A. Grandin

Redetermination of the β-Ca2P2O7 structure

β Dicalcium diphosphate is composed of eclipsed P 2 O 7 groups linked through Ca 2+ cations. The P-Ob bonds involving the bridging O atoms arelonger [average 1.620 (5) A] than the others [average 1.517 (5) A]. As the «O 4 » tetrahedra are almost regular the displacement of the P atoms involves two sets of O-P-O angles: O-P-O≃112.7 (2) and O-P-O b ≃106.0 (2) o

Two new diphosphates with SrV2(P2O7)2 structure: mercury and lead phases

Two new diphosphates of the AV2(P2O7)2 series involving a trans configuration of the bidentate P2O7 groups have been synthesized for A = Hg or Pb. Their structure, determined from a single-crystal X-ray diffraction study is similar to that of the Sr-phase. They crystallize in the P space group, Z= 1, a= 4.848(1)A, b= 6.892(1)A, c= 8.077(2)A, α= 92.65(1)°, β= 93.26(1)°, γ= 106.23(1)°, for the Hg phase and a= 4.804(1)A, b= 7.113(1)A, c= 7.898(2)A, α= 89.78(1)°, β= 92.62(1)°, γ= 106.10(2)°, for the Pb phase. The crystal structures, solved by the heavy-atom method, were refined. The final agreement factors are R= 0.027 and 0.033 for the Hg and Pb phases, respectively.A comparison with the other isotypic members of this series A = Sr, Cd and NaxMoP2O7(0.25 < x < 0.50) is presented. The great similarity between the lead and strontium phases is shown, demonstrating the absence of a lone-pair effect of PbII in this structure. The particular behaviour of the Hg phase is emphasized, due to the ability of this element to adopt the dumbbell (or 2 + 4) coordination; the strong covalent character of HgII induces a significant distortion of the PO4 tetrahedra, rarely observed to date in transition-metal phosphates.

(NH4)2(VO)(VP)O7, a Layered Structure Comprising Tetrahedral VPO7 Groups

The new mixed-valence ammonium vanadophosphate (NH 4 ) 2 (VO)(VP)O 7 (diammonium phosphorus divanadium octaoxide) is isostructural with K 2 VO(X 2 O 7 ) (X = P, V). Its structure consists of [V 2 PO 8 ]∞layers of corner-sharing VO 5 pyramids and VPO 7 groups, interleaved with NH 4 + ions. The unusual features of this structure are the existence of the VPO 7 groups involving VO 4 and PO 4 tetrahedra, and the simultaneous presence of V IV O 5 and V V O 4 polyhedra, observed here for the first time in this class of compounds. The VPO 7 groups are statistically distributed in the framework. Magnetic measurements and bond-valence calculations confirm the mixed-valence character of this phase and show that the VO 5 pyramids and VO 4 tetrahedra are occupied by V IV and V V , respectively.

CaV2O(PO4)2, isotypic with the Cd phase

The new V III monophosphate, calcium divanadium oxodiphosphate, CaV 2 O(PO 4 ) 2 , is isotypic with the Cd phase. Its structure consists of [V 2 PO 7 ]∞ layers, built up from [VO 4 ]∞ rutile chains and [VPO 8 ]∞ chains, interconnected through PO 4 tetrahedra. This framework delimits tunnels running along (011), at the intersection of which the calcium ions are located in a «6 + 1» coordination mode

An MoV monophosphate, Rb2Mo2P2O11

Molybdenum rubidium phosphate, Rb 2 Mo 2 P 2 O 11 , has been synthesized and the growth of single crystals allowed its structure to be determined. This compound is isotypic with Tl 2 Mo 2 P 2 O 11 and K 2 Mo 2 P 2 O 11 , and contains a framework built from Mo 2 P 2 O 15 units formed by one Mo 2 O 11 group sharing four of its corners with two PO 4 tetrahedra, leading to [Mo 2 P 2 O 13 ]∞ double layers. The [Mo 2 P 2 O 11 ]∞ framework forms two different intersecting tunnels along [001] and where the Rb + ions are located. Although different from Rb 1.5 Mo 2 P 2 O 11 , the two structures exhibit some close relationships

Single-crystal structure of K3Nb8O21

Tripotassium 21-oxooctaniobate, M r =1196.53, hexagonal, P6 3 /mcm, a=9.1584 (5), c=12.0404 (4) A, V=874.6 (1) A 3 , Z=2, D x =4.54 Mg m -3 , λ(Mo Kα)=0.71073 A, μ=5.65 mm -1 , F(000)=1106, T=294 K, R=0.044, wR=0.047 for 2540 reflections with I>3σ(I). The framework is built up from [Nb 6 O 24 ] units stacked along c giving [Nb 6 O 21 ] @0 2 2 chains which are held together by single NbO 6 octahedra

CdV2(P2O7)2

Cadmium vanadium diphosphate, CdV 2 (P 2 O 7 ) 2 , is isotypic with AV 2 (P 2 O 7 ) 2 (A=Ca, Sr) and Na x MoP 2 O 7 (0.25<x<0.50). Its structure consists of similar [VP 4 O 14 ]∞ columns which are built up from VO 6 octahedra and P 2 O 7 pyrophosphate groups, and linked together through VO 6 octahedra. This framework delimits tunnels in which the Cd ions are located with octahedral coordination. A comparison with AV 2 (P 2 O 7 ) 2 (A=Ca, Sr) is given

Structure of Tl2Mo2P2O11

Dimolybdenum dithallium diphosphate, Tl 2 Mo 2 P 2 O 11 , is isotypic with K 2 Mo 2 P 2 O 11 . Its framework is built from the original Mo 2 P 2 O 15 unit, formed by one Mo 2 O 11 group sharing four of its corners with two PO 4 tetrahedra, leading to [Mo 2 P 2 O 13 ]∞ chains running along c. The [Mo 2 P 2 O 11 ]∞ framework forms two different intersecting tunnels running along c and , where the Tl + ions are located

Structure of a new siliconiobate K6.5Ca0.75Nb14Si4O47

A new siliconiobate, potassium calcium siliconiobate, K 6.5 Ca 0.75 Nb 14 Si 4 O 47 , has been isolated which is isotypic with the oxide Ba 6+x Nb 14 Si 4 O 47 [Evans & Katz (1973). J. Solid State Chem. 8, 150-158]. Its host lattice consists of an intergrowth of two struc tural types, K 3 Nb 8 O 21 [Groult, Chailleux, Choisnet & Raveau (1976). J. Solid State Chem. 19, 235-244] and Ba 3 NbaSi 4 O 26 [Shannon & Katz (1970). Acta Cryst. B26, 105-109]. In this cage structure, the K and Ca atoms are distributed over two kinds of sites which are characterized by fifteen- and twelvefold coordination, respectively

NaTi5NbO13, an A2Ti6O13‐Type Phase Deficient in Alkali Metal Cations, Synthesized Under Mild Conditions.

Thermolysis of H2NaTi5NbO14·3 H2O (prepared by cation exchange) at 300 °C yields the title compound.