide excellent guideposts for the designed synthesis of new materials. Actual minerals, even though they may have intriguing physical properties, are often unsuitable for study because of the persistent impurities inevitably present in natural samples. Hydrothermal fluids at relatively high temperatures provide access to large, high quality single crystals of structures with mineral-like structures. This enables the study of physical properties like ionic conduction, magnetic spin frustration and non-linear optical behavior. Some fundamental considerations of the hydrothermal technique are discussed in the context of synthesizing mineralogically-inspired materials. The metal vanadates provide a surprisingly rich and diversified range of compounds and are selected to illustrate many of the concepts described here. A series of low dimensional mineral analogs featuring isolated units, chains, and layers have been prepared in the laboratory as large single crystals using a high temperature hydrothermal synthetic methods, and their physical properties are under investigation. The metal silicates are also highlighted as another promising field of exploration, since their hydrothermal synthesis surprisingly lags behind the enormous literature of the natural silicate minerals. The introduction of heteroelements, such as boron to make borosilicates, appears to also open the door to additional new materials. Many of these new materials have direct equivalents in the mineral kingdom, while others have no known analogs but are reminiscent of minerals and can be classified in the same ways. From these initial results there appears to be a very rich vein of synthetic minerals waiting to be unearthed in the laboratory using the high temperature hydrothermal method.