Hydrothermal Synthesis of Open-Framework Borophosphates with Tunable Micropore Sizes, Crystal Morphologies, and Thermal Stabilities

Thermal stabilities of zeolitic frameworks are important parameters for many applications. Two decades of research have produced only a very small number of zeolitic borophosphates such as Na₂[VO­(B₂O)­(PO₄)₂(HBO₃)]·<i>x</i>H₂O (<i>x</i> <b> ≈ </b> 2.92) (denoted as <b>B</b>_<b>3</b><b>P</b>_<b>2</b>), which shows the onset dehydration and a complete decomposition at 200 and 400 °C, respectively. In order to enhance thermal stabilities of borophosphate frameworks, a water-deficient hydrothermal route with phosphoric acid as the sole solvent has been developed and led to controlled syntheses of <b>B</b>_<b>3</b><b>P</b>_<b>2</b> and a new vanadium borophosphate, K_1.33Na_0.67[VO­(B₂O)­(PO₄)₂(HPO₄)]·<i>x</i>H₂O (<i>x</i> <b> ≈ </b> 1.63) (denoted as <b>B</b>_<b>2</b><b>P</b>_<b>3</b>). The latter is the first-ever borophosphate possessing the zeolite RHO-type net and is characterized by superlarge spherical cages, including 16-ring and 8-ring channels along the axes and 12-ring channels along the diagonals of the cubic cell. The new compound <b>B</b>_<b>2</b><b>P</b>_<b>3</b> has larger structural cages and higher thermal stability than <b>B</b>_<b>3</b><b>P</b>_<b>2</b>, where the enhanced thermal stability is attributable to different bonding arising from the substitution of [BO₂(OH)] by [PO₃(OH)] in the framework. This is the first demonstration that the micropore size, crystal morphology, and thermal stability of zeolitic borophosphates can be tuned by changing the fundamental building units of their frameworks via adjusting the B/P ratios in the starting materials

Multiple Fluorine-Substituted Phosphate Germanium Fluorides and Their Thermal Stabilities

Anhydrous compounds are crucially important for many technological applications, such as achieving high performance in lithium/sodium cells, but are often challenging to synthesize under hydrothermal conditions. Herein we report that a modified solvo-/hydro-fluorothermal method with fluoride-rich and water-deficient condition is highly effective for synthesizing anhydrous compounds by the replacement of hydroxyl groups and water molecules with fluorine. Two anhydrous phosphate germanium fluorides, namely, Na₃[GeF₄(PO₄)] and K₄[Ge₂F₉(PO₄)], with chainlike structures involving multiple fluorine substitutions, were synthesized using the modified solvo-/hydro-fluorothermal method. The crystal structure of Na₃[GeF₄(PO₄)] is constructed by the common single chains _∞¹{[GeF₄(PO₄)]^3–} built from alternating GeO₂F₄ octahedra and PO₄ tetrahedra. For K₄[Ge₂F₉(PO₄)], it takes the same single chain in Na₃[GeF₄(PO₄)] as the backbone but has additional flanking GeOF₅ octahedra via an O-corner of the PO₄ groups, resulting in a dendrite zigzag single chain _∞¹{[Ge₂F₉(PO₄)]^4–}. The multiple fluorine substitutions in these compounds not only force them to adopt the low-dimensional structures because of the “tailor effect” but also improve their thermal stabilities. The thermal behavior of Na₃[GeF₄(PO₄)] was investigated by an in situ powder X-ray diffraction experiment from room temperature to 700 °C. The modified solvo-/hydro-fluorothermal method is also shown to be effective in producing the most germanium-rich compounds in the germanophosphate system

Multiple Fluorine-Substituted Phosphate Germanium Fluorides and Their Thermal Stabilities

Anhydrous compounds are crucially important for many technological applications, such as achieving high performance in lithium/sodium cells, but are often challenging to synthesize under hydrothermal conditions. Herein we report that a modified solvo-/hydro-fluorothermal method with fluoride-rich and water-deficient condition is highly effective for synthesizing anhydrous compounds by the replacement of hydroxyl groups and water molecules with fluorine. Two anhydrous phosphate germanium fluorides, namely, Na₃[GeF₄(PO₄)] and K₄[Ge₂F₉(PO₄)], with chainlike structures involving multiple fluorine substitutions, were synthesized using the modified solvo-/hydro-fluorothermal method. The crystal structure of Na₃[GeF₄(PO₄)] is constructed by the common single chains _∞¹{[GeF₄(PO₄)]^3–} built from alternating GeO₂F₄ octahedra and PO₄ tetrahedra. For K₄[Ge₂F₉(PO₄)], it takes the same single chain in Na₃[GeF₄(PO₄)] as the backbone but has additional flanking GeOF₅ octahedra via an O-corner of the PO₄ groups, resulting in a dendrite zigzag single chain _∞¹{[Ge₂F₉(PO₄)]^4–}. The multiple fluorine substitutions in these compounds not only force them to adopt the low-dimensional structures because of the “tailor effect” but also improve their thermal stabilities. The thermal behavior of Na₃[GeF₄(PO₄)] was investigated by an in situ powder X-ray diffraction experiment from room temperature to 700 °C. The modified solvo-/hydro-fluorothermal method is also shown to be effective in producing the most germanium-rich compounds in the germanophosphate system

Two Isotypic Transition Metal Germanophosphates <i>M</i>^II₄(H₂O)₄[Ge(OH)₂(HPO₄)₂(PO₄)₂] (<i>M</i>^II = Fe, Co): Synthesis, Structure, Mössbauer Spectroscopy, and Magnetic Properties

Synthetic, structural, thermogravimetric, Mössbauer spectroscopic, and magnetic studies were performed on two new isotypic germanophosphates, <i>M</i>^II₄(H₂O)₄[Ge­(OH)₂(HPO₄)₂(PO₄)₂] (<i>M</i>^II = Fe, Co), which have been prepared under hydro-/solvo-thermal conditions. Their crystal structures, determined from single crystal data, are built from zigzag chains of <i>M</i>^IIO₆-octahedra sharing either trans or skew edges interconnected by [GeP₄O₁₄(OH)₄]^8– germanophosphate pentamers to form three-dimensional neutral framework structure. The edge-sharing <i>M</i>^IIO₆-octahedral chains lead to interesting magnetic properties. These two germanophosphates exhibit a paramagnetic to antiferromagnetic transition at low temperatures. Additionally, two antiferromagnetic ordering transitions at around 8 and 6 K were observed for cobalt compound while only one at 19 K for the iron compound. Low-dimensional magnetic correlations within the octahedral chains are also observed. The divalent state of Fe in the iron compound determined from the Mössbauer study and the isothermal magnetization as well as thermal analyses are discussed

Two Isotypic Transition Metal Germanophosphates <i>M</i>^II₄(H₂O)₄[Ge(OH)₂(HPO₄)₂(PO₄)₂] (<i>M</i>^II = Fe, Co): Synthesis, Structure, Mössbauer Spectroscopy, and Magnetic Properties

Synthetic, structural, thermogravimetric, Mössbauer spectroscopic, and magnetic studies were performed on two new isotypic germanophosphates, <i>M</i>^II₄(H₂O)₄[Ge­(OH)₂(HPO₄)₂(PO₄)₂] (<i>M</i>^II = Fe, Co), which have been prepared under hydro-/solvo-thermal conditions. Their crystal structures, determined from single crystal data, are built from zigzag chains of <i>M</i>^IIO₆-octahedra sharing either trans or skew edges interconnected by [GeP₄O₁₄(OH)₄]^8– germanophosphate pentamers to form three-dimensional neutral framework structure. The edge-sharing <i>M</i>^IIO₆-octahedral chains lead to interesting magnetic properties. These two germanophosphates exhibit a paramagnetic to antiferromagnetic transition at low temperatures. Additionally, two antiferromagnetic ordering transitions at around 8 and 6 K were observed for cobalt compound while only one at 19 K for the iron compound. Low-dimensional magnetic correlations within the octahedral chains are also observed. The divalent state of Fe in the iron compound determined from the Mössbauer study and the isothermal magnetization as well as thermal analyses are discussed

Dimensional Reduction From 2D Layer to 1D Band for Germanophosphates Induced by the “Tailor Effect” of Fluoride

The “tailor effect” of fluoride, exclusively as a terminal rather than a bridge, was applied successfully to design low-dimensional structures in the system of transition metal germanophosphates for the first time. Two series of new compounds with low-dimensional structures are reported herein. K­[<i>M</i>^IIGe­(OH)₂(H_0.5PO₄)₂] (<i>M</i> = Fe, Co) possess flat layered structures built from single chains of edge-sharing <i>M</i>^IIO₆ and GeO₆ octahedra interconnected by HPO₄ tetrahedra. Their fluorinated derivatives, K₄[<i>M</i>^IIGe₂F₂(OH)₂­(PO₄)₂(HPO₄)₂]·2H₂O (M = Fe, Co), exhibit band structures of two four-membered ring germanium phosphate single chains sandwiched by M^IIO₆ octahedra via corner-sharing. Both of these structures contain anionic chains of the condensation of four-membered rings built from alternating GeO₄Φ₂ (Φ = F, OH) octahedra and PO₄ tetrahedra via sharing common GeO₄Φ₂ (Φ = F, OH) octahedra, the topology of which is the same as that of the mineral kröhnkite [Na₂Cu­(SO₄)₂·2H₂O]. Note that the switch from the two-dimensional layered structure to the one-dimensional band structure was performed simply by the addition of a small amount of KF·2H₂O to the reaction mixture. This structural alteration arises from the incorporation of one terminal F atom to the coordination sphere of Ge, which breaks the linkage between the transition metal and germanium octahedra in the layer to form the band structure

Structural Assembly from Phosphate to Germanophosphate by Applying Germanate as a Binder

Structural assembly from phosphate to germanophosphate by applying germanate as a binder has been achieved. Two isotypic porous compounds, K₃[M^II₄(HPO₄)₂]­[Ge₂O­(OH)­(PO₄)₄]·<i>x</i>H₂O (<i>M</i>^II = Fe, Cd; <i>x</i> = 2 for Fe and 3 for Cd, denoted as <b>KFeGePO-1</b> and <b>KCdGePO-1</b>, respectively), contain a known transition-metal phosphate (TMPO) layer, _∞²{[<i>M</i>₂(HPO₄)₃]^2–}, which is built from chains of trans-edge-sharing <i>M</i>O₆ octahedra bridged by <i>M</i>O₅ trigonal bipyramids that were further linked and decorated by phosphate tetrahedra. The layers are bound by infinite chains of GeO₅(OH) octahedra, resulting in a 3D open-framework structure with 1D 12-ring channels that are occupied by K⁺ ions and water molecules. The curvature of the TMPO layers and shape of the 12-ring windows can be tuned by the transition metals because of their Jahn–Teller effect

Structural Assembly from Phosphate to Germanophosphate by Applying Germanate as a Binder

Structural assembly from phosphate to germanophosphate by applying germanate as a binder has been achieved. Two isotypic porous compounds, K₃[M^II₄(HPO₄)₂]­[Ge₂O­(OH)­(PO₄)₄]·<i>x</i>H₂O (<i>M</i>^II = Fe, Cd; <i>x</i> = 2 for Fe and 3 for Cd, denoted as <b>KFeGePO-1</b> and <b>KCdGePO-1</b>, respectively), contain a known transition-metal phosphate (TMPO) layer, _∞²{[<i>M</i>₂(HPO₄)₃]^2–}, which is built from chains of trans-edge-sharing <i>M</i>O₆ octahedra bridged by <i>M</i>O₅ trigonal bipyramids that were further linked and decorated by phosphate tetrahedra. The layers are bound by infinite chains of GeO₅(OH) octahedra, resulting in a 3D open-framework structure with 1D 12-ring channels that are occupied by K⁺ ions and water molecules. The curvature of the TMPO layers and shape of the 12-ring windows can be tuned by the transition metals because of their Jahn–Teller effect

Structural Assembly from Phosphate to Germanophosphate by Applying Germanate as a Binder

Structural assembly from phosphate to germanophosphate by applying germanate as a binder has been achieved. Two isotypic porous compounds, K₃[M^II₄(HPO₄)₂]­[Ge₂O­(OH)­(PO₄)₄]·<i>x</i>H₂O (<i>M</i>^II = Fe, Cd; <i>x</i> = 2 for Fe and 3 for Cd, denoted as <b>KFeGePO-1</b> and <b>KCdGePO-1</b>, respectively), contain a known transition-metal phosphate (TMPO) layer, _∞²{[<i>M</i>₂(HPO₄)₃]^2–}, which is built from chains of trans-edge-sharing <i>M</i>O₆ octahedra bridged by <i>M</i>O₅ trigonal bipyramids that were further linked and decorated by phosphate tetrahedra. The layers are bound by infinite chains of GeO₅(OH) octahedra, resulting in a 3D open-framework structure with 1D 12-ring channels that are occupied by K⁺ ions and water molecules. The curvature of the TMPO layers and shape of the 12-ring windows can be tuned by the transition metals because of their Jahn–Teller effect